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3 In Situ STM Studies of Model Catalysts Fan Yang and D. Wayne Goodman 3.1 Introduction The surface science approach to studying heterogeneous catalysis dates back to the pioneering work of Langmuir [1] in the 1910s that addressed the adsorption ofgases on catalyst surfaces. Since then surface science studies of catalytic processes have played a central role in our understanding of catalysis and have aided in the design and improvement ofcatalysts for energy and environmental uses. The goal ofsurface science investigations has been to provide structural and spectroscopic information of catalyst surfaces at the spatial and temporal limit. Scanning tunneling microscopy (STM), with the capacity to reach the spatial limit at the atomic level, has ignited considerable interest since its discovery and has become a widely used tool in catalytic science. By following a selected area or a molecule at a model catalyst surface, in situ STM can provide temporal measurements regarding the elementary steps of catalytic transformations. The capabilities of in situ STM allow one to follow the dynamic change of surface species and identify what Taylor described as the "active site" in catalytic reactions. Such studies also provide kinetic measurements at the atomic scale, enabling the most precise modeling of macroscopic reactions. With the development of STM techniques, a large body of in situ STM work has emerged in the past decade regarding catalytic processes such as adsorption and diffusion, surface reaction, and catalyst deactivation. These experiments provide invaluable insights into the fundamental issues of catalysis. The timescale ofcatalytically important processes ranges from 10- 12 to 104 s, with the chemisorptions and reactions taking place within picoseconds whereas catalyst deactivation occurs in seconds or minutes. Although the timescale of the funda mental process of adsorption and reaction is beyond the time resolution of STM, information about the pathway and energetics ofadsorption/reaction can be acquired with STM by monitoring the change in the spatial distribution ofsurface adsorbates. Indeed, in situ STM can be combined with femtolasers to probe surface processes at both the spatial and the temporal limits. The feasibility of combining these two Scanning Tunneling Microscopy in Surface Science. Nanoscience and Catalysis Edited by Michael Bowker and Philip R. Davies Copyright © 2010 WILEY·VO! Verlag GmbH & Co. KGaA. Weinheim ISBN: 978-3·527·31982·4 , ~. I ,1 I 561 3 In Situ STM Studies of Model Catalysts techniques has been demonstrated in a recent study by Bartels et al. [2]. Such studies are extremely promising with respect to our understanding of surface chemical processes. In this chapter, we show how in situ STM has helped to visualize elementary steps of chemical reaction and to elucidate mechanisms of catalyzed processes. Like most surface science techniques, conventional in situ STM studies have been carried out in UHV on model catalysts consisting ofextended planar surfaces. When extrapolating the information obtained in UHV surface science studies to real·world catalysis, two issues have generally concerned the catalysis community, namely. the pressure and material gaps. The pressure gap refers to the fact that surface science studies are conducted under UHV conditions (10- 10_10-- 14 bar). whereas industrial catalytic reactions typically are carried out at high pressures (l-toOO bar). Over 10 orders ofmagnitude difference in the pressure of reactant gases can drastically change the interaction of reactants on the catalyst surface. a process essential to a catalytic reaction. The material gap refers to the gap between the surface structure of metal single crystals often studied in surface science and that oftechnical catalysts. "Real-world" catalysts usually consist of small metal clusters ranging from 1 to 100 nm in size. finely dispersed onto a high· surface-area oxide support. These metal dusters can have structures and properties that are quite different from the bulk metal. In catalytic research, it is well documented that the reactivity and selectivity of catalysts often depend on the size and shape of supported metal clusters [3]. Furthermore, the presence of the oxide support can modify the structure and properties of supported metal clusters. The effect of cluster size and metal support interaction cannot be addressed in surface science studies on well-defined Single crystal metal surfaces. As a local structural probe. STM has the advantage ofaddressing these two issues. Although the operational range ofSTM extends from UHV to high pressures, there are challenges in maintaining the stability of STM at catalytically realistic operating temperatures and pressures. Nevertheless. it is possible to apply STM to study catalytic reactions under realistic conditions. To bridge the material gap, supported model catalysts. consisting of small metal clusters supported on planar oxide surfaces, have been introduced into surface science studies. STM is ideally suited to precisely charac terize the structure of these supported model catalysts. In this chapter, we show the recent progress in bridging the pressure and material gaps by applying in situ STM to the study of model catalysts under realistic reaction conditions. 3.2 Instrumentation To visualize the fundamental steps of chemisorptions and reactions that occur at surfaces, in situ STM investigations typically monitor the diffusion or transformation of adsorbed molecules. A series of snapshots of preselected surface regions, compiled into a STM movie, can reveal the evolution of surfac~ phenomena. On metal surfaces, the surface diffusion of adsorbates is usually so rapid that the surface temperature mu STM viewing. Low-ternl potentially allows the det enables the precise contI the STM tip. To measure reaction ki at temperatures relevant ature (VT) STM is requir' typical scan rate of on Considering the scannil acquisition time ofSTM the scanning component few STM groups adrieve model catalyst surfaces [ and using high perform pushed the scan rate abc: atomic resolution and 31 Recently, a few grmq investigations to high ~ over a wide pressure small changes at the ambient gas. Efforts temperatures and For in situ STM able to track a preirele~ junction instabilities contact with a approaches that ofthe particular areas developed where with the collimated monitor a pn$ellecte<lll pressure over 12 In addition to for in situ STM extensively studied tip is important for and in situ tip transition metals gases, especially ical and thermal 3.2 Instrumentation surface temperature must be lowered below room temperature (RT) for successful STM viewing. Low-temperature (LT) STM, developed in the mid-1990s, not only potentially allows the determination ofreaction intermediates and pathways but also enables the precise control and measurement ofthe bond activation processes using the STM tip. To measure reaction kinetics, STM should have the capability to resolve adsorbates at temperatures relevant to catalytic reactions. For this purpose, a variable temper ature (VT) STM is required, as well as capabilities for rapid scanning. VT STM with a typical scan rate of one frame per minute was developed in the mid-1990s. Considering the scanning probe is a mechanical probe driven by electronics, the acquisition time of STM images is typically restricted by the mechanical behavior of the scanning components and the performance ofthe electronics. In the mid-1990s, a few STM groups achieved a fast scan rate of approximately 20 frames/s on extended model catalyst surfaces [4-6]. Working on the compact design of the scanner probe and using high performance electronics, Frenken and coworkers [7] have recently pushed the scan rate above the video rate (",,50frame/s) on a graphite surface, with atomic resolution and an image with 256 x 256 pixels. Recently, a few groups have taken up the challenge to extend the in situ STM investigations to high pressures. A major challenge in imaging surfaces with STM over a wide pressure range is the sensitivity of the tunneling current to extremely small changes at the tunneling junction resulting from induced instabilities by the ambient gas. Efforts have emphasized the design of a STM that can work at high temperatures and pressures with greater stabilities [8-11]. For in situ STM studies at high temperature and pressures, the inability of being able to track a preselected surface area is often the limiting factor given the tunnel junction instabilities and sample drifts. To overcome this challenge and to maintain contact with a specific surface region, it is important to develop experimental approaches that pattern the surface without influencing the kinetics and dynamics of the particular areas under study. A "shadowing" technique (Figure 3.1a) has been developed where metal atoms are dosed with the STM tip in the tunneling position with the collimated metal flux creating a shadow of the tip on the substrate [12, 13]. For metal clusters supported on an oxide surface, tip manipulation is another method ofchoice (Figure 3.1c). This technique removes clusters from a specific area through aggressive scanning. Using the STM tip to pattern the surface, it is now possible to monitor a preselected surface area at elevated temperatures while changing the gas pressure over 12 orders of magnitude. In addition to the instrumental performance, the STM tip is ofprimary importance for in situ STM measurements. Methods for the preparation of STM tips have been extensively studied with a goal ofpreparing an atomically sharp tip [14-19]. The STM tip is important for high-pressure studies with respect to two aspects, tip selection and in situ tip regeneration. To ensure a continuous DOS near the Fermi level, transition metals are usually selected to prepare STM tips. In the presence ofreactant gases, especially under high.pressure and high-temperatur;;: conditions, the chem ical and thermal stability of the STM tip becomes the ultimate limit for reaction studies and thus the major concern in tip selection. Tungsten tips are very stable in 1 I 57 58/ 3 In Situ STM Studies of Model Catalysts (a) Gas purification i the surface is expo completely contami N2 can greatly imp ' studies and preven backfilling the STM large volume of gas carbonyl contamina i evaporator i o 0 sample 3.3 Visualizing the (e) • f· ' (cr): : ::_.: .:. . ~ . • •• ••••••• " " : " . ,:, ~ .... - ~ ;.::, -:" 1 j . .; i - ,'-'-,I:: .,r· Tip manipulation , ~" , ':1 .' . 1 / . : : , I, . : . •~ I · " t.. , : . -. ' . ." ~.,:I .I : _. __. ..a . , it ! .. . -. --,; , - _ - _ ...: .. " Of . -. ' '. • I . , . (.. With the introductil fun damental steps 01 and catalytic transfor! Wintterlin in 2000 [2. to the visualization 0 follOWing section, we STM for studying fu .._ , . • ," _. 2UII ;1!!' :Ie ~I 1i;"1 .: - . ":"': Figure 3.1 Method s of patterning the surface for in situ STM studies. (a) Schematics of "shadow" technique. (Reprinted with permiSSio n from Ref. [12]. Copyright 2002, Wiley, Inc.) (b) STM image of the surface created by "shadow" technique. The shadow area uncovered by metal clusters is distinguished from the area covered with metal clusters by the Path~ . wh ite dash line. (c) Schematics of the tip manipulation. (d) STM image of the su rface created by tip manipulation. The dash rectangle in (d) shows the area where most cl usters are picked up by the STM tip. This area with lower cluster densities can be distinguished from the rest of the surface .and serves as a nanomarker for in situ STM studies. CO but perform poorly in the presence of O 2 or mixtures of CO and O 2 , Platinum or platinum alloy tips are stable in O 2 but suffer from adsorption of CO, especially when the sample surface temperature is above 550 K [20]. Gold is stable in both CO and O 2 but unstable at high temperatures, especially in the presence of water. In addition to tip selection, in situ tip regeneration or cleaning is also critical for STM studies in the presence of high-pressure reactant gases because the STM tip is susceptible to picking up poorly conducting components during extended measurements at elevated pressures. Tn situ tip regeneration refers to the method of applying a large voltage pulse (from a few to hundreds ofvolts) between tip and sample while the tip is in tunneling range. !'his method induces field emission, which cleans and regen erates the STM tip . Wintterlin and cowurkers [2l.] recently reported a high-pressure STM study, in which tungsten tips were used to study ethylene oxidation on Ag(II1.); the tip could be recovered by applying high voltages to the tip (e.g. , + 300 V). 3.3.1 Imaging of Adsorbate STM can induce ad~ and with spatial con motion of adsorbate reaction intermediatt one of the most' imp Ho [25] have recend oxidation on Ag(ll 0 the Langmuir-Hinsr and a pair ofoxygen a atom pair was forme l the sample bias to is broken and the til Ag( 11 0) surface shol STM tip was then I repeatedly, causing t molecule moved dos form the o-co- o bias over the CO molt atom on the surface A second reaction f absorbed oxygen aton 3.3 Visualizing the Pathway of Catalytic Reactions Gas purification is another important issue for high-pressure STM studies. When the surface is exposed to high pressures, even highly diluted impurities may completely contaminate the surface. Purification of all reactant gases using liquid Nl can greatly improve the operating pressure range for high-pressure reaction studies and prevent the electrical breakdown often induced by humidity when backfilling the STM chamber [13J. For a flow-reactor system, where purification of large volu me of gases is required, a heated zeolite filter is effective in removing carbonyl contaminants from the gas flow [22, 23J. 3.3 Visuali~ing th e Pathway of Catalytic Reactions With the introduction of LT and VT STM, it is now possible to monitor the fu ndamental steps of chemical reactions, that is, reactant chemisorption, diffusion, and catalytic transformation. A detailed review covering this subject was published by Wintterlin in 2000 [24J. Since then, in. situ STM studies have flourished and expanded to the visualization of the reaction pathway and kinetics of surface processes. In the following section, we highlight selected examples of recent progress in using in situ STM for studying fundamental catalytic processes. 3.3.1 Imaging of Adsorbates and Reaction Intermediates STM can induce adsorption-desorption and dissociation processes nonthermally and with spatial control. At low temperatures, with limited surface diffusion, the motion of adsorbates can be controlled and allows the determination of surface reaction intermediates. The catalytic oxidation of CO on precious metal surfaces is one of the most important model reactions in heterogeneous catalysis. Hahn and Ho [25J have recently used in situ STM to visualize the reaction pathway of CO oxidation on Ag(ll 0) at 4 K. Figure 3.2 depicts the pathway believed to be operative: the Langmuir-Hinshelwood mechanism. Figure 3.2a and b shows a CO molecule and a pair ofoxygen atoms adsorbed on the Ag(ll 0) surface, respectively. The oxygen atom pair was formed by placing the tip over a molecularly adsorbed O 2 and raising the sample bias to 0.47 V. Subsequently, the bond between two oxygen atoms is broken and the two oxygen atoms adsorbed at the nearest fourfold sites of the Ag(ll 0) surface show slight elongation along the [1 I 0) direction in Figure 3.2b. The STM tip was then placed over the CO molecule with a + 0.24 V bias applied repeatedly, causing the molecule to diffuse across the surface. Eventually, the CO molecule moved close to the pair of 0 atoms (Figure 3.2c) and then joined them to form the O- Co- o complex (Figure 3.2e). With an additional pulse of the sample bias over the CO molecule, the O-Co-o complex is decomposed, leaving an oxygen atom on the surface with the CO 2 des orbing from the Ag surface. A second reaction pathway was also illustrated by moving a CO molecule toward the absorbed oxygen atoms. In Figure 3.3a, an STM tip with a CO molecule adsorbed atthe 159 60' 3 In Situ STM Studies of Model Catalysts (a) Cd) [110] (f) ' - [001] Figure 3.2 STM images obtain ed with a CO-terminated tip. Vl = 70 mV and It = 1 nA. (a) Iso lated CO molecule. (b) two 0 ato ms (adsorbed o n the nearest fourfold hollow sites alo ng the [1 I 0] di rectio n). (c) CO and two o atoms se parated by 6.1 A along th e [0 0 1] direction. and (e) O -CO -O complex. Grid lines are d rawn through the silve r surface atoms . Scan area of (a-c) and (e) is 25 Ax 25 A Schematic diagrams for adsorption geometries o f (c) and (e) are sh ow n in (d) and (f), res pectively; a line ar atop and a ti lted off-site CO are im pli cated. The black (red) circles represent ca rbon (oxygen ) atoms and the lar ge gray circles are sil ver atoms .·The sizes of the ci rcles are scaled to the atomic covalent radii_ (Reprinted with permission fro m Ref. [25] . Copyright 2001 . The Ameri can Physica l Society.) tip end was positioned over an oxygen atom, which lined up with another oxygen atom fro m the O 2 disso(jation. T he two oxygen atom s were separated fr om each othe r by two lattice spacings. With a pul se of + 0.47 V sample bias . the CO molecule is detached from the STM tip and reacted with the oxygen below th e ti p to form CO 2 . Figure 3.3b shows the two pul es of tunneling cunent. corresponding to the CO m olecule. impin ging on the surface and reacting with the adsorbed oxygen, and to CO 2 desorption . res pectively. Figure 3.3c shows the oxyge n atom left on the Ag(l 1 0) surface. The com bined im aging. manipulation. an d spectroscopic ca pabilities of the STM provide direct visualization of reaction pathways at the single molecule leve l. At low t mperatures. tip m anipulation has been regularly used tel promote smface diffusion, to activate bon ds. an d to synthesize molecules. Rece nt progress along these lines is desai bed by Hla an d Rieder [26 . 27J. Figure 3.3 Reaction fr om a CO-terminat. ad so rbed on the sur! with a CO-terminate separated by two tat alon g the [I 1 01dire th rough the silver su cu rren t during a 14) with the CO-ter mlna o atoms (denoted t 3.3.2 ,Imaging Chemisol Under catalytic re diffusion of ads or by in situ STM. C adsorbates are ill coverage is illCre, attractive interacr These adsorbate the SllIface. Eve~ islands grow-intG and by Wong et D surfaces depend coefficient measl surface cove rag ence on adsorba The structure relevant to earnl, found for CO ot1 3.3 Visualizing the Pathway of Catalytic Reactions (a) (b) 6 <4 .s 2n__--' 0.0 0.2 0.4 Time (s) (d) (e) Figure 3.3 Reaction of a CO molecule released from a CO-terminated tip with an 0 atom adsorbed on the surface. (a) STM image, taken with a CO-terminated tip, of two 0 atoms separated by two lattice spaci ngs (2 x 2.89 A) along the [1 I 0] direction. Grid lines are drawn through the silver surface atoms . (b) Tunneling current during a 1470 mV sample bias pulse with the CO-terminated tip over one of the two o atoms (denoted by "'''). Two current rises (f) (at 250 and 310 ms) indicate the moments of desorption and reaction of CO from the tip and the moment of desorption of CO 2 into vacuum . (c) STM image of the same area rescanned after the pulse, showing CO on the tip has reacted away. Scan area of (a) and (c) is 25 A x 25A. (d-f) are the schematic diagrams for (a-c). respecti vely. (Reprinted with permission from Ref. [25J. Copyright 2001 , The American Physi cal Society.) 3.3.2 Imaging Chemisorption on Metals Under catalytic reaction conditions, adsorbates are usually mobile on the surface. The diffusion ofadsorbates has been studied both on metal surfaces and on oxide surfaces by in situ STM. On metal surfaces, it has been shown that at low surface coverage, adsorba tes are mobile and are distributed randomly on the surface. As the surface coverage is increas d, the interaction betwcl'n adsorbates also changes such that an attractive interaction begins to appear, leading to the formation of adsorbate islands. These adsorbate islands are in equilibrium with the diffusing adsorbates (2D gas) at the surface. Eventually, with an increase in the adsorbate coverage, the adsorbate islands grow into an adsorbate overlay r. In situ STM studies by Wintterlin et al. [4J and by Wong et al. [281 both illustrat that the diffusion rate of adsorbates on metal surfaces depend on their coverage and/ or nearest neighbors. The surfac diffusion coeffici.entmeasured for adsorbates on metal surfaces is meaningful only at very low surface coverages where the adsorbate-adsorbate interaction has a negligible inA.u· ence on adsorbate diffusion. The structure of the adsorbate layer formed at high s urface coverage is more relevant to cat lytic reactions at high pressures. Besenbacher and coworkers [29-33] found for CO on I't(ll 0) and Pt(lll) and NO on Pd(111) that the structme ofhigh 161 621 3 In Situ STM Studies oj Model Catalysts adsorbate coverages, formed at low-temperature and low-pressure conditions, is identical to the structure formed at room temperature and high pressures . Th is finding suggests that on metal surfaces, reaction studies at high surface coverage conducted at low temperature likely connect with real catalytic processes at high pressures. By studying the diffusion of surface adsorbates (or adsorbate vacancies), in situ STM can be used to determine the active site for chemisorptions. Mitsui et al. [34-36] studied the process of hydrogen dissociation on Pd(l 1 1) using in situ STM. Pd is a catalyst widely used in hydrogenation and dehydrogenation reactions. xposure ofpd to H2 leads to dissociative adsorption. At approximately 65 K and in the presence of 2 x 1O- 7 1orr of H 2 , Pd(l 1 1) is nearly saturated with H atoms leaving only a few vacancies as sites for the dissociation of adsorbed Hz molecules (Figure 3.4). Due to the inversion of the image contrast caused by the adsorption ofH atoms on the ST M tip, the empty ~urface sites are imaged as protrusions. This surface can be used to model the Pd(l 1 1) surface under high -pressure Hz at room temperature. Figure 3.5 shows a sequence of STM images acquired for the same surface region. Figure 3.5a depicts a number of isolated vacancies (bright spots) separated by more than one Pd lattice, as well as two aggregat s of vacancies (marked by dashed circles). Each vacancy aggregate consists of a pair of vacancies (dimer) occupying neighboring fcc sites. The dimers in Figure 3.5 are always imaged as a three-lobed object because of the fast diffusion of a neighbor H atom, which can hop over bridging sites to occupy the vacancy pairs without getting close to other H atoms. The vacancy pairs or dimers are most frequ ently encountered in the STM study. Isolated vacancies can hop Figu re 3.5 STM im ag~ the forma tion, separa l H-vaca ncy clu sters. n (3 nm " 2.S nm) are r< ann otations. (a) Five \ are labeled (A-E) ana pairs (2V) are lTlarh d reference. (b) Vacanci, 2V cluster indica ted b the number "2". Vaca . Figure 3,4 The 6.5 nm x 6.5 nm STM image of Pd(l 1 1) with a H cove ra ge near one monolayer. Numerous H vacancies, visible as bri ght protrusions, are present. V, = 45 mV and I, = 2.7 nA. Stre aks and fractional protrusions are due to vacancies moving while the tip is scanni ng over them. (Reprinted with permission from Ref. [35]. Copyri gh t 2005, Springer.) randomly on the Pc Figure 3.Sb. VacJnc coaJesce to form a I several minutes an three-vaca ncy aggl (Figure 3.Sc). The aggrega te are OCelli 3.3 Visualizing the Pathway of Catalytic Reactions Figure 3.5 STM images from a movie showing the fo rmation, separation, and annihilation of H·vacancy cl usters. The image s on the left (3 nm x 2.5 nm) are repeated on the right with annotations. (a) Five vacancies near the cen ter are labeled (A-E) and two triangular vacancy pai rs (2V) are marked with dashed triangles for reference. (b) Vacancies A and 8 have formed a 2V cl uster indicated by the triangle containing the number "2". Vacancies C-E have formed a three-vacancy (3V) cluster, indicated by the larger triangle containing the number "3". (c) The 2V pair has separated into isolated vaca ncies A and 8, while the 3V cluster has been annihilated by dissociative adsorp tion of a Hz molecule , leaving a si ngle remaining vacancy C. The other 2V clusters separated a few frame s later. (Reprinted with permission from Ref. [3 4J. Copyright 2003, Nature Publishing Group.) randomly on the Pd surface and occasionally coalesce to form aggregates, as shown in Figure 3.Sb. Vacancies A and B aggregate to form a dimer while vacancies C, D, and E coalesce to form a tnree-vacancy aggr gate. The vacancy dimer remains together for several minutes and eventually disintegrates back to isolated vacancies. However, the three-vacan cy aggr gate disappears and leaves only one vacancy on the surface (Figure 3.Sc). The authors conclud d then that two vacancies in the three-vacancy ~ ggregate are occupied by H atoms from the clissociation ofadsorbed H 2 . The uthors 163 641 3 In Situ STM Studies oj Model Catalysts also found , in the presence of2 x 10- 7 Torr H2 at 65 K, vacancy aggregates with four or more vacancies are also filled by H atoms from the dissociation of H 2 within the aggregates and transformed into a single vacancy or totally annihilated. Through a series of ",TM movies on the diffu sion of surface hydrogen vacancies, the authors found that on ly an aggregate with three or more vacancies could be annihilated by H2 dissociation . rne dimers or isolated vacancies are never occupied by H atoms. Instead, the dimers always dissociate creating isolated vacancies with an average lifetime of 10 min at 65 K. From these data, the authors concluded that three or more empty palladium sites are necessary for the dissociation of H 2 molecules. This finding is rather surprising since it has traditionally been assumed that two neighboring empty sites are sufficient for the dissociation of a diatomic molecule. The discovery of the active sites for H 2 dissociation on Pd(lll) illustrates the power of in situ STM in addre ssing the elementary steps of surface reactions and in testing the conventional assumptions in catalysis. Yo [llOl~ [110] Figure 3.6 The b. 11 IT 3.3 .3 Determining the Sites for Chemisorption on Oxide Surfaces On reduced oxide surfaces , the diffusion of an adsorbate is often limited by the localized bonding, either ionic or covalent, between the adsorbate and the oxid e substrate. The relatively slow diffusion of adsorbates allows chemisorption and diffusion on oxides to be studied by STM at elevated temperatures. For example, Ti0 2 is an exc lIent photocatalyst for dissociation of wa ter and decomposition of organic molecules, critical to pollution control and the hydrogen economy. Studies of the adsorption and diffusion of wa ter, oxygen, and organic molecules on Ti0 2 are of pri mary impor tance to our understanding of photocatalysis by Ti0 2. Being th e m ost stable phase ofTi0 2, the rutile 1'i0 2(11 0) cryst<LI has been studied most extensiv ly by STM and other surface science techniques [37]. Figure 3.6 shows a structural model of the rutile Ti0 2(1 1 0)-(1 x 1) surface. 'The surface contains two types of tita nium atoms that form rows along the [0 0 1J direction. Rows of six coordinated Ti atoms alterna te with five-coordinated terminal Ti at.oms , which miss a single 0 atom perpendi cular to the surface. The surface also con tains two kinds of oxygen atoms, that is, three-coordin ated oxygen atoms, sitting in the surface plane, and bridging oxygen atoms, sitting above the surface plane a nd bonded to two six coordinated Ti atoms. Un dersaturated bridging oxygen atoms can be easily removed from the surface by annealing, electron bombardment, or ion sputte ri ng to form bridging xygen vacancies. Brid ging oxygen vacancies are tIl e most common and well-defined defects on the 1'i0 2 (1 1 0) surface. STM studies on the adsorption and diffu ion of small molecules on the 1'i0 2(11 0) surface began in the late 1990s [37-3 91 and have provided a g neral u nderstan ding of the impOltant role ofbri dgi ng oxygen vacancies in the dissociation of water, oxygen, and small organic molecul ' 5 . However, due to the difficulties in distinguishing active s urface siles and disso ·iation products, only recently have the ft.l ~da mental steps of ad sorption and di ssociation processes been understood with the help of in situ S1'M. - - - - - - - - - - - - - - (red) balls represent' coordinated surface ' coord ina ted Ti atoms single oxygen vacanCl f 5fTi row (Oat) are in Ref. [401 Copyright 2 Wendt el al. [40] water and O2 011 methods, aUow su atoms, smface h~ illu strates the din and adsorbed \Val, ST 1 resolves the bridging ox)'gen r oxygen vacancies oxygen vacancies found that appl}i atoms from the h; situ STM , Wendt bridging oxygen hydroxyl groups, \ at the neare t bric hut can be initiat; five-coo rdinated " cwes, hydrogen bridging oxygen Dohnalek ;md hydroxyl images (Figure 3.3 Visualizing the Pathway of Catalytic Reactions [110] ~ o atoms Ti atoms [110] Figure 3.6 The ball mode l of the Ti0 2 (1 1 0) surface . Large gray (red) ba lls represent 0 atom s, small light gray (gray) balls fi ve coordina ted s urfa ce Ti atoms (sf-Ti), and small black balls six coordinated Ti atoms (6f-Ti). The bfjdging oxygen ato ms (Ob,), si ngle oxygen vacan cie s (Vo) , and 0 atoms adsorbed o n top of the sf-Ti row (Oo,) are indicated. (Reprin ted with permission from Ref. [4 0J Copyright 2005 , Elsevier.) Wendt et al. [40] and Bikondoa et al. [41 J studied the ad sorption and dissociation of water and OL on TiO z(l 10). In situ STM studies, in parallel with the use of Drl method ,allow smface features such as bridging oxygen vacancies, adsorbed oxygen atoms, surface hydroxyls , and adsorbed water to be distinguished. Figure 3.7a illustrates th e difference between a bridging oxygen vacancy, a surface hydroxyl, and adsorbed water in their appearance in S M images. Due to electronic effects, STM resolves the five-coordinated terminal Ti rows as bright rows whereas the bridging oxygen rows are imag d as dark rows. The nuances in the appearance of oxygen vacancies and hydroxyls in STM images are distinguished by visualizing oxygen vacancies being transformed into OH species in situ. The authors have also found that applying voltage pulse ( ~ 3 V) over the hydroxyls desorbs individual H atoms from the hydroxyl groups while leaving the oxygen vacancies intact. Using in situ STM, Wendt et al. [421 demonstrated that water dissociation takes place at bridging oxygen vacancies of the TiOz(l 1 0) surface at 187 K and form pa ired hydroxyl groups, wilh one positioned at the oxygen vacancy site and the other ormed at the nearesl bri dging oll:ygen sit . The diffusion of these pairs is inhibited at 187 K but can be initiated in the presence of neighboring water m olecules adsorbed in the five-coordinated Ti trough. TIuough the interaction with neighboring wa ter mole cules , hydrogen atoms h om the paired hydroxyl grou ps are transferred to adjacent bridging oxygen rows, cau~ing the ross-row diffusion of hydroxyl groups. Dolmalek and coworkers [43, 44 ] hav' further m easured the diffu sion kinetics of hydroxyl groups (or H atom) at room tempera hlre an d above. In situ S M images (Figure 3.8) have confi rmed thal I fl O dissociates at the bridging oxygen 165 661 3 In Situ STM Studies of Model Catalysts ., - (a) • .' • .1 111 ,__ ' II , ( . (bl .t ---- II II • • • [0011 . '-- 1 - \ , . - Iliol 1. 5 .~ 1.0 ~, ,(c) I ! . • 1110 ,\ . 16() t\ • ! .~ ~ -. ~05 ~_' 0.0 ' .'-..-- .' • II I Ti 0 o __ 10 0 n 0 TI 15 0 20 TI TI 25 1) 0 5 0 TI 0 10 ci 15 20 Lcnglh along J iOI (A) TI 25 r o • • • 10 15 20 25 Fi gure 3.7 STM images (16 nm x 16 nm) of clean , reduced Ti0 2 (1 1 0) samples showing the difference between bridging oxygen vacancies, surface hydroxyls , and adsorbed water. The sample in (a) was less reduced than the sample in (b) . (c) STM height profiles along the 1 i 0] direction of species indicated in (b). (Reprinted with permission from Ref [40] . Copyright 2005 , Elsevier.) --I '" ,,;. , (a ) 1'# I (b) f .... '~I ::. •" , ... ,(e) .. ft ~ III I(d) ,. .... Figure 3.8 STM images of the same area on Ti0 2 (1 10) at 357K (Vt = 1.5 V, It =O. 1 nA) as a function of time (LH = 60 s): (a) clean Ti0 2 (1 1 0) with bridging oxygen (880) vacancies; (b) Ti0 2 (1 1 0) with a geminate hydroxyl pair formed by adsorption and dis socia tion of a water molecule. Hv marks the ~. "', . 'I • ., vacancies, producing I room temperature alor in the paired hydroxyl positioned at the heall one adsorbed at neight Hv and HB were meas estimated to be apprc these hydrogen atoms i inga repulsiveOH - OI state is responsible for measured kinetic para duced by OFf calculati The pathwayofdissc experiment aJso applie been used to study the Figure 3.9 shows a Sl exposure to bridging oxygen methanol led to the vacancies ofTiOl( l 1 group at the bridging hydroxyl group at the to the bridging higher than the entia ted from the .I OH hydrogen and He the hydrogen that s plit off from the OH ; (c) same area after a single hop of He; and (d) after su bsequent hop of Hv. Insets exhibi t the ball models illustrating the corresponding processes . (Reprinted with permission from Ref [44] . Copyright 2008, The America n Chemical Society.) The adsorption Wendt et al. [40] dissoci ate at the b and the other 0 previous vaca ncy 3. 3 Visualizing the Pathway of Catalytic Reactions vacancies, producing paired hydroxyl groups. Hydrogen atoms readily diffuse at room temperature along the bridging oxygen row. However, the two hydrogen atoms in the paired hydroxyl groups exhibit inequivalent diffusivity. The hydrogen atom positioned at the healed oxygen vacancy site (Hy) diffuses much slower than the one adsorbed at neighbor bridging oxygen sites (Hs). The different diffusion rates of Hyand HB were measured between 300 and 410K and the activation barrier ofHB estimated to be approximately 0.22 eV lower than Hy. The diffusion barrier of these hydrogen atoms increases with the separation between hydroxyl groups, suggest ing a repulsive 0 H -OH interaction. The authors speculated that a long-lived polaronic state is responsible for the inequivalent diffusion rates of Hy and H B . However, the measured kinetic parameters (prefactors and diffusion barriers) could not be repro duced by OFT calculations, suggesting a rather complex diffusion mechanism. The pathway ofdissociation and diffusion discovered in the above water adsorption experiment also applies to the adsorption of alcohols on Ti0 2 (1 10). In situ STM has been used to study the adsorption of methanol [45] and butanol on Ti0 2 (1 1 0) [46] . Figure 3.9 shows a series of STM images obtained on the same area following exposure to methanol. Figure 3.9a depicts the clean surface before exposure, with bridging oxygen vacancies marked as yellow circles. The exposure of 0.06 ML methanol led to the dissociative adsorption of methanol at the bridging oxygen vacancies ofTi0 2 (11 0) (Figure 3.%). The dissociation of methanol forms a methoxy group at the bridging oxygen vacancy, resolved as bright features in Figure 3. 9b, and a hydroxyl group at the nearest neighbor. The methoxy group has a similar appearance to the bridging oxygen vacancy in STM images, except that the methoxy group is 0.8 A higher than the bridging oxygen vacancy. The hydroxyl group could not be differ entiated from the bright features of neighboring methoxy groups in Figure 3.9b. However, with time, the hydrogen from the hydroxyl group (red dots in Figure 3.9c and d) diffuses along the bridging oxygen row and across the bridging oxygen rows through interactions with methanol molecules weakly bounded to the Ti trough. The diffusing hydrogen atoms were identified by their apparent height in STM images and by a tip desorption experiment (Figure 3.ge) proposed by Bikondoa et al. [41]. Figure 3.9f gives a graphic illustration of the dissociation and diffusion pathway of methanol on Ti0 2 (1 1 0), which was also observed in the adsorption experiment of 2-butanol (CH 3 CH 2 CH(OH)CH 3) on Ti0 2 (1 1 0) at room temperature [45J. The adsorption and dissociation of O 2 on Ti0 2 (1 1 0) have been investigated by Wendt et al. [40] and Du et al. [47]. Both observed that O2 molecules adsorb and dissociate at the bridging oxygen vacancies, with one 0 adatom healing the vacancy and the other 0 ada tom bounded to the neighboring five-coordinated Ti site. Du et aL. also analyzed the lateral distribution of the 0 adatoms upon dissociation and discovered a transient mobility of 0 adatoms along the Ti trough in the [0 0 1] direction. Unlike the dissociative adsorption of O 2 on metal surfaces where both adatoms have equal diffusivity, the diffusivity of 0 adatoms on Ti0 2 (11 0) was found to be inequivalent. While the 0 adatoms filling the vacancy are locked in the bridging oxygen row, 0 adatoms in the Ti trough are relatively free to move. A majority of 0 ada toms on the Ti trough (",81 %) were found separated from the 0 adatoms in the previous vacancy sites by two lattice constants. 167 • 681 3 In Situ STM Studies of Model Cata lysts Zhang et al. [4811 on Ti0 2 (1 10) usi n suggest that bridgi slow diffusion of ~ agreement with D chemistry ofTi01 ( for approximately 1 However, there of trimelhylacetic Ti0 2 (1 1 0) at roo AA toformT of the hydroxyl gr hydrogen atom wa the adjact'nt TMA coverage, TMAA ~ Wendtet al. [501 r in detail and sugge account for 0) diss approximately 10% a few Langmuirs of the influence of bri "perfect" Ti0 2 (J 1 temperature. H),dr oxygen vacancies, Figure 3.10c and groups and create previously suggest oxygen adatoms 0 However, tJle incr hydroxyl group h ......• .. • • CH 3 • • • • OH • . CH3 • CH O-H 6 3 /e '~ [001) . • o • Figure 3.9 STM images of s am e are a be fore an d after adsorpti o n of m etha nol o n reduced Ti0 2 (1 1 0) at 300 K (V, = 1. 0 ± 0. 3 V and I, <0. 1 nA): (a) bare su rface; (b) afte r 80 s exposure to m e tha nol; (c) after 110 s ex pos ure to metha nol; (d) taken on (c) after spo nta neous tip change; (e) after h igh bia s (3.0 V) sweep of (c); (f) schematic m odel of th e adso rpti on Ti io n dissociation chSTM results co state ofTi0 2(1 1 0) leading to a perfe state (Pigme 3, lOe) plo the evolution and sugg sts the Ti The authors suggl: the reduction of responsible for the of O 2, Indeed. the imp situ STM studies ~ [110] V Bridging-bond 0 ion 0 from methanol p rocess. Insets s how m ag ,fled areas ma rked by sq ua res . Yel low circl es show the pos itio n of brid gi ng oxygen vaca ncies. Bl ue ci rcles sh ow th e m thoxy group s o n oxygen vacan ies. Red squares s how H atoms diffUS ing o n bridging oxy en rows. (R pri nted with . pe rmission from Ref. [45 ]. Copyrigh t 2006 , The Ame rican Chem ical Society.) - - , .........II!I!! 3.3 Visualizing the Pathway of Catalytic Reactions Zhang et al. [48] have recently measured the stability of bridging oxygen vacancies on Ti0 2 (11 0) using in situ STM. Sequences of STM images between 340 and 420 K suggest that bridging oxygen vacancies migrate along the bridging oxygen row via the slow diffusion of bridging oxygen atoms with a diffusion barrier of 1.15 eV, in agreement with DFT calculations. All the above studies suggest that the surface chemistry ofTi0 2 (1 1 0) is dictated by bridging oxygen vacancies, which can account for approximatelyl0% of the bridging oxygen sites. However, there are disagreements. Lyubinetsky et al. [49] studied the adsorption of trimethylacetic acid ((CH 3 bCCOOH, TMAA), a photoreactive molecule, on TiO z(1 1 0) at room temperature. In situ STM found that the deprotonation of TMAA to form TMA does not necessarily occur at bridging oxygen vacancies. None of the hydroxyl groups was found during the adsorption of TMAA. instead, the hydrogen atom was bound to a pair of bridging oxygen atoms and stabilized by the adjacent TMA groups sitting on the five-coordinated Ti trough. At saturation coverage, MAA formed a (2 x 1) overlayer on the Ti 0 2 (1 1 0) surface. Wendt et al. [501recently studied the interaction between O 2 and Ti0 2 (11 0) surface in detail and suggested that bridging oxygen vacancies are only the minor sites that account for O 2 dissociation. Even though bridging oxygen vacancies account only for approximately 10% of surface bridging oxygen sites, exposing the clean Ti0 2 (11 0) to a fe w Langmuirs of O 2 could not fully remove all bridging oxygen vacancies. To isolate the influe nce of bridging oxygen vacancies in O 2 dissociation, the authors created a "p rfeet" Ti0 2 (1 1 0) surface by exposing the TiO z(1 1 0) surface to water at room tem perature. Hydroxyl groups, formed via water dissociation, covered all bridging oxygen vacancies, yielding a vacancy-free Ti0 2 (1 1 0) surface (Figure 3.1 0a). Figure 3.lOc and d illustrates that O 2 exposure can fully remove surface hydroxyl groups and create a Ti0 2 (I 1 0) surface with perfect bridging oxygen rows, as previously suggested in T PD studies [51 ]. With the titration of hydroxyl groups, oxygen adatoms on the five-coordinated li row (Ootl also increase (Figure 3.lOb) . However, the increase in oxygen adatoms does no t seem to stop even after all the hydroxyl groups have be -' n replaced with oxygen (Figure 3.IOe and d). Paired Oot atoms start to appear on the five-coordinated Ti row during extended O 2 exp osure. On the basis of these observations, the authors showed that a second and primary O 2 dissociation channel is operative on the five-coo rdinated Ti row. STM results combined with photoelectron spectroscopy (PE ) on the valence state ofTi0 2 (11 0) further show that the removal of all hydroxyl grou ps by oxygen, leadi ng to a perfect Ti0 2 (1 1 0) surface, only slightly attenuates the Ti 3d defect state (Figur 3.10e). The full attenuation of Ti 3d state requires 420 Lof0 2 . Figure 3.9f plots tlte evolutions of the Ti 3d defect state and the 0 I-I 30" state over O2 exposure and suggests tll e Ti 3d defect stOltI:' is not mainly caused b bridging oxygen acancies . he aut hors suggest that other types of defects, Ti3 ' interstitials thal form during the reduction of Ti0 2 (1 1 0) and are hidden beneath the surface, a.re primarily responsible for the formation of the Ti 3d defect stat and the dissociative ad orption of O2 , Inde d, the importance of T? ~ interstitials has also been realized in previous in situ STM studies of the reoxidation of Ti0 2 (1 1 0) [52- 54]. It is noted that Ti 3 + 169 70 I 3 In Situ STM Studies oj Model Catalysts Ti 3 + interstitiaJs ar oxygen vacancies are surprising to expect dissociation of adsor I worth noting that sue diffuse to the Ti0 2( Considering PES Wi attenuation of the Ti 3 have been oxidized a inlerstitials involves . eveliheless, the abo' active si tes, hidden 0 will stimulate more il their surface chemis 14 12 10 8 6 4 2 Binding Energy (eV) I gj 0.8 ::c -gN 0.6 ';ij 0.4 E 00.2 z • J * " . - il_ _ _ -l I Visualizing Reaction h • - a Ti3d __ \ OH3" ... o 2 3.3 .4 h-~i02(110) : oxygen exposur~ at room temperature 4 6 I J -~ 8 10 50 -- 100 02 Exposure (L) Figure 3.10 (a-d) STM image s (105 A >< 105 A) of the Ti02 (1 1 0) surface cove red with hydroxyls [h-Ti02 (1 1 O)J and then expo se d to in c rea sing amounts o f O 2 at room tempe ratu re. (e) Selected PES va lence- band spectra recorded on a n h-Ti02(1 10) s urface that was exposed to O 2 at RT. Arrows indicate t he representative STM im age s. (f) Normalized integra ted • - - 8 200 300 400 intensities of the O H 30 (red) and Ti 3d (blue) features for O 2 exposures up to 420 L from PES spectra; c ircl es indicate intensity values that were o bta ined from the spectra s how n in (e). (Repri n ted with permission from Ref. [50]. Copyright 2008, The Am erican Associati o n for the Advancement o f Science.) interstitials diffuse to the surface in the presence ofOz and form TiO x species, which serve as the building blocks for the regr O\vth ofT iO z(l l 0) plane. Using in situ STM, Bowker and cowo rkers [52] measured the reoxidation kinetics of reduced TiO z (ll 0) surface at temperatures b etween 573 and 1000 K and oxygen pressures of5 x 10- 8 to 2 X 10- 6 mba r. By mo nitoring the T iO x species that diffused and coalesced on the Ti 2(1 10) surface, the sur£1Ce growth rate co uld be measured th rough the change of island morphology. This growth rate was found to be linear with re sp ct to the oxygen partial pressure. The regrowth of T iOz(l 1 0) has a low activation energy of approximately 25 kl/mo], suggesting a high m obility of ~ interstitials in the presence of oxygen. Previous studies on the r eduction 0[1i0 2 (1 1 0) [37J have shown the ru tile TiO l( l 1 0) b ulk serves as a huge r se rvoir for T? I inte rstit ials during red uction, so that the surface is maintained near-stoichio m e try and is thermody nam ically stable. Te In situ STM ha alsa measure their kineti metals to produce w reaction is still a core I the oldest of catalytic 1 unclear, especially fc temperature ("- 170 ~ combination of disso hydroxyl groups tha formation of hydroxy. the h ydroxyl group, surface spectroscopic situ STM, that the r oxidation reaction or Wintterlin and co Pt(l 1 1) using in sitl which the surface v. subsequently expose prepared byexposi ng 22S K to di:;sociate 0 and monitored by ST by a (2 x 2)·O,dlayer, exposure to H2, a (Figure 3.11b). 'The! ordered layer with 1 phases are character l bonding. Additional 3.3 Visualizing the Pathway of Catalytic Reactions Ti 3 + interstitials are essentially oxygen vacancies within the bulk whereas bridging oxygen vacancies are basically undercoordinated Ti ions at the surface. It is not surprising to expect that Ti 3 + interstitiaIs in the subsurface playa role in the dissociation of adsorbed molecules. In the above STM study by Wendt et aI., it is worth noting that subsurface Ti 3 + interstitials were neither visualized nor seen to diffuse to the Ti0 2 (1 1 0) surface during O2 adsorption at room temperature. Considering PES usually probes the top few layers at the surface, the complete attenuation ofthe Ti 3d defect state suggests that Ti 3 + interstitials within those layers have been oxidized and therefore quenched. Details ofhow the excess charge ofTi 3 + interstitials involves in the bond breaking of O2 molecules remain to be elucidated. Nevertheless, the above studies demonstrate the power of in. situ STM in tracing the active sites, hidden or unhidden. The finding of mobile defects in a rigid Ti0 2(1 1 0) will stimulate more investigations on other reducible oxides and encourage revisiting their surface chemistry. 3.3 .4 Visualizing Reaction Intermediates and the Mechanism of H ydrogen Oxidation In situ STM has also been used to study the pathway of surface reactions and to measure their kinetics. The hydrogen oxidation reaction, catalyzed by Pt group metals to produce water, was the first catalytic reaction discovered in 1823. This reaction is still a core catalytic reaction at the heart of fuel cell technologies. Although the oldest ofcatalyti.c reactions, the mechanism ofca talytic hydrogen oxidation is still unclear, especially for its surprising reactivity at or below the water desorption temperature (~170K). It has been proposed that the reaction proceeds via the combination of dissociatively adsorbed 0 atoms (Oad) and H atoms (Had), forming hydroxyl groups that subsequently bind with another Had to form water. The formation ofhydroxyl groups has been postulated as the rate-limiting step. However, the hydroxyl group, as a reaction intermediate, could not be confirmed in early surface spectroscopic studies. It has not been until the past decade, with the help of in. situ STM, that the reaction mechanism has become apparent for the hydrogen oxidation reaction on metal surfaces. Wintterlin and coworkers [5 5, 56J studied the hydrogen oxidation reaction on Pt(l 1 1) using in. situ STM. The study was conducted as a titration experiment, in which the surface was precovered with Oad and then the O-terminated surface subsequently exposed to H2 molecules. The O·terminated Pt(l 1 1) surface was prepared by exposing the clean Pt(111) surface to 10 L of O2 , followed by annealing at 225 K to dissociate O2 , The surface was then exposed to 8 x 10- 9 mbar H2 at 131 K and monitored by STM , as shown in Figure 3.11 . The Pt(111) surface was precovered by a (2 x 2)·O"d layer, where Oad atom s were imaged as da rk dots (FiguTe 3.lla). Upon exposure to H 2 , a few bright islands form on top of the (2 x 2)-Oad layer (Figure 3.11 b). These islands continue to grow and eventually develop into an ordered layer with hexagonal and honeycomb phases (Figure 3.l1c). These two pha ses are charact rized as a surface hydroxyl (OHad) overlayer, fo=ed by hydrogen bonding. Additional diffusi ng islands seen in Figure 3.11 c are attributed to adsorbed 171 721 3 In Situ STM Studies oj Model Catalysts below the water de:; H 2 0 starts to desori cycle by stopping tl The autoca lytic apply to catal)'lic hy( the fir st to use S1' m eso scopic level, v. in surface reactions situ STM to visual i2 atomic resolution. 3.3.5 Measuring the Reac o OH Figure 3.11 Series of success ive STM images, recorded during dos in g of the O-covered Pt(l 1 1) su rface with Hz. (a-c) Frames (17 nm x 17 nm) from an experiment at 131 K [P(H 2 ) = 8 X 10- 9 mbarJ. The hexagonal pattern in (a) is the (2 x 2) -0 structure; 0 atoms appear as da rk dots and bright features are the initial OH is lands. In (c), the area is mostly covered by OH, wh ich forms ordered structures. The white, fuzzy features are H2 0 -covered areas. (d-f) Frames (220 nm x 220 nm) from an experiment at 112 K [P(H z) = 2 x 10- 8 mbarJ. In (d), the surface is mostly O-covered (not resolved). The bright dots are sma ll OH island s, most of which are concentrated in the expanding ring. H2 0 in the interior of the ring is not re so lved here. Thin , most ly vertical line s are atomic steps. (Reprinted with perm ission fro m Ref. [55J Copyrigh t 2001, The American Associat io n for the Advancement of Scie nce.) H 20 islands (H 2 0"d)' Figure 3.11a-c shows the formation of hydroxyl group as the reaction intermediate and reveal the atomic details ofhydrogen oxidation catalyzed by Pt(l 1 1). Figure 3.1 Id- f presents snapshots of STM images of the Oad-te rmin ated Pt(ll 1) surface exposed to 2 x 10- 8 mbar H 2 at 112 K. The imaged area includes several surface terrace s covered with numerous small bright dots and a bright r ing that expands with tim . These small bright dots h ave been assigned to small OH. d islands that appear after H ]. exposure . The white ring, termed as the rea ction fr ont, is concentrated with s mall O H ad islands and contin u s to grow as the reaction progress s, suggesting a fa st rea ction at the boundaries b etween Oad atoms and the diffusing Z . d· The fast reaction between ad and [-{ZOad produces two O H ad, which in tum forms HZOod through a rapid reaction with Had atom. Since the combination of Oad and H ad to form OHad is the r ate-lim iting s tep, th e presence of H ZO ad removes this kinetic limit and promotes an autocatalyti c cycl e until depletion of Oad atoms. STM images thus illustrate an autocatalytic reaction mechanism that accounts for the low activation barrier and high reactivity of hydrogen oxidation at or Another importa nt oxidation catalyzed on m eas uring the study ofll)Tdrogen ( as a titration expen then removed by e~ kinetics of surface' from STM images Win tterlin et aJ. Pt(111) using in si by exposing the S1. di ssociate 02' The exposed to 5 x 10 struc tures as a fun form an ordered (2 mainly covered by islands, scattered 0 surface oxygen ate The adsorbed CO . As time progresse: of the (2 x 2)-0 i' oxidation ca n be E (2 x 2)-0 islands. Figure 3.13 pic perimeter of oxyg( is linear with resp' oxidation main!: ( on the Pt(l 1 1 temperatures be of 0.49 eV and a parameters oblaill 3.3 Visualizing the Pathway of Catalytic Reactions below the water desorption tem perature. At temperatures higher than 170 K, where HzO starts to d ' sorb, the shortened lifetime of HZO ad breaks down the autocatalytic cycle by stoppin g the fast reaction between H 2 0 0u and Oad to form OH ad . The autocatalytic reaction mechanism apparent at low temperatures is expected to apply to catalytic hydrogen oxidation at high pressures. In addition, the above tudy is the first to use STM to observe the formation of dynamic surface patterns at the mesoscopic level, which had previously been observed by other imaging techniques in surface rea ti ons with no nlinear kinetics [57]. This study illustrates the ability of in situ STM to visualize rea tion intermediates and to reveal the reaction pathway with atomic resolution. 3.3 .5 Measuring the Reaction Kinetics of CO Oxidation Another important catalytic reaction that has been most exten sively studied is CO oxidation catalyzed by noble metals. In situ STM studies of CO oxidation have focused on measuring the kinetic parameters of this surface reaction. Similar to the above study ofhydrogen oxidation, in situ STM studies of CO oxidation are often cond ucted as a titration experiment. Metal surfaces are precovered with oxygen atoms that are then removed by exposure to a constant CO pr ssure. In the titration experiment, the kinetics of surface reaction can be simplified and the reaction rate directly measured fro m STM images. Wintterlin et al. [58] investigated the catalytic oxidation of carbon monoxide on Pt(J. 11) using in situ STM. Oxygen atoms were preadsorbed on the Pt(lll) surface by exposing the surface to 3 L Oz at 96 K, followed by a short anneal at 293 K to dissociate O2 , The oxygen·cov red Pt(1 1 1) surface was then cooled to 247 K and exposed to 5 x 10-8 Torr CO. STM was used to follow the change of surfac adsorbate stlUctures as a function of the CO exposure time (Figure 3.12). At 247 K, Oad atoms form an ordered (2 x 2) overlayer, imaged as dark dots . At t = 0, the Pt(11 1) surface is mainly covered by the (2 x 2)·0 layer together with empty Pt sites, imaaed as bright islands, scattered on the surlace. 'The addition ofCO molecules lowers the mobility of surface oxygen atom s and slowly compresses the (2 x 2)-0 layer into large islands. The adsorbed 0 molecules form ordered c(4 x 2) domains on the Pt(11 1) surface. As time progresse ,the areas of c(4 x 2) CO domains continue to grow at the expense of the (2 x 2)-0 island s. From the series of in situ STM images, the rate of CO oxidation can be estimated based on the reduction rate of the surface areas of the (2 x 2)-0 islands. Figure 3.13 plots the dependence of the reaction rate on the 'u rface a.rea or perimet r of oxygen domains, as a function of time. ApprOximately, the rea tion rate is linear with respect to th e perimeter ofthe surface oxygen domains, sugges tin g 0 oxidation mainly occurs along the boundary between the oxygen and the CO domains on the Pt(1 1 1) surface. The titration experiments were repeated at various temperatures between 237 and 274 K. An Arrhenius plot gives an activation energy of 0.49 eV and a prefactor of 3 x lOZl crn - 2 - \ in good agreem ent with the kinetic parameters obtained from m acroscopic m easurements. 173 • 741 3 In Situ STM Studies of Model Catalysts 30.,-----i C 20 ~ i5 15 + ..J 10 ~ ~ ~(ll a: au ~.. .~ ~ 5 °° " ' - -""------'- --:! 400 Figure 3.13 Reactio n rate. the change in the size of between successive pa nel Figure 3.12, norma lized t length of the boundary b CO doma ins (the fu ll line Figure 3.12 Series of STM images, recorded during reaction of adsorbed oxygen atoms with coadsorbed CO molecules at 247 K. all from the same area of a Pt(l 1 1) crystal. Before the experiment. a subm onolaye r of oxygen atoms was prepared and co was con tinuously supplied from th e gas phase (Peo = 5 x 10- 8 mbar). The times refer to the start of the CO exposure. The structure at the upper left corner is an atomic step of the Pt surface. Image sizes. 180A x l70A; V,=0.5 V; I, = 0 .8 nA . (Reprinted with permission from Ref. [58J. Copyright 1997, The American Association for the Advancement of Science.) On the Pd(l 1 1) s urface, Wintterlin and coworkers [59J have shown that CO oxidation goes through a different reaction pathway at low temperatures. Similar titration experiments were performed by exposing the Pd(l 1 1) surface precovered by (2 x 2)-0 overlayer to 2 x 10- B Torr CO at 143 K (Figure 3.14). Using STM to follow the same area of the Pd(l 1 1) surface, these authors found that CO does not react with surface oxygen at this temperature. Instead, CO molecules slowly occupy the surface sites of Pd(l 1 1) and compresses the (2 x 2) oxygen domains into the (2 x 1)-0 phase. This phase was imaged with a stripe pattern and exhibited an oxygen density twice that of the (2 x 2)-0 structure. The reaction kjnetics of CO titration was then measured on these (2 x 1)-0 islands between 144 arid 185 K. The (2 x 1)-0 phase shows a superior reactivjty over the (2 x 2)-0 phase that does not react with CO up to accelerated when the 0.3 ML (Figure 3.l5a) surface, the titration r linear relation with t islands, as shown in I occupation of CO on reaction. There was n phase based on the study unambiguously islands, especially wh The adsorption of 0 as is the case of (110) surfaces would also i accompanyi ng the oX) considerably reduced this reason, the CO tit of fcc metals in the el surface by Leibsle et a The experimen ts I I ' surface "'1th CO. Chen pha ses. wb idl. ill tur By monitoring the s that oxygen was reI 1.1. 1 OJdirection. Later. fcc(l 1 0) sy tems.511 3.3 Visualizing the Pathway of Catalytic Reactions 30 + , ~ 25 c ~ g 20 . :0 15 ... . ~ ..J ill 0; II 10 \ 5 + ' 't + - ~:! 0 0 - . f-- • .•• + • • e_ ~ • • •• -!t :; • I 400 • • •••i • _.__ ++.. ;_1- . 800 # . .. I '. ~ 'c 0.35 C :J :-7 ::J 040 0 .30 0 .25 0. 20 sg :0 g '0 (l) 0 ~ OJ 0.15 ro II 1200 1600 Time (5) Figure 3.B Reaction rates, determined from the change in the size of the (2 x 2) area between successive panels of the data of Figure 3.12, normalized to (squares) the length of the boundary between oxygen and CO domains (the full line is a linear fit) and (crosses) divided by eo (1 - eo), which is equal to eoe co if e = 1 implies maximum coverage of the respective phase (the broken line is only to guide the eye). (Reprinted with permission from Ref. [58J. Copyright 1997, The American Association for the Advancement of Science.) react with CO up to 180 K. More interestingly, the removal of oxygen islands accelerated when the surface coverage of (2 x 1)·0 islands decreased to below 0.3 ML (Figure 3.15a). Below 0.3 ML, unlike the previous study on the Pt(1 1 1) surface, the titration reaction rate with the (2 x 1)·0 islands on Pd(1 1 1) shows a linear relation with the surface area of oxygen, instead of the perimeter of oxygen islands, as shown in Figure 3.15b and c. The authors speculated that a transient occupation of CO on the oxygen island causes all 0 atoms to be accessible for the reaction. There was no direct evidence for the existence of this kind of mixed O/CO phase based on the STM images or other spectroscopic studies. Nonetheless, this study unambiguously illustrated the superior reactivity of compressed oxygen islands, especially when they become very smalL The adsorption of oxygen atoms often induces the reconstruction ofmetal surfaces as is the case of (110) surfaces of fcc metals. It is expected that CO titration on such surfaces would also involve the local transformation of metal substrates. Indeed, accompanying the oxygen· induced reconstruction, the mobility of surface oxygen is considerably reduced so that they can be resolved by STM at room temperature. For this reason, the CO titration experiments using STM were initiated on (110) srnfaces of fcc metals in the early 1990s. 0 oxidation was first visualized on a Rh(l 1 0) surface by Leibsle et al. [60J where a pronounced reaction anisotropy was observed. The experiments were carried out by titrating the oxygen precovered Rh(1 1 0) surface with CO. Chemisorption of oxygen on Rh(l 1 0) forms several reconstructed phases, which, in turn, were imaged as striped patterns along the [1 I OJdirection. By monitoring the surface changes during CO exposure, STM images revealed that oxygen was removed on the elongated stripes of the added rows in the [1 TOJ direction. Later, similar one-dimensional reactivi ty was also found on other fcc(1 1 0) systems, such as Cu [61-6 3], Ni [64], and Ag [65, 661. 175 761 3 In Situ STM Studies of Model Catalysts (a) 0.:1 :l • , 0.2 CD 0.1 .. 0.0 (a) 250 s (b) -200 430 s (b) 500 s (d) Figure 3.14 Se ries of STM images reco rded during CO dosing on the (2 x 2) -0-covered Pd (1 1 l)surface. T = 143 K, Pco = 2 X 10--8 Torr, all images are from the same area. Indicated is th e time elapsed since the start of the CO -6 , dosing. Vt = 0.3 V, I, = 2.2 nA , 240 A x 240 A. The two close-ups in (d) show details from the marked areas in frames (a) and (c). (Repr inted with permission from Ref (59J. Copyright 2005, The American Physical Soc ie ty.) While most in situ studi"s on the one-dimensional reactivity of fcc(I 1 0) metals remain qualitative, recent studies on the reactivity of oll.'Ygen-induced added rows of Ag(I 1 0) have provided quantitative measurement of one-dimensional reactivity. akagoe et al. [65, 66J conducted CO ti tration experiments on the added row recon structed Ag(I 1 O)(n x 1)-0 surfaces, where one-dimensional-Ag-O - chains arrange periodically along the [0 0 11direction. Figure 3.16 shows two series of in situ STM images, where Ag(l 10)(2 x 1)-0 surfaces were exposed to 1 x 10- 8 Torr CO at room t mperature. As a fun ction of time, Figure 3.16a-g depicts the continuous segmentation of AgO chains on the clean or carbon-containing Ag(l 1 0)(2 x 1)-0 surfaces. Figure 3.16h plots the remaining cov('nge of surface oxygen as a function of CO exposure_ Clearly, the reaction rate accompanying the segmentation of AgO chains is significantly accelerated. The authors have gone furth er to study the structure fluctuation at various temperatures. Below 230 K, the AgO chains were found to be straight while the removal of AgO chains only occurs at the end of the -10+--..-- -4 (e) ::: M -I -2 CD ;: -3 -4 4----,.--""", o Figure 3.15 (a) Time (b) plotofl n[ - d8(2, t ?:: 0; (c) plot ofln (Reprinted wi th permi The American Phys ical 3.3 Visualizing the Pathway of Catalytic Reactions O. 0 .I...-~---,-----.---t---'-----'-----"T----' 200 400 -200 o Time (s) (b) -6 .;;) 01 ~ ~ -8 -10 -3 -4 -I -2 In6 (c) :: ~ ( 2.d) -\ -2 <P E -3 _4 +---~--~--~--~--~--~~~ ·L----, o JOO 200 300 400 Time (s ) Figure 3.15 (a) Time evolution of the (2 x 1)-0 coverage; (b) plot ofln[ - d8(2), 1)/dtl versus In 8(2 " 1) for the data from (a) for t ? 0; (c) plot of In 8(2x1) versus t for the data from (a) for t ? 0 (Reprinted with permission from Ref. [59J . Copyright 2005, The American Physical Society.) 177 781 3 In Situ STM Studies of Model Catalysts r I IL [001] (e)so ",0 ~O. 4 i!1 ~ 0.3 8 (6x 1) 0 0 .2 ,.. .~ <ii c: Q -. ]01 °0 20 40 CO Figure 3.16 Two series of STM images of 37nm x 27 nm continuously taken at RT under a nominal CO pressure of 1 x 1O- 8 Torr for clean (a-d) and C-containing (e-g) Ag(l 1 0) (2 x 1)-0 surfaces (i, = 0.2nA, V"p= l.4V). Schematic models of the regio ns are als o shown for (a-d). (h) Titration curves obtained for both clean (red solid circles) and C-containing 60 80 100 exposure (L) 12g (empty circles) Ag(l 1 0) (2 x 1)-0 surfaces. Thick red and black curves are the least square fit s o btained by assuming second-o rder kinetics . The relative number of segments for the clean surface is also pl otted (blue triangles and curve) . (Reprinted with permission from Ref. [65J. Copyright 2003, The American Physical Society.) Figure 3.17 Evolution (j o su rface during expos chains and exhibits zero-order kinetics to CO exposure. The Arrhenius plot gives an activation barrier of 41 kJ mol- 1 and a prefactor of l.7 x 10 3 cm - 2 S-1 The results below 230 K agree with the previous study by Wintterlin et al. on Pt(1 1 1), where the reaction takes place at the periphery of oxygen domains. On the contrary, at room temperature , the reaction rate is drastically accelerated as the AgO chains become segmented and the shape of the AgO chains begins to fluctuate. It is clear there is a direct correlation between the surface structure and the reaction kinetics , although such correlation cannot be quanti.tatively described by the first- or second-order kinetic models. The nonlinear kinetics of CO titration on oxygen precovered surface has been found to take place not only on the oxygen-reconstructed one-dimensional wires but also on the two-dimensional surface oxides. [(lust and Madix [67J have recently studied the reduction of the Ag(1 1 1)-p(4 x 4)-0 surface by CO titration. The Ag (1 1 1)-p(4 x 4)-0 surface was prepared by exposing Ag(1 1 1) to NO z at 500 K. The reduction ofthi s surface was mon itored at room temperature by S M in the presence of 10- 8 mbar CO (Figure 3.17). With time, the surface areas covered by p(4 x 4)-0 continue to shlink while the bright (1 x 1) islands continue to grow on top of the p (4 x 4)-0 overlayer. Figure 3.18a-c illustrates the atomic structures of p(4 x 4)-0 phase, the oxygen-free (1 x 1) Ag islands, and the remnant dots of Ag surface oxide . The authors found that the reaction rate does not correlate with the perimeter of the temperature. The imagl surface area exposed to CO durin g imaging (aJ befo re CO exposure an , boundary layers but nonlinear increase il led the authors to s either at the bound oxygen atoms re l ea~ speculation of this I Jn sitl' CO ti tratic systems, that is , ir compared the CO ti (Ill) surface coven surface covered with the three surface \ temperature. III litll 10- 8_10 - 7 mbar CC surface with large V 3. 3 Visualizing the Pathway of Catalytic Reactions 179 • Figure3 .17 Evolution oftheAg(l 11) -p(4 >( 4) o sunface during exposure to CO at room temperature. The images show the same sunface area exposed to increasing amounts of CO during imaging. (a) Shows the surface before CO exposure and (b). (c) , and (d) at 15, 30, and 45 min after exposure start, respectively. Image (d) shows the final state of the sunface, no changes were observed after 45 min exposure to CO. (Reprinted with permission from Ref. [67J. Copyright 2007, The American Institute of Physics.) boundary layers but increases more rapidly with CO exposure (Figure 3.18d). The nonlinear increase in the reaction rate approximately scales with the reacted area and led the authors to speculate that CO reacts with undercoordinated oxygen atoms, either at the boundary between the Ag surface and the p(4 x 4) -0 phase or with oxygen atoms released onto the Ag surface. Due to the invisibility of such species, speculation of this kind is difficult to verify_ In situ CO titration experiments have also been conducted on multicomposition systems, that is, inverse model catalyst. Schoiswohl et al. [68J in their studies compared the CO titration reaction on three surfaces: clean Rh(1 1 1) surface, Rh (Ill) surface covered with large 2D V309 islands (mean size >50 nm), and Rh(1 1 1) surface covered \-vith sma112D V30 9islands (mean size <15 nm) . Prior to CO titration, the three surfaces were exposed to 10-- 7 mbar O2 to form a (2 x 1)-0 phase at room temperature. In situ STM was used to follow the titration reaction in the presence of 10- 8_10--7 mbar CO . CO titration on the clean Rh(1 1 1) surface or the Rh(1 1 1) surface vv'ith large V309 islands exhibits similar reaction kinetics. Figure 3.19 shows 80 I3 In Situ STM Studies of Model Catalysts 1.0 (d) ~08 m OJ ~ 06 OJ to al ~ 0.4 o Figu re 3.18 (a) Ag(l 1 1) islands and pits surrounded by the Ag(l 1 1)-p(4 x 4)-0 structure. The white arrow points to the remnants of the surface oxide that are occasion ally observed in the pits. The atomically resolved STM images show (b) the Ag(l 1 1)-p(4 x 4)-0 surface and (c) a small area of the (1 x 1) structure obtained on the island shown in (a) that appeared during CO oxidation. The black 15 30 time (min) 45 square on the island shown in (a) marks the approximate scan area of image (c). (d) Development of the reacted surface area during the titration reaction. The curve shows an exponential function fitted to the data. (Reprinted with permission from Ref [67J. Copyright 2007, The American Institute of Physics.) the adsorption of CO on the Rh(111)-(2 x 1)-0 surface occupies the on-top sites and reacts with half of the oxygen in the (2 x 1)-0 phase, leading to the formation of a coadsorbed (2 x 2) 0 + CO phase. The islands of (2 x 2) 0 + CO phase grow at the expense of the (2 x 1)-0 layer upon CO exposure. The titration reaction stops at approximately 30 L of CO and removes half of the surface oxygen atoms. Further removal of the adsorbed oxygen is kinetically inhibited on these two surfaces at room temperature. In contrast, CO titration reaction on the Rh(lll) surface covered with small V309 islands is significantly accelerated and could proceed further to remove all chemisorbed oxygen atoms on the Rh(lll) surface (Figure 3.20). The fuzzy edges of the V309 islands in Figure 3.20 suggest participation of the periphery of small V309 islands in CO oxidation via promotion of the CO oxidation reaction at the metal-oxide phase boundary. In summary, in situ STM studies of CO titration on the oxygen precovered metal surfaces have demonstrated atomic details of CO oxidation on metal surfaces and have shown excellent agreement with macroscopic kinetic measurements. Moreover, in situ studies have revealed an interesting but not well-understood, nonlinear behavior of reaction kinetics. The accelerated reaction rate observed takes place only when surface oxygen islands, either compressed oxygen islands or surface oxide islands, are reduced to the nanometer size. The nonlinear reactivity of these nanoislands is in stark contrast with the large adsorbate layer and requires further investigations. Figu re 3.1 9 Series of 5 0 1 "A) recorded du su rface covered wi th la (a) 1 L; (b) 8 l; (c) 15 L; large V30, island is seer (Repri nted with pennls El sev,er.) I,~ 3.4 Metal Surfaces at Hi The above studies ; composition and str has played a critica i I UH V to atm osp heri The pioneering}ll surface restructure pressUJ s and at 42 ~ surface using a flow surface ofPI(11 0) ex of CO lifts this recoil STM shows the expo to a bul k-like (1 x l Figure 3.21 shows s al 425 K. t he local [( phase upon CO exp, 3.4 Metal Surfaces at High Pressures Figure 3.19 Series ofSTM images (25 nm x 25nm, V, = 1.5 V, I, = 0.1 nA) recorded during dosing CO on the (2 x l)O-Rh (1 1 1) surface covered with large (mean size or ~5 0 nm) V3 0 9 islands: (a) 1 L; (b) 8 L; (c) 15 L; (d) 22 L; (e) 30 L; (f) 600 L. A fraction or a large V30 9 isla nd is seen in the upper right-hand side orthe image. (Rep rinted with permission rrom Ref. [68J. Copyright 2005, Elsevier.) 3.4 Metal Surfaces at High Pressures The above studi s show that the chemisorptions on metals could often alter the composition and structure of metal surfaces. 10 bridge the pressure gap, in situ STM has played a critical role in observing the dynamic behavior of catalytic surfaces from UI:-IV to atmospheric pressures . The pioneering high-pressure STM study by dntyre et al. [69Jshows the Pt(l 1 0) surface res tmctures in single-component gases of H2 , O2 , and CO at atmospheric pressures and at 425 K. Hendriksen et al. [22, 23J took one ste p further to view tbjs su rface using a flow-reactor STM under high-pressure 0 or a COj02 m ixture. The surface of Pt(ll 0) exhibits a (1 x 2) missing·row reconstruction in HV The exposure ofCO lifts this reconstruction even at low pressures. In the presence of1 bar 0 , in situ STM shows the exposure ofhigh-pressure CO not only lifts the (1 x 2) reconstruction to a bulk-like (1 x 1) phase but also causes the coarsening of Pt(l 1 0) surface. Figure 3. 21 shows sequences of snapshots on the Pt(l 1 0) surface in 1.25 bar CO at 425 K. The local rearrangement caused by the transition from the (1 x 2) to (1 x 1) phase upon CO exposure leads to the fragmentation of Pt(l 1 0) terraces and a high 181 3.4 Metal Surfaces at High Pressures Figure 3.1 9 Series of STM images (25 nm x 25 nm, V, = 1.5 V, ',= 0.1 nA) recorded du ring dosing CO on the (2 x l)O-Rh(l 1 1) surface covered wi th l arge (mean size of ~ 50 nm) V309 islands: (a) 1 L; (b) 8 L; (c) 15 L; (d) 22 L; (e) 30 L; (f) 600 L. A fraction of a large V109 isla nd is seen in the upper righ t-ha nd side of the image. (Reprinted with perm issio n from Ref. [68J. Copyright 2005, Elsevier.) 3.4 Metal Surfaces at High Pressures The above sht dies show that the chemisorptions on m etals could often alter the composition and struc:ture of m etal surfaces. To bridge the pressure gap. in situ STM has played a critical role in observi ng the dynamic behavior of catalytic surfaces £i-om !-IV to atmospheric preSSLU('s. The pioneering high-pressure STM study by Mdntyre et al. [69J shows the Pt(l 10) surface restructures in single-com ponent gases of H 2 , O2 , and CO at atmospheric pressures and at 42 K. Hendriksen et al. l22, 23J took one step further to view this SUIface using a flow-reactor STM under high-pressure C or a COI0 2 mixture. The surface ofPt(11 0) exhibits a (1 x 2) missing-row reconstruction in U H V. The e"'Posure of CO lifts Lhis reconstruction even at low pressures. In the presence ofl ba r CO, in situ STM shows the exposure ofhigh· pressure CO not only lifts the (1 x 2) reconstruction to a bulk·lik (1 x 1) phase but also causes the coarsening of Pt(l 1 0) : urface. Figure 3.21 shows sequences of snapshots on the Pt(l 10) surface in 1.25 bar CO at 425 K. The local rearran gement caused by the transition from the (1 x 2) to (1 x 1) phase upon CO exposure leads to the fragmentation of Pt(l 1 0) terraces an d a high 181 821 3 In Situ STM Studies of Model Catalysts Cd ) Figure 3.21 Series of (140n m x 140 nm) immediately after in in the reactor·STM at Figure 3.20 Series of STM images (25 nm x 25 nm, V, = 15 V, I, = 0.1 nA) recorded during dosing CO on the (2 x l)O-Rh (1 1 1) surface covered with small (mean size of ~ 15 nm) Vj 0 9 is lands: (a) 1 L; (b) 21 L; (c) 31 L; (d) 108 L; (e) 204 L; and (f) 270 l. Several small irreg ular shaped Vj 0 9 is lands phase are visible . The areas labeled (2 x 2)-A, (2 x 2) -8, and COs", correspond to the (2 x 2)-0 + CO, (2 x 2)-CO, and CO saturation layer phase s, respectively. (Reprinted with permission from Ref. [68] Copyright 2005 , Els evier.) density ofsurface steps (Figure 3.21a). To reduce the total surface energy, coarsening of the surface steps takes place (Figure 3.21b-f) whereas a much slower (1 x 1) phase transition occurs upon CO exposure . With time the curved Pt islands are smoothed, forming rounded large islands covered by CO on the Pt(l 1 0) surface. The CO-covered Pt(l 1 0) surface \vas then exposed to a mixture of CO/0 2 gases, with the ratio of CO/0 2 adjusted by flow meters. The pressure changes ofCO , O 2 , and the reaction product, CO 2 , were monitored by leaking the gases from the flow reactor to a quadrupole mass spectrometer (QMS) attached to the flow-reactor STM. The surface structure and reactivity of Pt(l 1 0) could be measured simultaneously with the combination of STM and QMS. Figure 3.22 plots the real-time pressure of CO , O 2 , and CO 2 , as well as the corresponding sna pshots of in situ STM images on Pt(l 1 0). The production of CO 2 starts right away with the presence of both CO and O 2 in the reactor. As the reaction proceeds, two stages of reaction rates exist, as evidenced by the step increase in CO 2 pressure in Figure 3.22. At the stage of low reaction rate (see B, F, and H in Figure 3.22), the corresponding STM images show no apparent change in the surface structure, indicating the Pt(l 1 0) surface remains metallic. At the stage of high reaction rate (see D and G in Figure 3.22, about three times higher than the low reaction rate) , the corresponding STM images suggest a rough ened surface, indicating the reaction rate (Figure in surface roughnesi Subsequent high-pr group [70] verified assumed to be resp suggest that the th periodicity. It is 11'01 only during the CO Combined with tl' gas phase could dim relatively high reacti "Mars-Van Krevelen on Pt group metals i! proceeds via (1) the (2) surface diffusion The authors furth, STM and SXRD Al oxidation. The gradl surface oxides and a high pressure is in I shows a strong orie : 3.4 Metal Su1aces at High Pressures Figure 3.21 Series of STM snapshots (140 nm x 140 nm) taken on Pt(l 10) , starting immediately after introduction of 1.25 bar CO in the reactor-STM at 425 K. The "tiger skin" pattern in the fir st panel shows that the (1 x 2) to (1 x 1) transition has divided the s urface in two levels, each 50%, and a high density of steps. Subsequent images show the progressive reduction of the step density by coarsening of the s tep pattern. The elapsed time in minute s is indicated in each panel. The two ball models indicate the atomic-scale geometries characteristic for the starting and end situations . (Reprinted with permission from Ref. [22]. Copyright 2005, Springer.) indicating the formation of surface Pt oxide. At the point of the step increase in reaction rate (Figure 3.22C), the STM image suggests a modest and uniform increase in surface roughness, which the authors assigned as a commensurate Pt oxide film. Subsequent high-pressure surface X-ray diffraction (SXRD) studies by the same group [70] verified the formation of this commensurate oxide film, which was assumed to be responsible for the increased reaction rate. Further SXRD results suggest that the thickness of this oxide film is one monolayer with a (1 x 2) periodicity. It is worth noting that roughening of the oxide surface was obs erved only during the CO oxidation reaction but not in 1 bar pure O2, Combined with their kinetic measurements, the authors proposed CO from the gas I hase could directly react with oxygen atoms in the surface oxides, accounting for relativ ly high reactivity of this phase for CO oxidation. This mechanism, termed as "Mars-Van Krevelen mechanism," challeng s the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of O2 and (2) surface diffusion of COad and Oad atoms to ultimately form CO 2, The auU10rs further tested the Pt(1 1 1) and Pd(1 10) surfaces [71, 72Jusing in situ STM and SXRD. All these single crystals show a similar kinetic behavior in CO oxidation. The gradual rough ening of the surface corresponds to the formation of surface oxides and a higher CO oxidation rate. The structure insensitivity observed at high pressure is in contrast with the results obtained in UHV, where the reactivity shows a strong orientational dependence. 183 841 3 In Situ STM Studies of Model Catalysts ,. ' ro .~ .. ~ ..... -.. .. . ... . ...... ~. ,' ~ _. - - - - - -- - " ~ 0.1 ;£ ~ :::J en 0.01 en ~ Q ro ro 1: 1E-3 Q 1E-4 0 1000 2000 3000 4000 5000 6000 7000 time [S] (A ) can lead to uncerta l global reaction rate. STM measnreme ing the surface stm further prevents pre application and devi provide more precis theless. the above sh studies in model cat effect of lowering rep rod uce the same the restricted difTus surface structures UJ UHVexperiments. 3.5 In Situ Studies ofSu Figure 3.22 QMS signa ls and STM images sim ultaneously measured during CO oxidation onthe Pt(ll 0) surfaceatatemperatureof425 K in a 3.0 ml min - 1 flow of mixtures of CO and/or O 2 at 0.5 bar. (Upper panel) QMS signals of O 2 , CO, and CO z, measured directly from the reactor cell. Label s (A) - (H) correspond to the STM images in the lower panel. R,ow and Rhigh denote the low and high CO z production rate branches. P'h indica tes the threshold value of the CO press ure at which the rate switched from R ,ow to R high and the surface changed simultaneously from smooth to rough . (Lower panel) STM images (210 nm x 210 nm) from an STM movie (65 s/image, I, = 0.2 nA. V, = 80 mV). The images were differentiated to enhance the contrast. Images (A). (B), (E). (F), and (H) show flat terra ces separated by steps of the Pt lattice. This co rres ponds to the metallic, CO-covered surface. Image (C) shows the change in the surface accompanying the step in activity at t = 2109 s (the image was built up from bottom to top). Images (D) and (G) show the rough surface consisting of protrusions. with heights of 0.2-0.4 nm , and pits (see inset) . (Reprinted with permission from Ref [23]. Copyright 2002. The American Phys ical Society.) Due to the increased complexity at high pressure (e.g.. the impinging reactant gases are no longer under molecular flow, causing the gas environment near the surface to be markedly different from the ambient gases). much more work is required before establishing a firm correlation between the surface structure and the surface reactivity. The difficulty in u sing STM to measure the reaction rate directly In an effort to brid catalysts have emel clusters supported a STM. The tremendc remarkable catalytic ago [73-801. It was g' The TiO l support G clusters in the 2-4n reactivity for low-ter Although the cause supported model A understandil1g of tt support. In this secti the synthesis of SUP! metal support inter; supported Au calaly. 3.5 .1 Monitoring the Grow Supported model ca atoms onto a planar ( metal clusters depeJ deposition rate. and, model catalysts, it i, - - -- -- ---- - - - 3.5 In Situ Studies of Supported Model Catalysts can lead to uncertainties in correlating local surface structure with the measured global reaction rate. STM measurements during high-pressure reaction present challenges for resolv ing the surface structures at high resolution. Rapid diffusion of surface adsorbates further prevents precise characterization of surface-active species. In this case, the application and development of the fast-scanning technique is essential and could provide more precise surface information during the high-pressure reaction. None theless, the above study demonstrates the necessity ofexpanding high-pressure STM studies in model catalysis. The effect of high pressure intuitively is equivalent to the effect of lowering the temperature. Low-temperature UHV experiments might reproduce the same adsorbate structures as do high pressures. However, due to the restricted diffusion of metal atoms at low temperature, the dynamic change of surface structures under catalytically realistic conditions could not be observed in the UHVexperiments . 3.5 In Situ Studies of Supported Model Catalysts In an effort to bridge the material gap, in situ STM studies of supported model catalysts have emerged in recent years. Among supported model catalysts, Au clusters supported on Ti0 2 (1 1 0) are the most investigated model system by in situ STM. The tremendous interest in studying this model system originates from the remarkable catalytic reactivity of supported Au clusters discovered a few decades ago [73-80J. It was generally believed that Au, the noblest metal, is inert as a catalyst. The Ti0 2 support alone also exhibits limited catalytic reactivity. In contrast, Au clusters in the 2-4 nm size range supported on Ti0 2 exhibit a remarkable catalytic reactivity for low-temperature CO oxidation and selective hydrogenation reactions. Although the cause of the extraordinary activity of Au clusters is still under debate, supported model Au catalysts are playing a key role in the development of our understanding of the relative importance of cluster morphology and the cluster support. In this section, we highlight a few recent in situ STM studies in monitoring the synthesis of supported model catalysts , revealing the mechanism of the "Strong metal support interaction" (SMSI) effect and measuring the sintering kinetics of supported Au catalysts. 3.5.1 Monitoring the Growth Kinetics of Supported Metal Catalysts Supported model catalysts are frequently prepared by thermally evaporating metal atoms onto a planar oxide surface in UHV The morphology and growth ofsupported metal clusters depend on a number of factors such as substrate morphology, the deposition rate , and the surface temperature . For a controlled synthesis of supported model catalysts, it is necessary to monitor the growth kinetics of supported metal - - - - - - -- - - - 185 • 86 1 3 In Situ STM Studies of Model Catalysts Figure 3.23 STM imagesofthesameareaofaTi0 2 (11 0) surface after the deposition of (a) 0.17 M L Au; (b) 0.34 M L Au; (c) 0.51 M L Au; (d) 0.69 M L Au; (e) 0.86 M L Au; and (f) 1.3 M L A u. All images have the same d imensions of 100 nm x 100 nm. The circle highlighted in white in each image indicates the identical area. (Reprinted with permission from Ref. [811 . Copyright 2003, The Japan Society of Applied Physics .) clu sters. Figure 3.23 shows a series of STM images as a function of increased Au coverage obtained on the same area of a Ti0 2 (1 1 0) surface at room temperature [81]. At the very early stages of growth, Au clusters preferentially decorate the step edges (Figure 3.23a). With an increase in Au coverage, the prevailing role of the step edges as major nucleation sites decreases , whereas new clusters begin to nucleate on terraces (Figure 3.23b-f). Au clusters grown at step edges grow much faster than those at terraces. A quantitative analysis of Au cluster distribution suggests a bimodal size distri bution for the growth of Au clusters at step edges and terraces. In situ STM studies allow the growth mode of Au clusters to be followed on a cluster-to-cluster basis. The results demonstrate that step edges playa dominant rol in the growth of Au clusters on Ti0 2 (1 1 0) at room temperature. Using in situ STM also makes it possible to monitor the growth of supported alloy model catalysts. The simplest way to synthesize supported alloy model catalysts is to Figure 3.24 STM images deposition on an Ag preco after deposi tion of (b) 0.1 precovered (0.033 ML) TiC (e) 0.17MLAu, (f)O.S 1 M I (e) show the appearance· permission from Ref [821 evaporate both metal a this method does not g clusters or separate to this problem to be add catalysts. In our study, Ag- Au and the grOWtll kinetic Au clusters in the prest carried out as a funct A particular precover~ saturate the step edge ~ in the absence of their 3.5 In Situ Studies of Supported Model Catalysts 187 • Figure 3.24 STM images (100 nm x 100 nm) during Au depos ition on an Ag precovered (0.08 ML) Ti0 2 (1 1 0) surface (a) after deposition of (b) 0.17 ML Au. (c) 0.85 ML Au; or on an Ag precovered (0. 033 ML) Ti0 2 (1 10) surface (d) after deposition of (e) 0 17 M L Au. (f) 0.51 M L Au . T he white circles in image (d) and (e) show the appearance of new Au cluste rs. (Reprinted wi th permission from Ref [82J. Copyr ight 2004 , Elsevier BV) evaporate both metal atoms in UHV directly onto an oxide support surface. However. this method does not give information on whether the t\'vo metals mix and form alloy clusters or separate to form tvvo types of monometallic clusters. In situ STM allows this problem to be addressed by investigating the gro\oV1:h kinetics of supported alloy catalysts. In our study, Ag-Au alloy clusters ofvaryin g ratios were synthesized on Ti0 2 (11 0) and the gro\vth kinetics for the clusters determined [82J . To understand the grO\oV1:h of Au clusters in the presence ofAg clusters, sequential in situ STM mea surements were carried out as a fun ction of Au coverage on a Ag precovered Ti0 2 (J I 0) surface. A particular precoverage of 0.08 ML Ag (Figure 3.24a) was chosen to essentially saturate the step edge sites with Ag clusters, allowing the investigation of Au growth in the absence of their preferred nuclea tion sites. The series of in situ ST M images 881 3 In Situ STM Studies of Model Catalysts (Figure 3.24a-c) show a preferential growth of Au on the existing Ag clusters. As the Au coverage is increased, new Au clusters begin to appear on the surface. In another set of experiments, the Ti0 2(1 1 0) surface was pre covered with less Ag so that a fraction of empty step edge sites remain open for the nucleation of new Au clusters (Figure 3.24d). The series of in situ STM images (Figure 3.24d-f) show even a small amount of Au deposition (0.17 M L) leads to the nucleation of new Au clusters. These systematic experiments of Au growth on Ti0 2 (1 1 0) precovered with Ag clusters show the versatility of in situ STM experiments in controlling the synthesis of supported alloy model catalysts. Simply by blocking step edge sites to a varying extent, either monometallic clusters or bimetallic clusters could be selectively synthesized and monitored. By changing the sequence of metal deposition, the surface structure of alloy clusters can also be altered. Adopting the same in situ approach, the synthesis of supported alloy model catalysts has been extended to Pdj Au clusters on Ti0 2 (1 1 0) [83] and PtjRh clusters on Ti0 2 (1 1 0) [84]. It is expected that this method will be generally adopted for the preparation and characterization of supported model catalysts. 3.5.2 Studies of the SMS I Effect [n the late 1970s, Tauster et al. [85-87] discovered the unusual properties of group VII[ metal. for example, Pt and]Y, when supported on Ti0 2 and reduced at relatively high temperatures. The Pt and [r clusters supported on Ti0 2 show a suppressed CO and H2 chemi sorption and an increased methanation catalytic activity. "S trong metal support interaction" has since been introduced to describe the unusual properties of group VIll metal supported on reducible oxides such as Ti0 2 . A few reasons for the SMS[ effect have been postulated, including alloy formation, altered electronic effects at the interface, or encapsulation of supported metal clusters. Bowker et al. [88 ,89] have investigated the SMSI effect using insitu STM. As shown in Figure 3.25, Pd clusters with a mean si ze of approxima tely 4 nm were supported on Ti0 2 (11 0) and heated to 673 K. The surface was then exposed to 5 x 10- 8 mbar O 2 and monitored by STM as a function of time. In the presence of oxygen, the reoxidation of the Ti0 2 (1 1 0) surface proceeds. Since the dissociation probability of O 2 on Pd is about three magnitudes higher than on Ti0 2 (1 1 0), O 2 dissociation mainly occurs on supported Pd clusters. The dissociated oxygen atoms subsequently diffuse onto the adjacent surface, that is, spillover, and coalesce with Ti interstitials from the bulk, causing the enrichment ofTiOx species surrounding the Pd clusters. The regrowth of Ti0 2 layers thus accelerates at the periphery of Pd clusters. Eventually, the growth of Ti0 2 layers surpass the height of the Pd clusters and fully encapsulates them v.rithin the Ti0 2 overlayers, that is, in situ STM study verifies that encapsulation is a major contributor to the SMSI effect. This is also the first study to have directly observed in situ the spillover, an elementary step in catalytic process. Figure 3.25 Asequence at 673 K of oxygen spillovt an amb ient pressu re of 5 (doubled before image (f): to oxygen from Image (a)· of L): 114, 178.237, 282, spillove r begins With the fc 3.5.3 Sintering Kinetics of SL Although supported A such as removing CO for the commercializal rapidly [78, 90, 91]. S consisting of Au cluste has been shown to con the sintering of suppor Ti0 2 (1 1 0) in UHV ar The first issue in si process. The sinterin e modes: (a) the migrati monomers (single 1m cluster migration, ent, coalesce with neigh bor monomers dissociate clusters [93, 94). 3. 5 In Situ Studies of Supported Model Catalysts 189 • Figure 3.25 A sequence of STM images taken at 673 Kof oxygen spillover from Pd clusters in an ambient pressure of 5 x 10- 8 mbar of O 2 (doubled before image (f)). The total exposures to oxygen from image (a}-(f) were (in units of L): 114, 178, 237, 282. 344, and 53l. The spillover begins with the formation of one layer around the Pd clusters, which then becomes multilayer growth, eventually burying the Pd cluster with Ti0 2 . In image (f), a total of seven layers ofTi0 2 have grown around the Pd on top of the original Ti0 2 surface. (Reprinted with permission from Ref. [89J. Copyright 2000, Elsevier) 3.5.3 Sintering Kinetics of Supported Au Clusters Although supported Au catalysts have great potential in technological applications , such as removing CO selectively from hydrogen fuel for fuel cells, a major problem for the commercialization of supported Au catalysts is that these catalysts deactivate rapidly [78, 90. 91J. Similar deactivation has been observed for model catalysts consisting of Au clusters deposited on a reduced Ti0 2 (11 0) where rapid deactivation has been shown to correlate with sintering of the Au clusters [91. 92J. To understand the sintering of supported Au catalysts. we have studied the sintering kinetics of Aul Ti0 2 (1 1 0) in UHV and in the presence of reactant gases. The first issue in sintering kinetics is to determine the primary mass transport process. The sintering of supported nanoclusters can occur in either one or two modes: (a) the migration and coalescence of whole clusters or (b) the migration of monomers (single metal atoms or metal complexes). The first mode, known as cluster migration, entails the migration of whole clusters on the surfa ce that then coalesce with neighboring clusters. In the second mode, known as Ostwald ripening, monomers dissociate from small clusters, diffuse to, and coalesce with large clusters [93. 94J. 90 I3 In Situ STM Studies of Model Catalysts In situ STM allows one to monitor individual clusters under realistic conditions and to directly determine the mass transport mode controlling the kinetics. Mitchell et al. studied Au clusters supported on Ti0 2 (1 1 0) at approximately 750 K as a function of time [95]. Their STM results show Au clusters are mostly static while some small clusters with size below 3 nm diffuse along the surface. The diffusion of small clusters does not behave in a continuous manner. Instead, the clusters stick for a relatively long time before taking a sudden jump. This unconventional diffusion of clusters, termed as Levy flight, leads to the speculation that Au clusters sinter in the mode of cluster migration. Such results are in agreement with the in situ ST M study by Kolmakov et al. [12], who examined the stability of AujT i0 2(1 1 0) after thermal treatment at 950 K. Dramatic morphological changes were observed. Accompanying the decomposition of surface steps, the density of Au clusters decreased by 20%, whereas the volume of the Au clusters increased. Most Au clusters changed their positions on the Ti0 2 (11 0) surface. Clearly, cluster migration has played a role in the thermally induced sintering of supported Au clusters under UHV. Interestingly, the above studies were both conducted at the temperature under which the Ti0 2 (1 1 0) substrate becomes unstable and starts to decompose. To fully explore the sintering mode induced by thermal treatment, it is necessary to conduct such in situ experi ments at intermediate temperatures where the Ti0 2 (1 1 0) lattice remains stable. The sintering of AujTi02(1 1 0) was also studied by our group in the presence of reactant gases [96J. Figure 3.26 shows the morphological changes of 0.5 ML Au clusters supported on Ti0 2 (1 1 0) in the presence of COj02 at 300 K. Under UH V conditions, the Au clusters remain immobile and show no detectable change ofeither cluster size or shape for more than 4 h. Sintering of Au clusters began immediately upon the introduction ofa 0.1 Torr COj02 gas mixture (1: 1 ratio). Figure 3.26 shows no apparent change of cluster positions before and after gas exposure; however, the small Au clusters gradually decay whereas the large Au clusters grow in size. The gradual change in the cluster size and static cluster positions are consistent with an Ostwald ripening process. Most Au clusters with a height around 4.6 A or less (assuming the height of Au clusters of two-layer thickness is 4.6 A [97]) disappear within 2 h of CO oxidation reaction. Gold clusters supported on Ti0 2 (1 10) were also exposed to O 2 and CO separately to investigate the influence of the chemisorbed reactants. Minimal changes were observed for 0.5 ML Au clusters supported on Ti0 2 (11 0) after an exposure of 0.1 Torr O 2 or 3 Torr CO for more than l.5 h at 300 K. Therefore, a synergetic effect of COj02 was found to induce and accelerate the sintering of Au clusters on Ti0 2 (1 1 0). The synergetic effect of a CO and O 2 mixture can be explained by a reaction induced mechanism, which might involve hot electrons generated by CO oxida tion [98- 100], inducing the detachment ofAu monomers from supported Au clusters and ini tiating the sinterin g process. On the basis of the sintering model of Ostwald ripening, the activation energy for Au clusters with diameters around 3 nm is estimated to be approximately 10kJmol- 1 , a value that closely tracks the activation energy of CO oxidation on supported Au clusters. The large difference in comparing this number with the activation energy ofsintering for supported Ali clusters in UH V, approximately 280 kJ mol 1 [101, 102], demonstrates that CO oxidation itself does (e) Figure 3.26 75 nm x : of0.5 MLAudusterssu surface in the presenc mixture at 300 K. (a- f at the same surface ar indeed influence ( oxidation over oth( 3.6 Outlook This chapter re iel From revealing rea UHVa nd on exten r questions in catal; 3. G OlAtlook (e) (f) Figure 3.26 7S nm x 7S nm in situ STM images of 0.5 MLAu ciusterssupported ontheTi0 2 (1 1 0) surface in the presence of 0.1 Torr CO and O 2 mixture at 300 K. (a-f) are consequently taken at the same surface area. The time intervals are (a) 0 min; (b) 28 min ; (c) 42 min ; (d) 63 min; (e) 120 min ; and (f) 280 min . Tunneling parameters are V, = 2 V and = 0.1 nA. (Reprinted with permission from Ref. [96J. Copyright 2009, The American Chemical Society.) 't indeed influence cluster sintering. This effect may very well be general fo r CO oxidation over other supported metal catalysts. 3.6 Outlook This chapter reviews the recent progress in in. situ STM studies of model catalysts. From revealing reaction pathways to delineating active sites, in. situ STM studies in UHVand on extended surfaces have demonstrated their power to solve fundamental questions in catalysis and enhance our understanding of the elementary steps of 191 92 / 3 In Situ STM Studies of Model Catalysts surface catalytic processes. Recent efforts in expanding in situ STM studies to high pressures and to supported model catalysts illustrate the need to study model catalysts under realistic conditions. The studies discussed in this chapter have shown surface structures are extremely dynamic under reaction conditions, consistent with the hypothesis promoted by Somorjai [103-107]. The h ypothesis, termed as the "flexible surface," supposes that metal surfaces with low coo rdinated metal atoms or curved cluster surfaces are highly fluxional during reaction. Those atoms with high diffusivity are responsible for high activity of metal surfaces. To this end, it is critical to compare the timescale and energetics of metal atom diffusion with their catalytic reactivity. With the develop ment of fast scan techniques, in situ STM now provides a great opportunity to test this hypothesis. The studies to date have shown that small can be quite different and less can sometimes mean mo re. In the coming years, there is no doubt that in situ STM studies on well-defined nanostructures will exponentially increase and contribute invaluably to an atomic-level understanding of reactions catalyzed by solid surfaces. Acknowledgments We gratefully acknowledge the support for this work by the Department of Energy (DOE) , Office of Basic Energy Sciences, Division of Chemical Sciences , and the Robert A. Welch Foundation. 14 Melmed, A.) . (199 9, 601. 15 Oliva, A. I. , Rom Anguiano, E., an Sci. hlstnml.. 67, 16 Weinstein, v..Sl u and Ben jacob, E. 66, 307 5. 17 Ren. B., Picardi, a (20 04) Rev. Sci. [;1 18 Fried, G.A. , Wang (1 993 ) Rev. Sci. In 19 lwami , M.. Ueh' (1998) Rm Sci. In 20 Schroder. U., Mci M.. and Somorjai, 333. 33 7. 21 Reiw el.t. R.. Gun Win tterlin, )., Ku Schlogl , R. (2007) Phys.. 9. 35 90. 22 Hendr iksen. B. L. and Frenkel1, ).\V. 36, 43 . 23 Hendriksen, B.L. \ (2002) Phl'S. Rev. L 24 Wintterlin. j. (20 References 25 Hahn , j.R. and Ho Lett.. 87. 1661 02. Langmuir, I. (191 5) Phys. Rev., 6, 79. 2 Bartels, L , Wang. F., Moll er. D.. Knoesel, E.•and Hein z, TF. (2004) Science. 305, 648 . 3 Che. M. and Bennett, C O. (1989) Adv. Cata!.. 36. 55 . 4 Wintterhn, )., Trost, j .. Renisch. S.. Schu ster. R., Zambelli, T . and Ert!. G. (1997) Suif. Sc i , 394, 159. Kuipe rs . L. . Hoogeman. M.S. , and Frenken. j.W.M . (199 3) Phys. Rev. Lett., 71. 3517. 6 Ludwig, C, Gompf, B.. Glatz. w.. Peterse n. ) .. Eisenmenger. w.. Mobus, M.. Zimmermann, U., and Karl, N. (1992) Z . Phys. B. 86, 397 . 7 Rost, M.).. Crama, L.. Schake\. P., Van To!. E., Van Ve\zen -Williams, G. B.E.M ., Overgauw, CF.. Ho rst. H. , Dekker, H., Okhuijs en, B.. Seynen , M., Vi jfti gschild, A. , Han, P., Katan . A. ) ., Schoots, K. , Schumm, R., Van Loo, w.. Oosterkamp, TH. , and Frenken, ). W.M . (200 5) Rev. Sc i. instrum., 76 , 053710. 8 Rossler, M. , Geng, P., and Wintterlin , ). (2005) Rev. Sci. Instrum., 76. 023705. 9 La egsga ard , E.. Osterlund. L , Thostrup. P., Rasmu ssen. P.B. , Stensgaard , 1.. and Besenbacher, F. (2001) Rev. Sc i. IItSt rum., 72, 353 7. 10 Rasmussen, P.B., Hendriksen, B.L.M. , Zeijlemake r, H., Ficke . H.G .. and Frenken. j.W.M. (1998) Rev. Sci. Instrum .. 69 . 3879. 11 Mcintyre, B.) ., Salmeron. M.. and Somorjai. GA (199 3) Rev. Sci. instrum. , 64 . 687. 12 Kolmakov, A. and Goodman, D.W. (2002) Chern. Rec., 2, 446. 13 Kolmakov. A. and Goodman, D.W. (2003) Rev. Sci. Instrum., 74, 2444 . 26 Hla, S.w. and Riedl Rev. Ph ys. (hem ., 5, 27 Rieder, K.H.. Meye l Moresco, F., Braun K., Repp, ). , Foelscl (20 04) Phi/os. Tram. 1207 28 Wong, K. L.. Rao, B. Ulin-Avila, E., and . J Chern. Pill'S., 123. 29 Longwitz, S.R.. Sch EX, Yang, R.T. La , gaa rd, I., Brune, H. (2004) J Phys. Chu 30 Vestergaard, EX , 1 Laegsgaa rd. E., S t~r B., and Besenbache Lett., 88, 259601. 31 Osterlund, L. , Rasn Thostrup, P., Laegs References 14 Melmed, A. J. (1991)). Vac. Sci. Technol. B, 9, 60l. lS Oliva, A.I. , Romero, A. , Pena, j.L. , Anguiano, E., and Aguilar, M. (1996) Rev. Sci. Instrum. , 67, 1917. 16 Weinstein, v., Slutzky, M., Arenshtam, A. , and Benjacob, E. (1995) Rev. Sci. Instrum ., 66, 3075. 17 Ren, 8. , Picardi, G., and Pettinger, B. (2004) Rev. Sci. Instrum., 75, 837. 18 Fried, G.A ., Wang, X.D., and Hipps, KW (1993) Rev. Sci . Ins trum., 64, 1495 . 19 iwami , M., Uehara, Y., and Ushioda, S. (1998) Rev. Sci . instrum., 69 , 4010. 20 Schroder, U., Mcintyr e, B.j. , Salmeron, M. , and Somorj ai, G.A. (1995) Surf Sci., 333,337. 21 Reichelt, R. , Gun the r, S., Rossler, M., Wintterlin, j., Kubias, B. , jakobi, B., and Sch log! , R (2007) Phys. Chem . Chem. Phys., 9, 3590. 22 Hendriksen, B.L.M. , Bobaru, S.c. , and Frenke n , j.WM. (2005) Top . Ca tat., 36, 43. 23 Hendriksen , B.L.M. and Frenken, j.WM. (2002) Phys. Rev. Lett., 89, 04610l. 24 Wintterlin, J. (2000) Adv. Catal., 45 , 131. 2S Hahn, j.R and H o, W (2001) Phys. Rev. Lett., 87, 166102. 26 Hla, S.W. and Rieder, KH. (2003) Annu. Rev. Phys. Chem., 54, 307. 27 Rieder, KH. , Meyer, G., Hla, S.W, Moresco, F. , Braun, KF., Morgenstern, K., Repp , j., Foelsch, S., and Bartels, L. (2004) Philos. Tra ns. Roy. Soc. , 362, 1207. 28 Wong, KL. , Rao, BV, Pawin, G., Ulin·Avila, E.. and Bartels, L. (2005) ). Chem. Phys., 123, 201102. 29 Longwitz, S.R., Schnadt, j., Ves terga ard, E.K. , Yang, R.T. , Laegs gaard , E. , Stens· gaard , I., Brune, H ., and Besenbacher, F. (2004)). Phys. Chem . B, 108, 14497. 30 Vestcrgaard, E.K. , Thostrup, P. , An, 1., Laegsgaard , E. , Stensgaard, L, Hammer, B., and Besenbacher, F. (2002) Phys. Rev. Lett., 88, 25960 l. 31 Osterlund, L. , Rasmussen, P.B. , Thostru p, P., Laegsgaard, E. , Stensgaard, I., and Besenbacher, F. (2001) Phys. Rev. Lett. , 86, 460. 32 Thostrup , P. , Ves tergaard, E.K , An , 1., La egsgaard , E., and Besenbacher, F. (2003)). Chem. Phys., 118, 3724. 33 Yang, R.T., Wang, j .G ., Knudsen, j. , Schnadt, j., La egsgaard , E., Stensgaa rd , I. , and Bese nbacher, F. (2005 )). Phys. Chem. B, 109 , 14262. 34 Mi tsui, 1., Rose , M.K , Fomin, E., Ogletree , D.F. , and Salmeron, M. (2003) Nature, 422, 70S. 35 Salm eron, M. (2005) Top. Catat. , 36, 55. 36 Mits ui, 1., Rose, M.K, Fomin, E. , Ogletree, D.F. , and Salme ron, M. (2003) Surf Sci ., 540, 5. 37 Diebold, U. (2003) Surf Sci. Rep., 48, 53. 38 Diebold , U. , Lehman , j., Mahmoud, 1., Kuhn, M., LeonardeIli, G., Hebenstreit, W, Sc hmid , M., and Varga, P. (1998) Surf Sci., 411, 137. 39 Schaub, R. , Thostrup , R, Lopez , N., Laegsgaard, E., Stens gaard , L, Norskov, ).K , and Besenbacher, F. (2001) Phys. Rev. Lett., 87, 266104. 40 We ndt, S., Schaub , R , Mattbiesen, j., Ves tergaard, E.K. , Wa hlstrom, E., Rasmu ssen , M.D. , Thostrup, P. , Molina , L.M., Laegsgaard, E., Stensgaard, I. , Ha mmer, B., and Besenbacher, F. (2005) Surf Sci. , 598, 226. 41 Bikondoa , 0 ., Pang, c.L., lthnin, R., Muryn, c.A. , O nishi , H ., and Thornton , G. (2006) Nat. Mater., 5, 189. 42 Wendt, S., Matthiesen, )., Schaub, R., Veste rgaard , E.K., Laegsgaard , E., Besenbacher, F., and H ammer, B. (2006) Phys. Rev. Lett., 96, 066107. 43 Zhang, Z., Bondarchuk, 0 ., Kay. B.D., White, j.M .. and Dohnalek, Z. (2006) J. Phys. Che m. B, 110, 21840. 44 Ii, S.c., Zhang, Z. , Shepp ard, D., Kay, B.D., White, j.M., Du, Y., Lyubioe tsky, I. , Henkelman, G., and Dohnalek, Z. (2008) ). Am. Chem. Soc. , 130, 9080. 45 Zhang, Z.R , Bondarchuk, 0., White , j .M. , Kay, B.D. , and Dohn alek, Z. (2006) ). Am. Chem. Soc., 128, 4198. 193 941 3 In Situ STM Studies of Model Catalysts 46 Zhang, Z.R" Bondarchuk, '., Kay, B.D., White, J .M., and Dohnalek, Z (2007) J. Phys. Chem. C, 111, 302l. 47 Du, YG. , Dohnalek, Z., and Lyubinetsky, I. (2008) J. Phys. Che m. C, 112, 2649. 48 Zhang, Z , Ge, Q., Li, S.C, Kay, B.D., White, ).M., and Dohnalek, Z. (2007) Phys. Rev. Lett., 99, 126105. 49 Lyubine tsky, I. , Yu, Z.Q. , and Henderson, M.A. (2007) J. Phys. Chem. C, 11 1, 4342. 50 Wend t, S., Sprunge r, PT, Lira, E., Madsen, G.KH ., Li , Z.S ., Hansen , ).0 ., Matthiese n, )., Bfekinge-Rasmussen. A., Laegsgaard, E. , Hammer, B., and Bese nbacher, F. (2008) Science, 320, 1755. 51 Henderson, M.A., Epling, WS., Peden, CH.F., and Perkins, CL (2003) J. Phys. Chem. B, 107, 534. 52 Smi th , R.D., Bennett, RA., and Bowker, M. (2002) Phys. Rev. B, 66, 7. 53 Park, KT, Pan, M., Meunier, V., and Plumm er, E.W (2007) Phys. Rev. B, 75, 245415. 54 Onishi, H. and [wasawa, Y (1996) Phys. Rev. Lett., 76, 79l. 55 Sachs, C, Hilde brand, M., Volkening, S., Wintterlin . I. , and Ertl, G. (2001) Science, 293, 1635. 56 Volkening, S., Bedurftig, K , Jacobi , K , Wintterlin, j. , and Ertl, G. (1999) Phys. Rev. Lett., 83. 2672. 57 1mbihl, R. and Ertl, G. (1995) Chem. Rev. , 95, 697. 58 Wintterlin, J., Volkcning, S., Janssens, TV.W , Zambelli, T, and Ertl, G. (1997) Science, 278, 1931. 59 Kim, S.H., Mendez, )., Winttcrlin , J., and Frt! , G. (2005) Phys. Rev. B, 72, 155414. 60 Leibsle, F.M., Murray, P.W, Francis, S.M .. Thornton , G .. and Bowker, M. (1993) Nature, 363, 706. 61 Le ibsle, F.M., Francis, S.M. , Hag, S. , a nd Bowker, M. (1994) Surf Sci., 318, 46. 62 Le ibsle. F.M. , Francis, S.M. , Da vis, R., Xiang. N., Hag, S., and Bowker, M. (1994) Phys. Rev. Lett .. 72, 2569. 63 Crew, WW and Madix, R.J. (1996) Surf Sci., 349, 275. 64 Ruan, L . Stensgaard, I. , Laegsgaard, E.. and Bese nbache r. F. (1994) Surf Sci.. 314, L873 . 65 Nakagoe, 0 .. Watanabe, K, Takagi, N.. and Mats umoto, Y (2003) Phys. Rev. Lett., 90 .226105. 66 akagoe. 0., Watanabe , K., Takagi. N., and Mats um oto. Y (2005) J. Phys. Chem. B, 109. 14536. 67 Klus t, A. and Madix, R.). (2007) J. Chem. Phys., 126, 084707. 68 Schoiswohl, I., Eck, S., Ramsey, M.G. , Andersen, I.N ., Surnev. S., and Netzer, F.P. (2005) Surf Sci.. 580, 122. 69 Mcintyre. B.)., Salmeron, M., and Somorjai, G.A. (1993) J. Vac. Sci. Tech., 11 , 1964. 70 Acke rma nn. M.D .. Pedersen, TM., Hendriksen, B.L.M., Robach. 0 .. Bobaru, S.C. Popa, I., Quiros, C, Kim, H., Hammer, B.. Ferrer. S., and Frenken. JWM. (2005) Phys. Rev. Lett., 95, 255505. 71 Hendrikse n, B.L.M., Bobaru. S.C, and Fre nken, IWM. (2004) Surf Sci.. 552. 229 . 72 He ndriksen, B.LM., Bobaru, S.C . and Fre nke n, I.WM. (2005) Catal. Today, 105, 234. 73 Freund , H .J., Baumer, M., and Kuhlenbeck, H. (2000) Adv. Catal., 45 , 333. 74 Henry. CR (1998) Sulj Sci. Rep. , 31, 235. 75 Campbell, CT (1997) Surf Sci. Rep. , 27, l. 76 Rainer, D.R. and Goodman, D.W (1998) J. Mol. Catal. A, 131, 259. 77 Campbell, CT, Grant, A.W., Starr. D.E. , Parker, S.C, and Bondzie, VA. (2001) Top . Catal., 14, 43. 78 Haruta, M. and Date, M. (2001) Appl. Catal. A. 222, 427. 79 Corti, CW , Hollida y. R.) ., and Thompson, DT (2007) Top. Catal. , 44, 33l. 80 Bond . G.C and Thompson, DT (1999) Catal. Rev., 41, 319. 81 Santra, A.K, Kolmakov, A., Yang, F. , and Goodman, DW (2003)jpn. J. Appl. Phys. Part 1. 42, 4795. 82 Sanrra, A. K. , Yang. J DW (2004) Sur}, Sc 83 Han, P. and Goodm j. Phys. ehem. 11 84 Park. J.B .. Ratliff. J. ! Chen, DA (2006) 5 85 Tausler.S .J.( 1987} At 86 Ta uster. S.)., Fung. : and Horsley, ).A. (1' c. 1121. 87 ·[auster. S.J .. Fung. S (1978) JAm. Cktm 88 Bowker. M (2007) I Phys., 9, 3514. 89 Bowker, M .. Bowker Stone, P.. and Ra mi J Mol . Catal. A. 16: 90 Haruta, M. (1997) ( 91 VaIden. M., Lai. X.• (1998) Scimc<. 281. 92 lAi. X., St Clair. TP Goodman, D.W (19' 25. 93 Chakravc rty, B.K. (1 Sol. 28, 2401 94 Wynbl att. P. and Gj Prog. Solid Stare ell References 82 $anlra, A. K.. Yan g, F., and Goodman, D.W. (2004) Surf Sci.. 548, 324. 83 Han, P. and Goodman, D.W. (2008) j. Phys. Chern. C, 112, 6390. 84 Park, ).B., Ratliff. ).5., Ma. 5. , and Chen, D.A. (2006) Surf. Sci. , 600, 2913. 85 Tau$ter, S.). (1987) Acc. Chern. Res., 20, 389 . 86 auster. 5. )., Fung. S.c., Baker, RT K., and Horsley, ) A. (1981) Science, 211 , 1121. 87 Tau ster, 5.)., f ung, S.c. , and Garten, R.L. (19 78) j. Arn. Chern. Soc, 100, 170. 88 Bowker, M. (2007) Phys. Chern Chern. Phys. , 9, 3514. 89 Bowker, M. , Bowker, L.). , Be nnett, R. A.. Stone, P., and Ramirez-Cuesta, A. (2000) J. Mol. Catal. A, 163, 221. 90 Haruta, M. (1997) Catal. Today, 36, 15 3. 91 Vaiden , M., Lai. X., and Goodman, D.W. (1998) Science, 281, 1647. 92 Lai , X., St Clair, TP .. Vaid en, M .. and Goodman , D.W. (1998) Prog. Suif. Sci ., 59, 25 . 93 Chakraverty, B.K. (1967) J. Phys. Chern. Sot, 28, 2401. 94 Wynblatt, P. and Gjostein, NA (1975) Prog. Solid State Chern., 9, 21.. 95 Mitchell, C.E.)., Howard , A., Carney, M., and Egdell , R.G. (2001) Suif. Sci., 490, 196 . 96 Yang, F. , Chen. M.S., and Goodman, D.W. (2008) J. Phys. Chern. C. 113, 254. 97 Cosandey, F. and Madey. TE. (2001) SUI! Rev. Lett ., 8, 73. 98 ji, X. Z. , Zuppero, A.. Gidwani, j.M., and Somorja i, G.A. (2005) Nan o Lett ., 5, 753. 99 Ii, X.Z., Zuppero, A.. Gidwani , ).M. , and Somorjai , GA (200 5) J. Arn. Chern. Soc. , 127, 5792. 100 Ii , X.Z. and omorjai, G.A. (2005 ) J. Ph)'s Chern. E, 109, 22530. 101 Parker, S.c. and Campbell, CT (2007) Phys Rev. E, 75, 035 430. 102 Campbell. C.T, Parker, S.c.. and Starr, D.E. (2002) Science, 298,811. 103 Somorjai, GA (1991) Langmuir, 7. 3176. 104 Somorjai, G.A. (1992) CataL Lett .. 12, 17. lOS Somo rj ai . GA (1994) Anlm. Rev. Phys. Chern., 45 , 721. 106 Somorjai, G A (1996) J. Mol. CataL A, 107, 39. 107 Somorjai, G .A. and Rupprechte r, G. (1998) J. Chern. Educ., 75, 161. 195