Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
J. Phys. Chem. 1993,97, 683689 683 Structural and Chemisorptive Properties of Model Catalysts: Copper Supported on Si02 Thin Films Xueping Xu and D. W . Goodman' Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received: September 8, 1992; In Final Form: November 9, 1992 The structural and chemical properties of model silica-supported copper catalysts have been investigated with infrared reflection-absorption spectroscopy and temperature-programmed desorption using C O as a probe molecule. The isosteric heats of adsorption of carbon monoxide on the model catalysts were measured at pressures between 10-8 and 10-3 Torr and temperatures between 180 and 300 K. The model catalysts were prepared by evaporating copper onto a planar silica thin film (- 100 A) which, in turn, was supported on a Mo(110) surface. When deposited at 100 K, copper initially forms a two-dimensional structure on silica; however, annealing induces the ultrathin copper films (<3 ML)to form small clusters. The unannealed copper films have a significant density of low-coordinated copper sites, whereas the annealed films consist of copper clusters with structures similar to low-index [( 1 1 l), (1 IO)] and high-index copper planes [(211) or (3 1l)], The distribution of the facets depends upon the initial copper coverage. The dispersion of the copper on the planar silica films is high, -0.5, for the annealed films with Bcu 2 ML. Carbon monoxide desorbs in a single peak centered a t -210 K from the unannealed copper film, but in several peaks centered between 150 and 220 K from the annealed film. The CO adsorption energy strongly depends on the C O coverage, varying from 17 kcal/mol at the low coverage limit to 10 kcal/mol near C O saturation. The copper structure, the dispersion, the particle size distribution, and the heats of C O adsorption for the model systems are remarkably similar to the corresponding values for silica-supported copper catalysts prepared by the ion-exchange method. These similarities demonstrate that the method of preparation described here using a dispersed metal on an oxide film produces an excellent model for the analogous supported metal catalyst. - - Introduction Model catalysts used in conjunction with surface science techniques have provided valuable new insights into many fundamental issues of catalysis.' Typically,single-crystal metal surfaces have served as model catalysts in that transition and noble metals are the components of many important catalyst systems. However, practical metal catalysts are usually supported on high surface area oxides. It is well recognized that there can be metal-support interactionsthat alter catalytic properties? even for relatively inert supports such as silica.3 To investigate these metal-support interactions, oxides have been deposited ontosinglecrystal metal surfacesand their modified reactivities studiedusing surfacescience techniq~es?,~ The metal catalysts used in industry, however, are dispersed on the support material; therefore, a preferred approach to model these systems involves metal deposition onto the support material. There have been relatively few investigations of metals on oxide supportsdue to experimental difficulties associated with surface charging, sample mounting, and sample heating/cooling.6-8 Recently, we have developed a new approach to modeling supported metal catalysts that circumvents these experimental difficulties? This approach utilizes the deposition of metals onto a silicon dioxide thin film (- 100A), which, in turn, is supported on a Mo( 110) surface. In addition to supporting the Si02 thin film, the Mo( 110) substrate provides a metallic reflecting surface which facilitates the use of infrared reflection-absorption spectroscopy ( I U S ) . The Mo(110) substrate, however, does not alter the chemical properties of the -100-A Si02 film. The Si02 film was prepared by evaporating silicon onto Mo(110) in -1 X 10-5 Torr of oxygen followed by an anneal to 1500 K.'O Auger electron, X-ray photoelectron, electron energy loss,infrared reflection-absorption, and thermal desorption spectroscopies show that the Si02 films have similar electronic and vibrational properties to those of fused silica, and that these films are stable to >1600 K . l i J 2 Therefore, the Si02 thin film provides a good model for a silica support. In previous studies, we have found that the initial sticking probability of copper on silica film is significantly less than unity, -0.6 at 90 K and less than 0.1 at a surface temperature of >400 K.13 During temperature-programmed desorption,copper desorbs from the silica film in two peaks: a metallic copper sublimation peak at -9W1200 K and a peak at 1200-1400 K derived from a strongly adsorbed copper species! The initial desorption energy for copper sublimation strongly depends on the effective copper coverage, ranging from -60 kcal/mol for 0.5monolayers (ML), Bcu = 0.5,to 75 kcal/mol for 8cU= 7 ML, and approaching the value of the copper sublimation energy of 79.3 kcal/mol14 for higher copper coverages. The strongly adsorbed copper species, which is partially oxidized, corresponds to an effective copper coverage less than 0.1 ML. Carbon monoxide does not adsorb on the partially oxidized copper species at 90 K under ultrahighvacuum ~ n d i t i o n s . ~ In this work, we have investigated the structural and chemisorptive properties of model silica-supported copper catalysts, whose industrial counterparts are used in methanol synthesis and water gas shift reactions.15 Infrared reflection-absorption spectra of adsorbed carbon monoxide has been used to address the structure of the supported copper on the silica film, as well as to measure the isosteric heats of carbon monoxide adsorption on these model copper catalysts. The results are compared to the corresponding measurements on copper single-crystal surfaces and silica-supported copper catalysts. Scanning tunneling microscopic (STM) studies of the copper/silica/Mo( l 10) model system have been described elsewhere.I6 Experimental Section The experiments were performed in an ultra-high-vacuum chamber equipped with an Auger electron and quadrupole mass spectrometersand a contiguous high-pressureinfrared cell. This 0022-3654/58/2091-0683S04.00/0 Q 1993 American Chemical Society Xu and Goodman 684 The Journal of Physical Chemistry, Vol. 97, No. 3, I993 TPD, W/CU/SD~/MO(~~O) @ TPD, e(cu)-1.1ML CO/Cu/SiO Annealed to 700 K - J CO/CUlSi(lWA e(cu) 15 ML \ ,/ 15ML 7ML AI n CL F 10- I/ I= I I I I 8 I A 0 , I I , l l l 5- 1.1 ML \ / l t l l ? 1 , 100 300 500 700 900 Annealing Temperature (K) Figure 2. Integrated TPD area for CO on silica-supported copper as a function of the preannealing temperature. The experimental conditions were the same as those in Figure 1. 1 .o 1 Cu/SiO, (100A)/Mo(I IO) 1o--- 3 R I F . d I ( 1 ML I4Gj-3.4 ML Annealed to 700 K .-0 ~ 0 S(Cu)-15ML o,8[OS- 2 4 0 4 0, Anne 0.01 0 I I I 2 4 6 I 8 I I I 10 12 14 Copper Coverage (ML) Figure 3, Dispersion of copper on silica as a function of the effective copper coverage. The inset shows the dispersion as a function of the annealing temperature for three copper films. Figure 1b shows the CO TPD spectra as a function of copper coverage, subsequent to an anneal to 700 K followed by a saturation CO exposure at 100 K. At low copper coverages, CO desorbs in two peaks with peak maxima located at 160 and 180 K. As the copper coverage is increased, a new CO TPD feature develops at -210 K. The copper films nucleate into 3D clusters upon annealing. The saturation coverage of CO, determined by TPD, decreases with an increase in the sample annealing temperature (Figure 2). The copper surface area (based on the CO TPD peak area) decreases approximately by a factor of 2 as the film is annealed from 100 to 900 K. In addition, the pronounced changes in the CO TPD spectra (Figure la) and in the IR reflection-absorption spectra (described below) indicatesthedegreeof structural change taking place in the supported copper upon heating. The dispersion of the copper (Figure 3) was determined using temperature-programmeddesorption. The total amount of copper can be accurately defined by desorbing the Cu from silica and referencing its corresponding TPD peak area to the Cu TPD peak area obtained from 1 ML of copper on Mo(llO).I3 The surface copper also can be measured with CO TPD, assuming the saturation coverage of CO on Cu to be -0.5 ML.'a-20 A planar, multilayer copper film was used to establish the reference for the copper CO TPD area. An atomic force microscopic examination of a 30 A thick copper film (annealed to 700 K) shows it to be continuous.16 The copper surface area for the 30-A film was - ReSultS A. Temperature-F'rogrammedDesorption of Carboo Monoxide. Figure 1 shows temperature-programmed desorption spectra for CO adsorbed on the model silica-supported copper catalysts. Copper was deposited onto the silica film at 100 K and annealed to the indicated temperatures. The sample was then cooled to 90 K and CO adsorbed to saturation. For all preparations, CO desorbs from the supported copper in the temperature range 140240 K. Figure l a shows the CO TPD from a copper film (1.1 ML) as a function of the annealing temperature. A single CO TPD peak is observed at -210 K for the unannealed copper sample. For the annealed (500-900 K) sample, CO TPD exhibits two peaks at 160 and 180 K. The CO TPD spectra for other copper coverages (0.5-60 ML) are qualitatively similar to those shown in Figure la. The unannealed copper samples exhibit a single CO TPD peak at -220 K, whereas the TPD for the annealed films shows several peaks between 160 and 210 K. The peak desorption temperature of CO from the as-deposited copper film is higher than that from the annealed samples. Silica-Supported Copper Catalysts 00-6 0.25% I L I A 2099 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 685 CO/CU/S~O~ (1 e(cu)-2.2 ML Ts-90 K -PI A 1 c o / C u / S i ~(I, 00 A) Cu deposited at 100 K I 2106~-C1l 41 I fi I-- I 2200 , 2108 , 2150 2072 700K ~ - 2100 2050 ,900Ki 2000 1950 Frequency (cm -') Figure4. Infrared reflection-absorptionspectra of CO on silica-supported copper as a function of the preannealing temperature. Copper was deposited a t 100 K and annealed to the indicated temperatures, followed by CO adsorption to saturation at 90 K. k & '? j: 0.5 ML 0.3 ML 2091 - 2200 2150 2100 2050 2000 1950 Frequency (cm-') Figure 5. IRAS of CO on silica-supported copper as a function of copper coverage. Copper was deposited a t 100 K, followed by CO adsorption to saturation. CO/Cu/SiO2(100 ) -50% larger than for a 70-A film (annealed to 700 K), whereas the surface area for the 70-A film was the same as that for 150-A copper film (annealed to 700 K). Accordingly, the 150-A copper film was considered to be flat and to have an atomic density of -1.4 X lOI5 atoms/cm2.21 Any roughness in the thick copper film would lead to an underestimation of the copper dispersion. Sincethe highest dispersion calculated was 0.83 for an unannealed 1.1-MLcopper film, the error in the copper dispersion is less than 20%. Figure 3 shows the copper dispersion as a function of the effective copper coverage for copper films deposited at 100 K and annealed to 700 K. The dispersion is high (0.5) for copper coverages less than 1.5 ML and gradually decreaseswith increase in the copper coverage. The dispersion of the unannealed film isevenhigher, -0.8 fora 1.1-MLcopperfilmat 100K(theinset of Figure 3), suggesting that copper wets silica at 100 K. Annealing the film decreases the copper dispersion, consistent with the formation of larger 3D copper clusters. B. Infrared Reflection-Absorption Spectra (W).The structure of copper on silica was investigated with IRAS using CO as a probe molecule. Figure 4 clearly shows the structural changes in the supported copper (2.2 ML) as a function of the anneal temperature. On the as-deposited (100 K) copper film, CO exhibits an IR absorption band centered at 2099 cm-I with a small shoulder on the low-frequency side of the peak. Upon annealing to 300-500 K, the 2099-cm-l band shifts to 2097 cm-I and a new band appears at 2070 cm-I. Further heating to 700900 K results in a splitting of the 2097-cm-1 feature into two peaks at 2108 and 2094 cm-I. These CO absorption bands are attributed to several distinct atop CO adsorption sites, since bridging and hollow adsorptionsites on Cu( 111) have CO stretch frequencies in the range 1814-1834 cm-1.22.23 The structure of the supported copper also depends on the copper coverage. Figure 5 shows the IRAS for saturation CO on theunannealedcopper samplesas a function of copper coverage. At very low effective copper coverages (0.3 ML), two CO IR absorption bands are observed at 2091 and 2049 cm-I. As the coverage increases, the high-frequency band gradually blue-shifts to 2106 cm-1 concomitantly with an increase in its intensity, whereas the intensity of the 2049-cm-1 band remains essentially Frequency (cm - l ) Figure 6. IRAS of CO on silica-supported copper as a function of copper coverage. Copper wasdepositedat 100Kand annealed to 900 K, followed by C O adsorption to saturation at 90 K. unchanged. At higher copper coverages (>2.2 ML), the lowfrequency band appears as an asymmetric tail. The full width at half-maximum (fwhm) of the CO feature is -20 cm-I, much larger than the intrinsic - 5 cm-l width of CO on single crystal copper surfaces.20 Figure 6 further demonstrates the effects of copper coverage and the anneal temperature on the structure of the supported copper. The CO IRAS spectra (Figure 6) for the annealed (900 K) copper films are dramatically different from the unannealed films (Figure 5 ) . On low-coverage copper films (<3.4 ML), CO exhibits three resolved IR absorption bands at -2108,2094, and 2076 cm-I. The lowest frequency CO band gradually shifts from 2062 cm-1 on a 0.3-ML copper sample to 2076 cm-1on a 3.4-ML copper film. More apparent, the intensity of the 2094-cm-l band Xu and Goodman 686 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 TABLE I: Literature Values of CO Stretching Frequencies on Copper SingleCrystal Surfaces and Films surface low 8co Cu(ll1) Cu(ll0) Cu(100) Cu(211) Cu(311) Cu(755) Cufilms 2080 2088 2079 2109/2095 2109/2093 2111 2102 a frequency (cm-I) medium 8coa saturn Bco 2074 2093 2085 2100 2102 2098 2102 2070 2094 2088 2110 2104 2106 2102 1I 0.03% ref A CO/Cu(O.5 ML)/Si02(100A) Annealed to 500 K Ts-90 K CO Exposure A 100 L - 18,22-24 19,24-26 20,24,27 24 24 24 24,28 CO coverage at maximum surface potential. grows with the copper coverage, whereas the 21 10-cm-l band remains approximately the same and gradually appears as a shoulder on the 2094-cm-1 feature. On a medium-coverage (1 5 ML) copper film, the 2094-cm-1 CO absorption band has disappeared, replaced by an intense IR absorption at 2102 cm-I and two small unresolved shoulders at 2087 and 2068 cm-I. The fwhm for the 2102 cm-1 band is -14 cm-1, narrower than that for the unannealed film. On the high-coverage (>30 ML) copper samples, CO exhibits three resolved IR absoprtion features at 2102,2087, and 2076 cm-I. The stretch frequenciesfor CO adsorbed on the annealed copper films are remarkably similar to the atop CO features found on single-crystal copper surfaces at similar CO saturation conditions (Table I). CO adsorption on copper single-crystal surfaces has been extensivelydocumented;ls-20~22-28 thus, using the thud column of Table I for saturation CO adsorption, the resolved IR bands in Figure 6 can be assigned to CO adsorption on specific singlecrystal facets. Accordingly, the 2074- and 2087-cm-l bands are assigned to CO adsorption on the (111) and (100) facets, respectively,of the thick copper films (>15 ML). The 2102-cm-l band is attributed to step/edge sites of the polycrystalline copper film. For very thin films (<15 ML), the copper clusters are relatively small (<200 A) and thus may not be crystalline. Nevertheless, the well-defined CO IR absorption bands suggest that the thin films have surface structural similar to the (1 10)plane (2094 cm-I), the (1 11)-plane (2074 cm-I), and the highindex (211) and (311) planes (2108-2110 cm-I). The IR absorption band at 2062 cm-I, observed at very low copper coverages (<0.5 ML), is lower than that for CO on any extended surfacesand is attributed to a particle size effect (see Discussion). It has been shown that the CO coverage influences the CO stretch frequency via a dipole coupling and a chemical shift mechanism.19,29q30 It is noteworthy that the frequency shift trends depend upon the specific crystal orientation. Figure 7 shows the IRAS of CO on a 0.5-ML copper film as a function of the CO exposure. Thedataareconsistent with (1 11)-likeandother highindex facets composing the majority facets at low copper coverages. At low exposures (<1 L), two CO IR absorption bands are observedat 2108and2083cm-I. AstheCOexposureisincreased (1-14 L), the intensity of the 2108-cm-l feature increases and gradually red-shifts to 2099 cm-1. The intensity of the 2083-cm-1 band also increases as the peak broadens and red-shifts to 2060 cm-I. A further increase in the CO exposure to 100 langmuirs causes an abrupt blue-shift of the high-frequency band from 2099 to 2107 cm-I. The frequency shift with coverage of the band at 2099-2108 cm-l on the supported copper is essentially identical to that shift observed on the high-index Cu(211) and Cu(755) ~urfaces.2~ The IR absorption band for CO on Cu(755) shifts continuously from 21 11 cm-I at the lowest Cu coverage to 2098 cm-I at the surface potential maximum and then shifts to a higher frequency (2106 cm-I) with the growth of the compressed CO str~cture.2~ The IRAS for CO on Cu(211) and Cu(3 11) exhibits features very similar to those observed for CO on Cu(755), with the exception of additional bands at -2095 cm-1.24 Figure 8 further illustrates thedependence of the IRAS on the CO coverage. Carbon monoxide was adsorbed at 90 K on a 2150 2100 2050 2000 Frequency (cm-1) Figure 7. IRAS of C O on 0.5-ML copper supported on silica as a function of C O exposure. 190 K I 2150 2100 I I I 2050 2Ooo Frequency (cm -1) Figure 8. IRAS of C O on 2.2-ML copper supported on silica as a function of temperature. Copper was deposited a t 100 K and annealed to 900 K, followed by C O adsorption to saturation at 90 K. The same was then warmed to the indicated temperature. 2.2-ML copper film to saturation and the surface temperature then gradually raised. At 90 K,three resolved CO IR absorption bands are observed at 2108,2094, and 2074 cm-I. The two highfrequency bands gradually merge into a single peak at 2097 cm-' during an anneal to 130 K, and the 2074-cm-I band shifts slightly to 2070 cm-I. The total IR absorbancedecreasesslightly between 90 and 130 K. On increasingthe temperature to 140-1 50 K,the IR absorbance decreases due to CO desorption. The 2099-cm-1 band red-shifts somewhat to 2097 cm-I, whereas the 2070-cm-1 band blue-shifts slightly to 2077 cm-I. Upon further annealing the sample to 190 K,the intensity of the high-frequency band decreases significantly and the peak position gradually shifts to 2 104 cm-I . Silica-Supported Copper Catalysts The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 607 103 - 5 10'- h t e? 3 v) v) e? I 10.' 10.8 1o-6 1o .~ 1o.4 1o .~ CO PRESSURE (Torr) Figure 9. A series of isotherms for CO adsorption onto 2.0-ML copper supported on silica. IRAS intensity was used to monitor the COcoverage. 1 8 $1 10'6I 4.0 I co/cu/siq(io0 A)/Mo(~io) 3.5 / I I 4.5 I 5.5 5.0 I 6.0 1000/T ( K - ' ) B(Cu)-O.B ML %I- \ \ \ / I Figure 11. Adsorption isosteres for C O on 2.0-ML copper supported on silica at three CO coverages. 18 L O(Cu)=l5 ML Integrated CO TPD Area (Arbitrary Unit) Figure 10. Relationship between the IRAS intensity for CO on copper/ silica (100 A)/Mo(I 10) and the amount of CO adsorbed for two copper films. The amount of CO was determined by temperature-programmed desorption and was not normalized to the copper surface area. c a 10 I The shift of the 2070 cm-1 band with temperature (coverage) in Figure 8 is consistent with this band corresponding to CO adsorption onto Cu( 111)-like facets. The IR absorption band for CO on Cu( 111) appears at 2078 cm-l at very low coverages, red-shifts slightly to 2074 cm-I at CO coverage of 0.33 ML,and decreases to 2070 cm-1 at a saturation CO coverage of 0.52 ML.18.22The bands at 2094 and 2108 cm-I are derived from two majority facets, as is demonstrated in Figure 6. As the temperature is increased (such that the CO coverage decreases slightly), the IR bands for CO adsorption onto the high-index facets abruptly red-shifts 10 ~ m - ' , 2causing ~ these two bands to merge into one. C. Isosteric Heats of Adsorption of CO. The isosteric heats of CO adsorption on the model silica-supportedcopper catalysts were measured using infrared reflection-absorption as a monitor of CO coverage. The isosteric heats of adsorption of CO on Cu(100) with CO pressure up to 10 Torr have been measured previously using IRAS.3' Figure 9 shows the adsorption isotherms of CO on an annealed copper film (2 ML)supported on silica. The data were collected in the pressure range of 10-8-10-3 Torr and in the temperature range of 175-300 K. The isotherms are expressed as a plot of integratd IR absorbance versus the logarithm of the CO pressure. The IR reflection-absorption intensity depends on both CO coverage and the copper cluster size. Figure 10 shows the relationship between the intensity of the CO IR band and the amount of CO adsorbed (measured by the CO TPD area). The IR intensity is not linear with CO TPD area for the copper coverages larger than 1 ML.The CO coveragesfor the adsorption isotherms are determined from IR intensities and calibrated by - 0.0 0.1 I 0.2 0.3 I 0.4 I I I I I 0.5 0.6 0.7 0.8 0.9 .o CO Coverage e()/, Figure 12. Isosteric heats of adsorption for C O on silica-supported copper as a function of the C O coverage for two different copper coverages. the IR/TPD curves similar to those in Figure 10 for the specific copper coverages. The isosteric heats of adsorption ( M a d s ) were obtained from the slope of the isosteric plots of log (Pco) versus 1/ T a t constant CO coverage (Figure 1l), according to the Clausius-Clapeyron equation: Mads = 2+303Rd(log PC,)/d( / nI.9 Figure 12 shows the isosteric heats of adsorption of CO on the model silica-supported copper catalysts as a function of CO coverage. Data for two copper coverages are shown, one for a relative thick polycrystalline copper film (1 5 ML)and the other for a 2-ML copper film annealed to 700 K. The isosteric heats of adsorption decreases with an increase in the CO coverage, in agreement with previous work on single-crystal Within experimentalerror, the heats of adsorption do not change with copper coverage at the same CO coverage. Discussion The data presented in the previous section clearly establish that the structure of copper supported on silica depends on both the copper coverage and the annealing temperature. The dispersion of the unannealed copper is very high, -0.8 for monolayer films (Figure 3),indicating that the copper initially Xu and Goodman 688 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 forms two-dimensional structures during deposition on silica at 100 K. Annealing inducesthe formation of 3D Cu clusters, whose surface area and dispersion decrease with the annealing temperature (Figure 2). Furthermore, the annealed copper films have several distinctive CO IR absorption bands corresponding to CO adsorption onto various copper crystalline facets, whereas the unannealed films exhibit a single band. - The single IR absorption band at -2100 cm-I for the unannealed copper films indicates that the morphologies of the unannealed films are quite similar. Temperature-programmed desorption show only one CO desorption peak on the unannealed copper films, whereas several peaks are seen for the annealed films. However, dipole coupling between CO molecules tends to transfer the IR absorption intensity of the low-frequency features to the high-frequency ones when these dipoles are adjacent to each ~ t h e r . ~ The ~ . ~magnitude ' of the intensity transfer inversely depends on the separation in distance and frequency between the dipoles.33 The low-frequency tails in Figure 5 demonstrate that the CO adsorption sites are not all identical on the unannealed copper films. It is noteworthy that, at low effective copper coverages (<1 ML), the unannealed copper films exhibit two resolved CO IR band at 2050 and 209 1 cm-' (Figure 5). This indicates that there are at least two types of small copper islands on the silica surface at copper coverages less than 1 ML. Otherwise, dipole coupling and intensity transfer could lead to only one CO band.33 As the copper coverage is increased, the small islands are merged into an extended smooth surface, and intensity transfer causes the 2049-cm-I band to appear as a tail (Figure 5). On the basis of CO stretch frequencies, we propose that copper atoms are initially aggregated into (1 1 1)-like islands (2050 cm-I) and islands with a surface similar to the high-index planes during deposition of Cu onto silica at 100 K. Annealing thecopper films causes the formation of 3D clusters. The well-resolved CO IR absorption bands suggest that there are severaltypes of isolated copper clusters for the annealed ultrathin (0.3-3.4 ML) copper films (Figure 6). If the annealed film is continuous, IR intensity transfer will diminish the resolution, producing a singleband. Indeed, scanning tunneling microscopic studies revealed several unique cluster structures for a 1 .O-ML film.I6 For a medium coverage film (-15 ML), atomic force microscopy showed that the surface consisted of a continuous copper grain of -200A.16 IRAS for CO on this surface exhibits a large peak at 2102 cm-1 with two unresolved shoulder at low frequencies (Figure 6). The CO vibrational frequencies at 2074, 2094, and 2 108 cm-I suggest that the copper clusters have surface structures resembling (1 1 l), (1 lo), and other high-index planes [(211) and (311)] of copper single crystals (Table I). Indeed, a copper cluster with a (3 1 1) surface structure was observed with STM.16 The distribution of copper clusters for the annealed film varies with coverage (Figure 6). At very low copper coverages (<0.5 ML), the annealed films mainly consist of (1 1 1)-like and (21 1)/ (31 1)-like facets. As the copper coverage is increased, a (1 10)like facet grows and becomes the major facet at an effective coverage of 3.4 ML. On increasing the Cu coverage to 15 ML, the copper film become continuous,I6 and the IRAS spectrum exhibits a CO feature corresponding to adsorption on step/edge sites (2102 cm-I). The adsorption bands corresponding to (1 11) and (100) facets (2074 and 2087 cm-I) only appear as shoulders due to intensity transfer to the 2102 cm-1 band. With a further increase in the copper coverage to 65 ML, the intensity of the 2076- and 2087-cm-l bands shows the growth of the (1 11) and (100) facets. At this coverage, the (1 11) or (100) facet is large enough such that the IR intensity transfer to step/edge sites no longer dominates. The nature of the preferential crystallinefacet growth of copper on silica is not well understood. Minimization of the surface free - energy of the metal particles and the substrate atomic structure are the important contributing factors. For the thick copper film (65 ML), the majority facets are (11 1 ) and (100) due to their lower surface free energy. For the low copper coverages (small particle sizes), the structure of the silica substrate may play a role. Although the silica film is macroscopically disordered, the silica surface structure on a small scale may be ordered, and may resemble certain copper facets, servingas a template for nucleation of copper atoms. CO on evaporated copper films has been studied previously by IRAS.24J8 However, only a single band at -2102 cm-1 was observed in the earlier studies, in contrast to this work where several well-resolved IR bands were detected. This discrepancy is attributed to the method of preparation of the copper films. In the previous work, copper films were evaporated onto the substrate at 1300 K. Figure 4 clearly shows that annealing to 700 K is required in order to produce several resolved IR bands. In addition, the copper coverage plays an important role in determining the IR spectra of adsorbed CO. It is noteworthy that a CO IR absorption band at 2050-2060 cm-I is observed for ultrathin copper (0.3 ML) on silica. This band has a much lower frequency than that for CO on singlecrystal copper surfaces. It is proposed that this unusual lowfrequency band at 2050-2060 cm-] is due to the fact that dipole coupling on very small particles is less than that on extended surfaces, whereas the chemical interaction remains essentially the same. It is well recognized that the shift of the CO vibrational frequency with respect to the CO coverage has two components, a dipole shift and a chemical On copper single crystals and on copper fiims, the dipole coupling and chemical shift virtually cancel each other, resulting in a small CO frequency shift (<lo cm-I) with coverage. For example, the dipole shift is 26 and 53 cm-1, and the chemical shift is -27 and - 6 3 cm-l for CO on Cu( 1 11) at coverages of 0.33 and 0.52 ML, respectively.18 Calculations have shown that the dipole shift decreases with the size of the CO islands on the extended Cu( 1 1 1 ) surface, from 25 cm-l for a d 3 X d 3 infinite CO overlayer, to 18 cm-' for a -27 A diameter CO ( d 3 X d 3 ) island on Cu( 1 1 l), to 9 cm-l for a -9 A diameter CO island.34 Therefore, if the copper clusters on silica are sufficientlysmall, thedipolecoupling and the resulting shift should be smaller than that on the extended surface. That is, the CO island sizes are limited by the copper clusters. As a result, the CO stretch frequency decreases with smaller cluster sizes. Scanning tunneling microscopic studies have revealed that a -0.6-ML film has a particle size of 10-30 & I 6 in the range where the dipole shift is minimal. The effect of particle size on the chemical shift should be small compared to the dipole shift, since the chemical interactionis relativelyshort range. In contrast, dipole coupling interactions are characteristically long range. The relationship between the IR absorption intensity and the CO coverage further supports the premise that dipole coupling is smaller for small copper particles (Figure 10). For small particles (low copper coverages), the IR absorbance is approximately linear with the CO coverage. However, for extended surfaces, the slope of the IR intensity versus CO TPD is much smaller at high CO coverages than at low CO coverages (Figure 10). The IR peak intensity is related to the dipole coupling by35 In (z/z,,)a a,e/(i + a,q2 where a"and a,are the vibrational and electronic polarizabilities, respectively, and T is the sum of the dipole coupling. Clearly, when the dipole coupling is small, the IR absorbance is proportional to the CO coverage. The isosteric heats of adsorption of CO on supported copper (Ocu > 2 ML, annealed to 700 K) strongly depend on the CO coverage. The heats of adsorption are 16-17 kcal/mol for Bco < 0.1 and decrease to 14-15 kcal/mol for 0.1 < 8 ~ <0 0.3. At high CO coverages, the isosteric heats of adsorption further Silica-Supported Copper Catalysts The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 689 - decrease to 10 kcal/mol. These values are in good agreement with values for single-crystal copper surfaces. For example, the isosteric heats of adsorption, measured with work function, are 14 kcal/mol on Cu( 1 and 12 kcal/mol on Cu(1 11)l8 at low CO coverages and decrease to -9 kcal/mol at saturation coverages. On Cu( loo), the adsorption energy is 17 kcal/mol at the lowest CO coverage, decreases to 13-14 kcal/mol at dco = 0.1-0.4, and falls to -10 kcal/mol at saturation CO co~erage.~IJ* The high adsorption energy on Cu(100) at the low coverage limit has been attributed to CO adsorption onto defect sites.31 Therefore, the high heats of adsorption at low CO coverages are attributed to CO adsorption on step/edge sites of the supported copper, whereas the low values at a saturation CO coverage are attributed to repulsive interactions between CO molecules. The model system, Cu/Si02(100 A)/Mo( 1lo), is an excellent low surface area model for high surface area silica-supported copper catalysts. The model Cu catalysts are shown to contain both low-index planes [( 11l), (1 lo), and (loo)] and high-index planes [(211) or (31 l)]. The sizes of the copper clusters in the most highly dispersed model systems are on the order of 10-30 A (ec, < l),l6 In highly dispersed silica-supported copper catalysts, prepared by ion-exchangetechnique, the typical particle sizes are from 10 to 30 A . 3 6 1 3 ~Both low- and high-index copper planes are believed to be present on the high surface area copper ~atalysts.3~ Furthermore,the heats of CO adsorption on the model systems are approximately the same as those found on the more realistic silica-supportedcopper ~ a t a l y s t s . 3In ~ ~both ~ ~ systems, the CO adsorption energy decreases with an increase in the CO coverage, from 14 to 16 kcal/mol at low CO coverages to less than 10 kcal/mol at high CO coverages. - - - Conclusion Copper initially forms two-dimensional structures when deposited on silica at 100 K; however, annealing induces twodimensional copper to form small, three-dimensional clusters. The unannealed copper films have a significant density of lowcoordinated copper sites, whereas the annealed films consist of copper clusters with structures similar to low-indexcopper planes [ ( l l l ) , (llO)] and high-index planes [(211) or 311)]. At high copper coverages (- 15 ML), copper clusters of -200 A coalesce to form an extended film. The distribution of cluster facets is strongly dependent upon the copper coverage. The dispersion of annealed (700 K) copper on the flat silica film is high, -0.5, for ,,e C 2. Carbon monoxide desorbs in a single peak centered at -210 K from the unannealed copper film but in several peaks centered between 150 and 220 K from the annealed film. The CO adsorption energy strongly depends upon the CO coverage, varying from 17 kcal/mol at the low coverage limit to 10 kcal/mol at saturation CO coverage. The copper structure, the high dispersion, the particle size, and the heats of CO adsorption for the model system are remarkably similar to those for silicasupported copper catalysts prepared by the ion-exchange method, - - demonstrating that this method of dispersing a metal on a thin oxide film yields an excellent model for a supported metal catalyst. Acknowledgment. We acknowledge with pleasure the support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. References and Notes (1) See, for example: (a) Friend, C. M.; Xu,X . Annu. Rev. Phys. Chem. 1991, 42, 251. (b) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Reports 1991, 14, 1. (c) Surface Science of Catalysis; Dwyer, D. J., Hoffmann, F. M., Eds.; American Chemical Society: Washington, DC, 1992. (2) See, for example: (a) Strong Metal-Support Interactions; Baker, R. T. K., et al., Eds.; American Chemical Society: Washington, DC, 1986. (b) Metal-Support Interactions in Catalysis, Sintering and Redispersion; Stevenson, S.A., et al., Eds.; Van Nostrand Reinhold: New York, 1987. (3) (a) Lamber, R.; Jaeger, N.; Schulz-Ekloff, G. Surf. Sci. 1990,227, 268. (b) Katrib, A.; Petit, C.; Legart, P.; Hilaire, L.; Maire, G. Surf. Sci. 1987,189/190,886. (c) Lamber, R.; Romanowski, W. J . Caral. 1987, 105, 213. (4) See, for example: Vurens, G. H.; Salmeron, M.; Somorjai, G. A. Prog. Surf. Sci. 1989, 32, 333. (5) Williams, K. J.; Boffa, A. B.; Lahtinen, J.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. Caral. Lett. 1990. 5, 385. (6) Wu, M. C.; Msller, P. J . Surf. Sci. 1989, 224, 250. (7) Guo, Q.;Msller, P. J. Surf.Sci. 1991, 244, 228. (8) Gautier, M.; Duraud, J. P.; Pham Van, L. Surf. Sci. 1991,249,L327. (9) Xu, X.;He, J. W.; Goodman, D. W. Sug. Sci., in press. (10) Xu, X.; Goodman, D. W. Appl. Phys. Lea. 1992, 61, 774. (11) Xu, X.;Goodman, D. W. Surf. Sci., in press. (12) He, J. W.; Xu,X.;Goodman, D. W. Surf. Sci., in press. (13) Xu, X.;Goodman, D. W. Appl. Phys. Lett. 1992, 61, 1799. (14) Hultren, R.;Desai, P. D.;Hawkins, D.T.;Gleiser, M.;Kelley, K. K. Selected Values of the Thermodynamic Properties offheElements; American Society for Metals: Metals Park, OH, 1973. (1 5 ) See, for example: Campbell, I. M. Catalysis at Surfaces; Chapman and Hall: New York, 1988. (16) Xu,X.; Vesecky, S.M.; Goodman, D. W. Science 1992. 258, 788. (17) Leung, L. W. H.; He, J. W.; Goodman, D. W. J . Chem. Phys. 1990, 93, 8378. (18) Hollins, P.; Pritchard, J. Surf. Sci. 1979, 89,486. (19) Woodruff, D. P.; Hayden, B. E.; Prince, K.; Bradshaw, A. M. Surf. Sci. 1982, 123, 397. (20) Ryberg, R. Surf. Sci. 1982, 114, 627. (21) The surface atomic density is estimated using the average values of the (loo), (110) and (111) surfaces. (22) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M. Surf. Sci. 1985, 155, 553. (23) Chesters, M. A.; Parker, S. F.; Raval, R. Surf. Sci. 1986, 165, 179. (24) Pritchard, J.; Catterick, T.; Gupta, R. A. Surf. Sci. 1975, 53, 1. (25) Hollins, P.; Davies, K. J.; Pritchard, J. Surf. Sci. 1984, 138, 75. (26) Horn, H.; Hussain, M.; Pritchard, J. Surf. Sci. 1977, 63, 244. (27) Horn, K.; Pritchard, J. Surf. Sci. 1976, 55, 701. (28) Dumas, P.; Tobin, R. G.; Richards, P. L. Surf. Sci. 1986, 171, 579. (29) Mahan, G. D.; Lucas, A. A. J. Chem. Phys. 1978,68, 1344. (30) Scheffler, M. Surf. Sci. 1979, 81, 562. (31) Truong, C. M.; Rodriguez, J. A.; Goodman, D. W. Surf. Sci. 1992, 271, L385. (32) Tracy, J. C. J . Chem. Phys. 1970, 56, 2798. (33) Browne, V. M.; Fox, S.G.; Hollins, P. Caral. Today 1991, 9, 1. (34) Hollins, P. Surf. Sci. 1981, 107, 75. (35) Hollins, P.; Pritchard, J. Chem. Phys. Leu. 1980, 75, 378. (36) Kohler, M.A.; Curry-Hyde, H. E.; Hughes, A. E.; Sexton, B. A.; Cant, N. W. J. Caral. 1987, 108, 323. (37) Kohler, M. A.; Cant, N. W.; Wainwright, M. S.;Trimm, D. L.J . Carol. 1989, 117, 188. (38) Monti, D. M.; Cant, N. W.; Trimm, D. L.; Wainwright, M.S.J . Caral. 1986, 100, 17.