Download Electronic and chemical interactions between boron and carbon

Electronic and chemical interactions
on Rb(0001)
between boron and carbon monoxide
Jo& A. Rodriguez, Charles M. Truong, W. Kevin Kuhn, and D. Wayne Goodman@
Department of Chemistry, Texas A&M University, College Station, Texas 77843-325.5
(Received 22 July 199 1; accepted 18 September 199 1)
The interaction between B and CO on Ru (Oool ) has been studied by means of thermal
desorption mass spectroscopy, Fourier transform infrared reflection absorption spectroscopy,
and x-ray photoelectron spectroscopy. Boron adatoms poison CO chemisorption
approximately on a one-to-one basis. No reaction or direct bonding between B and CO was
observed. The Be * *CO interaction is repulsive due to the electron-acceptor nature of both
adsorbates. Boron adatoms modify the electronic and chemical properties of first and second
nearest-neighbor metal atoms. In the presence of B, the CO desorption temperature decreases,
whereas the 0( 1s) binding energy and C-O stretching frequency increase. These trends are a
consequence of ( 1) a reduction in 2a* back donation caused by competition for metal
electrons between CO and B and (2) repulsive electrostatic interactions between the negative
charges on CO and B. The infrared results indicate that metal atoms strongly affected by B are
only occupied when no more unperturbed Ru sites are available on the surface, or when the
high temperature of the system favors CO migration (due to entropic effects) onto these
energetically less favorable sites.
I. INTRODUCTION
The addition of impurity atoms to a metal surface can
produce dramatic changes in its catalytic activity for reactions that involve the conversion of CO.’ There have been a
number of recent studies which indicate that the coadsorption of CO with “additive” atoms on metal surfaces can have
a profound influence on the structure of the adsorbed molecule and on the kinetics of surface reactions. l-5 For example,
coadsorption with alkali metals substantially increases the
heat of adsorption of CO molecules and reduces the C-O
stretch frequency.2-5 Opposite phenomena are observed
when CO is coadsorbed with highly electronegative species
such as oxygen and sulfur.1*3.5 A key question in coadsorption studies concerns the relative importance of local (or
steric) vs long-range (or electronic) effects. “Additive”
atoms that are pure site blockers exert only a short-range
local effect in which adsorption sites that are within - 1
bond length of the surface impurity are poisoned.6*7 In the
other extreme are “additive” atoms that induce electronic
perturbations over large distances and modify the chemical
properties of several surface atoms.le5
In the present study, we investigate the interaction between boron and CO on Ru (000 1) by means of thermal desorption mass spectroscopy (TDS), x-ray photoelectron
spectroscopy (XPS), and Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS). This work adds
to the few studies in which the coadsorption of boron and
CO on metal surfaces has been examined with the modern
techniques of surface science.’ The investigation of the surface interactions between B and CO is interesting because B
has a larger electronegativity than alkali metals, but a lower
electronegativity than S, Cl, and 0.
a) Author to whom correspondenceshould be addressed.
The adsorption of CO on Ru (000 1) has been previously
investigated by TDS,9 low-energy electron diffraction
(LEED),”
XPS,” ultraviolet photoelectron spectroscopy
(UPS), ‘* electron stimulated desorption ion angular distribution (ESDIAD), ‘* high-resolution electron energy loss
spectroscopy ( HREELS ) , l3 and FT-IRAS. l4 Carbon monoxide adsorbs on Ru (000 1) molecularly, bonded through its
C-end, with the molecular axis perpendicular to the surface
and with a saturation coverage of 0.68 molecules per Ru
surface atom. It desorbs without any dissociation in the temperature range between 300 and 500 K. TDS and XPS have
been employed to study the interaction between boron and
Ru(OOO~).~~ The boron adatoms were produced by dosing
B, H, to the metal surface (B2 H6,g +2B, + 3H,,*;
.T surf = 500 K) . The saturation coverage of B on Ru( 0001)
is - 1.1 ML. At this coverage, part of the B adlayer can be
removed from Ru ( 000 1) by heating to 1250 K, with a large
fraction of the B atoms still on the surface at 1450 K.15
II. EXPERIMENTAL
The experiments were carried out in two separate ultrahigh vacuum chambers. One of the instruments (base pressure < lo- lo Torr) has capabilities for Auger electron spectroscopy (AES), XPS, TDS, and LEED.“j The B(ls),
O( Is), and Ru(3d) spectra of Sec. III were recorded with
Al Ka radiation. The variations in the binding energies of
the B( 1s) and 0( 1s) XPS regions were determined by referencing against the Ru( 3d,,, > and Ru( 3p,,* ) peaks for clean
Ru ( 000 1) , which were set at binding energies of 280 and 483
eV, respectively. 11*17Detection was normal to the surface in
AES and XPS.
The second UHV chamber used in this study was
equipped with a Mattson Cygnus 100 Fourier-transform in-
@ 1991 American institute of Physics
0021-9606/92/010740-08!$06.00
J. Chem. Phys. 96 (l), 1 January 1992
740
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Rodriguez &al.: Boron and carbon monoxide on Ru(0001)
frared spectrometer with a liquid nitrogen-cooled mercury
cadmium telluride detector that covers the frequency range
between 4000 and 800 cm- ’. ‘* The FT-IRAS spectra were
acquired at a resolution of 4 cm- ‘.
The Ru( 0001) crystal was cleaned following procedures reported in the literature:”
oxidation in oxygen
( 1 X 10 - ’ Torr, 800 K) and anneal in vacuum to 1600-1700
K. The cleanliness and long-range order of the surface were
verified by means of AES, XPS, and LEED. The crystal was
mounted on a manipulator capable of resistive heating to
1500 K, electron beam heating to 2500 K, and cooling to 90
K. A W-5%Re/W-26%Re
thermocouple was spot-welded
to the sample edge for temperature measurements. A heating rate of 5 K/s was used in the thermal desorption experiments.
In this work, adsorbate coverages are reported with respect to the number of Ru( 0001) surface atoms
( 1.57 X lOI5 atoms/cm*). One adatom (or admolecule) per
substrate surface atom corresponds to 19= 1.0 ML. It has
been reported that a saturated monolayer of CO on
Ru(OOO1) yields an absolute coverage of 0co = 0.68 ML.14
This value was used to calibrate CO coverages determined
by measuring the area below the CO-TDS peaks or in the
O( 1s) XPS signal. The B coverages were estimated from the
B( ls)/Ru( 3d) XPS and/or B( 179)/Ru( 273) AES intensity ratios following the procedure described in Ref. 15.
Ill. RESULTS
A. Boron poisoning
of CO chemisorption
Figure 1 shows the effect of B precoverage upon the
saturation coverage of CO on Ru(OOO1). Boron adatoms
poison the metal surface toward CO chemisorption. In a
simple Langmuir-like adsorption mode1,19*20the saturation
coverage of the CO molecule varies with B precoverage as
0, = 0 “Co( 1 - me, )“, where 0 so is the saturation coverage of CO on clean Ru(OOO1 ), n is the number of sites required for adsorption of the molecule, and m is the number
of sites poisoned by a boron adatom. The reported saturation
741
coverage for CO on clean Ru (C)001), 0 “,, = 0.68 ML,14 implies that - 1.5 Ru surface atoms are required for each CO
adsorption site. The TDS and XPS data in Fig. 1 are fit
well
by
the following
mathematical
expression:
&, = 0.68( 1 - 1.458,) I.‘. This implies that on average
each boron adatom poisons - 1.5 metal surface atoms.
Therefore, we can conclude that B exerts a short-range local
effect on the chemisorption of CO, in the sense that only
surface metal atoms that are close to the impurity atoms are
poisoned. The TDS, XPS, and FT-IRAS results presented
below indicate that a boron adatom also modifies the electronic and chemical properties of first and second nearestneighbor Ru atoms.
B. CO/B/Ru(OOOl):
Thermal desorption
spectra
A series of thermal desorption spectra for CO adsorbed
on B/Ru(OOOl) is shown in Fig. 2. In a typical experiment
the boron adlayer was generated by dosing diborane to
Ru(OOO1) at 500 K,” then the surface was cooled to 90 K
and saturated with CO. With no boron present, CO desorbs
from Ru(OOO1) at temperatures between 300 and 500 K.
The line shape and desorption temperatures in the spectrum
are in good agreement with results presented in the literature
for the CO/Ru(OOOl ) system.9,21 The presence of boron
adatoms induces a reduction in the area below the CO-TDS
peaks and a significant decrease in the desorption temperature of the CO molecule.
Figure 3 displays the evolution of the CO thermal desorption spectra for different B precoverages and a fixed CO
coverage of - 0.15 ML. On clean Ru (000 1) , the CO-TDS
peak is centered at -485 K. The width of the CO desorption
peak increases with increasing B coverage. At 0, = 0.48
ML, peaks at - 300 and 395 K are observed. Assuming first-
CO/B/Ru(OOOl)
- 0~0 = SATURATION
CO/B/Ru(OOOl)
BORON
COVERAGE,
ML
FIG. 1. Effect of B coverage upon the saturation coverage of CO on
Ru(0001) at 90 K. The amount of CO present on the surface was determined by measuring the areasbelow the CO-TDS or O( 1s) XPS peaks.The
solid curve represents theoretical values calculated from the expression:
e, =0.68(1 - 1.458,)“.
1
150
I
250
,
I
350
I
I
450
TEMPERATURE,
I
I
550
I
I
650
K
FIG. 2. TDS for CO adsorbedon Ru(OC01) surfaceswith different B precoverages.The spectra correspond to saturation CO coveragesat 90 K.
J. Chem. Phys., Vol. 96, No. 1, I January 1992
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Rodriguez eta/.: Boron and carbon monoxide on Ru(0001)
742
CO/B/Ru(0001)
e,,.15~
$
z
3
:”
s
II
2
E
a
g
8
1
TEMPERATURE,
K
FIG. 3. TDS acquired after dosing 0.15 ML of CO to Ru(ODO1)surfaces
with different precoveragesof B at 90 K.
538
534
BINDING
532
530
528
ENERGY, eV
FIG. 4.0( 1s) XPS spectra for saturation coveragesof CO on B/Ru(OOOl )
surfaces.Temperature = 90 K.
order desorption kinetics with the pre-exponential factor of
lOI s- ’ reported for low coverages of CO on Ru(OOO~),~ a
Redhead analysis” yields desorption activation energies of
22.2 and 29.4 kcal/mol for the new CO adsorption states
induced by boron. These values are considerably smaller
than the activation energy of 36.3 kcal/mol observed for
0.15 ML of CO on Ru(OOO1). Thus, the Be * *CO interaction
reduces the adsorption energy of carbon monoxide on
Ru(OOO1).
C. CO/B/Ru(OOOl):
X-ray photoelectron
spectra
0( Is) XPS spectra for CO adsorbed on Ru( 0001) and
B/Ru(OOOl) (0, = 0.12,0.28, and 0.42 ML) are shown in
Fig. 4. The spectra were taken at 90 K after saturating the
surfaces with CO. The intensity of the O( 1s) signal decreases with increasing B precoverage. Simultaneously,
there is a shift in the peak position toward higher binding
energy. The O( 1s) feature for CO on clean Ru(OOO1) appears at - 1.5 eV lower binding energy than for the adsorption of CO on a surface covered with 0.42 ML of B.
No O( Is) signal was observed after annealing the
CO/B/Ru( 0001) surfaces to 500 K. This result indicates
that CO does not dissociate on clean Ru(OOO1) or on
B/Ru( 0001) surfaces under ultrahigh vacuum conditions.
Boron-oxygen adlayers are stable on Ru(OOO1) up to temperatures above 1000 K.15
Figure 5 illustrates the effect of CO adsorption on the
B( 1s) binding energy of B layers adsorbed on Ru(OOO1).
For 0, ~0.3 ML, the presence of CO induces a small shift
( -0.4 eV) toward higher binding energy in the B ( 1s) peak
position. At $ = 0.42 ML, CO produces both a broadening
of the B( Is) signal and features at higher binding energy
than those in the corresponding spectrum of B/Ru (000 1) .
The effects of CO adsorption on the B ( 1s) peak position of
the B/Ru (000 1) surfaces were reversible. The B ( 1s) binding energy found after desorption of the CO molecule in
most cases was identical to that seen before adsorption.
D. CO/B/Ru(OOOl):
FT-IRAS spectra
Our FT-IRAS results for CO on clean Ru (000 1) agree
well with published results for this system.14 Figure 6 shows
infrared spectra for different coverages of CO on clean
Ru (000 1) . The spectra were acquired at a temperature of
220 K. After each IR spectrum was recorded, the relative
CO coverage was determined from integration of the flash
B(ls) X P S
CO/B/Ru(0001)
Bco = SATURATION
192
190
BINDING
108
104
188
ENERGY,
eV
FIG. 5. Effect ofC0 adsorption on the B( 1s) XPS spectra of B/Ru(OCOl)
surfaces.Temperature = 90 K.
J. Chem. Phys., Vol. 96, No. 1,l January 1992
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Rodriguez Hal.: Boron and carbon monoxide on Ru(0001)
743
FT-IRAS
CO/B/Ru(OOOi)
0co = SATURATION
IO
, '"yq
2050
1600
2doo
21bo
2100
0.11
WAVENUMBER,
2000
WAVENUMBER,
woo
cm-’
1950
cm*’
CO COVERAGE,
ML
FIG. 7. FT-IRAS spectra for CO adsorbed on Ru(OCO1)surfaces with different precoveragesof B. Temperature = 220 K.
FIG. 6. Left: infrared spectra for different coverages of CO on clean
Ru(0001) at a temperature of 220 K. Right: IR-frequency and integrated
IR-intensity for CO/Ru(OOOl) at 220 K as a function of CO coverage.
desorption curve. In the spectra, the CO stretch frequency
appears between 2070 and 2000 cm - ‘, the range usually expected for terminal (or linear) CO on metal surfaces. In
agreement with previous HREELS and FT-IRAS studies
for CO/Ru(OO01),‘3*‘4 we did not see any evidence for existence of bridge-bonded CO in this adsorption system.
In the FT-IRAS spectra of CO on clean Ru(OOO1 ),
there was a slight temperature dependence in the peak position. At all temperatures, there was a monotonic shift of the
peak position to higher wave numbers with increasing CO
coverage. This shift is mainly a consequence of dipole-dipole
coupling and a reduction in electron back-donation from Ru
into the CO(2?r*) orbitals. l4 In general, there was not a
linear relationship between the integrated IR intensity and
the CO coverage. In Fig. 6(b), a maximum in the IR-intensity is observed at -0.35 ML of CO. The subsequent decrease
in IR intensity with increasing &, has been attributed to
strong lateral interactions between the molecules in the adlayer (i.e., dipole-dipole coupling, dipole screening, and
changes in the dynamic polarizability ) .14
FT-IRAS spectra for CO adsorbed on Ru( 0001) surfaces with different precoverages of B ( Br, = 0.07,0.18,0.33,
and 0.40 ML) are displayed in Fig. 7. The spectra were recorded at 220 K, after saturating the surfaces with CO. On
clean Ru(OOO1 ), a single absorption band is observed at
2062 cm - ‘. As the B precoverage is increased above 0.15
ML, a peak at 2072 cm - ’ appears. The intensity of this new
absorption band increases with the B coverage and is the
dominant feature in the spectrum at 8, = 0.4 ML. For
0.15 < 8, < 0.35 ML, the infrared spectra are characterized
by the presence of at least two peaks. The peak at 2072 cm - ’
is probably associated with CO bonded to Ru sites that are
strongly affected by the presence of B adatoms, whereas the
peak with a frequency below 2060 cm - ’ likely corresponds
to CO on Ru atoms weakly perturbedor unaffectedby B. At
0, = 0.4 ML, a large fraction of the Ru surface atoms is
blocked by boron and the remaining free sites show large
perturbations in its chemical and electronic properties. The
IR results of Fig. 7 are consistent with a model in which
boron adatoms exert only short-range electronic perturbations, modifying the chemisorption properties of their first
and second nearest-neighbor metal atoms.
Figure 8 shows the integrated IR intensities for saturation coverages of CO on B/Ru (0001) surfaces. The intensities have been normalized to the value found at eB = 0. 0co
was determined by measuring the area below the CO-TDS
peaks. The IR intensity is enhanced as B is added, until a
maximum is reached at 0.18<8, ~0.25 ML. This increase
can be attributed to a reduction in the saturation coverage of
8co
,.3,
0.51
0.0
0;"
SATURATION,
ML
0;5 0.4Q-,92
I
I
0.2
BORON
0;
I
I
0.4
COVERAGE,
I
I
0.6
ML
FIG. 8. Integrated IR-intensities ( 1950-2150 cm - ’) for saturation coveragesof CO on B/Ru (000 1) surfaces at 220 K. The CO coveragewas determined from thermal desorption experiments. The intensities were normalized to the value found at 0s = 0 and 0cc = 0.68 ML.
J. Chem. Phys., Vol. 96, No. 1,l January 1992
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744
Rodriguez et&:
r-
FT-IRAS:
CO/B/Ru(0001)
I
2072
2042
2020
0.10
A
I
2100
Boron and carbon monoxide on Ru(0001)
I
2600
WAVENUMBER,
1900
cm-’
FIG. 9. FT-IRAS spectra acquired after dosing different CO coveragesto a
Ru(OOO1)surface precovered with 0.26 ML of B. Temperature = 220 K.
The CO coveragewas determined from thermal desorption experiments.
CO (see Fig. 6). For 8, > 0.25 ML, there is a linear decrease
in the IR intensity with increasing B coverage. Under these
conditions the CO coverage is low and the effects of
CO. * *CO interactions upon the IR cross section are negligible.
Figure 9 shows FT-IRAS spectra acquired after dosing
different amounts of CO to a surface with 0.26 ML of B at
220 K. For all CO coverages the IR absorption band shows
two peaks. Both features shift to higher frequency when the
CO coverage increases, probably as a consequence of an enhancement in dipole-dipole coupling and a reduction in rback-donation. In all the cases examined, the measured peak
positions are at higher frequency (2040 cm-‘) than the
corresponding values for CO on clean Ru( 0001) . For
&o < 0.2 ML, the peak at lower frequency presents the larger intensity, whereas at saturation (0co = 0.31 ML) the
high frequency peak dominates in the IR spectrum. This
type of behavior indicates that the CO adsorption state
weakly perturbed by boron adatoms is populated first. This
is not surprising if one takes into consideration that such a
state probably has the largest heat of adsorption on the
B/Ru (0001) surface (see trends in Figs. 1 and 2).
Figure lO( a) illustrates the effect of temperature upon
the FT-IRAS spectrum of a Ru (0001) surface with 0.12 ML
of B and 0.51 ML of CO (saturation). The B/Ru(OOOl)
surface was saturated with CO at 90 K, and the spectra were
acquired in a sequential way after annealing to the indicated
temperatures for 2 min. For 0, = 0.12 ML and 19,~ = 0.51
ML only minor changes are observed in the IR spectrum
when the surface temperature is increased from 90 to 170 K.
The peak position is centered around 2055 cm - i. At 220 K a
shoulder toward higher frequency starts to appear and the
main peak shifts to 2050 cm-‘. The spectrum at 270 K
shows a new peak at 2070 cm-‘, with the main peak centered now at 2046 cm - i. Thus, the IR data indicate the existence of an inhomogeneous surface on which the population
of the different adsorption states changes with temperature.
When the temperature is increased, the CO molecules mi-
I
2100
2000
WAVENUMBER
2100
2000
(cm-‘)
FIG. 10. Effectoftemperatureupon theCO-ET-IRASspectraofRu(OOO1)
surfaceswith 0.12 and 0.35 ML of B. The surfaces were saturated with CO
at 90 K, and the spectra were acquired after annealing to the indicated temperatures during 2 min.
grate (due to entropic effects) from Ru sites weakly affected
by B (peak at 2050-2055 cm- i ) onto metal atoms largely
perturbed by the “additive” (peak at - 2070 cm - ’) .
In Fig. 10(b), the spectrum for 6a =0.35 ML and
0co = 0.20 ML (saturation) shows again different line
shapes at 90 and 220 K. For the lowest temperature, the
absorption band displays two unresolved peaks, with the one
at higher frequency centered at - 2066 cm - ‘. As the temperature is raised, intensity shifts from the peak at lower
frequency into the peak at higher frequency. Above 220 K,
two well-defined peaks are observed with frequencies of
2072 and - 2050 cm - I. In general, the FT-IRAS results of
Figs. 9 and 10 show that at low CO coverages and/or low
surface temperatures, Ru atoms which are moderately perturbed by B are preferentially populated. Metal atoms
strongly affected by B are only occupied when no more unperturbed Ru sites are available on the surface or when the
high temperature of the system allows CO adsorption on
these energetically less favorable sites due to entropic effects.
IV. DISCUSSION
The results of Fig. 1 indicate that boron adatoms poison
the adsorption of CO on Ru (0001) more or less on a one-toone basis. This agrees well with data for the
CO/B/Mo( 100) system,’ in which half a monolayer of boron reduces the saturation coverage of CO by half (t9a = 0.5
ML, e,, = 0.5 ML). Thus it appears that the poisoning
effects of boron are short-range in nature, very different
from the long-range poisoning found when additives with
large electronegativities (O,S,CI) were adsorbed on several
transition metal surfaces.’ On Ru (000 1) , however, no longrange poisoning of CO chemisorption has been observed for
different “additives” investigated in the past.21V23-26Cu at-
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Rodriguez eta/: Boron and carbon monoxide on Ru(0001)
tenuates CO chemisorption over Ru (0001) via a simple site
blocking mechanism. 21 In thermal desorption experiments,
there is a monotonic decrease upon addition of Cu of the CO
structure associated with Ru and an increase of the structure
corresponding to Cu. a For CO/O/Ru(OOOl ),23 a lowering
in the saturation amount of absorbed CO by 50% is found at
0, = 0.25 ML. An oxygen-saturated Ru(OOO1) surface
coo = 0.5 ML) adsorbed 35% as much CO as a clean surface.23 Finally, sulfur precoverages of 0.18 and 0.33 ML reduced the saturation coverage of CO on Ru( 0001) by 29%
and 73%, respectively. 24 Carbon monoxide did not adsorb
onto a Ru(OOO1) surface with 0.5 ML of sulfur.
The TDS, XPS, and FT-IRAS results presented in the
previous section show that boron adatoms can affect the
chemical properties of the neighboring metal atoms. This
phenomenon can be a consequence of electronic perturbations induced by the “additive” on the Ru surface atoms
(through-metal interaction) or a product of direct electrostatic interactions between B and CO (through-space interaction). In order to understand the effects of B on the surface
chemistry of CO on Ru(OOO1 ), we must examine first the
nature of the Ru-CO and Ru-B adsorption bonds.
The bonding mechanism of CO on metal surfaces involves B donation of electron density from CO into the unoccupied metal orbitals, and r-back-donation from occupied
metal orbitals into the lowest unoccupied molecular orbital
(27r*) of the CO mo1ecule.27-30Theoretical results with the
constrained space orbital variation (CSOV) method show
that, in this type of synergistic bond, r-back-donation is energetically more important in determining the character of
the bond than is a donation. 28 Inverse photoemission results
have supported the predominant importance of 2zr*-backdonation in CO chemisorption on Ru.~’ The population of
the antibonding 2~ orbitals of CO upon adsorption on
Ru(OOO1) is consistent with results of FT-IRAS, which
show that the C-O stretching frequency of adsorbed CO is
lower than that of free CO [ 2143 cm- ’ (Ref. 32) 1. It is wellknown that CO acts as a net electron acceptor when adsorbed on transition metals.33*34Theoretical calculations for
CO adsorbed on typical transition metals indicate a charge
transfer of 0.2-0.6e - from the substrate to C0.33,34 Experimental results for CO/Ru( Of!01 ) show an increase of 0.6 eV
in the work function of Ru(OOO~),~~ suggesting a partial
negative charge on adsorbed CO.
Recently, we have examined the bonding interactions
between atomic boron and Ru(OOO1) by performing quantum-chemical calculations based on the molecular orbital
self-consistent field method referred as IND0/1.36 Previous
studies have shown that INDO is a very good quantumchemical method for addressing the bonding mechanism of
species adsorbed on metals.” The Ru(OOO1) surface was
modeled by two-layer clusters containing between 18 and 15
metal atoms, and boron was adsorbed on the threefold hollow sites.36 In general, the results of the calculations showed
an adsorption bond dominated by the interaction between
the B (2~) orbitals and the valence 46 orbitals of Ru. For
bulk iron borides (Fe, B and FeB), photoemission experiments and ab initio calculations show that the Fe-B bonds
are mainly a consequence of the mixing of B (2~) states with
745
iron 3d valence states. 38 For B/Ru ( 000 1) , the percentage of
the adsorption bond in which each type of Ru orbital participated was approximately as follows:36 5s, 15%; 5p, 25%;
and 4d, 60%. From the viewpoint of the adsorbate, the
B( 2p) orbitals were involved in - 70% of the chemisorption
bond, whereas the B( 2.~) orbitals participated in only
30%.36 Adsorption of B induced a strong hybridization between the B( ;Is) and B( 2p) levels.36 Boron adatoms acted as
moderate electron acceptors on Ru( 0001) with a charge of
- - 0.37e calculated for adsorbed B.36 The direction of
charge transfer predicted by INDO/l
for the B/Ru( 0001)
system is in good agreement with experimental measurements that show an increase in the work function of
Mo( 100) when boron is adsorbed.’
Our results of Sec. III B show that coadsorption with
boron significantly decreases the desorption temperature of
CO on Ru(OOO1). At 0, = 0.48 ML, the activation energy
for desorption of CO is reduced by - 10 kcal/mol. We suggest that this is a product of ( 1) a reduction in 2rr*-backbonding caused by a competition for metal electrons between CO and B, and (2) repulsive electrostatic interactions
between the negative charges on CO and B. Both types of
phenomena seem to be of short range nature, affecting only
CO molecules bonded to Ru atoms that are first and second
nearest-neighbor to B atoms.
The XPS data of Sec. III C show that the 0( 1s) feature
of CO appears at higher binding energy for adsorption on
boron-covered Ru(OOO1) than for adsorption on clean
Ru(OOO1). An identical trend was observed for the
CO/Mo( 100) and CO/B/Mo( 100) systems.’ We can tentatively attribute the shift toward higher binding energy seen
in the 0 ( 1s) XPS spectra of CO on B/Ru (000 1) to a simple
initial-state stabilization of the 0( 1s) electronic energy level
caused by a reduction in electron density on the adsorbed
molecule. In a similar way, the CO-induced shift in B( 1s)
peak position observed in Fig. 5 is probably a consequence of
a decrease in the negative charge on the boron adatoms.
When boron and CO are coadsorbed on Ru( 0001 >, they
compete for metal electrons, and this produces adsorbates
that have a lower electron density and higher core level binding energies than those found for CO/Ru(OOOl)
and
B/Ru(OOOl).
The infrared data of Figs. 7, 9, and 10 show that for a
given CO coverage the C-O stretch frequency is larger on
B/Ru ( 0001) than on Ru (0001) . The strong dependence of
the frequency with coverage (see Fig. 6) makes it difficult to
separate quantitatively the effects of B. The CO coverages
reported in Figs. 7-10 represent an average over the whole
surface, and the real local coverages are probably different.
It is not difficult to imagine that the CO molecules will pack
preferentially on Ru sites not strongly perturbed by B, resulting in local coverages that are larger than the “average”
coverage determined from TDS. However, it is clear that a
large part of the B-induced shift in C-O stretch frequency is
due to changes in the Ru-CO bond: the frequency of 2072
cm - ’seen for a surface with 0.4 ML of B and 0.17 ML of CO
(saturation) is larger than that of 2062 cm - ’found for saturation CO on clean Ru(OOO1) (&, = 0.68 ML). This increase is probably caused by a reduction in 2rr*-back-dona-
J. Chem. Phys., Vol. 96, No. 1,i January 1992
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746
Rodriguez eta/.: Boron and carbon monoxide on Ru(0001)
tion from Ru to CO and is consistent with the XPS and TDS
results.
Boron is less electronegative than 0, S, and Cl, but more
electronegative than the alkali metals.39 The type of interaction seen in the TDS, XPS, and FT-IRAS spectra for
CO/B/Ru( 0001) is similar to that found for coadsorption
of CO and electronegative “additives” [0, S, Cl (Refs. 23,
24, and 29)], and contrary to that seen when CO is coadsorbed with electropositive “additives” [ Li, Na, K (Refs.
25, 26, and 29) 1. In general, electronegative “additives”
produce a substantial reduction in the heat of adsorption of
CO and an increase in the C-O stretch frequency.23,24,29’4042 The standard picture usedz9 to explain these effects is an
extension of the basic Blyholder model for CO chemisorption on metals. *’ Electronegative “additives” withdraw
electrons from the metal, becoming partially negatively
charged. As a result, r-back-donation to the 2n* orbitals of
CO is reduced. This weakens the metal-CO bond and increases the strength of the C-O bond. In a few coadsorption
cases, this general picture has been to some extent substantiated by calculations with different quantum-chemical
methods. 29*43-46In another type of mode1,47 the coadsorption effects of the “additives” are a consequence of changes
in the local density of occupied states near the Fermi level
rather than a product of variations in the total charge density
at the adsorption site. According to this mode1,47 the “additive”-induced total charge density vanishes beyond the immediately adjacent metal atom sites due to the effects of
screening by the metal electrons. However, the Fermi-level
density of states, which is not screened, and which governs
the ability of the surface to respond to the presence of other
species, is substantially reduced by electronegative “additives” even at nonadjacent sites.47 Finally, an alternate model, which produces the same final results, involves an electrostatic, through-space (as opposed to through-metal)
repulsive interaction between the charge distribution of the
coadsorbed species.34
Our results indicate that the B- . -CO interaction is repulsive due to the electron-acceptor nature of both adsorbates. In a previous study, l5 we observed that B increases the
adsorption energy of ammonia (an electron-donor species)
on Ru (000 1) . The B * * *NH, interaction was attractive.15’36
Adsorbed boron atoms create Ru sites on the surface with
partial positive charge, which facilitates electron donation
from ammonia to these metal sites. In addition, there is a
stabilizing through-space interaction between the partial
negative charge on the boron adatoms and the partial positive charge on the ammonia molecules. We did not see any
evidence that indicates direct bonding or reaction between B
and CO on Ru( 0001) . A similar result was found for the
NH, /B/Ru (000 1) system under UHV conditions.” It appears that B behaves as an inert “additive” when coadsorbed
with species that have a large chemical stability. A totally
different picture emerges when boron is coadsorbed with
reactive molecules.8~‘5~48-51 B oron adatoms react extensively
with N, H, (Ref. 49) and C, N, (Ref. 48) to produce species with strong B-N bonds. Boron enhances the ability of
the Ru( 0001) surface to chemisorb oxygen. l5 For
0, /B/Ru( 0001 ), XPS results show the formation of boron
oxides that are stable on the surface up to temperatures
above 1400 K.15
V. CONCLUSIONS
( 1) Boron adatoms poison CO chemisorption on
Ru (000 1) approximately on a one-to-one basis. At 100 K,
CO does not adsorb on Ru (000 1) surfaces with B coverages
above 0.7 ML. No reaction or bonding between B and CO
was observed.
(2) Boron atoms modify the chemical properties of first
and second nearest-neighbor metal atoms. The CO desorption temperature decreases, whereas the O( 1s) binding energy and C-O stretching frequency increase.
(3) The Be * *CO interaction is repulsive due to the electron-acceptor nature of both adsorbates. The trends in TDS,
XPS, and FT-IRAS are a consequence of ( 1) a reduction in
2r*-back-donation
caused by competition for metal electrons between CO and B and (2) repulsive electrostatic interactions between the negative charges on CO and B.
( 4) The FT-IRAS results for CO on B/Ru (0001) show
that at low CO coverages and/or low surface temperatures,
Ru atoms which are weakly perturbed by B are preferentially populated. Metal atoms strongly affected by B are only
occupied when no more unperturbed Ru sites are available
on the surface, or when the high temperature of the system
favors CO migration (due to entropic effects) onto them.
ACKNOWLEDGMENTS
We acknowledge with pleasure the support of this work
by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
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or copyright, see http://jcp.aip.org/jcp/copyright.jsp