2978
J. Phys. Chem. 1984, 88, 2978-2985
Stoichiometric Adsorbate Species Interconversion Processes In the Chemisorbed Layer. An Infrared Study of the CO/Pd System Patrick Gelin, Allen R. Sedle,+ and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: January 19, 1984)
Infrared spectroscopy has been employed to study the chemisorption of CO by Pd crystallites supported on S O 2 . At 80 K, it has been found that the adsorption of terminally bound CO species on Pd saturated with bridged-CO species results in the stoichiometric conversion of two CO-bridged species to terminal-CO species for each terminal CO adsorbed. The local character of this stoichiometric conversion process suggests that local donor-acceptor concepts operate at the sites experiencing the conversion. Comparisons of behavior on small Pd crystallites and on Pt( 1 11) are made, and it is found that remarkable similarities exist as well as differences.
Introduction Because of the ability of infrared spectroscopy to discriminate between the different modes of bonding of a chemisorbed molecule, we have applied this method to the detailed investigation of surface species-interaction phenomena. The system CO/Pd was chosen for this investigation because of its rich distribution of adsorbate states, and also because of the thorough investigations made to date for CO chemisorption on various Pd single-crystal planes.14 A generalization which may be drawn from the CO/Pd literature is that at 300 K C O prefers to initially bond as a bridging species rather than a linearly bound species. Thus, the dominant carbonyl stretching frequencies observed are below 2000 cm-' both on single crystals and on supported Pd surfaces. We have shown in an earlier communication5that, at adsorption temperatures below 300 K, a linear-CO species may be extensively populated and that this adsorption process is accompanied by stoichiometric conversion of preexisting bridged-CO species to linear-CO species. This paper is a full report of these observations. The connection between this work on adsorbed CO species and the behavior of C O ligands in metal cluster carbonyls will also be explored. In addition, the close connection between these studies on SiOz-supported Pd crystallites and similar work on a Pd( 111) single crystal will be described. Experimental Section The samples of Pd supported on SiO, (10% Pd by weight) were prepared following a standard procedure previously described for the preparation of Rh/A1203samplese6 Briefly, the SiO, support (Aerosil Degussa, 200 m2/g) was impregnated with 0.1 M aqueous solution of Pd(NO3),-2H20. An acetone-H20 (80% acetone) slurry of SiO, impregnated with the Pd" was sprayed out onto one half-section of a circular CaF, supporting plate heated to 353 K to flash evaporate solvents. The volume of the liquid phase was 45 mL/g PdI1-SiO2. By a masking procedure previously de~ c r i b e d Pd-free ,~ SiO, was deposited on the other half-section. The sample was then introduced into the IR cell7 and degassed overnight at 423 K under vacuum. The H2 reduction (400 torr of H,) was carried out at 150 OC and followed by subsequent degassing at the same temperature. The reduction outgassing sequence was repeated 4 times before the final outgassing of the sample at 453 K overnight. The Pd/SiOz sample contained 15.3 X mol of Pd and was deposited at a surface "density" of 6 x g/cm2. The infrared cell was equipped with a liquid-nitrogen-cooled sample holder permitting the establishment of any temperature in the range 80-300 K by using a controlled coolant flow rate. The infrared cell was attached to a grease-free stainless steel bakeable ultrahigh-vacuum system (base pressure = 1 X lo4 torr). The system was equipped with a liquid-nitrogen-cooled zeolite Science Research Laboratory, 3M Central Research Laboratories, St. Paul, MN 55144.
0022-3654/84/2088-2978$01.50/0
pump and an ion pump to maintain hydrocarbon-free conditions. Pressures were measured in the higher pressure range with Baratron capacitance manometers in the range 10-4-103 torr. Infrared spectra were recorded by transmission at a resolution of 3.0 cm-' with a Perkin-Elmer Model 580B spectrophotometer equipped with a fully computerized data station allowing data storage and treatment. All spectra presented here involved data acqusition times of 1 s/cm-l. All the spectra were treated with a 19-point smoothing function. A movable cell holder was used to position one side or the other of the divided sample in the IR beam and thus to examine both SiOz and Pd on SiO, following the same experimental treatment conditions and to completely compensate for the CO(g) spectrum. Reagent-grade CO(g) was obtained in break-seal flasks from the Matheson Co. and was used without further purification. Isotopically labeled I3CO (99% I3C) was obtained from Merck Isotopes and used without further purification.
Results CO Adsorptioh Isotherm and Pd Particle Size Estimation. By means of pressure measurements in the vacuum system of known volume, the isotherm of the chemisorption of carbon monoxide on the Pd sample at room temperature was measured, as shown in Figure 1. In the first increments, C O adsorbs completely and almost instantaneously on the surface, so that no residual C O pressure can be detected (points a-c). At higher C O coverage, the equilibrium pressure was slowly reached after contacting the sample with CO(g) overnight (point e) and then 30 min for the next points. Beyond an equilibrium pressure of about 0.2 torr of CO, errors due to volumetric and pressure uncertainties became too great for accurate continuation of the isotherm. The points a-f on the isotherm in Figure 1 correspond to the labeled infrared spectra in Figure 2. From Figure 1, it is seen that beyond an equilibrium pressure of torr, the 300 K isotherm for C O chemisorption on Pd/Si02 exhibits a plateau indicating the formation of the monolayer on the Pd surface. The metallic dispersion of the palladium, D, defined as the ratio of the total number of surface atoms (N,) to the total number of metal atoms present (A$), can be estimated from the total amount of chemisorbed C O on the Pd surface, assuming a C O chemisorption stoichiometry at monolayer coverage. On the basis of
-
(1) A. M. Bradshaw and F. Hoffmann, Surf. Sci., 72, 513 (1978). (2) F. M. Hoffmann and A. M. Bradshaw, Proc. Int. Vac. Congr., 7th, 1167 (1977). ( 3 ) A. Ortega, F. M. Hoffmann, and A. M. Bradshaw, Surf: Sci., 119,79 (1982). (4) A. Ortega (A. M. Bradshaw), Ph.D. Thesis, Free University of Berlin, West Berlin, West Germany. ( 5 ) P. Gelin and J. T. Yates, Jr., Surf.Sci., 136, L1-L8 (1984). ( 6 ) J. T. Yates, Jr., T. M. Duncan, S. D.Worley, and R. W. Vaughan, J . Chem. Phys., 70, 1219 (1979). (7) H. P. Wang and J. T. Yates, Jr., J . Phys. Chem., 88, 852 (1984).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2979
IR Study of the CO/Pd System
I
T=300K
1
=A
Pc 0 (Torr 1
=
I
I
131 44
a 05i
1: I
17
t 0
I
1
0.I
0 2
0.6
Pco ( T o r r )
2200
Figure 1. Typical isotherm for CO chemisorption on Pd/SiO2
1
,
I
I
1
2100
2000
I900
1800
1700
W a v e n u m b e r ( c m - )’ Figure 3. IR spectra of CO chemisorbed on Pd/Si02-high-pressure region. The half-plate IR cell design (ref 7) completely eliminates interference of CO(g) spectral features in these measurements at high CO
pressures.
W
u z Q m a
0 VI
(0
Q
I
2200
2100
2000
-
I
0.20
1900
WAVENUMBER
1800
1700
(ern-')
Figure 2. IR spectra of CO chemisorbed on Pd/Si02 for increasing
coverage. the measurement of hydrogen irreversibly bound to the Pd surface for a 9 wt % Pd/Si02 sample, and assuming a ratio H/Pd, = 1, it has been reported that C O irreversibly chemisorbed on the Pd surface in a ratio CO/Pd, = 0.75.* Assuming the same stoichiometry for CO chemisorption on our Pd sample, the dispersion D derived from the 300 K measurement of C O chemisorption at monolayer is
D = (1.7 X 10-6)(1/0.75)/(15.3
X 10”)
= 0.148
Assuming spherical particles, the mean diameter of the Pd particles, d, is related to the dispersion by
d = 6 (vPd/APd)D
(1)
( 8 ) A. Palam, C . C . Chang, and R.J. Kokes, J . Card., 36, 338 (1975).
where Apd is the average area occupied by a Pd atom in the surface, and vpd the volume per metal atom in the bulk. The value mz can be calculated for a Pd polycrystalline for Apd= 0.79 X surface, assuming equal proportions of the main low index planes, while vpd is given by Mw/(pNo)where M , is the atomic weight, p is the density of the metal, and No is Avogadro’s number. The calculation indicates a mean particle diameter of 75 8, for our supported Pd sample. Infrared Spectra of l2C0 Adsorbed on Pd/SiO, at Room Temperature. Following each adsorption point in Figure 1, the infrared spectra shown in Figure 2 were recorded. At low coverage, C O chemisorption on Pd/Si02 exhibits three IR bands at 1800, 1888, and 1968 cm-’, ascribed to bridged-CO species. Increasing C O coverage causes both a sharp intensity increase and a shift toward higher frequency of the band initially at 1968 cm-’ together with depletion of the 1888-cm-I band. The observed frequency shift parallels the general coverage-dependent frequency shift of the C O stretch vibration for C O on Pd single-crystal planes.’ This shift has been ascribed to dipolar coupling arising from purely electrodynamic “through space” interaction, and also to “through substrate” interactions of a chemical n a t ~ r e . ~ At higher CO coverages, a new band at -2050 cm-I forms and shifts to higher frequency with increasing coverage. This band has already been observed on SiO+upported Pd catalysts and At monolayer ascribed to linearly bonded CO specie^.^,^ (spectrum f), CO chemisorption exhibits mainly a strong sharp band at 1990 cm-’, together with a shoulder at 1950 crn-’ and a very broad feature near 1800 cm-I. These features below 2000 cm-’ indicate that CO predominantly chemisorbs on the Pd surface as bridged-CO species in agreement with previous but that, at high C O pressures, some linear CO can also be formed. Figure 3 shows the infrared spectra of CO chemisorbed on Pd/SiOz at room temperature as the CO pressure above the sample was further increased gradually to 131 torr of CO. The sharp feature at 1990 cm-I is not observed to shift nor to increase further, but instead decreases slightly in intensity, while the linear-CO-species band simultaneously increases in intensity and shifts to higher frequency. Decreasing C O pressure gradually restores the intensity of the 1990-cm-’ band while the linearCO-species band simultaneously decreases in intensity and shifts (9) (a) R. P. Eischens, S.A. Francis, and W. A. Pliskin, J . Phys. Chem., 60, 794 (1956); (b) M. A. Vannice and S. Y.Wang, ibid., 85, 2543 (1981).
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
2980
Gelin et al.
1I I
T = 80K
I
1
I
I
I
1995 cm-'
I
0.2
I W
0
z
W
0 .I
(z
W LL
k 0 W 0
0
2 U
m (z
W
0 v,
0
z a
m
a -.0.1
m (r
0
c
I!
m
m
a
1979 c m - '
- 0.2 2200
I
I
1
I
2100
2000
1900
1800
1700
(cm-') Figure 5. Difference spectra for CO chemisorption on Pd/SiOz at low temperature. The difference spectra are obtained by the indicated subtraction of spectra in Figure 4. WAVENUMBER
11
2200
2100
I1
1 1
2000
1900
WAVENUMBER
I
1
1800
1700
1 crn-' )
Figure 4. Additional CO adsorption on Pd/SiOz at low temperatures. Spectrum a: Follows saturation CO coverage at 300 K, evacuation for 30 min at 300 K, and cooling to 80 K. Spectra b-h: Incremental addition to small quantities of CO to cooled Pd/SiOz.
toward lower frequency. Assuming no variation of extinction coefficients the simultaneous changes of the bridged- and linear-CO-species bands with C O pressure are related to a bridgedto-linear interconversion process. This process is totally reversible since decreasing C O pressure restores the IR spectra to their original conditions. Infrared Spectra of Additional lZCO Adsorption at Low Temperature. After being exposed to a CO pressure of 131 torr a t 300 K, the Pd sample was subsequently evacuated at 300 K for 30 min and then cooled down to 80 K. Evacuation caused a shift of the 1990-cm-' sharp feature to 1979 cm-'. Simultaneously, a strong decrease of the linear-CO-species band indicates the removal of most linear species. A weak band at 2050 cm-l is still observed, indicating that some linear-CO-species are more strongly bound to the Pd surface at room temperature. These results are consistent with previous studies* where linear-CO species on Pd/Si02 were found to be almost completely desorbed under vacuum at room temperature. At this point (spectrum a, Figure 4), small quantities of l 2 C 0 were sequentially added to the sample at 80 K and were observed to be totally adsorbed by the sample. The spectral changes are shown in Figure 4. A sharp intense band at 2103 cm-' (designated L-CO) develops, while the 1979-cm-' band (designated B,-CO) decreases in intensity and a new feature at 1995 cm-' (designated B2-CO) forms, first as a shoulder and then as a sharp intense peak. During the last stages of C O adsorption at 80 K, a small band at 1883 cm-' develops, due possibly to some conformational changes in bridging-CO species induced by increasing C O coverage. A similar band at 1893 cm-' is shown for high CO coverage at 80 K on Pd(ll1) as will be discussed later. The interrelated changes of B1- and B2-CO species are more clearly seen in Figure 5 which shows difference spectra for additional C O adsorption
1
1
1
1
0 05
01
0 15
02
BRIDGED C O , B, (*"l995)
Figure 6. Evidence for stoichiometric interconversion of adsorbed CO
species. at 80 K. Here the relationship between the formation of L-CO and the interconversion of B1-CO to B2-CO is clearly seen. The peak absorbances of the L, B,, and B2 features, taken from Figure 5 , are plotted against each other in Figure 6. The strict linear interrelationships suggest that a stoichiometric conversion process is occurring between the surface C O species. An expanded view of bridged-CO-species spectra in the 2020-1940-cm-' region is shown in Figure 7. The existence of an invariant point at 1989 cm-', the isosbestic point, suggests that only two bridging420 species are involved, namely B1 and B, interconverting in a stoichiometric process as L-CO species form with increasing CO coverage. Figure 8 shows a plot of the integrated absorbance changes for L-CO (2120-2010 cm-') and bridged-CO species (2010-1700 cm-l) as a function of increasing CO coverage. Except in the first stages of the 80 K adsorption experiment, a linear relationship is observed for the two integrated absorbances, plotted vs. the additional C O coverage changes. This suggests a stoichiometric conversion process involving a conversion of bridged-CO to linear-CO species as CO coverage increases. The slight increase in bridged-CO-species intensity observed in the first stages of the experiment indicates a slight initial increase in bridged-CO-species coverage as CO is adsorbed. This result is
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2981
IR Study of the CO/Pd System
a, V
C
0
e
0
n
a
2020
2000
1980
I940
1960
-
Wavenumber (ern-') Figure 7. Expanded view of Bl-CO B2-C0 conversion as L-CO adsorbs. The presence of an isosbestic point at 1989 cm-' is indicative of the presence of the B1 and B, species which interconvert as L-CO adsorbs.
B2 region, which can best be ascribed to a stoichiometric interconversion process induced by L-CO adsorption. Additional Adsorption of CO at Low Temperature, Using Isotopic Species. After saturation by l2C0at room temperature and subsequent evacuation, the Pd surface was further exposed to small doses of CO at 80 K, by using isotopically labeled 13C0. The spectral changes are shown in Figure 9. For the bridgedcarbonyl region, the same changes as previously observed with I2C0 occurred, mainly depletion of the 1979-cm-l feature accompanied by the formation of the 1995-cm-I bridged CO. Above 2000 cm-I, it may be seen that in addition to the expected adsorption of I3CO in linear configuration (giving L* at 2050 cm-'), a large proportion of linearly bonded l2C0species is formed (at 2103 cm-I). These species can only originate from the preadsorbed I2CO-bridgedspecies, and this observation provides direct evidence for a CO-coverage-induced bridged-to-linear conversion process. The reverse low-temperature isotopic experiment has also been carried out, starting with the Pd sample saturated at 300 K with 13C0. The spectral changes resulting from sequential 80 K addition of small amounts of l 2 C 0 are shown in Figure 10. Additional quantities of I2CO adsorbed on the surface induce the formation of both l2C0and I3CO linear species at 2100 and 2050 cm-'. Simultaneously, B, B2 conversion occurs as suggested by the depletion of the 1940-cm-l band. However, the formation of B2 species expected at about 1950 cm-' is partially masked by the development of a broad intense band at 1970 cm-'. This latter band could be attributed to the presence of some isotopic exchange with l2C0. Measurements of the relative intensities of labeled (L*) and nonlabeled (L) linear-CO species are plotted in Figure 11 for isotopic experiments carried out in both orders of isotopic addition as a function of L (respectively L*) development. The intensity measurements were corrected for an effect observed in which a saturated layer of "CO yielded linear- and bridged-CO-species absorbances which were respectively 0.65 and 0.79 of those found for a monolayer of "CO. Using these two values as an absorbance correction factor yields an upper and lower limit for L/L* values, as shown in Figure 10. The ratio L/L* (respectively L*/L in the reverse isotopic experiment) was found to be approximately equal to 2 within experimental errors over the entire range of the experiment. Assuming no isotopic exchange in the adlayer, this result indicates that the adsorption of one CO molecule, L* (respectively L), results in the conversion of 2B1 2L. Since some exchange of bridged adspecies has been shown (Figure 10) to occur at 80 K, the measured L* intensity may be lower than it should be without any exchange, and consequently L/L* plotted in Figure 11 is possibly overestimated. Search for Surface Modifications at High CO Coverages. In order to exclude the attribution of the changes observed in CO adsorption states at 80 K to any modification of the Pd surface structure, the following experiments were carried out. After exposure of the Pd sample at 80 K to small I2C0 doses so that an IR spectrum of CO adspecies identical with spectrum h in Figure 4 could be obtained, the Pd sample was further contacted with increasing C O pressure at 80 K. The different spectra are shown in Figure 12. Both bridged- and linear-CO species at respectively 1995 and 2105 cm-' are observed to shift slightly to higher frequency with increasing CO pressure. No change in the intensities of chemisorbed CO species bands can be detected, indicating the saturation of the C O chemisorption adlayer. However, new intense bands develop successively at 21 59 and 2142 cm-', as CO pressure is increased. These bands are due to physisorbed C O layers on the SiOz support. At this point, the Pd sample was heated gradually from 90 to 300 K, while the C O pressure was maintained constant (Pco = 17 torr), and the IR spectra recorded at different intermediate temperatures are shown in Figure 13. Heating the sample to 165 K did not markedly affect the CO chemisorption spectrum, except for a small shift of L- and B2-CO species to lower frequency, while the C O physisorption peaks disappeared. Further increase of the Pd surface temperature resulted in both the decrease of L species band intensity and the concomitant increase of B species bands, accom-
-
-
1 0
I
I
0.1
0.2 lo6 x
I
0.3
Nc,
I
I
0.4
0.5
I
(mole)
Figure 8. Evidence for stoichiometricconversion of bridged- to linear-CO species for increasing CO coverage at 80 K.
consistent with the fact that some bridged-CO species have been evolved from the surface during the prior evacuation treatment at room temperature. The measurement of integrated intensity below 1940 cm-' reveals almost no variation, indicating that the changes in integrated absorbance for b r i d g e d 4 0 species must be attributed only to processes involving the B1-and B2-CO species. Assuming no coverage-dependent variation in the extinction coefficients for B1-, B2-, and L-CO species, the results shown in Figures 6-8 suggest that the three species are involved in a stoichiometric interconversion process induced by changes in surface C O coverage. In this model, B1 species are characteristic of the Pd surface mainly covered by bridged-CO species, and B2 is ascribed to bridged-CO species interacting with neighboring linear CO. The same interpretation has been proposed for 300 K experiments by Palazov et a1.* to explain a shift in the bridged-CO maximum toward higher frequency as linear-CO species simultaneously developed around 2095 cm-'. Our results, obtained at 80 K , indicate a sharper distinction between species, giving intensity changes at constant wavenumber in the B, and
2982
”he Journal of Physical Chemistry, Vol. 88, No. 14, 1984
=‘“o
Gelin et al.
I
n = I3c0
DIFFERENCE SPECTRA
I
BZ
0.05.A
W
V
z U
m 0:
O m
m
a
2; 0
21dO
I
I
I
2000
1900
1800
I
1700 2200
I
I
1
I
2100
2000
1900
1800
1700
Figure 9. Direct evidence for bridged-to-linear CO conversion-isotopic CO experiments. ‘*COis preadsorbed to saturation coverage at 300 K. The Pd/Si02 is evacuated at 300 K; the P d / S i 0 2 is cooled to 80 K, and I3CO is incrementally added.
panied by a small shift of the bands toward lower frequency. Simultaneously, the small band at 1883 cm-’ totally disappeared. The spectrum recorded at 300 K is identical with that shown in Figure 3 for the same C O pressure. These results indicate that the bridged-to-linear interconversion process is totally reversible. Furthermore, this process cannot be attributed to any irreversible reconstruction of the Pd surface but instead to a reversible interconversion process within the adsorbed layer which is induced by increasing C O coverage. Effect of Temperature on IR Line Shapes for Chemisorbed CO on Pd/SiOz. Small effects on the sharpness of the IR bands for chemisorbed C O species are observed during variation in the substrate temperature in this work. This effect is shown in Figure 14, where a comparison of the spectrum at 90 and 300 K is made. Sharpening at the lower temperature occurs in the three major spectral regions showing bands due to different chemisorbed CO species. These effects permit an increase in our ability to resolve spectral features at lower temperatures.
Discussion of Results Stoichiometric Bridged CO to Linear CO. Interconversion during Low-Temperature Adsorption. A schematic one-dimensional model for the surface conversion process observed in this work is given in Figure 15. Here we see an array of bridging-CO species, B , , arranged at a coverage of 1 CO/Pd. The closest packed Pd-Pd distance is 2.70 A, which is ideal for the formation of bridged CO.’O The 2.70-A spacing of the CO molecules in the schematic model requires some squeezing of the CO species, based on the van der Waals diameter for CO(g) of about 3.1 A.11 However, if such (IO) E. W. Plummer, W. R. Salaneck, and J. S . Miller, Phys. Rev., B, 18, 1673 (1978). (1 I ) W. J. Moore in “Physical Chemistry”, 2nd ed., Prentice-Hall, Engelwood Cliffs, NJ, 1955, p 179.
repulsive interaction effects are present, they can be avoided by the CO species by inclining neighbor CO molecules out of plane. This necessity suggests that a full-CO coverage will be less than 1 CO/Pd on the two-dimensional surface, a result compatible with known saturation LEED structures on Pd single crystals.I2 The addition of a linear-CO species to an array of Bl-CO species is shown in step 2 of Figure 15; the intermediate species, designated by $, represents a highly crowded state at the point of linear-CO adsorption, causing a local surface conversion of bridged-B1species to linear species (step 2). Again, it is necessary to invoke outof-plane orientations of the various C O species to avoid overcrowding. The stoichiometric process observed here is somewhat reminiscent of an effect observed by ESDIAD for CO adsorption on the Pd(210) crystal plane.13 At 80 K, CO first occupies bridged-CO sites, producing nonnormal CO bond directions characteristic of the favorable CO bridge bonding sites having 2.70-A Pd-Pd distance. At the highest CO coverages at 80 K, ESDIAD ion emission in the normal direction is observed and may be due to the population of linear-CO species, possibly by bridged linear conversion processes like those seen in this work. Stoichiometry of the Species Conversion Process. The data of Figures 6-8 clearly indicate that a quantitative interconversion effect is being observed here, assuming that large variations in the C O extinction coefficient are not occurring. In Figure 6, accurately linear peak absorbance relationships over the full range of linear-CO intensity are seen, suggesting that a local stoichiometry governs the interconversion process. This view is strengthened by the observations in Figure 7, where an isosbestic point is detected at about 1990 cm-l. The existence of an isosbestic
-
(12) (a) A. M. Bradshaw, Appl. Surf. Sci., 11/12. 712 (1982); (b) R. F. Willis, A. A. Lucas, and G. D. Mahan in ‘The Chemical Physics of Solid Surfaces”, D. A. King and D. P. Woodruff, Eds., Elsevier, Amsterdam. (13) T. E. Madey, J. T. Yates, Jr., A. M. Bradshaw, and F. M. Hoffmann, Surf. Sci., 89, 370 (1979).
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2983
IR Study of the CO/Pd System
PCO
(Torr ) 0
107.8
C
O
n L
0
45.8
v)
n
a 16.5
I
"c z .-
3.97
e
L
0 w
o
0.27
O C
n L 0
0.I3
n
a
-1
0.07
I 2200
I
I
1
2000
1900
1800
I
2100
1700
W a v e n u m b e r (crn-l )
Figure 10. Bridged-to-linearCO conversion-reverse isotope addition. Evidence for isotopic exchange of bridged-CO species at 80 K.
-
-0
( 2 K+CO-31
2200
2100
2000
I800
1900
1700
W o v e n u m b e r (crn-' )
Figure 12. Development of physisorbed CO layers on Pd/Si02. The invarience of the chemisorbed CO species intensity in the presence of physisorbed CO indicates that saturation CO coverages have been reached for the chemisorbed species.
0
LL
T
T
1 Pco=
I
17 Torr
I
A T (K)
0
L/L*
0
L*/ L
300 273 250
235
220
Absorbance o f Converted L - C O
-
Figure 11. Ratio for stoichiometricinterconversion process. The data suggest that the process 2 bridged CO + CO 3 linear CO occurs by using either I2CO(ads)+ "CO(g) or I3CO(ads)+ I2CO(g).
205 190
point in Figure 7 is indicative of a simple species conversion process and is often observed in chemistry for stoichiometric processes, although such effects have not been reported on surfaces. The integrated intensity data of Figure 8 confirm that bridged CO is being destroyed in concert with the production of linear CO; over the full range, linear integrated absorbance interrelationships are observed. We have attempted to estimate the approximate stoichiometry involved here (Figure 11). In this figure, an approximate 2:1 relation between bridged linear events and linear adsorption events is found. Since the surfaces being studied in this work are polyfaceted, this stoichiometry relationship is an average over the various crystallographic planes involved and should not necessarily
-
165
I30
90 I
2200
2100
2000
1900
1800
1700
W o v e n u m b e r (ern-' )
Figure 13. Changes in CO adsorption state distribution on heating CO/Pd/SiO,. These results, compared to the adsorption results in Figure 3, indicate complete reversibility of surface species population.
2984
The Journal of Physical Chemistry, Vo1. 88, No. 14, 1984 I
I
I 90 K
I
2200
I
i
-1
I
I
2 100
2000
Gelin et al.
Wavenumber
1
I
I
1900
1800
1T=
190-+273 K
I
1700
ern-' I
Figure 14. Effect of temperature on IR line shapes for chemisorbed CO on Pd/Si02.
0
0
A /\
/C\ Pd
Pd Bi
Pd
Pd
0 Pd
0 0 C /' \ ic\
0
/c\ Pd
Pd
'3
61
Bi
r o j
0
Pd B
I
+ CO(9)
0
0
c
C
C
C
Pd
Pd
e
Pd
5
Bl
4 0
0 Pd
Pd
6
7
Infeqraled Linear - C O Intensity (crn-I)
Figure 16. Effect of heating on CO-binding states distribution. The data indicate a reconversion from linear to bridged CO as the surface is heated under vacuum. This behavior is entirely reversible.
1 Pd
L
J
I 0
0
0
c
C
2\
Pd
0
0
Pd
6,
Bl
I
I/
Pd
Pd
Pd
L
L
L
___-------OVERALL
48,
-
0
c
0
\A Pd
B2
Pd
Bl
STOICHIOMETRY
+
L
3~
+
2B,
Figure IS. Schematic one-dimensional model for bridged-to-linearCO
species conversion process.
be expected to be an integral number. Nevertheless, it does appear as if about two bridged-CO species are destroyed by the adsorption of a single linear CO over the entire range of linear-CO coverages. This result suggests that the effect observed here is probably best understood as a local phenomenon, rather than a long-distance surface effect. The local nature of effects governing C O chemisorption on Pd has been proposed by Sachtler et al.,I4from studies of C O adsorption on Pd-Ag alloys. Reuersibility of the Stoichiometric Process on Heating. Figure 16 illustrates the overall observation of integrated absorbance changes which occur upon heating the fully covered CO/Pd surface from 190 to 273 K under vacuum. A reconversion to bridged C O occurs as linear CO desorbs. The adsorption-desorption sequence may be repeated many times without permanent change in behavior. Similar experiments involving very high exposures of the Pd to CO (under multilayer CO) are shown in Figures 12 and 13; again, no permanent changes are observed in cycling between 80 and 300 K. These observations suggest that we are not observing effects due to permanent changes in Pd crystal structure upon C O adsorption. The possibility of a reversible change in the local structural character of the Pd adsorption sites cannot be eliminated however, and we must await further experiments involving techniques such as EXAFS for the answer to this question. (14)
Y. Soma-Noto, and W. M. H. Sachtler, J . Catal., 32, 315 (1974).
I
2100
I
I
2000
I
1
1900
I
I800
Wavenumber (cm'l
Figure 17. CO Chemisorption on Pd( 111). These data are obtained from ref 4 and illustrate a remarkable similarity to the results obtained on Pd/Si02 (see Table I).
Comparison to CO Adsorption Studies on Pd( 111) Using Refection IR Methods. Recent results for the system CO/Pd(1 1l), obtained by Ortega? make an interesting comparison with the data obtained in this work on supported Pd/Si02 surfaces. In Figure 17, the sequence of spectra for increasing CO coverage is shown. Striking resemblances at 80-90 K are seen in the spectra of Figure 17 [Pd( 11l ) ] and the spectra of Figure 4 (Pd/Si02), and these are listed in Table I. It can be seen that there are strong similarities in behavior for Pd( 111) and Pd/Si02, accompanied by some differences. First of all, linear CO only appears during the last stages of CO adsorption in both cases. Secondly the appearance of linear C O causes a decrease in intensity of a bridged-CO species [ 1962 cm-I, Pd( 111); 1979 cm-' (Pd/Si02)]. Finally, at the last stages, a low-frequency bridged-CO band appears in both cases [ 1893 cm-', Pd(l1 l), 1883 cm-', (Pd/Si02)]. The close relationship between B,-CO and B2-CO is not seen in Pd( 111). It is possible that on Pd( 11l), long-range effects are more likely to occur, leading to long-range order and to subtle interactional
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2985
IR Study of the CO/Pd System TABLE I: Comparison of CO Spectra for Pd(ll1) and Pd/SiO, at 80 K"
adsorptn condition highest CO coverage before linear-CO adsorpn
vco obsd for chemisorbed CO. cm-' Pd(l1 l ) b Pd/Si02e 1996 1979 (broad shoulder at 1962 lower u )
onset of linear-CO adsorpn
2097 1996 1962
t
last stages of linear-CO adsorpn
2097 1996 1962 1893
f
2103 1995 1979
4
2103 1995 1979 1883
f
t
t 1 t 1
"Arrow indicates direction of change in absorbance. bReference4. cThis work. differences between Pd(l1 l), and our 75-APd crystallites. These differences could be responsible for some of the behavior differences observed. In addition, our Pd/Si02 preparation probably contains crystallites which exhibit a variety of Pd facets, and adsorption on this mixture of facets may be responsible for some of the differences observed. Relationship between the Stoichiometric Process on Pd/Si02 and Ligand Interaction Effects in Organometallic Chemistry. The carbonyl interconversion processes observed here on small palladium particles can be rationalized in the context of structure and bonding formalisms employed in organometallic ~hemistry,'~ and the now well-appreciated analogies between metal clusters and surfaces.I6 For palladium, precise molecular-scale models are particularly difficult to find: binary platinum carbonyls, [Pt3(p-CO)3(C0)3],", adopt columnar form" and binary nickel carbonyl clusters may have either columnar or polyhedral geometries.18 However, only phosphine-substituted derivatives of palladium carbonyls are known1gand the stereodynamic properties of them are largely unexplored. The surface reaction
4B1+ CO
-
3L
+ B,
may be rationalized in electronic terms. It is known that a bridging carbonyl is a better a-acceptor than a terminal CO, in agreement with which vco(bridge) < vco(terminal) ?O In molecular clusters, substitution for C O of other ligands leads to rearrangement of the remaining carbonyls. For example, Ir4(CO)12has Td geometry with all carbonyls in terminal positions, whereas Ir4(CO)1lX- (X = C1, Br, I, CN, SCN) and Ir4(CO)12-n(PPh3),( n = 2, 3) have structures with bridging carbonyls in the basal plane.21 These substituting ligands are, relative to carbon monoxide, better udonors and poorer a-acceptors so a terminal-to-bridge carbon shift may be considered to occur under conditions where a-donation takes place locally. Just the opposite happens on the surface of the small palladium particles: one is adding a strong a-acceptor, CO. In this case, one can argue that a-acceptor capability of two bridging carbonyl ligands is similar to the a-acceptor capability (15) J. P. Collman and L. S. Hedgedus, "Principles and Applications of Organotransition Metal Chemistry",University Science Books, Mill Valley, CA. ---,1980. - --
(16) E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Chem. Rev., 79, 91 (1979). (17) G. Longoni and P. Chini, J. Am. Chem. Soc., 98, 7225 (1976). (18) A. Ceriotti, P. Chini, R. D. Pergola, and G. Longoni, Znorg. Chem., 22, 1595 (1983), and references cited therein. (19) R. Goodard, P. W. Jolly, C. Kruger, K.-P. Schick, and G. Wilke, Organometallics, 1, 1709 (1982), and references cited therein. (20) V. Guttmann, Coord. Chem. Reu., 15, 207 (1975). (21) P. R. Raithby in "Transition Metal Clusters", B. F. G. Johnson, Ed.. Wiley, New York, 1980.
of three linear-CO species, and so a bridged-to-terminal rearrangement ensues. It is significant that, since separate infrared bands are observed for the B2 and L states, their interconversion is slow on the bond vibrational time scale. Even slower processes, however, are possible and, indeed, bridge-linear transformations are known to be involved in intramolecular CO migration in molecular clusters such as Rh4(C0)12.22The literature provides few clues as to anticipated rearrangements on palladium. In solution phase, columnar [Pt3(p-CO)3(CO)3],2-( n = 2-5) exhibit rotation of intact Pt3(C0)6units about the pseudothreefold ~ ~ in contrast, axis with no bridge-terminal C O i n t e r ~ h a n g ebut, there is bridge-terminal scrambling on the outer layers of metal triangles in nickel carbonyl clusters at 380 0C.24 If the transition state, $, shown in Figure 15 is higher in energy than the ground state by an amount less than Eads,it could plausibly be involved in the lateral migration of C O over the Pd crystallite surface by a bridge-terminal exchange analogous to that observed in molecular clusters. It is well-known that lateral migration of C O occurs on Pd at 80 K from ESDIAD data,13 where specific bridging CO sites are initially occupied at low coverages as mobile CO species sample the surfaces.
Summary The following general conclusions about the chemisorption of C O on Si02-supported Pd crystallites have been reached: A. CO initially bonds to Pd surfaces in a bridged-CO structure involving coordination to two or more Pd surface atoms. B. A more weakly bound terminal (or linear) CO species may be adsorbed in the later stages of monolayer development, leading to the production of an infrared absorption band at about 2100 cm-l. C. The adsorption of this additional C O leads to a local conversion of bridged-CO species to linear-CO species. The stoichiometry observed suggests that about two bridged-CO species are locally converted into terminal-CO species upon the adsorption of a terminal CO onto a surface region covered with bridged-CO species. D. Specific local interactions between linear- and bridged-CO species lead to the production of a high-frequency bridged-CO stretching mode near 1995 cm-I at the expense of a 1979-cm-' species. E. The local stoichiometric conversion may be rationalized along the following lines. Terminal-CO species involve less aacceptor behavior per molecule than does bridged CO. As weakly bound terminal CO is adsorbed on a surface saturated with bridged CO, the d-a donation capability of Pd drops to a level which triggers local conversion of bridged-CO species into terminal420 species via a process which appears to be remarkably stoichiometric. F. Carbonyl moiety rehybridization effects due to ligand-induced electronic perturbation of metal cluster compounds closely parallel the effects observed for C O chemisorbed on Pd. G. A strong similarity in the sequence of CO adsorbed species development exists in comparing Pd( 111) with Pd/SiO,. H. These observations illustrate well the high utility of IR spectroscopy for detailed investigation of interactional effects within the chemisorbed layer, especially when performed at low temperatures. Acknowledgment. We gratefully acknowledge support of this work by The Science Research Laboratory and the 3M Central Research Laboratories. Registry No. CO, 630-08-0; Pd, 7440-05-3. (22) E. Band and E. L. Muetterties, Chem. Reu., 78, 639 (1978). (23) C. Brown, B. T. Heaton, P. Chini, A. Fumagalli, and G. Longoni, J . Organomet. Chem., 181, 233 (1979). (24) G. Longoni, B. T. Heaton, and P. Chini, J . Chem. Soc., Dalton Trans., 1537 (1980).
