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Structural and catalytic properties of model silica- supported palladium catalysts: a comparison to single crystal surfaces
ELSEVIER
Catalysis Today 21 (1994) 57-69
catalysis today
Structural and catalytic properties of model silicasupported palladium catalysts: a comparison to single crystal surfaces X u e p i n g Xu, Jfinos Szanyi, Q i a n g Xu, D. W a y n e G o o d m a n * Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
Abstract
The structural and catalytic properties of model silica-programmed desorption, infrared reflectionabsorption spectroscopy of adsorbed CO, scanning tunneling and atomic force microscopies (STM and AFM), and catalytic CO oxidation at both low pressure and elevated pressure conditions. The CO oxidation reactions on Pd(111), Pd(110) and Pd(100) have also been investigated. By evaporating palladium onto silica thin films (100 /~), followed by an anneal to 900 K, the size of the palladium particles can be controlled in a range of 30-500 ,~. The surface of the palladium particles consists mainly of ( 111 ) and (100) facets, and exhibit catalytic activity similar to palladium single crystals for CO oxidation at both low pressure (10-8-10 -6 Torr) and high pressure (15 Torr) conditions. At low pressures, the rate of CO oxidation increases with temperature, reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover frequency for the CO oxidation reaction on the model catalysts compare favorable with analogous results from single crystal and high-surface-area catalysts. Thus, this system of metal particles supported on a silica thin film provides an excellent model to bridge between single crystal and high-surface-area catalysts. The CO oxidation reactions on Pd( 111 ), Pd(110) and Pd(100) have similar, but distinctive activation energies (28.1 _ 0.4, 30.7 + 0.5 and 29.4 + 0.3 kcal/mol, respectively) and turnover frequencies, indicating a subtle structure-sensitivity for CO oxidation on different crystal planes of palladium.
I. Introduction Understanding the basic phenomena of heterogeneous catalytic processes is a crucial step toward developing new, highly active and selective catalysts. With the development of surface science techniques in the last two decades, many catalytic reactions have been modelled in the ultra-high vacuum (UHV) environment on * Correspondingauthor. 0920-5861/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI0920-5861 (94)00033-X
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X. Xu et aL / Catalysis Today 21 (1994) 57-69
metal single crystal surfaces [ 1 ]. However, there is a lingering concern with respect to the application of surface science to heterogeneous catalysis because of the extreme differences in the reaction conditions [2]. Surface science studies under well-defined conditions are capable of providing insights into fundamental surface processes. Due to a pressure difference of more than 10 orders of magnitude between UHV conditions and real catalytic reaction conditions, certain important surface processes during catalytic reactions may not be accessible in UHV studies. In addition, most surface science studies are performed on well-defined single crystal surfaces, whereas the practical catalysts are dispersed on supports. It has been documented that the interaction between metal and its support may alter the catalytic reactivity [ 3 ]. Furthermore, the reactivity and selectivity for some catalytic reactions vary with the particle size of metal catalysts [4]. In order to model oxide-supported metal catalysts, metals of catalytic interests have been deposited onto single crystal oxides such as ZnO and TiO2 [ 5 ]. However, these model systems are much less than ideal due to non-uniform heating and cooling, and their susceptibility to charging when probed with charged-particle beams. An alternative model system for oxide-supported metal catalysts is a metal single crystal covered partially with a support oxide [6]. These model systems still differ decidedly from the industrial metal catalysts which are dispersed onto the supporting materials. These supported metal particles may have catalytic properties distinctly different from the bulk metals. Recently, we have developed a new approach to modelling supported metal catalysts that circumvents the experimental difficulties associated with bulk oxides. These model systems, which very closely mimic the actual catalysts [7-9], consist of an oxide thin film, such as silica, deposited onto a refractory metal surface. Metals of catalytic interest, such as copper or nickel, are then evaporated onto the oxide film. The refractory metal surface [e.g. M o ( l l 0 ) ] not only supports the oxide film, but also provides a metallic reflecting surface which facilitates the use of infrared reflection-absorption spectroscopy (IRAS) [9]. The Mo(110) substrate, however, does not alter the chemical properties of the ca. 100 ,~ silica film. Furthermore, silica films on Mo(110) are better models than thermally grown SiO2 films on silicon substrate, because SiO2 on Mo(110) is stable up to 1500 K [ 10], whereas SiO2 on Si decomposes at relatively low temperatures [ 11]. The higher thermal stability of silica on Mo(110) facilitates the thermal desorption of the supported metal catalyst from the silica film. In this paper, we present the structural and catalytic properties of model silicasupported palladium catalysts. The structure of the palladium particles are characterized with scanning tunneling microscopy (STM), atomic force microscopy (AFM), Fourier transform infrared reflection-absorption spectroscopy of adsorbed CO molecules (FT-IRAS) and dispersion measurements using the chemisorption of oxygen and carbon monoxide. The catalytic properties of the model silicasupported palladium catalysts have been studied with CO oxidation at both low
X. Xu et al./ Catalysis Today 21 (1994) 57-69
59
(10-8-10 -6 Torr) and elevated pressures (15 Torr) [ 12], and are compared to results from single crystal palladium surfaces.
2. Experimental All experiments were carried out in UHV chambers coupled to a high pressure reaction cell [ 12 ]. The chambers are equipped with an Auger electron spectrometer (AES), a quadrupole mass spectrometer, low energy electron diffraction (LEED) and a Mattson Cygnus 100 infrared spectrometer. The high pressure reaction cell is directly coupled to the UHV chamber via a Teflon sliding-seal interface. The high pressure cell is outfitted with flange-mounted CaF2 windows for in situ fourier transform infrared reflection-absorption spectroscopy (FT-IRAS). The IRAS spectra were acquired with single beam optics adjusted for 85 ° incident angle. The Pd( 111 ), Pd(110) and Pd(100) single crystals were cleaned with cycles of annealing in oxygen at 900 K and a subsequent flash to 1200 K in vacuo. The surfaces were analyzed with AES and LEED before and after the high pressure reactions. The model silica-supported palladium catalysts were prepared by evaporating palladium onto a silica thin film (ca. 100 ,~) and annealing to 900 K [12]. The palladium evaporation source consisted of a 0.25 mm Pd wire (99.997%, Johnson Matthey Chemical Limited) wrapped around a tungsten filament, and a line-ofsight mass spectrometer to monitor and control the palladium flux. Palladium was also deposited onto a clean Mo (110) surface. Temperature-programmed desorption of Pd/Mo(110) shows two distinct peaks at 1500 K and 1300 K, corresponding to monolayer and multilayer desorption, respectively [ 13 ]. The monolayer peak was used to calibrate the palladium flux. The amount of palladium on the silica film was controlled by flux and deposition time, and determined by temperature-programmed desorption of Pd/silica/Mo(110). The initial sticking probability of palladium on silica was determined to be unity at substrate temperatures below 500 K, in contrast to copper which has an initial sticking probability of ca. 0.3 at room temperature [ 14]. The preparation and characterization of silica thin films on Mo(110) have been described in refs. [ 10,15,16]. Briefly, the silica films, which have the properties of vitreous silica [ 10,16], were prepared by evaporating silicon in ca. 1 × 10 -5 Torr oxygen and subsequently annealing to 1400 K. The steady-state rate of CO oxidation at low pressure (10-8-10 -6 Torr) was measured by monitoring the CO2 intensity with mass spectrometry. Carbon monoxide and oxygen were admitted to the UHV chamber through two separate leak valves to the desired partial pressure. Since the system is continuously pumped, the CO2 formation rate is proportional to the increase in the CO2 pressure above background. The rate of CO oxidation in the high pressure ( 15 Torr) batch reactor was measured via gas phase CO2 IR absorption and the change in the total reactor pressure. The rate of CO oxidation in the latter measurement is proportional to the
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rate of the pressure decrease. The two methods yielded comparable results; however, rates obtained via the pressure drop method are, generally, more accurate and convenient. The CO oxidation rate on a passivated crystal, coated with a silica thin film on both faces, was one order of magnitude smaller than the silica-supported catalysts, demonstrating that background reaction on the heating leads was minimal. The surface area of the palladium single crystals was calculated using both the front and back faces of the crystal, whereas the surface area of the supported palladium catalysts was determined via temperature-programmed desorption of oxygen and carbon monoxide.
3. Results and discussion
3.1. Characterization of the model Pal/silica catalysts Fig. 1 shows the temperature-programmed desorption of palladium from a silica thin film. Palladium desorbs in a single peak in the temperature range of 10001300 K. This peak corresponds to the sublimation from the metallic palladium particles. The activation energy for the sublimation, calculated from the leading edge of the desorption profile, increases with palladium coverage (particle size) from 55 kcal/mol for 0pd = 1 monolayer (ml) to 76 kcal/mol for 0pd = 5 ml. The heat of sublimation of bulk palladium is 90 kcal/mol [ 17]. A similar behavior in the variation of sublimation heats with metal coverage has been observed for Cu/ Pd/SiO2 (100/~)
A
e-
co E 0
I
1000
I
I
1100
I
1200
I
I
1300
I
I
1400
I
1500
Temperature (K) Fig. l. Temperature-programmed desorption spectra for palladium on silica( 100 /k)/Mo(110). The nominal palladium coverages are 0.6, 0.9, 1.5, 3.6 and 4.7 monolayers referencing to clean Mo(110). One monolayer corresponds to 1.3 × 1015 atoms/cm 2.
X. Xu et al. / Catalysis Today 21 (1994) 5 7 4 9
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TPD, O/PdlSiO2(I (30A)IMo(II O)
0
cl} (/}
I
b 500
700
I
I
900
I
I
1100
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I
1 00
I
1500
T e m p e r a t u r e (K)
Fig. 2. Temperature-programmed desorption spectra for O/Pd/silica/Mo(110). The palladium coverage was 6 × 10 t5 atoms/cm 2. Oxygen was adsorbed at 100 K to saturation.
silica [8 ]. The lower sublimation energy for smaller coverages arises due to the lower coordination of the surface metal atoms on the smaller particles [ 8 ]. The average size of the palladium particles was determined using chemisorption, STM and AFM. STM and AFM, performed in air after the samples were prepared in the UHV chamber, yield a direct image of the metal clusters. Chemisorption of oxygen and carbon monoxide measures the amount of metal exposed based on temperature-programmed desorption. The saturation coverage for CO is ca. 0.5 monolayers on palladium at room temperatures [ 18,19 ]. For oxygen the saturation coverage was assumed to be 0.33 ml, the average of the saturation oxygen coverage on single crystal palladium surfaces when oxygen is adsorbed below room temperatures [ 20-22]. Desorption from saturation CO/Mo (110) ( 0co = 0.5 ml) [ 23 ] was used as a calibration for carbon monoxide. Oxygen desorption from a palladium oxide (PdO) film on silica, prepared in situ by evaporating palladium in 1 × 10 -6 Torr oxygen [ 24], was used as a reference for oxygen. Fig. 2 shows the oxygen temperature-programmed desorption from O/Pd/silica/ Mo(110). Oxygen desorption occurs primarily in the temperature region 700K900 K, in agreement with oxygen desorption from palladium single crystal surfaces and foils [ 20-22 ]. In addition, a small oxygen desorption peak is observed at 12001300 K, which is in the same temperature range of palladium sublimation. From these TPD experiments the amount of oxygen and palladium can be quantified. These results suggest that a small percentage of surface oxygen is diffused into the bulk of palladium particles during heating. Oxygen diffusion into bulk palladium has been inferred in earlier palladium single crystal studies [ 25 ]. Fig. 3 shows the dispersion of palladium on silica thin films as a function of coverage based on the oxygen TPD measurements; measurements based on CO TPD yield similar results. Palladium was deposited at 100 K followed by an anneal to 900 K. The dispersion decreases monotonously with the palladium coverage from ca. 0.33 for 0po = 0.5 ml to ca. 0.05 for 0po = 9 ml. The inset of Fig. 3 shows the effects of annealing on the palladium dispersion. For palladium films deposited
X. Xu et al. / Catalysis Today 21 (1994) 57-69
62
P d / S i O 2 (100/~)
0.4-
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-mll • O(Pd).-0.7ML
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Annealing Temperature •
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i
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4
6
8
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Pd C o v e r a g e (ML) Fig. 3. Dispersion, defined as the ratio of surface atoms over total atoms, as a function of palladium coverage on the silica film. The dispersion was measured with oxygen chemisorption. The palladium was deposited at 100 K and annealed to 900 K. The inset shows the dispersion as a function of annealing temperature for two palladium coverages.
at 100 K, annealing decreases the dispersion, consistent with the aggregation of the palladium particles. The size of the palladium particles can be estimated based on the dispersion. Assuming a spherical shape for the particles, the average particle diameter (d) is approximately 11.2/D A where D is the dispersion [ 26]. For example, for 0r,d= 0.5 ml, the average particle size is ca. 30 ,~. STM and AFM can be used to measure directly the particle size and distribution. The STM and AFM studies were carded out in air using a NanoScope II (Digital Instruments). The particle size measured with STM and AFM agrees with the value estimated from the dispersion measurements. Fig. 4 shows STM images for a bare silica thin film on Mo(110) and for palladium particles ( 0pa = 0.5 ml) supported on the silica thin film. The bare silica thin film is essentially flat (Fig. 4a), such that the features observed for Pd/silica can be unequivocally attributed to palladium clusters. Fig. 4b shows the STM images of clusters in the size range of 20-40 ,~, in agreement with the dispersion measurement of ca. 30 ,~. For other coverages, the cluster size measured with STM and AFM also falls in the range of the dispersion estimate. AFM images for Pd/silica have been reported previously [26]. It is noted that STM is superior for small clusters whereas AFM is ideal for larger clusters. Unfortunately, no atomic resolved images were observed for the palladium particles, although atomic resolution for Cu clusters on silica has been reported [ 7 ]. The surface structure of the supported palladium clusters was investigated with infrared reflection-absorption spectroscopy of adsorbed CO. Fig. 5 shows IRAS spectra for CO on palladium particles annealed to the indicated temperatures. Palladium was deposited at 100 K and the IRAS spectra were collected for the model catalysts at 100 K in a background of 1 X 10- 6 YolT CO. For the as-deposited palladium film, two broad bands are observed at 2110 and 1990 c m - 1, correspond-
Xo Xu et al. / Catalysis Today 21 (1994) 57-69
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X. Xu et aL / Catalysis Today 21 (1994) 57~9
ing to CO adsorption in the atop and bridging sites. Annealing the films to above 500 K induces a new band at 1890 c m - 1, corresponding to the CO adsorption onto three-fold hollow sites. In addition, the peak width for the atop and bridging bands also narrows for the annealed films. The broad CO bands for films with annealing temperature less than 500 K indicate the inhomogeneity of the palladium film. Annealing leads to the formation of thermally stable facets, as indicated by the narrowing of the CO bands. These data also illustrate that there is a kinetic barrier for the growth of the more stable palladium clusters at low temperatures. For the large palladium particles that have been annealed, the majority surface facets are (111) and (100), as demonstrated in Fig. 6. At 100 K and 10 -6 Tort, CO adsorbs in the two-fold bridging sites on Pd(100) with a stretch frequency of 1995 c m - t [ 27]. On Pd( 111 ), at identical conditions, CO forms a (2 × 2) structure with 0.5 ml on the three-fold hollow sites and 0.25 ml on the atop sites, with stretching frequencies of 1895 and 2110 c m - 1, respectively [ 19]. The IRAS spectrum for CO on 0pd = 15 ml is essentially a combination of the spectra for CO/ Pd(100) and C O / P d ( 111 ) with a slight broadening due to CO adsorption at the facet boundaries. Decreasing the size of the palladium clusters increases the relative intensity of the atop CO band at 2110 c m - 1 and significantly broadens the bridging CO bands. The most stable palladium clusters can be described as cubo-octahedra as evidenced by the majority ( 111 ) and (100) facets. As the cluster size decreases, the density of edge atoms increases, leading to a broadening of the CO bands. 0.08% ted
2200
CO/Pd/Si02 (1O0/~)1 O(Pd)=15 ML | 1 Pc° =1°6 T°rr/ ~/ Ts-100K ~
2120 2040 1960 1880 1800
Frequency(cm-1) Fig. 5. Infrared reflection-absorption spectra for CO on a model silica-supported palladium catalyst (0pa = 15 monolayer) as a function of pre-annealing temperatures. The spectra were collected at 100 K and in 10 - 6 Tort CO background. The surface were annealed to 100 K, 300 K, 500 K, 700 K and 900 K, respectively. Fig. 4. Unprosessed STM images for (a) bare silica thin film ( ~ 100 ~ ) on Mo(110) and (b) palladium particles on the silica/Mo(110). The palladium ( 0pa = 0.5 monolayer) was deposited at room temperature and annealed to 900 K. The STM images were taken in air with a tunneling current of 1 nA and bias voltage of 100 mV.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
65
Pco-lO6Torr T-100K
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2200 21O0 2000 1900 1800 Frequency (cm-1) Fig. 6. Infrared reflection-absorption spectra for CO on Pd(100), Pd( 111 ) and model silica-supported palladium catalysts. The palladium coverages were 1, 7 and 15 monolayers. The model catalysts were annealed to 900 K. All spectra were collected at 100 K and in 10 -6 Torr CO background.
3.2. CO oxidation at low pressures
Fig. 7 shows the steady-state rate of CO oxidation on a silica-supported palladium catalyst as a function of pressure in the temperature range of 350-1000 K. The gas
Pd/SiO2 PC°/P°2=2 Ptotal/Torr e-I
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300 400 500 600 700 800 900 1000 Temperature(K) Fig. 7. Steady-state rate of CO oxidation on a Pd(5 ml)/silica catalyst as a function of the catalyst temperature and total pressure. The ratio of CO and 02 partial pressure is 2:1 and the total pressures were in the range of 2.2 × 10 -8 to 1.6 x 10 -6 Ton'.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
66
phase CO and 0 2 ratio was 2:1 and the total pressure was 2.2 X 10-8-1.6X 1 0 - 6 Torr. Decreasing the total pressure from 1 0 - 6 Torr to 1 0 - 8 Torr decreases the reaction rate above 400 K, but does not alter the trend of the rate as a function of temperature. The data were collected continuously while heating the sample at a rate of 1 K/s. The reactions are at steady state since data collected during sample cooling and heating are essentially identical. Clearly, there are three regions of the steady-state rate of CO oxidation on Pd/SiO2. At low temperatures, the reaction rate increases as a function of temperature to a maximum at ca. 500 K. The reaction rate is at its maximum and nearly constant between 500 K and 600 K. At high temperatures ( > 600 K), the reaction rate decreases with an increase in the reaction temperature. These trends are observed for all sizes of palladium particles. The pattern of CO oxidation rate on Pd/SiO2 at different temperatures is in excellent agreement with the work of Engel and Ertl [ 28 ] and Coulston and Hailer [ 29 ]. The CO oxidation reaction proceeds through a Langmuir-Hinshelwood mechanism. A generic rate law has been derived in a previous study from this laboratory, which adequately describes the observed kinetic behavior of the CO oxidation reaction [ 12]. Within the low temperature range, because the surface is covered with CO, the rate-limiting step for CO oxidation is the adsorption of oxygen. Increasing the temperature increases the CO desorption rate and the oxygen adsorption rate, thus the oxidation rate increases. At high temperatures, the CO oxidation rate decreases with a increase in temperature because of the decrease in the CO surface residence time. 3.3. CO oxidation at evaluated pressure (15 Torr) Fig. 8 shows the Arrhenius plot of CO oxidation on the model silica-supported palladium catalysts for three palladium coverages. The initial reaction pressures Temperature (K) 550 500
600 I
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Fig. 8. Arrhenius plot of the rate of CO oxidation on palladium ( 111 ), (110), (100) and silica-supported palladium catalysts. See text for details.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
67
were 10.0 Torr for CO and 5.0 Torr for 02; the reaction temperatures were between 540 K and 625 K. Conversions were maintained at less than 50%. The absolute rate was measured by the pressure decrease in the static reactor of known volume (750 cm3). The number of the reactive sites was determined by both carbon monoxide and oxygen temperature-programmed desorption. The reaction rate is expressed in terms of turnover frequency (TOF), i.e., molecules per site per second. The activation energies of CO oxidation on the model silica-supported palladium catalysts only vary slightly with the palladium coverage, i.e., particle size. For example, for the average particles larger than 75 A, the activation energy is constant at ca. 27 kcal/mol; for smaller particles (ca. 30 ,~) this value is somewhat lower (25 kcal/mol). The reaction rate (TOF) does not change with palladium particle size at these conditions, in agreement with previous studies showing that CO oxidation is structure-insensitive [ 30]. Both the activation energy and TOF for the CO oxidation on the model catalysts agree with the values found for single crystal and high-surface-area Pd catalysts. The CO oxidation reaction on single crystal palladium catalysts was also investigated in this study using the pressure-decrease method [ 31,32]. These data are also shown in Fig. 8 for CO oxidation on Pd(100), Pd(110) and Pd( 111 ) in the temperature range of 470-570 K, and Pco = 1 Torr and Po2 = 0.5 Torr. Each plot consists of more than 30 data points to assure the accuracy of the measurements of activation energy and turnover frequency. These measurements agree with previous results, and yield data with significantly smaller standard deviations. The apparent activation energies for each of the three Pd single crystals are similar, but distinctive. The activation energies are 29.4 ___0.3 kcal/mol for Pd (100) [ 31 ], 30.7 ___0.5 kcal/ mol for Pd(110) and 28.1 ___0.4 kcal/mol for Pd( 111 ) [ 32]. Although these numbers are close, they do fall well outside the standard deviation. Furthermore, the TOF for CO oxidation on Pd( 111 ) is more than a factor of three smaller than on Pd(100). These data suggest a degree of structure-sensitive of the CO oxidation reaction. In addition, there are two reports of CO oxidation on single crystal surfaces [Pd(110) and Pd(100) ] at high pressure conditions that have found the activation energy and reaction rate to vary with the crystal orientation [ 33,34]. Furthermore, numerous studies of CO oxidation on supported palladium catalysts have yielded activation energies in the range of 10-37 kcal/mol at high pressure ( > 1 Torr) conditions [ 35-38 ]. The reaction rate of CO oxidation at low temperature and high pressure conditions is governed by the expression o
Rate
k Po2 K~ Pco
(1)
where k is the oxygen adsorption rate constant and Ke is the CO equilibrium constant. At these reaction conditions, the CO coverage is near saturation, and forms a compressed overlayer at high pressures and low temperatures [ 19,271. The equilibrium constant is largely determined by the heats of adsorption and the surface
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X. Xu et al. /Catalysis Today 21 (1994) 57-69
order. The activation energies for CO oxidation, indeed, track the isosteric heats of adsorption of CO at the same CO coverage on Pd( 111 ) and Pd(100). The isosteric heats of adsorption have been measured in our laboratory using infrared reflectionabsorption spectroscopy [39]. The frequency and intensity of the IR absorption features are directly related to the CO coverage. A correlation has been found between the CO heats of adsorption and the activation energy for CO oxidation on Pd( 111 ) and Pd(100). At the reaction conditions utilized, IRAS spectra demonstrate that the coverage of CO on the palladium surfaces is ca. 0.5 monolayers. Although the CO equilibrium adsorption constant varies with different palladium surfaces, the degree of variation is small, since the heats of CO adsorption are largely determined by the interaction between CO and palladium and the lateral interaction in the compressed CO overlayers. As a result, the rates and apparent activation energies for CO oxidation on the Pd(100), Pd(110) and Pd( 111 ) are similar, but distinctive.
4. Conclusion Model silica-supported palladium catalysts can be prepared by evaporating palladium onto silica thin films, and can be characterized with an array of surface science techniques including STM and AFM. The size of the palladium particle is quite uniform based on STM and AFM images. The metal cluster size increases with the coverage from ca. 30 ,~ for 0.5 ml to 500 ,~ for higher coverages. The desorption energy of palladium from a silica thin film increases with the particle size. The surface of the large palladium particles consists mainly of (111 ) and (100) facets. The model silica-supported palladium catalysts exhibit catalytic activity similar to palladium single crystals for CO oxidation at both low (10 - 6 Torr) and high pressures (15 Torr). At low pressures, the rate of CO oxidation increases with temperature reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover frequency for the CO oxidation reaction on the model catalysts fall within the same range as for the single crystals. Our work on single crystal palladium surfaces, however, indicates that CO oxidation on palladium is structure sensitive to a certain degree. The turnover frequency and activation energy for CO oxidation on Pd(111), Pd(110) and Pd(100) are similar, yet distinctive.
Acknowledgements 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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Catalysis Today 21 (1994) 57-69
catalysis today
Structural and catalytic properties of model silicasupported palladium catalysts: a comparison to single crystal surfaces X u e p i n g Xu, Jfinos Szanyi, Q i a n g Xu, D. W a y n e G o o d m a n * Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
Abstract
The structural and catalytic properties of model silica-programmed desorption, infrared reflectionabsorption spectroscopy of adsorbed CO, scanning tunneling and atomic force microscopies (STM and AFM), and catalytic CO oxidation at both low pressure and elevated pressure conditions. The CO oxidation reactions on Pd(111), Pd(110) and Pd(100) have also been investigated. By evaporating palladium onto silica thin films (100 /~), followed by an anneal to 900 K, the size of the palladium particles can be controlled in a range of 30-500 ,~. The surface of the palladium particles consists mainly of ( 111 ) and (100) facets, and exhibit catalytic activity similar to palladium single crystals for CO oxidation at both low pressure (10-8-10 -6 Torr) and high pressure (15 Torr) conditions. At low pressures, the rate of CO oxidation increases with temperature, reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover frequency for the CO oxidation reaction on the model catalysts compare favorable with analogous results from single crystal and high-surface-area catalysts. Thus, this system of metal particles supported on a silica thin film provides an excellent model to bridge between single crystal and high-surface-area catalysts. The CO oxidation reactions on Pd( 111 ), Pd(110) and Pd(100) have similar, but distinctive activation energies (28.1 _ 0.4, 30.7 + 0.5 and 29.4 + 0.3 kcal/mol, respectively) and turnover frequencies, indicating a subtle structure-sensitivity for CO oxidation on different crystal planes of palladium.
I. Introduction Understanding the basic phenomena of heterogeneous catalytic processes is a crucial step toward developing new, highly active and selective catalysts. With the development of surface science techniques in the last two decades, many catalytic reactions have been modelled in the ultra-high vacuum (UHV) environment on * Correspondingauthor. 0920-5861/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI0920-5861 (94)00033-X
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X. Xu et aL / Catalysis Today 21 (1994) 57-69
metal single crystal surfaces [ 1 ]. However, there is a lingering concern with respect to the application of surface science to heterogeneous catalysis because of the extreme differences in the reaction conditions [2]. Surface science studies under well-defined conditions are capable of providing insights into fundamental surface processes. Due to a pressure difference of more than 10 orders of magnitude between UHV conditions and real catalytic reaction conditions, certain important surface processes during catalytic reactions may not be accessible in UHV studies. In addition, most surface science studies are performed on well-defined single crystal surfaces, whereas the practical catalysts are dispersed on supports. It has been documented that the interaction between metal and its support may alter the catalytic reactivity [ 3 ]. Furthermore, the reactivity and selectivity for some catalytic reactions vary with the particle size of metal catalysts [4]. In order to model oxide-supported metal catalysts, metals of catalytic interests have been deposited onto single crystal oxides such as ZnO and TiO2 [ 5 ]. However, these model systems are much less than ideal due to non-uniform heating and cooling, and their susceptibility to charging when probed with charged-particle beams. An alternative model system for oxide-supported metal catalysts is a metal single crystal covered partially with a support oxide [6]. These model systems still differ decidedly from the industrial metal catalysts which are dispersed onto the supporting materials. These supported metal particles may have catalytic properties distinctly different from the bulk metals. Recently, we have developed a new approach to modelling supported metal catalysts that circumvents the experimental difficulties associated with bulk oxides. These model systems, which very closely mimic the actual catalysts [7-9], consist of an oxide thin film, such as silica, deposited onto a refractory metal surface. Metals of catalytic interest, such as copper or nickel, are then evaporated onto the oxide film. The refractory metal surface [e.g. M o ( l l 0 ) ] not only supports the oxide film, but also provides a metallic reflecting surface which facilitates the use of infrared reflection-absorption spectroscopy (IRAS) [9]. The Mo(110) substrate, however, does not alter the chemical properties of the ca. 100 ,~ silica film. Furthermore, silica films on Mo(110) are better models than thermally grown SiO2 films on silicon substrate, because SiO2 on Mo(110) is stable up to 1500 K [ 10], whereas SiO2 on Si decomposes at relatively low temperatures [ 11]. The higher thermal stability of silica on Mo(110) facilitates the thermal desorption of the supported metal catalyst from the silica film. In this paper, we present the structural and catalytic properties of model silicasupported palladium catalysts. The structure of the palladium particles are characterized with scanning tunneling microscopy (STM), atomic force microscopy (AFM), Fourier transform infrared reflection-absorption spectroscopy of adsorbed CO molecules (FT-IRAS) and dispersion measurements using the chemisorption of oxygen and carbon monoxide. The catalytic properties of the model silicasupported palladium catalysts have been studied with CO oxidation at both low
X. Xu et al./ Catalysis Today 21 (1994) 57-69
59
(10-8-10 -6 Torr) and elevated pressures (15 Torr) [ 12], and are compared to results from single crystal palladium surfaces.
2. Experimental All experiments were carried out in UHV chambers coupled to a high pressure reaction cell [ 12 ]. The chambers are equipped with an Auger electron spectrometer (AES), a quadrupole mass spectrometer, low energy electron diffraction (LEED) and a Mattson Cygnus 100 infrared spectrometer. The high pressure reaction cell is directly coupled to the UHV chamber via a Teflon sliding-seal interface. The high pressure cell is outfitted with flange-mounted CaF2 windows for in situ fourier transform infrared reflection-absorption spectroscopy (FT-IRAS). The IRAS spectra were acquired with single beam optics adjusted for 85 ° incident angle. The Pd( 111 ), Pd(110) and Pd(100) single crystals were cleaned with cycles of annealing in oxygen at 900 K and a subsequent flash to 1200 K in vacuo. The surfaces were analyzed with AES and LEED before and after the high pressure reactions. The model silica-supported palladium catalysts were prepared by evaporating palladium onto a silica thin film (ca. 100 ,~) and annealing to 900 K [12]. The palladium evaporation source consisted of a 0.25 mm Pd wire (99.997%, Johnson Matthey Chemical Limited) wrapped around a tungsten filament, and a line-ofsight mass spectrometer to monitor and control the palladium flux. Palladium was also deposited onto a clean Mo (110) surface. Temperature-programmed desorption of Pd/Mo(110) shows two distinct peaks at 1500 K and 1300 K, corresponding to monolayer and multilayer desorption, respectively [ 13 ]. The monolayer peak was used to calibrate the palladium flux. The amount of palladium on the silica film was controlled by flux and deposition time, and determined by temperature-programmed desorption of Pd/silica/Mo(110). The initial sticking probability of palladium on silica was determined to be unity at substrate temperatures below 500 K, in contrast to copper which has an initial sticking probability of ca. 0.3 at room temperature [ 14]. The preparation and characterization of silica thin films on Mo(110) have been described in refs. [ 10,15,16]. Briefly, the silica films, which have the properties of vitreous silica [ 10,16], were prepared by evaporating silicon in ca. 1 × 10 -5 Torr oxygen and subsequently annealing to 1400 K. The steady-state rate of CO oxidation at low pressure (10-8-10 -6 Torr) was measured by monitoring the CO2 intensity with mass spectrometry. Carbon monoxide and oxygen were admitted to the UHV chamber through two separate leak valves to the desired partial pressure. Since the system is continuously pumped, the CO2 formation rate is proportional to the increase in the CO2 pressure above background. The rate of CO oxidation in the high pressure ( 15 Torr) batch reactor was measured via gas phase CO2 IR absorption and the change in the total reactor pressure. The rate of CO oxidation in the latter measurement is proportional to the
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rate of the pressure decrease. The two methods yielded comparable results; however, rates obtained via the pressure drop method are, generally, more accurate and convenient. The CO oxidation rate on a passivated crystal, coated with a silica thin film on both faces, was one order of magnitude smaller than the silica-supported catalysts, demonstrating that background reaction on the heating leads was minimal. The surface area of the palladium single crystals was calculated using both the front and back faces of the crystal, whereas the surface area of the supported palladium catalysts was determined via temperature-programmed desorption of oxygen and carbon monoxide.
3. Results and discussion
3.1. Characterization of the model Pal/silica catalysts Fig. 1 shows the temperature-programmed desorption of palladium from a silica thin film. Palladium desorbs in a single peak in the temperature range of 10001300 K. This peak corresponds to the sublimation from the metallic palladium particles. The activation energy for the sublimation, calculated from the leading edge of the desorption profile, increases with palladium coverage (particle size) from 55 kcal/mol for 0pd = 1 monolayer (ml) to 76 kcal/mol for 0pd = 5 ml. The heat of sublimation of bulk palladium is 90 kcal/mol [ 17]. A similar behavior in the variation of sublimation heats with metal coverage has been observed for Cu/ Pd/SiO2 (100/~)
A
e-
co E 0
I
1000
I
I
1100
I
1200
I
I
1300
I
I
1400
I
1500
Temperature (K) Fig. l. Temperature-programmed desorption spectra for palladium on silica( 100 /k)/Mo(110). The nominal palladium coverages are 0.6, 0.9, 1.5, 3.6 and 4.7 monolayers referencing to clean Mo(110). One monolayer corresponds to 1.3 × 1015 atoms/cm 2.
X. Xu et al. / Catalysis Today 21 (1994) 5 7 4 9
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TPD, O/PdlSiO2(I (30A)IMo(II O)
0
cl} (/}
I
b 500
700
I
I
900
I
I
1100
I
I
1 00
I
1500
T e m p e r a t u r e (K)
Fig. 2. Temperature-programmed desorption spectra for O/Pd/silica/Mo(110). The palladium coverage was 6 × 10 t5 atoms/cm 2. Oxygen was adsorbed at 100 K to saturation.
silica [8 ]. The lower sublimation energy for smaller coverages arises due to the lower coordination of the surface metal atoms on the smaller particles [ 8 ]. The average size of the palladium particles was determined using chemisorption, STM and AFM. STM and AFM, performed in air after the samples were prepared in the UHV chamber, yield a direct image of the metal clusters. Chemisorption of oxygen and carbon monoxide measures the amount of metal exposed based on temperature-programmed desorption. The saturation coverage for CO is ca. 0.5 monolayers on palladium at room temperatures [ 18,19 ]. For oxygen the saturation coverage was assumed to be 0.33 ml, the average of the saturation oxygen coverage on single crystal palladium surfaces when oxygen is adsorbed below room temperatures [ 20-22]. Desorption from saturation CO/Mo (110) ( 0co = 0.5 ml) [ 23 ] was used as a calibration for carbon monoxide. Oxygen desorption from a palladium oxide (PdO) film on silica, prepared in situ by evaporating palladium in 1 × 10 -6 Torr oxygen [ 24], was used as a reference for oxygen. Fig. 2 shows the oxygen temperature-programmed desorption from O/Pd/silica/ Mo(110). Oxygen desorption occurs primarily in the temperature region 700K900 K, in agreement with oxygen desorption from palladium single crystal surfaces and foils [ 20-22 ]. In addition, a small oxygen desorption peak is observed at 12001300 K, which is in the same temperature range of palladium sublimation. From these TPD experiments the amount of oxygen and palladium can be quantified. These results suggest that a small percentage of surface oxygen is diffused into the bulk of palladium particles during heating. Oxygen diffusion into bulk palladium has been inferred in earlier palladium single crystal studies [ 25 ]. Fig. 3 shows the dispersion of palladium on silica thin films as a function of coverage based on the oxygen TPD measurements; measurements based on CO TPD yield similar results. Palladium was deposited at 100 K followed by an anneal to 900 K. The dispersion decreases monotonously with the palladium coverage from ca. 0.33 for 0po = 0.5 ml to ca. 0.05 for 0po = 9 ml. The inset of Fig. 3 shows the effects of annealing on the palladium dispersion. For palladium films deposited
X. Xu et al. / Catalysis Today 21 (1994) 57-69
62
P d / S i O 2 (100/~)
0.4-
0.5 .-~ 0.4
-mll • O(Pd).-0.7ML
P
0.3
c: 0.3 .0
E
•
0.2
1
e(Pd]-3.8 ML
0.1 0.0
a. 0.2
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Annealing Temperature •
0.1
0.0
0
] 2
OO
i
I
I
4
6
8
10
Pd C o v e r a g e (ML) Fig. 3. Dispersion, defined as the ratio of surface atoms over total atoms, as a function of palladium coverage on the silica film. The dispersion was measured with oxygen chemisorption. The palladium was deposited at 100 K and annealed to 900 K. The inset shows the dispersion as a function of annealing temperature for two palladium coverages.
at 100 K, annealing decreases the dispersion, consistent with the aggregation of the palladium particles. The size of the palladium particles can be estimated based on the dispersion. Assuming a spherical shape for the particles, the average particle diameter (d) is approximately 11.2/D A where D is the dispersion [ 26]. For example, for 0r,d= 0.5 ml, the average particle size is ca. 30 ,~. STM and AFM can be used to measure directly the particle size and distribution. The STM and AFM studies were carded out in air using a NanoScope II (Digital Instruments). The particle size measured with STM and AFM agrees with the value estimated from the dispersion measurements. Fig. 4 shows STM images for a bare silica thin film on Mo(110) and for palladium particles ( 0pa = 0.5 ml) supported on the silica thin film. The bare silica thin film is essentially flat (Fig. 4a), such that the features observed for Pd/silica can be unequivocally attributed to palladium clusters. Fig. 4b shows the STM images of clusters in the size range of 20-40 ,~, in agreement with the dispersion measurement of ca. 30 ,~. For other coverages, the cluster size measured with STM and AFM also falls in the range of the dispersion estimate. AFM images for Pd/silica have been reported previously [26]. It is noted that STM is superior for small clusters whereas AFM is ideal for larger clusters. Unfortunately, no atomic resolved images were observed for the palladium particles, although atomic resolution for Cu clusters on silica has been reported [ 7 ]. The surface structure of the supported palladium clusters was investigated with infrared reflection-absorption spectroscopy of adsorbed CO. Fig. 5 shows IRAS spectra for CO on palladium particles annealed to the indicated temperatures. Palladium was deposited at 100 K and the IRAS spectra were collected for the model catalysts at 100 K in a background of 1 X 10- 6 YolT CO. For the as-deposited palladium film, two broad bands are observed at 2110 and 1990 c m - 1, correspond-
Xo Xu et al. / Catalysis Today 21 (1994) 57-69
63
64
X. Xu et aL / Catalysis Today 21 (1994) 57~9
ing to CO adsorption in the atop and bridging sites. Annealing the films to above 500 K induces a new band at 1890 c m - 1, corresponding to the CO adsorption onto three-fold hollow sites. In addition, the peak width for the atop and bridging bands also narrows for the annealed films. The broad CO bands for films with annealing temperature less than 500 K indicate the inhomogeneity of the palladium film. Annealing leads to the formation of thermally stable facets, as indicated by the narrowing of the CO bands. These data also illustrate that there is a kinetic barrier for the growth of the more stable palladium clusters at low temperatures. For the large palladium particles that have been annealed, the majority surface facets are (111) and (100), as demonstrated in Fig. 6. At 100 K and 10 -6 Tort, CO adsorbs in the two-fold bridging sites on Pd(100) with a stretch frequency of 1995 c m - t [ 27]. On Pd( 111 ), at identical conditions, CO forms a (2 × 2) structure with 0.5 ml on the three-fold hollow sites and 0.25 ml on the atop sites, with stretching frequencies of 1895 and 2110 c m - 1, respectively [ 19]. The IRAS spectrum for CO on 0pd = 15 ml is essentially a combination of the spectra for CO/ Pd(100) and C O / P d ( 111 ) with a slight broadening due to CO adsorption at the facet boundaries. Decreasing the size of the palladium clusters increases the relative intensity of the atop CO band at 2110 c m - 1 and significantly broadens the bridging CO bands. The most stable palladium clusters can be described as cubo-octahedra as evidenced by the majority ( 111 ) and (100) facets. As the cluster size decreases, the density of edge atoms increases, leading to a broadening of the CO bands. 0.08% ted
2200
CO/Pd/Si02 (1O0/~)1 O(Pd)=15 ML | 1 Pc° =1°6 T°rr/ ~/ Ts-100K ~
2120 2040 1960 1880 1800
Frequency(cm-1) Fig. 5. Infrared reflection-absorption spectra for CO on a model silica-supported palladium catalyst (0pa = 15 monolayer) as a function of pre-annealing temperatures. The spectra were collected at 100 K and in 10 - 6 Tort CO background. The surface were annealed to 100 K, 300 K, 500 K, 700 K and 900 K, respectively. Fig. 4. Unprosessed STM images for (a) bare silica thin film ( ~ 100 ~ ) on Mo(110) and (b) palladium particles on the silica/Mo(110). The palladium ( 0pa = 0.5 monolayer) was deposited at room temperature and annealed to 900 K. The STM images were taken in air with a tunneling current of 1 nA and bias voltage of 100 mV.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
65
Pco-lO6Torr T-100K
Pd(lO0) 8
. . . .
¢ ¢1
Pd/Silica/Mo(110)j
J
<
I
I
I
I
I
I
I
2200 21O0 2000 1900 1800 Frequency (cm-1) Fig. 6. Infrared reflection-absorption spectra for CO on Pd(100), Pd( 111 ) and model silica-supported palladium catalysts. The palladium coverages were 1, 7 and 15 monolayers. The model catalysts were annealed to 900 K. All spectra were collected at 100 K and in 10 -6 Torr CO background.
3.2. CO oxidation at low pressures
Fig. 7 shows the steady-state rate of CO oxidation on a silica-supported palladium catalyst as a function of pressure in the temperature range of 350-1000 K. The gas
Pd/SiO2 PC°/P°2=2 Ptotal/Torr e-I
1.6xl 0 -6 4.8xl 0"7 1.6x10 -7
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®
rr O~ LL~ .o_
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i I
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I
I
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300 400 500 600 700 800 900 1000 Temperature(K) Fig. 7. Steady-state rate of CO oxidation on a Pd(5 ml)/silica catalyst as a function of the catalyst temperature and total pressure. The ratio of CO and 02 partial pressure is 2:1 and the total pressures were in the range of 2.2 × 10 -8 to 1.6 x 10 -6 Ton'.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
66
phase CO and 0 2 ratio was 2:1 and the total pressure was 2.2 X 10-8-1.6X 1 0 - 6 Torr. Decreasing the total pressure from 1 0 - 6 Torr to 1 0 - 8 Torr decreases the reaction rate above 400 K, but does not alter the trend of the rate as a function of temperature. The data were collected continuously while heating the sample at a rate of 1 K/s. The reactions are at steady state since data collected during sample cooling and heating are essentially identical. Clearly, there are three regions of the steady-state rate of CO oxidation on Pd/SiO2. At low temperatures, the reaction rate increases as a function of temperature to a maximum at ca. 500 K. The reaction rate is at its maximum and nearly constant between 500 K and 600 K. At high temperatures ( > 600 K), the reaction rate decreases with an increase in the reaction temperature. These trends are observed for all sizes of palladium particles. The pattern of CO oxidation rate on Pd/SiO2 at different temperatures is in excellent agreement with the work of Engel and Ertl [ 28 ] and Coulston and Hailer [ 29 ]. The CO oxidation reaction proceeds through a Langmuir-Hinshelwood mechanism. A generic rate law has been derived in a previous study from this laboratory, which adequately describes the observed kinetic behavior of the CO oxidation reaction [ 12]. Within the low temperature range, because the surface is covered with CO, the rate-limiting step for CO oxidation is the adsorption of oxygen. Increasing the temperature increases the CO desorption rate and the oxygen adsorption rate, thus the oxidation rate increases. At high temperatures, the CO oxidation rate decreases with a increase in temperature because of the decrease in the CO surface residence time. 3.3. CO oxidation at evaluated pressure (15 Torr) Fig. 8 shows the Arrhenius plot of CO oxidation on the model silica-supported palladium catalysts for three palladium coverages. The initial reaction pressures Temperature (K) 550 500
600 I
I
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1000
¢0
100
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,
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I 1.70
I 1.90
1000/1" (K -1)
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Fig. 8. Arrhenius plot of the rate of CO oxidation on palladium ( 111 ), (110), (100) and silica-supported palladium catalysts. See text for details.
X. Xu et al. / Catalysis Today 21 (1994) 57-69
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were 10.0 Torr for CO and 5.0 Torr for 02; the reaction temperatures were between 540 K and 625 K. Conversions were maintained at less than 50%. The absolute rate was measured by the pressure decrease in the static reactor of known volume (750 cm3). The number of the reactive sites was determined by both carbon monoxide and oxygen temperature-programmed desorption. The reaction rate is expressed in terms of turnover frequency (TOF), i.e., molecules per site per second. The activation energies of CO oxidation on the model silica-supported palladium catalysts only vary slightly with the palladium coverage, i.e., particle size. For example, for the average particles larger than 75 A, the activation energy is constant at ca. 27 kcal/mol; for smaller particles (ca. 30 ,~) this value is somewhat lower (25 kcal/mol). The reaction rate (TOF) does not change with palladium particle size at these conditions, in agreement with previous studies showing that CO oxidation is structure-insensitive [ 30]. Both the activation energy and TOF for the CO oxidation on the model catalysts agree with the values found for single crystal and high-surface-area Pd catalysts. The CO oxidation reaction on single crystal palladium catalysts was also investigated in this study using the pressure-decrease method [ 31,32]. These data are also shown in Fig. 8 for CO oxidation on Pd(100), Pd(110) and Pd( 111 ) in the temperature range of 470-570 K, and Pco = 1 Torr and Po2 = 0.5 Torr. Each plot consists of more than 30 data points to assure the accuracy of the measurements of activation energy and turnover frequency. These measurements agree with previous results, and yield data with significantly smaller standard deviations. The apparent activation energies for each of the three Pd single crystals are similar, but distinctive. The activation energies are 29.4 ___0.3 kcal/mol for Pd (100) [ 31 ], 30.7 ___0.5 kcal/ mol for Pd(110) and 28.1 ___0.4 kcal/mol for Pd( 111 ) [ 32]. Although these numbers are close, they do fall well outside the standard deviation. Furthermore, the TOF for CO oxidation on Pd( 111 ) is more than a factor of three smaller than on Pd(100). These data suggest a degree of structure-sensitive of the CO oxidation reaction. In addition, there are two reports of CO oxidation on single crystal surfaces [Pd(110) and Pd(100) ] at high pressure conditions that have found the activation energy and reaction rate to vary with the crystal orientation [ 33,34]. Furthermore, numerous studies of CO oxidation on supported palladium catalysts have yielded activation energies in the range of 10-37 kcal/mol at high pressure ( > 1 Torr) conditions [ 35-38 ]. The reaction rate of CO oxidation at low temperature and high pressure conditions is governed by the expression o
Rate
k Po2 K~ Pco
(1)
where k is the oxygen adsorption rate constant and Ke is the CO equilibrium constant. At these reaction conditions, the CO coverage is near saturation, and forms a compressed overlayer at high pressures and low temperatures [ 19,271. The equilibrium constant is largely determined by the heats of adsorption and the surface
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X. Xu et al. /Catalysis Today 21 (1994) 57-69
order. The activation energies for CO oxidation, indeed, track the isosteric heats of adsorption of CO at the same CO coverage on Pd( 111 ) and Pd(100). The isosteric heats of adsorption have been measured in our laboratory using infrared reflectionabsorption spectroscopy [39]. The frequency and intensity of the IR absorption features are directly related to the CO coverage. A correlation has been found between the CO heats of adsorption and the activation energy for CO oxidation on Pd( 111 ) and Pd(100). At the reaction conditions utilized, IRAS spectra demonstrate that the coverage of CO on the palladium surfaces is ca. 0.5 monolayers. Although the CO equilibrium adsorption constant varies with different palladium surfaces, the degree of variation is small, since the heats of CO adsorption are largely determined by the interaction between CO and palladium and the lateral interaction in the compressed CO overlayers. As a result, the rates and apparent activation energies for CO oxidation on the Pd(100), Pd(110) and Pd( 111 ) are similar, but distinctive.
4. Conclusion Model silica-supported palladium catalysts can be prepared by evaporating palladium onto silica thin films, and can be characterized with an array of surface science techniques including STM and AFM. The size of the palladium particle is quite uniform based on STM and AFM images. The metal cluster size increases with the coverage from ca. 30 ,~ for 0.5 ml to 500 ,~ for higher coverages. The desorption energy of palladium from a silica thin film increases with the particle size. The surface of the large palladium particles consists mainly of (111 ) and (100) facets. The model silica-supported palladium catalysts exhibit catalytic activity similar to palladium single crystals for CO oxidation at both low (10 - 6 Torr) and high pressures (15 Torr). At low pressures, the rate of CO oxidation increases with temperature reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover frequency for the CO oxidation reaction on the model catalysts fall within the same range as for the single crystals. Our work on single crystal palladium surfaces, however, indicates that CO oxidation on palladium is structure sensitive to a certain degree. The turnover frequency and activation energy for CO oxidation on Pd(111), Pd(110) and Pd(100) are similar, yet distinctive.
Acknowledgements 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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