Cluster size dependent kinetics for the oxidation of CO on a PdMgO(100) model catalyst

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Surface Science 352-354 (1996) 457-462

Cluster size dependent kinetics for the oxidation of CO on a Pd/MgO(100) model catalyst C. Becker * ,1, C.R. Henry CRMC2-CNRS, Campus de Luminy, F-13288 Marseille Cedex 9, France

Received 5 September 1995; accepted for publication31 October 1995

Abstract

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A kinetic study of the dependence of the oxidation of CO on the size of Pd clusters supported on Mg0(100)has been carried out using molecular beam relaxation spectroscopy (MBRS) and transmission electron microscopy (TEM). The reaction was studied in the temperature range from 400 to 600 K and in the cluster size range from 2.8 nm to 13 nm at O 2 and CO partial pressures of about 2 × 1 0 -5 Pa. It has been found that the steady-state reaction rate displays a strong dependence on the clusters size and cluster density. This effect is mainly due to spillover of CO from the support t0 the clusters. Even though this suggests that the reaction proceeds quasi-identical for the different cluster sizes large differences in the shape of the MBRS signals, that is the kinetics of the reaction, have been found as a function of cluster size. The simulation of the reaction rate as a function of temperature for different cluster sizes shows that the ~pillover cannot explain the data for small cluster sizes. Therefore, we conclude that a size effect exists for this particular reaction. Furthermore; we have been able to show that for temperatures below 500 K a second strongly boundiCO adsorption state exists with a residence time that is about 10 times larger than that of the known CO state on single crystal surfaces. Keywords: Carbon dioxide; Carbon monoxide; Catalysis; Clusters; Magnesiumoxides; Oxygen;Palladium;Surface chemical reaction

1. Introduction

The oxidation of CO is one of the key reactions in cleaning hydrocarbon combustion exhausts. Therefore, much attention has been payed in the past to understand the underlying processes. The mechanism of the reaction on Pd and Pt single crystal surfaces has been thoroughly studied by Engel and Ertl [1,2] and is now well established. These authors have

* Corresponding author. Fax: +49 228 7..32551. i Permanentaddress: Insitut fiir Physikalischeand Theoretische Chemie der UniversitatBonn, D-53115 Bonn, Germany.

found that the reaction rate as a function Of tempera= ture is independent of the crystallographic orientation of the surfacel Rea&ions displaying this behavior w e r e said to be structure insensitive, that is, should not depend on the size 0f the catalyst particles [3]. A number of investigations of Pd particles on different supports seem to confirm :this hypothesis [4-7]. Later it was, however, s h o w n that the CO adsorption energy shows a strong increase for Cluster sizes below 5 n m [8-10]. This increase in adsorption energy can be translated ~to higher CO equilibrium coverage. As the CO adsorption and coverage . pre= sents the l i m i t i n g factor f o r the reaction rate at temperature > 500 K, we w o u l d expect to find a

0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)01179-X

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C. Becker, C.R. Henry/Surface Science 352-354 (1996)457-462

size effect in the reaction rate and kinetics in this temperature range. The goal of our investigations was to demonstrate that there exists, besides the apparent size effect caused by the spillover, a real size effect for the catalytic oxidation of CO on a P d / M g O ( 1 0 0 ) model catalyst.

2. E x p e r i m e n t a l procedure The experiments were performed in an U H V system with a base pressure of 3 x 10 -s Pa. The system is equipped with facilities for in situ cleavage and preparation of the MgO(100) supports, a Knudson cell for Pd evaporation, an Auger electron spectrometer (AES) and a differentially pumped modulated supersonic molecular beam source for reactive scattering (MBRS). The equivalent pressure o f the CO beam at the sample surface is 3.5 X 10 -5 Pa. The oxygen can be introduced isotropically into the system using a standard U H V leak valve. For the experiments presented here the oxygen pressure was 1.3 X 10 -5 Pa. The CO and CO 2 desorption rates are measured by a differentially pumped mass spectrometer. The Pd clusters were produced by evaporation of Pd onto the substrate and subsequent annealing of the sample. B y using different substrate temperatures during the evaporation and changing the total amount Pd deposited different cluster sizes and densities can be produced. After each reaction experiment the cluster size distribution and cluster densities were measured ex situ using transmission electron microscopy (TEM). For the details of the experimental set-up and the preparation of the clusters the reader is referred to a previous paper [8]. By the procedure described above three different cluster collections were prepared. Type 1 with a mean diameter D = 13 nm and a number density n s = 1.5 × 1011 cm -2, Type 2 with a mean diameter D = 6.8 nm and a number density n s = 1.5 X 101~ cm -2, and Type 3 with a mean diameter D -- 2.8 nm and a number density n s = 2 x 1012 cm -2. The calibration of the total area exposed of the clusters was done in two ways. The first way was to calculate this area from the TEM images assuming a certain shape and using a fixed height to diameter ratio for the clusters. This is, however, not very precise because it

is based on a couple of assumptions. The best w a y t o get the total Pd area is by a CO titration technique. The surface was exposed to oxygen saturation. This oxygen covered surface then was exposed to the CO beam. By recording and integrating the signal of the desorbing CO 2 the exact amount of CO 2 produced by a saturation coverage oxygen can be calculated. Therefore, all turnover numbers in this communication are given in oxygen monolayers per second (ML o s - l ) . The two calibration methods yield, within 10%, the same results assuming that the oxygen saturation coverage on the clusters is 0.25 M L as on the P d ( l l l ) and Pd(100) surfaces [2].

3. Results Fig. 1 shows part of a TEM image of a cluster distribution of Type 3 ( D = 2.8 nm, n s = 2 X 1012 c m - 2 ) . To obtain the size distribution shown in Fig. 2 several TEM images were analyzed, taking into account about 2000 particles. As one can see the size distribution is rather narrow with a half width of about 25% of the mean diameter. Diffraction measurements have shown that the particles are in epitaxy on the substrate for all types of distributions presented here.

Fig. 1. TEM image of Pd clusters of Type 3. The horizontal bar in the image corresponds to 10 nm.

C. Becket, C.R. Henry/Surface Science 352-354 (1996) 457-462

Fig. 3 shows MBRS spectra for two different mean cluster sizes obtained at 433 K. The top graph corresponds to small Pd particles of Type 3. The spectrum is characterized by three different regions. Just after the CO beam is switched on the CO 2 production rate rises rapidly to a maximum value and starts then to fall again. It reaches its equilibrium value in region II. After the CO beam is stopped the CO 2 production rate drops almost to zero but starts rising again to finally drop to zero about 20 s after the end of the CO pulse (region III). The transition region originates from the fact that we have allowed for the build-up of a complete oxygen layer (0.25 NIL) on the clusters before opening the CO beam. Therefore, under the conditions used for the spectra in Fig. 3, we start the reaction in region I with an excess oxygen coverage that yields a CO 2 production rate that is higher than the equilibrium production rate in region II. After the end of the CO pulse we reach region III where the CO 2 production rate initially decreases. This decrease is due to the use up of adsorbed O and CO. After a few seconds, however, the rate goes up again. This can be explained by a reaction of CO that remained on the surface with oxygen that is adsorbed from the gas phase after the end of the pulse. A couple of important conclusions can be drawn from this observation. Since the reaction goes on for more than 10 s after the end of the CO pulse, the lifetime of this adsorbed CO species has to be fairly

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large compared with the lifetime of CO on a large particle [8] or an extended Pd(111) surface [1], which is 0.3 s at this temperature. For particles of mean diameter D = 2.8 nm previous measurements gave at low CO coverage ( < 0.02 ML) an adsorption energy of 138 kJ m o l - i and a frequency factor of 5 × 10 ]5 s - 1 [10] The lifetime at 433 K for these values is 9 s in good agreement with the observations presented here. Furthermore, the equilibrium CO coverage must be larger than the equilibrium oxygen coverage under these reaction conditions, otherwise all CO would rapidly react to form CO 2 and the peak in region III would not appear. This is actually the case at higher

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C. Becker, C.R. Henry/Surface Science 352-354 (1996) 457--462

temperatures (T > 480 K) where the equilibrium CO coverages are smaller than the equilibrium oxygen coverages. Assuming that the production rate of CO 2 is proportional to the oxygen coverage, one can also conclude, that 0 2 adsorption is the rate limiting step of the process under these experimental conditions. For larger clusters of Type 1 we find qualitatively the same behaviour (Fig. 3 bottom). All the features that have been discussed for the small clusters are also found for large clusters, however, in a less pronounced way. The comparison of the two spectra shows that there are in fact quantitative differences for the two cluster types. The ratios of the reaction rates in the regions I, II, and III are totally different. This suggests that the kinetics of the reaction are not the same, that is, a size effect for the oxidation of CO on supported Pd clusters exists. MBRS spectra like the ones shown in Fig. 3 were taken for the different types in temperature steps of 20 K. From these spectra the steady-state reaction rates were deduced. The resulting experimental values are presented in Fig. 4, which shows a plot of the reaction rate as a function of temperature. It can easily be seen that the number of CO 2 molecules produced per unit time shows an apparent size effect. Previous studies suggested that the differences in production rates for different cluster sizes can be explained by a spillover of CO from the support to the clusters assuming that the reaction rates on the Clusters show no size effect [7-11 ]. Given the difference in the shape of the MBRS spectra for different cluster types this result is quite surprising. We have tried to model the reaction rate curves with a model that has been proposed by Henry [11,12]. Due to space limitations only a brief description o f the model is given here. For the details the reader is referred to the original publication. It is a collection zone model that treats the CO molecule diffusion on the substrate to the clusters. This diffusion will eventually increase the net arrival rate of CO molecules on the cluster and thus change the reaction rate. The shape of the clusters is assumed to be hemispherical. Each cluster is in the center of a circular collection zone. The turnover number (TON), or reaction rate, is given by: TON=

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Fig. 4. Turnover number as a function of temperature for clusters of Type 1 (O), Type 2 (,x), and Type 3 ([3). The top graph shows in addition to that the calculated turnover numbers for clusters of Type 1 (solid line), Type 2 (dashed line), Type 3 (dotted line), and single crystals (dash-dotted line). The bottom graph shows the best fits to the experimental data.

where N O = 1.3 × 1015 c m - 2 is the mean surface density of Pd atoms, J the impinging CO flux, S(T) the reaction probability, a the sticking probability on the substrate, X s the mean diffusion distance of a CO molecule on the substrate, R the cluster radius, and P(x, y) the function that describes the diffusion process. As has been shown in [11] X s can be expressed in terms of the distance between adjacent adsorption sites on the substrate a = 3.0 nm and the difference between the adsorption energy E a and the diffusion energy E d of CO on the substrate: xs = a exp[(ea

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C. Becker, C.R. Henry/Surface Science 352-354 (1996) 457-462

The function P ( x , y) can be expressed by the modified Bessel functions (In, Kn): 11(y) K I ( x ) -- K I ( y ) I I ( x )

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Here L is the distance between two clusters. The original model was based on a reaction probability S ( T ) that was an approximation of the universal reaction rate curve proposed Ertl [13] in the vicinity of the maximum of the reaction rate (500 K). Instead of this function we introduce here a reaction probability that is in close agreement with the curve over the whole temperature range S ( T ) = S0{1 - 2.2 × 1 0 - 3 ( T - To) - 4.61

× 10-2exp[ - 1.83 × 1 0 - 2 ( T - T0)l}.

(5) In Eq. (5) we have only two adjustable parameters. One is the maximum reaction probability So, the other one is the temperature TO that allows to shift the position of the maximum. To compare the experimental results to the theoretical model we have adjusted these parameters to give the best fit for the largest particles because in this case the difference to single crystal data should be smallest. The parameters obtained by this procedure So = 4.46 × 10 .2 and To = 584.5 K were used to calculate the turnover number for the smaller cluster sizes. As can be seen in Fig. 4 (top) the agreement between the experimental data and the model is excellent for the largest cluster size. This confirms the validity of the spillover model for clusters of Type 1. The dash-dotted curve in Fig. 4 (top) is a plot of the first term of Eq. (1), hence without spillover. Comparison of the two curves shows that the spillover increases the turnover number by about 50%. For the smaller clusters the calculations do not fit the experimental data. The most pronounced difference is the rapid fall off at higher temperatures. Furthermore, the reactivity of the small clusters is higher than predicted by the model. To check if the experimental data can be explained by the proposed model using other values SO and T0, a fit of these parameters to the experimental data had been undertaken. The result of this

461

procedure is displayed in Fig. 4 (bottom). Even under these conditions no agreement between the experimental data and the model has been found. Again the rapid fall-off of the experimental turnover number at higher temperatures poses the biggest problem. Therefore, the shape of the curve S ( T ) cannot be universal for different cluster sizes as it is for different Pd single crystal surfaces. This means that there must be a size effect for the catalytic oxidation of CO on small ( < 6.8 nm) Pd clusters on MgO(lO0).

4. Conclusion It has been shown that the steady-state catalytic oxidation of CO on large (13 nm) Pd clusters on MgO(100) can be explained by the a spillover model, thus, the reaction proceeds as on Pd single crystal surfaces. This model fails for smaller Pd cluster sizes (2.8 and 6.8 nm) which can only be interpreted in terms of a Pd cluster size effect. Furthermore, a strongly bound CO species has been found. This could be due to CO adsorbed on special sites (like edges), the proportion of which increases as the cluster size drops. That would explain the size effect observed in CO adsorption experiments on small Pd particles [8,9].

Acknowledgement The financial support of the project by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References [1] T. Engel and G. Ertl, J. Chem. Phys. 69 (1978) 1267. [2] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 1. [3] M. Boudart, Adv. Catal. 20 (1969) 153. [4] S. Ladas, H. Poppa and M. Boudart, Surf. Sci. 102 (1981) 151. [5] E. Gillet, S. Channakhone, V. Matolin and M. Gi|let, Surf. Sci. 152/153 (1985) 603. [6] E. Gillet, S. Channakhone and V. Matolin, J. Catal. 97 (1986) 437.

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C. Becker, C.R. Henry / Surface Science 352-354 (1996) 457-462

[7] F. Rumpf, H. Poppa and M. Boudart, Langmuir 4 (1988) 722. [8] C. Duriez, C.R. Henry and C. Chapon, Surf. Sci. 253 (1991) 190. [9] C.R. Henry, C. Chapon and C. Duriez, Z. Phys. D 19 (1991) 347.

[10] C.R. Henry, C. Chapon, C. Goyhenex and R. Mouot, Surf. Sci. 272 (1992) 283. [11] C.R. Henry, Surf. Sci. 223 (1991) 519. [12] C.R. Henry, C. Chapon and C. Duriez, J. Chem. Phys. 95 (1991) 700. [13] G. Ertl, Pure Appl. Chem. 52 (1980) 2051.