Al2O3 model catalysts: influence of the support structure

Surface Science 433–435 (1999) 215–220 www.elsevier.nl/locate/susc

The interaction of carbon monoxide with Rh/Al O model 2 3 catalysts: influence of the support structure V. Nehasil a, *, S. Zafeiratos b, S. Ladas b, V. Matolı´n a a Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic b Department of Chemical Engineering, University of Patras and ICE/HT–FORTH, Patras, Greece

Abstract Thermal desorption spectroscopy (TDS ) has been used to investigate the interaction of CO with rhodium particles evaporated by electron bombardment onto differently oriented single-crystal alumina supports. Rhodium particle populations of similar particle size and density were deposited onto the (0001), (1-102) and (11-20) a-alumina substrates. The CO TDS results depend on the crystallographic orientation of the support. The CO desorption peak shapes and activation energies, as well as the CO production which accompanies CO desorption, are compared. The 2 data set obtained enables a discussion of the influence of the alumina support structure on the adsorptive and catalytic activity of the rhodium particles. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Aluminium oxide; Carbon monoxide; Clusters; Rhodium

1. Introduction The interaction of highly dispersed metals, e.g., rhodium, palladium and platinum, deposited on oxide substrates, with gas-phase molecules plays an important role in heterogeneous catalysis. Although catalysts are widely used in industry, the basic mechanisms of catalytic reactions are still under intensive study. Model catalysts, that is metal particles deposited under well-defined conditions on well-characterized substrates, are often used to investigate the influence of particle size and morphology on the reaction mechanism [1]. The influence on the adsorption of CO of the particle size of rhodium deposited on Al O has 2 3 * Corresponding author. Fax: +42-2-688-5095. E-mail address: [email protected] ( V. Nehasil )

been studied previously [2,3]. It was reported that the activation energy of desorption generally decreases with decreasing particle size and increasing CO coverage on the rhodium surface. The partial dissociation of CO by rhodium particles has been also discussed with regard to particle size [3–5]. All these effects have been found to be dependent on particle size. It is possible to affect the epitaxial growth of rhodium particles by the choice of substrate and deposition conditions [6 ]. That is why we have prepared similar rhodium particle populations on different orientations of single-crystal Al O substrates. 2 3 The aim of this work was to compare the influence of orientation of the Al O substrate on 2 3 CO/Rh adsorption. The results, obtained by means of thermal desorption spectroscopy ( TDS ), exhibit clearly the dependence of CO adsorption on the orientation of the substrate surface.

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2. Experimental The experiments were performed in an ultrahigh vacuum ( UHV ) system equipped with a TDS spectrometer consisting of a computer-controlled quadrupole mass spectrometer (QMS ), a linear sample heating system, a simple molecular beam system (MB) for CO and O adsorption and an 2 evaporation cell for in situ deposition of rhodium particles. The entrance orifice to the differentially pumped QMS was 3 mm in diameter and the sample was placed very close to it so that the background signal was minimized. The temperature of the sample holder was monitored by a chromel–alumel thermocouple and recorded in real time by the computer operating the QMS. The sample temperature is probably lower than that measured by the thermocouple, which would affect the estimated activation energies of desorption, E . Since the goal of this paper was to compare a the CO adsorption on rhodium particles deposited on different orientations of alumina and not to determine E accurately, no temperature correction a was attempted. Since all substrates had the same thickness, the error can be assumed to be the same in all experiments. The substrates were Al O single-crystalline 2 3 plates of dimensions 10 mm×10 mm×0.5 mm. The set of orientations (0001), (1-102) and (11-20) has been used. The substrates were cleaned chemically and heated in air for 2 h at 1570 K. Rhodium was evaporated in situ from the micro electron-beam source. During deposition the pressure was typically 3×10−7 Pa, and the substrate temperature 580 K. The evaporation rate estimated by the quartz thickness monitor was 1 nm/45 s and, for a deposition time of 60 s, an average thickness of 1.3 nm for the deposited film can be expected for all samples. The real particle size and density of the rhodium deposits were measured after TDS by transmission electron microscopy ( TEM ) using the method of transparent carbon replicas. Prior to the TDS experiments, the rhodium particles were stabilized in a CO–O atmosphere 2 at a partial pressure of 3×10−6 Pa for both gases and at about 650 K. This procedure was performed to avoid the morphology change of freshly depos-

ited particles stimulated by the first adsorption or heating. Subsequently, two saturation adsorptions of CO and TDS were performed to reduce adsorbed oxygen.

3. Results and discussion Some parameters of the samples studied, as determined by TEM, are presented in Table 1. We have tried to deposit the same amount of rhodium onto all substrates. It can be seen that the deviation of the calculated amount of deposited rhodium atoms from the average value does not exceed 25%. On the other hand, differences can be seen in the particle size and density. The curves of E estimated by Redhead analysis a [7] versus relative CO coverage, H =H/H , are rel max presented in Fig. 1. The relative coverage is the ratio of the number of CO molecules, H, actually adsorbed on the rhodium surface and the maximum number of CO molecules that can be adsorbed, H . It can be calculated from the ratio max of the CO desorption peak and the saturation desorption peak areas. The absolute value of E is a not important in this paper, because the goal is to compare the results obtained on different substrates. In addition, the estimated temperatures of the TDS peak maximum are probably shifted to higher values as described in Section 2. It can be easily recognized that E generally a decreases with H as reported previously [3] and rel that the values obtained on the (1-102) and (11-20) alumina substrates are similar, the former being slightly higher. This difference reflects qualitatively the rhodium particle size on both samples, as it has often been reported that E decreases with a decreasing particle size [2,3]. The estimated values Table 1 Parameters of the rhodium particles studied, as estimated by TEM Al O 2 3 substrate

Particle size (nm)

Density (cm−2)

Deposit (atoms cm−2)

(0001) (1-102) (11-20)

3.5 2.6 2.2

1.3×1012 4.5×1012 5×1012

1.3×1015 1.7×1015 1.1×1015

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Fig. 1. The dependence of E on H in different samples. a rel

for the (0001) substrate are surprisingly lower by about 15 kJ mol−1, although the average particle size obtained by TEM is higher on this sample. The development of the CO desorption peak with CO exposure can be seen in Fig. 2. Saturation occurs at an exposure of about 6 L (1 L is exposure to 1×10−6 Torr for 1 s). The peak shapes for the (1-102) and (11-20) samples are similar, but for the (0001) sample it is completely different. The desorption peaks obtained on rhodium particles deposited on the (0001) substrate are generally wider and they seem to be more symmetric over the whole range of H than those measured on the rhodium particles rel deposited onto the other substrates. With increasing exposure the peaks become wider towards lower temperature. Above 2 L the development of a second peak can be recognized at lower temperatures. On the other hand, the peaks measured on the (1-102) and (11-20) substrates are narrower. Upon increasing CO exposures a low-temperature tail appears that results in a more asymmetric shape of the TDS peak. We can compare the TDS peak shapes obtained

on different alumina surfaces with those from the literature [2,3]. It can be easily recognized that the peaks obtained on the (0001) substrate correspond to those obtained on small particles, in contrast to the peaks obtained on (1-102) and (11-20), corresponding to those obtained on larger particles. It must be noted that the CO desorption peak shape can be strongly influenced by the particle morphology, as found for palladium particles in [8]. The third effect studied is the production of CO during CO desorption, as in [3]. It was 2 concluded that this is a result of CO dissociation on the rhodium surface followed by the CO+OCO reaction [3]. The CO production 2 2 has been found to depend on the rhodium particles’ morphology, the ratio of the amounts of desorbed CO and CO molecules being higher in the case of 2 atomically rougher rhodium surface. No CO pro2 duction was measured on the small rhodium particles. This was explained by the lower E on the a small particles, which results in a low CO concentration at a temperature sufficient to drive the CO+OCO reaction and consequently in a 2 lower probability for CO oxidation. The peaks of CO production obtained after 2 L CO exposure 2

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Fig. 2. The development of the TDS peak with increasing CO exposure. The substrate orientation is marked near the curves. The selected TDS peaks (from the bottom: CO exposures of 0.5 L, 1 L, 3 L and 6 L) are plotted for each substrate orientation. The curves are shifted vertically for clarity.

are plotted in Fig. 3, the results obtained after different exposures being similar. Production of CO can be observed on the (1-102) and (11-20) 2 substrates but not on the (0001) substrate. This is consistent with the lower E values obtained on a

(0001) (Fig. 1). This effect has been previously connected with lower particle size due to lower desorption energy for CO. In this work we have obtained similar results on the (0001) substrate where the particle size was larger.

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Fig. 3. Temperature dependence of CO production during CO desorption for different samples. The present peaks were obtained 2 after 2 L CO exposure. The curves are shifted vertically for clarity.

In summary, three parameters (energy of desorption, desorption peak shape and CO pro2 duction) on the (0001) substrate correspond to the behaviour of small rhodium particles, although the particle size was the largest. On the other hand, the rhodium particle size on the (1-102) and (1120) substrates was smaller but their behaviour corresponds to that observed previously for larger particles deposited on c-alumina [3]. The effects described above are probably caused by the different epitaxial growth of rhodium particles on the (0001) substrate compared with the other substrates. The crystallographic structure of the (0001) surface is hexagonal and differs strongly from that of the other substrates, which is rectangular. Thus the particle shape, especially the crystallographic planes enclosing the particle, are conditioned by the substrate structure. The particle population parameters, especially the size and density, are determined by the preparation conditions, the diffusion of deposited atoms on the surface, and a particle adhesion energy. The preparation conditions were kept constant. Diffusion can influence the particle density

because, if some critical density is reached, new particles are not formed on the substrate and it is only the particle size that increases (see [9] for example). It can be seen in Table 1 that the particle density estimated on the (0001) sample is about three or four times lower than on the other substrates used in this work. So the diffusion length of rhodium atoms on the (0001) substrate is longer, which also results in a different morphology of the rhodium particles. The results on the (0001) substrate are very different from those obtained on the (1-102) and (11-20) substrates. The difference in particle size can be neglected in comparison to the size differences in [2,3] (2 nm versus 7 nm and 2.5 nm versus 5 nm, respectively). Furthermore, we have found the opposite dependence on the particle size for all effects studied (E , TDS peak shape and CO a 2 production). Thus in our case it is not the particle size that is the most important factor in CO adsorption on rhodium particles, but the particle shape. The shape can vary even if the particle size is the same or similar. It seems to be strongly influenced by the substrate’s crystallographic ori-

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entation and morphology, and in this way the substrate can be very important in the adsorptive behaviour of the rhodium/alumina system.

programme KONTAKT: Czech–Greek scientific and technical collaboration. This work was also supported by the Czech Grant Agency, project number GACR 202/97/1166, and by Charles University, project number GAUK 35/97/B.

4. Conclusion Rhodium particles have been deposited on different single-crystalline surfaces of Al O . 2 3 Adsorption of CO molecules on these rhodium/ alumina samples was investigated. Contrary to previously published results which showed a dependence on the particle size, our results illustrate the influence of the substrate orientation.

Acknowledgements The common experiments performed by the Czech–Greek team were realized thanks to the

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