Spatial coupling between kinetic oscillations on different regions of a cylindrical Pt single crystal

Vacuum/volume41/numbers 1-3/pages 272 to 274/1990 Printed in Great Britain

0042-207X/9053.00 + .00 ~) 1990 Pergamon Press plc

Spatial coupling between kinetic oscillations on different regions of a cylindrical Pt single crystal M S a n d e r , M R B a s s e t t , R I m b i h l a n d G Ertl, Fritz-Haber-lnstitut der Max-Planck-Gesellschaft,

Faradayweg 4-6, D- 1000 Berlin 33, West Germany

Kinetic oscillations in the catalytic CO oxidation have been investigated in the 10-s and 10 -4 torr range on a cylindrical Pt single crystal whose axis is oriented parallel to the [O01]-direction. Two Kelvin probes and measurements of the reaction rate served to follow the local and the integral behavior of kinetic oscillations. The cylindrical surface exhibits the orientations (100), (110) and (210) whose oscillatory behavior has been studied before. The orientational dependence of the existence region for kinetic oscillations was investigated at T = 480 K. A strong broadening of the oscillatory region in parameter space as compared to a flat surface was detected for the (100) orientation which is attributed to coupling effects between different regions of the surface. In the vicinity of the (100) orientations, propagating reaction fronts provide the spatial coupling between different regions, while the (110) orientations communicate via gas phase coupling. Prolonged oscillation experiments cause an increase in catalytic activity around the (110) orientation, whereas the catalytic activity of the (210) orientation decreases.

The occurrence of kinetic oscillations in the oxidation of CO on several platinum single crystal surfaces, e.g. Pt(100), Pt(110) and Pt(210), was the subject of extended investigations in recent years*. In the case of Pt(100) and Pt(ll0) the oscillations are linked to a CO-induced surface phase transition. The lifting of the surface reconstruction by CO modulates the oxygen sticking coefficient and hence the catalytic activity. Spatial synchronisation on Pt(100) is achieved via propagating reaction fronts of macroscopic dimensions which result from the coupling between CO diffusion and the reactive removal of adsorbed CO. On Pt(ll0), which exhibits a much narrower existence region for oscillations, spatial coupling proceeds via the partial pressure changes in the gas phase, i.e. a periodic depletion of CO during oscillations as a consequence of mass balance in the reaction. The Pt(ll0) surface in contrast to Pt(100) therefore oscillates spatially homogeneously. In this work a cylindrical platinum single crystal with its axis parallel to [001] was used to study the coupling between different oscillating regions. Since the [001]-zone comprises the (100), (110) and (210) orientations which exhibit autonomous oscillations, the existence of different synchronization mechanisms and their dependence on the surface structure should determine the oscillatory behavior of the cylindric Pt surface. Due to the fourfold symmetry of the sample it is possible to differentiate between the two coupling mechanisms, since symmetry equivalent orientations can only communicate via the gas phase. On the other hand, coupling via CO diffusion can only take place between adjacent orientations. Two Kelvin probes, of which one was rotatable, served to follow local work function changes simultaneously at two different regions of the cylinder. The lateral resolution of the work function measurements is determined by the size of the Kelvin probe and averages over an angle of ~ 7 ° on * For kinetic oscillations on Pt(100) and Pt(110) see refs 1, 2, for Pt(210) ref 2.

see

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the cylinder surface (diameter 17 mm). The integral behavior of the cylinder could be followed by measurements of the reaction rate via a differentially pumped mass spectrometer. In order to establish a relation between surface structure and oscillatory behavior we studied first the orientational dependence of the adsorption properties. While CO adsorption is practically insensitive to a change in surface structure, the oxygen sticking coefficient varies over more than two orders of magnitude with a change in orientation as is reflected by the data of Figure 1. The oxygen sticking coefficient S°o2 follows exactly the work function of the clean surface q~° in its orientational dependence, such that the orientation with the highest work function exhibits the lowest S°o2. The lowest S°o: corresponds to the reconstructed Pt(100) surface, while the highest S°o~ value has been measured for the (210) orientation. The work function of the clean surface is determined by the openness of the surface structure, i.e. the step density, which has a maximum for (210), but is very low on the densely packed reconstructed Pt(100) surface. LEED shows that the surface region between (110) and (210) exhibits a continuous change in the surface structure as the density of steps with (100) orientation increases up to a maximum at the (210) orientation where (110) terraces and (100) steps alternate. In between the (210) and the (I00) orientation the surface of the cylinder is faceted into (210) and (100) orientations. Figure 2 sho~vs the existence region for oscillations on the cylinder surface for a fixed Po~ of 5 x 10- 5 torr and T = 480 K. As known from previous studies the oscillatory Pco range is very broad for Pt(100) but narrows down to a few % of Pco in the vicinity of Pt(110). Towards the (210) orientation both existence regions are shifted to higher Pco, since the sticking coefficient for oxygen increases in the same direction. The Pt(210) orientation itself does not exhibit oscillations in the parameter range investigated here. The comparison with the results of previous investigations on flat single crystal surfaces which are indicated by bars in Figure 2 reveals rather strong differe,ces. The parameter space

Spatial coupling between kinetic oscillations

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existence of propagating reaction fronts on the Pt cylinder can be simply followed by the time delay between the oscillating signal measured by two adjacent Kelvin probes. Since the density and/ or the size of the reactive (210) facets increases with the distance from the ideal (100) orientation one observes an unidirectional propagation of reaction fronts from the (210) towards the (100) orientation. The results obtained with the Pt cylinder thus confirm the previously drawn conclusion that spatial synchronization on Pt(100) is accomplished via propagating reaction fronts. The negligible influence of gas phase coupling on oscillations in the vicinity of Pt(100) can be shown directly by following the oscillations simultaneously at two symmetry equivalent (100) orientations. These oscillate uncorrelated to each other as expected from the proposed mechanism. In order to follow the influence of gas phase coupling on the oscillations of Pt(110), conditions were chosen where the oscillations were limited to a small range of about + 2 ° around each of the four (110) orientations. This is the case at T = 530 K and Po2 = 5 x 10-5 torr where regular fast (period of several seconds) oscillations on the Pt(ll0) surface exist which are not complicated by the additional influence of faceting. A coupling between two symmetric Pt(ll0) orientations was in fact observed, but mostly complex oscillation patterns prevailed. These presumably result from insufficient synchronization, since the small surface area causes only Pco changes which are below the detection limit of ~ 1% in our system. A second important factor is certainly the non-uniformity of the oscillating (110) regions on the cylinder. Since the existence region for oscillations on a flat (110) surface is very narrow ( < 10 % of Pco), structural differences will spread out the response of the oscillating (110) regions and render an effective synchronization difficult. At higher pressure (Po2 = 1.5 x 10 - 4 torr and at T = 480 K) conditions were found where almost all orientations of the cylinder with the exception of small regions around (100) were oscillating. The comparison with the results of previous studies showed that only the (210) orientation was then in its genuine existence region for autonomous oscillations L2. Since the concomittant changes in Pco reach values up to 25 %, periodicforcing

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o 3 2 for oscillations on both orientations is shifted towards higher Pco and on Pt(100) the oscillating Pco-range is broadened by a factor of roughly 2.5. The origin of the different behavior of the Pt cylinder can be attributed to coupling effects between different orientations. As a consequence more reactive parts can trigger oscillations in less active parts and thus cause a broadening as well as a shift of the oscillating region in parameter space. The influence of surface inhomogeneities is particularly strong on Pt(100), as the reaction fronts nucleate at reactive defect sites and propagate then over the rest of the surface area. The

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273

M Sander et al: Spatial coupling between kinetic oscillations

through the gas phase of the orientations without autonomous oscillations appears to be the driving mechanism for this overall synchronization 4. But another effect is probably of equal importance, namely the slow ( ~ 2 h) change of the catalytic activity of the different orientations which converge towards an intermediate activity. In this way the oscillation properties of the individual orientations become more similar and synchronization is achieved more easily. The change in catalytic activity after maintaining kinetic oscillations for 3 h is depicted in Figure 3 for the various orientations of the Pt cylinder. Convergence of the different behavior of the orientations is achieved via an increase in catalytic activity for the (110) region, while the more active region around (210) decreases in its catalytic activity. The first process associated with an increase of the reaction rate is due to a facetting of flat (110) regions towards the formation of (210) facets which has been extensively studied in the 10 - 4 torr region 3. The increase in catalytic activity has to be traced back to the formation of steps which exhibit a higher Sos than (110)

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terraces. The decrease in catalytic activity in the (210) region is also due to a faceting process in which the very open and reactive (210) surface is transformed into more close-packed and less reactive orientations. This has been demonstrated in experiments with a Pt(210) single crystal which under the influence of the catalytic CO-oxidation faceted in (110) and (310) orientations.

Acknowledgements One of the authors (MRB) was supported by the North Atlantic Treaty Organization under a grant awarded in 1988. The authors thank S Wasle for technical assistance.

References i M Eiswirth, P Mrller, K Wetzl, R Imbihl and G Ertl, J Chem Phys, 90, 510 (1989). 2M Ehsasi, J H Block, K Christmann and W Hirschwald, J Vac Sci TechnoL A5, 821 (1987); M Ehsasi, Thesis, FU Berlin (1989). 3 S Ladas, R Imbihl and G Ertl, Surface Sci, 197, 153 (1988). 4 M Eiswirth and G Ertl, Phys Rer~ Lett, 60, 1526 (1988).