Sensitivity of catalysis to surface structure: The example of CO oxidation on Rh under realistic conditions J. Gustafson, 1, * R. Westerström, 2 A. Mikkelsen, 2 X. Torrelles, 3 O. Balmes, 4 N. Bovet, 5 J. N. Andersen, 2 C. J. Baddeley, 1 and E. Lundgren 2 1 EaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, United Kingdom 2 Department of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden 3 Instituto de Ciencia de Materiales de Barcelona (CSIC), 08193 Bellaterra, Barcelona, Spain 4 ESRF, 6 rue Jules Horowitz, F-38043 Grenoble Cedex, France 5 MAX-lab, Lund University, Box 118, SE-221 00 Lund, Sweden Received 17 March 2008; published 22 July 2008 Using a combination of surface x-ray diffraction and mass spectrometry at realistic pressures, the CO oxidation reactivity of Rh111 and Rh100 model catalysts has been studied in conjunction with the surface structure. The measurements show that the presence of a specific thin surface oxide is crucial for the high activity of the Rh based CO oxidation. As this oxide is readily formed on all Rh facets, we conclude that the specific Rh crystal planes exposed during catalysis will not directly influence the reactivity. This is fortified by the very close similarity between the Rh111 and the Rh100 results. DOI: 10.1103/PhysRevB.78.045423 PACS numbers: 82.65.+r, 61.05.cp, 68.47.Gh, 82.80.Ms I. INTRODUCTION Transition-metal based catalysts, consisting of dispersed active metal nanoparticles on an insulating oxide support, form the basis of much of modern chemistry. These nanopar- ticles, or nanocrystals, expose various facets, the most abun- dant of which, in the case of an fcc metal, are the 111 and 100 surfaces due to their low surface energies, according to the Wulff construction. 1 The catalysis related properties of different facets have therefore been extensively explored un- der ultrahigh vacuum UHV conditions, using model single- crystal surfaces, demonstrating that for many reactions, the surface orientations present on the nanoparticle have a strong impact on the catalytic activity. 2–6 At more realistic pres- sures, the effects of the surface orientation on the catalytic properties is in principle unexplored, both experimentally and theoretically. A very recent in situ reflectance absorption infrared spec- troscopy RAIRS study of CO oxidation on Pt-group metals under relevant conditions shows the existence of a “hyperac- tive” oxygen-covered phase. 7 This could agree well with ear- lier studies showing that the CO oxidation reaction over Pt and Pd model catalysts is more efficient when a thin oxide is present on the substrate surface, 8–10 a situation which may correspond to that of a real catalyst at work. Although such thin oxides have been shown to exist on a number of transition-metal surfaces, 11 their role in catalysis at realistic conditions is under debate. 12 In the case of Rh, one of the active components in auto- motive catalytic converters, 13,14 no such studies have been performed previously. It has, however, been shown that in high oxygen partial pressures, a similar thin trilayer O-Rh-O surface oxide is formed on all Rh surface orientations inves- tigated so far. 15–19 These studies include low-index surfaces as well as the 553 and 223 high-index surfaces, where the stepped surface structure completely vanishes during the sur- face oxide formation. 18,19 A very recent density-functional- theory study by Mittendorfer et al. 20 showed that this also applies to Rh nanoparticles. In the present paper, we have used surface x-ray diffrac- tion SXRD and mass spectrometry to investigate the rela- tion between the presence of oxide structures and changes in the CO oxidation activity over Rh111 and Rh100 surfaces in situ at catalytically relevant pressures. Although there are some minor differences, the general behavior is practically identical for the two surface orientations. Starting with the surface oxide in pure O 2 , introducing CO reduces the oxide and leaves the surface in a metallic phase. At this point, mass spectrometry reveals a low CO 2 production. As the reaction proceeds, the O 2 / CO ratio in the chamber rises and at one point the surface oxide is reformed. Concurrently, a large increase in the CO oxidation rate can be observed. The results show that for CO oxidation, the Rh surface is much more active in the surface oxide phase than in the metallic phase. Since a similar surface oxide is formed on all Rh surfaces, including nanoparticles, our results strongly suggest that the reactivity of a Rh based CO oxidation cata- lyst is not sensitive to the specific crystal planes exposed but is governed by the surface oxide formation. This conclusion is fortified by a very close similarity between the Rh111 and the Rh100 results. II. EXPERIMENT The measurements were performed in the high-pressure chamber 21 at the surface diffraction beamline ID3 Ref. 22 at the European Synchrotron Radiation Facility ESRF in Grenoble, France. The wavelength of the incident x rays was set to 0.724 Å. The sample was aligned according to the bulk Bragg reflections of the Rh substrates. The coordinates H , K , L in reciprocal space refer to a basis b 1 , b 2 , b 3  with b 1 and b 2 spanning the surface lattice of the Rh substrate, as shown in Figs. 1b and 1c for 111 and 100, respec- tively, and with b 3 perpendicular to the surface plane. The CO oxidation measurements were performed in a so- called batch reaction chamber, in which the system is first stabilized in the presence of pure O 2 . CO is then introduced PHYSICAL REVIEW B 78, 045423 2008 1098-0121/2008/784/0454236 ©2008 The American Physical Society 045423-1