Carbon monoxide oxidation over well-deﬁned Pt/ZrO 2 model catalysts: Bridging the material gap Attila Wootsch a, * , Claude Descorme b , Sophie Rousselet c , Daniel Duprez d , Claude Templier c a Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, Budapest H-1525, Hungary b Institut de Recherches sur la Catalyse (IRC), CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne Cedex, France c LMP-UMR 6630, CNRS-Universite ´ de Poitiers, SP2MI, Boulevard Marie et Pierre Curie, BP 30179, F-86962 Futuroscope Chasseneuil Cedex, France d LACCO-UMR 6503, CNRS-Universite ´ de Poitiers, 40 Avenue du Recteur Pineau, F-86022 Poitiers Cedex, France Received 15 July 2005; received in revised form 24 November 2005; accepted 2 February 2006 Available online 15 March 2006 Abstract Four different Pt/ZrO 2 /(C/)SiO 2 model catalysts were prepared by electron beam evaporation. The morphology of these samples was examined before and after the catalytic reaction by Rutherford back-scattering (RBS), transmission electron microscopy (TEM) and grazing-incidence small- angle scattering (GISAXS). The catalytic behavior of such model catalysts was compared to a conventional Pt/ZrO 2 catalyst in the CO oxidation reaction using different oxygen excess (l = 1 and 2). The so-called material gap was observed: model catalysts were active at higher temperature (620–770 K) and resulted in higher activation energy values (E a = 77–93 kJ mol 1 at l = 1 and 129–141 kJ mol 1 at l = 2) compared to the powdered Pt/ZrO 2 catalyst (370–470 K, E a = 74–76 kJ mol 1 ). This material gap is discussed in terms of diffusion limitations, reaction mechanism and apparent compensation effect. Diffusion processes seem to limit the reaction on planar samples in the reactor system that was shown to be appropriate for the evaluation of the catalytic activity of powder samples. Kinetic parameters obeyed the so-called apparent compensation effect, which is discussed in detail. Langmuir–Hinshelwood-type of reaction, between CO ads and O ads , was proposed as the rate-determining step in all cases. Pt particles deposited on planar structures can be used for modeling conventional powdered catalysts, even though some limitations must be taken into account. # 2006 Elsevier B.V. All rights reserved. Keywords: CO oxidation; Platinum; Pt; Model catalysts; Electron beam evaporation; Pt/ZrO 2 ; RBS; GISAX; Compensation effect; Material gap 1. Introduction Real powder catalysts always appear to be very hetero- geneous in morphology, structure, local composition, etc. As a result, it is always very difﬁcult to extract the exact inﬂuence of any isolated parameter. Only average information might be derived unless one might work with well-deﬁned model systems [1,2]. The characterization of such model systems is usually easy because they have well-deﬁned structures [1,3]. However, they do not always behave as conventional powdered or industrial catalysts in catalytic reactions [1–3]. This phenomenon is the so-called ‘‘material gap’’, which was already observed for example in the CO oxidation reaction on Pt [1,3], Pd [1,3], Ru [4] and Au [5,6] catalysts. The original goal of the present research was to prepare some well-deﬁned model catalysts that would behave in the exact same way as conventional powder catalysts. They were prepared using physical methods instead of chemical ones [1,3]. A strict control was kept over (i) the metal particle size, with a narrow particle size distribution and (ii) the distance in between particles, with a homogeneous distribution of the particles on the support. The initial idea was to investigate the inﬂuence on the oxygen mobility on oxides of both the metal particle morphology and the distance between particles on the surface. This thematic is related with the oxygen storage and diffusion process on automotive three-way catalysts [7]. In fact, it was observed earlier that the oxygen activation on noble metals could be strongly inﬂuenced by the metal particle size. www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 1310–1322 * Corresponding author. Tel.: +36 392 2222/3172; fax: +36 1 392 2533. E-mail address: wootsch@iki.kfki.hu (A. Wootsch). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.006