Carbon monoxide oxidation over well-defined Pt/ZrO2 model catalysts: Bridging the material gap

Four different Pt/ZrO2/(C/)SiO2 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/ZrO2 catalyst in the CO oxidation reaction using different oxygen excess (λ = 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 (Ea = 77-93 kJ mol−1 at λ = 1 and 129-141 kJ mol−1 at λ = 2) compared to the powdered Pt/ZrO2 catalyst (370-470 K, Ea = 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 COads and Oads, 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.