CO oxidation over small Pd particle model catalysts. A static secondary ion mass spectrometry study

Abstract

Static SIMS has been used to monitor low-pressure CO oxidation over model palladium catalysts consisting of Pd films and small alumina-supported Pd particles in the size range 2–30 nm. We have shown that by combining thermoprogrammed reactions and SSIMS measurements the changing gas-phase composition under catalytic conditions can be correlated with the surface concentration and coordination of adsorbed species, monitored using ion-intensity ratios. For example, the surface coverage of reactant or intermediate can be selectively monitored on the Pd active phase via∑(Pd_nX⁺/Pd⁺ _n). Although the CO oxidation reaction occurs predominantly by a similar Langmuir–Hinshelwood mechanism between adsorbed CO and oxygen on both single-crystal and small-particle Pd surfaces, the study reveals some significant differences in behaviour between the two. CO_ads coordination is almost entirely linear on the small particles and CO dissociation takes place to a significant degree; the resulting surface carbon seems to provide a further CO oxidation route. On larger particles and thick supported (111) film the coordination at high CO coverage is 2 or 3 fold and little CO dissociation occurs. Under oxidation reaction conditions in the presence of adsorbed oxygen the coordination of CO_ads on all surfaces is entirely linear. The observations are discussed in terms of the reaction mechanism and the special electronic characteristics of small Pd particles.