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∑(PdnX+/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. COads 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 COads 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.