Size effects in the CO-oxidation over supported Pd particles

Low pressure CO oxidation was studied over small supported Pd particles by means of static secondary ion mass spectrometry (SSIMS). The SSIMS spectra of adsorbed layers on Pd particles with diameters below ∝7 nm contained, besides other species, PdnX+ ions (X=C, O, CO; n=1-3). In experiments with Pd films, PdnC+ species were much less abundant or even absent from the mass spectra. We conclude that CO decomposed in appreciable amounts only on Pd particles. No significant influence of the support material (mica, MgO, α-Al2O3) was seen and posthumous TEM analysis revealed well defined particle morphologies with largely (111) and (100) facets. At total pressures below 10-4 Pa, the relative carbidic intensities PdnC+/Pdn+, which were taken as proportional to the carbon surface concentrations, first increased with increasing temperatures and then decreased after they passed a maximum at T=400-440 K. The smaller the particle size, the more intense were the maxima. For T> or approx.440 K, carbon was removed from the surface via reaction with oxygen. Strong hysteresis effects in the Cad and COad concentrations occurred during temperature variation mesurements. The PdnC+/Pdn+ ratios displayed maxima during heating but not during cooling. While the PdnCO+/Pdn+ ratios were always high at 300 K and always low at T>550 K, they were more intense in the medium temperature range when subjecting the samples to heating. These hysteresis effects are discussed in terms of asymmetric adsorption and reaction behaviour of CO/O2 mixtures, involving inhibition of CO2 formation by adsorbed carbon and carbon monoxide. (orig.)