CO DISPROPORTIONATION OVER SUPPORTED PD PARTICLES - A TPD AND STATIC SIMS STUDY

Temperature programmed desorption (TPD) has been used to monitor the decay in CO adsorption capacity during cycles of CO adsorption/desorption over small palladium particles deposited on both MgO and alumina substrates. It is found that CO desorption is accompanied by CO2 production as a result of CO disproportionation on the Pd surface. The decay in CO adsorption capacity is caused by a strong inhibition of CO adsorption by surface carbon which results from the CO disproportionation. The formation of surface carbon is accompanied by the appearance of PdnC+ ions in the static secondary ion mass spectroscopy (SSIMS) signal. We show, by combining TPD and SSIMS measurements, that the surface carbon coverage can be monitored using the ion intensity ratio PdnC+ Pd+n. The ability to monitor the carbon coverage in such a way makes it possible to investigate t kinetics of surface carbon formation while heating the particles in CO atmosphere. It is shown that the rate of surface carbon formation increases with both surface temperature and the CO coverage. The CO disproportionation rate depends on the particle size. The smaller the particle size, the higher the carbon formation rate. © 1990.