Palladium clusters: H2, D2, N2, CH4, CD4, C2H4, and C2H6 reactivity and D2 saturation studies

Abstract

In this paper both deuterium saturation measurements and gas-phase kinetic measurements of chemisorption of H2, D2, N2, CD4, CH4, C2H4, and C2H6 on neutral palladium clusters are reported. Saturation studies with D2 show that small palladium clusters can bind up to three deuterium atoms per palladium atom in the cluster, in contast to H/M ratios near unity typically reported for metal surfaces. In addition, the small palladium clusters exhibit pronounced discontinuities in deuterium uptake which may be indicative of structural transformations or selective desorption of deuterium. From the kinetic studies we find that, in general, the rate constants for a given size cluster towards different reagents tend to order as D2, H2>N2>C2H4>CD4, CH4, C2H6. The shape of the reactivity pattern with the different reagents varies strongly with cluster size for clusters containing less than 25 atoms. Finally, an inverse hydrogen isotope effect is observed for both hydrogen and methane, i.e., the D2 and CD4 rate constants are significantly larger than those of H2 and CH4, respectively.