Methane activation and partial oxidation on free gold and palladium clusters: mechanistic insights into cooperative and highly selective cluster catalysis.

The catalytic activation, dehydrogenation, and direct oxidative conversion of methane into more valuable products such as larger hydrocarbons, alcohols, aldehydes etc. are of considerable industrial interest. To investigate the energetics and kinetics of elementary bond-breaking and bond-formation processes, free metal clusters can serve as versatile catalytic model systems. In this context, temperature dependent reactions of small cationic gold clusters Au(x)+ with methane as well as a mixture of methane and molecular oxygen have been performed in an octopole ion trap under multi-collision conditions and compared with the corresponding reactions on palladium clusters Pd(x)+ (x = 2-4). Binding energies of methane to all investigated cluster cations are determined from kinetic measurements via statistical analysis. Furthermore, among the gold clusters, the dimer Au2+ is found to be able to dehydrogenate methane and to convert it into ethylene in a highly selective catalytic reaction. In contrast, all investigated palladium clusters activate methane under non-selective formation of a variety of dehydrogenated products. Most interestingly, methane dehydrogenation is observed for Pd(x)+ and Au2+ only, if a cluster specific 'critical number' of methane molecules is pre-adsorbed. This emphasizes the importance of cooperative coadsorption effects in the dehydrogenation process on these clusters. Finally, the reaction between Au2+ and both O2 and CH4 yields a low temperature product of the stoichiometry Au2(C3H8O2)+ that clearly contains activated O2 and dehydrogenated methane indicating a possible C-O bond formation process. The palladium dimer Pd2+ on the other hand exhibits only the mere coadsorption of molecular oxygen and non-dehydrogenated methane.