Comparison of methane activation and catalytic ethylene formation on free gold and palladium dimer cations: product binding determines the catalytic turnover

The gas-phase dimers Au-2(+) and Pd-2(+) have been shown to readily activate methane and to facilitate its dehydrogenation eventually leading to the selective formation of ethylene at room temperature in the case of Au-2(+). Ion trap mass spectrometric investigations under multi-collision conditions reveal similar product ion distributions at room temperature for both dimer cations in reaction with methane. Yet, the corresponding kinetics disclose considerable differences in their catalytic properties. A detailed evaluation of the rate constants associated with the individual catalytic reaction steps in conjunction with temperature dependent reactivity studies allow for the determination of turnover frequencies, critical and rate-determining reaction steps, as well as activation barriers. These results emphasize that the propensity for the final liberation of the formed product from the metal center decisively determines the superior catalytic properties of the gold dimer compared to palladium.