Size-Dependent Reactivity of Rhodium Cluster Anions toward Methane

Metals are important industrial catalysts for activation and transformation of methane. Investigations on the reactivity evolution of metal particles along size change to determine the critical size at which the metal can exhibit excellent reactivity are helpful for design of efficient metal-based catalysts. Herein, the rhodium cluster anions Rh-n(-) with n = 1-10 have been generated by the laser ablation method and a quadrupole mass filter has been used to mass-select cluster ions with a specific size for reacting with methane in an ion trap reactor under thermal collision conditions. The reactant and product ions have been detected by using a time-of-flight mass spectrometer. While the clusters of Rh-2-6(-) can react with a single methane molecule, giving rise to RhnCH2- ions, the dehydrogenation product is absent for the reaction systems of Rh-1,7-10(-) unless two or three CH4 molecules are coadsorbed. Reaction kinetics analysis demonstrates that although apparent oscillation of reaction rates appears across different sizes, the reactivity of Rh-3(-) and Rh-4(-) clusters is substantially higher than the smaller and larger ones by 2 or 3 orders of magnitude. The mechanistic aspects for the size-dependent reactivity of Rh-n(-) have been discussed on the basis of quantum chemistry calculations.