Dependence on Size of Supported Rh Nanoclusters in the Decomposition of Methanol

The decomposition of methanol catalyzed with Rh nanoclusters supported on an ordered thin film of Al₂O₃/NiAl(100) became enhanced on decreasing the size of the clusters. The decomposition of methanol (and methanol-d₄) proceeded through dehydrogenation; the formation thereby of CO became evident above 200 K, depending little on the cluster size. In contrast, the production of CO and hydrogen (deuterium) from the reaction varied notably with the cluster size. The quantity of either CO or hydrogen produced per Rh surface site was unaltered on clusters of diameter >1.5 nm and height >0.6 nm, corresponding to about 65% of methanol undergoing decomposition on adsorption in a monolayer on the clusters. For clusters of diameter <1.5 nm and height <0.6 nm, the production per Rh surface site increased with decreasing size, up to 4 times that on the large clusters or Rh(100) single-crystal surface. The reactivity was enhanced largely because, with decreasing cluster size, the activation energy for the scission of the O-H bond in the initial dehydrogenation became smaller than the activation energy for the competing desorption. The property was associated with the edge Rh atoms at the surface of small clusters.