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

The decomposition of ethanol on Rh nanoclusters supported on thin film Al₂O₃/NiAl(100) was significantly enhanced with a decreased size of the clusters. The process started with the O-H bond scission, followed predominantly by the C_α-H bond scission to form surface acetaldehyde as a major intermediate and subsequently by further C_α-H and C-C bond scission, and ultimately produced gaseous CO, CH₄, and H₂. The gaseous production per adsorbed ethanol increased by 3.5-8.8 times (for varied products) when decreasing the cluster diameter from 3.5 to 1.1 nm, indicating a size-dependent reaction probability. The reaction probability of adsorbed ethanol remained near 35% on the clusters of a diameter ≥1.8 nm, whereas it was promoted with decreased cluster diameter <1.8 nm, up to nearly 100%. The promoted reactivity arose largely from the increased concentration of corner Rh atoms on small clusters, where the activation energy for the scission of the O-H bond of adsorbed ethanol, the initial dehydrogenation, became evidently smaller than that for the competing desorption. Moreover, the heavily produced surface CO obstructed the dehydrogenation of methyl intermediates, leading to the pronounced production of gaseous CH₄ on small clusters, contrasting with a limited one on Rh single crystals.