Catalyzed Decomposition of Methanol-d4 on Vanadium Nanoclusters Supported on an Ultrathin Film of Al2O3/NiAl(100)

The decomposition of methanol- d 4 on vanadium (V) nanoclusters grown by vapor deposition of V on an ordered thin film of Al 2 O 3 /NiAl(100) was studied under ultrahigh-vacuum conditions and with various surface probe techniques and calculations based on density functional theory. The V clusters had mean diameter 1.55–2.10 nm and height 0.46–0.66 nm evolving with their coverage; they grew in a bcc phase and primarily in orientation (001); the lattice contracted 4% (relative to the bulk) to match structurally better the alumina surface. The methanol- d 4 adsorbed on the V clusters decomposed largely through the formation of methoxy- d 3 (denoted as CD 3 O*) at 175–225 K and subsequent cleavage of the C–O bond, yielding methyl- d 3 (CD 3 *), at temperature ≥350 K; CD 3 * either combined with surface deuterium (D*) and desorbed as methane- d 4 (CD 4(g) ) or dehydrogenated to supply more D* to assist the formation of molecular deuterium (D 2(g) ). As a measure of the reactivity, the quantities of CD 4(g) and D 2(g) produced per surface V site exhibited an evident dependence on the cluster size. Both these productions were inhibited on small clusters but increased with the cluster size; the production of D 2(g) per surface site was saturated about that from V thin films, whereas that of CD 4(g) attained a maximum at a cluster diameter near 2.0 nm but decreased, with further increasing size, to a value near that from V thin films. We argue that the inactivity of small V clusters arose from an increased energy barrier for scission of the C–O bond of CD 3 O*, which is a critical step of methanol- d 4 decomposition, on two-dimensional structures of small clusters; the separate trends of production of CD 4(g) and D 2(g) on larger clusters are attributable to the competition of CD 3 * and D* for D*.