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The decomposition of methanol on Pt nanoclusters grown from vapor deposition onto an ordered Al2O3/NiAl(100) thin film was investigated under ultrahigh vacuum conditions with various surface probe techniques. The Pt clusters had mean diameter near 2.3 nm and height 0.4 nm before their coalescence; consisting of phase fcc, the clusters grew with their facets either (111) or (001) parallel to the θ-Al2O3(100) surface, depending on the temperature of growth. More than half the adsorbed monolayer of methanol on the Pt clusters decomposed via two channels: dehydrogenation to CO and C–O bond scission. The dehydrogenation was dominant and induced first at low-coordinated Pt sites, at 150 K on Pt(001) clusters and 200 K on Pt(111) clusters, whereas both low-coordinated and some terrace Pt sites exhibited reactivity, despite the cluster size. On average, one CO was produced per surface Pt site, for a monolayer of methanol on either Pt(111) or Pt(001) clusters. In the other reaction, scission of the C–O bond occurr...

The adsorption of methanol altered structures of Au–Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100). Methanol adsorbed on the Au–Pt intermixed bimetallic clusters, of which the surfaces consist of both Au and Pt, induced a segregation of Au from Pt. This segregation state was unstable, as the clusters returned to the initial Au–Pt intermixed state upon desorption or decomposition of adsorbed methanol. Ethanol and cyclohexene were adsorbed on Au–Pt bimetallic clusters for comparisons, indicating that the interaction of the hydroxyl group of methanol with the clusters accounts for the structural modifications.

Au-Pt bimetallic nanoclusters on a thin film of Al(2)O(3)/NiAl(100) undergo significant structural evolution on variation of the temperature. Au and Pt deposited sequentially from the vapor onto thin-film Al(2)O(3)/NiAl(100) at 300 K form preferentially bimetallic nanoclusters (diameter ≦ 6.0 nm and height ≦ 0.8 nm) with both Au and Pt coexisting at the cluster surface, despite the order of metal deposition. These bimetallic clusters are structurally ordered, have a fcc phase and grow with their facets either (111) or (001) parallel to the θ-Al(2)O(3)(100) surface. Upon annealing the clusters to 400-500 K, the Au atoms inside the clusters migrate toward the surface, resulting in formation of a structure with a Pt core and an Au shell. Annealing the sample to 500-650 K reorients the bimetallic clusters--all clusters have their (001) facets parallel to the oxide surface--and induces oxidation of Pt. Such annealed bimetallic clusters become encapsulated with the aluminium-oxide materials and a few Au remain on the surface.

With a variety of surface probe techniques, we investigated low-temperature decomposition of methanol on Au nanoclusters formed by vapor deposition onto an ordered Al(2)O(3)/NiAl(100) thin film. Upon adsorption of methanol on the Au clusters (with mean diameter 1.5-3.8 nm and height 0.45-0.85 nm) at 110 K, some of the adsorbed methanol dehydrogenates directly into carbon monoxide (CO); the produced hydrogen atoms (H) begin to desorb near 125 K whereas most of the CO desorbs above 240 K. The reaction exhibits a significant dependence on the Au coverage: the produced CO increases in quantity with the Au coverage, reaches a maximum at about 1.0-1.5 ML Au, whereas decreases with further increase of the Au coverage. The coverage-dependence is rationalized partly by an altered number of reactive sites associated with low-coordinated Au in the clusters. At least two kinds of reactive sites for the low-temperature decomposition are distinguished through distinct C-O stretching frequencies (2050 cm(-1) and 2092 cm(-1)) while the produced CO co-adsorbs with H and methanol.