Decomposition of methanol-d 4 on Rh nanoclusters supported by thin-film Al 2 O 3 /NiAl(100) under near-ambient-pressure conditions.

The decomposition of methanol-d 4 (CD 3 OD) on Rh nanoclusters grown by the deposition of Rh vapors onto an ordered thin film of Al 2 O 3 /NiAl(100) was studied, with various surface-probe techniques and largely under near-ambient-pressure (NAP) conditions. The results showed a superior reactivity of small Rh clusters (diameter < 1.5 nm) exposed to CD 3 OD at 5 × 10 -3 -0.1 mbar at 400 K; the gaseous production of CO and D 2 from decomposed methanol-d 4 per Rh surface site on the small Rh clusters with diameters of ∼1.1 nm was nearly 8 times that on large ones with diameters of ∼3.5 nm. The promotion of reactivity with decreased cluster size under NAP conditions was evidently greater than that under ultrahigh vacuum conditions. Moreover, the concentration of atomic carbon (C*; where * denotes adsorbate)-a key catalyst poisoner-yielded from the dissociation of CO* from dehydrogenated methanol-d 4 was significantly smaller on small clusters (diameter < 1.5 nm). The NAP size effect on methanol-d 4 decomposition involved the surface hydroxyl (OH*) from the little co-adsorbed water (H 2 O*) that was dissociated at a probability dependent on the cluster size. H 2 O* was more likely dissociated into OH* on small Rh clusters, by virtue of their more reactive d-band structure, and the OH* then effectively promoted the O-D cleavage of methanol-d 4 , as the rate-determining step, and thus the reaction probability; on the other hand, the OH* limited CO* dissociation on small Rh clusters via both adsorbate and lateral effects. These results suggest that the superior properties of small Rh clusters in both reactivity and anti-poisoning would persist and be highly applicable under "real-world" catalysis conditions.