Reaction of carbon monoxide and hydrogen on neutral Nb8 clusters in the gas phase

Reactions of neutral V-n, Nb-n, and Ta-n metal clusters (n <= 11) with CO+H-2 mixed gases and CH3OH in a flow tube reactor (1-50 Torr) are studied by time of flight mass spectroscopy and density functional theory calculations. Metal clusters are generated by laser ablation, and reactants and products are ionized by low fluence (similar to 200 mu J/cm(2)) 193 nm excimer laser light. Nb-n clusters exhibit strong size dependent reactivity in reactions both with CO+H-2 and CH3OH compared with V-n and Ta-n clusters. A magic number (relatively intense) mass peak at Nb8COH4 is observed in the reaction of Nb-n clusters with CO+H-2, and CH3OH is suggested to be formed. This feature at Nb8COH4 remains the most intense peak independent of the relative concentrations of CO and H-2 in the flow tube reactor. No other Nb-n, Ta-n, or V-n feature behaves in this manner. In reactions of CH3OH with metal clusters M-n (M=V, Nb, and Ta, n=3-11), nondehydrogenated products MnCOH4/MnCH3OH are only observed on Nb-8 and Nb-10, whereas dehydrogenated products MnCO/CMnO are observed for all other clusters. These observations support the suggestion that CH3OH can be formed on Nb-8 in the reaction of Nb-n with CO+H-2. A reaction mechanism is suggested based on the experimental results and theoretical calculations of this work and of those in the literature. Methanol formation from CO+H-2 on Nb-8 is overall barrierless and thermodynamically and kinetically favorable. (c) 2008 American Institute of Physics.