Methanol Formation from Carbon Monoxide and Hydrogen on Neutral Nb$_{8}$ Clusters in the Gas Phase

Reactions of neutral V$_{n}$, Nb$_{n}$, and Ta$_{n}$ metal clusters ($n \le $ 11) with (CO + H$_{2})$/He mixed gases and CH$_{3}$OH/He in a flow tube reactor (P $\sim $ 14 Torr) are studied by time of flight mass spectroscopy. Metal clusters are generated by 532 nm laser ablation and reactants and products are ionized by low fluence ($\sim $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 CH$_{3}$OH compared with V$_{n}$ and Ta$_{n }$ clusters. A remarkably strong mass peak Nb$_{8}$COH$_{4}$ is observed in the reaction of Nb$_{n}$ clusters with the mixed gases CO + H$_{2}$ at various concentration of H$_{2}$. This suggests a stable, low energy CH$_{3}$OH structure may form on an Nb$_{8}$ cluster. Methanol formation is not found on other Nb$_{n}$ ($n \ne $ 8), V$_{n}$, and Ta$_{n}$ clusters. In reactions of CH$_{3}$OH with metal clusters $M_{n }(M $= V, Nb, Ta, $n$ = 3-11), molecularly adsorbed products ($M_{n}$CH$_{3}$OH) are only observed on Nb$_{8}$ and Nb$_{10}$, whereas dehydrogenated products ($M_{n}$CO) are observed for all other clusters. This observation supports the suggestion that CH$_{3}$OH can be formed on Nb$_{8}$ in the reaction of Nb$_{n}$ with CO + H$_{2}$. Reaction mechanisms are discussed based on the experimental results in this work and those in the literature. Theoretical calculations are carried out to confirm our experimental results and suggested reaction mechanisms.