On the reactivity of neutral metal oxide clusters in the gas phase: Detection through 118 nm single photon ionization

Single photon ionization by a vacuum ultra-violet (VUV, 118 nm) laser is successfully employed for the study of reactions of \textit{neutral} metal oxide clusters (Ti$_{m}$O$_{n}$, Fe$_{m}$O$_{n}$, Co$_{m}$O$_{n})$ with various simple molecules (CO, NO, SO$_{2}$, H$_{2}$O) in the gas phase. Neutral clusters are generated by reaction of laser ablation generated metal plasma with O$_{2}$ in a supersonic expansion. Clusters are reacted with reactant gases in a flow tube reactor. Detection of neutral clusters and products is through ionization with 118 nm laser radiation and time of flight mass spectroscopy. Rich neutral cluster chemistry is observed: (1) Ti$_{m}$O$_{2m}$ and Ti$_{m}$O$_{2m+1}$ absorb one or more H$_{2}$O molecules for $m \quad \ge $ 2 and $m \ge $ 1, respectively; (2) FeO$_{2}$, FeO$_{3}$, and possibly FeO are reactive with CO while Fe$_{2}$O$_{4}$ and Fe$_{2}$O$_{5}$ are less reactive; (3) Fe$_{2}$O$_{5}$ is reactive with NO and SO$_{2}$, but FeO$_{2}$ is much less reactive with them; and (4) small Co$_{m}$O$_{n}$ clusters ($m \quad \le $ 4 and $n \quad \le $ 6) are more reactive than large clusters (6 $\le \quad m \quad \le $ 12 and 8 $\le \quad n \le $ 17) with CO, and among these small clusters, Co$_{3}$O$_{4}$ is particularly reactive. A detailed quantum chemistry study of Fe$_{m}$O$_{n}$ reactions with CO is in progress. Preliminary calculations indicate that reactions of FeO and FeO$_{2}$ with CO to produce CO$_{2}$ are overall barrierless, in agreement with the experimental observations.