Formation, stability, and reactivity studies of neutral iron sulfide clusters

Different methods are used to generate neutral iron sulfide clusters to study their formation, stability, and reactivity, employing a time of flight mass spectrometer (TOFMS) with VUV (118 nm) radiation single photon ionization (SPI). Neutral Fe$_{m}$S$_{n}$ ($m \quad =$ 1-4, $n \quad =$1-6 ), and hydrogen containing Fe$_{m}$S$_{n}$H$_{x}$ ($x$ \textgreater 0,$ n$ \textgreater $m)$ clusters are generated by the reaction of seeded H$_{\mathrm{2}}$S in a helium carrier gas with laser ablated iron metal within a supersonic nozzle. The observed strong signal of association products Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$(SH)$_{\mathrm{0,1}}M$ ($M \quad =$ CO, C$_{\mathrm{2}}$H$_{\mathrm{4}}$, C$_{\mathrm{3}}$H$_{\mathrm{6}})$ suggest that the Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$(SH)$_{\mathrm{0,1}}$ clusters have the high activity for interactions with these small molecules. In order to avoid the effect for reactivity from hydrogen containing clusters, pure Fe$_{m}$S$_{n} $clusters are generated through laser ablation of a mixed iron/sulfur target in the presence of a pure helium carrier gas. (FeS)$_{m}$ ($m \quad =$ 1-4) is observed to be the most stable series. Reaction of CO and H$_{\mathrm{2}}$ on neutral (FeS)$_{\mathrm{1,2}} $clusters is farther investigated both experimentally and theoretically. A size dependent reactivity of iron sulfide clusters \quad toward CO is characterized. The reaction FeS $+$ CO $\to $ Fe $+$ OCS is found for the FeS cluster. Products Fe$_{\mathrm{2}}$S$_{\mathrm{2}}^{\mathrm{13}}$COH$_{\mathrm{2}}$ and Fe$_{\mathrm{2}}$S$_{\mathrm{2}}^{\mathrm{13}}$COH$_{\mathrm{4}}$ are identified for reactions of $^{\mathrm{13}}$CO and H$_{\mathrm{2}}$ on Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$ clusters: this suggests that the Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$ cluster has a high catalytic activity for hydrogenation reactions of CO to form formaldehyde and methanol. DFT calculations are performed to explore the potential energy surfaces for the two reactions: Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CO $+$ 2H$_{\mathrm{2}} \quad \to $ Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CH$_{\mathrm{3}}$OH; and Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CO $+$ H$_{\mathrm{2}} \quad \to $ Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CH$_{\mathrm{2}}$O.