Reactions of sulfur dioxide with neutral vanadium oxide clusters in the gas phase. II. Experimental study employing single-photon ionization.

Single-photon ionization through vacuum ultraviolet (VUV, 10.5 eV) and soft X-ray (extreme ultraviolet, EUV, 26.5 eV) laser radiation is successfully employed for the study of the reactions of neutral vanadium oxide clusters (V(m)O(n)) with sulfur dioxide (SO2) in the gas phase. V(m)O(n) clusters are generated by reaction of a laser-generated vanadium plasma with O2 in a supersonic expansion. The clusters are cooled in the expansion and are reacted with SO2 in a fast-flow reactor. Detection of neutral clusters and products is through ionization employing VUV and EUV laser radiation and time-of-flight mass spectrometry. Many association reaction intermediates [V(m)O(n)SO2 and V2O4(SO2)2] are observed. Isolated SO is also observed, as a product as predicted by theoretical studies presented in part I (J. Phys. Chem. A 2007, 111, 13339). A weak feature at the SO3 mass channel (80 amu) is suggested to be present in the product mass spectra. Further reactions of the intermediates with O2 are positively identified for VO2SO2, V3O7SO2, and V5O10SO2. Reaction mechanisms are interpreted on the basis of the observations and preliminary theoretical calculations. Molecular level reaction mechanisms for oxidation of SO2 to SO3 facilitated by condensed-phase vanadium oxides as catalysts are suggested.