Gas-Phase Reactions of Transition-Metal Ions with Molecular Oxygen: Room-Temperature Kinetics and Periodicities in Reactivity

Abstract

An Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic survey of room- temperature reactions of O2 with 29 transition-metal ions. The atomic ions are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic ions from Sc+ to Zn+, of second-row atomic ions from Y+ to Cd+ (excluding Tc+) and of third-row atomic ions from La+ to Hg+. Both O-atom abstraction and O2 addition were observed as primary reaction channels. Periodicities in reactivity are identified and are compared with periodicities in the O-atom affinity of the atomic ion. O-atom abstraction was observed to be efficient when exothermic and inefficient when endothermic. NbO+, MoO+, TaO+, and WO+ reacted further in a second O-atom transfer reaction with O2. ReO2+ produced by direct addition of O2 to Re+ reacted further by both O-atom abstraction and O2 addition. Sequential O2 addition was observed to produce the higher oxides: MO3+ (M = Ti, V, Y, Zr, Hf, Os), MO4+ (M = Cr, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, Pt, Au), MO5+ (M = V, Re, Os), MO6+ (M = Nb, W, Re) and MO7+ (M = V). Novel termolecular reactions second-order in oxygen were identified for Os+ and ZrO+ at high oxygen concentrations to produce O2+ and OsO+ from Os+ and a neutral metal oxide from Zr+.