KINETICS OF NEUTRAL TRANSITION-METAL ATOMS IN THE GAS-PHASE - OXIDATION OF SC(A2D), TI(A3F), AND V(A4F) BY NO, O2, AND N2O

The oxidation kinetics of gas-phase, ground-state Sc(a2D), Ti(a3F), and V(a4F) atoms by NO, O2, and N2O is studied in 0.80 Torr of He buffer gas at 300 ± 5 K. The metal atoms are created in a hollow cathode sputtering source before entering a fast flow reactor. The reactions are monitored by laser-induced fluorescence of the metal atom reactant. All nine M + OX → MO + X bimolecular oxygen atom transfer reactions are inefficient at 300 K with rate constants in the range 0.45 × 10-12 to 10 × 10-12 cm3 s-1. This indicates activation energies not larger than 2-4 kcal mol-1 in all cases. The substantial range of reaction rates for a particular metal with the three different oxidants contrasts with the remarkable similarity of rates for all three metal atoms with each oxidant. The rate constants for reaction of each metal with the three oxidants fall in the order kNO > kO2 > kN2O- opposite to the normal expectation of decreasing activation energy with increasing exothermicity. The rate constant ordering kSc > kV > kTi for each oxidant and the small activation energies are interpreted in terms of an electron-transfer mechanism from neutral M + OX reactant surfaces to ion-pair M+O- + X product surfaces. © 1990 American Chemical Society.