The reaction of CH 3NO 2 on O-covered Mo(1 1 0): the effect of oxygen on product distribution

Abstract

The reactions of CH 3NO 2 on Mo(1 1 0) containing 0.66 and 0.40 ML of chemisorbed oxygen are studied using temperature programmed reaction (TPR) and reflection absorption infrared spectroscopy (RAIRS). Nitromethane decomposes on the surface with 0.66 ML of O––occupying both twofold and quasi-threefold coordinated sites––below 300 K via dissociation of N–O bonds to form methylimido, CH 3N(a). The oxygen is deposited on the surface populating terminal sites while the methylimido yields gas-phase HCN, H 2O, and methyl radical between ∼550 and 800 K. In addition, some methylimido decomposes nonselectively to adsorbed N and C. The reaction on the 0.40 ML surface (containing only twofold coordinated oxygen) also affords HCN, H 2O, CH 3, and adsorbed O, N, and C. The product distribution is different when the initial oxygen coverage is 0.4 ML––the amount of HCN and H 2O is ∼10% greater, the yield of CH 3 is ∼10% less, and the amount of adsorbed C and O is substantially increased. In addition, the desorption temperature of the C–H bond scission products is ∼20 K lower on the 0.4 ML oxygen surface than on the saturated surface. The reactions on the chemisorbed oxygen overlayers are compared to those of CH 3NO 2 on a thin film oxide of Mo(1 1 0). The observed differences in reactivity as a function of oxygen coverage and as a function of the presence of subsurface oxygen are discussed in terms of surface preparations which favor either C–H bond scission or oxygenate evolution.