Surface science studies of nitrogen oxides and related species.

摘要

In this work we present ultrahigh vacuum surface science studies of species relevant to the alkane-assisted reduction of NO_x and to the partial oxidation of hydrocarbons. We use these studies to understand how variations in surface oxidation state, oxygen coverage, vacancy density, and interfacial interactions change the reactivity of catalytically important species.; Interfacial interactions between a substrate and a metal overlayer induce electronic perturbations that can alter reaction selectivity. Adsorption of nickel on O-covered Mo(110) decreases the selectivity for complete dehydrogenation products. This shift in reactivity allows for the evolution of formaldehyde, a product not previously observed on any other clean nickel surface or bulk nickel.; Surface oxygen passivates Mo(110) toward hydrocarbon bond activation. The presence of oxygen on Mo(110) decreases the amount of 2-propen-1-ol reaction indicating that surface oxygen diminishes O-H bond activation. Surface oxygen also decreases the amount of C-H and C=C bond activation leading to decreased nonselective decomposition of 2-propen-1-ol. Further, nitromethane reaction on Mo(110) covered with 0.40 ML of oxygen is more selective for C-H bond scission products than the same reaction on Mo(110) covered with 0.66 ML of oxygen.; High surface oxidation state favors evolution of molecules containing intact C-O bonds and also prevents nonselective decomposition. Formaldehyde desorbs during the reaction of nitromethane on highly oxidized Mo(110) but not during the reaction of nitromethane on O-covered Mo(110). A small amount of nitrogen dioxide decomposes nonselectively on O-covered Mo(110) whereas no nitrogen dioxide decomposes nonselectively on highly oxidized Mo(110).; The presence of oxygen vacancies favors the complete reduction of nitrogen dioxide to dinitrogen. The thin film oxide of Mo(110) induces more decomposition to dinitrogen than does the O-covered Mo(110) surface. The enhanced reduction on the thin film oxide is correlated with the greater vacancy density.