Probing the role of surface oxygen in catalytically relevant reactions on Mo(110) and oxygen-modified Mo(110).

摘要

The studies reported herein utilize a variety of surface science techniques to investigate the role of surface oxygen in catalytically important reactions--specifically, partial oxidation of alkanes and reduction of nitric oxide--on single crystal Mo(110). By varying oxidation protocols we vary not only the coordination sites occupied by atomic oxygen but also the overall extent of surface oxidation, thus addressing fundamental aspects of the chemistry of MoO{dollar}sb3{dollar}-based catalysts. The complementary surface vibrational techniques of high-resolution electron energy loss spectroscopy and infrared reflectance absorbance spectroscopy allow identification of atomic and molecular surface intermediates formed en route to evolution of gas-phase products identified via temperature programmed reaction.; Oxygen coordination site is found to influence alkane oxidation markedly, since gas-phase methyl radicals add preferentially to oxygen in high-coordination, low-symmetry sites to form surface methoxy. Studies of methoxy formed via methanol reaction on oxygen-modified Mo(110) demonstrate that the subsequent reactivity of this intermediate depends strongly on the overall extent of surface oxidation, with the formation of surface oxygen vacancies dominating irreversible methanol reaction on more highly oxidized Mo(110). The unique reactivity of methoxy on oxygen-modified Mo(110) (homolytic C-O bond scission to evolve methyl radicals), as compared to its reaction on the clean surface, is attributed to a decrease in the facility for dehydrogenation on the oxygen-covered surfaces, rather than to significant changes in the C-O bond potential with increasing surface oxidation.; Infrared spectroscopy reveals coupling of nitric oxide (NO) molecules through a dinitrosyl intermediate on Mo(110), found to lead to nitrous oxide (N{dollar}sb2{dollar}O) and some dinitrogen formation at low temperature. Surface oxygen favors NO desorption over NO reduction, with distinct mononitrosyl species correlated with increasingly higher-temperature NO desorption features as surface oxidation is increased. The conversion of some of these nitrosyls to dinitrosyls is observed with infrared spectroscopy on a surface oxygen overlayer of {dollar}sim{dollar}0.75 ML. On highly oxidized Mo(110), infrared spectroscopy provides evidence for the formation of a perturbed condensed-phase-type NO dimer, evidence for another pathway to NO coupling on transition metal surfaces. Finally, the interaction of surface-bound NO with gas-phase methyl radicals is studied to identify potential pathways for NO reduction by methane.