Selective oxidation of methane over Sn 1− xGe xO 2

Abstract

The effects of germanium ion incorporation into tin oxide on the selectivity for the selective oxidation of methane in the presence of NO over Sn 1− x Ge x O 2 ( x=0–0.5) were examined with the use of a conventional catalytic reaction and theoretical calculations. The reactivity of the tin oxide for methane oxidation was decreased by the incorporation of germanium. The product selectivity was measured at 10% methane conversion. The product was CO 2, CO, and a small amount of C 2 compounds in the absence of germanium, but we obtained C 1-oxygenates over every germanium ion-incorporating catalyst. The main product of the C 1-oxygenates was formaldehyde. The selectivity of formaldehyde increased with the content of germanium ( x) and had a maximum value at x = 0.2 . We considered two reaction routes of HCHO formation: a heterogeneous reaction and a homogeneous one. Since a larger proportion of CO 2 selectivity in comparison with CO selectivity was found in the presence of Sn 0.9Ge 0.1O 2, we assumed that the heterogeneous reactions over the catalyst were dominant in a lower temperature region below ca. 860 K. We suggested that the C 1-oxygenates were produced on the Sn 0.9Ge 0.1O 2 catalyst, not in the gas phase. We theoretically predicted that the substituted germanium ion occupied the site for fourfold Sn 2+ on the topmost layer by the DFT theoretical calculations. From the results of calculation, we assumed that the coverage of chemisorbed oxygen on the Ge/SnO 2 (110) surface was expected to be smaller than that on the SnO 2 (110) surface through the reactions, and we concluded that the smaller amount of chemisorbed oxygen through the reactions and the obstruction of oxygen migration over the Ge/SnO 2 was favorable to an increase in the selectivity for C 1-oxygenates in the reaction.