NiO-alkaline earth oxide catalysts for oxidative methane-to-syngas conversion: influence of alkaline earth oxide on the surface properties and temperature-programmed reduction/reaction by H₂ and methane

Abstract

Temperature-programmed reaction of pure methane from 100-900°C with NiO-MgO, NiO-CaO, NiO-SrO, and NiO-mixed alkaline earth oxides (viz., MgO-CaO, MgO-SrO, MgO-BaO, CaO-SrO, CaO-BaO, and SrO-BaO) with Ni/alkaline earth element(s) ratio of 1.0 (calcined at 930#176;C for 4 h) has been studied. Temperature-programmed reduction of these catalysts and NiO-MgO with different Ni/Mg ratios by H₂ from 100°C to 900°C has also been thoroughly investigated. The NiO-CaO, NiO-MgO-CaO, and NiO-MgO catalysts show increasingly superior performance in the oxidative conversion of methane to CO and H₂ at a very low contact time (1 ms). However, the presence of higher alkaline earth oxide (SrO and/or BaO) causes a large decrease or almost complete deactivation of the NiO-containing alkaline earth oxide(s) catalyst for the reaction. The catalysts before and after reduction (by H₂ at 500°C) are characterized for their relative surface composition of Ni and alkaline earth element(s) by XPS. Because of the reduction, the surface Ni/alkaline earth(s) ratio for all the catalysts is decreased; the decrease is, however, very large for the NiO-SrO and all the BaO containing catalysts.