Modeling of Active Sites and Mechanism of Oxidative Coupling of Methane over Alkali Tungstate Catalysts

There has been an increase in demand worldwide for efficient use of natural gas. Thus, studies have been investigating the oxidative coupling of methane (OCM) to higher hydrocarbons, which can directly form a C–C bond in a single reactor at high temperatures. However, the OCM reaction still has a drawback of low product selectivity due to overoxidation of the C2 products. Among the OCM catalysts investigated, alkali-based catalysts, especially Na2WO4 and K2WO4, have demonstrated a high C2 yield via a unique response to H2O, one of the dominant products of the OCM reaction. It was postulated that the catalyst forms highly reactive OH radicals from O2 and H2O, which is responsible for its high selectivity and conversion rate. Previous studies with near-ambient-pressure X-ray photoelectron spectroscopy suggested the involvement of alkali (su)peroxide formation. Based on this, density functional theory calculation was applied to examine an unmolten K2WO4 surface (in contrast to molten Na2WO4). To construct the crystal structure for calculation, in situ X-ray diffraction measurement was used, which provided the crystal phase of hexagonal K2WO4 at temperatures relevant to OCM. This phase resulted from the transformation of monoclinic K2WO4under ambient conditions. Generally, a stable surface was found to occur with K atoms on the outer surface rather than with the O- or W-exposed surfaces. After considering various intermediates with different positions, a plausible surface pathway that leads to H2O2 through adsorptions of H2O and O2 was suggested. The calculation results support the hypothesis of the involvement of OH radicals in the OCM through a unique water-activation pathway.