Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na2WO4/SiO2

Abstract

Na2WO4/SiO2, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH4, C2H6, C2H4, C3H6, C6H6) in the presence of gas-phase dioxygen at 883–983 K. Hydrogen combustion rates exhibit a near-first-order dependence on H2 partial pressure and are zero-order in H2O and O2 partial pressures. Mechanistic studies at 923 K using isotopically-labeled reagents demonstrate the kinetic relevance of H−H dissociation and absence of O-atom recombination. In situ X-ray diffraction (XRD) and W LIII-edge X-ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second-shell coordination with respect to W6+ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na2WO4 (971 K) during SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox-active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.