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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 H₂partial pressure and are zero‐order in H₂O and O₂partial 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 L_III‐edge X‐ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na₂WO₄crystallinity 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/SiO₂SHC 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. 

The requirement for C₂H₂concentrations below 2 parts per million (ppm) in gas streams for C₂H₄polymerization necessitates its semihydrogenation to C₂H₄. We demonstrate selective chemical looping combustion of C₂H₂in C₂H₄-rich streams by Bi₂O₃as an alternative catalytic pathway to reduce C₂H₂concentration below 2 ppm. Bi₂O₃combusts C₂H₂with a first-order rate constant that is 3000 times greater than the rate constant for C₂H₄combustion. In successive redox cycles, the lattice O of Bi₂O₃can be fully replenished without discernible changes in local Bi coordination or C₂H₂combustion selectivity. Heterolytic activation of C–H bonds across Bi–O sites and the higher acidity of C₂H₂results in lower barriers for C₂H₂activation than C₂H₄, enabling selective catalytic hydrocarbon combustion leveraging differences in molecular deprotonation energies. 

Abstract Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO₂, PtCoO₂) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 μm. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we report a different approach to PdCoO₂crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios ( > 440). Nevertheless, detailed mass spectrometry measurements on these materials reveal that they are not ultrapure in a general sense, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (∼1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a sublattice purification mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites.