Tuning the Interplay of Metal and Oxygen Environments within Mixed Metal Oxide Catalysts for Oxidative Methane Conversions

Abstract Mixed-metal oxides demonstrate notable catalytic activity for a wide library of oxidative transformations. Bulk oxides, however, often have heterogeneous active site ensembles that obscure the construction of robust structure–function relationships necessary for the design of sustainable reaction systems. This work focuses on ordered Ruddlesden–Popper (R-P) phase oxides (A2BO4) with tunable A- and B-site cation identities and defined atomistic arrangements as a modular platform. We utilize this platform to elucidate materials design principles for selective catalysts within the context of oxidative coupling of methane (OCM), a direct pathway to form valorized C2+ products. Here, we systematically tune B-site cation identities within LaSrBO4 (B = Fe, Co, Ni, Al) oxides to deconvolute catalyst B-site cation property contributions from fluid-phase events involved in C2 versus carbon oxides (COx) formation. Variations in B-site identity yield disparate C2 selectivities at isoconversion conditions (<10%; 1073 K), with the Al variant yielding the highest C2 selectivity. However, literature remains ambiguous whether the abundance or electronic nature of reactive lattice oxygen species versus other (OH-mediated) pathways dictate C2 selectivities here and in other catalytic architectures. The kinetic and spectroscopic assessments reported herein show that C2 selectivities are governed by an interplay between lattice oxygen mobility and OH-mediated pathways, with B-site electronegativity and OH adsorption energies serving as appropriate descriptors. Finally, water cofeed studies suggest that higher C2 production rates are conferred as gas-phase OH concentrations increase and affect C2 product distributions (ethane, ethylene) by reducing the probability of overoxidation events to COx via lattice oxygen or O2. Overall, this work unravels how cation identity in mixed-metal oxide catalysts can be leveraged for informed design of atom-efficient catalysts for selective hydrocarbon oxidation to chemicals and fuels.