Mixed-metal oxides demonstrate notable reactivity, selectivity, and stability for a wide library of oxidative transformations. Bulk oxides, however, are often characterized by heterogeneous distributions of active sites that obscure the construction of robust structure-function relationships. As such, this work focuses on ordered Ruddlesden-Popper (R-P) phase oxides (A2BO4) with tunable A- and B-site cation identities within the context of oxidative coupling of methane (OCM), a direct pathway to form valorized C2 products. Here, we systematically tune A-site and B-site compositions to assess changes in reactive oxygen species involved in C2 product formation versus unproductive carbon oxides (CO2, CO). Although variations in the B-site composition of R-P oxides yield disparate (and stable) C2 selectivities, it remains ambiguous in literature whether the electronic nature of reactive lattice oxygen species or hydroxyl-mediated pathways dictate C2 selectivities within these mixed-metal oxides. We find that C2 selectivities are facilitated by hydroxyl-mediated pathways, as hydroxyl adsorption energies serve as unifying descriptors for measured C2 selectivities compared to descriptors related to the nature (electronegativity/oxophilicity) or mobility of lattice oxygens. However, the nature/mobility of lattice oxygens influences selectivity between complete (CO2) versus incomplete (CO) combustion pathways. Here, the concentration and mobility of lattice oxygen have limited impact on intrinsic CO formation rates within a given B-site series but contribute to CO2 formation rate differences. Overall, this work combines synthetic and kinetic approaches to unravel how A- and B-site compositions tune oxygenated reactive species, which provides valuable insight for informed design of future, atom-efficient catalysts for hydrocarbon oxidation processes to chemicals and fuels.
