Carbonate dimorphism, and the reinterpretation of rates of lattice and excess oxygen-driven catalytic cycles

Abstract

Catalytic implications of local deviations from nominal oxide stoichiometry remain critical to consider yet challenging to elucidate in the context of bulk oxide-mediated light alkane oxidation reactions, in part due to a lack of a priori knowledge of surface active oxygen site counts. Carbonate dimorphism, i.e., differences in carbonate speciation resulting from CO2 adsorption onto unsupported nickel oxide surfaces, can be exploited toward quantifying the surface density of lattice and excess oxygens, and to further deconvolute their respective contributions toward specific steps within the ethane oxidation reaction network. Density functional theory, volumetric gas sorption, in situ titration, in situ spectroscopy, and transient kinetic data interpreted in light of quantitative estimates of surface excess oxygen density confirm the involvement of excess oxygen in partial oxidative turnovers producing ethene from ethane and O2. These collective studies cast a more nuanced interpretation of site requirements pertaining to nickel oxide-mediated alkane oxidation. Our findings reveal that excess oxygen atoms can be titrated under reaction conditions, and that these sites are (on average) more selective to ethene than lattice oxygens. Tools and methodologies employed herein connect seemingly disparate elements of bulk oxide catalysis research – the need for quantitative estimates of oxide surface non-stoichiometry on the one hand, and the propensity of unsupported oxides toward carbonate formation on the other – and provide a template for the possible broader application of quantitative analyses of probe-molecule binding characteristics toward elucidation of active site density and speciation in catalytic partial oxidation reaction systems of commercial relevance.