Engineering Morphology and Ni Substitution of NixCo3–xO4 Spinel Oxides to Promote Catalytic Combustion of Ethane: Elucidating the Influence of Oxygen Defects

Transition metal oxides are potential alternatives to noble metal catalysts for oxidation reactions. Co-based spinel oxides, in particular, have attracted significant attention. Herein, Ni x Co 3– x O 4 catalysts were synthesized to elucidate the influence of oxygen vacancies on catalyst activities and reaction mechanisms for ethane combustion. A correlation between the activity and the population and properties of O defects was developed, with an increased number of O defects typically resulting in higher activity. Also, the shape-induced facet effect is related to the amount of Ni that is incorporated into the octahedral sites of Co oxide. The substituted Ni atoms altered the redox ability of Ni x Co 3– x O 4 by changing O vacancy formation and C–H bond dissociation. The NiCo 2 O 4 -TM catalyst (6.0 mmol mL –1 h –1 ) exhibits the highest activity for ethane oxidation compared with NiCo 2 O 4 -PC (0.5 mmol mL –1 h –1 ) and NiCo 2 O 4 -OL catalysts (1.3 mmol mL –1 h –1 ) at 330 °C, and its activation energy (E a ) is 70.9 kJ mol –1 . No activity decay is observed after the initial transition stage of the reaction in a long-term stability test up to 500 h on the NiCo 2 O 4 -TM-coated monolith, either with or without water addition. A vacancy-mediated pathway was proposed according to in situ diffuse reflectance infrared Fourier transform (DRIFT) and density functional theory (DFT) calculations over the NiCo 2 O 4 (311) facet. Findings from this study expand our understanding of the facet-dependent catalytic behavior and ultimately enable the rational design of high-performance catalysts.