Engineering Morphology and Ni Substitution of Ni<i><sub>x</sub></i>Co<sub>3–<i>x</i></sub>O<sub>4</sub> Spinel Oxides to Promote Catalytic Combustion of Ethane: Elucidating the Influence of Oxygen Defects
Shida Liu, Haiyan Wang, Shuang Wang, Yiling Dai, Bing Liu, Liu Yi, Feixiong Dang, Kevin J. Smith, Xiaowa Nie, Shuandi Hou, Xinwen Guo
Abstract
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.