(738b) Kinetics of Ethane Oxidative Dehydrogenation over Bulk NiO-Based Catalysts

Ethylene production through ethane oxidative dehydrogenation (ODHE) is a less energetically demanding process compared to the steam cracking. While Ni-M-O (M = Nb, W, etc.) materials have been studied as potential ODHE catalysts, which promote C₂H₄ selectivity (S_C2H4) compared to pure NiO, questions about NiO-based catalysts regarding 1) elementary steps mediating ODH turnovers and 2) properties that determine ODH rates and selectivity have not been well-established. In this respect, the effect of Ni-M-O non-stoichiometry on improving selectivity was obfuscated by the presence of heterogeneous oxide domains and multiple facets. Decoupling the effects of non-stoichiometry and aliovalent metals are essential for deconvoluting their catalytic functionalities to engineer these catalysts towards better performance. In this context, we report for the first time the use of well-defined, thermally stable NiO {100} terminated cubes for elucidating structure-ODHE catalytic property relationships.

In this effort, we first demonstrated that the degree of non-stoichiometry can be tuned by thermal treatment (T_ps) at 400-1000 °C without changing the cubic crystal habit of NiO. Crucially, non-stoichiometric oxygen (NSO) densities do not change substantially under reaction conditions irrespective of NiO_Tps, and catalytic studies point to a positive correlation between NSO densities and S_C2H4. We hypothesize that there exist at least two types of active sites (one being more selective than the other), and that their relative densities evolve with increasing T_ps, thereby resulting in a monotonic decrease in S_C2H4. To substantiate this hypothesis and extend our understanding to Ni-M-O systems, we performed kinetic studies on NiO and Ni-Nb-O. Our findings reveal that CO₂ inhibits ODH rates on the NiO but not Ni-Nb-O, providing further clues as to the basis for improvements in catalyst performance upon incorporating niobium. Collectively, these results provide molecular-level insights into the role of non-stoichiometric oxygen and aliovalent doping on NiO ODHE catalytic performance.