(391e) Nanoscale Influence of Dopant on the Selectivity of Ethane Oxidative Dehydrogenation over NiO-Based Catalysts

Light olefins are critical chemical building blocks in the petrochemical industry, but the current industrial processes have significant shortcomings involving energy intensity and coke formation. Thus, ethane oxidative dehydrogenation (EODH) is an attractive route for ethylene production. NiO-based catalysts show promise for EODH reactions at relatively low temperatures, and the ethylene selectivity can be greatly increased via incorporation of transition metal promoters. However, the catalytic effect of the promoter is difficult to predict a priori due to the lack of insights into the working catalyst surface structure. Here, we address this challenge by predicting the in situ structure of a series of M-NiO catalysts, simultaneously determining the effect of facet ((100), (110)), promoter element (Al, Mo, Nb, Sn, Ti, V, W, Zr), promoter placement (surface, subsurface), defects formation (oxygen vacancy, Ni vacancy), and oxygen adsorption (O*, O₂*) from density functional theory (DFT) calculations. After identifying dominant N-NiO surfaces under EODH conditions, the promoter effect on selectivity determining steps was evaluated based on four ethane-derived species (CH₃CH₂*, CH₂CH₂*, CH₃CH*, CH₂CH*). For desired C-H scission to occur on NiO surfaces, CH₃CH₂* must adsorb onto Ni sites, with O site adsorption leading to undesired C-H/C-C scission steps and CO₂ formation. Furthermore, the C-H scission transition state is highly charged, and the activation energies are closely related to the charges of the C_xH_y* fragment and Ni site. The introduction of transition metal promotor into NiO systems as well as distinct surface defects leads to surface charge redistribution, particularly for each active site, that significantly affects the activation barriers for selectivity determining steps. Overall, our work both enables the identification of surface structures present during EODH and sheds light on the intrinsic nature of how different promoters control CH₂CH₂* formation among selectivity determining steps with better EODH performance for the design of future NiO-based catalysts.