(675h) Determining Promoter Effects in the Nanoscale Surface Structure and Stability of NiO-Based Catalysts for Ethane Oxidative Dehydrogenation

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 screening the stability of a series of M-NiO catalysts, simultaneously determining the effect of facet ((111), (100), (110)), promoter element (V, Mo, Sn, W, Nb, Ti, Zr), promoter placement (surface, subsurface), and defects formation (oxygen vacancy, Ni vacancy). This study combines density functional theory and ab initio phase diagrams. Using (100) facet as an example (Figure 1), the stability of each M-NiO surface was determined by calculating the Gibbs free formation energy of formation over a wide range of temperatures and pressures. From the phase diagrams, we can see that the V, Mo, and Nb promoters’ preferential configuration is in the subsurface under EODH conditions (300-500 ^oC, ~1 bar), while the Sn, Ti, and Zr promoters prefer locating in the surface layer. Notably, the majority of thermodynamically preferred M-NiO structures have no vacancies under EODH environments, indicating the promoted (100) surfaces have maintained a fully oxidated state. The W promoter is an exception as it shows a similar preference for both the fully oxidized and single surface oxygen vacancy under EODH conditions. Overall, this approach both enables the identification of surface structures present under EODH conditions and reduces the number of structures required for sampling the EODH potential energy surface in future sequential studies.