(683c) Unravelling the Active Site Speciation of Highly Tunable Oxy-Carbide Catalysts with Atomic Level Detail

Metal carbides are an exciting class of catalysts for dry reforming, dehydration, and biomass hydro-deoxygenation. Under reaction conditions, the carbide surface dynamically forms oxy-carbide phases with the surface oxygen to carbon ratio dictated by the external environment. Such oxy-carbide phases unexpectedly mitigate coking during the dry reforming of methane.^[1] Modulating the oxygen to carbon ratio on the surface further presents exquisitely tunable acid and redox sites.^[2] Although knowledge about the active site speciation of oxy-carbides under reaction conditions is emerging, we are yet to unravel how the external redox environment steers the surface state of oxy-carbides. To plug this knowledge gap, we employ first principles thermodynamics to determine the active site speciation under reaction conditions. We first calculate surface energies of prototypical stepped and terrace crystal planes for V_xC_y phases in vacuum. We show that the most stable crystal plane is VC (100). Under reaction conditions however, the thermodynamic stability is governed by adsorption energies of O*/C*, co-adsorbate interactions between O* and C*, and the chemical potential of the oxidizing (CO₂) and reducing (CH₄) gases. We conceptualize an accelerated scheme to rapidly determine the adsorption energies and co-adsorbate interactions thus exploring a vast configurational space with minimal density functional theory (DFT) derived inputs. This scheme unites site-specific scaling relations between the active site stability and adsorption energies, together with additive pair-wise interactions between co-adsorbates. After extensively validating our scheme on prototypical V_xC_y crystal planes (mean averaged error < 0.10 eV), we unleash this approach to determine external conditions (temperature, pressure of CO₂/CH₄) which stabilize coking resistant oxy-carbide phases during dry reforming. We also quantify the C*/O* coverage, which implicitly governs the nature of acid and redox active sites. The proposed scheme serves as a blueprint for understanding the structural dynamics of oxy-carbides under reaction conditions.