(513q) A Comparative Multiscale Computational Study of Methane Dry and Tri Reforming on Nickel Catalysts

Methane dry reforming (DR) and tri-reforming (TR) are gaining attention for their use of CO₂ to produce syngas. A popular reforming catalyst choice is Ni due to its high activity, despite poor stability. The main cause of catalyst deactivation is the formation of coke, yet less coking has been observed in TR experiments. It is extremely challenging to perform in-situ experimental characterization, reaction pathway analysis and to measure kinetics of elementary reaction steps to compare processes in terms of their reaction networks and catalyst stability. Hence, computational tools need to be implemented with higher level analyses to gain fundamental insights into reaction mechanisms, catalyst coverages, conversions, and deactivation. Hence, in this work, comprehensive networks of 38 and 53 elementary reactions were developed for DR and TR, respectively, including direct oxidation of CH_x^* and CO₂^*; the Boudouard reaction; as well as O₂ and H₂O adsorption and dissociation. The activation barrier, reaction energy, and kinetic rate constants were calculated for each reaction using density functional theory (DFT). DFT calculations were performed using VASP with the rPBE-vdW functional as it predicts CO and CO₂ adsorption best. Combining statistical thermodynamics, transition state theory, and ideal packed bed reactor molar balances allowed us to build microkinetic models (MKM) describing the change of surface coverages and conversion over space-time. The addition of O₂ and H₂O in TR increases the supply of oxidants on the surface available to oxidize any of the CH_x* species, thereby possibly limiting coke formation and/or increasing the possibility of coke oxidation. As such, the dominant reaction path for DR and TR were identified through the lowest energy pathway and MKM sensitivity analysis, then compared. Gaining multiscale understanding of DR and TR brings us one step closer to commercializing these processes through the rational design of active, selective and stable catalysts.