(560fd) Combined DFT and Microkinetic Study of Dry Reforming of Methane on Ni and B Promoted Ni Surfaces

Dry reforming of methane (DRM) has incited significant academic and industrial attention in the past couple of decades. Although Ni-based catalysts have shown good activity for DRM, deactivation due to carbon deposition is a serious concern. Several strategies are proposed, including doping by other metals and metalloids, to improve the stability of Ni, and hence Boron doped Ni is investigated in this work as a potential catalyst for DRM. The effect of Boron doping on the catalytic activity and selectivity can be evaluated by identifying the elementary reaction steps and by computing reaction energetics in the conversion of CO₂ and CH₄ on Ni and NiB surfaces. In this work, combined DFT and microkinetic modelling are performed to identify the dominant reaction pathways and kinetically relevant steps of the DRM reaction system on Ni and NiB surfaces. The accuracy of DFT methods is crucial and hence benchmarking study was first performed to assess the accuracy of DFT functionals and rPBE-vdW functional, with a correction of 28 kJ/mol for gas phase CO₂ energy, was found to predict the adsorption and reactions accurately. We considered a detailed reaction network involving multiple CO₂ dissociation routes, CH₄ dissociation routes, side reactions (H₂O formation, Boudouard reaction) and desorption of products on Ni and NiB surfaces. Our calculations suggest that both Ni and NiB share similar CO₂ (direct dissociation to CO*) and CH₄ (CH₄ dehydrogenates to C* and then C* is oxidized by OH* to COH* which dissociates to CO*) activation routes. Compared to Ni, the CO₂ activation barrier on NiB surface is higher but the barriers in CH₄ activation routes are significantly lower on NiB. Besides this, CO formation by Boudouard reaction (CO₂*+C*→2CO*) also has a lower barrier on NiB compared to Ni. The dominating reaction pathway on Ni involves direct dissociation of CO₂* to CO* and atomic O*, and CH₄ dissociation to C*, followed by OH* oxidation and finally COH* decomposition to form CO* and the rate-limiting step is C* oxidation by OH*. On NiB, the dominant reaction pathway is CH₄ dissociation to C* and then C* reacts with CO₂ to produce CO*. Here, the rate-limiting step is C* reacting with CO₂*. Subsequently, the DFT calculated energies were used to derive kinetic rate constants of the elementary reaction steps. These rate constants were employed to develop a microkinetic model and the DFT results were further extended to calculate conversions and coverages under reaction conditions. From the current study, we establish that the presence of B alters the DRM reaction pathway with a considerable reduction in the rate limiting barrier. Consequently, by making the alternate pathway which consumes the formed carbon fast enough, this NiB surface can make a better catalyst. Thus, we believe that NiB can be a potential catalyst that can prevent deactivation with an improvement in the catalytic activity for the DRM process.