Catalytic methane dissociation and its non-oxidative coupling in metal-dispersed molten salt media: an ab initio molecular dynamics investigation

Abstract

Methane dehydrogenation (CH_4(g) → C_(s) + 2H_2(g)) in molten media is an emerging technology to produce CO₂-free hydrogen and solid carbon. However, molten salts exhibit little catalytic activity for methane dissociation. In this study, we propose a catalytically active solid metals dispersed molten salt for the non-oxidative dehydrogenation of methane, investigating both the sequential dehydrogenation of methane and its non-oxidative coupling, which can produce more valuable C₂ products over solid carbon. Four different solid metals, namely, nickel, boron-doped nickel, copper, and boron-doped copper are investigated for their activity, stability against coking, and selectivity towards C₂ products in the molten sodium bromide (NaBr) salt. The catalytic reactions in the explicit condensed phase, with finite temperature effects are studied using ab initio molecular dynamics (AIMD) and metadynamics simulations at 1200 K. Our investigation demonstrated that at high temperatures, microstructural changes in the Cu catalyst are more pronounced than those in the Ni catalyst, enhancing the activity of Cu significantly. Moreover, these operando structural changes in the catalyst at high temperatures can only be captured by AIMD simulations, and not by ground-state DFT calculations. Our calculated free energy barriers for methane dehydrogenation indicate that boron doping in Ni and Cu catalysts lowers the CH₄ activation barrier by 39 kJ mol⁻¹ and 60 kJ mol⁻¹, respectively in comparison with Ni catalysts. Furthermore, we found that the CuB–NaBr system kinetically promotes non-oxidative C–C coupling reactions over the competing dehydrogenation of CH_x intermediates, whereas other metal-dispersed systems primarily favor complete CH₄ dehydrogenation to form carbon. Interestingly, the carbon generated as a byproduct diffuses into Ni and Cu, leading to deactivation, but boron-doped systems prevent this diffusion, making them promising candidates that are stable against catalyst deactivation. Moreover, we have not observed any leaching of metal atoms from the catalyst into the molten salt medium, nor the diffusion of boron from the subsurface to the on-the-surface at these elevated temperatures, ensuring the stability of the system under these conditions. This first-principles-based study revealed that heterogeneous catalysts in molten salts have the potential to catalyse the non-oxidative dehydrogenation of CH₄, and that boron-doped Cu in molten NaBr is a promising system for non-oxidative C–C coupling reactions.