Understanding the Mechanism of CO2 to CH4 Conversion and Hydrogen Evolution Reaction on Mg Nanoparticles in Water under ambient conditions: A DFT Perspective

Of late, experiments have demonstrated that CO2 can be reduced to Methanol and Methane (major product) accompanied by H2 evolution in water at room temperature using Magnesium (Mg) nanoparticles. Herein using Density Functional Theory (DFT) we have unearthed the underlying reaction pathways through which these important reductions occur on nanometer scale Mg clusters (Mg22 and Mg56). Our studies reveal that chemisorbed water on Mg NPs dissociate to form Mg-H and Mg-OH bonds. H2 evolution reaction (HER) occurs through the combination of a surface adsorbed hydride with a proton from a surface bound H2O molecule at a barrier of 3.1 kcal/mol. Moreover, we show that multiple alternating proton transfer from Mg-OH2 and hydride transfer from Mg-H facilitate surface bound CO2 reduction to chemisorbed CH3OH with the highest barrier being 22.5 kcal/mol. Notably, we find that Methanol C-O bond activation is facile (activation barrier 16.9 kcal/mol) which leads to formation of Mg-CH3 species. Subsequently, a low barrier reverse C-H activation type reaction between Mg-CH3 and proton from Mg-OH2 enables release of CH4.