Synthesis of methanol from CO₂ hydrogenation promoted by dissociative adsorption of hydrogen on a Ga₃Ni₅(221) surface

Catalytic carbon dioxide (CO₂) hydrogenation to liquid fuels including methanol (CH₃OH) has attracted great attention in recent years. In this work, density functional theory (DFT) calculations have been employed to study the reaction mechanisms of CO₂ hydrogenation to CH₃OH on Ga₃Ni₅(221) surfaces. The results show that all intermediates except for the O atom prefer to adsorb on Ni sites, and dissociative adsorption of hydrogen (H₂) on the Ga₃Ni₅(221) surface is almost barrierless and highly exothermic, favoring CO₂ hydrogenation. Moreover, the presence of Ga indeed enhances the dissociative adsorption of H₂, and this is verified by the projected density of states (PDOS) analysis. Importantly, three possible reaction pathways based on formate (HCOO) and hydrocarboxyl (COOH) formations and reverse water gas shift (rWGS) with carbon monoxide (CO) hydrogenation have been discussed. It is found that CO₂ reduction to CH₃OH in these pathways prefers to occur entirely via the Langmuir-Hinshelwood (L-H) mechanism. COOH generation is the most favorable pathway because the HCOO and rWGS with CO hydrogenation pathways have high energy barriers and the resulting HCOOH intermediate in the HCOO pathway is unstable. In the COOH reaction pathway, CO₂ is firstly hydrogenated to trans-COOH, followed by the formation of COH via three isomers of COHOH, its hydrogenation to trans-HCOH, and then the production of CH₃OH via a CH₂OH intermediate.