Density functional study of carbon dioxide hydrogenation on molybdenum carbide and metal Original Research Article

The density functional theory with a cluster approach was used to study the relative stabilities and reactivities of the surface species of molybdenum carbide and metal during the hydrogenation of carbon dioxide. The adsorption of CO2 on a Mo4C2 cluster produces an optimized structure with the adsorption energy of −212.2 kJ/mol, higher than that on the Mo4 cluster. The first hydrogenation of CO2 on the Mo4C2 cluster led to through the cleavage of Csingle bondO and O of the adsorbed carbon dioxide with hydrogen addition. This step has a lower activation energy than that for the Mo4 cluster to HO2Csingle bondMo4, which was formed by hydrogen addition to the adsorbed carbon dioxide without Csingle bondO cleavage. The second hydrogenation of to involved a higher activation energy than the cleavage of Csingle bondO and Osingle bondH of HO2Csingle bondMo4 with hydrogen to yield . The difference in the adsorption stabilities of CO2 on the Mo4C2 and Mo4 clusters was also explained by the difference in the electronic structures and the orbital interactions near the highest orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. The experimental result of the CO2 hydrogenation showed that Mo carbide preferentially catalyzed the reverse gas shift reaction to yield CO and H2O compared to Mo metal. The density functional theory (DFT) calculation accounts for the higher activity of molybdenum carbide for CO2 hydrogenation being higher than that of molybdenum metal.