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. Theadsorption Of CO2 on a Mo4C2 cluster produces an optimized structure with the adsorption energy of -212.2 kJ/mol, higher than that on the Mo-4 cluster. The first hydrogenation Of CO2 on the Mo4C2 cluster led to GRAPHICS through the cleavage of C-O and O of the adsorbed carbon dioxide with hydrogen addition. This step has a lower activation energy than that for the Mo-4 cluster to HO2C-Mo-4, which was formed by hydrogen addition to the adsorbed carbon dioxide without C-O cleavage. The second hydrogenation of GRAPHICS involved a higher activation energy than the cleavage of C-O and O-H of HO2C-MO4 with hydrogen to yield GRAPHICS The difference in the adsorption stabilities Of CO2 on the Mo4C2 and Mo-4 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. (c) 2004 Elsevier B.V. All rights reserved.