Mechanisms of the Water-Gas Shift Reaction Catalyzed by Ruthenium Carbonyl Complexes

Density functional theory (DFT) is employed to study the water-gas shift (WGS) reaction in the gas phase for two complexes, Ru-3(CO)(12) and Ru(CO)(5). Here we report four mechanisms of ruthenium carbonyl complexes catalyzed for WGS reaction. The energetic span model is applied to evaluate efficiency of the four catalytic pathways. Our results indicate that mechanism C and D show a good catalytic behavior, which is in agreement with results from the literature. The mechanism C and D not only include the important intermediate Ru-3(CO)(11)H- but also exclude the energy-demanding OH- desorption and revise an unfavorable factor of the previous mechanism. Two complexes along mechanisms B have the highest turnover frequency (TOF) values. The trinudear carbonyl complexes-Ru-3(CO)(12) is preferred over mononuclear carbonyl Ru(CO)(5) by comparing TOF due to the fact that metal metal cooperativity can enhance activity to the WGS reaction. In this work, the nature of interaction between transition states and intermediates is also analyzed by the detailed electronic densities of states, and we further clarify high catalytic activity of ruthenium carbonyl complexes as well. Our conclusions provide a guide to design catalysts for the WGS reaction.