Theoretical Investigation of Water Gas Shift Reaction Catalyzed by Iron Group Carbonyl Complexes M(CO)5 (M = Fe, Ru, Os)

We have investigated the mechanism of M(CO)(5) (M = Fe, Ru, Os) catalyzed water gas shift reaction (WGSR) by using density functional theory and ab initio calculations. Our calculation results indicate that the whole reaction cycle consists of six steps: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 2. In this stepwise mechanism the metals Fe, Ru, and Os behave generally in a similar way. However, crucial differences appear in steps 3 -> 4 -> 5 which involve dihydride M(H)(2)(CO)(3)COOH- (4') and/or dihydrogen complex MH2(CO)(3)COOH- (4). The stability of the dihydrogen complexes becomes weaker down the iron group. The dihydrogen complex 4_Fe is only 11.1 kJ/mol less stable than its dihydride 4'_Fe at the B3LYP/II(f)++//B3LYP/II(f) level. Due to very low energy barrier it is very easy to realize the transform from 4_Fe to 4'_Fe and vice versa, and thus for Fe there is no substantial difference to differentiate 4 and 4' for the reaction cycle. The most possible key intermediate 4'_Ru is 38.2 kJ/mol more stable than 4_Ru. However, the barrier for the conversion 3_Ru -> 4' Ru is 23.8 kJ/mol higher than that for 3_Ru -> 4_Ru. Additionally, 4'_Ru has to go through 4_Ru to complete dehydrogenation 4'_Ru -> 5_Ru. The concerted mechanism 4'_Ru -> 6_Ru, in which the CO group attacks ruthenium while H-2 dissociates, can be excluded. In contrast to Fe and Ru, the dihydrogen complex of Os is too unstable to exist at the level of theory. Moreover, we predict Fe and Ru species are more favorable than Os species for the WGSR, because the energy barriers for the 4 -> 5 processes of Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas 140.5 kJ/mol is calculated for the conversion 4' -> 5 of Os, which is significantly higher. In general, the calculations are in good agreement with available experimental data. We hope that our work will be beneficial to the development and design of the WGSR catalyst with high performance.