Theoretical Investigationof Water Gas Shift ReactionCatalyzed 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 densityfunctional theory and ab initio calculations. Our calculation resultsindicate 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 behavegenerally in a similar way. However, crucial differences appear insteps 3→ 4→ 5which involve dihydride M(H)2(CO)3COOH–(4′) and/or dihydrogen complexMH2(CO)3COOH–(4). The stability of the dihydrogen complexes becomes weaker downthe iron group. The dihydrogen complex 4_Feis only 11.1kJ/mol less stable than its dihydride 4′_Featthe B3LYP/II(f)ﯿ쇜똏ﲂ(f) level. Due to very low energy barrierit is very easy to realize the transform from 4_Feto 4′_Feand vice versa, and thus for Fe there is no substantialdifference to differentiate 4and 4′for the reaction cycle. The most possible key intermediate 4′_Ruis 38.2 kJ/mol more stable than 4_Ru. However, the barrier for the conversion 3_Ru→ 4′_Ruis 23.8 kJ/mol higher than that for 3_Ru→ 4_Ru. Additionally, 4′_Ruhas to go through 4_Ruto complete dehydrogenation 4′_Ru→ 5_Ru. The concerted mechanism 4′_Ru→ 6_Ru, in which the CO groupattacks ruthenium while H2dissociates, can be excluded.In contrast to Fe and Ru, the dihydrogen complex of Os is too unstableto exist at the level of theory. Moreover, we predict Fe and Ru speciesare more favorable than Os species for the WGSR, because the energybarriers for the 4→ 5processesof Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas140.5 kJ/mol is calculated for the conversion 4′→ 5of Os, which is significantly higher. Ingeneral, the calculations are in good agreement with available experimentaldata. We hope that our work will be beneficial to the developmentand design of the WGSR catalyst with high performance. [ABSTRACT FROM AUTHOR]