Theoretical study of TiO-catalyzed hydrogenation of carbon dioxide to formic acid

Density functional B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) calculations have been performed for the potential energy surface of the CO2/H-2/TiO system. The results demonstrate that titanium oxide can serve as a catalyst for the gas-phase hydrogenation of carbon dioxide to formic acid. The most favorable reaction pathways are CO2 + H-2 + TiO((3)Delta) --> H-2 + eta(2)-CO2-TiO --> TS2 --> eta(2)-CO2-HTiOH --> TS3 --> OC(H)OTiOH --> TS4 --> cyc-HCO2TiOH --> TS5 --> cis-HOC(H)OTiO --> cis-HCOOH + TiO((3)Delta) and (starting from the OC(H)OTiOH intermediate) OC(H)OTiOH --> TS6 --> trans-HC(O)O(H)TiO --> trans-HCOOH + TiO((3)Delta). In these mechanisms, the highest relative energies with respect to the initial reactants are found for the final products, 7.4 and 11.0 kcal/mol for the trans and cis conformers of formic acid, respectively, and all transition states have lower energies. Therefore, in the gas phase, titanium oxide can be an efficient catalyst; the activation energy for the TiO-catalyzed hydrogenation of CO2 coincides with its endothermicity. However, the highest barriers for individual reaction steps are lowered to 60.1 and 54.5 kcal/mol, respectively, as compared to 73.8 kcal/mol for the uncatalyzed CO2 + H-2 --> HCOOH reaction, indicating that in the condensed phase TiO would not be a particularly efficient catalyst.