Ab initio and density functional calculations of the potential energy surfaces for the Ni + CO2 → NiO + CO reaction in the lowest triplet and singlet electronic states have been carried out at the B3LYP/6-31G*, B3LYP/6-311G*, B3LYP/6-311+G(3df), CCSD(T)/6-311G*, and CCSD(T)/6-311+G(3df) theoretical levels. The reaction is calculated to preferentially occur in the triplet state and to proceed by the formation of a cyclic four-member ring C2v-symmetric NiOCO intermediate (t-cyc) that lies ∼19 kcal/mol above the reactants. The barrier for the initial reaction step is about 23 kcal/mol. From t-cyc the reaction continues via transition state t-TS2 toward the linear t-CONiO complex. The latter is stabilized by ∼10 kcal/mol with respect to the products, NiO (3Σ-) + CO, and can dissociate producing them without exit barrier. The highest barrier at the reaction pathway, about 53 kcal/mol, occurs at t-TS2. The reverse NiO (3Σ-) + CO reaction yielding Ni atoms and CO2 with exothermicity of 36 kcal/mol is shown to have a barrier of 15 kcal/mol relative to the reactants occurring at the second reaction step. On this basis, nickel oxide is expected to be less efficient for oxidizing CO to CO2 than the oxides of alkaline earth metals. Reduction of CO2 to CO can be significantly enhanced in the presence of Ni atoms due to much lower endothermicity (36−37 kcal/mol) and activation barrier (∼53 kca/mol) for the t-Ni + CO2 → NiO (3Σ-) + CO reaction as compared to those for the unimolecular decomposition of carbon dioxide. The accuracies of different theoretical methods for calculations of the reaction energies have been compared.
Journal of physical chemistry A 104(49), pp.11622-11627