Ab initio study of the reaction mechanism of CO2 with Ti atom in the ground and excited electronic states

Abstract

Density functional B3LYP/6-311+G(3df )//B3LYP/6-31G* calculations of potential energy surfaces (PES) have been performed for the Ti+CO2→TiO+CO reaction in the triplet, quintet, and singlet electronic states. The results indicate that in the ground triplet state the most favorable reaction mechanism involves insertion of the Ti atom into a CO bond [via a η2-C,O coordinated t-(TiOC)O complex] to produce a triplet t-OTiCO molecule with the energy gain of 43.9 kcal/mol and the latter can further dissociate to TiO(3Δ)+CO with the total reaction exothermicity of ∼30 kcal/mol. The addition mechanism leading to the same TiO(3Δ)+CO products via a metastable η2-O,O complex t-cyc-TiCO2 is also feasible at ambient temperatures since the highest barrier on the reaction pathway is only 4.7 kcal/mol. The reaction mechanisms in excited singlet and quintet electronic states have many similar features with the ground state reaction but also exhibit some differences. In the singlet state, the reaction can follow A″1 and A′1 pathways, of those the insertion via a s-(TiOC)O (1A′) complex leading to s-OTiCO (1A′) and then to TiO(1Δ)+CO does not have an activation barrier. The insertion mechanism on the A″1 PES depicts a low barrier of 1.8 kcal/mol and leads to s-OTiCO (1A″), which dissociates into TiO(1Δ)+CO. The addition pathways via η2-O,O coordinated complexes require to overcome significant barriers, 7.8 and 34.9 kcal/mol for the A″1 and A′1 states, respectively. In the quintet state, the reaction at low and ambient temperatures can proceed only by coordination of Ti(5F) toward CO2 with formation of η2-C,O q-(TiOC)O, η2-O,O q-cyc-TiCO2, and η1-O q-TiOCO bound by 9.7, 6.1, and 4.6 kcal/mol, respectively, relative to the reactants. The η2-C,O and η1-O coordinations occur without barriers, while the η2-O,O coordination has an entrance barrier of 4.2 kcal/mol. The calculated PESs show that the carbon dioxide reforming into CO in the presence of Ti atoms should take place spontaneously.