Invited Paper EH-MoM8
Activation of CO₂ using Heterogeneous Catalysts

Monday, December 12, 2016, 10:20 am, Room Lehua

Methanol synthesis by hydrogenation of CO₂ using Cu catalysts is one of the promising reactions to convert CO₂ into useful chemicals. Formate species is the pivotal intermediates formed as the initial step of CO₂ hydrogenation (CO₂ ＋ H_a → HCOO_a). The reaction rate of formate synthesis is very low and the reaction probabilities are about 10^-12 at 340K. The formate synthesis must be accelerated in order to convert CO₂ to methanol efficiently. Our previous kinetic measurements have suggested that formate is synthesized via Eley-Rideal typed mechanism, in which CO₂ molecules directly attack adsorbed hydrogen atoms on Cu surfaces. The structure insensitivity observed for formate synthesis experiments on Cu(111), Cu(100), and Cu(110) were well explained by the Eley -Rideal mechanism based on DFT calculations. In addition, sharp angular desorptions of CO₂ have been observed for formate decomposition as the reverse reaction of formate synthesis, indicating thermal non- equilibrium reaction. We thus performed molecular beam experiments to prove the Eley-Rideal typed mechanism, in which CO₂ molecules with controlling vibrational and translational energies were reacted with adsorbed hydrogen on cold Cu(111) and Cu(110) surfaces (TS =150-215 K). The formation of formate was found by infrared spectroscopy and temperature programmed desorption measurements of CO₂ and H₂ from decomposition of formate. We confirmed the formation of formate species on Cu(111) and Cu(110) with reaction probabilities of 10^-5 by heating nozzle above 1000 K, while no formate is formed at nozzle temperatures below 1000 K. The results indicate the Eley-Rideal typed mechanism with thermal non-equilibrium character. DFT calculations also reproduce the Eley-Rideal typed mechanism, in which translational and vibrational excitations of CO₂ are required to overcome the barrier of formate synthesis, 0.6-0.7 eV. The understanding on dynamics of CO₂ activation is expected to apply for the state-to state chemistry of methanol synthesis. That is, selective energy supply for an energy-efficient methanol synthesis process.