AVS 63rd International Symposium & Exhibition
    Fundamental Discoveries in Heterogeneous Catalysis Focus Topic Thursday Sessions
       Session HC+SS-ThM

Paper HC+SS-ThM2
Dynamics of Formate Synthesis from CO2 and Formate Decomposition on Cu Surfaces

Thursday, November 10, 2016, 8:20 am, Room 103A

Session: Dynamics of Gas-surface Interactions in Heterogeneous Catalysis
Presenter: Junji Nakamura, University of Tsukuba, Japan
Authors: J. Quan, University of Tsukuba, Japan
T. Kozarashi, University of Tsukuba, Japan
T. Ogawa, University of Tsukuba, Japan
T. Kondo, University of Tsukuba, Japan
J. Nakamura, University of Tsukuba, Japan
Correspondent: Click to Email
Much attention has been paid to methanol synthesis by hydrogenation of CO2 as a promising chemical conversion of CO2. It has been well-known that Cu-based catalysts show high activity for the methanol synthesis, in which the initial elementary step of CO2 is formation of formate species by the reaction of CO2 with surface hydrogen atoms on Cu. Previous kinetic measurements of the formate synthesis by hydrogenation of CO2 have suggested an Eley-Rideal (E-R) typed mechanism that CO2 directly attacks a hydrogen atom on Cu surfaces. We have thus tried to prove the E-R typed mechanism by molecular beam experiments assuming that the reaction between incoming CO2 molecules and Cu surfaces is thermally non-equilibrated. We first prepared hydrogen atoms on cold Cu(111), Cu(110), and Cu(100) kept around 200 K and then hot CO2 molecular beams were illuminated onto the surfaces with controlling its vibrational and translational energy. It is found that the formate synthesis proceeds significantly without heating Cu samples if vibrational and translational energies are supplied to gaseous CO2. The results clearly indicate the thermal non-equilibrium dynamics. Possibility of a tunneling mechanism between CO2 and a hydrogen atom on Cu was discarded because no significant H/D effect was observed on the reaction rate of formate synthesis. On the other hand, we have studied the dynamics of the formate decomposition as a reverse reaction of the formate synthesis. We measured the angular distribution and the translational energy of desorbed CO2 formed by the decomposition of formate on Cu(110) under a steady state reaction of HCOOH and O2. The angular distribution showed a sharp collimation, cos6θ, perpendicular to the surface. The translational energy of CO2 was independent of the surface temperature of Cu(110). It is thus found that the formate decomposition is also thermal non-equilibrium dynamics. However, the translational energy was as low as 100 meV, which is much lower than that required for formate synthesis, about 600 meV. The discrepancy will be discussed in the presentation.