Paper SS2-ThA2
Thermal Non-Equilibrium Desorption of ¹³CO₂ Formed by the Decomposition of Formate on Cu Surfaces

Thursday, October 31, 2013, 2:20 pm, Room 202 A

It is well known that formate (HCOO*) specie is the important intermediate for methanol synthesis by hydrogenation of CO₂ on copper catalysts. Because the formate synthesis by reaction of CO₂ with adsorbed hydrogen atom has been suggested to be an Eley – Rideal type mechanism ^[1], we try to clarify the dynamics of formate decomposition on Cu（110）, Cu（100） and Cu（111）surfaces in this study. That is, both formate synthesis and formate decomposition are expected to be thermal non-equilibrium processes. We have thus measured angle-resolved temperature programmed desorption (AR-TPD) of CO₂ formed by decomposition of formate. Formate was prepared by decomposition of ¹³C-formic acid (H¹³COOH) on oxygen pre-adsorbed or oxygen-free Cu(110), Cu(100) and Cu(111) surfaces. AR-TPD experiments were then carried out for ¹³CO₂. The signal intensity of ¹³C¹⁶O₂ in distribution was plotted for each polar angle in the range -80° to 80°( 5° as interval angle) from TPD spectrum. It is found that identical sharp distributions of ¹³CO₂ were observed Cu(110), Cu(100) and Cu(111) surfaces, showing distribution normal to the surfaces with the function of cos⁸θ. These results indicate that the decomposition of formate is a thermal non-equilibrium process, in which CO₂ molecules desorb with hyper-thermal energy due to a repulsive force from the surfaces. It is also suggested that the local structures just before the CO₂ desorption near the transition states (TS) are the same on Cu（110）, Cu（100） and Cu（111）. That is, ¹³C of ¹³COO is bound with a hydrogen adatom, while the O-Cu bond has been already broken just before the CO₂ desorption. The TS structures may be free to rotate and therefore desorbed ¹³CO₂ distribution is insensitive to the surface structure. The structure insensitivity is consistent with that measured for the kinetics of formate synthesis. Density functional theory (DFT) calculations are also consistent with the proposed model. [1] H. Nakano, J. Nakamura et al., J. Phys. Chem. B, 105 (2001) 1355-1365.