We have long studied the formation of formate (HCOOa) intermediates as an intermediates of methanol synthesis by hydrogenation of CO2 on Cu surfaces [1-4]. It has been suggested based on the kinetic analysis for the high pressure bulb experiments that the formate formation proceeds via Eley-Rideal type mechanism, in which CO2 directly reacts with adsorbed hydrogen atom on Cu. In order to clarify the mechanism and dynamics, we carried out CO2 molecular beam studies with the translational energy of 1.12-1.97 eV and the nozzle temperature of 800-1100 K. It was found that hot CO2 in the molecular beam reacts directly with pre-adsorbed hydrogen atoms on cold Cu(111) and Cu(100) surfaces at 120–220 K to form formate adspecies (CO2 + Ha → HCOOa). That is, even at the low surface temperatures, formate species is formed rapidly when supplying energy only to CO2. This indicates that the energy to overcome the reaction barrier comes from the hot CO2 molecule itself instead of the Cu surface. The vibrational energy of CO2 was much more effective for the reaction compared to the translational energy; and the reaction rate was independent of the surface temperature. The independence of surface temperature indicates the E-R type mechanism that the CO2 molecule is not thermally equilibrated with the Cu surfaces but directly reacts with Ha. If the impinging CO2 molecule chemisorbs on the Cu surfaces before reacting with Ha, the reaction rate should be dependent on the surface temperature, rather than the kinetic and the internal energies of CO2. We evaluated the barrier distribution factors (W) of so-called Luntz equation by analysis of the reaction probability curves, which shows significant efficacy of the vibrational energy on the reaction of CO2, which has not been observed before. DFT calculations are consistent with the experimental observations [5]. The direct reaction of CO2 with adsorbed hydrogen atom can be regarded as the first example of vibration-driven bond formation reactions on surfaces.

Vibrationally energy-driven reactions of CO2 on Cu surfaces

2019 Fall National Meeting

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