Structure-Dependent Kinetics for Synthesis and Decomposition of Formate Species over Cu(111) and Cu(110) Model Catalysts

The kinetics and mechanism of formate synthesis by hydrogenation of CO2 (CO2 + 1/2H2 → HCOOa) and the formate decomposition into CO2 and H2 (HCOOa → CO2 + 1/2H2) over Cu(111) and Cu(110) surfaces were studied by in-situ infrared reflection−absorption spectroscopy (IRAS) using a high-pressure reactor (∼1 atm). The reaction rates and the apparent activation energy of the formate synthesis were measured for Cu(111) and Cu(110), indicating that the formate synthesis on Cu was found to be structure-insensitive. The pressure dependence of CO2 and H2 on the initial formation rate of formate suggested an Eley−Rideal type mechanism, in which a gaseous CO2 molecule directly reacts with an adsorbed hydrogen atom on Cu. This is analogous to the well-known mechanism of formate synthesis by organometallic catalysts, in which CO2 is inserted into a Cu−hydride bond. The reaction rates and the activation energy of the decomposition were measured for Cu(111) and Cu(110). It was found that the formate decomposition on Cu was structure-sensitive in contrast to the formate synthesis. The promotional effect of coexisting H2 upon the rate of formate decomposition by 17 times at maximum was incidentally found only on Cu(111). Interestingly, the increase in the decomposition rate was due to an increase in the preexponential factor of the rate constant for the formate decomposition with the activation energy being constant. Furthermore, the decomposition kinetics of the formate prepared by adsorption of formic acid on O/Cu(111) was identical with the H2-promoted decomposition kinetics of the synthesized formate. The difference in the decomposition kinetics was ascribed to the ordered structure of formate based on the previous STM results, in which a chainlike structure of formate was observed for the synthesized formate, whereas no formate chain was observed for the formate prepared by adsorption of formic acid on O/Cu(111). The unique character of both the decomposition kinetics and the structure of formate observed only for Cu(111) was discussed from the viewpoint of the mass transport of copper atoms creating added formate chains.