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

Abstract

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.

Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst

“…These results indicate clearly that reduction in high pressure hydrogen/deuterium does not contribute significantly to the decay rate of bidentate copper formate on Cu/SiO 2 . Previous results have indicated a significant role for a hydrogenation channel in the reaction of surface-adsorbed formate on Cu (100) [11], Cu [111], Cu [110] as well as Cu/SiO 2 catalysts [16][17][18], in contrast to the present results. The presence or absence of the hydrogen effect is discussed further below.…”

“…The prominent band in H 2 / 12 CO 2 at 1,350 cm -1 is assigned to the O-C-O symmetric stretching mode (m s (CO 2 -)) [10,14,17,18,20,21,38,39], whereas the less intense, broad, 1,580 cm -1 mode is the corresponding asymmetric mode [20,21,26,28,29,38], both referred to as bidentate formate on copper. Of particular interest is the weaker feature at 1,365 cm -1 .…”

“…Previous studies of formate coverage on Cu/SiO 2 versus temperature have been performed by Nakamura and coworkers [16][17][18] at one atmosphere pressure, wherein the coverages were quantified by desorption of CO 2 product after reaction. Here we extend measurements of formate coverage to 6 bar total pressure conditions in both D 2 / 12 CO 2 and H 2 / 12 CO 2 atmospheres.…”

“…As a result, traditional surface analysis using elementary step by step investigations in ultra high vacuum can provide only limited information. Other studies have addressed surface formate reactivity on supported copper catalysts [16][17][18]. These studies have also observed the presence of surface-bound formate species under reaction conditions [16] and also have measured the stability of these species in inert gas and hydrogen [17,18].…”

“…Other studies have addressed surface formate reactivity on supported copper catalysts [16][17][18]. These studies have also observed the presence of surface-bound formate species under reaction conditions [16] and also have measured the stability of these species in inert gas and hydrogen [17,18]. Compared with single crystal copper surfaces, the surface species observed on the oxide supported polycrystalline copper system are more complex because of the multi-faceted copper surfaces presented by the supported particles [14,17,[19][20][21] and the presence of species adsorbed on the support materials (e.g., alumina and ceria) as well as the support-metal interface [22].…”