The adsorbed structure of carbonate species formed via CO₂ disproportion on Cu(111)

Abstract

Surface chemistry of carbon dioxide (CO₂) is one of the topics that have received much attention because of its impact on catalytic CO₂ reduction, including methanol synthesis, reverse water gas shift, Sabatier reaction, and so on [1]. Copper is an essential element for the methanol synthesis catalyst, and thereby, the reaction mechanism of methanol synthesis from CO₂ on the Cu-based catalyst surface has been intensively investigated [2]. One of the plausible mechanisms is the pathway where formate species play a critical role. While formate is recognized to be formed via the reaction of CO₂ with an adsorbed hydrogen atom [2], some studies have proposed the presence of carbonate (CO₃) species as a precursor for formate [3]. The recent in-situ spectroscopic studies on the well-defined Cu model catalyst have supported the CO₃-mediate pathway [4]. Despite such importance of CO₃, the works focusing on the detailed properties of CO₃ on the well-defined Cu surface have been little reported. In this study, we investigated the adsorption structure of CO₃ on the Cu(111) surface using scanning tunneling microscopy (STM) and in-situ infrared reflection absorption spectroscopy (IRAS).

The clean Cu(111) surface was prepared by several cycles of Ar⁺ sputtering and annealing at 670 K. In the present STM experiment, Cu(111) was exposed to 0.1 Pa of CO₂ at 300 K in a load-lock chamber. The CO₂-exposed sample was transferred to the STM chamber, and the STM measurements were carried out at 80 K. In the in-situ IRAS measurement, the vacuum chamber was separated from any vacuum pump, and CO₂ was introduced through a pulse valve. The IRAS spectra were continuously measured under CO₂ at 300 K. The pressure of CO₂ was kept at 0.01 Pa during the FT-IR measurement.

In the STM images after the CO₂ exposure for 1 min, the depletions having 3-fold rotational symmetry were observed. When the exposed time was increased to 4 min, the size of the depletion did not change, and the number of them increased. This indicates that the feature originates from the species formed by the surface reaction of CO₂. The most probable candidate is CO₃ having the molecular plane parallel to the surface. In in-situ IRAS spectra, the vibrational peak at 1288-1292 cm⁻¹ was developed with the elapsed time. The calculational results indicate that this peak can be assigned to the asymmetric mode of CO₃ with the molecular plane parallel to the surface. The absence of the peak at the 1500-1800 cm⁻¹ region, which is the feature of carbonyl group, also supported the parallel orientation of CO₃. The observation of asymmetric mode indicates that CO₃ adsorbed on Cu(111) belongs to the C_s point group rather than C_3v. The parallel orientation and the C_s symmetry strongly suggest that CO₃ is adsorbed at the bridge site of Cu(111) judging from the surface normal dipole selection rule.

The present STM and in-situ IRAS results are consistent with each other. Based on these results, we concluded that CO₂ is chemisorbed as carbonate Cu(111) at the near-ambient condition at 300 K. It is adsorbed at the bridge site with the molecular plane parallel to the surface. The contribution of carbonate species to methanol synthesis should be further investigated.

Reference

[1] U. Burghaus, Prog. Surf. Sci. 89, 161 (2014).

[2] G. Cui et al., Catalysts 14, 232 (2024).

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