It is important to understand the mechanisms of activation and hydrogenation of carbon dioxide (CO₂) in order to utilize the abundant CO₂ effectively as a chemical feedstock. One way of utilizing CO₂ is methanol synthesis on Cu-based catalysts. Metallic copper in the catalysts is considered to be an active site for methanol synthesis. So far, the adsorption and reaction of CO₂ on Cu surfaces have been widely investigated under ultrahigh vacuum (UHV) and under ambient pressure conditions.

Fig. 1. A series of XPS spectra on Zn(0.25 ML)/Cu(997) surface: (a) O 1s and (b) C 1s. The spectra of bare Zn-Cu(997) surface were measured under UHV. AP-XPS measurements were first performed at 299 K in the presence of 0.8 mbar CO₂, 0.4 mbar H2, and 0.05 mbar D₂O, then the sample was heated up to 473 K under this ambient-pressure condition.

Fig. 2. Schematic diagram of proposed reactions on the Zn-deposited Cu surface in the presence of CO₂, H₂ and H₂O.

In this study [1], the reaction of CO₂ on Zn-Cu(111) and Zn-Cu(997) was systematically studied using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at SPring-8 BL07LSU (2015B7496 and 2016A7401). AP-XPS allows adsorbate and substrate electronic states to be elucidated under reaction conditions. The aim of this study is to reveal the roles of Zn and water in the CO₂ reaction, and elementary steps of methanol synthesis by a series of systematic experiments performed under well-defined conditions at temperature between 299-473 K. We used Zn-deposited Cu(111) and Cu(997) single crystals as model catalytic systems. Stepped Cu surfaces are more reactive for dissociation of CO₂ compared to flat Cu surfaces. Recently, we have investigated adsorption states of CO₂ on Cu(997) surfaces under UHV, and near-ambient condition. In this study, we have focused on hydrogenation of CO₂ under near-ambient pressure conditions to clarify the effect of the step sites. The oxidation state of Zn-deposited Cu surfaces and the stability of reaction products depend strongly on the gas composition and sample temperature. The effect of water on the CO₂ activation was also examined by a set of control experiments.

In the presence of 0.8 mbar CO₂ and 0.4 mbar H₂ gases, hydrogenation products are not observed; only carbonate is formed on Zn-deposited Cu(111) and Cu(997) surfaces (not shown here). We found that the formation rate of carbonate at 299 K is significantly faster on Zn/Cu(997) than that on Zn/Cu(111), indicating that step sites are more reactive for CO₂ activation than terrace sites. On the other hand, the addition of water (D₂O) in the feed gas leads to hydrogenation of CO₂ to formate at sample temperatures around 400 K (Fig. 1). This suggests that hydroxyl produced from dissociative adsorption of water is a reactant in the CO₂ hydrogenation observed under the present reaction conditions. Figure 2 schematically shows the proposed reactions on the Zn-deposited Cu surface in the presence of CO₂, H₂ and H₂O. We also found that the reaction products such as carbonate and formate on the Zn/Cu(997) surface are more stable than those on the bare Cu(997) surface. In particular, formate remains on Zn-deposited Cu surfaces up to 473 K, which is close to the operation temperature of industrial Cu-ZnO catalysts. We conclude that an important role of Zn on Cu surfaces is the stabilization of reaction intermediates originated from CO₂.

References

• [1] T. Koitaya, S. Yamamoto, Y. Shiozawa, Y. Yoshikura, M. Hasegawa, J. Tang, K. Takeuchi, K. Mukai, S. Yoshimoto, I. Matsuda, and J. Yoshinobu, ACS Catal. 9, 4539 (2019).

Authors

• T. Koitaya^a, S. Yamamoto, I. Matsuda, and J. Yoshinobu
• ^aInstitute for Molecular Science (present address)