In situ transmission infrared spectroscopy of plasma-assisted CO₂ hydrogenation on ZnO: Role of catalyst support materials

Atsushi Saito^{1, 2}, Zunrong Sheng¹, Dae-Yeong Kim¹, Yutaka Imamura², Tomohiro Nozaki^{1, *}

^* The author to whom correspondence should be addressed.

¹ Department of Mechanical Engineering, Tokyo Institute of Technology, Japan
² Innovative Technology Laboratories, AGC Inc., Yokohama 230-0045, Japan

Abstract

Nonthermal plasma-assisted catalysis is shown to be capable of reducing carbon dioxide (CO₂) in the presence of hydrogen (H₂). Dielectric barrier discharge-type plasma-activated CO₂ and H₂ were irradiated on a zinc oxide (ZnO) catalyst, and the surface species and their reaction pathways were investigated via in situ transmission infrared absorption spectroscopy. In our previous study on in situ Raman spectroscopy of CO₂ hydrogenation on Cu/ZnO, we found the various surface species were uniquely identified on ZnO when DBD was irradiated. Therefore, this study aimed particularly to investigate the effects of nonthermal plasma on support materials (ZnO) without copper. The infrared absorption peaks of the surface species were analyzed via density functional theory (DFT) calculations. Formate (HCOO), formyl group (HCO), and formaldehyde (H₂CO) were generated as reaction products from the plasma and ZnO catalyst. Moreover, the process simultaneously produced plasma-activated hydrogen that adheres to ZnO and undercoordinated Zn in ZnO. These hydrogen adhesions and undercoordinated Zn sites did not appear in thermal reactions in the absence of plasma, suggesting that the plasma-activated hydrogen site in ZnO functions as an active site for the plasma catalytic reaction.

Keywords - CO₂ conversion, carbon recycling, in situ spectroscopy, plasma catalysis, nonthermal plasma.

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