Active Site Dependent Reaction Mechanism over Ru/CeO₂ Catalyst toward CO₂ Methanation

TLDR

Oxygen vacancies on metal‑oxide surfaces are key reactive sites in many catalytic reactions. This study uses operando spectroscopy to compare CO₂ methanation on Ru/CeO₂, which contains oxygen vacancies, with Ru/α‑Al₂O₃, which lacks them, in order to elucidate how the active site influences the reaction mechanism. Operando XANES, IR, and Raman spectroscopy were employed to monitor the formation of Ce³⁺, surface hydroxyls, and oxygen vacancies in Ru/CeO₂ under realistic reaction conditions, providing a detailed view of the active‑site‑dependent pathway. SSITKA‑type in situ DRIFT IR revealed that CO₂ methanation on Ru/CeO₂ proceeds via a formate route with formate dissociation to methanol catalyzed by oxygen vacancies as the rate‑determining step, whereas on Ru/α‑Al₂O₃ the reaction follows a CO route; catalytic tests confirm that the oxygen vacancy lowers the activation temperature from 250 °C to 125 °C.

Abstract

Oxygen vacancy on the surface of metal oxides is one of the most important defects which acts as the reactive site in a variety of catalytic reactions. In this work, operando spectroscopy methodology was employed to study the CO2 methanation reaction catalyzed by Ru/CeO2 (with oxygen vacancy in CeO2) and Ru/α-Al2O3 (without oxygen vacancy), respectively, so as to give a thorough understanding on active site dependent reaction mechanism. In Ru/CeO2 catalyst, operando XANES, IR, and Raman were used to reveal the generation process of Ce(3+), surface hydroxyl, and oxygen vacancy as well as their structural evolvements under practical reaction conditions. The steady-state isotope transient kinetic analysis (SSITKA)-type in situ DRIFT infrared spectroscopy undoubtedly substantiates that CO2 methanation undergoes formate route over Ru/CeO2 catalyst, and the formate dissociation to methanol catalyzed by oxygen vacancy is the rate-determining step. In contrast, CO2 methanation undergoes CO route over Ru surface in Ru/α-Al2O3 with the absence of oxygen vacancy, demonstrating active site dependent catalytic mechanism toward CO2 methanation. In addition, the catalytic activity evaluation and the oscillating reaction over Ru/CeO2 catalyst further prove that the oxygen vacancy catalyzes the rate-determining step with a much lower activation temperature compared with Ru surface in Ru/α-Al2O3 (125 vs 250 °C).