CeO ₂-Supported Single-Atom Cu Catalysts Modified with Fe for RWGS Reaction: Deciphering the Role of Fe in the Reaction Mechanism by In Situ/Operando Spectroscopic Techniques

Abstract

Reverse water–gas shift (RWGS) reaction has attracted much attention as a potential approach for CO ₂ valorization via the production of synthesis gas, especially over Fe-modified supported Cu catalysts on CeO ₂. However, most studies have focused solely on investigating the RWGS reaction over catalysts with high Cu and Fe loadings, thus leading to an increase in the complexity of the catalytic system and, hence, preventing the gain of any reliable information about the nature of the active sites and reaction mechanism. In this work, a CeO ₂-supported single-atom Cu catalyst modified with iron was synthesized and evaluated for the RWGS reaction. The catalytic results reveal a significant synergistic effect between CuCeO ₂ and Fe, demonstrating an activity up to three times higher than the combined catalytic activities of monometallic catalysts (Fe/CeO ₂ + CuCeO ₂) under identical conditions. Various ex situ and in situ/operando techniques are employed to unveil the concealed role of Fe in catalyst activity enhancement. The combined findings from hydrogen temperature-programmed reduction (H ₂-TPR) and operando electron paramagnetic resonance spectroscopy (EPR) reveal that the added Fe predominantly interacts with Cu-containing surface sites, resulting in the stabilization of higher proportions of Cu single sites. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando EPR results unveil a synergistic interplay of Fe with Cu-containing sites and CeO _x domains, efficiently enhancing both the reoxidation of Cu ⁺ in Cu ⁺–O _v–Ce ³⁺ moieties and the reducibility of Ce ⁴⁺ in CeO _x domains under RWGS conditions. Detailed mechanistic studies reveal that the RWGS reaction predominantly proceeds via the redox mechanism.