Guaiacol Hydrodeoxygenation over Iron-Ceria Catalysts with Platinum Single-Atom Alloy Clusters as a Promoter

A series of ceria-supported Fe catalysts with a trace amount of noble-metal modification (NM-Fe/CeO₂, NM = Pt, Ir, Pd, Rh, and Ru) were prepared by the co-impregnation method and used for guaiacol hydrodeoxygenation (HDO) to phenolic compounds at 673 K and atmospheric pressure. In the absence of H₂O, the addition of noble metal improved the initial activities but showed severer deactivation than Fe/CeO₂. Conversely, the addition of water showed little effect on the initial activity and helped to improve the stability of NM-Fe/CeO₂. Among all of the tested NM-Fe/CeO₂catalysts, Pt-Fe/CeO₂showed the highest guaiacol conversion. The X-ray absorption spectroscopy (XAS) characterization confirmed that the original structure of active FeOxspecies, probably Fe₄O₆, was mostly preserved after the addition of Pt. The Pt modifier was completely reduced to form Pt₁Fe₄single-atom alloy (Pt₁Fe₄SAA) clusters during the guaiacol HDO reaction. These Pt₁Fe₄SAA clusters probably promoted the reduction of FeOxspecies to form the coordinatively unsaturated sites (CUS), which were the active sites for the HDO reaction. The presence of Pt₁Fe₄SAA clusters also encouraged the dissociation of H₂O on Pt-Fe/CeO₂to maintain the catalytic activity under the H₂O-containing conditions, as shown by the temperature-programmed surface reaction with H₂O (H₂O-TPSR). Characterization of spent catalysts with Raman spectroscopy, scanning transition electron microscopy (STEM), and XAS showed that the Pt-Fe/CeO₂catalyst was deactivated by coke deposition and carburization of Fe₄O₆clusters in the absence of H₂O, while the growth of coke species and the formation of inactive iron carbide were suppressed in the presence of H₂O.