(589e) First-Principles Investigation of Direct Deoxygenation of Phenolic Compounds over Ru/TiO2(110)

Catalytic conversion of oxygenated aromatic compounds (OACs) into oxygen-free compounds, or bio-oil upgrading, suffers from both poor selectivity and low conversion, because the direct deoxygenation (DDO) is harder to activate than decarbonylation (DCN) or hydrogenation (HYD). According to recent studies, Ru/TiO₂ has been identified as promising catalyst with good activity and selectivity towards DDO products from OACs [1, 2]. However, the nature of the active site and roles of the Ru cluster and TiO₂ support remain by and large unknown.

Using density functional theory (DFT), we have explored the hydrodeoxygenation (HDO) pathways of phenol and m-cresol over the Ru(0001) surface, hydroxylated h-TiO₂(110), and a 10-atom cluster model for the Ru/TiO₂ interface. The comparison of the HYD vs. DDO pathways for phenol and m-cresol on Ru(0001) indicates that HYD is fast and kinetically preferred over DDO, suggesting that metallic Ru is unselective for direct C-O bond scission of oxygenated aromatic compounds. For the hydroxylated h-TiO₂ surface an oxygen vacancy is required to provide an adsorption site for phenol or m-cresol, but vacancy formation is kinetically limited by TiO₂’s weak interaction with gas-phase H₂ [3]. For the Ru₁₀/TiO₂ interface our results suggest that the presence of Ru on TiO₂ catalyzes hydrogen delivery to TiO₂(110) and facilitates oxygen vacancy formation at the Ru/TiO₂ interface. Phenol and m-cresol subsequently adsorb via their hydroxyl groups into the formed vacancy. The energy barriers for the following C-O bond scissions in phenol and m-cresol are 0.78 eV and 0.71 eV, respectively. The eliminated OH group heals the TiO₂ vacancy, and the aromatic rings of phenol and m-cresol remain on the Ru cluster. The energy barrier for C-O bond scission on TiO₂ and at the Ru/TiO₂ interface are not significantly different, indicating that the oxygen vacancy site plays a similar role in catalyzing the DDO pathway of phenol and m-cresol. Our theoretical analysis is consistent with experimental evidence, and we conclude that the role of Ru is to activate hydrogen and an oxygen vacancy site near the Ru/TiO₂ interface is required for selective C-O bond scission.

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