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Pd catalysts supported on TiO 2 with different crystalline phases were prepared with formaldehyde as reducing agent and examined for hydrodeoxygenation (HDO) of guaiacol. Their properties were characterized by N 2 adsorption, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Compared to the carbon-supported Pd catalysts, TiO 2 -supported Pd catalysts exhibited higher C−O bond scission ability, which may be attributed to the presence of partially reduced titanium species originating from the reduction of Ti 4+ by spillover hydrogen from Pd at 200 °C on the surface of TiO 2 . Guaiacol was hydrogenated on Pd sites to give 2-methoxycyclohexanol, which diffused to partially reduced titanium species and subsequently reacted with hydrogen from Pd to generate cyclohexane. Anatase TiO 2 -supported Pd catalyst gave the highest HDO activity of guaiacol among the Pd catalysts supported on three types of TiO 2 (anatase, rutile, and their mix, P25), suggesting that more partially reduced titanium species are in favor of the HDO reaction because anatase is facile to reduce by H 2 at 200 °C. Higher selectivity of cyclohexane for Pd/TiO 2 reduced at 500 °C than that reduced at 200 °C further confirmed that the enhanced C−O bond scission ability of Pd/TiO 2 is mainly attributed to the partially reduced titanium species on the surface of TiO 2 .

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TiO2–ZrO2 composite oxides with a series of different Ti/Zr atomic ratios were synthesized by using a deposition precipitation method and employed as supports to prepare Ru catalysts. The physicochemical characteristics of Ru catalysts were tested by N2 adsorption, CO chemisorption, X-ray diffraction, H2 temperature-programmed reduction, scanning electron microscopy, transmission electron microscopy, NH3 adsorption, and X-ray photoelectron spectroscopy, and their catalytic hydrodeoxygenation (HDO) performance was evaluated by using guaiacol as a model compound. The Ru/TiO2 catalyst exhibits low HDO activity due to the surface modification of Ru particles with Ti3+ species formed during the reduction of Ru/TiO2. The HDO activity of Ru catalysts was significantly promoted by ZrO2 in the supports because it can hinder the migration of Ti3+ species onto the surface of Ru particles and thus more Ru active sites are exposed. The higher selectivity of benzene for Ru catalysts supported on TiO2–ZrO2 composite oxides suggests that they are promising supports for Ru catalysts in the HDO reaction of guaiacol.

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