Synthesis and Characterization of Pd over Novel TiO₂ Mixtures: Insights on Metal-Support Interactions

Palladium nanoparticles were supported on unusual mixtures of anatase, TiO₂ (II) and rutile titania phases (not commercially available) by wet impregnation, obtaining catalysts with metal contents of 0.25 wt% labeled Pd/Ti5, Pd/Ti45 and Pd/Ti120. Crystalline structures were confirmed by X-Ray Diffraction (XRD) and Raman Spectroscopy. Pd loadings were quantified by Inductively Coupled Plasma (ICP-OES) whilst particle sizes in the range 4-20 nm were obtained by Transmission Electron Microscopy (TEM). Low-values of external surface area (S_BET) in the range 10-17 m² g^-1, measured by Brunauer-Emmet-Teller (BET) method, were higher enough to achieve a good distribution of palladium over titanium oxide outer surface, as evidenced by Energy-Dispersive X-ray Spectroscopy (EDS) elemental mapping. Pd⁰/Pd^δ+ atomic ratio measured by X-Ray Photoelectron Spectroscopy (XPS) showed a decrease from Pd/Ti5 to Pd/Ti120, in line with the decrease in anatase phase present in the catalysts. This behavior suggested that palladium tended to form more TiPd_xO structures in Pd/Ti5 whilst PdO_x structures were more likely to be present on supports with greater amounts of TiO₂ (II) and rutile phases, due to the distinct metal-support interactions. An increase in reducibility and oxygen mobility from Pd/Ti5 to Pd/Ti120 was observed by Temperature Programmed Measurements (TPM), which could be associated to the different defective structures achieved in the supports previously synthesized by high-energy ball milling. Catalysts with improved properties reported herein could exhibit an excellent performance in oxidation reactions, e.g. liquid-phase glycerol selective oxidation.