Fabrication of palladium-titania nanofiltration membranes via a colloidal sol-gel process

Titania nanofiltration (NF) membranes with a molecular weight cut-off (MWCO) of ∼1000 Da impregnated with noble metal Pd have been fabricated via the colloidal sol-gel route. The process of calcination under air atmosphere before reduction under hydrogen atmosphere was utilized for complete removal of organics, and the as-prepared Pd-doped TiO₂ materials were denoted as PdO-TiO₂ and Pd-TiO₂, respectively. XRD patterns and TEM images confirmed that, under the air atmosphere, the Pd doping restrained TiO₂ grain growth and A → R transformation, which may be due to the substitution of Pd²⁺ into the TiO₂ lattice, suggesting that the release of deformation energy stabilized the anatase phase. Combining N₂ adsorption-desorption results, the PdO-TiO₂ materials with a Pd concentration of 30 dwb% and a calcination temperature of 400 °C were optimized with a pore size of 1.7 nm. After hydrogen reduction, the inhibitory effect of Pd doping on phase transformation was weakened, with a looser pore structure achieved for the Pd-TiO₂ materials with a pore size of 2.4 nm, indicating the inhibitory effect of PdO, Pd⁰ and Pd²⁺, the three chemical states of elemental Pd, on TiO₂ crystal growth and A → R transformation improved in turn. Finally, the Pd-TiO₂ NF membranes were fabricated with a water permeability of ∼10 L/(m² h bar), showing high ion retention rates, while the undoped TiO₂ membranes exhibited much lower retention properties.