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

• Pd-TiO2 NF membranes were prepared via a colloidal sol–gel method.
• The doped Pd metal inhibited grain growth and A → R transformation of TiO2 materials.
• The inhibitory effect of PdO, Pd0 and Pd2+ on phase transition improved in turn.
• The Pd-TiO2 NF membranes exhibited higher performance than the undoped membranes.

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.

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