Pd Nanoparticles Loaded on Ceramic Membranes by Atomic Layer Deposition with Enhanced Catalytic Properties

Atomic layer deposition (ALD) was adopted for the first time to load Pd nanoparticles on Al2O3 ceramic membranes (CMs) for fabricating catalytic membranes (Pd/CMs). The membrane surface was functionalized with the deposition of TiO2 by ALD and subsequent calcination under different atmospheres [CM-TiO2-A, obtained by calcination under an atmosphere of pure argon; CM-TiO2-H, obtained by calcination under a mixed atmosphere of H2 and Ar (H2/Ar = 1:9 in volume)]. The influence of the TiO2 deposition on the microstructure of catalytic membranes and their catalytic performance in p-nitrophenol reduction to p-aminophenol were investigated in detail. The catalytic activity of Pd/CM-TiO2-H improved by 1.9 times compared to Pd/CM, and 100% conversion of p-nitrophenol to p-aminophenol can be achieved. The X-ray photoelectron spectroscopy (XPS), inductively coupled plasma emission spectroscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy results indicate that the ALD of TiO2 and subsequent calcination under a 10% H2 atmosphere can produce more surface oxygen vacancies, providing more active sites for Pd(hfac)2 adsorption and then promoting the loading of Pd nanoparticles. The XPS and H2 temperature-programmed reduction results suggest that calcining TiO2 under the 10% H2 atmosphere is more beneficial for the removal of the hfac ligands and the reduction of Pd(hfac)2, leading to a higher Pd0 ratio. These two aspects contribute to the superior catalytic activity of Pd/CM-TiO2-H.