Abstract
Composites of TiO2 (Hombikat, P25, sol-gel synthesis) and zeolite ZSM-5 (nSi/nAl = 55) with mass fractions from 25/75 to 75/25 were prepared by mechanical mixing, solid-state dispersion and sol-gel synthesis. Characterization of the composites by X-ray diffraction (XRD), N2-sorption, scanning electron microscopy (SEM), and UV-Vis spectroscopy show that mechanical mixing and solid-state dispersion lead to comparable textural properties of the composites. A homogeneous distribution and intimate contact of small TiO2 particles on the crystal surface of zeolite ZSM-5 were achieved by sol-gel synthesis. The composites were studied in the photocatalytic oxidation (PCO) of NO in a flatbed reactor under continuous flow according to ISO 22197-1. The highest NO conversion of 41% at an NO2 selectivity as low as 19% stable for 24 h on-stream was reached over the TiO2/ZSM-5 composite from sol-gel synthesis with equal amounts of the two components after calcination at 523 K. The higher activity and stability for complete NO oxidation than for pure TiO2 from sol-gel synthesis, Hombikat, or P25 is attributed to the adsorptive properties of the zeolite ZSM-5 in the composite catalyst. Increasing the calcination temperature up to 823 K leads to larger TiO2 particles and a lower photocatalytic activity.
Highlights
The increasing consumption of fossil fuels and the worldwide industrial growth has resulted in increasing emissions and air pollution
By varying the calcination temperature of TiO2 prepared by the hydrolysis of titanium alkoxide, Hashimoto et al [19] have shown that a higher photocatalytic activity in the photocatalytic oxidation (PCO) of NOx can be achieved over smaller TiO2 crystals in comparison to a commercial TiO2
When the composite is calcined at 623 K or higher, the photocatalytic activity decreases with the calcination temperature as evident from the lower NO conversion and the lower amount of NO removed, at lower selectivity for NO2 than for the material calcined at 423 K
Summary
The increasing consumption of fossil fuels and the worldwide industrial growth has resulted in increasing emissions and air pollution. By varying the calcination temperature of TiO2 prepared by the hydrolysis of titanium alkoxide, Hashimoto et al [19] have shown that a higher photocatalytic activity in the PCO of NOx can be achieved over smaller TiO2 crystals in comparison to a commercial TiO2. The role of zeolite H-beta in composites with TiO2 in the photocatalytic degradation of an aqueous propoxur solution was shown to rely on concentrating the reactants and intermediates within its cavities and a subsequent and continuous supply to the TiO2 surface [28] It was, the aim of this work to study composites of TiO2 and zeolite ZSM-5 over a broad range of mass ratios for the photocatalytic oxidation of NO with respect to the activity of pure TiO2 and pure zeolite ZSM-5. Varying the calcination temperature of the composites and, the crystallite size of the TiO2 and the zeolite surface hydrophobicity, respectively
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