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however, increasingly large mesopores within the TiO2 phase are formed, reducing the contact area to the zeolite crystals. The activity of the composites in the photocatalytic oxidation (PCO) of NO in a flatbed reactor under continuous flow according to ISO 22197-1 is distinctly different from that of the pure TiO2 materials. While the NO conversion is lower, the selectivity for the undesired NO2 is lower and a higher fraction of nitrate and nitrite may be formed. The role of the zeolite can, thus, be understood to be an adsorptive component for NO2, which is continuously released to the photocatalytically active TiO2 particles and further converted to the desired nitrate. Another function of the zeolite may be to adsorb the nitric acid formed by photooxidation of NOx, which otherwise deposits on the TiO2 surface and leads to deactivation by blocking of active sites. This function, would, however, be limited by the overall adsorption capacity of the zeolite which, in the experiments conducted here, was not reached. The highest NO conversion of 41% at a NO2 selectivity as low as 19% stable for 24 h on-stream was reached over the TiO2/ZSM-5 composite with equal amounts of the two components after calcination at 523 K from sol-gel synthesis. This activity and stability for complete NO oxidation was even higher than for pure TiO2 from sol-gel synthesis, Hombikat, or P25. These favorable photocatalytic properties could be clearly attributed to the adsorptive properties of the zeolite ZSM-5 in the composite photocatalyst. They have to be balanced, however, with the photocatalytic function provided by the TiO2 and its amount in the composite. The approach of utilizing zeolites as adsorptive components in photocatalytic composites for oxidative NOxremoval may be applied to other zeolites and photoactive catalyst components. Further studies towards increasing the effectiveness of these composites should, besides long-term stability studies under the application conditions, target the balance of surface hydrophobicity/hydrophilicity with respect to adsorption capacity, i.e., specific pore volume and surface area. As the contact between the photocatalytically active and the sorptive function is of prime importance, the effect of the diameters and the shape of the zeolite crystals may also be adjusted for improving the activity and stability of composites catalysts for photooxidative NOx removal. Acknowledgments: The authors are grateful to Sebastian Zimmermann, Institut of Chemical Technology, Universität Leipzig, for help with the photocatalytic apparatus and to Gerald Wagner, Institute for Mineralogy, Crystallography and Material Science; Universität Leipzig, for the SEM pictures. The authors acknowledge the Ministry of Higher Education and Scientific Research-Libya for financial support. Author Contributions: Roger Gläser provided the idea and design for the study. Akram Tawari performed the experiments. Akram Tawari, Roger Gläser and Wolf-Dietrich Einicke analyzed the data. Akram Tawari drafted the manuscript. Wolf-Dietrich Einicke and Roger Gläser revised it. Conflicts of Interest: The authors declare no conflict of interest. 172PDF Image | Zeolite Catalysis
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