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The highest NO conversion of 41% at an NO2 selectivity as low as 19% is achieved over the composite from sol-gel synthesis (Table 1). This activity is even higher than that of the pure TiO2 from sol-gel synthesis or Hombikat. In addition, a steady state is reached on these catalysts within less than 2 h of irradiation on-stream (Figure 5f). This steady state of high NOx removal rate remains constant over the whole 24 h of the experiment without any detectable deactivation. This result can be attributed to the high dispersion and intense interaction of the TiO2 particles with the surface of the crystals of zeolite ZSM-5 as confirmed by N2 sorption, XRD and SEM (see Section 2.1). Besides serving as an adsorbent and reservoir for the NO2 formed in the first step of NO oxidation, the zeolite might as well act as an efficient adsorbent for the nitrate and nitrites in solution, i.e., nitric and nitrous acid, from the terminal NOx oxidation. Thus, the well-dispersed TiO2 particles over the ZSM-5 crystals from sol-gel not only decrease NO2 selectivity, but also lead to an overall increase of the NOx removal. In order to further verify the role of zeolite ZSM-5 in the composite, non-porous sea sand with negligible adsorption capacity was used in the sol-gel synthesis as a 50/50 composite (denoted as TSSSG, for data from characterization by XRD, N2 sorption and SEM, Figures S2–S4). This composite shows, however, very high NO2 selectivity and low NOx removal similar to the pure zeolite-free TiO2 catalysts. The improvement in photocatalytic activity and reduction in NO2 selectivity of the zeolite-containing composites are, therefore, due to the high adsorption capacity in the micropores (and possibly the hydrophilicity) of the zeolite ZSM-5. As seen also from the results in Table 1, the photocatalytic activity of HZMM, HZSSD and TZSG decreases with increasing mass fraction of zeolite ZSM-5 and decreasing TiO2 content in the composites. This suggests that a higher loading of ZSM-5 may lower the amount of TiO2 available on the ZSM-5 surface and, therefore, decreases the photooxidation activity. Furthermore, increasing the TiO2 content with decreasing ZSM-5 content at a mass ratio of 75/25 does not lead to a proportional increase of NOx removal. This implies that a higher loading of TiO2 forms thicker layers on the ZSM-5 crystallite surface where the excess amount of TiO2 is inaccessible to the UV irradiation. It can be concluded that for the TZSG material a mass ratio of 50/50 is the optimum ratio to achieve the highest overall NOx removal, even higher than for pure P25. Supporting this conclusion, the highest amount of nitrate, more than twice as high as for P25, is found on this sol-gel derived catalyst (Table 2). Note, however, that the photocatalytic results obtained here are rather different from those for the photooxidation of NO over TiO2/zeolite composites reported by Guo et al. [31], Hashimoto et al. [32] and Ichiura et al. [34]. NO conversions between 25% and 96% were achieved in these studies, however at largely different conditions of the photocatalytic experiments with respect to, e.g., irradiation time and intensity, relative humidity and feed concentration and low rates. For instance, the 165PDF Image | Zeolite Catalysis
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