Simultaneous catalytic ozonation of NO and dichloromethane on Mn/H-ZSM-5 catalysts: Interaction effect and mechanism

Catalytic ozonation is a promising method for simultaneous removal of NOx and Cl-VOCs, but needs to clarify their interaction mechanism and the influence of catalyst acidity. In this paper, the simultaneous catalytic ozonation of NO and dichloromethane (DCM) on Mn/H-ZSM-5 molecular sieve catalysts were investigated experimentally. Results show that the overall acidity, acid sites type and intensity have a significant impact on the degradation efficiency, the conversion path of Cl element, and the interaction of NO/DCM adsorption-degradation. Nevertheless, regardless of catalysts, NO could be preferentially oxidized by ozone to generate NO2 in co-ozonation process, which inhibited and even shielded DCM ozonation at O3/DCM ratio <1.7. In addition, the highly active oxidizing species such as NO3/N2O5, produced by the deep ozonation of NO2, exhibited a synergistic effect on the conversion of DCM and intermediates, which in turn weakened NO2 deep oxidation. Specifically, NO addition caused a general decrease in the HCl selectivity, and a slight increase in the CHCl3 selectivity of all samples, while the Cl2 selectivity was determined by the overall catalyst acidity. The samples with higher overall acidity exhibited lower activity for DCM degradation. In particular, for samples with the weak overall acidity but strong acid sites, the sum selectivity of HCl, Cl2, and CHCl3 was significantly improved under the interplay effect of NO, indicating that strong acidic sites were beneficial to the complete degradation of DCM. In-situ DRIFTS revealed that aldehydes and carboxylates were the key intermediates of DCM ozonation. In the co-ozonation, NO and its oxidation products (such as nitrates) could promote the formation and conversion of these intermediates, and further converted into CO and CO2 by the active oxidant from ozone. Finally, the interference of H2O and SO2 on the NO/DCM co-ozonation were revealed.