Mechanisms of catalytic ozonation: An investigation into superoxide ion radical and hydrogen peroxide formation during catalytic ozonation on alumina and zeolites in water

Authors: Amir Ikhlaq, David R. Brown, Barbara Kasprzyk-Hordern

Abstract:
This study aims to investigate mechanisms of ozonation in the presence of ZSM-5 zeolites and - alumina in water. Four ZSM-5 zeolites with varying silica to alumina ratios and with both hydrogen and sodium counter ions were used in the study (Z1000H:SiO₂/Al₂O₃ = 1000, Z900Na:SiO₂/Al₂O₃ = 900, Z25H:SiO₂/Al₂O₃ = 25 and Z25Na:SiO₂/Al₂O₃ = 25). The formation of reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) and superoxide ion radical (^oO₂⁻) was investigated during ozonation in the presence of ZSM-5 zeolites and alumina using amplex red and 4-chloro-7-nitrobenzo-2-oxa-1,3- dizole (NBD-Cl) as probe molecules. To the authors’ knowledge, this is the first report utilising NBD-Cl and amplex red to study mechanisms of catalytic ozonation. The results showed that alumina promotes much higher formation of ROS in aqueous solutions when compared to ozonation alone and ozonation in the presence of ZSM-5 zeolites. The process was found to be pH dependent. Furthermore, alumina showed its highest activity at a pH close to its point of zero charge. The presence of tertiary butyl alcohol (TBA) and phosphates in the reaction solution did not have a significant effect on ROS production in the presence of ZSM-5 zeolites. However, in the case of alumina, the presence of phosphates significantly lowered ROS formation. This indicates the critical importance of surface hydroxyl groups of alumina in ozone decomposition and ROS formation. In contrast to H₂O₂ formation, TBA did not have a significant effect on ^oO2⁻ production in the case of alumina. This suggests that ^oO₂⁻ plays a significant role in the formation of hydroxyl radicals. Furthermore, both zeolites and alumina were found to catalyse the removal of NBD-Cl from aqueous solution. Therefore, it is suggested that alumina operates through a radical mechanism leading to the production of ROS. On the other hand, zeolites serve as reservoirs of ozone and adsorbents of organic compounds, which interact via direct ozonation pathways. The activity of zeolites depends on the silica to alumina ratios of the zeolite and is independent of the nature of the zeolite counter ions.

Keywords:
Catalytic ozonation
Mechanisms
Water
Superoxide ion radical
Hydrogen peroxide ZSM-5 zeolites
Alumina

Published in: Applied catalysis B: Environmental  (Volume 129, Pages 1-614, 17 January 2013)

Publisher: Elsevier  

ISSN Information: 0926-3373

Mechanisms of catalytic ozonation: An investigation into superoxide ion radical and hydrogen peroxide formation during catalytic ozonation on alumina and zeolites in water

Bình luận của bạn

*

*

*

*

 Captcha

Gửi bình luận nhập lại