Vacuum UV catalytic oxidation for efficient degradation of volatile organic compounds at room temperature : process investigation and mechanistic study

Abstract

Air pollutants are getting more and more severe with the fast development of industry, among which, volatile organic compounds (VOCs) are one of the most concerning air pollutants nowadays. Great number of technologies have been studied in the area of VOCs elimination. Vacuum Ultraviolet (VUV) catalytic oxidation process is considered to be one of the most promising technologies to remove VOCs as several advanced oxidation processes (AOPs) are involved in this system including VUV photolysis, photocatalytic oxidation (PCO), ozone catalytic oxidation (OZCO) as well as the combination of photocatalysis and ozone catalytic oxidation (UV-OZCO). However, some limitations of the VUV catalytic oxidation impede such technology for wide application. First, the efficiencies of VOCs conversion and the elimination of generated ozone still need to be improved. Besides, composite materials for realizing different catalytic functions in this system are complicated to synthesize and the uniformity of the composite catalysts is difficult to ensure. Furthermore, there are still no mechanistic studies involving all the processes in this technology. The contributions and synergetic effects of these processes are also lacking in previous studies. In this thesis, several catalysts were developed to enhance the efficiencies of toluene degradation in the VUV catalytic oxidation system. Characterization analyses, including SEM, TEM, XRD, BET, XPS, Raman, etc., were carried out to estimate the physical and chemical properties of the synthetic catalysts. To have a deeper insight into the VUV catalytic oxidation technology, different pathways of toluene degradation were also proposed through the study of the intermediates. Process investigation and mechanistic studies of the VUV catalytic oxidation were conducted with different catalysts as follow. • MnO2-rGO composite catalysts were developed for the system consisting of VUV photolysis and OZCO of toluene. The performance and mechanism of the MnO2-rGO catalysts were studied. Two reaction pathways were proposed and mechanism of toluene degradation was summarized. • CeO2 supported Mn-TiO2 catalysts were synthesized to improve the performance of VUV catalytic oxidation system. The composite catalysts possess excellent performance in toluene degradation and ozone elimination. Hydroxyl radicals were reported to have a great contribution to the toluene oxidation. • Three kinds of pure CeO2 with different structures were synthesized which possesses both the catalytic activities of photocatalysis and ozonation in the VUV catalytic oxidation system. Ce3+ ratios were found to have critical effects on the catalytic performance. Contributions of different VUV involved processes were also investigated for different CeO2 samples. • The mechanisms of VUV catalytic oxidation system were further investigated by using CeO2 nanorods. Synergetic effects of UV-PCO and OZCO were manifested. The mechanism of toluene degradation in VUV catalytic oxidation system was proposed based on the radical formations. Overall, by developing the composite catalysts including MnO2-rGO and Mn-TiO2/CeO2, efficiencies of the VUV catalytic oxidation technology were greatly enhanced. A single-component catalyst, CeO2 was innovatively applied to achieve multiple catalytic functions in this technology. By investigating the process contribution over the pure CeO2 catalysts, the mechanism of this technology was further clarified. Perspectives of future works were also proposed in the end of the thesis. (499 words)