H₂-O₂ Synergistic Effect On Heterogeneous Photocatalysis And Its Mechanism

TiO₂-based photocatalytic oxidation has received wide interest as a promising technique for environmental remediation,because the complete mineralization of many organic pollutants can be conducted at ambient conditions. However, the low quantum efficiency ( 4 %) of photocatalytic process has awfully hindered the industrialization. In this paper, the H₂-O₂ synergistic effect on photooxidation of volatile organic compounds (VOCs) on Pt/TiO₂ and Pt/TiO₂-xNx has been investigated for the first time. Mechanisms are proposed to elucidate this promoting effect of the H₂ addition. Hopefully, this work may open a new door towards improving the quantum efficiency of pollutant photodegradation. Titanium dioxide sol was prepared by a sol-gel technique. Wet impregnation method was applied to prepare Pt/TiO₂ and Pt/TiO₂-xNx photocatalysts. Powder X-ray diffraction (XRD), nitrogen sorption (BET) at 77 K, UV-Vis diffuse reflectance spectra (DRS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and composition of samples including crystal phase composition, crystalline sizes, typical specific surface area, optical absorption, and surface chemical state, respectively. Taking account of the characterization and photocatalytic analyses, relationship among structure, composition and photocatalytic performance of catalysts was studied. Temperature programmed desorption (O₂-TPD and H₂-TPD) was used to investigate the physisorption and chemisorption of hydrogen and oxygen on samples. Surface photovoltage spectroscopy (SPS), electric field induced surface photovoltage spectroscopy (EFISPS), time-resolved photoconductivity (TRPC), photoluminescence (PL), spin-trapped electron paramagnetic resonance (EPR), terephthalic acid photoluminescence probing technique (TA-PL) and Fourier transform infrared spectroscopy (FT-IR) were used to study H₂-O₂ synergistic effect on the separation efficiency of photogenerated electron-hole pairs, formation of surface oxygenous radicals and product distribution. Results show that unprecedented photocatalytic activity and durability of Pt/TiO₂ for decomposing benzene have been obtained by adding trace H₂ into an O₂-rich photooxidation system. In pure O₂ atmosphere, the conversion of benzene is very low (3%), and no detectable CO2 is observed at 30oC. Surprisingly, when the feeding gas contains both H2 and O2 (H2/O2 ratio: 0.02), the photocatalytic conversion and mineralization ratio of benzene rapidly increase up to 71% and 78%, respectively. Exceptional stability of Pt/TiO2 in H2-O2 atmosphere is demonstrated by repeated use of the catalyst four times or successional operation for 50 h. The complete decomposition of benzene is achieved when the H2/O2 ratio is 0.31. The calcination temperature and the content of Pt for Pt/TiO2 have remarkable effects on the photocatalytic performance of the catalyst. When calcined at 300 oC, Pt/TiO2 exhibits the highest photocatalytic performance in H2-O2 atmosphere (H2/O2 ratio: 0.02). The conversion and mineralization of benzene are 76 % and 82 %, respectively. While the content of Pt ranges from 5¹2 wt.%, complete mineralization of benzene is observed on Pt/TiO2. The order of photodegradation efficiencies for VOCs is cyclohexane < acetone < benzene < toluene < ethylbenzene. Among Ni, Pd, Pt, Cu, Ag, Au, Fe, Co, Ru and Rh modification on TiO2, the H2-O2 synergistic effect is only observed for Pd/TiO2 and Pt/TiO2 samples. The reason may be ascribed to the dissociative adsorption of H2 and O2 on the surface of Pt particles. Moreover, the more H2 and O2 are adsorbed on catalyst, simultaneously, the higher photocatalytic activity is exhibited. Pt(0) seems to be the active site on Pt/TiO2. With an increase in the content of Pt(0), more and more benzene can be photodegraded. The H2-O2 synergistic effect in the photocatalysis is studied by FT-IR, spin-trapping EPR, SPS and PL. Results demonstrate that the introduced H2 has several beneficial effects on heterogeneous photocatalysis, namely, a comparative clean surface of Pt/TiO2 with no persistent aromatic intermediates, an increased surface hydroxyl radical in photocatalytic process, and an enhanced separation efficiency of photogenerated electron-hole pairs. When it turns to Pt/TiO2-xNx catalyst irradiated by visible light, results also indicate that the high photocatalytic activity and durability of the photocatalyst for decomposing benzene have been obtained in H2-O2 photooxidation system at 30 oC. The photocatalytic conversion and mineralization of benzene rapidly increase up to 18 % and 80 %, respectively, compared with no photodegradation in O2 atmosphere. The order of photodegradation efficiencies for volatile organic compounds is cyclohexane < benzene < toluene < ethylbenzene < acetone < ethylene.The H2-O2 synergistic effect on the Pt/TiO2-xNx photocatalytic system is studied by EFISPS and TA-PL. Results demonstrate that the introduced H2 has severalbeneficial effects on heterogeneous photocatalysis, including an increased surface hydroxyl radical in photocatalytic process, and an enhanced separation efficiency of photogenerated electron-hole pairs.