Chinese Journal of Catalysis

Chinese Journal of Catalysis ›› 2017, Vol. 38 ›› Issue (12): 2076-2084.DOI: 10.1016/S1872-2067(17)62951-6

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Humic acid-mediated visible-light degradation of phenol on phosphate-modified and Nafion-modified TiO2 surfaces

Longhui Zhenga, Xiaojuan Yua, Mingce Longa,b, Qilin Lic   

  1. a School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China;
    b Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China;
    c Department of Civil and Environmental Engineering, Rice University, Houston, Texas 77005, United States
  • Received:2017-10-09 Revised:2017-10-30 Online:2017-12-18 Published:2017-12-29
  • Supported by:

    This work was supported by the National Natural Science Foundation of China (21377084), Special Fund for Agro-Scientific Research in the Public Interest (201503107), and Shanghai Municipal International Cooperation Foundation (15230724600).

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Abstract:

Although humic acid (HA) can inhibit TiO2 photocatalysis, it can sensitize TiO2 and induce significant visible-light (VL) activity in phenol degradation. This favorable effect of HA was negligible on phosphate-modified TiO2 (P-TiO2), but significantly stronger on Nafion-modified TiO2 (Nf-TiO2). The reaction rate constants for phenol degradation on Nf-TiO2 increased from (0.003±0.001) to (0.025±0.003) min-1 when the reaction was performed in the presence of 20 mg/L HA. The different effects of HA on P-TiO2 and Nf-TiO2 photocatalysis cannot be attributed to adsorption changes, because the adsorption capacities of P-TiO2 and Nf-TiO2 were only slightly lower than that of TiO2 at an initial HA concentration of 20 mg/mL. Scavenger tests, electron paramagnetic resonance spectroscopy, and H2O2 detection were taken to understand the low VL activity of the P-TiO2/HA suspension. The main active species for phenol degradation in the TiO2 and Nf-TiO2 suspensions were superoxide radicals. There were negligible amounts of superoxide radicals in the P-TiO2/HA suspension, possibly because a direct four-electron oxygen reduction reaction occurred. The better VL activity of Nf-TiO2 was rationalized on the basis of Mott-Schottky and electrochemical impedance plots. Nafion modification resulted in cathodic shifts of the energy band positions, increased electron density, and less resistance to electron transfer across the interface between TiO2 and electrolytes. All these factors facilitated electron transfer and improved the production of active species. Phosphate modification therefore did not improve the VL response of HA sensitized TiO2, and low concentrations of HA can facilitate VL photocatalytic degradation of organic pollutants on Nafion surface-modified TiO2.

Key words: Titanium dioxide, Humic acid, Nafion, Phosphate, Surface modification