The reaction mechanism of plasmonic photocatalytic NO oxidation on Bi-metal/BiPO<sub>4</sub>

Abstract

With the rapid development of economy, factories and vehicles make the air pollution became a worldwide focused issue. As one of the major air pollutants, nitric oxide (NO) would lead to serious atmospheric problems such as acid rain, haze, and photochemical smog. However, low concentration nitric oxide is hard to be removed by the way of industrialization. Photocatalysis, as a green and effective technology, has become one of the most promising technologies for solving air pollution problems and that works on low concentration air pollutant. As one of the typical photocatalyst, BiPO4 consists of alternating layers of [Bi2O2]2+ and PO43− groups and has the better UV-light photocatalysis performance when compare with TiO2, emerging as an attractive photocatalyst. At present, the study to enable BiPO4 respond visible light which comes up with many ways, such as element doping, build heterojunction and surface hybridization. The recent investigation shows that metal Bi possess the property of surface plasmon resonance (SPR) as well as noble metal like gold could enhance the photocatalysis activity by increasing light absorption and raising the separation efficiency of photogenerated electron-hole pair. In this study, Bi/BiPO4 composite photocatalyst was synthesized via a two steps method. Firstly, the hexagonal phase BiPO4 was prepared by a precipitation method. And then, the metal Bi was deposited on the surface of BiPO4 in the presence of NaBH4. The as-prepared photocatalyst was applied to the photocatalytic removal of low concentration of NO in air. The microstructures of the catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-visible diffuse-reflectance spectrum (UV-vis DRS) and electron spin-resonance spectroscopy (ESR). The in situ FT-IR was used to analyze the reaction intermediates of photocatalytic NO oxidation under visible light irradiation. On the basis of free radicals capture from ESR, the reaction mechanism was proposed. The results show that the surface plasmon resonance of Bi metal promoted the visible light absorption and separation of photogenerated charge on BiPO4. The DFT calculation results indicated that the presence of oxygen defects induced the formation of the intermediate energy level between the valence band and the conduction band of the bismuth phosphate, which is beneficial to the electron transition from valence band. The results of electronic structure calculations manifest that the metal Bi also plays a role of storing and transferring electrons from the conduction band of BiPO4. The generation and rapid transfer of photogenerated carriers could decrease the photogenerated electron-hole pair recombination rate. As a result, the BiPO4 can be converted into a high performance visible light photocatalyst. The present work could provide new insights into the understanding of the Bi-based plasmonic photocatalyst and the mechanism of gas-phase photocatalytic reaction.