Photocatalytic Oxidation Of Nitrogen Oxides Over Pt Doped TiO₂Photocatalyst

Nitrogen oxides （NO_x） are major contributors to air pollution. Photocatalysis is apromising and environmental friendly de-NO_xtechnology. Among all semiconductors, TiO₂isthe most widely studied photocatalyst, due to its strong redox ability, chemical stability andcheapness. However, TiO₂absorbs only the small ultraviolet part （3⁵%） of solar light due toits wide band-gap （3.2eV for anatase form）. Moerover, easy recombination ofphoto-induced electrons and holes resulting to its low quantum efficiency. Therefore,developing efficient visible-light-responsive photocatalysts has become one of the mostimportant topics in photocatalysis research. To overcome this shortcoming, many researchershave made much effort on the modification of TiO₂to utilize sunlight more effectively. Oneapproch to achieve this goal is doping transition metal ions into TiO₂structure. However, thedoping metal increases carrier-recomination rates on TiO₂and reduces the thermal stability.Thus, metal-doped TiO₂usually shows an increased visible light activity at the expense of UVactivity. While noble metals surface modification for TiO₂can only improve its photocatalyticefficiency in a certain kind of light. Considering the above factors, Pt metal was doped intoTiO₂to make its absorbing wavelength move to the visible light region and improve itsphotocatalytic performance both under ultraviolet light and visible light irradiation.Various Pt-doped TiO₂with different doping ratios were prepared by a sol-gel methodand investigated with respect to their behavior in photocatalytic oxidation of nitrogen oxides（NO_x） under visible light irradiation. The catalysts were characterized by XRD, UV-vis andPL. From the results, it was found that all the Pt-TiO₂showed an anatase crystallite structure.The absorption edge of Pt-TiO₂shifted to the visible light region upon Pt doping. ThePt-doped TiO₂showed an obvious photocatalytic activity for NO_xoxidation under visiblelight irradiation, and the optimal doping ratio was0.4%. The doping of Pt resulted to theintroduction of impurity energy level and the efficient separation of photo-generated electronsand holes, thus contributed to the improvement of photocatalytic efficiency.Pt modified TiO₂was prepared by sol-gel method and impregnation methods. XRD,UV-vis, XPS, HRTEM-EDS and PL spectra were used to characterize the photocatalyst. Theresults showed that the sol-gel method can dope Pt⁴⁺ions into TiO₂crystal lattice, to form thedoping energy level and oxygen vacancies, and there was also a small part of Pt²⁺located onthe catalyst surface in the form of PtO. As for the impregnated sample Pt/TiO₂, all Pt elementswere dispersed on surface of catalyst in the form of PtO. Photocatalytic tests showed that theNO_xphotocatalytic oxidation efficiency of Pt-TiO₂was higher than that of Pt/TiO₂. This may attribute to co-existence of Pt⁴⁺and Pt²⁺in the Pt-TiO₂sample, while Pt⁴⁺was located in thecrystal lattice of TiO₂, Pt²⁺was located on surface of TiO₂. And Pt⁴⁺may capture thepohoto-induced electrons more timely, thus promote the migration of photo-induced holes tothe catalyst surface, therefore, improve the photocatalytic activity. At last, photocatalyticoxidation of NO_xwere conducted over Pt-TiO₂under irradiation of ultraviolet light, simulatesunlight and visible light, respectively. The results showed that compared to P25and pureTiO₂, the photocatalytic activity of Pt-TiO₂was improved apparently both under ultravioletlight, simulated solar light and visible light irradiation, showing an increas in NOconversion, decrease in NO2selectivity and a increase in total NO_xremoval amount. The NO_xphotocatalytic oxidation mechanism under various light sources were also dicussed.