ZnTi layered double hydroxides as photocatalysts for salicylic acid degradation under visible light irradiation

A series of ZnTi layered double hydroxides (LDH) with different Zn/Ti ratios are prepared and used as catalysts for photodegradation of salicylic acid (SA) under visible light. The catalysts are characterized by X-Ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, UV–vis diffuse reflectance spectroscopy, thermogravimetry, electron paramagnetic resonance and N2 adsorption-desorption. The results show that SA anions bind to the LDH surface and that an electron can be excited from the HOMO in the adsorbed molecules to the conduction band of the LDH under visible light illumination. This charge transfer further leads to an effective photodegradation and mineralization of SA with better conversion results than on P25 titania. Key factors influencing the charge-transfer process in LDH are the high surface area and the Ti/Zn ratio of the LDH materials. The combination of highly dispersed Zn2+ and Ti4+ cations in the brucite-like sheets of the LDH allows for a better charge separation, which also accounts for the high photocatalytic activity. The present results show that superoxide radicals play a role in the visible-light induced degradation of SA on LDH, while no •OH radicals are formed. In contrast to LDH, the light-induced degradation pathway of SA over P25 titania leads to the formation of CO2−, a relatively stable anion that may hamper further conversion to CO2 and hence limit the photocatalytic performance. The introduction of an electron acceptor, such as peroxydisulfate, further improves the degradation and mineralization of SA over LDH, but care should be taken not to use an electron acceptor that can easily adsorb to the LDH surface, such as H2O2. ZnTi LDH are thus very promising alternatives to TiO2 for the photodegradation of colorless organic pollutants, such as SA, under visible light irradiation.