Ti–Ni and Ti–Co Mixed Oxides Supported on Y Zeolite with Different Porosity as Photocatalysts in Degradation of Amoxicillin ^†

Zeolites are very useful supports for synthesis of catalysts. Many of these materials with photocatalytic properties contain active species with TiO₂ homogeneously dispersed [1] and often a co-dopant in order to enhance their efficiency. Among different transition metal ions, Ni and Co have been used in photocatalytic degradation of organic pollutants. The present work reports the synthesis of Y zeolite with microporous and hierarchical structure containing Ti–Ni and Ti–Co mixed oxides as photocatalysts for degradation of amoxicillin from water. The effects of supports, TiO₂ loading and type of the second immobilized metal (Ni or Co) on formation of reactive species and photocatalytic performances and mechanism were studied.

In the first step, zeolite supports (Y zeolite, named Y, and hierarchical Y zeolite, named hY) were obtained. In the second step, these supports were impregnated with active species (Ti and Co or Ni). Firstly, different Ti amounts (5%, 10% TiO₂) were supported, followed by loading of Ni/Co species (5% NiO, Co₃O₄). The obtained samples were named YT5N, hYT5N, YT10N, YT5C, hYT5C and YT10C and were characterized by XRD, N₂ sorption, UV-Vis absorption, Raman spectroscopy and H₂-TPR. Photocatalytic activity was evaluated in degradation of amoxicillin (AMX) as a model test. The photocatalytic mechanism was investigated using ethanol, p-benzoquinone and KI as •OH, •O₂⁻ radicals and hole (h+) scavenger. Formation of •OH radical on the samples’ surface under irradiation was investigated by fluorescence technique.

X-ray diffraction evidenced that the crystallinity of the zeolite Y was not altered after impregnation with Ti and Co/Ni species. In addition, the XRD results evidence the presence of TiO₂ as anatase on zeolite materials. The intensity of the anatase characteristic peak increased with TiO₂ loading. N₂ adsorption-desorption results show type IV isotherms for hY samples and a combination of type IV and I isotherms for Y samples. UV–Vis diffuse reflectance spectra presented characteristic peaks of TiO₂, NiO and spinel Co₃O₄ species. The absorption edge of photocatalysts shifted to the visible region with increasing Ti loading and especially with Co and Ni addition. TPR profiles indicated the presence of bulk NiO and some Ni sites incorporated into the zeolite framework in all the samples modified with Ni. Furthermore, Ni ions located inside the channels of zeolite were evidenced only for the samples with hierarchical zeolite support. Photocatalytic evaluation evidenced the effect of support, TiO₂ loading (5% was the optimal concentration), Ni or Co immobilization and irradiation wavelength. Better degradation efficiencies were obtained for Ni containing photocatalyst, as a result of p-n heterojunction.

In conclusion, new active photocatalysts were obtained by dispersion of TiO₂ and Ni/Co oxides on zeolite. The support properties, TiO₂ amount and immobilization of Co or Ni species influenced their dispersion and metal-titania interaction, leading to the formation of different reactive species responsible for the photocatalytic degradation of amoxicillin. In fact, degradation efficiency is a result of active radicals and depends on their number and time of the photocatalytic process.