Synthesis of porous TiO2/ZrO2 photocatalyst derived from zirconium metal organic framework for degradation of organic pollutants under visible light irradiation

Highlights

• Porous TiO₂/ZrO₂ photocatalyst derived from UiO-66 and Titania hybrids was synthesized.

• The decolorization kinetics followed first-order kinetic model under visible light.

• The highest rate constant (k) was found 0.013 min⁻¹ for degradation of Rhodamin B.

• After four times recycling, the regenerated photocatalyst showed high stability and photodegradation ability (90%).

Abstract

Zirconium based metal-organic frameworks (Zr-MOFs) are promising candidates for photocatalytic wastewater treatment due to their excellent properties such as high chemical and thermal stability and high photodegradation ability. Herein, we report a novel porous TiO₂/ZrO₂ photocatalyst derived from UiO-66 and Titania hybrids. UiO-66 nanoparticles was synthesized through solvothermal method and utilized as catalyst support to grow TiO₂ particles on its surface. The prepared Titania/MOF nanocomposite was calcined to obtain porous TiO₂/ZrO₂ photocatalyst for degradation of organic pollutants from colored wastewater under LED visible light. The prepared materials were fully characterized with FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS and ICP analysis. The results showed that the developed TiO₂/ZrO₂ enhanced photodegradation ability of Rhodamin B (RhB) in comparison with the mixture of prior UiO-66 and TiO₂ and was found to affect the photocatalytic activity by increasing the adsorption of photons in visible region and enhanced the transfer and separation of produced charge. The decolorization kinetics followed first-order kinetic model. In addition, after four times recycling, the regenerated nanocomposite still showed high stability and photodegradation ability (90%).

Introduction

In recent years, water pollution and the crisis of water shortage, are the most important problems that humans have been faced. A great amount of water pollution comes from chemical industries such as textile or paper industry [1]. Wastewater treatment of textile industry is the most difficult one among others since textile industry utilizes more than 7 $\times$ 10⁵ tones dyes annually and 2% of the amount of dyes, which has been used, is discharged into the effluent and 10% is lost during the dyeing processes [2,3]. Removal of dyes form colored wastewaters is more essential because a small amount of dyes in wastewater is harmful for humans’ environment and ecosystem [4]. Furthermore, in comparison to other contaminants, dyes are more resistant to conventional methods of wastewater treatment.

Several methods such as physical, biological and chemical processes are employed for water purification and removal of different contaminants from wastewaters [5,6]. Among different treatment methods, chemical treatments especially advanced oxidation processes (AOPs) counts as one of the most effective methods in dye removal from wastewater [7,8]. Some advantages of AOPs are that these processes operate at or near ambient temperature and pressure and convert approximately all the organic compounds contaminating effluents into less hazardous products [9]. Processes using H₂O₂ or UV irradiation [10], Fenton and photo-Fenton catalytic reactions as some examples of AOPs [11], are efficacious methods based on the generation of hydroxyl radicals which oxidizes a wide range of organic pollutants in wastewater non-selectively.

Using photocatalytic degradation processes under UV/visible irradiation is a newborn and green method in wastewater treatment especially in the field of dye degradation. Several photocatalysts such as TiO₂, ZrO₂, ZnO, etc. have been used for destroying pollutants [[12], [13], [14], [15]]. Among different photocatalysts, TiO₂ has attracted a great attention due to the fact that it is not toxic and harmful for the environment and has a low-cost synthesis. Besides, there are many studies on TiO₂ among other photocatalyst; Although, the band gap of TiO₂ is relatively wide (3.2 eV) which makes it not effective enough under visible light [15]. However, whether semiconductors combined with some other porous materials may enhance the photocatalytic performance and make the photocatalytic processes more sufficient. One of the new emerged compounds named metal organic frameworks (MOFs) have attracted significant attention as a photocatalyst [16,17].

MOFs as organic-inorganic hybrid materials are a new kind of porous compound which have been surveyed in several fields such as catalysis [[18], [19], [20]], gas storage [21], sensors [22], chemical separation [23], adsorption [16], etc. These materials have attracted great attention because of their high surface area, high porosity, and low density [24]. Several studies have been conducted on MOFs, results of which have shown that these structures can act as a photocatalyst under UV or visible irradiation in Rhodamin B (RhB) degradation process [[25], [26], [27], [28], [29]]. As one of Zr-based MOFs, UiO-66, possesses high thermal and mechanical stability [30]. Nevertheless, the band gap of UiO-66 is about 3.6 eV which limit its optical adsorption in the visible light region. To achieve this, a narrow band gap semiconductors coupling formed a heterojunction method is an ideal choice. The heterojunction can form an inner electric field to facilitate the transfer/separation of photo-generated electron/hole and inhibit the recombination of electrons and hole so as to enhance the photocatalytic activity [31]. So, it is rational to expect that by hybridizing UiO-66 with another excellent semiconductors, a more stable and efficacious MOF composite with photocatalytic activity would be attained.

In this study, we are reporting porous TiO₂/ZrO₂ derived Titania/MOF nanocomposite as a photocatalyst for degradation of RhB under LED visible irradiations for the first time. The functional group, crystalline structure, morphology and photocatalytic activity of prepared material was investigated by FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS, and ICP analysis. The results showed that incorporation of Zr-MOF and TiO₂ increased photodegradation ability of RhB. Effects of different factors such as initial solution pH, amounts of photocatalyst and dye concentration were investigated in dye degradation process.