Synthesis, characterization and catalytic evaluation of tungstophosphoric acid immobilized on Y zeolite

Abstract

The degradation of chemicals present in wastes by heterogeneous photocatalysis is an important issue and its study is a constantly growing field. An increasing number of papers deal with TiO2 (titania) as one of the most appropriate semiconductor materials to be employed as a photocatalyst, due to its high activity in the photodegradation of organic compounds, low cost, low toxicity, and chemical stability [Herrann 2010, Sclafani et al. (1991), Antonelli et al. (1995), Sakthivel et al. (2004), Yeung et al. (2003), Yu et al. (2000), Wang et al. (1998), Van Grieken et al. (2002)]. It is generally reported that titania performance in the photodegradation of contaminants contained in wastes is influenced by the crystal structure, crystallinity, surface area, porosity, and band gap energy [Li et al. (1999), Kyoko et al. (1997), Sakulkhaemaruethai et al. (2005)], among other factors. The fast recombination of the photoinduced electrons and holes can lead to a low photocatalytic activity. Transition metals or metal oxides were used to avoid the recombination of the electron?hole pairs of TiO2-based catalysts [Lee et al. (1992), Sclafani et al. (1991), Lee et al. (1993), Ikeda et al. (2001)], improving the photocatalytic activity. Additionally, different heteropolyoxometalates (POM) have been added to TiO2 suspensions [Ozer and Ferry 2001] or anchored to TiO2 by chemical interactions [Yang et al. (2004)], incorporated either into TiO2 colloids [Yoon et al. (2001)] or into the titania matrix during the TiO2 gel synthesis [Fuchs et al. (2008), Blanco and Pizzio 2010, Fuchs et al. (2009)] with the purpose of reducing the charge recombination in UV-illuminated TiO2. The capacity of POM as acceptors of the electrons of UV-irradiated TiO2 suspensions, generated in the conduction band (ecb-) together with holes in the valence band (hvb+), was demonstrated by Park and Choi [Park and Choi 2003] using a photoelectrochemical method. Heteropolyoxometalates are widely used as oxidation as well as acid catalysts [Okuhara et al. (1996), Fuchs et al. (2008), Pizzio et al. (1998)]. They are also employed as effective homogeneous photocatalysts in the degradation of organic pollutants in water [Mylonas et al. (1996), Mylonas and Papaconstantinou 1996, Hu et al. (2000), Ozer and Ferry 2000].