Ruthenium(ii)–tris-bipyridine/titanium dioxide codoped zeolite Y photocatalysts: II. Photocatalyzed degradation of the model pollutant 2,4-xylidine, evidence for percolation behavior

Abstract

A considerably arduous test of a novel class of composite materials consisting of [Ru(bpy)₃]²⁺ and TiO₂ codoped zeolites Y is presented here. The [Ru(bpy)₃]²⁺ and TiO₂ codoped zeolites Y served as photocatalysts in the oxidation of the model compounds 2,4-dimethylaniline (2,4-xylidine) by H₂O₂ in an acidic aqueous medium. Zeolite-embedded TiO₂ (nano)particles play an important role in the degradation mechanism. The first step in this complex mechanism is the photoelectron transfer from photoexcited [Ru(bpy)₃]²⁺*, located inside the supercage of zeolite Y, to a neighboring TiO₂ nanoparticle. During this electron transfer process, electron injection into the conduction band of TiO₂ is achieved. The second decisive step is the reaction of this electron with H₂O₂, which was previously chemisorbed at the surface-region of the TiO₂ nanoparticles. In this reaction, a TiO₂ bound hydroxyl radical (TiO₂–HO˙) is created. This highly reactive intermediate initiates then the oxidation of 2,4-xylidine, which enters the zeolites framework in its protonated form (Hxyl⁺). The formation of 2,4-dimethylphenol as first detectable reaction product indicated that this oxidation proceeds via an electron transfer mechanism. Furthermore, [Ru(bpy)₃]³⁺, which was created in the initiating photoelectron transfer reaction between [Ru(bpy)₃]²⁺* and TiO₂, also takes place in the oxidation of Hxyl⁺. [Ru(bpy)₃]²⁺ is recycled in that reaction, which also belongs to the group of electron transfer reactions. In addition to the primary steps of this particular Advanced Oxidation Process (AOP), the dependence of the efficiency of the 2,4-xylidine degradation as a function of the [Ru(bpy)₃]²⁺ and TiO₂ loadings of the zeolite Y framework is also reported here. The quenching of [Ru(bpy)₃]²⁺* by H₂O₂ as well as the photocatalytic activity of the [Ru(bpy)₃]²⁺ and TiO₂ codoped zeolite Y catalysts both follow a distinct percolation behavior in dependence of their TiO₂ content.