The effect of methyl methacrylate transformations during photocatalysis in the presence of RbTe_1.5W_0.5O₆ on the change of the complex oxide surface

Abstract

The features of the surface changes of the RbTe1.5W0.5O6 photocatalyst during polymerization processes involving methyl methacrylate (MMA) under irradiation with visible light λ= 400–700 nm at a temperature of 20–25 °C, as well as regeneration conditions for its reuse, have been studied. The realization of a chemical reaction in a mixture of a photocatalyst and a monomer is determined by its kinetic parameters and the concentration of reacting particles. The formation of OH• radicals, which are active in radical polymerization, occurs in this case by both ways: interacting between water molecules adsorbed on the surface, as well as during the recovery of oxygen dissolved in water. Due to the high reactivity of the hydroxyl radical, an active process of radical polymerization of MMA could be expected. However, the formation of polymethyl methacrylate (PMMA) in the reaction takes place with a low conversion – the formation of 5–10% polymer, which is related to changes on the surface of the complex oxide RbTe1.5W0.5O6 during the reaction. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods have been used to investigate the catalyst surface. The presence of organic and polymer substrates was revealed on the catalyst surface after the reaction. As a result of the catalyst treatment by ultrasound in an aqueous emulsion, polymer macromolecules are not identified on the surface, but chemically adsorbed monomer and oligomers formed under the ultrasound destruction of the polymer are presented. Other methods of cleaning the catalyst surface by washing in different solutions as chloroform and tetrahydrofuran also are not effective and cause the adsorbates transformations on the surface. It leads to decreasing the RbTe1.5W0.5O6 catalyst's activity for repeated polymerization. Thus, to successful regeneration of the catalyst's powder not only ultrasound treatment is required, but also heating it at 300–400 °C to remove organic substrates.