Reaction Mechanism Of Photocatalytic Degradation Of Organic Dye Over Iron (Hydr) Oxides

Photocatalysis technology based on TiO₂ has provided an effective and promising means for remediation of environmental pollutants in air and water.However,its application for industrial water treatment still remains on the laboratory stage, mainly due to low efficiency in photoenergy utiliziation.In this thesis,a series of iron oxides and hydroxides have been studied as a photocatalyst,which has been rarely reported in the literature.The thesis is divided into four sections,as follows:Sectionâ… .Photoreduction of silver ion over six different iron(hydr)oxides,i.e,α-Fe₂O₃,γ-Fe₂O₃,Fe₃O₄,α-FeOOH,γ-FeOOH andδ-FeOOH,was studied with UV light in an aerated aqueous suspension.All the catalysts were effective for the photoinduced reduction of Ag(â… ),but the rate constant decreased in the order of PT-TiO₂ >α-Fe₂O₃>α-FeOOH >γ-Fe₂O₃ > AT-TiO₂ >γ-FeOOH >δ-FeOOH. Moreover,the reactions withα-Fe₂O₃,γ-Fe₂O₃ andα-FeOOH only occurred when the amount of Ag(â… )adsorption reached about half of the maximum surface coverage.It is proposed that the Ag(â… )reduction occurs through a photogenerated conduction band electron or through a photogenerated surface Fe(â…¢)species.Sectionâ…¡.Photodegradation of Orangeâ…¡in aerated aqueous suspension at neutral pH over different iron(hydr)oxides was studied either with UV or visible light.All the catalysts could effectively initiate the dye photodegradation.It was observed that the initial rate of dye photodegradation was proportional to the initial amount of dye adsorption,and the reaction rate was significantly enhanced upon the addition of H₂O₂,AgNO₃ and NaF into the suspensions.Detailed studies reveal that that the observed dye photodegradation with all the iron(hydr)oxides mainly occur through semiconductor photocatalysis,but not the photoinduced ligand-to-metal charge transfer within a surface complex,or(photo)-Fenton reactions,as previously debated.Sectionâ…¢.Photodegradation of Orangeâ…¡overα-Fe₂O₃ in aqueous suspension was studied in terms of catalyst calcination temperature.It was found that both the dye adsorption and the catalyst crystallity were important.Below 400℃,the degree of catalyst crystallity was dominant,while beyond 400℃,the organic adsorption was more important.Immobilization ofα-Fe₂O₃ onto nanosized SiO₂ particles led to a decrease in the amount of organic adsorption,but an increase in the rate of dye photodegradation.This is mainly due to the well-dispersedα-Fe₂O₃ that promotes the diffusion of h_vb⁺ or reactive sites onto the catalyst surface where to react with the adsorbed dye substrate.Addition of H₂O₂ into the dispersion could accelerate the dye photodegradation both onα-Fe₂O₃ and on SiO₂-supportedα-Fe₂O₃.Sectionâ…£.Ferric species was successfully immobilized onto natural Cationic clay. It was observed that under visible light irradiation in the presence of H₂O₂,this composite was effective to initiate the degradation of both cationic and anionic-type organic dyes in water at netural pH.The dissolved iron was minor,and the catalyst could be used repeatedly.The reaction mechanism is proposed as heterogeneous photosensitization,followed by acceleration with H₂O₂ present in the suspension.