偶氮染料吸附和光催化氧化动力学

Adsorption and Photocatalytic Kinetics of Azo Dyes

以甲基橙和酸性大红两种偶氮染料为模拟污染有机物,对它们的暗吸附和光催化氧化行为进行研究.实验结果表明,两种偶氮染料的吸附受溶液酸碱度影响很大,酸性(pH=3)条件下,两种染料吸附量都很大,酸性大红吸附量更大;近中性(pH≈6)时两种染料的吸附显著减少;碱性(pH=9)条件下两种染料不发生吸附.光催化反应结果显示,碱性条件或酸性条件下两种染料降解速度都很快.说明在不同酸碱度条件下,光催化反应按不同机理进行.酸性条件下,反应在催化剂表面进行,在碱性介质中,光催化氧化在溶液中进行.提出了一个碱性条件下的动力学方程,经过进一步简化,可以得到表观一级方程,形式上和准一级L-H方程十分相似,但其含义不同.

A pseudo first order Langamuir-Hinswood expression is generally used to describe heterogeneous photocatalytic kinetics, but it still uncertain whether the oxidation process occurs on TiO2 surface or in solution. Adsorption behavior and photocatalytic oxidation of methyl orange and naphthalene scarlet 3R on P-25 TiO2 have been studied in order to understand the relationship of adsorption and reaction kinetics of the azo dyes. The results showed that the adsorption of both the azo dyes is obviously influenced by pH value in solution. Significant adsorption of both the azo dyes oil the catalyst has been detected at pH = 3, while the adsorption decreased obviously at pH approximate to 6. The adsorption amount of naphthalene scarlet 3R is larger than that of methyl orange under cases of pH = 3 and pH approximate to 6 due to the structural difference between the two azo dyes. No adsorption was observed at pH 9 for both the azo dyes. Rates of photocatalytic degradation of the azo dyes in different media were in the order the acidic > the basic > the neutral condition. These. facts indicated that the photocatalytic reaction carried out under diffeerent mechanism in different media. It is proposed that the photocatalysis occurs on catalyst surface under acidic condition, which may follow L-H mechanism, whereas the photocatalysis occurs in solution under basic condition. A kinetic expression for the photocatalysis under basic condition was proposed, which can be finally simplified to an apparent first order reaction kinetic equation, similar to the pseudo first order L-H expression. This may be one of the reasons that the photocatalytic degradation kinetics usually can be described by the pseudo first order L-H expression.