TiO2-mediated heterogeneous photocatalysis of saturated aliphatic C1 to C5 carboxylic acids : kinetics, degradation pathway, and process efficiency
In this study, we first look at the relation between pH and extent of dark adsorption for aqueous suspensions of linear C1 to C5 saturated aliphatic carboxylic acids, their branched analogues, and their 2-hydroxy and 2-keto derivatives, over TiO 2 Degusa P25. We demonstrate that the observed trends...
| Summary: | In this study, we first look at the relation between pH and extent of dark adsorption for aqueous suspensions of linear C1 to C5 saturated aliphatic carboxylic acids, their branched analogues, and their 2-hydroxy and 2-keto derivatives, over TiO 2 Degusa P25. We demonstrate that the observed trends between the extent of dark adsorption and pH can be rationalized by an electrostatic model. As a second step, we attempt without success to demonstrate that the primary degradation products of valeric acid are the alpha-hydroxy and the alpha-keto derivatives. However, we confirm the identification of 15 degradation products. The nature of the degradation products allows us to determine that the first step in the degradation of valeric acid involves hydroxylation of the carbon backbone. Evidences for the formation of keto derivatives from hydroxylated degradation products is also presented. Third, we investigate the possible changes in the nature of the surface of the photocatalyst upon light irradiation by determining the adsorption constant for a few selected acids from (1) adsorption isotherms and (2) Kinetically. A statistically significant difference between the constants obtained by the two methods is obtained. Finally, the overall quantum yield of formic acid is determined from plots of the reciprocal of photonic efficiencies vs. the reciprocal of catalyst load. The photon flow from the light source needed for the determination of Photonic efficiencies (defined as the ratio of the initial reaction rate to the photon flow from the light source) is determined with the chemical actinometer Aberchrome 540. Using the concept of relative photonic efficiencies and the limiting photonic efficiency of formic acid, we propose quantum yields for acetic acid, propanoic acid, butanoic acid, valeric acid, and 2-methylbutanoic acid. |
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