Photocatalytic oxidation of propan-2-ol by cadmium sulfide

Abstract

The photocatalytic oxidation of liquid propan-2-ol to propanone has been investigated using γ-Al₂O₃-supported CdS, platinised CdS and Zn²⁺ or Ag⁺ cation modified CdS. Reaction proceeded only in the presence of absorbed rediation and added water, although O₂ was also required with catalysts other than platinised CdS. Following confirmation of the activity of CdS based catalysts in ‘visible light’, measurements were made using UV radiation so that comparisons could be made with TiO₂-based catalysts. Rate measurements at 303 K were made to determine the activity dependence of CdS/Al₂O₃ on catalyst mass, oxygen pressure, radiation intensity and water content of the reaction mixture. Activity measurements under standard conditions over the range 283–308 K showed that CdS-based catalysts possessed a common activation energy of 53 ± 2 kJ mol^–1, which was unchanged if methanol or ethanol replaced propan-2-ol as the reactant. This value is appreciably higher than the 20 ± 1 kJ mol^–1 observed with TiO₂-based catalysts, which also differed in not requiring added water for reaction to proceed. The results are discussed in terms of a mechanism in which the photogenerated ˙OH radical is the primary oxidant. With TiO₂ it is generally accepted that ˙OH is generated by trapping a photohole at a surface hydroxide ion; with CdS it is proposed that ˙OH is generated by trapping a photohole at molecularly adsorbed water.