Adsorption effects in semiconductor photocatalysis

The influence of adsorption on the zinc sulfide catalysed photodehpdrodimerisation of 2,5 dihydrofuran (2,5-DHF) was investigated through application of the Hiemenz adsorption model. Assuming an ideally mixed solvent-solute monolayer, the adsorption constant was obtained as K-ad = 170 +/- 30 l mol(-1) and the area covered by one 2,5-DHF molecule as 10.2 Angstrom(2), in excellent agreement with the theoretical value of 9.1 Angstrom(2). At initial concentrations above ca. 60 mmol g(-1) multilayer adsorption is observed. Comparison of the concentration dependence of initial rate with the amounts of 2,5-DHF adsorbed reveals a good agreement. This and the square dependence of rate on coverage indicates that dimerisation of the intermediate dihydrofuryl radicals occurs in the solvent-solute surface monolayer.