While the mechanism of some semiconductor catalyzed transformations is rather well investigated for colloidal systems due to their pseudo-homogeneous nature which allows conventional optical spectroscopy, this is not true for powder suspensions which on the other hand are more important for chemical synthesis.
In both systems, however, two basic mechanistic questions could not be answered. Up to now only a few studies dealt with the question whether emissive (etr-, htr+) and reactive (er-, hr+) surface sites are identical or not. For the second question, whether the primary redox intermediates are transformed to the final products in adsorbed or fully solvated state, again only scarce experimental evidence is available.
In this contribution we try to answer these two questions for the ZnS catalyzed dehydrodimerization of 2,5-dihydrofuran (2,5-DHF, RH, Eq. 1), one of the rare examples where a new compound was obtained by semiconductor photocatalysis on a preparative scale [1].
It was assumed that the photogenerated electron-hole pair reduces water to hydrogen and OH- and oxidizes RH to R. and H+.

Fig. 1

Dimerization of R. affords a statistical mixture of regioisomeric dehydrodimers R - R (Equ. 1). In order to answer the two questions raised above, the effect of electron scavengers on emission properties and reactivity of various ZnS powders and the dark adsorption of 2,5-DHF were determined, and competition experiments between 2,5-DHF and THF were performed.

[1]	a) N.Zeug,J.Bücheler and H.Kisch, J.Am.Chem.Soc. 107 (1985) 1459-1465.
	b) R.Künneth, G.Twardzik, G.Emig and H.Kisch, J.Photochem. Photobiol. A: Chem. 76 (1993) 209-215.

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