Light-induced formation of 2,5-dihydroxy-p-benzoquinonefrom hydroquinone in photoirradiated silver-loaded zirconium phosphate suspension

Abstract

Silver-loaded zirconium phosphate [Ag_1-xH_xZr₂(PO₄)₃] has shown photocatalytic activity concerning the generation of OH and the hydroxylation of hydroquinone (HQ) to 2,5-dihydroxy-p-benzoquinone (DHQH₂). HQ was initially oxidized to p-benzoquinone (BQ) by Ag⁺ on the surface of Ag_1-xH_xZr₂(PO₄)₃ in the dark and by photogenerated OH during visible-light irradiation. As an intermediate, a semiquinone radical BQH was detected by EPR measurements in both cases. Furthermore, under visible-light irradiation, DHQH₂ was identified by its absorption spectrum and thin layer chromatography. The amount of DHQH₂ increased and BQ decreased with irradiation time. The total amount of BQ and DHQH₂ under visible-light irradiation agreed with that of BQ in the dark. Consequently, DHQH₂ appeared to form from BQH generated by the oxidation of HQ and the reduction of BQ by photogenerated e^-. From the XPS and FT-Raman technique analyses, it was found that the addition of the photogenerated OH to BQH occurred at the surface of Ag_1-xH_xZr₂(PO₄)₃. This indicated that both the OH and HBQ were stabilized on the surface of Ag_1-xH_xZr₂(PO₄)₃.