Voltammetry of phosphotungstic acid immobilized in templated silica gel

Silica prepared by the acid-catalyzed hydrolysis of tetramethyl orthosilicate is used as a host for the Keggin-type polyoxometalate, phosphotungstic acid (PTA), which is being investigated as an electrochemical reduction catalyst. The general voltammetric behavior of PTA in silica in the absence of a contacting liquid phase is the same as that for PTA in aqueous solution as long as the solid is prepared to contain pore water. Specifically, stepwise, quasi-reversible reduction of the twelve W-VI centers in PTA is seen; in the potential window of 0.0 to - 0.6 V versus Ag \ AgCl, two one-electron processes are observed. When Triton X-114 is included above its critical micelle concentration, cmc, in the sol precursor, the resulting gel structure is apparently changed. Effective diffusion coefficients, D-eff, for PTA are higher than those seen in gels prepared without the surfactant. Depending on the age and water content of the gel, D-eff values are in the range 3 x 10(-6) to 2 x 10(-7) cm(2) s(-1) for gels prepared with Triton X-114 above its cmc; in the absence of the surfactant, the comparable range is 2 x 10(-8) to 7 x 10(-8) cm(2) s(-1). The observations are consistent with recent reports on the templating of the internal structure of silica by macrosystems such as micelles. (C) 1998 Elsevier Science S.A. All rights reserved.