VOLTAMMETRY OF PHOSPHOTUNGSTIC ACID IMMOBILIZED IN TEMPLATED SILICA-GEL

Silica prepared by the acid-catalyzed hydrolysis of tetramethyl orthos ilicate is used as a host for the Keggin-type polyoxometalate, phospho tungstic acid (PTA), which is being investigated as an electrochemical reduction catalyst. The general voltammetric behavior of PTA in silic a 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 po re water. Specifically, stepwise, quasi-reversible reduction of the tw elve 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 chan ged. Effective diffusion coefficients, D-eff, for PTA are higher than those seen in gels prepared without the surfactant. Depending on the a ge 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 abov e 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 w ith recent reports on the templating of the internal structure of sili ca by macrosystems such as micelles. (C) 1998 Elsevier Science S.A. Al l rights reserved.