UV-visible spectroscopy as a probe of heteropolyacid redox properties: application to liquid phase oxidations

UV-visible spectra of heteropolyacids (HPAs) in solution were measured as a simple method of correlating and predicting the redox properties of HPAs. The range of Keggin-type HPAs included H3PMo12O40, H3PW12O40, and variations of cations, heteroatoms, and/or the framework metals. The absorption edges of 0.01 molar aqueous HPA solutions were determined and compared with absorption edges of solid HPAs and with results from other methods that probe redox properties. The absorption edges of the solutions followed similar trends to the absorption edges of bulk solid HPAs, NDR (negative differential resistance) peak voltages from STM measurements on HPA monolayers, and reduction potentials measured electrochemically. HPAs that are stronger oxidizing agents are characterized by absorption edges at longer wavelength (lower energy), and NDR peaks and reduction potentials at more positive voltages. Comparison of edge energies measured in aqueous solution with the performance of HPA catalysts for liquid phase oxidations demonstrates threshold behavior for oxidation activity in each case examined. These results illustrate the potential use of UV-visible spectroscopy in solution as a simple diagnostic indicator in the selection of HPAs as oxidation catalysts.