Reduction potential, UV–visible absorption edge energy, and oxidation catalysis of niobium-containing H 3+ xPW 12− xNb xO 40 Keggin and H 6+ xP 2W 18− xNb xO 62 Wells-Dawson heteropolyacid catalysts

Abstract

Niobium-containing H 3+ x PW 12− x Nb x O 40 ( x = 0, 1, 2 and 3) Keggin and H 6+ x P 2W 18− x Nb x O 62 ( x = 0, 1, 2 and 3) Wells-Dawson heteropolyacids (HPAs) were prepared in this work to explore their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H 3+ x PW 12− x Nb x O 40 and H 6+ x P 2W 18− x Nb x O 62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy, respectively. The trend of reduction potential is well consistent with the trend of absorption edge energy with respect to niobium substitution in both series of H 3+ x PW 12− x Nb x O 40 and H 6+ x P 2W 18− x Nb x O 62 HPA catalysts. Absorption edge energy of H 3+ x PW 12− x Nb x O 40 and H 6+ x P 2W 18− x Nb x O 62 HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. In order to probe oxidation catalysis of H 3+ x PW 12− x Nb x O 40 and H 6+ x P 2W 18− x Nb x O 62 HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of H 3+ x PW 12− x Nb x O 40 and H 6+ x P 2W 18− x Nb x O 62 HPA catalysts.