Reversibility in the transformation between cationic and metallic silver and the generation of Brønsted-acid sites in the silver salt of dodecatungstophosphoric acid

Abstract

The ¹H FTNMR technique was applied to the characterization of protons in Ag₃PW₁₂O₄₀, which had been partially reduced with hydrogen. The intensity of the ¹H NMR peak varied reversible with variation of the pressure of hydrogen in the gas phase. The evidence of reversible transformation between Ag⁺ cations and silver metal particles was also revealed by XRD measurements of silver metal particles and IR measurements of CO chemisorbed on Ag^– cations. The catalytic activity of Ag₃PW₁₂O₄₀ supported on silica and partially reduced with hydrogen, for isomerization of hexane, was enhanced by the presence of hydrogen in the gas phase at 423 K. These results indicate that the reversibility in the catalytic activity is caused by the generation of acid sites with concomitant reduction of Ag⁺ cations into silver metal particles; the whole process can be reversed by eliminating hydrogen from the system.