Study of heteropoly compounds with the Keggin structure by temperature-programmed reduction and HD exchange

Heteropoly acids and salts with the Keggin structure were studied using temperature-programmed reduction (TPR) and temperature-programmed hydrogen-deuterium exchange (TPE) techniques. H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, and H₃PMo₁₂O₄₀ were characterized in TPR by peaks in identical positions to those observed in temperature-programmed desorption experiments. No distinct peaks due to reduction were observed for monovalent salts but continuous reduction occurred in TPR above 873 °K. Behavior similar to that shown by the acids in reduction was observed for divalent salts. Exchange occurred at lower temperatures than reduction for H₃PW₁₂ O₄₀ and H₄SiW₁₂O₄₀ whereas the opposite behavior was noted for H₃PMo₁₂O₄₀. A reduction mechanism for the acids was proposed in which rearrangement of the secondary structure which involved localization of the protons on a single lattice oxygen anion released other lattice anions so that they could act as sites for dissociative sorption of hydrogen and partial reduction of the acids.