SURFACE-TO-BULK SPILLOVER OF HYDROGEN (PROTON) IN THE CATALYSIS OF SOLID HETEROPOLY COMPOUNDS

Two examples of ''surface-to-bulk'' spillover phenomena found for the catalysis of solid heteropolyacids(HPA) are described. (1) By the spillover of hydrogen atom from Pd-carbon into the solid bulk of HPA, the reduction proceeded smoothly at room temperature. A two-step mechanism was confirmed for the reduction of HPA. The reducibility of HPA thus measured exhibited a fair correlation with the catalytic activity for oxidation. (2) The H-2 + D2 reaction over HPA at 573 K was shown to belong to bulk-type(II) catalysis. The dissociation of H-2 on the surface of HPA and its migration into the solid bulk are rapid, the water formation in the bulk being rate limiting. This was confirmed quantitatively by the analysis of H-2 + D2 reactions.