DYNAMIC MODIFICATION OF SURFACE ACID PROPERTIES WITH HYDROGEN MOLECULE FOR ZIRCONIUM-OXIDE PROMOTED BY PLATINUM AND SULFATE-IONS

The dynamic nature of the protonic acid sites on Pt/SO42-ZrO2 was studied by IR spectroscopy. The generation and elimination of the protonic acid sites in response to the presence and absence of molecular hydrogen in the gaseous phase were observed by monitoring the IR spectra of adsorbed pyridine. By heating in the presence of molecular hydrogen, the protonic acid sites were generated and the Lewis acid sites were weakened. By evacuation of molecular hydrogen, the surface acidic properties retrieved original states. The generation of the protonic acid sites involves dissociative adsorption of the hydrogen molecule on Pt, spillover of the H atom onto the SO42-ZrO2 surface, and electron transfer from the H atom to Lewis acid sites leaving H+ on the surface. The elimination of the protonic acid sites by evacuation of molecular hydrogen involves combination of the H+ with the electron trapped on Lewis acid sites to form an H atom, reverse spillover of the H atom to Pt, and association of atomic H to form H2 to be desorbed. Essentially the same dynamic nature was observed on the physical mixture containing Pt black and SO42-ZrO2, although the extent of the change in acidic properties with heating in the presence of hydrogen or evacuation of hydrogen was small compared to Pt/SO42-ZrO2. © 1992.