MORPHOLOGY AND SITE BLOCKING EFFECTS ON CHEMISORPTION PROPERTIES AND REACTIVITY OF PT/TIO2 AND SULFIDED PT/AL2O3 CATALYSTS

Hydrogen and carbon monoxide chemisorption as well as n-butane isomerization and hydrogenolysis were performed on Pt/η-Al2O3, taken as a reference, on sulfided Pt/η-Al2O3 and on Pt/TiO2 catalysts reduced either at low (513 K), medium (573 K), or high (773 K) temperature. The poisoning by sulfur or TiOx decoration leads to a complete loss of irreversible hydrogen or carbon monoxide chemisorption and a depression by three orders of magnitude of the original platinum surface activity. In both cases, the surface inhibition effect is similar and conveniently shown, for reactivity, on compensation plots. Since sulfur adatoms or TiOx fragments are supposed to induce a transfer of electrons in the opposite direction, one must conclude that both inhibitions are mainly due to geometric effects. In both cases, the surface inhibition appears to take place in two steps and can be reversed by oxygen treatment. First, an apparent flattening of the particles that leads to less active, low index planes exposure to reactants and to a compensation effect is suggested. This occurs at high temperature on Pt/η-Al2O3 and at medium temperature in the presence of sulfur or TiO2. Then, following a high temperature reduction, the second step, which is responsible for the major part of the inhibition, takes place by simple site blocking by sulfur or TiOx, moieties and does not lead to compensation. © 1991.