Morphology and site blocking effects on chemisorption properties and reactivity of Pt/Ti0 2 and sulfided Pt/Al 20 3 catalysts

Abstract

Hydrogen and carbon monoxide chemisorption as well as n-butane isomerization and hydrogenolysis were performed on Pt/η-Al 2O 3, taken as a reference, on sulfided Pt/η-Al 2O 3 and on Pt/TiO 2 catalysts reduced either at low (513 K), medium (573 K), or high (773 K) temperature. The poisoning by sulfur or TiO x 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 TiO x 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-Al 2O 3 and at medium temperature in the presence of sulfur or TiO 2. 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 TiO x , moieties and does not lead to compensation.