Structure and surface properties of ZrO2-supported WO3 nanostructures

Abstract

Isomerization reactions on ZrO 2 -supported WO x domains require catalyst pretreatment in air at high temperatures and the presence of H 2 during reaction. o-Xylene isomerization rates are proportional to H 2 pressure, suggesting a role of H 2 in reduction steps that form and maintain Bronsted acid sites. Consistent with this, O 2 reversibly inhibits isomerization reactions, apparently by titrating H atoms and re-oxidizing reduced W centers. NH 3 and pyridine adsorption showed that the Bronsted/Lewis acid site density ratio increases as WO x surface density increases (3.6–17.7 W/nm 2 ). The total concentration of acid sites (per Watom), however, decreases with increasing surface density as WO x species become increasingly inaccessible with increasing cluster size. The effect of H 2 on the density and type of acid sites is much weaker than its effect on isomerization rates. Isomerization rates show a distinct maximum near monolayer WO x coverages on ZrO 2 (10 W/nm 2 ) whether the rates are normalized by W-atoms, BET surface area, or acid site density. The Bronsted acid sites responsible for o-xylene isomerization (in H 2 ) appear to be present as a small fraction of those titrated by NH 3 or pyridine. Such sites are unusual in that they are rendered inactive by small amounts of oxygen and show higher turnover rates than Bronsted acids on H-ZSM5.