Chemisorption properties of platinum and iridium supported on TiO2$z.sbnd;Al2O3 mixed-oxide carriers: Evidence for strong metal-support interaction formation

Fresh Pt and Ir on A12O3 catalysts reduced at 773 K exhibit high HM ratios (1 to 2.5) indicative of highly dispersed metal phases. In contrast, TiO2-supported Pt and Ir catalysts display low HM ratios (<0.04) following hydrogen reduction at 773 K. The suppressed hydrogen uptakes typically exhibited by TiO2-based Group VIII metal catalysts result from a strong metal-support interaction (SMSI) and not from a poor metals dispersion. Following reduction at 773 K, the HM ratios displayed by Pt and Ir catalysts supported on TiO2Al2O3 mixed-oxide carriers are intermediate in value between those observed on TiO2 and Al2O3 supports. The extent of H2 uptake suppression is dependent upon the relative TiO2 and Group VIII metal concentrations on the Al2O3 surface. The magnitude of H2 uptake suppression (and thus SMSI formation) increases at higher TiO2/Group VIII metal ratios. Higher reduction temperatures and extended reduction times at a given elevated temperature also enhance SMSI formation. Taken together, these results imply that the generation of reduced Group VIII metal-TiO2−x species on the Al2O3 surface may be responsible for the SMSI effect.