Surface and Subsurface Platinum in Sulfated Zirconia Catalysts: Relation with Toluene Hydrogenation and n-Hexane Isomerization

Surface platinum of sulfated zirconia has been characterized by hydrogenation of toluene at 150°C and by carbon monoxide adsorption on a series of catalysts containing 0.025–0.8 wt% Pt. The variation of the hydrogenation rate with the Pt content showed two linear domains with a sharp increase at 0.15 wt% Pt. A very good correlation between hydrogenation activity and CO adsorption indicates that the low activity at low loading results from a lack of accessible platinum. We suggest that part of the platinum migrated to the subsurface of the zirconia matrix. After the saturation of the subsurface platinum sites, additional platinum is located on the surface with a constant particle size of about 1 nm. Combination of these results with those of the isomerization of n-hexane at 150°C indicates that very active surface platinum does not play the key role in isomerization. The influence of hydrogen on the isomerization activity nicely correlates with the estimated amount of platinum inserted in the subsurface of zirconia. Reduction of the catalysts at higher temperatures (up to 250°C) diminishes strongly the toluene hydrogenation activity of the samples, while n-hexane isomerization is hardly affected. This is further proof of the importance of subsurface platinum in isomerization and supports the kinetic model based on Lewis acid sites, where desorption of the isocarbenium ion is assisted by hydrides formed on subsurface platinum.