Quantifying accessible sites and reactivity on titania-silica (photo)catalysts: Refining TOF calculations

It can be difficult to determine the number of active atoms accessible to the fluid phase in mixed oxide catalysts, as required for obtaining true turnover frequencies (TOF). Here, we utilize the selective titration of surface Ti atoms with phenylphosphonic acid (PPA) on TiO₂-SiO₂ materials to estimate the number of reactant-accessible sites. TiO ₂-SiO₂ composites were synthesized over a range of Ti loadings from grafting of titanocene dichloride (Cp₂TiCl₂) or tetraethoxy orthotitanate (TEOT) on SiO₂ and sol-gel co-hydrolysis of Si and Ti alkoxides. The materials were characterized by DRUV-vis, XRD, BET, and XANES. Despite the significant morphological and electronic differences, materials prepared by Cp₂TiCl₂ and TEOT yielded a near-constant TOF of 0.14 h^-1 (±0.04) across Ti loadings, for benzyl alcohol photooxidation, when normalizing rates by sites titrated by PPA. The fraction of Ti atoms titrated by PPA was strongly dependent on synthesis method and surface density. PPA titration and benzyl alcohol photooxidation may be useful measures of surface accessibility in other supported oxides.