Counting active sites on titanium oxide-silica catalysts for hydrogen peroxide activation through in situ poisoning with phenylphosphonic acid

Quantifying specific active sites in supported catalysts improves our understanding and assists in rational design. Supported oxides can undergo significant structural changes as surface densities increase from site-isolated cations to monolayers and crystallites, which changes the number of kinetically relevant sites. Herein, TiO_x domains are titrated on TiO_x-SiO₂ selectively with phenylphosphonic acid (PPA). An ex situ method quantifies all fluid-accessible TiO_x, whereas an in situ titration during cis-cyclooctene epoxidation provides previously unavailable values for the number of tetrahedral Ti sites on which H₂O₂activation occurs. We use this method to determine the active site densities of 22 different catalysts with different synthesis methods, loadings, and characteristic spectra and find a single intrinsic turnover frequency for cis-cyclooctene epoxidation of (40± 7) h^-1. This simple method gives molecular-level insight into catalyst structure that is otherwise hidden when bulk techniques are used.