Transient technique for identification of true reaction intermediates: Hydroperoxide species in propylene epoxidation on gold/titanosilicate catalysts by X-ray absorption fine structure spectroscopy

In situ ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy was used in combination with in situ Ti K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo species during the gas-phase epoxidation of propylene with H-2 and O-2 at reaction conditions over a Au-Ba/Ti-SiO2 (Ti-TUD) catalyst. The in situ UV-vis measurements showed growth of a signal due to Ti-hydroperoxo species when the catalyst was put in contact with H-2/O-2/Ar (1/1/8) and C3H6/H-2/O-2/Ar (1/1/1/7) gas mixtures at 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the Ti K-edge XANES spectra of the catalyst were used to estimate the Ti-hydroperoxo species coverages (theta) under operating conditions. Transient Ti K-edge XANES experiments with H-2/O-2/Ar (1/1/8) and C3H6/H-2/O-2/Ar (1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving the determination of d theta/dt. It is shown that the Ti-hydroperoxo species are true intermediates because their initial rate of reaction measured from the in situ transient XANES data (3.4 x 10(-4) s(-1)) has the same order of magnitude as the steady-state turnover frequency for propylene epoxidation based on the total Ti (2.5 x 10(-4) s(-1)) measured in a catalytic flow reactor. This is the first use of XANES to measure the turnover rate of a catalyzed reaction.