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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−SiO 2 (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 C 3 H 6 /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 (θ) under operating conditions. Transient Ti K-edge XANES experiments with H 2 /O 2 /Ar (1/1/8) and C 3 H 6 /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 dθ/d t . 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 × 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 × 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.
Bravo-Suárez et al. (Tue,) studied this question.