In situ ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy was used in combination with in situTi K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo speciesduring 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-hydroperoxospecies when the
catalyst was put in contact with H
2/O
2/Ar (1/1/8) and C
3H
6/H
2/O
2/Ar (1/1/1/7) gas mixturesat 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the TiK-edge XANES spectra of the
catalyst were used to estimate the Ti-hydroperoxo species coverages (
![](/images/gifchars/theta.gif)
) underoperating conditions. Transient Ti K-edge XANES experiments with H
2/O
2/Ar (1/1/8) and C
3H
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 thedetermination of d
![](/images/gifchars/theta.gif)
/d
t. It is shown that the Ti-hydroperoxo species are true intermediates because their initialrate of reaction measured from the in situ transient XANES data (3.4 × 10
-4 s
-1) has the same order ofmagnitude as the steady-state turnover frequency for propylene epoxidation based on the total Ti (2.5 × 10
-4s
-1) measured in a
catalytic flow reactor. This is the first use of XANES to measure the turnover rate of a
catalyzed reaction.