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/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 × 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.

In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O(2) and H(2) over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O(2) and H(2). In order to obtain direct evidence for the different type of active oxygen species, in situ UV-vis and EPR measurements were carried out during the epoxidation of propylene with O(2) and H(2) over a Au/Ti-SiO(2) (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O(2)(-)) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H(2)O(2) produced from O(2) and H(2) adsorbed on the Au surfaces.

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The dynamic behavior and kinetics of the structural transformation of supported bimetallic nanoparticle catalysts with synergistic functions in the oxidation process are fundamental issues to understand their unique catalytic properties as well as to regulate the catalytic capability of alloy nanoparticles. The phase separation and structural transformation of Pt(3)Sn/C and PtSn/C catalysts during the oxidation process were characterized by in situ time-resolved energy-dispersive XAFS (DXAFS) and quick XAFS (QXAFS) techniques, which are element-selective spectroscopies, at the Pt L(III)-edge and the Sn K-edge. The time-resolved XAFS techniques provided the kinetics of the change in structures and oxidation states of the bimetallic nanoparticles on carbon surfaces. The kinetic parameters and mechanisms for the oxidation of the Pt(3)Sn/C and PtSn/C catalysts were determined by time-resolved XAFS techniques. The oxidation of Pt to PtO in Pt(3)Sn/C proceeded via two successive processes, while the oxidation of Sn to SnO(2) in Pt(3)Sn/C proceeded as a one step process. The rate constant for the fast Pt oxidation, which was completed in 3 s at 573 K, was the same as that for the Sn oxidation, and the following slow Pt oxidation rate was one fifth of that for the first Pt oxidation process. The rate constant and activation energy for the Sn oxidation in PtSn/C were similar to those for the Sn oxidation in Pt(3)Sn/C. In the PtSn/C, however, it was hard for Pt oxidation to PtO to proceed at 573 K, where Pt oxidation was strongly affected by the quantity of Sn in the alloy nanoparticles due to swift segregation of SnO(2) nanoparticles/layers on the Pt nanoparticles. The mechanisms for the phase separation and structure transformation in the Pt(3)Sn/C and PtSn/C catalysts are also discussed on the basis of the structural kinetics of the catalysts themselves determined by the in situ time-resolved DXAFS and QXAFS.

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ARTICLEnot simple but proceeds by three successive steps via two intermediates. This behavior may provide the Pt 3 Sn/γ-Al 2 O 3 catalyst with bifunctional and synergistic properties due to core-shell phase separation, its geometric location, changing oxidation states, geometric shape, lattice strain, and interaction at the boundary depending on the ambient.