Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy
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- 作者：Juan J. Bravo-Suá ; rez ; Kyoko K. Bando ; Jiqing Lu ; Masatake Haruta ; Tadahiro Fujitani ; S. Ted Oyama
- 刊名：Journal of Physical Chemistry C
- 出版年：2008
- 出版时间：January 31, 2008
- 年：2008
- 卷：112
- 期：4
- 页码：1115 - 1123
- 全文大小：293K
- 年卷期：v.112,no.4(January 31, 2008)
- ISSN：1932-7455
文摘
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 H2 and O2 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-hydroperoxospecies when the catalyst was put in contact with H2/O2/Ar (1/1/8) and C3H6/H2/O2/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 (
) underoperating conditions. Transient Ti K-edge XANES experiments with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar(1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving thedetermination of d/dt. 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 acatalyzed reaction.
) underoperating conditions. Transient Ti K-edge XANES experiments with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar(1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving thedetermination of d/dt. 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 acatalyzed reaction.