Transient technique for identification of true reaction intermediates: Hydroperoxide species in propylene epoxidation on gold/titanosilicate catalysts by X-ray absorption fine structure spectroscopy
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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DOI：
10.1021/jp077501s
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发表时间：
2008-01-31
影响因子：
3.7
通讯作者：
Oyama, S. Ted
中科院分区：
文献类型：
--
作者：
Bravo-Suarez, Juan J.;Bando, Kyoko K.;Oyama, S. Ted
采用原位紫外可见漫反射光谱(UV-Vis)和原位钛K边X射线吸收近边结构(XANES)相结合的方法，研究了在Au-Ba/Ti-SiO_2(Ti-TUD)催化剂上丙烯与H-2和O-2在气相环氧化反应中钛氢过氧物种的形成。原位UV-Vis测量表明，在423K和0.1 MPa下，催化剂与H-2/O-2/Ar(1/1/8)和C3H6/H-2/O-2/Ar(1/1/1/7)混合气体接触时，由于钛过氧物种的存在而产生了一个信号。利用催化剂钛K边XANES谱中以4968.9 eV为中心的前缘峰面积的变化来估算操作条件下的钛氢过氧物种覆盖度(Theta)。用H-2/O-2/Ar(1/1/8)和C3H6/H-2/O-2/Ar(1/1/1/7)混合气体进行的瞬时TiK边XANES实验，允许用一种新的方法来估计净环氧化速率。结果表明，钛氢过氧物种是真正的中间体，因为它们的初始反应速率与稳定的反应速率具有相同的数量级。基于催化流动反应器中测得的总钛(2.5×10~(-4)S(~(-1)，丙烯环氧化的状态转换频率。这是XANES首次用于测量催化反应的转化率。
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.