Abstract
A novel set-up has been developed in which two complementary spectroscopic techniques, namely operando Raman and UV-Vis-NIR spectroscopy, are combined. With this set-up it is possible to characterize catalytic materials under reaction conditions (high temperature, normal pressure) and in this way on can obtain mechanistic information on catalytic processes. The
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set-up was tested for the dehydrogenation of propane over a Cr / Al2O3 catalyst, a very important industrial process. It is shown that coke species, formed on the catalytic surface, can be monitored with Raman spectroscopy. Furthermore, it is shown that simultaneously measured UV-Vis-NIR spectra provide information about the chromium site on the catalyst. Unfortunately, the obtained information was qualitative and it would be very advantageous if the information could also be used in a quantitative manner. Especially since the operando Raman spectra, measured during dehydrogenation, suggested a role for coke in the dehydrogenation process. Therefore, a method was developed, in which the observed Raman intensity was corrected for the changing color (absorption) of the catalyst. By measuring simultaneously the changing color (absorption at a specific wavelength) of the catalyst via UV-Vis-NIR spectroscopy, a G(R?) correction factor could be determined. This factor was used to quantify the Raman signal. The results of the application of the G(R?) correction factor were compared with the quantification of the Raman data making use of BN as an internal standard. The latter approach is a more commonly known procedure to quantify Raman signals. It appeared that both methods showed good agreement, making the newly developed method preferable since the addition of an extra compound (namely the internal standard) is not required. This shows the value of combining these two spectroscopic techniques in one set-up. Separately, they provide qualitative information, whereas the combination is capable of providing quantitative information. In addition, the general applicability of the method is shown via the monitoring of Cr6+ in Cr / Al2O3 catalysts during temperature programmed reduction experiments. It appeared that the laser was capable of heating the sample locally and as a consequence; the color was not the same throughout the reactor. This made the application of the G(R?) correction not straightforward. Several attempts were made to circumvent this heat effect and every method had its own strengths and limitations. Furthermore, it is shown that if the experimental conditions are chosen carefully, Raman spectroscopy can be performed in a quantitative manner. In the last part of this thesis, the role of coke during the dehydrogenation was explored in more detail. It appeared that propene is a precursor for coke formed on the catalyst during the dehydrogenation. Initially, this coke enhances the propane adsorption and as a result the activity increases. Hereafter, coke starts blocking the active site, and as a result the activity will drop making a regeneration step necessary. Raman spectroscopy showed that during reaction the type of coke present changes from a more aliphatic to a more graphitic type of coke.
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