Time‐resolved in situ Raman spectroscopy of working catalysts: sulfated and tungstated zirconia

Abstract

AbstractThe spectra of time‐dependent in situ Raman experiments on sulfated and tungstated zirconia catalysts during the isomerization reaction of n‐pentane are presented and discussed. A purpose‐made in situ Raman cell which was used for this application is described. The choice of appropriate experimental parameters, namely laser power and laser wavelength, is discussed. During the experiments the activity and selectivity of the catalysts were determined simultaneously by on‐line gas chromatography. The sulfated zirconia catalyst shows structural changes in the sulfate region during the reaction. One of two different types of initially present sulfate species is eroded during the reaction, presumably due to a reduction to H2S. The activity shows a typical induction period followed by a fast deactivation. No coke formation is observed. Since fast deactivation occurs, coke formation cannot be the only reason for the deactivation of the catalyst. The tungstated catalyst shows strong darkening after initiation of the reaction with increasing time‐on‐stream (TOS). This leads to the disappearance of the catalyst bands. However, coke formation indicated by a broad band at 1590 cm−1 can be observed. Since darkening has a strong effect on Raman intensities, the time evolution of the observed bands is not obtained correctly. We propose a new method to correct for the effect of the darkening. The change of the diffuse reflectance of the catalyst is determined by the variation of the intensity of laser plasma lines. Based on an approximate equation proposed by Waters which correlates the Raman intensities and the diffuse reflectance of a sample, a correction factor for the spectra is obtained. After the intensity correction the spectra indicate that most of the coke is formed in the first few minutes of the reaction followed by a constant formation rate with TOS. Copyright © 2002 John Wiley & Sons, Ltd.