Sensitivity enhancement of X-ray absorption spectroscopy applied to Co-based Fischer-Tropsch synthesis catalysts
Doctoral thesis
Permanent lenke
http://hdl.handle.net/11250/248626Utgivelsesdato
2014Metadata
Vis full innførselSamlinger
Sammendrag
Fischer-Tropsch synthesis is one of the options to produce sulphur-free high quality fuels. This process is of a great interest to researchers because environmental regulations become increasingly stricter. In the present work cobalt based rhenium promoted Fischer-Tropsch synthesis catalysts have been investigated under different regimes with the use of X-ray absorption spectroscopy (XAS).
It has been found that under industrially relevant reaction conditions (483 K, 18 bar) no significant changes take place in the cobalt crystallites in the catalyst. A rise in the temperature and lowering of the pressure (673 K, 10 bar) led to the sintering of the cobalt particles and further reduction of the unreduced part of the catalyst.
Another issue is the location and state of rhenium in Co-Re/Al2O3 Fischer-Tropsch synthesis catalysts. Rhenium addition plays an active role to the catalyst performance. Both catalytic activity and desired selectivity can be enhanced by the addition of a small amount of rhenium. The exact mechanism of the Re performance and its influence on the catalytic activity is under discussion. A specially designed experiment with the use of high-energy X-rays has been realized. A high-energy X-ray experiment has been shown to be an attractive option for performing in situ catalytic studies because the results from such an experiment can be directly compared with the results from conventional laboratory scale fixed-bed reactors. Results obtained from the high-energy experiment (Re K edge) have been compared with the results from the traditional way of studying rhenium with X-ray absorption spectroscopy (Re LIII edge). Both experiments confirmed that Re is atomically distributed inside the bulk cobalt particles. The presence of Re atoms induce a local cobalt distortion while being in close contact with cobalt which can be a reason for the observed increase in catalytic activity.
Usually during in situ studies with the use of X-ray absorption spectroscopy, large data sets are collected. An extraction of the useful information therefore becomes very time consuming. A Multivariate Curve Resolution (MCR) method was applied to the XAS data set as a blind separation method. MCR was capable of extracting in a highly automated manner component spectra with distinct kinetic evolution together with their respective concentration profiles without the use of reference spectra. It has been demonstrated that the components extracted with MCR have a clear chemical meaning and together with their concentration profile can be used for the explanation of the chemical changes in the catalyst.
XAS is a technique that provides information about the local surroundings of the absorbing atom. Despite all the advantages of the XAS technique, it can be applied to surface studies only by using a special configuration of a hardware and sample (SEXAFS – Surface extended X-ray absorption fine structure). Due to the nature of Co based FT catalysts they cannot be studied efficiently with SEXAFS. To overcome this limitation, a new approach involving a software application to the XAS data has been established. Efforts were aimed at extraction of the surface component from the overall XAS signal. A multivariate curve resolution (MCR) method was applied to a series of XAS data sets in order to separate surface and bulk components from the total signal.