THE THEORY AND APPLICATION OF TEMPERATURE PROGRAMMED DESORPTION IN THE STUDY OF UNSUPPORTED IRON AND IRON-COPPER FISCHER-TROPSCH SYNTHESIS CATALYSTS

This thesis comprises two main sections: Section A (Chapters I - III) and Section B (Chapters IV and V). In Section A, the analysis of temperature programmed desorption spectra by the shape index method is proposed. Section B deals with the study of unsupported iron and iron-copper Fischer-Tropsch synthesis catalysts by temperature programmed desorption.; The determination of desorption order in TPD analysis is generally based upon the variation of the peak maximum (T(,m)) with changes in the initial fractional coverage. If T(,m)was invariant with (THETA)(,o), then first order desorption was assumed; and if it decreased with increase in fractional coverage, then second order desorption was assumed.; However, this criterion breaks down if the surface of the absorbent is energetically heterogeneous or if there is considerable readsorption. As porous heterogeneous catalysts may exhibit either or both of these qualifying behaviors, the inadequacy of this T(,m) approach for such porous catalysts prompted the development of an alternative method.; The shape index (a phrase coined by Kissinger (1.6)) is the magnitude of the ratio of peak inflection point slopes, i.e. the ratio of the maximum low temperature slope to the maximum high temperature slope. Previously developed for differential thermal analysis (DTA) peaks, it provided a quick and easy means of determining desorption order from a single peak. Employing the same principles, we extended the development to temperature programmed desorption from porous catalysts containing uniform (Langmuirian) sites (Chapter I). A criterion for estimating diffusional influences in TPD from porous systems is also proposed.; In Chapter II, the shape index of TPD from non-uniform surfaces is treated, based upon two models of non-uniformity. In one, the activation energy for desorption decreases linearly with fractional coverage; and in the other, nonintegral desorption order prevails, but with constant activation energy.; In this section, also, in order to test the applicability of the derived expressions to porous systems, an experiment was performed with porous vycor glass and acidized porous glass possessing well defined pore size distributions (Chapter III).; The results reported in Section A show that the shape index method provides an attractive TPD technique which clearly allows determination of desorption order. Of course, the peaks must be reasonably resolved.; In Section B, the study of unsupported iron and iron-copper catalysts by the adsorption and temperature programmed desorption of CO and CO(,2) is treated. Unsupported iron catalysts (sometimes promoted with copper) are used widely in the Fischer-Tropsch synthesis. In order to shift the selectivity to longer chain hydrocarbons or increase the synthesis activity of the catalyst, it is often promoted with potassium or "precarbided" or both. The aim of this study, therefore, was to investigate the effect of these additives on the adsorption and TPD of these gases. The experiments were performed in an atmospheric pressure, flow microreactor unit. The reported spectra are net thermal conductivity responses.; Both potassium and carbon were observed to convert some low energy adsorption sites to high energy adsorption sites. The observation on the effect of carbon contravenes that reported from single crystal studies but is substantiated by kinetic studies on similar unsupported catalysts. A possible explanation is advanced. Copper, while enhancing total adsorption, apparently inhibited CO dissociation and carbon deposition. Based on an x-ray/SEM study, a model is proposed according to which the surface layers become depleted of copper during pretreatment. The relevance of all these findings to the Fischer-Tropsch synthesis is discussed.