THE PREPARATION, CHARACTERIZATION, AND USE OF UNSUPPORTED IRON-COBALT CATALYSTS FOR FISCHER-TROPSCH SYNTHESIS

To understand the effects of Fe/Co bimetallic catalyst composition on the kinetics of the Fischer-Tropsch synthesis, a series of unsupported Fe/Co bimetallic catalysts were prepared along with their monometallic constituents. A multi-technique characterization of these materials throughout their genesis as well as before and after Fischer-Tropsch synthesis was employed to correlate the kinetic behavior of these catalysts with their physical and chemical properties. Two major conclusions were derived from this investigation: (1) the surface and bulk compositions of the alloys were identical; and (2) all iron-containing catalysts exhibited iron-like hydrocarbon selectivities. The bulk composition of each catalyst was determined by atomic absorption spectroscopy and electron microprobe analysis. X-ray diffraction spectroscopy (XRD) and Mossbauer spectroscopy were used to identify and characterize the bulk phases present in the catalysts. X-ray photoelectron spectroscopy (XPS) was utilized to ascertain the concentration and chemical state all species present in and on the surface layers of the catalysts. Catalyst homogeneity was determined by energy dispersive analysis of X rays (EDAX), XRD, and Mossbauer spectroscopy. Catalyst particle size determinations were made utilizing both the BET method and XRD line broadening. Atmospheric pressure kinetics were conducted at 250(DEGREES)C in a 3.3 H(,2)/CO reactant mixture using gas chromatographic analysis. The CO conversion level was maintained below four percent. The precipitation-calcination-reduction catalyst preparation technique employed during this investigation produced high purity, moderate surface area catalysts. The Fe/Co bimetallics produced by this method were shown to be alloys. However, not all of these alloys were completely homogeneous. The degree of inhomogeneity exhibited was greatest for the iron-rich samples, and resulted from the inability to quantitatively form the initial mixed metal ferrite. The reduced catalysts were very susceptible to room temperature oxidation. However, this oxidation was limited to the first few atomic layers. Cobalt exhibited the greatest resistance to oxidation and enhanced the reducibility of iron in the bimetallics. The mild surface enrichment in iron exhibited in XPS of the iron-rich bimetallics, was attributed mainly to inhomogeneity. Thus, the surface and bulk compositions of the alloys were found to be identical. Alloy composition had little effect on hydrocarbon selectivity. All bimetallics exhibited iron-like hydrocarbon selectivities. Hydrocarbon selectivity appeared to be controlled by the hydrocarbon adlayer associated with the catalyst\u27s surface. Hydrocarbon precursors generated on iron atoms seemed to dominate the composition of this layer. Straight-chain olefins and paraffins were the major hydrocarbon products, with methane always the most abundant. Cobalt exhibited the highest methane selectivity. Olefins were shown to be the probable precursors of all higher hydrocarbons. However, a strong inverse relationship was shown to exist between relative olefin production and CO conversion level. This effect drastically limited olefin production for conversions above three percent. Carbon dioxide production was most predominant on iron-rich catalysts. Water was the major oxygen-containing product for cobalt-rich catalysts. Catalyst activity and deactivation were shown to be related to the formation of an inactive carbon layer. The thickness of this graphite overlayer was found to be inversely proportional to the activity of the catalyst. Graphitization did not occur on the cobalt catalyst. The iron catalyst produced (chi)-carbide during the reaction. Carbide formation in the iron-rich bimetallics resulted from inhomogeneity