COMPOSITIONAL ASPECTS OF IRON FISCHER-TROPSCH CATALYSTS: AN X-RAY PHOTOELECTRON SPECTROSCOPY STUDY

Iron catalysts employed in the Fischer-Tropsch synthesis consist of metallic, carbide, and oxide phases. During reaction, the relative amounts of these phases vary depending on catalyst pretreatment, synthesis conditions, and time on stream. The catalytic and compositional behavior of prereduced and unreduced iron catalysts were investigated in this study. Catalytic behavior was evaluated by measuring the rates of hydrocarbon formation in a 3:1 H(,2)/CO mixture at one atmosphere and 250(DEGREES)C. Iron phases which evolved near the catalyst surfaces were characterized by x-ray photoelectron spectroscopy, and bulk phases present following the synthesis reactions were determined by Mossbauer spectroscopy.; At low conversion levels, prereduced iron catalysts were gradually converted to iron carbide. No significant surface oxide phases were detected. Synthesis activities increased steadily over the first few hours of reaction, during which time carbidic and graphitic carbon accumulated rapidly on the catalyst surfaces. At longer reaction times, prereduced catalysts invariably exhibited a loss of synthesis activity, resulting from continued accumulation of inactive surface carbon. Olefin selectivities also increased with time on stream, suggesting that the growing carbon adlayer hindered readsorption of primary hydrocarbon products.; At total CO conversion levels in the range of 30 to 40 percent, prereduced catalysts were converted primarily to iron carbide, although some surface oxide phases also formed. The principal effect of these higher conversions was an inhibition of the synthesis rate, apparently due to water adsorbed on the catalyst surfaces. The rate of surface carbon accumulation was also suppressed under these synthesis conditions.; Unreduced Fe(,2)O(,3) exhibited no initial synthesis activity, but underwent gradual activation and eventually became more active than the prereduced catalysts. The conversion level and olefin selectivity both increased during this activation, suggesting that some paraffins were formed as primary products. Prereduced and unreduced catalysts yielded essentially identical product chain length distributions. The oxide catalysts were completely reduced to Fe(,3)O(,4) under synthesis conditions, and metallic phases which formed were rapidly converted to iron carbide. Compared to the prereduced catalysts, the oxide accumulated considerably less surface carbon, and did not exhibit any loss of synthesis activity for reaction times up to 48 hours.