DOE OSTI2011
Argonne National Laboratory carried out a research program to create, prepare, and evaluate catalysts to promote Fischer-Tropsch (FT) chemistry-specifically, the reaction of hydrogen with carbon monoxide to form long-chain hydrocarbons. In addition to needing high activity, it was desirable that the catalysts have high selectivity and stability with respect to both mechanical strength and aging properties. It was desired that selectivity be directed toward producing diesel fraction components and avoiding excess yields of both light hydrocarbons and heavy waxes. The original goal was to produce shape-selective catalysts that had the potential to limit the formation of long-chain products and yet retain the active metal sites in a protected 'cage.' This cage would also restrict their loss by attrition during use in slurry-bed reactors. The first stage of this program was to prepare and evaluate iron-containing particulate catalysts. Such catalysts were prepared with silica-containing fractal cages. The activity and strength was essentially the same as that of catalysts without the cages. Since there was no improvement, the program plan was modified as discussed below. A second experimental stage was undertaken to prepare and evaluate active FT catalysts formed by atomic-layer deposition [ALD] of active components on supported membranes and particulate supports. The concept was that of depositing active metals (i.e. ruthenium, iron or cobalt) upon membranes with well defined flow channels of small diameter and length such that the catalytic activity and product molecular weight distribution could be controlled. In order to rapidly evaluate the catalytic membranes, the ALD coating processes were performed in an 'exploratory mode' in which ALD procedures from the literature appropriate for coating flat surfaces were applied to the high surface area membranes. Consequently, the Fe and Ru loadings in the membranes were likely to be smaller than those expected for complete monolayer coverage. In addition, there was likely to be significant variation in the Fe and Ru loading among the membranes due to difficulties in nucleating these materials on the aluminum oxide surfaces. The first series of experiments using coated membranes demonstrated that the technology needed further improvement. Specifically, observed catalytic FT activity was low. This low activity appeared to be due to: (1) low available surface area, (2) atomic deposition techniques that needed improvements, and (3) insufficient preconditioning of the catalyst surface prior to FT testing. Therefore, experimentation was expanded to the use of particulate silica supports having defined channels and reasonably high surface area. An effective FT catalyst consisting of ALD-deposited Co and Pt on a silica support has been prepared and demonstrated. This catalyst was more effective than a similar catalyst deposited upon a support of ALD-deposited Al 2 O 3 on silica. This result implies that the deposition of Al 2 O 3 to form a support is not as effective as desired. The addition of Pt as a Co-containing catalyst promoter has been demonstrated; it appears to primarily affect the catalyst pre-conditioning step. Co on Al 2 O 3 catalyst prepared by the Center for Applied Energy Research (CAER) is more effective than Argonne-prepared ALD-deposited Co on ALD-deposited Al 2 O 3 catalyst. The FT activity of ALD-coated Co catalyst on Al 2 O 3 is about linear with Co level from about 9 to 25%. A cooperative research effort was undertaken to test the deposition of platinum on Co FT catalysts; this Pt influences the effectiveness of catalyst conditioning and its continuing activity. In summary, the ALD Pt at a low concentration (0.1 wt %) was as effective as that of the wet chemical deposition technique of CAER (specifically incipient deposition on a Co catalyst that had been prepared and calcined before the Pt deposition.) The ALD technique appeared to be nominally better than the incipient wetness technique that involved co-deposition of Pt and Co prior to calcination. The activation energy of the rate of CO conversion was tightly grouped about an average of 29.2 Kcal/mol when all of the Co-containing catalysts other than those with high Pt promoter levels were taken into account; this implies a uniform reaction mechanism. Catalysts containing Pt and Ru that were ALD-deposited on an ALD-Al 2 O 3 coated catalyst support were found to be relatively inactive. Additional tests were made with a low concentration (0.1 wt %) of Ru or Ir deposited on the reference Co catalyst. The Ir coated catalysts were particularly effective. In support of the above, there was an opportunity to undertake a study of cobalt/promoter/support interaction using the Advanced Photon Source (APS) of Argonne. A number of catalysts (including reference cobalt oxide and iron oxides) were tested using temperature programmed EXAFS/XANES experiments.