Iron carbide catalyst

Wang, Y., and Davis, B. H. 1999. Fischer-Tropsch synthesis Conversion of alcohols over iron oxide and iron carbide catalysts. Applied Catalysis A General 180 277-85. 

Figure 19.6 XRD results for iron carbide catalyst synthesized by laser pyrolysis at various times of the Fischer-Tropsch Synthesis (1-Fe304, 2-x-Fe5C2, 3-e -Fe2 2C).

The typical BET surface area of the freshly prepared iron carbide catalyst is approximately 70 m2 g 1. The surface area of the precipitated catalyst and ultrafine catalyst before pretreatment was 140 m2 g 1 and 250 m2 g 1, respectively however, following pretreatment with CO the surface areas dropped to 32 m2 g 1, and 64 m2 g-1, respectively. The particle sizes of the iron carbide and precipitated catalysts after 170 h, determined by X-ray line broadening, were 27 nm and 30 nm, respectively. The ultrafine catalyst had an average particle size of 25 nm after 240 h of synthesis. These particle sizes correspond to a surface area of about 40 m2 g-1. 

Figure 19.2 Synthesis gas conversion as a function of time for the iron carbide catalyst synthesized by laser pyrolysis (weight = 12.0 g, Sg = 70 m2 g ). O, CO , H2 O, CO +...

Hydrocarbon production and selectivities at comparable CO conversion are given in Table 19.2. The ultrafine iron oxide catalyst had a very poor C2-C4 olefin selectivity while the olefin selectivity of the precipitated catalyst was slightly higher than the iron carbide catalyst. This is surprising because Rice et al. report higher olefin selectivity for a similar iron carbide catalyst than a conventional Fe/Co catalyst.6 Soled et al. have subsequently reported that the conventional catalyst contains acidic sites which... 

It was found that an iron carbide catalyst produced by laser pyrolysis and a commercially available ultrafine iron oxide catalyst are not as active for FTS as a precipitated iron catalyst. Operating under industrial conditions, it was found that the unpromoted precipitated catalyst had a hydrocarbon productivity 93% of that reported by Kolbel while the novel catalysts were far below Kolbel s benchmark. It was found, however, that at similar CO conversion, the iron carbide catalyst had a higher hydrocarbon production rate and had a better selectivity for C5+ hydrocarbons. 

The details of the preparation of the iron oxide catalyst precursors are described elsewhere [15-17], For those carbides made by exsitu carburization, the oxide was loaded into a 1 diameter quartz tube and heated in a 1/1 H2/CO mixture at a space velocity of 10,000 v/v/hr at 350°C for 24 hours. Iron carbide catalysts were also prepared by laser pyrolysis of iron carbonyl and ethylene using a 150 watt continuous wave CO2 laser to provide both a rapid high temperature reaction (, 1 sec with T 1000°C) and quench [18],... 

The gas and catalyst linear velocities are much lower in the FFBs than those in the CFBs, particularly in the narrower sections (see Fig. 2). The gas compression costs are consequently lower. Because the iron carbide catalyst is very abrasive, the narrower sections of the CFB reactor are ceramic lined and regular maintenance is essential. This problem is absent in the lower velocity FFB reactors and this allows longer on-line times between maintenance inspections, leading to higher production rates and lower maintenance costs. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Dr. Moeller We have done this, and we compared an iron catalyst used for the Fischer-Tropsch plant and a nickel catalyst used in the methanation plant. By the same x-ray techniques, we found no nickel carbide on the used methanation catalyst, but we did find iron carbide on the used Fischer-Tropsch catalyst. 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

To verify that steady state catalytic activity had been achieved, the catalyst was allowed to operate uninterrupted for approximately 8 hours. The catalyst was then removed from the reactor and the surface investigated by XPS. The results are shown in Figure 2c. The two major changes in the XPS spectrun were a shift in the iron 2p line to 706.9 eV and a new carbon Is line centered at 283.3 eV. This combination of iron and carbon lines indicates the formation of an iron carbide phase within the XPS sampling volume.(J) In fact after extended operation, XRD of the iron sample indicated that the bulk had been converted to FecC2 commonly referred to as the Hagg carbide.(2) It appears that the bulk and surface are fully carbided under differential reaction conditions. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Temperature-Programmed EXAFS/ XANES Characterization of the Impact of Cu and Alkali Promoters to Iron-Based Catalysts on the Carbide Formation Rate... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

An example of activity developing with a Co catalyst is shown in Figure 9.9 (right). CO-conversion (respectively the yield of products) increases with time by a factor of about 10, from ca. 4% to ca. 55%.7,17 Figure 9.9 (left) shows the time dependence of FT with an iron catalyst. There are a strong initial carbon deposition (referring to iron carbide formation) and fast water gas shift reaction, and FT... 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

After calcinations, the precipitated iron catalyst is composed of a mixture of iron oxide phases before activation. The exact nature of this phase is not critical for the discussion and will be referred to in general as an oxide phase. During activation the catalyst is subjected to a reducing environment that will lead to the formation of either metallic iron if pure hydrogen is used or some iron carbide if the reduction is done with either CO or syngas. During reduction with a gas... 

Therefore, when operating in the filter cake mode, the axial velocity should be maintained at a level such that an adequate shear force exists along the filter media to prevent excessive caking of the catalyst that could cause a blockage in the down-comer circuit. For the separation of ultrafine catalyst particles from FT catalyst/wax slurry, the filter medium can easily become plugged using the dynamic membrane mode filtration. Also, small iron carbide particles (less than 3 nm) near the filter wall are easily taken into the pores of the medium due to their low mass and high surface area. Therefore, pure inertial filtration near the filter media surface is practically ineffective. 

When reduced Fe/Ti02 is used as a catalyst for the reaction between CO and H2 to form hydrocarbons (the Fischer-Tropsch synthesis) the spectrum changes entirely. All metallic iron has been converted into a new phase. The spectrum is that of a crystallographically well-defined iron carbide, namely the Hagg carbide, or %-Fe5C2. Apparently the strongly reducing atmosphere has affected the unreduced iron as well all ions are now present as Fe2+. 

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