Iron catalysts pretreatment

Figure I. Percent solid carbon formed as a function of reaction time for iron catalysts pretreated at various H2S levels prior to reaction in CO/H2 (4 1) at 6Q0°C H2S/H2 (a) None (b) 2xl0 7 (c) 2x10 6 (d) 1.1x10 5. anci (e) 5x10 5.

Iron catalysts used in Fischer-Tropsch synthesis are very sensitive to conditions of their preparation and pretreatment. Metallic iron exhibits very low activity. Under Fischer-Tropsch reaction conditions, however, it is slowly transformed into an active catalyst. For example, iron used in medium-pressure synthesis required an activation process of several weeks at atmospheric pressure to obtain optimum activity and stability.188 During this activation period, called carburization, phase... 

Figure 19.3 Synthesis gas conversion as a function of time for the precipitated iron oxide catalyst pretreated with CO (weight = 72.7 g, Sg = 32 m2 g ). O, CO , H2 O, CO + H2.

Changes in the morphological and structural characteristics of the carbon deposit resulting from pretreatment of the iron catalyst in H2S were determined from a combination of transmission electron microscopy techniques, X-ray diffraction, surface area measurements and controlled oxidation studies in CO2. Iron powder 200 mesh) was purchased from Johnson Matthey Inc. (99 99% purity) and had a BET surface area of 0.3 m2/g at -196°C, The gases used in this work CO (99 9%), hydrogen (99.999%), ethylene (99.999%), H2S/argon mixtures and helium (99,999%) were obtained from Alphagaz company and used without further purification. 

Studies of the Fischer-Tropsch synthesis on nitrided catalysts at the Bureau of Mines have been described (4,5,23). These experiments were made in laboratory-scale, fixed-bed testing units (24). In reference 5, the catalyst activity was expressed as cubic centimeters of synthesis gas converted per gram of iron per hour at 240°C. and at a constant conversion of 65%. Actually, the experiments were not conducted at 240°C., but the activity was corrected to this temperature by the use of an empirical rate equation (25). Conditions of catalyst pretreatment for one precipitated and two fused catalysts are given in Table IV. 

Accurate and reproducible Mossbauer spectra of supported iron catalysts require the prevention of the adsorption of impurities including oxygen and water onto the highly reactive sample surface. Cells designed to allow chemical reactions and in vacuo pretreatments at temperatures up to 673 K while the Mossbauer spectrum is being recorded have recently been reported and represent a significant and important development in the application of Mossbauer spectroscopy to catalytic and surface studies (109-111). 

To examine the effect of iron oxidation state on the dehydrogenation the activity of the iron oxide catalyst was compared as in various pretreatment. As presented in Figure 3, the catalyst pretreated with nitrogen showed rather high activity compared to those pretreated with carbon... 

A third example is the water-gas-shift reaction catalysed by iron catalysts. The active phase is an intermediate iron oxide (Fe304), distinctly different from the manufactured catalyst (Fe203). So, mild reduction should be carried out. In optimising the pretreatment procedure it should be realized that over-reduction is to be avoided because of the danger of carbon deposition and methane formation (highly exothermic). It has been found that a well-controlled reduction in a H2/H2O mixture is possible, whereas the use of H2 and steam separately should be avoided. 

In connection with their work on medium-pressure synthesis with cobalt catalysts, Fischer and Pichler began (1936-1937) some research work with iron catalysts too. The first positive results, comparable with results obtained with cobalt catalysts were achieved when an iron precipitation catalyst that had been in use at atmospheric pressure for several weeks was switched to operation at a synthesis gas pressure of 15 atm. The initial pretreatment of the catalyst at 1 atm. proved to be necessary for the successful use of the catalyst at higher pressure. The combination of proper pretreatment (reduction and carbonization) followed by synthesis at elevated pressures increased the yields of Cs+ hydrocarbons to more than double and the lifetime of the catalyst was... 

Pichler and Merkel (24) investigated the composition of iron catalysts at various stages of pretreatment and synthesis by chemical and thermo-magnetic analysis. Copper-free iron catalysts, carburized at 325°C. before medium-pressure synthesis, were virtually completely transformed to a ferromagnetic higher iron carbide with a Curie point of 265°C., whose formula corresponded to approximately Fe2C. 

D. Iron catalysts for medium-pressure require pretreatment with reducing gases. A very cautious reduction of fused iron catalysts with pure dry hydrogen was necessary while active catalysts could be pretreated with synthesis gas only. 

The physical structure, which can be changed by suitable methods of catalyst manufacturing, is. of decisive importance (promoters high-melting oxides supports kieselguhr of cobalt and nickel catalysts pretreatment low-temperature reduction which limits the size of the crystals, or carbon monoxide treatment of iron catalysts which increases the surface by breaking up the structure with carbon). 

Similar data on the effect of pressure on the durability of iron catalysts are presented by Pichler (37, 74, 86). Iron catalysts usually are much less durable at atmospheric pressure than are cobalt catalysts. As with the cobalt catalyst it is possible, however, by frequent flushings with hydrogen to keep a carefully prepared and pretreated iron catalyst active at atmospheric pressure for 6-12 months (37). The pressure coefficient of the synthesis on iron catalysts is shown in Fig. 11, prepared by Pichler (37) from... 

Two iron FTS catalysts with an atomic ratio of K Fe=10 100 and Be Fe=1.44 100 were prepared and utilized in this study. Precipitated iron catalysts were prepared using Fe(N03)3 9H20 tetraethyl orthosilicate, Cu(N03)2 3H20, and K2CO3 or Be(N03)2 was used as the promoter precursor. Details of the preparation procedure was given elsewhere (5). In this study, the potassium promoted iron catalysts were pretreated with CO at 270°C and 1.2 MPa for 24 hours. The CO flowed through a catalyst slurry in 300 ml of Ethylflow oil... 

Effect of particle size on turnover rate for ammonia sjmthesis. Small particles of metallic iron supported on magnesia were prepared by Boudart et The iron particle size could be changed between 1.5 run and 30 run and determined, in part, by electron microscopy. X-ray diffraction, magnetic susceptibility and Moss-bauer spectroscopy. Agreement was satisfactory with particle size values obtained by selective chemisorption of carbon monoxide (if 2 Fe for 1 CO). Two results are noteworthy, (i) The turnover rate for ammonia synthesis increases by a factor of 35 as the iron particle size increases (Table 2.12). (ii) A pretreatment of the iron catalyst with ammonia increases the turnover rate by only 10% for the larger particles, but quite appreciable for iron clusters (Table 2.13). 

Typical support precursors like the ethanol adduct of magnesium dichloride MgCl2 xEtOH also yield good substrates for iron catalysts, i.e., after pretreating by heating under vacuum and yielding alpha MgCl2 types [77]. The activity of a... 

The BIP iron catalyst on a support can be used to prepare in situ 1-olefins from ethylene. These can used to prepare PE-LLDs from ethylene through the action of a further catalyst. Fused silicas and MCM-41 were used as supports for the preparation of a catalyst capable of generating PE-LLD from ethylene. The catalyst was prepared by pretreating the silica particles with TMA and subsequently with gaseous H2O to obtain a Lewis acidic and alkylating layer [78]. Titanocene and/ or zirconenes in combination with a BIP catalyst were added either simultaneously or subsequently to the treated support. PE with several microstructures and distributions could be obtained in this way. Ethylene oligomerization with the supported iron catalyst is very effective (>50 ton molpe h ) and yields a products with of 1,430 and PDI of about 10. 

Minico, S., Scire, S., CrisafnUi, C., and Galvagno, S. Inflnence of catalyst pretreatments on volatile organic componnds oxidation over gold/iron oxide. Appl. Catal. B Environ. 2001, 34, 277-285. 

Iron-based catalysts are used in both LTFT and HTFT process mode. Precipitated iron catalysts, used in fixed-bed or slurry reactors for the production of waxes, are prepared by precipitation and have a high surface area. A sihca support is commonly used with added alumina to prevent sintering. HTFT catalysts for fluidized bed apphcations must be more resistant to attrition. Fused iron catalysts, prepared by fusion, satisfy this requirement (Olah and Molnar, 2003). For both types of iron-based catalysts, the basicity of the surface is of vital importance. The probability of chain growth increases with alkali promotion in the order Li, Na, K, and Rb (Dry, 2002), as alkalis tend to increase the strength of CO chemisorption and enhance its decomposition to C and O atoms. Due to the high price o Rb, K is used in practice as a promoter for iron catalysts. Copper is also typically added to enhance the reduction of iron oxide to metallic iron during the catalyst pretreatment step (Adesina, 1996). Under steady state FT conditions, the Fe catalyst consists of a mixture of iron carbides and reoxidized Fe304 phase, active for the WGS reaction (Adesina, 1996 Davis, 2003). 

The effect of oxidation pretreatment and oxidative reaction on the graphitic structure of all CNF or CNF based catalysts has been studied by XRD and HRTEM. From the diffraction patterns as shown in Fig. 2(a), it can be observed the subsequent treatment do not affect the integrity of graphite-like structure. TEM examination on the tested K(0.5)-Fe(5)/CNF catalysts as presented in Fig.2(b), also indicates that the graphitic structure of CNF is still intact. The XRD and TEM results are in agreement with TGA profiles of fi-esh and tested catalyst there is no obviously different stability in the carbon dioxide atmosphere (profiles are not shown). Moreover, TEM image as shown in Fig. 2(b) indicates that the iron oxide particle deposited on the surface of carbon nanofibcr are mostly less than less than 10 nm. 

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. 

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