Fe-based catalysts

Fig. 3 A shows the effluent NH3 concentration observed for Ru/MgO as a function of reaction temperature for three different Pn, / Phj / Paf ratios at 20 bar total pressure. It is obvious that the reaction orders for N2 and H2 have opposite signs. Fig. 3B illustrates that the reaction orders for N2 and H2 partly compensate each other in the kineticaliy controlled temperature regime. Hence an increase in total pressure with a constant Pnj / Phj 1/3 ratio does not lead to a significant increase in conversion at lower temperatures. For the plication of alkali-promoted Ru catalysts under industrial synthesis conditions, it is necessary to find a compromise between kinetics and thermodynamics by increasing the Pn, / Phj ratio. The optimum observed for Cs-Ru/MgO prepared from CS2CO3 at 50 bar is at about Pnj / Phj 40 / 60 [15]. The high NH3 concentration of about 8 % obtained with 0.138 g catalyst using a total flow of 100 Nml/min clearly shows that Ru catalysts have indeed the potential to replace Fe-based catalysts in industrial synthesis [15].

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

Five of the chapters in this volume can be considered directly related to this topic. First, Edd Blekkan, 0yvind Borg, Vidar Froseth, and Anders Holmen (Norwegian University of Science and Technology, Trondheim) review recent work on the effect of water on the Fischer-Tropsch reaction. Steam is both a reactant and product in this syngas-based process, and its effect on Co- and Fe-based catalysts is important in determining the activity and selectivity of the FT process. 

Gasteiger et al. reviewed the best performing Fe-based catalysts in the literature up to 2004 1. Even the best of these catalysts (Fe on pyrolyzed peryle-netetracarboxylic dianhydride) showed a corrected turnover frequency of 7% and a volume activity density of 0.2% of Ft. More recent work has focused on optimizing the metal, nitrogen, and carbon composition of the materials. 

Table 1. Properties of Co-, Mn-, and Fe-based catalysts after calcination at 500 C for...

Heteronuclear carbonyls containing Fe-Mn and Fe-Ru frames are available, with different metal ratios, and these are useful to study the promoter effects of both Mn and Ru on Fe-based catalysts. 

Rh, Ru, Pd) and oxides (<4wt% Fe jO4/Cr2O3, La2O3, SnO2, K2O) was recently performed by Lodeng et al. [134]. A comparison with Ni- and Fe-based catalysts was also addressed. It was found that addition of metal promoters, particularly Rh and Pt, enhanced the catalyst activity at low temperatures (which resulted in delayed extinction of the reaction during ramping at —1 Tmin ). However, addition of Ni promoted carbon formation. Addition of surface oxides typically promoted instability, deactivation and combustion (hence the formation of a stable Co metallic phase was hindered). It was found that Ni performed better than Co-based catalysts at all temperatures. However, Fe-based catalysts showed high combustion activity. 

Fischer-Tropsch (FT) process is used for the production of hydrocarbon fuels. The process uses synthesis gases CO and H2O. It is shown that cobalt/alumina-based catalysts are highly active for the synthesis. The process is also used to convert coal to substitute or synthetic natural gas (SNG). The use of Fe-based catalysts is also believed to be attractive due to their high FT activity. HRTEM has played a major role in the study of phase transformations in Fe Fischer-Tropsch during temperature programmed reduction (TPR) using both CO and H2 (Jin et al 2000, Shroff et al 1995). TiClj/MgC -based (Ziegler-Natta) catalysts are used for polymerization of alkenes (Kim et al 2000) and EM is used to study the polymerization (Oleshko et al 2002). 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

Bennissad et al. investigated CF formation on Fe-based catalysts using CH4-H2 mixtures at temperatures to 1423 Under these conditions thicker... 

In the absence of excess sulfur, the sulfated hematite did not show any activity. Post-reaction analysis of this catalyst showed that it was converted primarily into a-Fe. Addition of sulfur to the reaction mixture increased the activity of this Fe-based catalyst for all the reactions studied. XRD analysis of the spent catalyst indicates that it is transformed to a pyrrhotite (Fe1 A.S) phase. The results shown in Figure 27.6 and the values listed in Table 27.3 for the sulfated hematite, are those obtained with added sulfur. 

When light hydrocarbons terminate predominantly as paraffins (kh>>ko), or when a-olefins are rapidly hydrogenated in secondary reactions (ks>kr), we should obtain a light product distribution with a low and constant value of a. We describe below two such systems. A Fe-based catalyst (a-Fe2C>3) at very high H2/CO ratios (-9) gives only C to C5 paraffins with a constant chain growth... 

Figure 8 Flory plot of SASOL s Synthol process on Fe-based catalysts. Data from Ref. (17). (See legend for Figure 1).

Many studies address the effect of promoters such as K and Mn on Fe-based catalysts. Dry et al (23) suggest that the alkali promoter weakens the C-O bond and enhances its rate of dissociation it also strengthens the metal-C bond, the surface residence time of adsorbed chains, and the probability of chain growth. In the presence of Mn, termination to olefins predominates (24-26). Our results suggest that we must also consider the effect of promoters and of catalyst treatment on a-olefin readsorption. Perhaps the presence of alkali also enhances a-olefin readsorption reactions leading to heavier products whereas Mn does not. 

XAFS/WAXS Fischer-Tropsch synthesis over Fe-based catalysts [56]... 

One of the goals of green chemistry is the destruction of pollutants, an area in which oxidation chemistry can play a major role. CoUins has developed an extremely robust, efficient Fe-based catalyst that uses H2O2 as oxidant to oxidize a variety of pollutant materials. The ligand set features deprotonated amide, a very strong electron donor ligand, with extensive alkyl substitution to protect what would otherwise be sensitive CH bonds. The result is an extremely oxidation-resistant catalyst which has proved useful in commercial applications, such as oxidative degradation of dyes. 

Villers, D., Jacques-Bedard, X., and Dodelet, J.-R, Fe-based catalysts for oxygen reduction in PEM fuel cells, J. Electrochem. Soc., 151, A1507, 2004. 

Lefevre, M., Dodelet, J.P., and Bertr, R, Molecular oxygen reduction in PEM fuel cells evidence for the simultaneous presence of two active sites in Fe-based catalysts, J. Phys. Chem. B, 106, 8705, 2002. 

Faubert, G. et ah. Activation and characterization of Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells, Electrochim. Acta, 43, 1969, 1998. 

Non-Flory molecular weight distributions have also been attributed to the presence of several types of active sites with different probabilities for chain growth and for chain termination to olefins and paraffins (45). Two-site models have been used to explain the sharp changes in chain growth probability that occur for intermediate-size hydrocarbons on Fe-based catalysts (46,47). Many of these reports of non-Flory distributions may instead reflect ineffective dispersal of alkali promoters on Fe catalysts or inadequate mass balances and product collection protocols. Recently, we have shown that multisite models alone cannot explain the selectivity changes that occur with increasing chain size, bed residence time, and site density on Ru and Co catalysts (4,5,40,44). 

Cu-Zn-Cr oxides were prepared as follows. CuO was added to an aqueous solution of CrOj, and ZnO was added after 1 h aging. The obtained paste was dried without heating. Fe-based catalysts were prepared by the coprecipitation of the corresponding nitrates using sodium hydroxide. The precipitate was washed five times, dried at 120 C for 6 h and calcined at SSO C for 3 h. The composite catalysts were obtained by the physical mixing of the equal amounts of a methanol synthesis catalyst and a zeolite. HY [JRC-Z-HY4.8(2)] and NaY(JRC-Z-Y4.8) were provided from the Catalysis Society of Japan as the Reference Catalyst. 

Figure 2 shows the effect of method of Fe addition on product distributions. Cu0-Zn0/Ti02 (cat. A) was active for methanol synthesis, but it was not effective for the synthesis of hydrocarbons. This indicates that Cu species alone is not enough to produce hydrocarbons. On the contrary, Fe-based catalysts are known as hydrocarbon synthesis catalysts from CO, that is, Fischer-Tropsch reaction. However, Fe/TiOj catalyst (cat. C) showed poor... 

To synthesize ethanol more effectively from CO2, the Cu-Zn-Al-K mixed oxide catalyst was combined with the Fe-based catalyst. An F-T type Fe-Cu-Al-K mixed oxide catalyst, which has been developed already in our laboratory [1], converted CO2 to both ethanol and hydrocarbons, while the Cu-based catalyst converted CO2 to CO and methanol with high selectivity. Through the combination of these two catalysts, the three functions were harmonized C-C bond growth, partial reduction of CO2 to CO, and OH insertion to the products. Furthermore, combination catalyst of Fe- and Cu-based ones was modified with both Pd and Ga to maintain the desirable reduced state of the metal oxides during the reaction. As the result, the space-time yield of ethanol was enhanced to 476 g/l-h at SV=20,000 h ... 

Performances of each catalyst is shown in Figure 1. The ethanol synthesis catalyst (Fe-based catalyst. Cat. 1) have both functions of F-T synthesis and alcohol synthesis. The main products were hydrocarbons, ethanol and methanol. With the increase of CO in reaction gas, the yield of ethanol increased[l]. The Cu-based catalyst (Cat. 2) converted CO2 to CO with selectivity more than 70% at a temperature range from 270 to 370°C, and other products were methanol and a slight amount of methane. Ethanol and C2 hydrocarbons were not produced. In order to harmonize the three functions, C-C bond growth, partial reduction of CO2 to CO, and OH insertion to products, the mixed ratio of Fe-based catalyst to Cu-based catalyst was coordinated at the range from Cu/Fe =... 

Fig. 1 The peformance of catalysts for CO2 hydrogenation Cat. 1 Fe-based catalyst I Fe Cu Al K = 1 0.03 2 0.7 Cat. 4 Pd-modified (Fe-based + Ga-modified Cu-based) Cat. 2 Cu-based catalyst Cu Zn Al K = 1 1 1 0.1 Cat. 5 Pd-modified (Ga-modified Fe-based + Cu-based)...

The addition of Cu to K/Fe oxides catalyst enhanced its ability of ethanol production. K/Cu-Fe oxides catalysts, prepared by kneading KjCO, with Cu-Fe co-precipitate, gave the ethanol selectivity of more than lOC-% at 300°C. The combination of K/Fe and Cu-Zn gave remarkable results on ethanol production. In the reaction over K/Cu-Zn-Fe oxides catalyst, CO2 conversion of 44% and ethanol selectivity of 20C-% were obtained under the standard reaction conditions. Besides ethanol, hydrocarbons were produced with a selectivity of almost 45C-%. The formation of hydrocarbons seems inevitable as long as Fe-based catalysts are employed. 

Cobalt catalysts are reported about three times more active compared with Fe-based catalysts at typical operation conditions of 473K and 2.0 MPa.8 They demonstrate long lifetime and produce predominantly linear paraffins.9 This type of catalyst is primarily employed in low-temperature FT (LTFT) processes for the production of middle distillates and high-molecular-weight fuels, achieving high selectivity. At... 

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