Fe-and Co-based catalysts

Water is the main reaction product of the FTS over cobalt-based catalysts, the water is formed on the surface, and it is not surprising that the water has an influence on the kinetics of the FTS. Although the water effect was documented early, in a recent review,5 none of the reported kinetic equations describing the FTS over cobalt catalysts include a water-term, van Steen and Schulz41 have since suggested a common rate-equation for Fe and Co-based catalysts, including terms for water. 

It is interesting to note that the same effect of water, both on activity and C5 + and methane selectivity, is noted for a supported Ru catalyst.52 The same rate equation as was used for Fe and Co-based catalysts, including a term for water, was also valid for the Ru-based catalyst (eqn (4)). 

There has long been interest in investigating Fe- and Co-based catalysts for oxygen reduction because of their role as highly effective enzymes for oxygen transport and conversion in biological systems. More recently, additional interest has been centered on alternative precious metals, metal oxides, and metal carbides and nitrides as possible oxygen reduction catalysts. 

Fe and Co-based Catalysts. - Catalysts which are not Ni-based have low CF formation rates/yields. However, it is interesting to consider metals such as Fe and Co since the properties of the CF depend on the metal employed for catalyzing the methane decomposition reaction. 

The reactions were carried out in a fixed-bed micro reactor using Fe- and Co-based catalysts at reaction temperatures of 523 K and 463 K, respectively, a total pressure of 1 MPa, and a reaction gas flow of 30 ml/min (NTP). The H2/C ratio of the reaction gases was kept constant, 7/3 for the Fe-based and 6/3 for the Co-based catalyst, decreasing only the CO/CO2 ratio. Product samples were taken after steady state activity and selectivity were maintained for at least 24 hours. The catalysts were prepared by precipitation with an aqueous NH3 solution from the solutions of the metal nitrates, in which Aerosil (Aerosil 200, Degussa) was... 

The catalyst preparation procedure starting with the adsorption of a metal hydroxide followed by its reduction in a stream of H2 at 600 C and a further activation step in acetonitrile at 1,000°C was expanded to all transition metals of the first row. Catalysts for O2 reduction were obtained only with Cr, Fe, and Co. Cr203/C, Fe/C, and Co/C were detected after the reduction step in H2, while Co, Fe3C, and a chromium carbide-nitride (Cr6.2 C3 5 N0.3/C) were detected in the catalyst. The nominal loading of all metals was 10 wt%. Catalytic activity decreased as Cr > Fe > Co. Tests in fuel cells indicated that the Cr-based catalyst was not stable, while Fe and Co-based catalysts were stable (see Section 4 for details). 

Tafel slopes which are usually found for O2 reduction on Fe- and Co-based catalysts and on Pt. A list of Tafel slopes experimentally found for these catalysts is given in Table 3.4. 

The data and discussion that follow will be confined to the two commercially used catalysts, namely, Fe- and Co-based catalysts. For each class of catalyst, many apparently different kinetic equations can be found in the literature (see the sections Kinetics for Fe-Based Catalysts and Kinetics for Co-Based Catalysts and also Reference 4, p. 412). In all of these equations (for both Fe and Co catalysts), however, it is clear that the partial pressure of H2 has a greater, positive... 

Riedel T, Claeys M, Schulz H, Schaub G, Nam SS, Jun KW, Choi MJ, Kishan G, Lee KW (1999) Comparative study of Fischer-Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts. Appl Catal Gen 186 201-213... 

Industrially, transformation of syngas is normally carried out using Fisher-Tropsch (FT) method which utilizes either Fe or Co based catalysts to obtain useful fuels and chemicals. For wider applications, the usage of a support for these catalysts... 

We now discuss the main literature results on Mn-promotion for unsupported as well as supported Fe-, Ru-, and Co-based Fischer-Tropsch catalysts. Such comparison between Fe-, Ru- and Co-based catalysts has been shown to be very useful because it places the role of Mn as a promoter in F-T catalysis in a broader perspective. 

Transition metal catalysts are commonly employed for both categories. Fe and Co based FT catalysts are applicable for first type, where Co-based catalytic reaction occurs at lower temperature and Fe catalyst reaction takes place at high temperature [116-118]. Use of supports and promoters induce a significant impact on CO2 hydrogenation. Addition of Ce02 to Fe-based catalyst increases the hydrocarbon yield because of the facile redox properties... 

Al-Dossary et al., 2015), it is energy-intensive and requires elevated pressure and temperature and Fe- or Co-based catalyst. In addition, the FT process is sensitive to contaminants such as sulfur, and also a specific H2/CO ratio is normally required (Pansare and Allison, 2010 Bambal et al., 2014 Al-Dossary et al., 2015). 

Wallin M, Cruise N, Klement U, Palmqvist A, Skoglundh B. (2004). Preparation of Mn, Fe and Co based perovskite catalysts using microemulsions. Colloid Surface A. 238, 27-35. 

Proven, industrially used catalysts are mostly based on either iron or cobalt. Ruthenium is an active F-T catalyst but is too expensive for industrial use. Both Fe and Co are prepared by several techniques including both precipitation and impregnation of (e.g. alumina or silica) supports. The more noble Ni catalyst produces nearly exclusively methane and is used for the removal of trace of CO in H2. 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

The most active catalysts for NH3 decomposition are based on Ru, however, cheaper Fe, Co, Ni and alloy systems are also intensely investigated [148]. The impact of the support material is remarkable. In a study by Au et al., Ru/CNTs performed better than all oxide-supported systems, whereas activated carbon resulted in one of the lowest NH3 conversions (Tab. 15.6) [147]. The dispersion of the active component as well as basicity [147] and conductivity [149] of the support are discussed as the relevant factors for high catalytic efficiency. However, the difference between CNT and activated carbon support is still remarkable. Thus it is not surprising that even the residual catalyst material on commercial MWCNTs, which is basically based on Fe and Co, results is a high catalytic performance in NH3 decomposition [150]. 

Other bimetallic systems have been investigated. An Fe-Co-based catalyst [266] exhibits high activity (achieving complete acetic acid conversion at 400 °C), high H2 selectivity and good stability. 

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. 

Strong interest in late transition metal olefin polymerization catalysts resulted in the development of new five-coordinate Fe and Co systems (69) that afford highly linear, crystalline, high-density polyethylene.587-589 A new class of single-component, neutral Ni catalysts based on salicylaldimine ligands (70) was reported to be active in the polymerization of ethylene 590,591... 

Based on an experimental study the present investigation addresses for two different types of catalysts the effect of CO2 concentration in the reaction gas on carbon conversion rates, yields of organic products and selectivity in the carbon number range Cj to 20- Two catalysts on Fe- and Co-basis with significantly different CO shift reaction activity were characterized by parameters according to the previously developed model of non trivial surface polymerisation , based on extended Anderson-Schulz-Flory kinetics [2]. 

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 ... 

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