Iron-based catalysts models

The hydrogenation of atomic nitrogen (N- ) preadsorbed on an iron-based catalyst surface has been studied by Fastrup et al. [9]. For the sake of simplicity, the non-steady-state TPD cell during the TPSR experiment has been treated as a CSTR. In the present study, simulation results are shown using the proper PFR model. Additionally, experimental and simulation results obtained with a Cs-Ru/MgO catalyst are presented to illustrate the influence of the reactor model. 

Figure 2. N2TPD from an iron-based catalyst. Comparison of the modeling results using either the transient CSTR model (solid line B) or the PFR model (dashed line C). TTie inset shows the experimental trace (solid line A) obtained by Muhler et al.[10]...

Figure 3. N- TPSR data obtained with an iron-based catalyst (left) and a Cs-Ru/MgO catalyst (right). The modeling results demonstrate the influence of the reactor model.

Kinetic model is a key point to describe complex FTS reaction. Although much research has been done to deal with FTS, the kinetic model research stiU stay at lumped kinetic model (Anderson, 1956 Derosset et al., 1976 Feimer et al., 1981 Frohning and Comils, 1974 Kam et al., 1960 Thomas and Eckert, 1984), because the reaction mechanism of FTS is very complex containing series of surface reactions. The lumped kinetics can only explain syngas conversion rate, the distribution of products usually be described by the semiempirical model (like ASF model) (Wang et al., 1999). However, if the lumping kinetics use in the reactor simulate, only total conversion and distribution of temperature can be obtained. T able 2 (W ang et al., 1999) shows the lumped kinetics for fix-bed FTS on iron-based catalyst. 

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... 

Typical catalysts for SCR include supported vanadia, and iron or copper supported on zeolite. Here the application of a model to the design and understanding of vanadia catalyst systems is presented. Over the vanadia-based catalyst system, a Rideal-Eley approach has been adopted by most workers in the field, in which the first step is ammonia adsorption on the catalyst. This stored ammonia can then either react with NOx or be desorbed. Some important contributions to the SCR modelling literature are Andersson et al. (1994), Lietti and Forzatti (1994), Dumesic et al. (1996), Lietti et al. 

Hydrogenation and hydrocracking activity of iron catalysts has been extensively investigated using coal and model compounds (93 -95). Iron catalysts can hydrogenerate olefinic unsaturated bonds, while they are known to be less active for the hydrogenation of aromatic rings compared with molybdenum-based catalysts. 

Simple chemical oxidation of arsenite by ferric iron at acid pH has been questioned by Barrett et al. (37). They found experimentally that Fe could not oxidize As02 chemically at pH 1.3 at either 70 or 45°C in the presence of a mixed culture capable of growing on Fe and pyrite (FeS2). However, when they added pyrite to the reaction mixture, the bacteria did promote oxidation of arsenite at 45°C. They explained the effect of the pyrite as a heterogeneous catalyst, the role of the bacteria being the regeneration of a clean catalytic surface on the pyrite and the reoxidation of Fe + generated in the oxidation of arsenite by Fe. Mandl and Vyskovsky (38) developed a kinetic model for the catalytic role of pyrite in this form of bacterial arsenite oxidation by Fe. They performed the experiments on which they based their model with T. ferrooxidans strain CCM 4253. 

Many chemical model systems based on metalloporphyrin catalysts and mimicking cytochrome P450-dependent monooxygenases have been described during these last decade. Several review articles have been devoted to these systems 2-10. in that context, very recent results about the preparation and catalytic properties of new homogeneous and supported catalysts will be described in a first chapter. In the second chapter, some preliminary results showing that the oxidation of alkanes by a dioxygenase-like mechanism could occur in the presence of iron porphyrin catalysts activated either photochemically or thermally, will be reported. 

Several experimental studies have been performed to validate this model. This mechanism was mainly validated by Boreskov et al. [3]. They directly measured the oxidation and reduction of an iron oxide-based catalyst with CO and H2O. They concluded that the catalyst surface is reduced and oxidized by the CO and H2O and the rate of reactions is in good agreement with the rate conversion of CO in the WGSR. 

Light-driven acceptor side (H2 formation) assemblies employ both biomimetic catalysts modeled after the hydrogenase active site (Fe, Ni complexes) as well as other catalysts based on abundant metals like cobalt but with non-biomimetic structures. The catalyst shown in Fig. le is an example of an iron-iron [FeFe] hydrogenase active site model that has been employed in a system for photocatalytic... 

Fig. 25 Roth ferrate model (top-left) and the iron-based water oxidation catalysts of Bernhard (top-middle) and Lloret-Fillol-Costas (the rest, with ligand acronyms in parentheses).

DW Matson, JC Linehen, JG Darab, ME Buehler. Nanophase iron-based liquefaction catalysts synthesis, characterization, and model compound reactivity. Energy and Fuels 1994 8 10. 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

In 1994, Rayox developers generated cost estimates for the technology based on bench-scale studies. Using a proposed cleanup site in Canada as a model, researchers compared these estimates with the costs of using an air stripper/liquid carbon/catalytic oxidizer (air/carbon) option. Results indicated that UV/peroxide treatment, with or without an iron catalyst, was found to have an estimated capital cost equal to the air/carbon option at the site (D12302U). 

Finally, iron catalysts based on salen-type ligands have been used. These iron(III)-salen complexes were regarded as enzyme models, using PhIO as oxidant (Scheme 3.52) [162]. Initially, the corresponding active iron-oxo complexes were formed by reaction with PhIO and isolated before use. A stoichiometric amount of the iron-oxo complex allowed the efficient oxidation of a variety of aryl methyl sulfides in moderate to good yields. 

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