Iron-based catalysts experimental results

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

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. 

In previous papers by the authors [9,10], the temperature-programmed desorption of N2 from an iron-based catalyst has been studied experimentally. The microkinetic analysis of these results is based on the kinetic simulation of ammonia synthesis by Stoltze and Nprskov [22-24] using the approach by Dumesic and Trevino [2]. On Fe single crystal surfaces it was possible to detect a di-molecular precursor labelled a-N2 — forN2 dissociation... 

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]...

Based on his own experimental results and their analysis, Traube has made the correct (in terms of modem understanding) conclusion that H202 displays high reactivity in the presence of a catalyst in the system. This is the pathway of various substrate oxidations by hydrogen peroxide, proceeding in the Fenton system (iron ion + H202), and some biochemical processes. 

The need in new inexpensive, safe and effective processes for asymmetric sulfide oxidations is determined by pharmaceutical industry requirements [38], Recently, inexpensive and active. systems based on hydrogen peroxide as oxidant and non-toxic chiral iron(III) complexes as catalysts have been reported [39-41 ]. Different mctal-salen complexes have also been previously employed as catalysts for oxidation of sulfides with PhIO Mn "(salen) [42-44], salen) 45], salen) [46], The mechanism proposed in [46] involves intermediate formation of 0x0 iron(lV)-salcn cation radical, that seems doubtful based on the experimental results obtained. In this Chapter we present asymmcinc version of the latter system [(salen )Fc ClJ/PhlO (where salcn stands for the corresponding chiral Schiff base ligands. Scheme 5) and an NMR investigation of the active intermediates. 

In that work, the carbon number distribution of FT products on iron catalyst was studied by use of a modified ASF distribution with two chain-growth probabilities. Based on enol mechanism and two ASF distributions the complete set of elementary reactions is given. Using mechanistic kinetic studies of FTS reaction, the chain-growth probabilities ( i and 2) for two ASF distributions formulated. The calculated two ASF model are carefully fitted with experimental results at low carbon monoxide. Thus, the two ASF model is a useful model for prediction of products distribution on lanthanum-promoted iron catalyst in their experimental conditions at low carbon monoxide conversions. The results for higher carbon monoxide conversions deviate substantially because the FTS reaction highly depends on the hydrogen formed by the WGS as the carbon monoxide conversion increases. 

The above conclusion, based on a simplified model, must be considered in the light of the experimental results, according to the above conclusion that an iron catalyst for ammonia synthesis is about half-covered with nitrogen during the steady state reaction. Another way to look at these results is to consider them as expressions of the principle of Sabatier, who considered that the optimum catalyst was capable of adsorbing an intermediate compound sufficiently, but not to stably. 

Thereout, it is found that the catalyst has the highest activity among all the fused iron catalysts for ammonia synthesis when its chemical composition and crystal structure of the precmsor are those of wiistite (Fei xO)- It is called Fei xO or wiistite based ammonia sjmthesis catalysts, where the defect concentration x of iron ion is 0.04 experimental results break through the classical conclusion that lasted for more than 80 years, namely the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most close to those of magnetite. It also provides a new approach for a novelcat-alytic system — wiistite Fei xO system for improving the performances of the fused iron catalysts. 

Our quantum-chemical simulation of benzene oxidation reaction based on pseudospinel iron center (see Fig. 20.36, bottom) reveals the same structure. The characteristic feature of such intermediate is the presence of C(sp )-H bond. The presence of the C(5/7 )-H bond intermediate was confirmed by in-situ IR experiment of Panov et al. [84]. The IR band at 2874 cm appeared immediately after benzene was fed to the FeO catalyst. At the same time no phenol signals were detected. Heating of the sample resulted in complete disappearance of this band. According to our quantem-chemical simulation only the a-complex structure has the characteristic of this IR band. For benzene oxide, which also has two C(sp )-H bonds, this band is not present, since all of the vibrational frequencies are within narrow range of 3182-3218 cm . In the case of the benzene o-complex the calculated IR frequency for the C(sp )-H vibration is 2930 cm , while the other C-H vibrations are within 3178-3215 cm . Applying anharmonic scaling factor/= 0.96 one may obtain quite reasonable agreement 2813 em and 3050-3086 cm (theory estimation) versus 3037-3090 cm and 2874 em (experimental data). 

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