Iron characterization techniques

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

From the different characterization techniques, it follows that catalysts have been prepared that display a homogeneous distribution of the supported phase on the support pellets, with an increased interaction as compared with a physical mixture of iron oxide and titania. Tn the case of a pure anatase support, the interaction leads to the formation of a mixed oxide of iron and titanium. 

There is stiU a dispute as to whether the catalytic activity of iron-containing zeotype materials, for example, Fe-ZSM-5, should be attributed to isomorphously substituted framework iron or to extra-framework iron oxide or iron hydroxide species that are highly dispersed in the material. These extra-framework iron species are present for two reasons, either because they were not incorporated into the framework during the synthesis or because they were ejected from the framework during postsynthesis treatments (such as calcination or other heat treatments). The unresolved issue of the origin of catalytic activity continues to be the subject of research, whereby state-of-the-art characterization techniques are being applied. 

Mechanism of the High-Temperature WGSR Over Iron Oxide Catalysts Using Characterization Techniques... 

This abbreviated history of stainless steels illustrates the successes in the development of iron-based, corrosion-resistant alloys. Developments were accompanied by use of increasingly sophisticated experimental and characterization techniques from advances in allied fields. However, it was also a labor-intensive effort, and the time period—from recognition of the problem, to an understanding of its origin, to the development of the most resistant alloys— was on the order of 80 years. This was hardly an efficient process. 

The nature of the active sites in such kind of Me/N/C catalysts is not yet doubtlessly resolved, however, there is strong evidence that they consist of an iron ion which is coordinated to nitrogen atoms embedded into the carbon matrix. Active sites of the kind FeN4, FeN2+2 as well as FeN2 have been postulated based on multiple characterization techniques [34,42,43,46]. A simple model of the probable structure of an active site can be seen in Figure 10.2. 

Ceramic bond formation and grain growth by diffusion are the two prominent reactions for bonding at the high temperature (1100 to 1370°C, or 2000 to 2500°F, for iron ore) employed. The minimum temperature required for sintering may be measured by modern dilatometry techniques, as well as by differential scanning calorimetry. See Compo et al. [Powder Tech., 51(1), 87 (1987) Paiticle Characterization, 1, 171 (1984)] for reviews. 

For iron, cobalt, nickel, and their alloys, the most sensitive technique for characterizing the particle surface is the measurement of magnetic properties. Thus, we synthesized cobalt nanoparticles of 1.6 nm (ca. 150 atoms), 2 nm (ca. 300 atoms) and 4 nm (a few thousand atoms) mean size. The structure of the particles is hep in the latter case and polytetrahedral in the first two cases. The 4 nm particles display a saturation magnetization equal to that of bulk... 

These two examples illustrate how Mossbauer spectroscopy reveals the identity of iron phases in a catalyst after different treatments. The examples are typical for many applications of the technique in catalysis. A catalyst is reduced, carburized, sulfided, or passivated, and, after cooling down, its Mossbauer spectrum is taken at room temperature. However, a complete characterization of phases in a catalyst... 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

In reference 190, the authors describe the spectroscopic and X-ray crystallographic techniques they used to determine the pMMO structure. First, EPR and EX AFS experiments indicated a mononuclear, type 2 Cu(II) center hgated by histidine residues and a copper-containing cluster characterized by a 2.57 A Cu-Cu interaction. A functional iron center was also indicated by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). ICP-AES uses inductively coupled plasma to produce excited atoms that emit electromagnetic radiation at a wavelength characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element (iron in this case) within the sample. 

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