Iron-based catalysts spectroscopies

Iron-based Catalysts. - As indicated above, iron-based compounds have been the choice materials for catalysis of the first stage of DCL viz. coal depolymerization. In a recent paper, Huffman et al. have used a variety of analytical techniques to determine the structures of a large number of nanoscale iron-based catalysts before and after DCL experiments. In most of these experiments using iron oxide and iron oxyhydride catalysts, the material found in the residue after the DCL experiments is pyrrhotite, formed by the reaction of H2S with FeS2 present in the coals and with the added catalyst. A number of these catalysts have been used by Pradhan et al. in DCL experiments and by Ibrahim and Seehra in ESR experiments. We now compare the results of these experiments, since they provide the most direct use of ESR spectroscopy to date in DCL experiments. 

Proper understanding of the process of DCL in the presence of catalysts needs an understanding of the electronic changes a catalyst goes through during a DCL process. Huffman et al. have reviewed the use of Mdssbauer spectroscopy, magnetization, and x-ray absorption studies to determine the chemical status of the iron-based catalysts before... 

Iron modified zeolites and ordered mesoporous oxides have been studied as catalysts for the sulfur dioxide oxidation in sulfur rich gases. Both zeolitic materials and mesoporous oxides show very good activity in this reaction. Other than solid state or incipient wetness loaded MCM-41 materials, the zeolites do not show an initial loss of activity. However, they loose activity upon prolonged exposure to reaction conditions around 700°C. The zeolitic samples were analyzed via X-ray absorption spectroscopy, and the deactivation could be related to removal of iron from framework sites to result in the formation of hematite-like species. If the iron can be stabilized in the framework, these materials could be an interesting alternative to other iron based catalysts for the commercial application in sulfur rich gases. 

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... 

After performing FT synthesis on an unreduced iron oxide catalyst, Kuivila et al.12 observed 22% carbide in the bulk by Mossbauer spectroscopy, but only —3% carbide on the surface by XPS, and therefore concluded that a sub-surface carbide phase had formed beneath a magnetite surface layer. Based in part on this result, they conclude that magnetite is the active phase for FT synthesis. Reymond et a/.10 also observed substantial amounts of carbide by XRD, but little or no carbide by XPS. The observation of a 2-4 nm thick carbon layer on the carbide phase, but not on the magnetite, allows a reinterpretation of the data in these two papers. Sputtering of the surface carbon layer permits the XPS to see the underlying carbide, and therefore it is not necessary that the carbide be present beneath an oxide layer. Thus, measurement of low carbide signals by XPS cannot be interpreted to mean that carbide is absent from the catalyst surface, and therefore not an important phase in FT... 

A mechanistic proposal, which is based on the mthenium-catalyzed dehydration reaction reported by Nagashima and coworkers [146], is shown in Scheme 44. Reaction of a primary amine with hydrosilane in the presence of the iron catalyst affords the bis(silyl)amine a and 2 equiv. of H2. Subsequently, the isomerization of a gives the A,0-bis(silyl)imidate b and then elimination of the disiloxane from b produces the corresponding nitrile. Although the disiloxane and its monohydrolysis product were observed by and Si NMR spectroscopy and by GC-Mass-analysis, intermediates a and b were not detected. 

The formation of an intermediate, which is then reduced to form Mo—with either Ni or Fe acting as a catalyst, was also claimed, based on in-situ Raman spectroscopy studies.Although the exact composition of the intermediate was not identified in these studies, it was argued that at low cathodic polarization, the main species on the electrode surface were polymolybdates, that could be reduced to Mo(IV) at a higher cathodic polarization. The species of Mo(IV) could be further reduced to Mo atoms only when cations of the iron-group metal were present in the electrolyte. 

During the 1990s, van Veen and collaborators mainly studied the electrochemical kinetics of oxygen reduction. Their results are presented in Sect. 3. These mechanistic studies were, however, always based on the model in which the C0-N4 or Fe-N4 moieties of the respective macrocycles were retained intact at all pyrolysis temperatures. Their last contribution to the molecular structure of the catalytic site was a study in 2002 of catalysts obtained by adsorption of iron tetramethoxyphenyl porphyrin chloride (ClFeTMPP) on Vulcan XC-72, heat treated between 325 and 800°C in inert atmosphere, and characterized by EXAFS and Mossbauer spectroscopy, as well as by cyclic voltammetry". The loading of these catalysts was 7 wt% chelate ( 0.5 wt% Fe). 

---