Mossbauer spectroscopy supported Iron oxide

D. G. Rethwisch, J. A. Dumesic, Adsorptive and catalytic properties of supported metal oxides 1. Mossbauer spectroscopy of supported iron oxides, J. Phys. Chem. 90 (1986) 1863-1871. 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

The first and most studied Mossbauer nucleus, iron-57, displays specific catalytic behavior. Mossbauer investigations of supported microcrystallites of iron and its oxide have demonstrated the importance of the techniques in the investigation of surface structure and chemistry. The application to other nuclei that have important catalytic qualities indicates the potential importance of the study of supported microcrystallites by Mossbauer spectroscopy in future investigations of catalysts. Developments in experimental techniques enabling in situ investigations are enhancing the scope of the technique. 

With the development of nanoscaled catalysts, several recent studies have pointed out the great interest of recording Mossbauer spectra below 4.2 K up to 0.055 K (100,203,204) it allowed the identification at the surface of various support such as mesoporous silica or Zr02, the formation of nanometric iron oxide clusters this identification was not possible in classical low temperature studies conducted above 4.2K, which concluded to the presence of larger particles. The analysis by Mossbauer spectroscopy performed at lower temperature enabled to show that these larger particles were agglomerates of nanometric iron clusters and allowed to reach another level of resolution of ferric particles structures (204). 

The [TPPFeCl] -NaBITi system in diglyme has been shown to perform remarkably efficiently as a catalyst for the reduction of nitrobenzenes to anilines. Although both TPPCo and TPPMnCl also catalyze the reduction of p-chloronitrobenzene to jo-chloroaniline, their catalytic activities are much lower thanthatofTPPFeCl. Anothertype of reduction catalyst, in this case of dioxygen to water, begins with carbon-supported chloroiron(ni) tetramethoxyphenylporphyrin, which is then heat-treated at 900 °C for one h." This causes decomposition of the porphyrin to produce metallic, carbidic and oxidic iron, as detected by Mossbauer spectroscopy. The active... 

A new catalyst for the selective oxidation of hydrogen sulfide in Claus tail gas to elemental sulfur has been developed. The catalyst consists of highly dispersed iron oxide supported on a silica carrier. During operation the activity of this catalyst decreases due to transformation of iron(III) oxide into a less active component. X-ray diffraction, wet chemical qualitative analysts and Mossbauer spectroscopy reveal the component comprises iron(II) sulfate. Although the transformation of inon(III) oxide into iron(IQ sulfate causes deactivation, the increase in selectivity results in high sulfur yields (up to 94%). 

Progress in the biochemistry and spectroscopy of iron-only nitrogenase has occurred, with the most active, pure preparations reported in 1997. Mossbauer and EXAFS results support electronic and structural analogy between the eight-iron cluster in this enzyme (the FeFeco ) and the FeMoco. One notable difference is that the FeFeco is diamagnetic in its dithionite-reduced form, and thus corresponds to M° in the MoFe protein. Therefore, the oxidation states are thought to be Fe(II)4Fe(III)4 or Fe(II)6Fe(III)2, in analogy to the two most likely oxidation... 

The explanation of endotactic heterostructures in molecular dispersion for the X-ray anomalies of ammonia iron was proved by H. Topsoe by Mossbauer spectroscopy. The Mossbauer data imply the presence of small amounts of non-metal iron components which are present, however, as large particles of structural promoter oxides. They are located in grain boundaries and at the outer surface of the catalyst. This location also explains the SIMS data on Fe-Al-O fragments which were intended to support the hypothesis of endotactic heterostructures.The EXAFS data ° ° provide clear evidence for the identical average local coordination of iron in ammonia iron and normal iron. 

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