Mossbauer spectroscopy, hydrogen

Fe/Ir catalysts on silica and alumina Fe and Ir Mossbauer spectroscopy silica- and alumina-supported Fe-Ir catalysts formed by calcination in air contain mixtures of small particles of Fe(III) oxide and Ir(IV) oxide. IrOz is reduced in hydrogen to metallic Ir. a-Fe203 on SiOz is reduced in hydrogen to an Fe-Ir alloy, whilst supported on alumina stabilizes in hydrogen as Fe(II). Possible use for methanol formation is discussed... 

Fe/Ir catalysts In situ Fe and Ir Mossbauer spectroscopy of silica-supported Fe/Ir catalysts with different iron to iridium ratios following pretreatment in hydrogen show that the reduction of the Fe component is enhanced by the presence of Ir metal. The presence of Ir was found to increase the catalytic activity in hydrogenation of carbon monoxide and also to influence selectivity... 

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

However, Poulet et al. (79) reported that phosphorus induces a decrease of the S/Mo ratio in a MoP/ Al catalyst after sulfidation or after a subsequent hydrogen treatment. Topspe et al (98) interpreted the shift of the IR bands of NO adsorbed on (Ni)MoP/Al catalysts toward higher frequency as indicating less sulfidation of the molybdenum (and nickel) species. These authors also confirmed, by using Mossbauer spectroscopy, the presence of a less sulfided state of molybdenum when phosphorus is present. 

MFI ferrisilicates with different iron contents (Si/Fe ratios of 27.5, 35, 39 and 67) were investigated under reducing and catalytic conditions by in situ techniques (ESR and Mossbauer spectroscopies). Framework substituted and extra-framework iron species with different coordination and oxidation states were identified, transformations of various forms of iron in hydrogen and hydrocarbon treatments are compared. Results obtained from catalytic measurements in hydrocarbon conversion and data obtained from in situ techniques are correlated, and the roles of various iron species in catalytic processes are suggested. 

Effects of reduction (in hydrogen) and catalytic treatment (conversion of toluene) on coordination and oxidation states of iron in various species, identified by ESR and Mossbauer spectroscopies, were compared on ferrisilicates of various iron contents. Estimations are given for the reiative amounts of iron species in dependence on the iron content. Both treatments resulted in a significant Fe(lll)-Td Fe(ll)-Oh reduction. The removai of substituted iron from the framework was more pronounced upon reduction. At hydrocarbon conversion the changes in the coordination and oxidation states of iron were more reversible, a certain restoration of the framework was possibie by the removal of the attached reaction products. The possibie roles of... 

Well dispersed iron oxide on silica of a specific surface area of 50 m /g has proven to be a very suitable catalyst for the selective oxidarion of hydrogen sulfide to elemental sulfur. Under reaction conditions the iron oxide is transformed into ironCII sulfate as revealed by X ray diffraction, Mbssbauer spectroscopy, and wet chemical analysis. The presence of an iron(III) component as observed by ex situ Mossbauer spectroscopy can not be excluded. Although the transfomiation of iTon(]Il) oxide into iron(II) sulfate causes initial deactivation, the increase in selectivity (96%) results in high sulfur yields (up to 94%). 

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