Mossbauer spectroscopy sulfided catalysts

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... 

These tetrahedral distorted cobalt atoms can be observed by NMR as a pure phase on carbon supports in the absence of molybdenum and are thus stable these probably correspond to the Co sites observed by Topspe s group using Mossbauer spectroscopy because Craje et al. (93) found a similar Mossbauer doublet for both cobalt in CoMo catalysts and pure cobalt sulfide on carbon support. They are also active for HDS and confirm the findings of Prins and co-workers (94) and Ledoux (96). These different structures are in full agreement with the XANES experiments performed by Prins and co-workers (95) and Ledoux (96). These structures also led Ledoux et al. to an incorrect interpretation of the synergy effect (64). On poorly dispersed catalysts supported on silica or in bulk form, their presence and activity are large enough to explain the increase in activity when cobalt is added to molybdenum, but on well-dispersed catalysts i.e., on alumina or carbon support this interpretation is shown to be incorrect if the activity is carefully measured. 

The carbon support used was a Norit activated carbon (RX3 extra) having a surface area of 1190 m2.g 1 and a pore volume of 1.0 cm3.g . The Co/C catalyst (4.1 wt% Co) was prepared by pore volume impregnation with an aqueous solution of cobalt nitrate (Merck p.a.) followed by drying in air at 383 K (16 h). The promoted catalyst (1.5 wt% Co, 7.7 wt% Mo) was prepared in a special way to ensure a maximum amount of the Co-Mo-S phase (11). Mossbauer spectroscopy of this promoted catalyst clearly showed that only the Co-Mo-S phase was present after sulfiding (11) and furthermore that this Co-Mo-S is probably a Co-Mo-S type II phase, meaning a minor influence of active phase-support interaction (11,12). The catalytic activity of the sulfided catalysts was determined by a thiophene HDS measurement at 673 K and atmospheric pressure, as described elsewhere (10). The thiophene HDS reaction rate constant kHDg per mol Co present (approximated as a first order reaction) was found to be 17 10 s 1 for Co/C and 61 10 3s 1 for Co-Mo/C. 

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. 

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%). 

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%). 

A great deal is known about the current generation of hydroprocessing catalysts. Using in-situ Mossbauer emission spectroscopy and extended x-ray absorption fine structure (EXAFS), the Haldor-Topsoe group has conclusively shown that the active sites on the sulfided C0-M0-AI2O3 catalyst are vacancies associated with the edge sites in the Co-Mo-S phase. In fact, the thiophene desulfurization rate is linearly proportional to the concentration of the Co-Mo-S phase. A... 

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