Mwj Crajé

Reduction behaviour of Fe/ZrO2 and Fe/K/ZrO2 Fischer–Tropsch catalysts

Abstract The reduction behaviour (the activation process) of two iron-based Fischer–Tropsch catalysts viz. an unpromoted and a potassium-promoted zirconia-supported iron catalyst is investigated. The initial dispersion of both catalysts is very high. To establish the nature of the iron species under hydrogen atmosphere, experiments were performed at ambient pressure. It is demonstrated for both catalysts that during reduction the carbon originating from the citrate complex used in the preparation procedure is not completely removed, whereby the largest amount of the carbon species is encountered on the potassium-promoted catalyst. During reduction at ambient pressure this residual carbon reacts with metallic iron and forms cementite ( θ -Fe 3 C). The iron oxide of the potassium-promoted catalyst turns out to reduce more easily than the oxide of the unpromoted catalyst. Upon formation of divalent iron, this divalent species readily reacts with the support to give a stable mixed oxide. This mixed oxide is suggested to be a prerequisite in maintaining a high dispersion of the metallic iron particles. Based on the analyses presented here, a reduction model is proposed.

SO-CALLED "Co-Mo-S" PHASE OBSERVED IN CARBON- SUPPORTED Co AND Co-Mo SULFIDE CATALYSTS BY MOSSBAUER EMISSION SPECTROSCOPY

In-situ Mössbauer Emission Spectroscopy (MES) has been used to study the type of phases present in sulfided activated carbon-supported Co and Co-Mo hydrodesulfurization (HDS) catalysts. Most of the reported MES studies are performed on Co-Mo/Al2O3 catalysts. Co species in the so-called “Co-Mo-S” phase, sofar only observed in sulfided catalysts containing Co and Mo, should govern the HDS activity. The present observations show that the same Co species in sulfided Co/C and Co-Mo/C catalysts, with the same quadrupole splitting as “Co-Mo-S” can be formed. Furthermore, it turns out that the QS-value of the “active phase” in the sulfided Co/C and Co-Mo/C catalysts depends on the sulfiding temperature and Co content. Hence, it seems unlikely that there will be only one well defined active sulfide phase which governs the HDS activity.

The application of Mössbauer emission spectroscopy to industrial cobalt based Fischer–Tropsch catalysts

The application of Mossbauer emission spectroscopy to study cobalt based Fischer–Tropsch catalysts for the gas-to-liquids process was investigated. It was shown that Mossbauer emission spectroscopy could be used to study the oxidation of cobalt as a deactivation mechanism of high loading cobalt based Fischer–Tropsch catalysts. Oxidation was observed under conditions that are in contradiction with the bulk cobalt phase thermodynamics. This can be explained by oxidation of small cobalt crystallites or by surface oxidation. The formation of re-reducible Co3+ species was observed as well as the formation of irreducible Co3+ and Co2+ species that interact strongly with the alumina support. The formation of the different cobalt species depends on the oxidation conditions. Iron was used as a probe nuclide to investigate the cobalt catalyst preparation procedure. A high-pressure Mossbauer emission spectroscopy cell was designed and constructed, which creates the opportunity to study cobalt based Fischer–Tropsch catalysts under realistic synthesis conditions.

So-called "Co-Mo-S" phase observed in carbon-supported cobalt sulfide catalyst by Mössbauer emission spectroscopy

• A submitted manuscript is the authors version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publishers website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Sulfidation of alumina-supported iron and iron-molybdenum oxide catalysts

The transition of alumina-supported iron and iron-molybdenum catalysts from the oxidic precursor to the sulfided catalysts was systematically studied by means of in-situ Mossbauer spectroscopy at room temperature. This enabled the adjudgement of various sulfidic phases in the sulfided catalysts. The alumina support material prevents complete sulfidation of the iron phase. Some of the iron diffuses into the support material at temperatures of 573 K and higher. The relative amount of iron diffused into the alumina was reduced at higher iron content and in the presence of molybdenum. Together with the formation of elemental sulfur and a lower intrinsic activity, the incomplete sulfidation accounts for the poor HDS activity of alumina-supported iron and iron molybdenum sulfide catalysts.

On the so-called "Co-Mo-S" phase observed in carbon-supported cobalt sulfide catalysts : temperature dependence of the in-situ Mössbauer emission spectrum

• A submitted manuscript is the authors version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publishers website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Synthesis of highly dispersed zirconia-supported iron-based catalysts for Fischer-Tropsch synthesis

Zirconia-supported iron-based Fischer–Tropsch catalysts were prepared using incipient wetness impregnation. The choice of the precursor, in this case the chelating ammonium iron(III) citrate, and the applied calcination temperature determine the final distribution of the precursor on the zirconia support. Several techniques reveal that both an unpromoted and a potassium-promoted catalyst can be prepared, of which the iron(III) oxide exhibits a high dispersion and is, moreover, monodisperse.

Mössbauer emission study on 57Co doped carbon-supported Ni and Ni-Mo sulfide hydrotreating catalysts : the influence of phosphorus on the structure

Abstract In the present study it is demonstrated that Mossbauer emission spectroscopy (MES) can generate information on the various Ni phases present in sulfided Ni containing catalysts when a small amount of 57 Co is used as a probe for Ni. Application of MES to 57 Co:Ni(4.5)Mo(8.0)/C and 57 Co:Ni(5.6)/C revealed the formation of a so-called “Ni-Mo-S” phase in the former and a bulk sulfide in the latter catalyst. After addition of phosphorus a “Ni-(thio)phosphate” phase is found to be formed in both catalysts. The relation between the structure of these catalysts and their activity for thiophene HDS and quinoline HDN is discussed.

Sulphidation of carbon-supported iron-molybdemum oxide catalysts

Abstract Carbon-supported iron-molybdenum sulphide catalysts were characterized by means of Mossbauer spectroscopy at temperatures down to 4.2 K. Thiophene hydrodesulphurization (HDS) activity measurements were performed at 673 K in a flow microreactor operating at atmospheric pressure. The molybdenum content was 9.5 wt.-% whereas the iron content varied from 0.6 to 9.0 wt.-%. Sequential deposition (Molybdenum first) by pore-volume impregnation was employed to prepare oxidic catalyst precursors. The oxidic catalyst precursors were dried at 293 K in an air flow, followed by an additional hydrogen treatment up to 393 K. The type and relative particle sizes of the iron compounds present in the oxidic precursors and in the sulphided and reoxidized catalysts were determined by Mossbauer spectroscopy. It was demonstrated that after sulphidation for 4 h at 623 K, the composition of the sulphide catalyst depends on the iron content. Sulphided Fe-Mo/C catalysts contain a mixed “Fe Mo S” phase and “Fe-sulphide”. The former is responsible for the observed promoting effect toward thiophene HDS. From the temperature dependence of the resonant absorption areas, it was concluded that the iron atoms in the “Fe Mo S” phase are located at the surface of MoS2 microcrystals. The amount of “Fe-sulphide” present in the catalyst was found to increase with increasing iron content. This “Fe-sulphide” might partly cover the “Fe Mo S” phase, thus causing a decrease in the promoting effect.

Similarity of Co-species in Co and Co-Mo-sulfide catalysts supported on carbon and alumina : Moessbauer emission spectroscopic study

It is shown that, irrespective of the application of carbon or alumina as a support, the local structure of the “Co-sulfide” phase formed during sulfidation of Co-and CoMo-catalysts is the same. A relation is found between the quadrupole splitting (Q.S. value) of the “Co-sulfide” phase and its dispersion. The higher the dispersion, the larger the Q.S. value. The so-called “Co-Mo-S” doublet is observed in all cases and it turns out to be related to a highly dispersed “Co-sulfide” phase instead of a Co, Mo and S containing phase.