E. Gerkema

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.

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

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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.

A real support effect on the activity of fully sulphided CoMoS for the hydrodesulphurization of thiophene

It is shown that on Al2O3, SiO2, and C, a fully sulphided CoMoS phase can be prepared with similar degrees of dispersion, and that the specific activity of this phase for the hydrodesulphurization of thiophene is higher when this phase is supported on carbon than when it is supported on alumina or silica.

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.

Sulfidation of carbon-supported iron oxide catalysts

The sulfidation of carbon-supported iron oxide catalysts was studied by means of in-situ Mossbauer spectroscopy at temperatures down to 4.2 K. The catalysts were dried in two different ways and then sulfided in a flow of 10% H2S in H2 at temperatures between 293 and 773 K. Thiophene hydrodesulfurization (HDS) activity measurements were performed at 673 K in a flow microreactor operating at atmospheric pressure. The iron content varied from 1.1 to 9.0 wt.-% Fe. In the oxidic catalyst precursors, dried in air at 293 K, nitrate anions were still present. These anions could be removed by means of an additional H2 treatment up to 393 K. This treatment also led to an increase in the interaction strength between the iron (III) oxide particles and the carbon support. It is shown that the transition from iron (III) oxide to iron sulfide proceeded through two intermediate phases, viz. an oxidic high-spin Fe2+-phase and FeS2. A correlation between the HDS activity per mole of iron and the mean iron sulfide particle size was observed, with the mean particle size being dependent on the preparational treatments of the oxidic catalyst precursors.

Stability of the so-called "Co-Mo-S" phase in a carbon-supported Co-Mo sulfide catalyst at very low Co/Mo ratio

Several aspects of the stability of the so-called “Co−Mo−S” phase in a fully sulfided Co(0.04)Mo(6.84)/C (wt%) catalyst have been examined by Mössbauer emission spectroscopy. The “Co−Mo−S” parameters turned out to be unchanged when the catalyst is exposed for 23 days to ambient air at room temperature. However, if this exposure to air is extended for over 100 days, the so-called “Co−Mo−S” phase was found to be oxidized to a high-spin Fe2+ phase which has no strong interaction with the “MoS2” particles. After resulfidation of the catalyst the so-called “Co−Mo−S” phase is formed again.

The study on supported Fe and FeMo hydrotreating catalysts by combining Mössbauer spectroscopy and ordinary gamma-ray transmission

From in-situ Mössbauer spectra recorded at room temperature it is concluded that the support material (carbon or alumina) influences the degree of sulfidation of Fe and FeMo hydrotreating catalysts. For the latter a Fe−Mo−S phase is observed. By combining Mössbauer spectroscopy and ordinary γ-ray spectroscopy the sulfur uptake by the catalyst has been studied.

Mössbauer emission and absorption studies on the reduction behaviour of supported Co, Co−Mo, Fe and Fe−Mo hydrotreating catalysts

In carbon-supported Fe−Mo catalysts an Fe−Mo alloy can be formed during reduction in H2 at 673 K. In carbon-supported Co−Mo catalysts, instead of the formation of a Co−Mo alloy segregation of metallic Co is observed.In alumina-supported Fe and Fe−Mo catalysts, the interaction between the alumina support and the catalyst particles hinders reduction of the ions to an Fe0 phase. Most probably a surface “Fe(II)-aluminate” is formed.