Smam Stephan Bouwens

Carbon-covered alumina as a support for sulfide catalysts☆

Abstract Carbon-covered alumina carrier materials (10–35 wt.% carbon deposited) were prepared via pyrolysis (873–973 K) of cyclohexene or ethene on the surface of a γ-alumina and evaluated for their use as supports for cobalt sulfide hydrodesulfurization catalysts. Promising textural properties were obtained for the samples prepared: BET surface areas up to 334 m 2 g −1 , meso- and macropore surface areas reaching values of 190–270 m 2 g −1 , and narrow pore size distributions in the 2.5–10 nm pore radius range. XPS measurements showed that the alumina surface was not uniformly covered, probably due to diffusion limitations of the carbon forming hydrocarbons. The coverage could be improved (maximum value reached was 77%) by increasing the amount of carbon deposited as well as by an additional high-temperature (1073 K) treatment. The thiophene hydrodesulfurization activity of Co sulfide supported on the prepared carbon-covered aluminas was found to increase linearly with increasing alumina surface coverage by carbon. A threefold increase in activity compared to Co Al 2 O 3 catalysts was obtained, demonstrating the effective shielding by the carbon layer which reduces or eliminates the strong metal-alumina interactions. Oxidizing the carbon surface prior to the introduction of cobalt led to a further improvement of the catalytic activity.

EXAFS study of the local structure of Ni in Ni-MoS2/C hydrodesulfurization catalysts

To study the local structure of the Ni promoter atom, the Ni and Mo K edge EXAFS spectra of Ni-MoS2/C hydrodesulfurization catalyst were measured in an in-situ EXAFS cell at 77 K. The Ni atom is situated in a square pyramid of five S atoms at a distance of 2.21 Å from the S atoms. In addition an EXAFS contribution due to a Mo atom at 2.82 Å from the Ni atom could be identified. This local structure indicates that the Ni atoms are situated on top of the S4 squares at the MoS2 edges in millerite-type Ni sites. The Ni atoms are situated in the planes of the Mo atoms and not in the intercalation plane midway between successive MoS2 sandwich layers.

CARBON BLACK-SUPPORTED MOLYBDENUM SULFIDE CATALYSTS*

Four carbon black samples differing in surface area, pH and surface properties (oxygen functionality) were pore volume impregnated with aqueous molybdate solutions as to achieve a Mo loading of 0.5 Mo atoms per nm2 support surface area. Dispersion measurements obtained by means of X-ray photoelectron spectroscopy, dynamic oxygen chemisorption and transmission electron microscopy, indicated the presence of highly dispersed molybdate in all precursor samples, which upon sulfidation was converted into molybdenum sulfide with a particle size varying between 3.5 and 13.5 nm dependant on the type of carbon black support. To explain these dispersion differences the interaction between molybdate ions and the carbon surface was studied by means of FTIR and XPS. No major changes were observed in the oxygen functionality of the carbon black upon loading with molybdate. Some minor changes were, however, observed by means of FTIR which could point to a chemical reaction between an aryl ether functional group and the molybdate ions.

The influence of phosphorus on the structure and hydrodesulfurization activity of sulfided Co and Co-Mo catalysts supported on carbon and alumina

The thiophene HDS activities of phosphorus-containing Co and CozMo sulfide catalysts supported on γ-Al2O3 and activated carbon were measured at atmospheric pressure. Phosphorus decreased the thiophene HDS activity of the carbon-supported catalysts considerably but did not affect the activity of the alumina-supported catalysts. From XPS it was inferred that phosphorus improves the dispersion of the cobalt phase in the oxidic precursor state of both the carbon- and alumina-supported catalysts. In the sulfided state the interaction between phosphorus and the metal sulfide phase appeared to depend on the support. In the sulfided carbon-supported catalysts, phosphorus formed a Co-phosphate phase, which is responsible for the decrease in HDS activity, while on alumina phosphorus did not influence the structure of the metal sulfide phase.

The structure of the cobalt sulfide phase in carbon-supported Co and Co-Mo sulfide catalysts as studied by exafs and xanes

An X-ray absorption spectroscopy study has been carried out on a sulfided Co/C and a Co-Mo/C catalyst, consisting of a fully sulfided Co-Mo-S phase. By comparing the EXAFS and XANES spectra of the catalysts with those of pure Co9S8 and CoS2 reference compounds, it is shown that the cobalt ions in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt ions in the Co/C catalyst have a larger fraction of octahedral cobalt than Co9S8.

The role of Ni in sulfided carbon-supported Ni-Mo hydrodesulfurization catalysts

Abstract The thiophene hydrodesulfurization activities of Ni and NiMo sulfide catalysts supported on activated carbon were measured at atmospheric pressure and the catalyst structures were studied by means of X-ray photoelectron spectroscopy, dynamic oxygen chemisorption, and chemical sulfur analysis. The Ni/C catalysts contained a Ni 3 S 2 -like phase which, due to the impregnation method, was considerably enriched in the outer shell of the carbon support grains. Both for the Ni/C and NiMo/C catalysts no correlation was observed between the dynamic oxygen chemisorption and the catalytic activity and for the Ni/C catalysts indications for full oxidation of the Ni 3 S 2 particles were obtained. The specific catalytic activity per Ni atom was lower in the Ni/C catalysts than in the NiMo/C catalysts. Since the local Ni structure in these catalysts is different, however, it proved not possible to decide whether the Ni atoms in NiMo/C catalysts are the actual active sites or not.

The influence of phosphorus on the structure and hydrodesulfurization activity of sulfided Fe and Fe-Mo catalysts supported on carbon and alumina

Phosphorus-containing Fe and Fe−Mo sulfide catalysts supported on γ-Al2O3 and activated carbon were evaluated for their thiophene HDS activities at atmospheric pressure. The thiophene HDS activity of the carbon-supported catalysts decreased considerably in the presence of phosphorus, while the activity of the alumina-supported catalysts was not affected. It was shown by Mössbauer spectroscopy that in both the oxidic carbon- and alumina-supported catalyst precursors the presence of phosphorus resulted in an improved dispersion of the Fe phase. In the sulfided carbon-supported Fe and Fe−Mo catalysts, the presence of phosphorus resulted in the formation of an “Fe(II)-phosphate” phase, which was held responsible for the decrease in HDS activity. With regard to the sulfided alumina-supported catalysts, it was found that phosphorus did not influence the structure of the metal sulfide species.