Jpr Vissers

Effect of the support on the structure of Mo-based hydrodesulfurization catalysts: Activated carbon versus alumina☆

The structure of oxidic and sulhded MO catalysts supported on activated carbon was studied by means of X-ray photoelectron spectroscopy (XPS), temperature programmed sulfiding (TPS), and suIfur analysis measurements. In the oxidic state the MO phase was highly dispersed as isolated or polymerized monolayer species at MO loadings helow 3 wt% and as very tiny three-dimensional particles at higher loadings. Upon sulfiding particle growth took place, although the size of the sulfide particles remained below 4.6 nm even in the sample with the highest MO loading (14.1 wt%). TPS patterns showed that sulfiding proceeded via a mechanism of 0-S substitution reactions and was completed at temperatures below 560 K. In the suhided catalysts only MO(W) was detected by XPS and S/MO stoichiometries determined by XPS, TPS, and chemical sulfur analysis varied between 1.5 and 2.0, demonstrating that MO& was the major phase present after sulfidation. The higher catalytic activity for MO/C compared to Mo/A120, is explained by differences in the structure of the sulfide phases present and in the interaction between these phases and the respective supports.

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.

Phosphorus poisoning of molybdenum sulfide hydrodesulfurization catalysts supported on carbon and alumina

Phosphorus-containing MO sulfide catalysts supported on y-A1203 and activated carbon were evaluated for their thiophene HDS activities. Phosphorus was added as phosphoric acid to the carrier material prior to the molybdenum component. The thiophene HDS activity of the carbonsupported catalysts was strongly decreased by phosphorus, while alumina-supported catalysts were not poisoned by phosphorus when present at moderate contents. The structural characteristics and degree of dispersion of the sulfided carbon-supported catalysts were determined by X-ray photoelectron spectroscopy and dynamic CO chemisorption. The cause of the phosphorus poisoning could not be related to a decrease in active phase dispersion or to incomplete sulfidation of the oxidic precursor catalyst. CO chemisorption revealed that in a phosphorus-containing catalyst anion vacancies were blocked. It was suggested that phosphorus poisoning can be related to phosphine (PH,), created by reduction of phosphate, probably during the presulfiding treatment. The poisoning effect can be explained as resulting from the adsorption of phosphine on the anion vacancies. The fact that alumina-supported catalysts are not poisoned by phosphorus can be explained by the strong interaction of phosphate with the alumina support. Due to this strong interaction, phosphate will not be reduced to phosphine under the sulfiding and reaction conditions

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.

Carbon black composites as carrier materials for sulphide catalysts

Various carbon blacks differing in particle size and surface area, amongst other properties, were used to prepare carbon black composites by mixing the carbon particles with a poly(furfuryl alcohol) binder and subsequently carbonizing the binder material at elevated temperatures. The textural properties of the composites were found to vary according to the properties of the carbon black substrate. The composites demonstrated promising textural properties (BET surface area 120–730 m2 g−1; mesoporosity 50–90%) for use as supports for sulphide catalysts. Molybdenum sulphide catalysts were prepared on the composite supports and evaluated for their thiophene hydrodesulphurization activity at atmospheric pressure. Owing to the inert character of the composite surface, sintering of the Mo phase took place during sulphidation. Therefore, the composites were subjected to several oxidative treatments to increase their surface heterogeneity and thus their affinity towards the deposited Mo phase. Treatment with HNO3 yielded the most promising results.