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Preparation of heterogeneous catalysts

Synthesis of highly dispersed solids and their reactivity

Journal of Thermal Analysis and Calorimetry
Author:
B. Delmon

Abstract  

The preparation of heterogeneous catalysts has been for many years a dynamic field of sub-nanotechnology and remains so nowadays. The approach to preparation must be examined in function of the specific demands concerning (i) activity and (ii) selectivity, that both depend on the arrangement of atoms at a scale smaller than 0.02 nm. Adequate access of reactants to the surface must be provided. Most catalysts are used in the form of pellets or cylinders obtained by pressing, extrusion or other techniques. This implies a control of texture at dimension scales extending from a fraction of a nanometer to several millimetres (and sometimes more). A third demand (iii) is resistance to ageing. In particular, stability at relatively high temperatures is necessary. The strategy adopted in the majority of cases is to start from a material that is homogeneous in composition at the Angström scale, generally a liquid or a solid of complex composition, frequently amorphous. A general objective is to locate the different constituting atoms at precise positions. The main difficulty is to transform the starting precursor into a highly porous solid without segregation of different elements that would produce tiny parts with different properties. The specific approach to catalyst preparation is based on the general concepts used for controlling the reactivity of solids. Typical methods of general use will be examined. Chemical bonds of practically any kind can retain the elements constituting the future catalyst at the position they have in the precursor. The so-called ‘citrate process’ and its variants are of wide application. More elaborate approaches start from molecules or polymers associating the necessary elements.

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Abstract  

A57Co doped unsupported CoMo sulfide catalyst with atomic composition ratio r=Co/(Co+Mo)=0.3 was prepared by the homogeneous sulfide precipitation method and exposed to a series of reduction-sulfidation treatments. The treated samples were analyzed by Mössbauer emission spectroscopy using very long accumulation times. Computer decomposition of the spectra revealed the presence of five different cobalt species which were identified as Co9S8, CoS1+x, and three species related to the Co–Mo–S structure. Reduction of the sample under atmospheric pressure of H2 at (and above) 573 K causes an increase of the amount of Co9S8 at the expense of all other species. These results afford a new insight into the stability of the Co–Mo–S structure and of the sulfur rich CoS1+x phase under hydrotreating conditions.

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Precursors of unsupported NiMo and FeMo sulfide hydrodesulfurization catalysts with concentration ratiosr=Ni(Fe)/(Ni(Fe) + Mo) ranging from 0.1 to 0.3 were prepared by three methods: homogeneous sulfide precipitation (HSP), inverse HSP and coprecipitation. Differential thermal analysis was used to study the decomposition under argon, and the reduction/sulfidation under 15% H2S—H2 of the precursors and the subsequent oxidation under air of the samples obtained after these reactions. The reactivity of the solids varies as a function of the preparation method, the nature of the promoter and the concentration ratio. The degree of sulfidation of the precursor and the presence of either NH4NO3 or NH2Cl formed from group VIII metal salts and (NH4)2S may affect the thermal behaviour of samples during DTA.

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