PHYSICAL DEACTIVATION AND SINTERING

Physical Deactivation and Sintering 225 Table 6.4 Reactor Point Effectiveness Uniform Sintering te = k/(0... 

Since several supported catalytic systems have sufficient activity for the POM reaction, the main topic of research is the stability of the catalysts. There are two main processes for the deactivation of the catalyst carbon deposition and sintering of the metal. Carbon deposition is due to the process of decomposition of CH4 and CO (reactions (3) and (4)). Two different kinds of carbon can be formed on the surface of the catalyst encapsulated carbon, which covers the metal particle and is the reason for physical-chemical deactivation and whiskers of carbon, which do not deactivate the particle directly but may produce mechanical plugging of the catalytic bed. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

Sintering is an important mode of deactivation in supported metals. The high surface area support (carrier or substrate) in these catalysts serves several functions (l) to increase the dispersion and utilization of the catalytic metal phase, (2) to physically separate metal crystallites and to bind them to its surface, thereby enhancing their thermal stability towards agglomeration, and (3) in some cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

Sintering is an important mode of deactivation in supported metals that involves complex microscopic physical and chemical phenomena, e.g., dissociation, emission, diffusion, and capture of metal atoms and crystallites. The relative importance of these different processes may change with reaction conditions and catalyst formulation. Modeling and prevention of sintering processes require an understanding of these basic processes as well as quantitative measurements of sintering rates. 

As mentioned above, an area in which the concepts and techniques of statistical physics of disordered media have found useful application is the phenomenon of catalyst deactivation. Deactivation is typically caused by a chemical species, which adsorbs on and poisons the catalyst s surface and frequently blocks its porous structure. One finds that often reactants, products and reaction intermediates, as well as various reactant stream impurities, also serve as poisons and/or poison precursors. In addition to the above mode of deactivation, usually called chemical deactivation (2 3.), catalyst particles also deactivate due to thermal and mechanical causes. Thermal deactivation (sintering), in particular, and particle attrition and break-up due to thermal and mechanical causes, are amenable to modeling using the concepts of statistical physics of disordered media, but as already mentioned above the subject will not be dealt with in this paper. 

Samples of catalyst were removed from the pilot-plant reactor at various times during Run >3. Physical and chemical analyses were carried out on these samples and the results were compared with measurements on freshly-reduced catalyst, prior to exposure to synthesis gas. The analyses included BET surface area, S. Cl, Fe and Ni concentration on the catalyst. Cu and ZnO crystallite sizes, and Cu/Zn and Cu VCu ratios on the catalyst surface. The only strong correlation between the rate constant and any of these parameters is shown in Fig. 2, which reveals a striking dependence of the rate constant on the BET surface area. This relationship suggests that sintering of the overall catalyst surface is responsible for a large part of the observed deactivation. 

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