The causes of deactivation are basically threefold: chemical, mechanical and thermal. deactivation''. This process is both of chemical and physical nature and occurs simultaneously with the main reaction. Deactivation is inevitable, but it can be slowed or prevented and some of its consequences can be avoided.

There are three fundamental reasons for catalyst deactivation, i.e.

1. Poisoning,

2. Coking or fouling and

3. Ageing,Sintering or phase transformation

Poisoning can be reversible or irreversible, and with geometric or electronic effect. It can also be selective, nonselective and ant selective, depending on catalyst/poison affinity and kinetics. Catalyst deactivation is the loss of catalyst activity and/or selectivity as a function of time on stream (TOS) and presents a major challenge in many industrial catalytic processes

Kinetics of coking is determined by both mechanism of the coking reaction and its diffusion restrictions. Sintering is the main cause for catalyst ageing.

It appears in two forms: thermal or chemical, depending on prevailing reaction parameters, i.e., temperature or concentration. To cope with deactivation two approaches are offered: either to avoid it when possible, like in the case of feed purification, or accept it but with an effort to minimize its effects. Accelerated deactivation tests can be powerful tools for studying catalyst deactivation in a relatively short time. By proper selection of reaction parameters and applying deactivation compensation approach, reaction and deactivation kinetics can be separated. Based on obtained deactivation kinetics parameters, and by applying appropriate modeling and simulation, the life time of a catalyst and its performance in the commercial reactor at any time can be predicted.

For example Ammonia converter catalyst most dangerous poison is Oxides. Upon poisoning the overall catalyst activity may be decreased without affecting the selectivity, but often the selectivity is affected, since some of the active sites are deactivated while others are practically unaffected. This is the case of ``multifunctional'' catalysts, which have active sites of different nature that promote, simultaneously, different chemical transformations. The Pt/Al2O3 reforming catalysts are typical examples: the metal participates in the hydrogenation± dehydrogenation reactions whereas alumina acts both as support and as acid catalyst for the isomerization and cracking reactions. Hence basic nitrogen compounds adsorb on the alumina acid sites and reduce isomerization and cracking activity, but they have little effect on dehydrogenation activity.

Dear;

hope that you are well.

deactivation of solid catalysts is a complicated phenomena. physical and chemical factors are being involved. modification of the physical structure by sintering and carrier aging are serious factors in loosing he catalyst activity degradation of active surface area and pore volume and modification pore volume & size distribution are also important and articular.

on the other face of the problem chemical deposits altering the composition of the active surface of the catalyst also of special importance. these deposits may be poisons, poisoning the active centers neutralizing the catalytic sites or when metals they may cause selectivity malfunctions catalyzing other undesired reactions, or they may plug the entrance of the pores thus isolating the interior effective area of the these deposits can deactivate in two ways, reversible and irreversible deactivation.

Prof.

Jamal Amous

https://pubs.acs.org/doi/pdf/10.1021/ba-1972-0109.ch007

Catalysts desactivation may be caused by pore mouth plugging, poisoning, fouling, sintering, and attrition.

Temperature, pollution and side reactions are the most important factors.