Poisoning thermal deactivation

It is simple to demonstrate that a catalyst operated in the absence of poisons (54, 55) still can show significant activity loss, albeit to a much smaller degree than in their presence. This deactivation process is induced by thermal effects. A separation of chemical and thermal deactivation requires considerable efforts. 

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. 

However, when we wish to prolong the catalyst life, it is necessary to pay attention not only to thermal deactivation but also to the deactivation by trace amounts of poisonous elements which are not problematie for automobiles due to the shorter life time required. 

For the present purpose, a high Rh surface area even after thermal deactivation is desirable. Therefore, one of the improved catalysts, in which a relatively high amount of Rh is loaded, GEC-01 (Pt = 3 g/1, Rh = 0.6 g/1), was selected as a long life catalyst which would have high poison resistance. Figure 6 shows the activity of model Pb-poisoned catalysts. The characteristics of the activity of Pb-poisoned catalyst are clearly observed on the conventional catalyst, but hardly observed on GEC-01. 

An improved catalyst, which can maintain a high Rh surface area even after thermal deactivation by separating high Pt/Rh ratio particles from low Pt/Rh ratio particles was found to have high poison-resistance in activity tests using model Pb-poisoned catalysts as well as field tests, and its durability has been proven over 30,000 h. 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

The chemical, thermal, and mechamcal stability of a catalyst determines its lifetime in industrial reactors. Catalyst stabUity is influenced by numerous factors, including decomposition, coking, and poisoning. Catalyst deactivation can be followed by measuring activity or selectivity as a function of time. 

Slow decay of adsorbents due to irreversible adsorption of trace conponents or thermal deactivation of active sites is also common. When this occurs, operating conditions must be adjusted accordingly. Because of this poisoning, adsorption processes, which use surface phenomena, are often much more sensitive to trace chemicals than distillation and other separation techniques that rely on bulk properties. An occasional wash step or extreme regeneration step maybe needed. A short life for the sorbent, which can be a problem in biological operations, often makes the process uneconomical. Long-term pilot plant tests with the actual feed from the plant are useful to determine the seriousness of these problems. 

The cost-efFectiveness of an industrial catalytic process is strongly influenced by the stability, long-term activity, and life time of the catalyst The two main causes of life-time reduction of a Cu/Zn-methanol catalyst are catalyst poisoning and thermal deactivation. 

Deactivation in Process The active surface of a catalyst can be degraded by chemical, thermal, or mechanical factors. Poisons and... 

Coking is a severe thermal cracking process designed to handle heavy residues with high asphaltene and metal contents. These residues cannot be fed to catalytic cracking units because their impurities deactivate and poison the catalysts. 

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