Reversible and irreversible catalyst

Figure 2 and 3 give examples of causal loop diagrams for reversible and irreversible catalyst deactivation ... 

Schipper, P.H., and F.J. Krambeck, A Reactor Design Simulation with Reversible and Irreversible Catalyst Deactivation, presented at 9th International Symposium on Chemical Reaction Engineering (ISCRE-9), Philadelphia, May 18-21, 1986. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

The utility of SCFs for PTC was demonstrated for several model organic reactions - the nucleophilic displacement of benzyl chloride with bromide ion (26) and cyanide ion (27), which were chosen as model reversible and irreversible Sn2 reactions. The next two reactions reported were the alkylation and cycloalkylation of phenylacetonitrile (28,29). Catalyst solubility in the SCF was very limited, yet the rate of reaction increased linearly with the amount of catalyst present. Figure 5 shows data for the cyanide displacement of benzyl bromide, and the data followed pseudo-first order, irreversible kinetics. The catalyst amounts ranged from 0.06 (solubility limit) to 10% of the limiting reactant, benzyl chloride. 

The surface saturation by sulfur has to be compared to the irreversible adsorbed sulfur introduced by Menon and Prasad (22) and Apesteg-uia et al. (23). The study of H2S adsorption on supported catalysts was carried out by Menon and Prasad (22) and Apesteguia et al., Parera et al., and Barbier et al. and Marecot (23-25). For alumina supports, it was shown (23-25) that chlorine inhibits the adsorption of H2S on the support. Yet this adsorption on pure alumina is wholly reversible at 500°C, as is shown in Fig. 2. On Pt/Al203 at 500°C, only a fraction of the adsorbed sulfur is quickly desorbed in a hydrogen atmosphere. This result enabled the preceding authors (22-25) to develop the notion of reversible and irreversible adsorbed sulfur. The irreversible form, which does not exist on pure alumina, would interact with the metal. The quantity of irreversible sulfur, determined after 30 h of desorption under hydrogen flow at 500°C, does not depend on the sulfiding conditions (Table I). 

For a sufficiently reducing reactant like ethanol both reversible and irreversible deactivation can be neglected. For MGP however unable to keep the catalyst in a low oxidation state particle growth occurs most probably via an Ostwald-ripening mechanism resulting in an irreversible decrease of the platinum surface area exposed. 

This value was verified in a continuous laboratory reactor used to study the catalyst deactivation in long time kinetic runs[2]. On the basis of experimental observations, we recognized that the palladium catalyst is subjected to both reversible and irreversible poisoning. Water beiing responsible for reversible poisoning of the catalyst. Thus, we suggested, the following mechanism ... 

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

When in situ dosing onto two different samples of Pt/silica (45, 48) and onto Cu/MgO was used (49), no evidence for spillover was found from NMR. Only one detailed study based on fully relaxed spectra led to observation of a non zero spillover (4T). In a Ru/Si02 catalyst, the silanol protons w ere exchanged for deuterons, the sam.ple was evacuated at 623 K, and a reference NMR spectrum w as taken at room, temperature. The sample was then exposed to 20 Torr of H2, an NMR spectrum was taken, and the difference with respect to the reference was calculated (line in Fig. 14). This represents the sum of reversible and irreversible hydrogen on the metal (resonating at -65 ppm) and spilled over on the support (at about 3 ppm). Then the sample was pumped out at room temperature for 10 min, and again a difference spectrum with the reference state was obtained (dashed line in Fig. 14) this represents irreversible hydrogen both on the support and on the metal. Similar in situ NMR techniques were used... 

Regarding the Rh/Ce02 (N) catalyst. Table 4.3 shows that the overall concentration of species is actually determined by the addition of two different components. They have been referred to as reversible and irreversible contributions (195). The first one, is associated with the hydrogen chemisorbed on the ceria support. This hydrogen form may be eliminated, with inherent reoxidation of the catalyst, by simple evacuation, it being restored by a further treatment with H2. In accordance with Table 4.3, for a mean surface area catalyst (49 m. g ), reduced at a moderate temperature (623 K), the reversible contribution represents about two thirds of the total concentration of species. This observation is in full agreement with the magnetic... 

The presence of H2O in the feed stream during NO reduction with C3H8 caused kinetic inhibition of the main reactions as well as the apparent activation of solid state phenomena possibly associated to Cu migration. As a result, boA reversible and irreversible deactivation for NO and CsHg conversions were observed. Catalyst deactivation followed a nearly linear trend at short times on stream (t < 12 h), but after long times we observed an unexpected sharp increase in deactivation. The drop in activity was a function of both temperature and H2O concentration, and a low steady state conversion was finally reached. It appears that the main reason for the deactivation of Cu-ZSM-5 is the mobility of Cu + in the presence of H2O rather than ZSM-5 dealumination. 

The experiments clearly indicated the existence of reversible deactivation on the catalyst. It was possible to explain this behavior by a simple model including reversible and irreversible types of coke, and a mechanistic model based on the results was proposed in Figure 4. The transient approach, in which the reaction conditions were changed a number of times during each run, was powerful and yielded abundant information in a limited number of runs. Transient experiments were necessary in order to distinguish between the two types of deactivation. However, interpretation was difficult and it requires extensive numerical evaluations. 

The optimal temperature policy in a batch reactor, for a first order irreversible reaction was formulated by Szepe and Levenspiel (1968). The optimal situation was found to be either operating at the maximum allowable temperature, or with a rising temperature policy, Chou el al. (1967) have discussed the problem of simple optimal control policies of isothermal tubular reactors with catalyst decay. They found that the optimal policy is to maintain a constant conversion assuming that the decay is dependent on temperature. Ogunye and Ray (1968) found that, for both reversible and irreversible reactions, the simple optimal policies for the maximization of a total yield of a reactor over a period of catalyst decay were not always optimal. The optimal policy can be mixed containing both constrained and unconstrained parts as well as being purely constrained. 

Figure 25 Reversible and irreversible sulfur adsorption as a function of surface Pt atoms. 0.065% H2S in the sulfurizing mixture. T = 773 K. catalyst with 1.90% Pt, = catalyst with 0.93% Pt, X= catalyst with 0.53% Pt, = chlorided AI2O3.79...

Reversible/Irreversible Poisoning- Further definition, however, is needed to clarify the related concepts of reversible and irreversible deactivation, and poisons versus simple competitive adsoiption. Deactivation, which may involve the loss of the catalyst s conversion ability, activity or selectivity, over time is often irreversible. If irreversible, continued in situ operation of the catalyst in the absence of the deactivating agent does not restore the catalyst to its original activity. 

Sulfur is a catalyst poison and will damage catalyst in fuel-cell heating appliances reversible and irreversible. All types of electro catalyst of the different fuel cells will be more or less damaged by sulfur compounds. Therefore the sulfur substances of the natural gas have to be removed. For the desulfurization different processes were developed. In fuel-ceU heating appliances two processes are common. 

Example of a catalytic cycle with catalyst precursors and both the reversible and irreversible formation of inactive species. 

There are many substances that can deteriorate the performance of the reforming catalysts. These substances are called poisons and can cause reversible and irreversible deactivation. The reason why one or more of these substances can appear in a reforming unit is because of an improper feed pretreatment. Alternatively, the poisons can enter the unit through the water or chloride injection streams. 

The reasons for the deterioration of ceU performance can be distinguished in reversible and irreversible power loss. Inevitable irreversible performance loss is caused by carbon oxidation, platinum dissolution, and chemical attack of the membrane by radicals [7]. Reversible power loss can be caused by flooding of the cell, dehydration of the membrane electrode assembly (MEA), or change of the catalyst surface oxidation state [8]. If corrective actions are not started immediately, reversible effects lead to irreversible power loss that we define as degradation. In this chapter, we focus on the degradation of the catalyst layer due to undesired side reactions. 

Catalysts increase the rates of both reversible and irreversible reactions. Catalysts do not alter the position of equilibrium (Chapter 7), they only increase the rate at which equilibrium is achieved. In other words, the presence of a catalyst does not increase the yield of products but increases the rate of their production. This is because a catalyst lowers the activation energy barrier, E, for both the forward and reverse reactions, increasing the rates of both forward and reverse reactions to the same degree. (This effect of a catalyst on forward and reverse energy barriers is known as the principle of microscopic reversibility.) Hence, to find a good catalyst for a particular reaction it is sufficient to look for a good catalyst for the reverse reaction. 

Unfortunately, all catalysts will deactivate under the same reaction conditions, but at different rates of deactivation. Catalyst designers need to continue to find ways that could render the catalyst being used most economically for a longer fife. Deactivation can be divided into two types reversible and irreversible. Both types of deactivation are not desirable. Even for the former case, the deactivated catalyst has to be taken out of the production line for reduction. 

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