Rate equation, catalytic deactivation

Prior to the kinetic experiments, possible deactivation phenomena of the catalytic system were checked by recycling experiments with prenal and citral as substrates. These results provide not only important hints on the form of the rate equation, but also on which reaction is convenient for long-term investigations in the loop reactor. After the reaction, the aqueous and organic phases were separated and the catalyst phase was reused without further purification. Results on the hydrogenation of prenal are shown in Fig. 7. The reaction rate clearly decreases if the catalyst phase is reused. According to GC analysis and H-NMR studies, this can be attributed to the fact that the product of the reaction, prenol, is highly soluble in water. Consequently, a simple phase... 

Using the resin, asphalt (R+AT) and aromatics (AR) separated from an atmospheric rcsid oil (ARO) as fc stocks, we have investigated the effects of catalytic coke additive coke (Cgdd) on the cracking activity of a commercial FCC catalyst in a fixed bed (FB) and a rixed fluid bed (FFB) pilot units. Correlations between catalyst activity (a) and coke on catalyst (Q.) have been developed. A catalyst deactivation model, which is useful in modeling of cracking reaction kinetics, has been derived through rate equations of coke formation. 

The predominance of kinetic studies have assumed uniform sites on the catalyst surface. However, it has long been recognized that many catalyst surfaces exhibit non-uniform sites. Boudart and Djega-Mariadassou [3] have discussed the kinetics of non-uniform surfaces and conclude that "a non-uniform surface behaves catalytically. .like a uniform surface..", and that "rate equations are similar for a given mechanism on a uniform or non-uniform surface". This result justifies "the common practice of neglecting non-uniformity of catalytic surfaces in kinetic studies". However, it appears that uniform catalyst sites catmot adequately explain catalyst deactivation phenomena. The objective of the present study was to explain deactivation in terms of a model based on a variable activation energy site distribution on the catalyst. 

This comes closest to reality for aging mechanisms, whose rates are indeed largely independent of the composition of the contacting fluid. For example, in sintering [65,66], the crystallites migrate regardless of any adsorbates, and decay can be represented reasonably well as a reaction of second order in total catalytic sites. Provided deactivation is slow compared with the desired reaction, such models allow the activity to be calculated separately as a function of time, and then used in the rate equation for the desired reaction. For example, for second-order sintering at constant temperature ... 

A series of Cu- and Cu-Ln-ZSM-5 (Ln= Ce, Sm) were prepared by ionic-exchange of ZSM-5 with various copper salts. These catalysts were subjected to TG-DSC measurements coupled with mass spectrometry analysis in the presence of a 30 ml min flow of NO. The fitting of these experimental curves with rate equations led to the determination of the kinetic parameters. The correlation of these parameters with catalytic, FTIR, and XPS data provided a model of deactivation of these catalysts in NO decomposition. 

The conversion in a catalytic reaction performed under constant conditions of reaction often decreases with time of run or time on stream. This phenomenon is called catalyst deactivation or catalyst decay. If it is possible to determine the kinetic form of the reaction and, thus, to measure the rate constant for the catalytic reaction k, it is sometimes possible to express the rate of deactivation by an empirical equation such as... 

The additivity treatment also allows one to evaluate the influence of substituents which are otherwise obtainable only with difficulty. The study of the non-catalytic bromination of the halo-substituted poly-methylbenzenes by Illuminati and Marino (1956) allowed the evaluation of the partial rate factors for the highly deactivating m- and p-halogens. These data for the slow, highly selective bromination are inaccessible by other techniques. Analysis of the relative rates is made by application of the additivity equations (5) and (6) as described in Section I. An important aspect of the chemistry of the substituted polymethyl-benzenes, in contrast to the monosubstituted benzenes, is the large difference in p for bromination. The partial rate factors derived for each reaction are correlated with good precision by the tr4 -constants (Figs. 11 and 19). Yet the susceptibility of the reactions to the influence of substituents is altered by more than 25%. As already noted, this aspect of the problem is not well defined and is worthy of additional attention. 

In an early paper on the subject Szepe and Levenspiel [refe 10) introduced the notion of separability. The equations (1), (2) and (3), In which the deactivation function is a variable factor multiplying the initial rate of a reaction, correspond to their definition of separability. It may be useful to remind here that any kinetic treatment assuming ideal surfaces or accepting an average activity for the catalytic sites is bound to lead to such a form, provided, of course, there is no shift in rate determining steps. The question whether the deactivation is separable or not reduces to the question generally encountered in kinetic studies is it necessary to account explicitly for non uniform activity of the catalytic sites ... 

Figure IB displays relative catalytic activity (RA) - in terms of pseudo first-order rate constants, corrected for coke content, related to the fresh, sulfided catalyst vs carbon content. The individual HDS, HVD and CNH activities all decrease with increasing carbon content, the order of deactivation being HYD < HDS < CNH. (The results for relative HDN activities followed closely those of CNH, and are not shown). Relative activities fall off less sharply as coke content increases. Because of the limited set and scatter of the data, a definitive deactivation correlation could not be obtained. Best fit curves to the data were constructed from a power-deactivation equation in C (1), and are shown by the solid curves in Fig. IB.

Catalyst deactivation is always a problem in catalyst and catalytic reactor design. Empirical equations to represent deactivation rates in design calculations are reported by Weekman (1968), Sadana and Doraiswamy (1971), and Doraiswamy and Sharma (1984). Here, we briefly touch upon the... 

In the simplest case, the catalytic activity is proportional to the number of active sites Nj, intrinsic rate constant and the effectiveness factor. Catalyst deactivation can be caused by a decrease in the number of active sites, changes in the intrinsic rate constant, e.g. changes in the ability of surface sites to promote catalysis and by degradation in accessibility of the pore space. When the reaction and deactivation rates are of different magnitudes, the reactions proceed in seconds while the deactivation can require hours, days or months, and moreover the deactivation does not affect the selectivity, the concept of separability is applied. The reaction rates and deactivation are treated by different equations. A quantity called activity, (a) is introduced to account for changes during the reaction. 

Equation (8.117) took into account self- regeneration. The rate constant of this step is difficult to extract from catalytic experiments, therefore a notion of stationary activity is often used. This activity corresponds to a situation when the deactivation and self-regeneration are... 

In our water effect experiment, a primarily reversible decrease in CO conversion was observed when up to 30 vol% of water was added to the feed for this catalyst [9], With the 0.5% Pt-15%Co/Al203 catalyst, a reversible water effect was obtained at a lower volume percent of water addition but irreversible deactivation occurred at > 25% vol. water addition [7], One possibility for the effect of water is that the amount of catalytic active sites (i.e., surface cobalt metal atoms) available for the FT reaction changes with partial pressure of water, perhaps by a temporary oxidation process for cobalt [9], Alternatively, competitive adsorption of water may decrease the surface concentration of CO and/or H2 [9], Thus, the following equation is proposed to described the reversible impact of water on the CO reaction rate ... 

Part of the rearranged Cu species is then completely blocked as nitrite-nitrate species. These species exhibit no catalytic activity. TG curves may be fitted using equations of type (II) The values of km associated to this step account to the deactivation of the catalysts Small rates in NO accumulation correspond actually to a low deactivation of these catalysts. The deactivation of the catalysts was found to be influenced on the metal loading, the copper precursor salt and the Si/AI ratio in the used zeolite. The presence of a second species like Ce or Sm in a small loading prevents the deactivation. 

Catalyst deaetivation is one of the most vexing problems in catalyst and catalytic reactor design. We shall not be concerned with this in the present ehapter beyond using empirical equations to represent deactivation rates for use in design calculations (see, e.g., Weekman, 1968 Sadana and Doraiswamy, 1971 Doraiswamy and Sharma,... 

Ammonia decomposition over Fe, Cu, Ag, Au, and Pt Hydrolysis of starch to glucose catalyzed by acids Mixture of coal gas and air makes a platinum wire white hot Measurements on the rate of H2O2 decomposition Selective oxidation of ethanol to acetic acid over platinum Comprehensive paper on the H2 + O2 reaction on platinum foils, including reaction rates, deactivation, reactivation, and poisoning Definition of catalysis, catalyst, and catalytic force First quantitative analysis of reaction rates Systematic studies on the concentration dependence of reaction rates First concise monograph on chemical kinetics Definition of order of reaction Arrhenius equation k = u exp (-Ea/RT)... 

In a simple A B reaction, each catalytic turnover corresponds to one mole of B being formed per mole of catalyst. The catalytic rate is often given as a turnover frequency (TOP), the number of catalytic cycles completed per unit time (usually h ). Catalyst lifetime is measured by the turnover number (TON), the number of cycles before deactivation, assuming excess substrate still remains. The TON and TOP depend on the conditions, which must therefore be stated. Since the TOP continually varies with the elapsed time, the maximum TOP during the catalytic run is often cited. This often occurs at the outset of the reaction, and we often see the initial rate reported as a TOP. Comparison of the TOPS can tell us which catalyst has the best rate, while comparison of the TONS tells us which catalyst is the most robust. Conversion (%) measures how much substrate has been converted at a given point, typically after the reaction has come to a halt. Yield (%) measures the amount of any one product relative to the theoretical maximum yield derived from the chemical equation, given the conversion achieved. Selectivity (%) measures the amount of the desired product relative to the theoretical maximum yield. This means the yield is the conversion times the selectivity. 

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