Decay, of catalytic activity

When the amount of coke formed as a function of time on stream is compared to the decrease in catalytic activity (see Fig. 3), two regimes of deactivation can be noticed for the strongly deactivating catalysts, i e, a slow initial deactivation which is followed by a rapid loss of activity This first phase is characteristic of a slow transformation of the reactive carbon into less reactive coke. The second phase is attributed to carbon formed on the support which accumulates there and rapidly covers the Pt particles when its amount reaches a critical value causing the sudden decay of catalytic activity. 

Catalyst deactivation is of major concern in catalyst development and design of packed bed reactors. Decay of catalytic activity with time can be caused by several mechanisms such as fouling, sintering and poisoning. Although much fundamental experimental work has been done on deactivation,very little attention has been focused on modelling and systematic analysis of nonadiabatic fixed bed reactors where a deactivation process occurs. 

Iguchi [266] considered a mechanism in which growing chains are occluded, and thus become non-propagating, by a process kinetically analogous to the interaction of crystallites growing from the polymer melt and following Avrami kinetics. (It was assumed that chemical transfer and termination reactions do not occur.) The polymerization rate is given by the concentrations of monomer and active centres at any time, i.e. Rp = kp [C ], [M],. If the decay of catalytic activity follows the relationship... 

Fig. 16. Decay of catalytically active base in the anionic polymerization of caprolactam [135]. The concentration of effective base is expressed in percent of the initial concentration of sodium caprolactam. Concentration of benzoylcaprolactam, 0.0360...

The most reported catalytic experiment has been the dehydration of cyclohexanol over a mixed magnesium sulfate-sodium sulfate catalyst, and the key observations have been the decay of catalytic activity with the decay of the radioactivity and the lack of any comparable effect of external radiation of similar quality and intensity. The first of these is... 

Considering the fact, that the procedure of washing and repacking could be completed in about 1 h from the overall average decay of catalytic activity indicated, by the broken line a catalytic half-life of could be determined. 

A low decay of catalytic activity was observed for a catalyst sample after successive runs. To make sure that the activity level was the same for tests under different experimental conditions the catalyst sample was changed, following the procedure detailed in [5]. 

It is of interest to note that the catalytic activity (1 — y) decays exponentially with time according to Eq. 5.77 if the bulk-fluid concentrations are constant. In well-mixed fluidized-beds, concentrations can be considered to be uniform everywhere, approaching CSTR behavior. This exponential decay of catalytic activity has been found (Weekman 1970) satisfactory for the simulation of a fluidized-bed catalytic cracker. 

Finally, experimental procedures differing from that described in the preceding examples could also be employed for studying catalytic reactions by means of heat-flow calorimetry. In order to assess, at least qualitatively, but rapidly, the decay of the activity of a catalyst in the course of its action, the reaction mixture could be, for instance, either diluted in a carrier gas and fed continuously to the catalyst placed in the calorimeter, or injected as successive slugs in the stream of carrier gas. Calorimetric and kinetic data could therefore be recorded simultaneously, at least in favorable cases, by using flow or pulse reactors equipped with heat-flow calorimeters in place of the usual furnaces. 

As previous studies had suggested that the selective oxidation of ethane might occur through the formation and further reaction of ethoxide, it seemed useful to investigate the effects of these molybdate catalysts in the decomposition of ethanol. The decomposition of ethanol at 603 K yielded acetaldehyde (64-69%), ethane (25-26%), ethylene (3-5%) and small amounts of methane and CO. A decay in catalytic activity was observed for all catalysts. At the steady state, neither the activity nor the selectivity differed significantly for these molybdates. 

MAS NMR experiments characterizing catalysts in reaction environments in flow systems may be carried out under conditions close to those of industrial processes. The formation of catalytically active surface species and the cause of the deactivation of catalysts can be characterized best under flow conditions. When flow techniques are used for the investigation of reactions under steady-state conditions, a continuous formation and transformation of intermediates occurs. The lifetime of the species under study must be of the order of the length of the free-induction decay, which is ca. 100 ms for " C MAS NMR spectroscopy. 

Treatment of TPPFe(III)Cl or raeso-tetra-o-tolylporphinato-iron(III) chloride [TTPFe(III)Cl (10)] with iodosylbenzene caused rapid oxidation of the porphyrin and loss of catalytic activity for hydrocarbon oxidation. Figure 1 shows changes in the visible absorption spectrum upon treatment of 10 with iodosylbenzene. These data indicate that shortly after the addition of iodosylbenzene (Scan b, Figure 1) a new porphyrin species (11) is formed, which then rapidly decays to oxidized porphyrin products. The kinetics of this decay process are approximately first order (Figure 2). 

Bartholomew and co-workers also measured the loss of catalytic activity with time of Ni and Co bimetallics (157, 194), Ni-molybdenum oxide (23, 113), and borided Ni and Co catalysts (161) during methanation in the presence of 10 ppm H2S. Typical activity versus time plots are shown in Figs. 25 and 26. Activity is defined as the ratio of the mass-based rate of methane production at any time t divided by the initial rate. The activitytime curves are generally characteristic of exponential decay some catalysts decay more slowly than others, but all catalysts suffer at least two orders of magnitude loss in activity within a period of 100-150 hr. Accordingly, it does not appear that other metals or metal oxides in conjunction with Ni significantly change the sulfur tolerance defined in terms of steady-state activity of Ni. These materials can, however, influence the rate at which the... 

Rate decay is mainly ascribed to a chemical deactivation of active centers. Nevertheless, in the case of ethylene, it appears that diffusive phenomena play also a certain role in the drop of the polymerization rate88 94. Moreover, diffusivity of monomer in the reaction medium may restrict polymerization rate, as can be concluded from the dependence of catalytic activity on catalyst concentration 95... 

We will now consider three reaction systems that can be used to handle systems with decaying catalyst. We will classify these systems as those having slow, moderate, and rapid losses of catalytic activity. To offset the decline in chemical reactivity of decaying catalysts in continuous-flow reactors, the following three methods are commonly used ... 

To the extent to which it can be accomplished, the attribution of the induced catalytic activity to a particular defect is straightforward. ESR signals are available as markers for many of the defects in magnesium oxide, and one of these was found to be formed in parallel with the introduction of catalytic activity by ultraviolet, and to decay in parallel with the activity at 0° and at 30°. Since the annealing experiments did not suggest the presence of more than one catalytic site, there is no need to look farther than the one center already tied to the activity. 

The different smface area could reasonably be attributed to the presence of radioactivity during the precipitation. Colloid-chemical effects of this sort are well documented (171), and are explicable in terms of radiolysis of the solvent with consequent modification of conditions at the surface of the growing crystallites, or perhaps by nucleation. The decay in catalytic activity of both kinds of catalyst was followed as long as 227 days. No explanation was advanced for the decay, but it is, of course, not uncommon in catalysis. 

The decay of catalytic performance in the Beckmann rearrangement with time on stream is a major and common problem associated with all types of catalyst. Two main reasons are suggested for the deterioration in activity-coke formation and irreversible adsorption of basic products. 

One of the most important parameters of an immobilized-carrier complex is stability of its activity. Catalytic activity of the complex diminishes with time because of leakage, desorption, deactivation, and the like. The half-life of the complex is often used to describe the activity stabihty. Even though there may be frequent exceptions, hn-ear decay is often assumed in treating the kinetics of activity decay of an immobilized complex. 

Various investigators have tried to obtain information concerning the reaction mechanism from kinetic studies. However, as is often the case in catalytic studies, the reproducibility of the kinetic measurements proved to be poor. A poor reproducibility can be caused by many factors, including sensitivity of the catalyst to traces of poisons in the reactants and dependence of the catalytic activity on storage conditions, activation procedures, and previous experimental use. Moreover, the activity of the catalyst may not be constant in time because of an induction period or of catalyst decay. Hence, it is often impossible to obtain a catalyst with a constant, reproducible activity and, therefore, kinetic data must be evaluated carefully. 

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