The Deactivation Process

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

The use of aprotic. (and therefore totally unbuffered media in analytical studies of sulphones may lead to wrong conclusions. The voltammetric determination of the benzyl phenyl sulphone26 was aimed to clearly exemplify the deactivation process in dry dimethylformamide. So, in aprotic or low acidity solvents, two main steps (equivalent to the transfer of one electron each) can be seen (Figure 6, curve 1). These two steps were shown at the potential of the cleavage (— 1.7 volt) to correspond, for acidic sulphones, to the following processes ... 

If the activity of the catalyst is slowly modified by chemisorption of materials that are not easily removed, the deactivation process is termed poisoning. It is usually caused by preferential adsorption of small quantities of impurities (poisons) present in the feedstream. Adsorption of extremely small amounts of the poison (a small fraction of a monolayer) is often sufficient to cause very large losses in catalytic activity. The bonds linking the catalyst and poison are often abnormally strong and highly specific. Consequently, the process is often irreversible. If the process is reversible, a change in the temperature or the composition of the gas to which it is exposed may be sufficient to restore catalyst... 

The catalytic cycle with Ni catalysts is generally similar. The essential difference is the deactivation process, which in this case occurs not via the formation of a precipitate of Ni°, but rather due to interception of the highly reactive Ni° species by any fortuitous oxidant, such as oxygen. As Ni11 is not so easily reduced to Ni° as Pdn is to Pd°, Ni-catalyzed systems often require the addition of a stoichiometric reducing agent (Zn, DIBAL-H, other hydride transfer agents, BuLi, etc.). 

During the process described in problem 12-17, it was discovered that the enzyme used to produce gluconic acid was subject to deactivation, with a half-life of 12 days. It appears that the deactivation process is first-order, such that decreases exponentially with time. 

Radiationless transitions (Chapter 5), where no emission of electromagnetic radiation accompanies the deactivation process. 

Consider the deactivation processes of the excited Si state in the presence of a quencher, Q ... 

The results for PtSn-BM and PtSn-OM catalysts (Figure 6.15) indicate that the addition of tin substantially improves their stability, almost inhibiting the deactivation processes. In the case of PtSn-OM, no deactivation is observed and only a slight loss in the conversion level is observed in the case of PtSn-BM. Nevertheless, in the latter case, catalyst regeneration in air at 773 K allows the original catalytic phase to be obtained, since it recovers its initial activity and selectivity. 

The deactivation process of enzymes can be classified generally into first-order and non-first-order processes. The first-order model is often sufficient for describing the deactivation, especially in case of highly stable enzymes in microemulsions, like lipases. For a lot of enzymes the activity decay in microemulsions has to be fitted to the deactivation model which involves an active intermediate in order to describe the deactivation processes qualitatively [34,100-102] ... 

The independence of the fluorescence yield of temperature and pressure cannot be explained in terms of competing deactivation and emission processes, although the importance of the deactivation process is demonstrated by the fall in the yield of decomposition products with increase in pressure. 

The limit at C is almost independent of the surface of the vessel thus in accordance with the considerations of the earlier section on critical limits, it represents the point at which some deactivation process in the gas phase becomes marked enough to prevent branching chains from developing explosively. It is suggested that the deactivation process in question is the mutual destruction of H202 molecules, which between B and C are the centres from which explosive chains develop, e. g. 

Palmer and Padrick (790) have determined rate constants kl30 < 5 x 10"15 and k13, = 2x 10-l3cm3 molec-1 sec-1, that is, the deactivation process is more important than the chemical reaction. Mains and Lewis (659) have measured the quantum yield of methane production in low and high intensity photolysis of CH3I in the near ultraviolet. The quantum yield of CH4 is a function of pressure and ranges from 0.1 to 0.001 in the high intensity photolysis and from 0.02 to 0.05 in the low intensity photolysis. 

Breckenridge and Taubc (144) have studied the photolysis of OCS + CS2 and OCS + N20 mixtures at 2288 and 2537 A. They have demonstrated that the primary yield of production ofS( D) [process (Vl-25a)] is 0.74 0.04 and 0.25 for S(3P) production [process (Vl-25b)] in agreement with the results of Gunning and Strausz (430). The deactivation process [process (VI-28)] must be about one third that of the total reaction of S( >) with OCS in order to be consistent with their finding that 50"- of the S atoms formed in the primary process react as S(3P)(144). 

In the course of the reaction, all the catalysts deactivated by deposition of coke. Several deactivation laws were fitted with the experimental data. The Voohries law, r = r t n, often claimed to represent ageing of acid catalysts did not apply. The best fit for the rate law of the deactivation process was obtained with -dr/dt = kd>r°<, with o(= 1+0.2 depending on the catalyst. Therefore a first order deactivation rate applies which takes the integral form r = rQ expt-k. t). In most cases, correlation coefficients better than 0.9 were obtained when determining kd (refs.5,15). 

Assuming that the deactivation process is much slower than the reaction represented by Section 5.4.4, scheme 1, and that enzyme E will deactivate faster than the enzyme in the bound state (i.e. ES complex), then equation 5.32 may be written as ... 

Some predictions beyond the theory of electrocatalysis for pure metals seem indeed possible. It is, however, necessary to stress again that the applicability of a cathode depends on the impact of many factors, the most outstanding ones being the intrinsic stability and the resistance to poisoning. This is probably still the weak point of cathodes. Their life-time appears to be lower than for anodes, although the deactivation process for cathodes is slower and less abrupt than for anodes. 

The validity of a first-order decay law over time for the activity of enzymes according to Eq. (2.19), with [E]t and [E]0 as the active enzyme concentration at time t or 0, respectively, and kd as the deactivation rate constant, is based on the suitability of thinking of the deactivation process of enzymes in terms of Boltzmann statistics. These statistics cause a certain number of active protein molecules to deactivate momentarily with a rate constant proportional to the amount of active protein [ for evidence for such a catastrophic decomposition, see Craig (1996)]. 

It is important to understand that equations such as Eqs. (5.74) or (5.77) do not provide any mechanistic detail about the deactivation process. 

Thus the measured unimolecular radiative lifetime is the reciprocal of the sum of the unimolecular rate constants for all the deactivation processes. The general form of the equation is given by... 

Turning to the basic kinetic aspects of the deactivation process, the equation describing the time-dependence of the excited species population, noted M (/), is ... 

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