Chemical deactivation

Resistance to antimicrobial agents is of concern as it is well known that bacterial resistance to antibiotics can develop. Many bacteria already derive some nonspecific resistance to biocides through morphological features such as thek cell wall. Bacterial populations present as part of a biofilm have achieved additional resistance owkig to the more complex and thicker nature of the biofilm. A system contaminated with a biofilm population can requke several orders of magnitude more chlorine to achieve control than unassociated bacteria of the same species. A second type of resistance is attributed to chemical deactivation of the biocide. This deactivation resistance to the strong oxidising biocides probably will not occur (27). 

Another additive used is a metal deactivator to chemically deactivate any catalytic metals such as copper accidentally dissolved in the fuel from metal surfaces. Uless they are chemically deactivated, dissolved metals cause the loss of good stability quality. 

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

Chemical deactivation. In chemical deactivation the active surface area changes by strong chemisorption of impurities in the feed, by blocking of active sites by heavy products formed in parallel or sequential reactions, etc. The most important chemical causes of deactivation are poisoning by impurities in the feed and deposition of carbonaceous material, usually referred to as coke . 

Feibush, B., A new chemically deactivated silica-based reversed phase/ion exchange support, /. Liq. Chromatogr. Relat. Technol, 19(14), 2315, 1996. 

When the air flow was temporarily substituted by a nitrogen flow for 15-20 minutes in the reaction represented by Figure 5a, the rate of alcohol oxidation did not increase. These experiments also prove that the reason of catalyst deactivation is not the over-oxidation of Pt° active sites, but a partial coverage of active sites by impurities (chemical deactivation). 

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

Photochemistry is the branch of chemistry that deals with the causes and courses of chemical deactivation processes of electronically excited particles, usually with the participation of ultraviolet, visible, or near-infrared radiation [1]. The photochemist is interested in both the modes of excited-state formation processes (direct photoexcitation, energy transfer, etc.) and the deactivation pathways of excited atoms, molecules, and ions. 

Current photochemical research is strongly linked with the study of photophysical behavior of excited particles. Data on photophysical processes (such as luminescence, internal conversion, intersystem crossing, intramolecular energy dissipation) assist photochemists in the identification and interpretation of chemical deactivation modes. Most of the data related to the elementary steps within deactivation of excited particles have been obtained by fast flash techniques in nano-, pico-, and femtosecond time domains. Photophysics is, in general, as rich a branch of science as photochemistry, and both the parts of excited-state research deserve comparable attention and extent. In the present review, some results on photophysics will be mentioned where suitable and necessary. We will restrict our discussion, however, predominantly to photochemical behavior of metallotetrapyrroles. 

Photochemists who rely more on experimental data and proofs than on hypotheses know that there are still blank areas in solving the most general problem of photochemistry interrelationships between the nature of the photo-reactive excited state of a molecule and the pathways and efficiency of its chemical deactivations [29]. The categorization of photoreactions of metal-lotetrapyrroles, presented in this article, stems from available published data, and the opinions and deductions of their authors. 

It should be pointed out that the nature of the primary photochemical step(s) is still obscured and can depend, even for the same complex, on experimental conditions. Thus, Fen,(Por)N3 converts under irradiation in the solid state at low temperatures [162,163] into FevN(Por) in some solution systems [133] the formation of azidoradicals N3 has been detected by spin-trapping EPR no information on the heterolytic splitting of the Fe-N3 bond yielding NJ anion has been described in the literature (for azido complexes of some other central atoms the photosubstitution of the coordinated N3 ligand is a dominant chemical deactivation mode [1]). In addition, at particular conditions, the... 

The analysis system consisted of a Shimsdzu QC-6A gas chromatograph, a chemically deactivated four-way valve for solvent ventilation, a heated transfer tube interface, a Beenakker-type TM0i0 microwave resonance cavity, and an Ebert-type monochromator (0.5m focal length). 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

At low coke content, pore plugging is still negligible and decay is mainly due to site coverage. Consequently, the variation of deactivation function with coke content is only due to chemical deactivation, and it is proportional to the remaining activity ... 

Figure 2. Chemical deactivation versus initial Figure 3 Effectiveness factor ratio versus coke content (legends in fig, L) initial coke content (legends in fig. 1)...

Chemical deactivation of the LC excited states sometimes leads to ligand isomerization, as in the case of /ac-[Re(CO)3(NN)(irans,-L)]+ (NN = polypyridyl, L = stilbene), when trans-stilbene is transformed into e/lv-stilbene [26],... 

Biologically hazardous material must always be inactivated before it can be disposed of, or even removed from the production facility. This will normally require a validated heat or chemical deactivation system for aque-... 

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. 

Water/soap wash Chemical warfare agents have a generally low solubility and slow rate of diffusion in both fresh water and seawater. Therefore, the major effect of water and water combined with soap (especially alkaline soaps) is via a slow breakdown of the compound (i.e., hydrolysis) or through dilution of the agent and the mechanical force of the wash. When other chemical deactivation means are not available, washing with water or soap and water is a good alternative. 

Chemical solutions In the event of an emergency you may be directed to perform decontamination with other chemical deactivation agents. These vary depending on the chemical warfare agent and may include alkaline solutions of hypochlorite. 

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... 

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

From the above results, it is clear that the rate decay must be attributed to a chemical deactivation of the polymerization centers with time. Different mathematical expressions have been proposed, for those catalyst systems most widely studied in the literature, in order to express the law of the decay. For propylene polymerization with TiCyMgCl2—AlEt3/EB or with TiCyEB/MgCl2 - Al Et3/EB, Spitz 45-97) proposed an expression of the following type ... 

Most of the emphasis of this chapter is on the mixed-oxide solid solution oxygen storage materials that comprise the advanced catalyst formulations in use today and are still under development. In particular, we focus on their durability, both with respect to thermal and chemical deactivation, while also briefly reviewing special uses of these and other oxygen storage materials in automotive applications. 

Also the activity decay with time of many catalytic systems may be attributed to their incapsulation by the polymer and to the consequent monomer diffusion hindrance. On the other hand, the activity decay can also be explained on the basis of chemical deactivation of active centres Other well known phenomena, listed below, connected to Ziegler-Natta polymerization, provide further experimental evidences which could agree with the two last models proposed ... 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

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