Deactivation mechanism

The activity of catalyst degrades with time. The loss of activity is primarily due to impurities in the FCC feed, such as nickel, vanadium, and sodium, and to thermal and hydrothermal deactivation mechanisms. To maintain the desired activity, fresh catalyst is continually added to the unit. Fresh catalyst is stored in a fresh catalyst hopper and, in most units, is added automatically to the regenerator via a catalyst loader. 

Finally, if the phosphorylation of myosin is the activation mechanism, then dephosphorylation is likely to be the deactivation mechanism, and so it seems. However, there are several myosin phosphatases in smooth muscle cells and they have some range of substrate specificities. Thus, there are several possible candidates for a regulatory role. 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

Mennucci B, Toniolo A, Tomasi J (2001) Theoretical study of the photophysics of adenine in solution tautomerism, deactivation mechanisms, and comparison with the 2-aminopurine fluorescent isomer. J Phys Chem A 105 4749... 

Gepshtein R, Huppert D, Agmon N (2006) Deactivation mechanism of the green fluorescent chromophore. J Phys Chem B 110 4434-4442... 

It may also happen that an association equilibrium exists between the luminescent indicator and the quencher. Non-associated indicator molecules will be quenched by a dynamic process however, the paired indicator dye will be instantaneously deactivated after absorption of light (static quenching). Equation 2 still holds provided static quenching is the only luminescence deactivation mechanism (i.e. no simultaneous dynamic quenching occurs) but, in this case, Ksv equals their association constant (Kas). However, if both mechanisms operate simultaneously (a common situation), the Stem-Volmer equation adopts more complicated forms, depending on the stoichiometry of the fluorophore quencher adduct, the occurrence of different complexes, and their different association constants. For instance, if the adduct has a 1 1 composition (the simplest case), the Stem-Volmer equation is given by equation 3 ... 

Figure 5.15. Deactivation Mechanism of Rh -TPPTS Catalyst (Ar = S-CytLtSOsNa)...

The purpose of this review is to integrate the literature on this topic, along with some of the work we have performed, to provide a clearer understanding on the role of carbon as a deactivation mechanism. The minimization of carbon by promotion, regeneration of catalysts, and some selectivity implications will also be briefly discussed. 

Carbon formation/deposition is a difficult deactivation mechanism to characterize on cobalt-based FTS catalysts. This is due to the low quantities of carbon that are responsible for the deactivation (<0.5 m%) coupled with the presence of wax that is produced during FTS. Furthermore, carbon is only detrimental to the FT performance if it is bound irreversibly to an active site or interacts electronically with it. Hence, not all carbon detected will be responsible for deactivation, especially if the carbon is located on the support. 

Moodley, D. J., van de Loosdrecht, J., Saib, A. M., Overett, M. J., Datye, A. K., and Niemantsverdriet, J. W. 2009. Carbon deposition as a deactivation mechanism of cobalt-based Fischer-Tropsch synthesis catalysts under reahstic conditions. Appl. Catal. A, 354 102-10. 

Schanke, D., Hilmen, A.M., Bergene, E., Kinnari, K., Rytter, E., Adnanes, E., and Holmen, A. 1995. Study of the deactivation mechanism of Al203-supported cobalt Fischer-Trospch catalysts. Catal. Lett. 34 269-84. 

There has been little insight into potential decomposition pathways for the Ni(II) system due to sparse experimental evidence. Polymerization results with catalysts bearing different alkyl and fluorinated substituents have suggested that a C-H activation process analogous to that occuring with the Pd(II) catalysts is unlikely with Ni(TT) [28], Instead, side reactions between Ni and the aluminum coactivator, present as it is in such large excess, have been implicated. The formation of nickel dialkyl species and their subsequent reductive elimination to Ni(0) is one possible deactivation mechanism [68]. 

Considerably more work is needed to clarify the passivation mechanism for deep levels. No information is yet available on the susceptibility of these levels to passivation by other species such as the alkali metal ions Na+, Li+, K+ or species like F. Such experiments may shed more light on the deactivation mechanism. DeLeo et al. (1984) have predicted that alkali metal impurities will not passivate vacancy dangling bonds. This is experimentally testable in a relatively straightforward fashion—both Li and F can be introduced into Si at high concentration by a number of methods. 

P. J. van Berge, J. van de Loosdrecht, S. Barradas and A. M. van der Kraan, Oxidation of cobalt based Fischer-Tropsch catalysts as a deactivation mechanism, Catal. Today, 2000, 58, 321-334. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

A final issue that faces this class of catalysts is stability in the fuel cell environment. Deactivation of materials in a fuel cell environment has been shown to be minimal in some studies,31,137 and severe in others.128,142 More active catalysts seem more susceptible to deactivation. Deactivation has been linked to the formation of peroxide and the loss of metal from the catalyst.128 On the other hand, demetallization has also been observed in pyrolyzed samples that did not lose activity with time.84 Another possible mode of deactivation could be due to the oxidation of the carbon surface. However, it seems reasonable that a complete understanding of the deactivation mechanism would first require a well-developed understanding of the active site. 

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), 24 111-114 Attenuation, 77 132-133 Attenuation length (AL), 24 87-89 Attrition, catalyst deactivation mechanism, 5 256t... 

Solid soil detergency, 8 423-424, 428-433 Solid-solid reactions, catalyst deactivation mechanism, 5 256t, 278-280 Solid-solid sedimentation, 22 50 Solid solutions... 

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

For a zeohtic catalyst where Pt, Pd or other transition metal might be present to provide metal activity, STEM can be used to determine whether the metal is agglomerated and to what extent, whether the metal is in the zeolite or present on the geometric exterior or whether the metal is associated with the zeolite or binder. As an example of the utility of the technique. Figure 4.15 shows the growth of Pt clusters for fresh and spent faujasite zeolite catalyst. After time under reaction conditions, the Pt clusters have grown from Inm to 2nm. The clusters have remained in the channels of the faujasite. Pt agglomeration can be concluded as the deactivation mechanism. 

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

Three series of LaCoi. CuxOs, LaMni.xCuxOs, LaFei x(Cu, Pd)x03 perovskites prepared by reactive grinding were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O2, NO + O2, and CsHg in the absence or presence of H2O, Fourier transform infrared (FTIR) spectroscopy as well as activity evaluations without or with 10% steam in the feed. This research was carried out with the objective to investigate the water vapor effect on the catalytic behavior of the tested perovskites. An attempt to propose a steam deactivation mechanism and to correlate the water resistance of perovskites with their properties has also been done. 

Scheme 1. Deactivation mechanism in the case of (a) dense agglomerates and (b) looser agglomerates [18].

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