Mechanism of Deactivation

Analysis of deactivated samples of a Pt/AbOa catalyst by scanning electron microscopy and energy dispersive X-ray analysis indicates that silicon is dispersed across the metal surfaces rather than on the alumina support material. In another study comparing Pt on different supports, it was impossible to determine whether deposition occurred on the silica or zeolite supports.  

Colin et aV studied the adsorption and decomposition of HMDS in the absence of oxygen on various platinum surfaces at different temperatures at low pressure by XPS, ultraviolet Photoelectron Spectroscopy (UPS), and Thermal Diffuse Scattering (TDS). They also used Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) as surface characterization tools. 

The carbon residue was readily removed by an oxygen treatment at moderate temperature. Almost no silicon or oxygen remained on the surface above 300K. This result cannot explain the poisoning effect of HMDS on platinum sensors used to detect methane in coal mine and other applications. It shows that the mechanism of decomposition of HMDS in the absence of oxygen and at the low 

CH2ads CHads t Hods Hads + Hads H2g CHsads + Hads CH4g 

Further dehydrogenation reactions, amorphous carbon and graphitization, 

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The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

The concentration of atoms in the excited state is measured by monitoring their spectral deactivation to the ground state. The radiant power of this mechanism of deactivation is given by ... 

In the preceding sections, we have covered the kinetic behavior of enzyme deactivation. We now discuss the molecular basis for enzyme deactivation. Alexander Klibanov s group at MIT (Cambridge/MA, USA) analyzed the mechanisms of deactivation for ribonuclease A. They drew the conclusion that an enzyme can deactivate irreversibly for two kinds of reasons (Ahern, 1985, 1988) ... 

Furthermore, the organometallic compounds (of which nickel and vanadium are the principal constituents) that are present to varying degrees in all residua and in the majority of heavy oils cause catalyst deterioration. Deposition of these metals in any form on to the catalyst leads to catalyst deactivation but the exact mechanism of deactivation is still subject to speculation. Nickel tends to be deposited throughout the catalyst whereas vanadium is usually more concentrated in the outer layers of the catalyst. In either case, catalyst deactivation is certain whether it be by physical blockage of the pores or destruction of reactive sites. 

Figure 16.5 Illustration of biocidal nanotubes absorbing onto and penetrating the wall of an . co// cell (A), and completely covering the surfaces of several E. coli cells (B). E. coli cells were used as model biological weapon agents. The mechanism of deactivation is not yet completely understood. From Lee eta/., [976], Copyright 2004, American Chemical Society.

A possible mechanism of deactivation is change in the oxidation state of Mb as induced by sulfur deposition. 

Coke formation is a possible mechanism of deactivation in the runs with feed C. Although carbon balances were within 5% for both B and C, even small amounts of coke can lead to pore plugging and loss of activity. Air calcination of the chromia/alumina after use for 1,1-dichloroethylene oxidation, in separate experiemts under similar conditions, produced observable amounts of C02, probably originating from carbon on the surface [7]. 

The value of E0 is probably a little high if coking were the only mechanism of deactivation, thus it probably reflects sintering or a solid state transformation (with water vapor ) as being an additional important factor. 

The mechanism of deactivation of supported Wacker catalysts in the oxidation of 1-butene... 

Evaluation of the suitability was carried out by investigating the catalysts and catalytic performances, putting emphasis on the deactivation behavior. The present paper reports on the properties of titania supported iron oxide catalysts. This system was chosen to extend the concept to other supports, since a priori titania appeared to be a suitable material. Although textural properties were satisfactory, the strong interaction of titania with the applied components resulted in a rapid deactivation. However, the mechanism of deactivation was completely different from systems investigated earlier. 

We have shown that the changes in the shape selectivity can be explained by changes in diffusivity by using ZSM-5 (MFI type) and Y type zeolites as model zeolites. However, it is very difficult to derive the model equations for representing the deactivation mechanisms for every types of zeolites, since each type of zeolite has different pore structure Hence, the mechanism of deactivation should be clarified for each type of zeolites. Reports on the activity of zeolites which were determined experimentally are omitted here. However, it is still impossible to evaluate physicochemical properties of a catalyst from the spectrum of ammonia TPD, which is usually employed to evaluate the acidic properties of a catalyst, since the spectrum is affected by various factors. Therefore, it is difficult to obtain the exact relationship between acidic properties and the change in activity due to deactivation. However, if an accurate method to evaluate the acidic properties is developed, it is expected that we can clarify whether the coverage of acid sites or pore blockage is the dominant factor of decrease in the activity due to coke deposition. 

Nevertheless, it is possible that such analytical fittings of the catalyst decay curve are too oversimplified to take into account the complexity of the phenomena which take place during polymerization. On the other hand, the kinetic studies are only able to measure the average constants of the reaction and not those for each individual species. Thus, although the mechanism of deactivation of the active centers, or part thereof, has clearly been shown to be of a chemical nature, it can only be explained in hypothetical terms. In agreement with the 2nd order decay law they had proposed, Keii and Doi98 99) speculated on a bimolecular disproportionation of the active species with a consequent reduction of Ti3+ to Ti2 1 due to the action of the cocatalyst. 

Coke fonnadon though not diiecdy observed, cannot be ruled out as a mechanism of deactivation in the oxidation of stream B on this catalyst. Although the catbon balance was within i 5 % for both streams A and B, the retention of even a small amount of coke on the catalyst surface can cause pom plugging In the long run and hciKC a loss in activity. For example, Ramanathan and Spivey observed that an aJr stream passed over this same chromia-alumina catalyst after being used for the oxidation of IJ-dichloroelhylcnc under similar reaction conditions contained cartxm dioxide, probably formed by the oxidation of carbon on the catalyst surface. 

A second mechanism for removing neurotransmitters from the synapse is called rcuptake. Neurotransmitters arc taken back up into the terminal button after they have been released—hence the term reuptake. This is an economical mechanism of deactivating transmitters because the ncurotransmitter molecule is preserved intact and can be used again without the expense of energ) involved in the manufacture of new transmitters. Some drugs (notably cocaine) exert some of their action by blocking the reuptake process. 

It should be remembered that these thermal waves can be large enough to cause catalyst sintering, thus activating another mechanism of deactivation. Did the observer at the end of the bed see any evidence of deactivation Of course not, because the active reaction zone was actually confined to a small part of the bed, conversion is very good there, and until the thermal wave passes out of the end of the reactor, there is no evidence as to what is going on. Then immediately conversion goes to zero and the unprepared observer may find it necessary to seek some other sort of employment. 

Solid state transformations like sintering or phase changes, which we have only mentioned in this report, remain probably the least well understood of the mechanisms of deactivation. One would hope that the next decade will see advances here comparable to those made with respect to poisoning in the last. 

The kinetic model reproduces satisfectorily experimental results. Deactivation experiments seems to indicate that the mechanism of deactivation changes with the nature of the contaminant used. When a strong poison for active acidic sites like pyridine is used, the catalyst gets totally deactivated when its concentration is over 250 ppm. In this case, the deactivation is fester than with CS2, hut not as fest as an acid base reaction should be. The behavior can be explained assiuning that the pyridine reaction with acidic sites is a diffesion controlled phenomenon enhanced by its molecular size, which is very near to the zeolite pore size. The presence of a mixed mechanism of deactivation and inhibition is also evident. 

The use of composite catalyst fillings is the industrial standard when treating resids in a modem refinery. The grading can be made both with respect to size, shape and catalyst properties in order to optimize the performance, obtain the desired product quality and the longest catalyst life. The optimization is often a careful balancing of catalyst deactivation and performance, and it is therefore important to understand the mechanisms of deactivation. For this reason, a brief introduction to the various aspects of catalyst deactivation will be given what deactivates the catalysts and how is catalyst deactivation generally tied in with catalyst properties ... 

In the HDS mode of operation, the cycle length is longer (typically one year). Guard reactor plug ng and dry sludge formation only tend to occur towards end of run, when the temperature reaches similar levels as applied in MHC mode. Besides metal tolerance, coke formation on the tail end catalyst is a predominant mechanism of deactivation in the HDS mode. 

Formation of cyclic acetals by sugar hydroxyls generally retards nucleophilic displacements at the anomeric centre of the same sugar residue. In the case of 4,6-benzylidene derivatives, the mechanism of deactivation appears to be that the dipole of the C6-06 bond is constrained with its positive end directed towards the anomeric centre.In the case of the 1,2-diketals, the deactivation arises from the increased difficulty of forming half-chair or boat conformations in a six-membered ring, which is part of a traw -fused decalin structure. 

The proposed complex is assumed to be stable over a certain pH range which thus corresponds witii the pH range for maximum selectivity. Due to the competing mechanisms of deactivation by adsorbed acids and over-oxidation, the optimum pH is difficult to ascertain from the given data (a possible future way to determine this value would be to measure the stability of... 

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