Mechanism catalyst sintering

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. 

In our mechanism, coke formation is due to the presence of olefins, which occur as intermediate species during the reforming reactions. As discussed in Section II, these olefins can go either to products or to coke precursors. The deactivation caused by feed poison, catalyst sintering during regeneration, or improper regeneration techniques is not considered in this development. 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

On the other hand, rate constants for 0.6 and 5% Pt/alumina catalysts sintered in H2 at 973 K (see Table 1) of 0.53 and 0.84 h 1 are not substantially different. This result is not altogether unreasonable, as the number of crystallites per unit area of support surface and the metal surface area would be about the same in both 0.6 and 5% catalysts because of the much lower dispersion of the 5% catalyst. Nevertheless, it is fascinating that these two catalysts sinter at much different relative rates in air (see discussion above), a fact suggesting that different mechanisms (i.e., atomic migration vs. crystallite migration) may be involved in air versus H2 atmospheres as proposed by Wynblatt and Ahn [5J. 

Effect of Oxygen on the Platinum Functions. On the basis of the mechanism of sintering in a non-hydrogen atmosphere (2) and in order to protect the platinum functions of the catalysts, we treated them at 482 494°C with different oxygen concentration from 0 to 21% in N2 for 4 8 hrs. The results are shown in Table III. 

It is clear from the above discussion that despite the importance of alumina as a catalyst support, the detailed mechanism of sintering is far from clear. There is good understanding of the effect of different factors on sintering, but the detailed ntechanlsra of phase transformations is, in many cases, open to considerable uncertainty. 

There are four principal ways in which catalysts undergo deactivation (1) poisoning, (2) fouling, (3) sintering, and (4) volatilization. Mechanistically these processes can be classified as chemical, mechanical, or thermal. These mechanisms of catalyst deactivation are described and discussed in detail in several recent reviews and books. This review focuses on some impOTtant scientific facets of one of these important mechanisms, namely sintering. 

One of the principal objectives of model catalyst studies is to relate observed changes in the structure, con sition, and size of single crystallites or of small collections of crystallites to mechanisms of sintering and redispersion. It may be possible under favcaable circumstances to observe direcdy mechanistic processes such as crystallite migration and spreading. In this section, mechanistic evidence frtan model catalysts studies is Hcsented and discussed. 

This type of deactivation mechanism often applies catalyst sintering and coke deactivation. The deactivation rate constant is expected to have an Arrhenius dependence on temperature. 

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. 

A formal derivation of steady state particle size distributions during catalyst sintering has not been made before. Their existence adds validity to the use of steady state dispersions during the kinetic analysis of sintering data. At the same time, this provides an interesting perspective for reanalyzing the present ideas about sintering mechanisms. 

The mechanism for sintering is migration and coalescence of nickel particles on the carrier surface, leading to a smaller surface area. Sintering is a complex process influenced by several parameters including chemical environment, catalyst structure... 

An idealized platelet picture of catalyst structure as related to possible mechanisms for sintering in vacuum and in steam is presented in Fig. 10. Extreme cases of sintering are shown schematically. Structure representations of this type may aid in the ultimate interpretation of sintering... 

The hot gas recycle process made it necessary to use iron catalysts of adequate mechanical strength. Sintered catalysts showed better resistance against the erosive influence of fast moving gases than highly active precipitation catalysts. 

The deactivation of bulk iron oxide during methane combustion has been studied. The observed deactivation behaviour has been explained as the result of two simultaneous deactivation mechanisms. In the initial phase of reaction both are in operation and the activity drops rapidly as a consequence of both catalyst sintering and of the depletion of lattice oxygen in the outer layers, due to a partial reduction of the catalytic surface. In later stages, catalyst deactivation is almost exclusively due to sintering imder reaction conditions. A kinetic model of deactivation is presented, together with the physicochemical characterization of fresh and partially deactivated catalysts. 

Mechanism of sintering-redispersion of supported platinum catalysts can be found in several articles (38,145,147). 

A nttmber of degradation mechanisms have been proposed for PBI-based MEAs, such as phosphoric acid loss from the membrane, faster catalyst dissolution in the hot acid meditrm, Pt catalyst sintering, thermal stress on fuel cell parts, thermal degradation of the catalyst support and carbon support corrosion. In particttlar, phosphoric acid loss has been specrrlated as a major degradation... 

In short, several mechanisms, not just a single mechanism, are responsible for the overall deactivation of Co catalysts. At the molecular level, normally, DFT has rarely worked on the problems of catalyst sintering and catalyst fouling by wax or attrition. Therefore, in the following section, we review DFT calculations about the first three deactivation mechanisms of FTS over Co catalysts. 

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. 

Prepare a solution of 41 g. of anhydrous palladium chloride (1) in 10 ml. of concentrated hydrochloric acid and 25 ml. of water (as in A). Add all at once 60 ml. of 6iV-sulphuric acid to a rapidly stirred, hot (80°) solution of 63 1 g. of A.R. crystallised barium hydroxide in 600 ml. of water contained in a 2-htre beaker. Add more 6iV-sulphuric acid to render the suspension just acid to htmus (5). Introduce the palladium chloride solution and 4 ml. of 37 per cent, formaldehyde solution into the hot mechanically stirred suspension of barium sulphate. Render the suspension slightly alkaline with 30 per cent, sodium hydroxide solution, continue the stirring for 5 minutes longer, and allow the catalyst to settle. Decant the clear supernatant hquid, replace it by water and resuspend the catalyst. Wash the catalyst by decantation 8-10 times and then collect it on a medium - porosity sintered glass funnel, wash it with five 25 ml. portions of water and suck as dry as possible. Dry the funnel and contents at 80°, powder the catalyst (48 g.), and store it in a tightly stoppered bottle. 

---