Catalyst sintering rate

From data analyzed using second-order GPLE kinetics it is possible for the first time to quantitatively correlate effects of sintering conditions and catalyst properties on catalyst sintering rates. 

A variation of eq. (5.453) has been proposed by Fuentes et al. (1991) for quantitatively determining the effect of temperature, time, and atmosphere on the sintering rate of supported metal catalysts ... 

Nevertheless efforts to understand, treat and model sintering/thermal-deactivation phenomena are easily justified. Indeed deactivation considerations greatly influence research development, design and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover it may be possible under selected circumstances to reverse the sintering process through redispersion (the increase in catalytic surface area due to crystallite division or vapor transport followed by redeposition). 

Studies of sintering and redispersion of supported metal catalysts have been reviewed by several authors [M8] most of these reviews focus on early kinetic studies of sintering of supported metal catalysts using a simplified power law expression (SPLE). Unfortunately this crude approach does not permit sintering kinetics to be presented in a consistent way nor does it enable (1) useful extrapolation of the data to other conditions (2) useful quantitative comparisons between different studies, or (3) physically meaningful kinetic parameters to be obtained. The ultimate result has been confusion regarding the effects of reaction parameters such as atmosphere and temperature and of catalyst properties such as support promoters, etc., on sintering rates. 

From previous experimental studies of sintering [2,9 11 12] it is evident that sintering and redispersion are strong functions of temperature time atmosphere and support. Sintering/redispersion rates are also significantly affected by choice of metal and/or promoter metal loading, and catalyst preparation. The discussion below of previous work will focus on how sintering rates are affected by these variables. 

Figure 4. Effects of hydrogen and oxygen atmospheres and of metal loading on sintering rates of 0.6% and 5% Pt/alumina catalysts [28,331.

Second Order Sintering Rate Constants and Activation Energies for Pt Catalysts... 

The first or second order rate constant ks is in principle a kinetic parameter that provides a direct quantitative measure of sintering rate and is a function only of temperature. However, it is clear from first principles that rate constants for different experiments or catalysts can be compared with validity only for the same value of m or sintering order. Moreover, it follows from a careful analysis of the data of this study that rate constants of the... 

A similar comparison of rate constants for 0.6 and 5% Pt/alumina and 1,8% Ag/alumina catalysts at 673 K in oxygen atmosphere reveals that sintering rates for the Ag catalyst are roughly 40-50 times higher than for either Pt catalyst. Thus, Pt is clearly much more thermally stable than Ag under oxidizing conditions. These results are consistent with those from a model catalyst study [44] of sintering of Pt and Ag on alumina in vacuum in which it was observed that Pt/alumina was thermally stable in vacuum to about 873 K, above which temperature liquid-like particle migration was observed, while Ag/alumina was stable to only 723 K, above which temperature evaporation of the metal was observed. This latter result is... 

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. 

Sintering is an important mode of deactivation in supported metals that involves complex microscopic physical and chemical phenomena, e.g., dissociation, emission, diffusion, and capture of metal atoms and crystallites. The relative importance of these different processes may change with reaction conditions and catalyst formulation. Modeling and prevention of sintering processes require an understanding of these basic processes as well as quantitative measurements of sintering rates. 

Table 3 Second-order sintering rate constants, normalized dispersions and activation energies for Ni/alumina catalysts ...

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. 

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