Sintering redispersion

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. 

What kinds of experimental data are needed to useffilly describe and model sintering/ redispersion processes What kinds of data are available frmi model catalyst studies and what are their limitations ... 

Table 2 Summary of sintering/redispersion experiments with model-supported metal catalysts ...

Surface thermodynamic considerations can be helpful in an understanding of the complex phenomena which occur in supported metal catalysts. Indeed, the physical and chemical interactions between metal, substrate and atmosphere lead to wetting and spreading phenomena (of the active catalyst over the substrate and of the substrate over the metal) which are relevant for the physical (sintering, redispersion) as well as chemical (suppression of chemisorption, modification of selectivity, enhanced activity) manifestations of supported metal catalysts. 

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

From the electron microscopy study reported by Cfes et al, the modification of the chemical properties mentioned above was not accompanied by parallel changes either in the total dispersion, D, or in the D -j parameter, which, as already mentioned, would account for the contribution to D of Au surface sites with CN < 7, i.e. those which might be expected to be active for CO adsorption. Therefore, the authors concluded that metal sintering/redispersion effects... 

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

Stevenson S, Dumesic JA, Baker RTK, Ruckenstein E (1987) (eds) Metal-support interactions in catalysis, sintering and redispersion. Van Nostrand Reinhold, New York Ferrari AM, Neyman KM, Mayer M, Staufer M, Gates BC, ROsch N (1999) J Pbys Chem B 103 5311... 

Stevenson, S.A., Dumesic, J.A., Baker, R.T.K. and Ruckenstein, E. (1987) Metal-Support Interactions in Catalysis, Sintering and Redispersion, Van Nostrand Reinhold, New York. 

The preparation of real supported catalysts will involve the deposition of a precursor salt followed by decomposition and/or reduction to the final metallic state. We shall consider the influence of the precursors and the effect of oxidative pretreatments later. First, we consider how the shapes of supported metal particles will vary with time under reducing conditions, since this represents the working condition for most metal catalysts. A comprehensive review of sintering and redispersion in supported metals has been presented by Ruckenstein and Dadyburjor.232... 

The effect of Cl in Ar is negligible and, in H2, sintering is only slightly slower when Cl is added. It is proposed that a Pt-alumina-Cl complex is formed which inhibits sintering. H2 destroys the complex by removal of Cl Cl destroys the complex, but a volatile chlorocomplex is formed instead which allows redispersion to take place. 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

Pt-Ir Temperature, gas flow rate, oxychlorination studied. Careful control necessary. Sintered catalysts could be redispersed somewhat by oxychlorination, especially with higher Cl levels.89,90 ... 

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. 

Species such as, Cl, or F that increase the mobility of metal atoms may cause either redispersion or increased sintering rates. The role of Cl in redispersion has been discussed elsewhere [16]. There is evidence that S and F poisons increase rates of sintering. For example, Erekson and Bartholomew [57] found that an unsupported Ni powder with particles having diameters of 2-6 pm was relatively stable during reduction in H2 at 725-775 K over a period of 18 h. However, after exposure for just 6 h to 0.2 ppm H2S/H2 at either 725 or 775 K, (but not below 725 K) most of the small particles had agglomerated to large (100-250 pm)... 

R.T.K. Baker, C.H. Bartholomew, and D.B. Dadyburjor, in J.A. Horsley (Editor and Project Leader), Stability of Supported Catalysts Sintering and Redispersion, Catalytic Studies Division, 1991. 

C.H. Bartholomew, "Model Catalyst Studies of Supported Metal Sintering and Redispersion Kinetics," Catalysis, Specialist Periodical Report, Royal Society of Chemistry, Thomas Graham House, Cambridge, UK, Vol. 10,1992,... 

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