Platinum agglomeration

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

Figure 4. Platinum Agglomeration and Dispersion Measured by Ion Scattering.

The results from both types of experiments lead to the same conclusion platinum agglomerates on the filter act as an NO oxidation catalyst. The resulting NO2 subsequently oxidises the soot. The observation that platinum treated filters are more effective after a high-temperature treatment is supported by studies rejtorted by Xue etal. [17]. There is a direct relation between the size of platinum particles and the effectiveness as an NO-oxidation catalyst. 

Kinoshita, K, Routsis, K, Bett, J.A.S. and Brooks, C.S. (1973) Changes in morphology of platinum agglomerates during sintering. Electrochim. Acta 18, 953-961. 

The next step of the UOP method of CCR regeneration is oxidation and chlorination. In this step, the catalyst is oxidized in air at about 510°C. A sufficient amount of chloride is usually added as an organic chloride, such as trichloroethane, to restore the chloride content and acid function of the catalyst to that of the fresh catalyst. If the platinum crystaUites ate smaller than about 10 nm, sufficient chlorine is present in the gas to completely tedispetse agglomerated platinum on the catalyst, as a result of the Deacon equUibtium ... 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

Honji A, Mori T, Tamura K, Hishinuma Y. 1988. Agglomeration of platinum particles supported on carbon in phosphoric acid. J Electrochem Soc 135 355-359. 

Figure 20 demonstrates by transmission electron microscopy the distribution of platinum microcrystals on (transparent) graphitized soot. Soot particles form agglomerates that measure around 0.1 /u-m in diameter. 

These agglomerates, after activation by dispersed platinum, are bonded and externally hydrophobized by an appropriate amount of PTFE. This bonding is achieved by mixing a PTFE emulsion (particle diameter of dis-... 

Agglomeration of Pt crystallites due to Brownian motion can really be observed and it can also be shown that, indeed, the interaction between the Pt particles and the supporting soot in the presence of the electrolyte, phosphoric acid, is weak enough to allow for relatively free movement of the Pt particles. This fast process obviously is also the reason for the nonobservability of slower surface diffusion-induced Ostwald ripening. Fortunately alloy catalysts composed of platinum and nonnoble metals seem to show a reduced tendency to agglomeration as their deterioration and activity loss is much slower than that of the pure platinum catalyst. 

According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

Fig. 21. EDAX maps of the distribution of (a) platinum and (b) rhodium for the cross section of an area of a Pt/Rh sample covered with large agglomerates produced during catalytic etching in a H2/02 mixture (51).

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