Platinum-alumina catalysts sintering

L. L. Hegedus, and K. Baron (1975) Effects of poisoning and sintering on the pore structure and diffusive behavior of platinum/alumina catalysts in automotive converters, Journal of Catalysis 37(1) 127-132... 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

The addition of LaA103, is shown to increase both the dispersion and the resistance to sintering of the platinum supported alumina catalyst. Moreover, lanthanum hexa-aluminate (La-p-A C ) is present in the platinum catalyst fired at 1150°C. 

J. Bournonville and G. Martino, "Sintering of Alumina Supported Platinum", in Catalyst Deactivation, eds. Delmon Froment, Elsevier, Amsterdam, 1980, pp. 159-166. 

A typical converter is made up of multiple furnaces, each of which contains 8 to 10 reactors. Each reactor is made up of 10 to 30 sintered alumina tubes lined with platinum. The furnaces are direct fired with natural gas to 1200—1300°C. A typical furnace can produce about 125 t per month of hydrogen cyanide. Catalyst life is approximately 10,000 h. 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

Similar results were found by Bozo [44]. Palladium deposited onto ceria-zirconia Ceo67Zro3302 solid solution showed very high activity in methane combustion (T50 close to 300 C) but similar to that of palladium deposited onto alumina. Like for the case of platinum a deactivation is observed during tests at temperatures comprised between 200°C and 400 C (Fig. 13.3). However when aged at 1000 C under an air+water mixture this catalysts showed superior resistance compared to classical catalysts as far as activity is considered. Despite a severe sintering of both metal (dispersion is now 1%) and support, whose surface area is close to 4 mVg, T50 was shifted to 420 C, i.e. 120°C only, still much lower for platinum deposited on the same support which showed a TSO close to 620°C. Calculation of specific activities in the 200-300°C range have clearly evidenced that ceria-zirconia support does not have any influence upon performance of PdO in... 

The catalytic layer of monolithic automotive reactors usually consist of active metals (Pt, Pd, Rh) supported on alumina. One of the most important problems set by these catalysts is the decrease in their activity after thermal exposure to the exhaust gas itself (Ref.l). It is well known that this thermal deactivation is directly related to the sintering of the active components. Moreover, this modification of the supported metal is drastically enhanced by structural changes of the support. Thus using TEM experiments, Chu et al (Ref. 2) have reported rapid sintering of platinum during the structural transition Y-AI2O3 to (X-AI2O3. 

The experimental results show that the thermal stability of both alumina and platinum catalysts is improved by the presence of lanthanum. In the case of the support, this improvement can be related to the formation of LaAlOy with 8-AI2O3. For the platinum catalyst, lanthanum has additional roles promoting a better dispersion of the metal and improving its resistance to sintering. 

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