Palladium Associated with Ceria

Amongst all noble metals palladium represents the most attractive active phase for methane combustion. It is now well established that it works as oxide PdO having a limited stability range from 400°C to roughly 750/800°C, and so a powerful oxygen 

On the contrary, for propane combustion on Pd/AljOs modified by ceria addition, Shyu and co-workers observed that ceria allows to maintain Pd in a more stable oxidised state less prompt to react with propane [78]. Catalytic oxidation which occurred from 200°C to 350°C is now delayed to higher temperature by 50°C to 100°C from 250°C to 450°C. More dramatically is the decrease of the conversion at 350°C from 100% to 20% as the partial pressure of oxygen increases beyond the stoichiometric ratio up to 8 times. They concluded from their studies that ceria and palladium are in close interaction, may be in a Pd-O-O-Ce-0 model via a 02 species. 

Similar results were found by Bozo [44]. Palladium deposited onto ceria-zirconia Ceo67Zro.3302 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 T50 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 

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Of the noble metals, palladium exhibits the highest activity for CO oxidation.As already mentioned for Pd-promoted CeCoO catalysts, the reaction over a noble metal supported on ceria involves a cooperative effect between the metals and the oxide, the so-called dual site mechanism. A recent study by temporal analysis of products (TAP) experiments for Pt supported over ceria showed two independent sites with different activities. The high activity site was associated with the metal/support interface and the low activity one was located on the support. The different sites were characterized by two different activation energies. Moreover, at variance with the stable number of high activity sites, the number of the low active sites increased with reaction temperature. 

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