PALLADIUM-CERIA CATALYSTS - REVERSIBILITY OF HYDROGEN CHEMISORPTION AND REDOX PHENOMENA

The interactions of hydrogen with palladium-ceria catalysts have been studied by volumetric and magnetic susceptibility measurements. The ratio H-irr:Pd-total, where H-irr stands for irreversibly adsorbed hydrogen, lies in the range 2.5-3.5; this result is ascribed to hydrogen spillover from palladium to ceria. The uptake of hydrogen by ceria leads to its complete surface reduction at room temperature, whereas bulk reduction begins at 473 K. Magnetic measurements show that: (i) a large amount of hydrogen is reversibly adsorbed on ceria, (ii) evacuation at 373 K induces the reverse migration of hydrogen (back spillover), which leads to the reoxidation of Ce3+ ions. The reoxidation ratio depends on the outgassing temperature whereas the amount of irreversibly formed Ce3+ increases with the reduction and the outgassing temperatures. However, the estimation of the amount of hydrogen retained by ceria from the quantity of Ce3+ formed does not allow determination of the metal fraction exposed (MFE) by hydrogen adsorption at 373 K.