Oxygen Titration of Spill-Over Hydrogen in Ceria and Ceria-Alumina Supported Platinum-Rhodium Catalysts:Application to the Determination of the Ceria Surface in Contact with Metal

摘要

Successive O_2/H_2O_2 chemisorption measurements were performed at room temperature over Pt/CeO_2 catalysts of different BET surface areas after reduction at 573 K under H_2.It has been determined that about 4 #mu#atom of oxygen per m~2 of ceria is requred to fill the oxygen vacancies created by the reduction in agreement with the value deduced from the TPR profiles. The same value was also obtained from the H_2O_2 titration, which shows that oxygen is able to titrate the hydrogen which has spilled over the ceria surface via the metallic particles. Thus, the consistency of the three techniques demonstrates the validity of this method of measuring both the total ceria surface area and the ceria surface in contact with the precious metals. The application of this methodology to the case of Pt and Pt-Rh /CeO_2-Al_2O_3 moldel three-way catalysts leads to a set of ceria surface areas deduced from O_2 chemisorption and O_2 titration much lower than those obtained by TPR. This behaviour has been attributed to the presence of chlorine ions originating form the prcursor and to the formation of stabilised Ce~(3+) ions at the CeO_2-Al_2O_3 interface during reduction. After an ageing treatment at 1273 K under N_2 +10% H_2O, an even greater difference was observed between the TPR ceria surface areas and those deduced from the chemisorption method. The results indicate an increased fraction of ceria without metal due to sintering and to the segregation between ceria and the metal particles.