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

Successive O-2/H-2/O-2 chemisorption measurements were performed at room te mperature over Pt/CeO2 catalysts of different BET surface areas after reduc tion at 573 K under H-2. It has been determined that about 4 mu atom of oxy gen per m(2) of ceria is required to fill the oxygen vacancies created by t he reduction in agreement with the value deduced from the TPR profiles. The same value was also obtained from the H-2/O-2 titration, which shows that oxygen is able to titrate the hydrogen which has spilled over the ceria sur face via the metallic particles, Thus, the consistency of the three techniq ues demonstrates the validity of this method of measuring both the total ce ria 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 /CeO2-Al2O 3 model three-way catalysts leads to a set of ceria surface areas deduced f rom O-2 chemisorption and O-2 titration much lower than those obtained by T PR. This behaviour has been attributed to the presence of chlorine ions ori ginating from the precursor and to the formation of stabilised Ce3+ ions at the CeO2/Al2O3 interface during reduction, After an ageing treatment at 12 73 K under N-2 + 10% H2O, an even greater difference was observed between t he TPR ceria surface areas and those deduced from the chemisorption method. The results indicate an increased fraction of ceria without metal due to s intering and to the segregation between ceria and the metal particles. (C) 2000 Academic Press.