The role of rare earth dopants in nanophase zirconia catalysts for automotive emission control

Rare earth (RE) modification of automotive catalysts (e.g., ZrO2) for exhaust gas treatment results in outstanding improvement of the structural stability, catalytic functions and resistance to sintering at high temperatures. Owing to the low redox potential of nonstoichiometric CeO2, oxygen release and intake associated with the conversion between the 3+ and 4+ oxidation states of the Ce ions in Ce-doped ZrO2 provide the oxygen storage capacity that is essentially to effective catalytic functions under dynamic air-to-fuel ratio cycling. Doping tripositive RE ions such as La and Nd in ZrO2, on the other hand, introduces oxygen vacancies that affect the electronic and ionic conductivity. These effects, in conjunction with the nanostructure and surface reactivity of the fine powders, present a challenging problem in the development of better ZrO2-containing three-way catalysts. We have carried out in situ small- to wide-angle neutron diffraction at high temperatures and under controlled atmospheres to study the structural phase transitions, sintering behavior, and Ce3+<->Ce4+ redox process. We found substantial effects due to RE doping on the nature of aggregation of nanoparticles, defect formation, crystal phase transformation, and metal-support interaction in ZrO2 catalysts for automotive emission control. (C) 2000 Elsevier Science S.A. All rights reserved.