CO OXIDATION FOR THE CHARACTERIZATION OF REDUCIBILITY IN OXYGEN STORAGE COMPONENTS OF 3-WAY AUTOMOTIVE CATALYSTS

Steady-state, CO-oxidation kinetics at 515 K have been measured on mod el, Pd catalysts, prepared by vapor deposition of Pd onto either zirco nia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rat e process (RE2), associated with both the metal and the oxide support and observed previously on ceria-supported catalysts in excess CO[7], was also found for Pd supported on ceria-zirconia, but neither zirconi a nor praseodymia had any effect on CO oxidation under the conditions of our study. For ceria and ceria-zirconia, deactivation, through the loss of RE2, caused by high-temperature calcination, was examined, wit h the Pd added after calcination so that the metal particle size was n ot a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 117 0 K. XRD showed that the loss was accompanied by a large increase in t he crystallite size. Results for ceria-zirconia showed that the loss i n this case was more gradual, with CO oxidation activity due to RE2 ma intained to much higher calcination temperatures. Taking the importanc e of RE2 as a measure of the ability of the catalyst to use oxygen fro m the oxide, the implications of these results with respect to oxide s tructure and the effect of aging on oxygen-storage properties of reduc ible oxides are discussed. (C) 1997 Elsevier Science B.V.