CO oxidation for the characterization of reducibility in oxygen storage components of three-way automotive catalysts

Steady-state, CO-oxidation kinetics at 515 K have been measured on model, Pd catalysts, prepared by vapor deposition of Pd onto either zirconia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rate 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 zirconia 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, with the Pd added after calcination so that the metal particle size was not a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 1170 K. XRD showed that the loss was accompanied by a large increase in the crystallite size. Results for ceria-zirconia showed that the loss in this case was more gradual, with CO oxidation activity due to RE2 maintained to much higher calcination temperatures. Taking the importance of RE2 as a measure of the ability of the catalyst to use oxygen from the oxide, the implications of these results with respect to oxide structure and the effect of aging on oxygen-storage properties of reducible oxides are discussed. (C) 1997 Elsevier Science B.V.