Characterization of model automotive exhaust catalysts: Pd on ceria and ceria–zirconia supports

Abstract

Pure cerias, silica-doped ceria, ceria–zirconia solid solutions, and ceria–zirconia solid solutions with partial incorporation of praseodymium in the structure were prepared by Rhodia as high-surface-area powders and used as supports in model Pd automotive three-way catalysts prepared at Ford. The catalysts were aged for 12 h at 1050°C, both in air and under redox conditions simulating automotive exhaust gases. Both the fresh and aged catalysts were characterized by a combination of techniques including oxygen storage capacity (OSC) measurements. After aging, catalysts prepared on the solid solution materials provided much greater OSC than those based on pure ceria or silica-doped ceria. Addition of 5 wt% praseodymia as a substitute for ceria improved the thermal stability of the ceria–zirconia, however, without increasing the OSC of the model catalysts. The ceria–zirconia based catalysts revealed a new temperature-programmed reduction peak, between 100°C and 200°C, after 1050°C aging, which is attributed to Pd-assisted bulk reduction of ceria. Significant differences in OSC were noted between catalysts prepared on a series of 70 wt% ceria–30 wt% zirconia supports prepared by different processes, despite virtually identical characteristics of the aged materials as judged by the other techniques. These observations indicate that different processing methods lead to different physical and chemical characteristics of aged catalysts, not readily discerned by conventional characterization techniques, but nonetheless affecting the performance.