Characterization and dynamic behavior of precious metals in automotive exhaust gas purification catalysts

Both the behaviors of sintering and reactivation of Pt and Rh on various metal oxide supports were investigated by TEM, CO pulse chemisorption and XAFS analysis. The results suggest that phenomenon of reversible sintering and re-dispersion to reduced, active metallic sites is related to the electron density of O atoms in support and to the crystal structure of support. As a result of in situ XAFS and in situ TEM analysis, Pt reversible sintering and re-dispersion phenomenon was observed on CeO2 based metal oxide. The Pt re-dispersion process proceeds by the repetition of (1) migration of Pt oxide from the surface of large Pt particles, (2) adsorption of Pt oxide on the surface of CeO2 based metal oxide by strong Pt–O–Ce interaction and (3) reduction Pt oxide and Pt–O–Ce bond. To achieve a good balance between activity and the sintering suppression, CeO2 based metal oxide is selected as Pt support, and ZrO2 is used as Rh support. The blended catalyst of Pt on ceria based oxide and Rh on ZrO2 has higher activity after aging.