For internal combustion engine vehicles, the deterioration of emissions, unlike fuel consumption and power output, cannot be perceived by the driver. For over 30 years, SAE has played a leadership role in ground vehicle emissions reduction and on-board diagnostics (OBDs). OBD in gasoline vehicles is detected by measuring the fluctuations in the rear oxygen sensor due to the degradation over time of the oxygen storage capacity (OSC) of cerium complex oxides (ceria) contained in automotive catalysts. Both the amount and rate of OSC in ceria are affected by the composition including dopants and precious metals supported on the surface, and many studies have been reported [1-3]. With more precise detection limits for OBD being required, a deeper understanding of ceria's OSC has become essential.
In this study, we took advantage of the high energy of the synchrotron radiation in SPring-8 to perform an operando analysis using time-resolved dispersive X-ray absorption fine structure (DXAFS) near the Ce K-edge (40,443 eV), which allows observations with more uniformity and less distortion than the Ce LIII-edge (5,723 eV) even in dispersive optics. This technique makes it possible to "directly observe changes in cerium valence", going far beyond conventional catalytic research, and can clarify the effect of supporting a precious metal on ceria by comparing not only the differences in species such as Pt, Pd, and Rh, but also the presence or absence of precious metals.
Taking this advantage, we performed a detailed analysis of how differences in boundary conditions, such as the type of precious metals, reducing atmospheres (CO, H2), environmental temperatures, and aging (simulated mileage deterioration), affect the amount and rate of OSC in a fluctuating redox atmosphere. The results of this research will not only provide guidance for more accurate OBD methods, but also provide fundamental scientific knowledge for catalytic emissions control.
References
1. H. Tanaka, M. Yamamoto, “Improvement in oxygen storage capacity”, SAE Special Publications, 1173, p.231-238, (1996) [SAE Paper , 960794 (1996)]
2. K. Yamada, H. Tanaka, M. Yamamoto, “Oxygen storage capacity on cerium oxide-precious metal system”, SAE Paper , 970464 (1997)
3. Y. Sakamoto, Y. Kizaki, Y. Yokota, H. Sobukawa, M. Sugiura, H. Tanaka, M. Uenishi, “New method of measuring the amount of oxygen storage/release on millisecond time scale on planer catalyst”, J. Catal., 211, p.157-164 (2002).