In Situ and Simultaneous Observation of Palladium Redox and Oxygen Storage/Release in Pd/Sr-Fe-O Perovskite Catalysts Using Dispersive XAFS
Masao Kimura, Yasuhiro Niwa, Kenichi Uemura, Toru Nagai, Yasuhiro Inada, and Masaharu Nomura : Materials Transactions, Vol.54 No.02 (2013) pp.246-254
We have succeeded in in situ and simultaneously observing the redox reaction of palladium and the oxygen storage/release process in a newly developed palladium-promoted Sr-Fe-O (Pd/Sr-Fe-O) catalyst during redox-gas cycles using dispersive X-ray absorption fine structure (XAFS) analysis at 673K with a time resolution of less than 20 ms. The Pd/SrFeO catalyst, which exhibits high performance for automotive emission control, has a unique “multi-phase-domain” structure, where a single grain is composed of nano-sized domains of three phases: SrFeO3-δ, Sr4Fe6O13-δ and SrFe12O19-δ. In situ observation has shown a strong correlation between the redox of the palladium and the oxygen storage/release in the Pd/Sr-Fe-O catalyst, and the correlation factors differ between the reduction and oxidation reactions. The oxide phases exhibit a type of “oxygen buffer” effect in the oxidation cycle and delay the formation of Pd-O, resulting in the high performance of the catalyst.
Fig. 1 Atomic structures of the co-existing phases in the Pd/Sr-Fe-O catalysts based on the fitted parameters: (a) SrFeO3-δ, (b) Sr4Fe6O13-δ and (c) SrFe12O19-δ. The red, orange and blue spheres indicate strontium, iron and oxygen, respectively. A gray octahedron shows the FeO6 unit, and PL and IL represent the perovskite (Sr-Fe-O) and iron (Fe-O) stacking layers, respectively.
Fig. 2 Spectral changes obtained by in situ DXAFS measurements during the redox reactions for an Pd/Sr-Fe-O catalyst with 3.0 mass% Pd that was measured under gas-closed conditions at T = 673K (a) in H2, P = 18.5 kPa, and (b) in O2, P = 47.2 kPa. The arrows in the figures show the time evolution of the spectra.
Fig. 3 Time evolution of the atomic fraction ratio (xval.) and the normalized density (ρnorm) during redox reactions for the Pd/Sr-Fe-O catalyst with 3.0 mass% Pd: (a) in H2 gas with P = 18.5 kPa at T = 673 K, and (b) in O2 gas with P = 47.2 kPa at T = 673 K. The redox reaction periods are classified as Red-I, Red-II, Ox-I, Ox-II and Ox-III according to the characteristics of the time dependence of xval. and ρnorm.
Fig. 8 Schematic illustration of the reaction mechanisms that provides a clear understanding of the difference in the reaction mechanisms for the different reaction periods: Red-I, II, and Ox-I, II, and III. For simplicity, the Fe-O structure is symbolized by a square of oxygen atoms with an iron atom at its center. Pink broken circles show oxygen vacancies. The illustration is drawn in 2D, and does not indicate the exact atomic configuration.