Synthesis, phase stability and catalytic activity of Pd-doped La-based perovskites during SCR-NO_x with C₃H₆

Kurzfassung

A wide variety of gperovskite combinations functions as host materials for precious metals (e.g. Pt, Rh, Pd, Ru). This is because the synergic interaction between the metal and the host perovskite structure occurs under redox conditions leading to improved catalytic properties [1]. As it is reported, a reversible diffusion of Pd ions in and out of the crystal lattice takes place under stochimetric combustion (λ = 1) conditions [2]. We studied the redox behaviour of LaFe_0.65Co_(0.35-x)Pd_xO₃ (x=0, 0.05) catalysts under lean-burn conditions and tested its catalytic performance during the SCR NO_x-reaction with C₃H₆. A modified citrate route was successfully employed to synthesize palladium substituted perovskites. XRD results show the presence of pure orthorhombic perovskite with composition LaFe_0.65Co_0.3Pd_0.05O₃ that begins to crystallize at approx. 600°C and completes its crystallization at temperatures slightly below 700°C. According to the crystallization behaviour we can postulate that our catalyst is metastable up to 700°C. Our XPS results show that this catalyst displays a reversible behaviour under redox conditions. The exposure of the LaFe_0.65Co_0.3Pd_0.05O₃ catalyst calcined preliminary at 700°C to 5 vol.% H₂ in N₂ at 600°C/1h leads to diffusion of Pd ions as metallic sites (10 to 15 nm size) which reversibly disappear after re-oxidation process in static air at 600°C/1h as detected with ex-situ SEM and TEM analysis. The catalytic test during the SCR-NO_x with C₃H₆ model reaction shows an improved catalytic performance of the Pd-doped perovskite (LaFe_0.65Co_0.3Pd_0.05O₃) catalyst compared to the Pd-supported (Pd/LaFe_0.65Co_0.35O₃-700°C) catalyst. The Pd-free perovskite LaFe_0.65Co_0.35O₃-700°C shows only a negligible activity under the same reaction conditions. The LaFe_0.65Co_0.3Pd_0.05O₃-700°C catalyst shows a NO conversion up to 40 % of at 275°C even at high space velocities (SV~220.800 hr^-1). [1] H. Tanaka, M. Misono, Current Opinion in Sol. State and Mat Sci. 5 (2001) 381-387. [2] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Kimura, T. Okamoto and H. Hamada, Nature 418, 164-167 (2002).