Synthesis and analysis of adaptive Pd-integrated perovskite catalysts for effective NO_x-reduction under lean conditions

Kurzfassung

The literature reports a series of precious metal integrated perovskite based catalysts revealing intelligent properties [1]. Lanthanum based perovskites are among these catalysts that are able to stabilize precious metal ions such as Pd, Pt, Rh, Ru, etc. in their crystal lattice. Precious metal ions in the catalyst respond reversibly to the changes in the exhaust gas composition by diffusing out of the perovskitic lattice as metallic nanoparticles under reducing conditions and by re-adsorbing into the crystal structure under oxidizing conditions. This behaviour improves the sintering resistance of precious metal particles and leads to enhanced NO_x-reduction capability of the catalysts. Moreover, additional active species are formed in these catalysts which require less precious metal consumption in automotive catalytic converters. The present study is devoted to the synthesis of La-based perovskite catalysts by the polymeric citrate route and their analysis to establish the adaptive behaviour of the precious metal ions. In order to investigate the state and behaviour of the palladium ions in the perovskite, the catalysts were calcined under redox conditions at different temperatures and analyzed via XRD, SEM, TEM and XPS. XRD analysis showed that the La-based perovskites form an orthorhombic phase above 700°C and palladium ions are soluble in the perovskite lattice up to this temperature. FE-SEM observations displayed that no segregation of the palladium particles or agglomerates occurs in the oxidized conditions of the perovskite based catalyst. TEM-analysis of the as-prepared perovskite confirmed the presence of palladium in the matrix (or perovskitic crystal lattice), but small oxidic palladium particles on the perovskite surface were found as well. This observation agrees well with the XPS analysis showing signals with a Pd 3d_5/2 binding energy of 336.5 eV, which correspond to Pd₂₊ in PdO, together with signals at higher binding energies, which can be assigned to intra-crystalline Pd²⁺at the surface an in the bulk of the crystalline [1]. TEM-EDX mapping analysis showed that lanthanum and iron were homo-geneously distributed in the perovskite matrix, however, few cobalt- and palladium- rich zones were also found. On reduction of a Pd-integrated perovskite catalyst in (20:1) N₂:H₂ atmosphere, palladium agglomerates in 10-15 nm sizes were detected indicating the diffusion of palladium ions out of the crystal structure to form Pd° as reported by Tanaka et al [1]. Binding energies of ~335.1 eV corresponding to Pd 3d_5/2 were measured by XPS thus supporting the TEM and SEM observations. A Pd-supported perovskite Pd-LaFe_(1-x)Co_xO₃ which was prepared by the classical impregnation method, clearly showed formation of palladium agglomerates in sizes up to 50 nm upon reduction treatment in 20:1 N₂:H₂ atmosphere. The XPS study suggested that some of the Pd-ions in the supported catalyst may have entered the first surface-layers of the perovskite lattice during the calcinations performed. The catalytic experiments demonstrated that these catalyst offer higher effi-ciency in NO_x-conversion than typical SCR-catalysts under lean conditions employing either hydrogen or propene as reducing agents. The Pd-integrated perovskite e.g. LaFe_0.65Co_0.3Pd_0.05O₃ displayed a maximum NO_x-conversion of 74 % and N₂-selectivity of 60 % at 206°C in the reduction of NO by H₂ in the presence of 5 vol-% O₂ which is better than the performance of typical platinum supported catalysts i.e. Pt/SiO₂ [2] which produce mostly N₂O under similar reaction conditions. These NO_x-conversions values were maintained in the presence of H₂O + CO₂. The challenge of the H₂-SCR of NO_x technology is to reduce NO_x at a relative wide temperature window less than 300°C. This issue can be solved by substitution of other elements at the A-site of the host perovskite structure. In fact, the addition of ceria into the A-site of the La-based perovskite resulted in improvement of the N₂-selectivity (73.7 %) at the maximum NO_x-conversion (72 %) in the presence of H₂O and CO₂ in the feed. Furthermore a prototype was developed for catalytic testing under near real reaction conditions. For this purpose, the Pd-integrated perovskite with composition LaFe_0.65Co_0.3Pd_0.05O₃ was coated on cordierite substrates. The catalytic converter displayed a NO_x-conversion of 20 % at 400°C even in the presence of 4 vol. % water vapour in the feed at high space velocity SV = 60000 h^-1 during the C₃H₆-SCR of NO_x reaction. [1] H. Tanaka, M. Misono, Curr. Opinion in Sol. State and Mat. Sci. 5 (2001) 381-387. [2] R. Burch, M. D. Coleman, Appl. Catal. B: Environ. 23 (1999) 115-121.