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 NOx-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 3d5/2 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 3d5/2 were measured by XPS thus supporting the TEM and SEM observations. A Pd-supported perovskite Pd-LaFe(1-x)CoxO3 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 NOx-conversion than typical SCR-catalysts under lean conditions employing either hydrogen or propene as reducing agents. The Pd-integrated perovskite e.g. LaFe0.65Co0.3Pd0.05O3 displayed a maximum NOx-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 NOx-conversions values were maintained in the presence of H2O + CO2. The challenge of the H2-SCR of NOx technology is to reduce NOx 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 NOx-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 LaFe0.65Co0.3Pd0.05O3 was coated on cordierite substrates. The catalytic converter displayed a NOx-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 NOx 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
