A hyperbranched polymer synthetic strategy for the efficient fixation of metal species within nanoporous structures: Application in automotive catalysis

In the present study, a novel synthetic approach, called assisted impregnation technique, is described for the production of highly active Pd monometallic three-way catalysts supported on SBA-15. The assistance in this method originates from a hyperbranched polymer (carboxy-methylated polyethyleneimine, Trilon-P), able to entrap metal ions and therefore act as a metal binding agent due to its unique chemical and chelating properties. A series of Pd / SBA-15 samples with different Pd loadings, varying from 0.13 to 3 wt%, were produced. All the as-developed materials, evaluated under simulated exhaust conditions at the stoichiometric point, exhibit an excellent CO and HC oxidation performance, providing even very high CH4 conversions at low temperatures. However, material with 3 wt% Pd loading stands out affording complete NO reduction at 300 °C. The positive effect of the employed synthetic procedure is clearly demonstrated by comparing the latter sample with its counterpart derived from conventional dry impregnation technique. As deduced by a combination of techniques, assisted impregnation technique succeeds to deliver very small highly dispersed PdO nanoparticles uniformly distributed within SBA-15 porous network, through metal-polymer complex formation. The derived PdO nanoparticles demonstrate an average size ranging between 2.1 and 2.4 nm with tight distribution and pronounced stability considering the negligible aggregation and particle growth phenomena after the catalytic reaction. On the contrary when dry impregnation is employed the size of PdO nanoparticles undergoes a two-fold increase from approximately 3 to 6 nm upon exposure at automotive exhaust conditions.