Synthesis of nanostructured lean-NO _x catalysts by direct laser deposition of monometallic Pt-, Rh- and bimetallic PtRh-nanoparticles on SiO₂ support

Monometallic Pt and Rh and bimetallic PtRh catalysts with a highly dispersed noble metal weight loading of ca. 1 wt% were produced via the direct deposition of nanoparticles on different SiO₂ supports by means of pulsed ultra-violet (248 nm) excimer laser ablation of Pt, Rh bulk metal and PtRh alloy targets. Backscattered electron microscopy (BSE), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) were employed to characterize the deposited nanoparticles, which were found to exhibit narrow size distribution centred around 2.5 nm. The catalytic activities for lean NO _x reduction of the monometallic and bimetallic catalyst samples were investigated in a flow reactor setup in the temperature range 100–400°C using a test gas mixture representative of oxygen rich diesel engine exhaust gas. For comparison a Rh/SiO₂ reference catalyst prepared by a conventional impregnation method was also tested. Further experiments were performed in which PtRh nanoparticles were deposited on a Rh/SiO₂ reference catalyst sample to study the possibility for controlled modification of its activity. The catalytic activity measurements revealed that among the samples solely prepared by laser deposition the PtRh–SiO₂ nanoparticle catalyst showed the highest activity for NO _x reduction at low temperatures 100–300°C. In addition, it could be demonstrated that the initially low NO _x reduction activity and the N₂ selectivity of the Rh/SiO₂ reference catalyst sample for temperatures below 250°C can be enhanced by post laser deposition of PtRh nanoparticles.