Detection, Origin and Effect of Ultra-Low Platinum Contamination on Diesel-SCR Catalysts

This paper discusses the poisoning of a selective catalytic reduction (SCR) catalyst by trace levels of platinum originating from an upstream diesel oxidation catalyst (DOC). A diesel aftertreatment system consisting of a DOC, urea based SCR Catalyst and a DPF was aged and evaluated on a 6.4 liter diesel engine dynamometer. The SCR catalyst system consisted of an Fe-zeolite catalyst followed by a Cu-zeolite catalyst. After approximately 400 hours of engine operation at varied exhaust flow rates and temperatures, deactivation of the SCR catalyst was observed. A subsequent detailed investigation revealed that the Cu catalyst was not deactivated and the front half of the Fe-based catalyst showed severe deactivation. The deactivated portion of the catalyst showed high activity of NH₃ conversion to NO_x and N₂O formation. The cause of the deactivation was identified to be the presence of trace Pt contamination. The contamination level was less than 0.002wt% that could not easily be detected by a conventional X-ray fluorescence (XRF) method. The detection of trace Pt was made at first using a newly developed ethylene hydrogenation reaction method. The reaction system was built in-house for measuring the conversion of ethylene to ethane at room temperature over a catalyst monolith core. The system allowed the detection of Pt metal below 0.001wt%. The presence of Fe, Cu, or Ni did not interfere with the ethylene hydrogenation technique. The sublimation of Pt from an upstream DOC onto Fe-based catalysts was demonstrated experimentally in the laboratory using the new technique. In the aftertreatment system studied, the Fe catalyst acted as a buffer to trap Pt sublimed from the upstream DOC shielding the Cu catalyst from Pt contamination. The effect of DOC aging conditions on Pt sublimation and subsequent Pt contamination on the downstream SCR catalyst was also investigated.