Impact and Prevention of Ultra-Low Contamination of Platinum Group Metals on SCR Catalysts Due to DOC Design

Diesel aftertreatment systems configured with a diesel oxidation catalyst (DOC) upstream of an urea selective catalytic reduction (SCR) catalyst run the risk of precious metal contamination. During active diesel particulate filter (DPF) regeneration events, the DOC bed temperature can reach up to 850°C. Under these conditions, precious metal (especially Pt) can be volatized and then deposited on a downstream SCR catalyst. In this paper, the impact of ultra-low contamination of platinum group metals (PGM) on the SCR catalyst was studied. A method based on precious metal volatilization of a Pt-rich DOC at 850°C and under lean gas conditions was employed to contaminate downstream FeSCR and CuSCR formulations. The contamination resulted in poor NOx conversion (via NOx remake) and excessive N₂O formation. The precious metal volatilization method was employed to screen various Pt/Pd based DOCs to avoid contamination of the downstream FeSCR. A correlation was developed that showed a reduction in SCR catalyst contamination as the Pt/Pd ratio decreased. The Pd-only DOC showed no impact to NOx conversion and no increase in N₂O formation. The Pt-rich DOC showed the most impact. DOC formulations with higher Pd content demonstrated enhanced CO and HC light-off stability. The higher Pd content served to stabilize Pt in the DOC formulations. The higher Pd content resulted in less Pt vaporization. This, in turn, reduced the subsequent contamination risk on the downstream SCR. The vaporization of Pd from the DOC formulations was determined to be very low. However, an incipient wetness technique was used to purposely dope Pd on the FeSCR formulation and Pt on the CuSCR formulation. Contamination up to 0.004wt% Pd was required to alter the NOx performance by 5–10% on the FeSCR formulation. However, contamination up to 0.008wt% Pt was required to alter the NOx conversion 5–10% on the CuSCR formulation. A second ethylene hydrogenation method was used on the contaminated SCR catalysts to determine presence of precious metals.