Noble metal volatilisation from diesel oxidation catalysts

Louise  Olsson, Chalmers University

Noble metal volatilisation from diesel oxidation catalysts

Kirsten Leistner1, Caroline Braga1, Ashok Kumar2, Krishna Kamasamudram2, Louise Olsson1*

1Chemical Engineering, Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

2 Cummins Inc., 1900 McKinley Ave, MC 50183, Columbus, IN 47201, USA

*louise.olsson@chalmers.se

 

One of the concern in real exhaust aftertreatment systems is the deactivation of SCR components by noble metal poisoning [1-3]. Noble metal-containing DOCs and DPFs are often placed upstream of an SCR catalyst. Small amounts of noble metals can thus volatilise and deposit on the downstream SCR component. Since noble metals are highly active for ammonia oxidation, even these very small amounts severely compromise the function of the SCR catalyst. Ammonia oxidation by the noble metals consumes ammonia needed for the SCR reaction, thus reducing the SCR efficiency.

While there exist some studies on the effect of noble metal poisoning on SCR efficiency [4, 5], there is to our knowledge no detailed studies available that have studied the way noble metals volatilise from a DOC.

 

In this study, we have reproduced the DOC-upstream-of-SCR configuration in the laboratory, with model Pt/Al2O3, Pd-Pt/Al2O3, Pd/Al2O3 and also a commercial DOC catalysts upstream of an alumina-washcoated core. Amounts of volatilised noble metal were determined by ICP-SFMS analysis of the “capturing monoliths”. Presence of noble metal on these monoliths was further investigated by testing their NH3 and CO oxidation ability. “Parent monoliths” were analyzed for effects of sintering by determining relative particle size via TEM and XRD. Volatilization experiments were performed in presence and absence of O2, at different temperatures and for different durations.

 

Results showed that only insignificant amounts of noble metal volatilised in an inert atmosphere, and that no palladium was detectable, thus confirming that the poisoning occurs via volatile platinum oxides. Volatilization from model DOCs increases with temperature, and the trend is exponential. We observe no difference in volatilization upon addition of Pd, even though Pd stabilises against sintering. The deposited Pt is initially prone to extensive sintering upon characterization by oxidation tests, with the oxidation ability increasing after each cycle due to particle size increase [6].

           

References

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