A common approach to reducing NOx emissions from diesel engines is the use of Selective Catalytic Reduction (SCR); the system converts NOx to nitrogen and water via the addition of ammonia. However, SCR only delivers satisfactory results at an exhaust gas temperature well above 200 ˚C. In a cold start, it takes a few minutes until the aftertreatment system is performing optimally. For the same reason, SCR can have reduced performance on cold winter days.
Now, a team at Switzerland’s Paul Scherrer Institut has used time-resolved X-ray absorption spectroscopy and transient experimentation to find that, depending on the temperature and operating conditions, different amounts of ammonia are required to reduce NOx optimally. A paper on their work is published in the journal Nature Catalysis.
The team examined the chemical processes in a copper-exchanged small-pore SSZ-13 catalyst used currently in the selective catalytic reduction ofNOx from the exhaust gas of diesel-fueled vehicles.
The time-spectroscopy study revealed that if there is too much ammonia in the system, it does not work optimally.
Ammonia is necessary to break down the nitrogen oxides, but if there is too much ammonia, the catalyst can only work to a limited extent.
—Davide Ferri, co-corresponding author
On the basis of their spectroscopic findings, the researchers next investigated the function of a copper zeolite catalyst under realistic operating conditions by adding different amounts of ammonia at varying temperatures. They measured which ammonia dosage gives the best results.
The observation of NH3 inhibition on the washcoated catalyst suggests benefits to improving the low-temperature activity by varying the NH3 dos-age, with the aim of controlling the influence of NH3 inhibition and thus NH3 slip at the catalyst outlet. This can be realized by progressively changing the NH3 concentration in the feed rather than dosing the same amount continuously. This operation mode resulted in an optimized NO conversion, especially in the low-temperature regime (around 20% NO conversion with standard dosage and 36% with optimized dosage at 200°C). Low NH3 dosage is desired in real applications; for example, catalytic converters of diesel vehicles, for cost-efficient operation of the system and to meet the maximum allowed NH3 concentration in the exhaust.
—Marberger et al.
Adrian Marberger, Andrey W. Petrov, Patrick Steiger, Martin Elsener, Oliver Kröcher, Maarten Nachtegaal & Davide Ferri (2018) “Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13” Nature Catalysis volume 1, pages 221–227 doi: 10.1038/s41929-018-0032-6