Electrochemical reduction of nitrogen oxides combining ion conducting ceramics with nitrogen oxide storing materials

Title data

Röder-Roith, Ulla ; Sahner, Kathy ; Moos, Ralf:
Electrochemical reduction of nitrogen oxides combining ion conducting ceramics with nitrogen oxide storing materials.
2008
Event: Junior Euromat 2008 , 14.-18.07.2008 , Lausanne, Schweiz.
(Conference item: Conference , Other Presentation type)

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Lean-burn engines, for example diesel engines that provide oxygen in excess during combustion, are excellent in terms of energy efficiency but emit a large quantity of nitrogen oxides (NOx) due to their operating principle. Since oxygen is present in excess in the lean exhaust, NOx cannot be completely reduced by conventional three-way-catalysts. In an alternative electrochemical approach, NOx is reduced electrochemically by pumping a current through a ceramic ionic conducting membrane. Depending on the nature of the ions, two options are available. In case of an oxygen ion conductor, eg., yttria stabilized zirconia, the oxygen partial pressure is decreased by pumping the oxygen and hence, NOx decomposes to nitrogen and oxygen. By using a proton conductor, eg., perovskite ceramics such as doped barium cerate, water in the exhaust is decomposed to form hydrogen and oxygen. The hydrogen then serves as an efficient reducing agent for NOx. However, the low effectiveness of both principles in presence of excess oxygen persists.

In the present contribution, we therefore propose a novel method to reduce NOx in lean exhausts using an all-ceramic set-up. To the ionic conducting oxide, we added a storage material that stores nitrogen oxides. Possible candidates are barium carbonate or potassium carbonate. NOx reduction then proceeds in a two-step process. During a storage period, NOx is stored by conversion of the carbonates to nitrates. When no more nitrogen oxide can be stored, a pumping voltage is applied to the ion conductor, thus reducing the stored NOx. The presence of the storage material in direct contact to the ion conductor limits the oxygen access. As a consequence, the parasite reactions related to excess oxygen, eg., oxygen pumping or formation of water, are limited. Our study aimed at demonstrating feasibility of this concept and optimizing the ceramic materials and operating conditions used.