Novel NOx reduction method combining NOx storing materials with electrochemical reduction of nitrogen oxides

Title data

Röder-Roith, Ulla ; Sahner, Kathy ; Moos, Ralf:
Novel NOx reduction method combining NOx storing materials with electrochemical reduction of nitrogen oxides.
2008
Event: 5th International Conference on Environmental Catalysis , 31.08.-03.09.2008 , Belfast, Northern Ireland.
(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. However, the effectiveness of these principles in presence of excess oxygen is low.

In this contribution, a modified method for electrochemical NOx reduction in lean exhausts is presented. Two different types of ionic conductors were used, oxygen ion conductors and proton conductors. To increase the efficiency of the NOx reduction in oxygen containing exhaust gases, we added a NOx storing material. Possible candidates are barium or potassium carbonate, materials known from NOx storage catalysts. NOx reduction then proceeds in a two-step process. During the first step, NOx is stored by the storing material. 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 and increases the NOx partial pressure compared to the oxygen partial pressure. As a consequence, the parasite reactions related to excess oxygen, eg., oxygen pumping or formation of water, are limited. The main challenge of this concept is to identify the optimum trade-off in operating temperatures. While an optimum temperature around 350 °C exists for storing NOx, ionic conductors present a better performance, i.e., conductivity, at considerably higher temperatures. Our study aimed at demonstrating feasibility of this concept by finding an appropriate combination of materials and operating conditionsto solve this problem.

This study proposes a novel aftertreatment method for lean automotive exhausts and fits therefore well within the scope of the automotive emission control section of the ICEC. In particular, the NOx reduction in lean exhaust treated herein is a highly significant, up-to-date issue that is closely related to the discussion on climate warming by CO2 emission.