Electrochemical removal of NO_x and hydrocarbons

This thesis comprise of investigations on an electrochemical cell for the reduction of NO by propene. The focus has been on the electrochemical enhancement of the selective reduction of NO by propene by using perovskites, which are known to be catalyst for this reaction in the composite electrodes.Two main types of composite electrodes, La0.85Sr0.15MnO3/Ce0.9Gd0.1O1.95 (LSM/CGO) and La0.85Sr0.15FeO3/Ce0.9Gd0.1O1.95 (LSF/CGO), were investigated as backbones for different kinds of impregnation materials.

The impregnation materials were used either to enhance certain properties of the electrode, or to contribute with entirely new properties to the system. The LSM was also tested with cobalt doping in the structure.Two types of electrochemical cells were tested. The simplest one being the symmetrical cell and the other a cell stack of porous alternating layers of electrode and electrolyte.

To evaluate the systems, conversion measurements has been done where polarisations were applied to the cells during gas analysis. The characterisation of the different materials was done using electrochemical impedance spectroscopy and cyclic voltammetry.This project revealed that the propene acts as an inhibitor on the electrodes during polarisation, probably because of strong adsorption of the hydrocarbon relative to NO.

On LSF/CGO electrode the impregnation of ionic conducting material increased the oxidation of NO to NO2 which is an important step before nitrogen formation. The propene inhibited this reaction because of the strong adsorption of the alkene on the surface. This electrode also showed a high catalytic oxidation of propene to CO2 but little electrochemical promotion of this reaction.

The main focus of the project was electrochemical reduction, but also catalysis experiments were conducted. These revealed that impregnation of the praseodymium doped ceria (CPO) in LSF/CGO electrode, enhanced the direct decomposition of NO in the presence of excess oxygen significantly.The study also demonstrated the possibility to enhance the NOx reduction with the application of a potential when propene was present in the gas.

This could only be done if the electrode was impregnated with BaO. The nitrate formation did not seem to be inhibited by the presence of the hydrocarbon. However, the oxidation of propene was inhibited by the BaO because the active sites for oxidations were partially covered by the BaO nanoparticles.The impregnation of platinum in very small amounts could enhance the electrochemical reduction of NO in the presence of propene even with high excess of oxygen.

A NEMCA of Λ = 6 was achieved for the CO2 formation