In the next few years, gaseous emissions related to the growing mobile transport (e.g. road and air traffic) as well as power generation systems will further increase. Due to the associated air pollution, the stringency of NOx- and other gas-emissions has been increased twice in the last decade and yet an additional increase has been recommended. Increases in engine efficiency and in engine operating temperatures have tended to offset the efforts to reduce NOx-emissions requiring the development of use of the best proven low-emission technologies. Current low-emission engines are based on in-furnace-control methods such as axially staged combustor with an LPP main module, dual annular combustor (DAC) as in CFM56 engine or those equipped with optimized injection systems and new diffuser design for lean-modules. As these technologies are still at low development levels to be considered for entry into service, there are some concerns about their applicability due to engine instabilities as well as a likelihood of a trade-off between NOx- and low power HC- and CO-emissions. When these and the issues such as impact of NOx at altitude, NOx-reduction during all phases of flight and difficulties in derivative engine design are merged, the importance of post-combustion NOx-reduction becomes significant. Combustion in post-modern lean-burn engines occurs under high air-to-fuel-ratios (A/F>15) and increases engine operating temperatures, leading to formation of higher NOx-emissions. Under these new conditions, the currently applied materials and technologies come to their uppermost limits. In order to ensure that NOx-emission is effectively reduced and thus, the environmental protection is enabled, it is essential to develop new catalytic materials i.e. catalytically active systems. Catalytic reduction of NOx can be achieved either by catalytic combustion as in primary systems or by selective catalytic reduction (SCR) as a part of the exhaust gas treatment technology which can be simultaneously controlled and monitored by gas sensors.
The principal aim of our team is to reduce NOx-emission in turbines of airliners which are today, compared to their predecessors, more fuel-efficient and cleaner relying on the advanced technology combustion systems. Considering the exhaust gas temperatures, pressure and velocity, use of ceramic coatings which can reduce toxic gases effectively under net-oxidizing conditions is unavoidable. The research topics are fundamentally focused on material science related developments of catalytic and sensing coatings and their functioning principals, essentially regarding the turbine conditions. Complex oxides and nano-phase noble metal particles embedded in ceramic matrices are especially interesting relying on their high-temperature stability and implementation as catalysis.
Our studies target both technology concepts for catalytic reduction of NOx; catalytic combustion in the primary systems and sensor controlled catalytic converters in the exhaust gas after-treatment systems. By the exhaust gas after-treatment technologies, the catalytic converter is replaced at the post-combustion area. Currently, many materials and principals are suggested for effective NOx-reduction in exhaust gases. In high oxygen-content exhaust gases such as those in lean-burn diesel engines and turbines, the state-of-the-art concepts and materials display a fraction of their performances compared to those detected under oxygen-free conditions. Therefore, there is a need for future applications to develop materials that are catalytically active and temperature stabile above 600°C. These can be complex oxides from binary compounds with perovskite and spinel structures and/or ternary compounds of magnetoplumbite structure. These can accommodate various catalytic and sensing elements in combination in the crystal structure and thus, are versatile and cheap to produce. Promising alternatives are catalysts having nano-sized noble metal clusters embedded in ceramic matrices and self-regenerative nobel-metal doped perovskites. The later contains less noble metal particles in sizes less than 10 nm and are more effective than the Pd-impregnated perovskite support-catalyst.
Our research on NOx-sensors targets the impedance sensors and the resistive sensors. The principle of the impedance sensor relies on the measurement of the variations of the AC-resistance at different frequencies which occur with the changes in gas environment and concentration. The selectivity of these sensors can be increased by the use of tailored sensor electrodes and elements. The best candidates are oxides and binary compounds of Co, Ni, Zn and Pd.
Development of catalytic materials is carried out primarily on powders and coatings synthesized by sol-gel route which is flexible and easy to handle for production of complex oxides. Successful materials are coated by means of jumping beam EB-PVD process for achievement of robust and highly porous layers. Thin coatings to employ as sensing element are to synthesis by Magnetron Sputtering which yields nano-sized, best quality and high homogeneity coatings. By applying masks and lithographical techniques, the nano-structuring of these layers can be achieved. Physical Vapor Electron-Beam Deposition (Coating processing) is another processing route which enables application of versatile fabrication strategies leading to system integrated manufacture of sensors, etc. The characterization of the material system is carried out by facilities located in Institute. These are Sensor und Catalyst Characterization Center (SESAM), thermal ageing unit under different atmospheres, SEM, EDX, TEM, XRD, XFA. The sensor und catalyst characterization center which is specially constructed for testing of these materials can mix up to eight individual gases in various complexities and heat those in a quartz glass reactor to temperatures up to 1200°C before entering into the test chamber. The volume of gas mixture can be individually varied, so that various test conditions can be created. Similarly, the test samples can have different geometries and sizes. The center contains a Solartron 1255 impedance spectroscope equipped with a potentiostate, a mass spectroscope and a NDIR- and UV-IR-analyzer as well as other diverse electrical devices. The characterization center is computer controlled to allow continuous and non-stop investigation during long-term experiments.