Hideaki Hamada, National Institute of Advanced Industrial Science and Technology (AIST), Japan
Catalytic after-treatment plays an important role to reduce harmful compounds in car emissions such as CO, hydrocarbon (HC), NOx and PM. Lately, various state-of-the-art catalytic technologies based on nanotechnology have been developed for gasoline and diesel cars in order to meet the recent stringent emission standards.
Three-way catalyst (TWC) now works quite effectively to reduce exhaust emissions from gasoline-fueled cars. Normally, a component with high OSC (oxygen storage capacity) is added to TWC to increase the A/F window. Toyota has recently developed homogeneous CeO2-ZrO2 solid solution materials covering whole Ce/Zr ratios and having high OSC. They have also succeeded in improving the catalyst durability by inhibiting the sintering of CeO2-ZrO2 by Al2O3.
"Intelligent catalyst" developed by Daihatsu is another example of the application of nanotechnology to exhaust catalyst. The catalyst is actually Pd catalyst supported on composite perovskite oxides featured by long catalyst life. While the particle size of active Pd grows steadily on Al2O3 causing catalyst deactivation, Pd on perovskite does not sinter because Pd metal is incorporated into the structure of perovskite under lean conditions and deposited again on perovskite under rich conditions. They have also succeeded in the preparation of new intelligent Pt- and Rh-based catalysts by manipulating the structure and composition of perovskite. On the other hand, Nissan and Mazda have commercialized TWC with less amount of platinum by using materials to inhibit its sintering.
Hydrocarbon adsorber TWC system has been developed by Nissan. This system can remove cold-start HC emission by using zeolite-based HC adsorber added to TWC. When the temperature of the exhaust gas is low at cold-start, the zeolite removes HC by adsorption, and after the temperature gets higher, the adsorbed HC is desorbed and reduced over TWC. In this system, the structure of zeolite is important to determine HC adsorbing capacity. Zeolites with larger pore size can adsorb large-size HC, indicating shape selectivity. The loading of Ag to zeolite was found effective to increase the HC adsorbing capacity.
The development of novel catalytic technologies to remove NOx emitted from diesel and lean-burn engines has attracted much attention, since there is a so-called trade-off problem between NOx emission and fuel economy concerning engine combustion improvement. Three types of catalytic NOx removal systems have been investigated: 1) NOx storage reduction (NSR) 2) SCR (selective catalytic reduction) of NOx with urea (urea-SCR), 3) SCR of NOx with hydrocarbons, H2 or CO (HC-SCR, H2-SCR, CO-SCR).
The NSR system originally developed by Toyota enables NOx removal by using a combination of TWC catalyst and NOx storage compounds such as alkali and alkaline earth metal oxides, in cooperation with engine control. Under lean conditions the NOx storage compound absorbs NOx, which are then desorbed and reduced by the function of TWC under stoichiometric-rich conditions. Recently Honda developed a new NSR catalyst containing CeO2 and zeolite-based materials as catalyst components. Their catalytic system is more efficient for NOx emissions at low temperatures, claiming the presence of NH3 intermediate. Urea-SCR using V2O5/TiO2 or ion-exchanged zeolite-based catalysts is an application of well-known NH3-SCR and expected as a practical NOx removal system mainly for heavy-duty diesel vehicles. HC, H2, and CO-SCR, found by Japanese researchers including our group, are considered as a more ideal method because the fuel and fuel-derivatives can be used as reducer. Since HC-SCR is comprised of a series of complicated parallel and sequential reaction steps, there are many factors affecting the catalytic activities. Although no catalytic systems have been put to commercial use regarding HC, H2, and CO-SCR, the development of high-performance catalysts is still desired.
Sergey Bredikhin, Institute of Solid State Physics Russian Academy of Science, Russia
The electrochemical reduction of nitric oxide in the presence of the excess oxygen is reviewed. To solve the problem of effective electrochemical reduction of nitric oxide in the presence of the excess oxygen the concept of artificially designed multilayer structure proposed. Our investigations have shown that substitution of traditional cathodes by the nanostructured multilayer electro-catalytic electrode leads to a dramatic decrease in the value of the electrical power required for NO decomposition. It was shown that nanostructured multilayer electro-catalytic electrode should consist at list from three main functional layers: Cathode; Electro-catalytic electrode; Covering layer, in order to operate as an electrode with high selectivity. The values of current efficiency in such reactors increase up to 20% and the values of the NO/O2 selectivity up to 25. These results indicate that this new type of nanostructured electro-catalytic reactor can be used for practical applications and such systems should substitute traditional catalytic systems for exhaust gas purification.
Masanobu Awano, National Institute of Advanced Industrial Science and Technology (AIST)
Ceramic electrochemical reactors, with high conversion efficiency and reactivity for both energy production and chemical conversion, could be realized in, for example, electric power generators (SOFC), synthesis of hydrogen, and the decomposition and purification of environmental pollutants. Significant improvements in the selective purification of NOx in exhaust gases containing oxygen, using a novel electrochemical reactor, have been achieved by means of nano-scale control of an electro-catalytic layer in the electrochemical cell through a self organization process. Furthermore, two new types of electrochemical reactor have been developed. Firstly, PM/NOx clean-up reactors make it possible to oxidize particulate matters (PM) in emissions simultaneously with deNOx, through redox reactions, in an electrochemical cell. Furthermore, an interactive ceramic reactor, composed of the deNOx electrochemical cells with a thermoelectric ceramic power module, operable without any external power supply, has been developed to apply the energy conversion from exhaust-waste heat to electric power.
Ceramic electrochemical SOFC-type reactors are envisaged as potential products, due to their high-efficiency and operability in the intermediate temperature range (under 650°C). Possible power densities of more than 2kW/liter are expected for applications such as auxiliary power units (APU), and small size cogeneration systems, by the improvement of materials, accumulation of fine parts, and assembly of those parts into a high performance module. Tubular SOFC’s, with sub-millimeter diameters, enable us to obtain a high volumetric power density when they are accumulated as ‘cubes’, were hundreds of these sub-millimeter SOFC tubes are precisely mounted in a porous electrode matrix in a volume of one cubic centimeter. Micro tubular SOFCs have been successfully fabricated and have been demonstrated to have excellent performance with a power density of 1W/cm2 under 600oC, and reaching 3W/cm3 in one sugar-cube size by means of nanoscale control. Furthermore, development of novel ceramic fabrication processes for integration of SOFC’s to cubes, and cube-accumulated prototype modules, has realized micro honeycomb type cell stacks with a cell integration density of 250 multilayered tubes.
Michael Stelter, Head of Department Modules, Systems and Environmental Technologies Fraunhofer Institute for Ceramic Technologies and Systems, Dresden, Germany
Ceramic technologies in general do contribute to modern energy technologies to a large extend, as ceramics provide a very broad and diverse range of material properties that can be used to face the current challenges. The talk will, in it’s first part, give some insight in how ceramics can be used to improve the eco-efficiency of transportation systems.
Example 1: Fuel cell auxiliary power units
Long-haul trucks need electrical energy even during standstill periods, either to provide air condition and electricity to the driver during rest periods, or to operate necessary electric equipment. From an environmental point of view, it is not desirable to idle the main engine during these periods. Instead, diesel fuel operated fuel cells, so called auxiliary power units (APUs) can take over this task using only a fraction of the fuel. However, high temperature fuel cells with a high power density require novel material solutions that can be produced at low cost. Nanotechnology has been employed to develop novel, redox stable and cheap ceramic cells and novel ceramic cell contact ribs that can be operated at 850°C.
Example 2: Cheap and effective solar cells
Battery powered electric vehicles only provide an ecologically friendly alternative to internal combustion engine driven cars if electricity can be produced in an effective and eco-friendly way. Solar cells can produce clean electricity. However, grid parity is not reached yet, partly because of still lacking efficiency. The efficiency and cost of manufacturing of solar cells can be improved in part by an improved front side contacting layer that uses less precious material such as silver, uses less energy to be produced and has a higher conductivity, thus covers less of the solar cell’s area. Nanotechnology has been employed to develop new inks with higher conductivity and lower sintering temperature.
Example 3: Automotive Exhaust Treatment Sensors
Upcoming environmental regulations in the automotive sector such as EU6 and US T2B5 will lead to cleaner combustion engines. However, a large amount of new sensors will be needed in future vehicle powertrains. To still provide mobility at an attractive and affordable price point, new technologies will be needed to produce complex exhaust sensors in large quantities at low prices. Nanotechnology has been used to demonstrate that a lot of crucial exhaust sensor principles such as diesel soot or NOx sensors can in future be produced in multilayer technology at low prices in mass production, making automotive exhaust aftertreatment systems affordable for emerging economies.
Outlook to Ceramic Nano Powder Processing and Conclusion
In the examples it will be shown, that using nano powders many ceramic systems relevant for green car technologies can be made cheaper or more reliable or can be improved in their functionality, which increases their eco-impact. It is characteristic to the ceramic processing, that a lot of health issues will arise from handling nano scaled ceramic powders in a production environment. These issues and how they can be handled will be highlighted in the talk. On the other hand, however, once the ceramic is sintered, the nano properties of the powders are replaced with the (improved) properties of the bulk ceramic. Effectively, this means that nano health issues in the world of ceramic are strictly limited to the rather controlled production phase, not the end user or the recycling phase in the product life cycle.
Improvment of PEM Fuel Cells for Car Application: From Stack Characterisation to Tailored Electrodes Nanostructured Materials
Patrick Achard, Centre Energétique et Procédés - Centre for Energy and Processes - MINES ParisTech
The research team « Energetics, Materials and Processes » has been established in 1990 within the research Center Energy and Processes of MINES-ParisTech. The work presented here is situated at the crossing of two main research axis of this team:
First of all an axis is devoted to energy storage and conversion mainly focusing on hydrogen technologies and fuel cells at system level – the lab has been equipped with test benches permitting to characterize fuel cells stacks with a rated power up to 15 kW. This lab was designed to participate to the first European project aiming at designing and realizing a hydrogen and fuel cell powered car in 1994 ie the FEVER project (Fuel cell Electrical Vehicle for Extended Range). This project was led by Renault, our lab was the only one university type laboratory.
Secondly, another axis is devoted to studies on materials permitting to solve energy problems or to improve energy efficiency. This axis is called Energy and Advanced Materials. Here the work is mainly focused on sol/gel process and the class of materials called aerogel like materials. An intense effort has been done on polymeric materials obtained thanks to that process permitting to get carbonaceous materials after pyrolisis. Those materials are nanostructured, nanoporous and monolithic and present a good electronic conductivity. Their nanostructure can be tailored by adapting the parameters of the sol/gel process., so one can get an appropriate pore size distribution for example. Those materials appear to be good candidates for electrodes of supercapacitors and PEM fuel cells. For those devices, noble metal catalysts have to be deposited within the nanostructure.
On those basis the lab acquired the know-how permitting to realise PEM fuel cells MEA’s (Membranes Electrode Assemblies) and developed it’s own MEA’s based on the referred materials and a test bench permitting to calibrate them. For the electrode itself, triple phase contact appears to be essential. The carbonaceous studied materials appear to be model materials permitting to evaluate the different contributions to the lowering of the efficiency of a PEM monocell.
Here one can understand how nanotechnology permits to improve the understanding of the behaviour of the energy conversion achieved within elements constituting a PEM fuel cell.