The Functional Assembly Technology Group at Advanced Manufacturing Research Institute of Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a new electrochemical reactor.
The reactor can efficiently purify nitrogen oxides (NOx) contained in diesel exhaust gas by breaking them down into N2 and O2 at low temperatures without using a catalyst.
The nanostructured electrode formed in the reactor made it possible to break down NOx in diesel exhaust gas containing a high concentration (approx. 20%) of oxygen at temperatures lower than 250°C. Moreover, the reactor can reduce the energy required to purify exhaust gas, thereby contributing to the improvement of fuel efficiency.
The reactor enables the system that allows the conservation of air environment while reducing CO2. Thus, it is likely that the reactor can replace the existing exhaust gas purification systems in diesel cars when controls on emissions are tightened.
In highly efficient engines such as lean burn and common rail diesel engines, NOx emission increases because the temperature and pressure of combustion tend to rise. In addition, the existing three way catalysts cannot sufficiently break down and purify NOx due to a high concentration of oxygen contained in exhaust gas. Thus, it is difficult to improve the fuel efficiency and the exhaust gas purification performance at the same time.
At present, NOx storage selective reduction catalyst, etc are intensively developed as a technique to purify NOx included in exhaust gas containing a high concentration of oxygen. But the techniques thus far developed involve a number of problems. For example, extra fuel is required as a reductant due to the lack of precise control over the reaction, and hazardous byproducts are emitted because of the use of ammonia.
Especially in Japan, drivers frequently drive their cars at low speeds for short distances in urban areas, where the engine is repeatedly halted and started at traffic lights. Therefore, there is an increasing demand for a technique to efficiently purify NOx contained in exhaust gas at low temperatures.
As for a new exhaust gas purification system, AIST succeeded in developing a solid electrolyte electrochemical reactor based on an electrochemical reaction in 2001. The reactor made it possible to break down and remove NOx included in a gas containing oxygen in a highly selective manner. However, this reactor was unsuitable for practical use because it was difficult to lower its operating temperature.
Thus, AIST designed a new electrode structure based on the analytical research on the NOx selective breakdown mechanism and developed a highly efficient NOx breakdown/purification reactor that is operable at low temperatures.
Koichi Hamamoto, a researcher of Functional Assembly Technology Group, developed a nanostructured working electrode with the use of an electrolyte with a high oxygen ion conductivity. As a result, the area of three-phase interface composed of electrode, electrolyte and gas, which acts as a reaction point, is increased, and the activity of the interface is enhanced.
At the same time, the operating temperature was significantly reduced by an atmosphere formed in the electrode section, where the NOx breakdown predominantly occurs.
The electrochemical reactor using this nanostructured electrode exhibited a high reactivity to NOx and was capable of purifying approximately 90% of 1,000ppm NO gas in an atmosphere containing 20% oxygen (an oxygen concentration higher than that in the actual diesel exhaust gas composition) and 80% nitrogen at a temperature of only 250°C. Under such conditions, it is extremely difficult to break down NOx with the existing catalyst.
AIST intends to further reduce the operating temperature and enhance the integration of reactive area. Concurrently, the organization plans to evaluate the durability and the influences of coexisting gases to consider the feasibility of the latest reactor.
AIST also plans to continue working on this subject in combination with the research on the high-sensitive high-speed response NOx sensor and the NOx/PM (particulate carbons) simultaneous purification device thus far developed. In this way, AIST aims to evolve the latest reactor into an integrated electrochemical exhaust gas purification device and establish it as a technology that helps solve the energy and environmental problems.
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