Scientists from Tokyo Metropolitan University have developed a low-temperature catalyst for removing nitrogen oxide gas from industrial exhausts using ammonia. The team was able to demonstrate the improvement in performance of the new catalyst and identify the reaction mechanisms responsible for the difference.
This research was carried out in collaboration with the Chugoku Electric Power Company, Incorporated and was partially supported by the Cooperative Research Program of the Institute for Catalysis, Hokkaido University.
Nitrogen monoxide (NO) and nitrogen dioxide (NO2), or nitrogen oxides (NOx), are common atmospheric pollutants created by burning fossil fuels, coal and natural gas. They are a major cause of photochemical smog and acid rain, which makes it important to remove them from vehicle and factory emissions. Nitrogen oxides can be removed from the air due to their reaction with ammonia via selective catalytic reduction (SCR), where NOx is rendered harmless as it is reduced to nitrogen and water. In particular, vanadium oxides supported on titania are known to have excellent selectivity for conversion to nitrogen, and have been successfully applied to stationary boilers.
A significant bottleneck for supported catalysts is the high temperature required for catalytic activity, which ranges from 200 to 400°C. This often results in units being placed close to areas with higher temperatures such as the boiler in power plants, where they can be easily damaged physically by ash as well as by the accumulation of ammonium sulfates. These deactivating factors can be avoided if the unit is placed downstream after an electrostatic precipitator for removing dust and a desulfation system to remove sulfate deposits. However, this approach requires high catalytic activity at lower temperatures, since the temperature of the exhaust gas has generally dropped to around 100°C by this point. A catalyst was needed that was able to remove nitrogen oxides at lower temperatures.
This resulted in a team led by Yusuke Inomata and Toru Murayama from Tokyo Metropolitan University developing a catalyst based on bulk vanadium oxides.
Vanadium oxide (V2O5) is a common state of vanadium oxide but the team was able to successfully synthesize a mixture of vanadium (V) and vanadium (IV) oxides, or “defective” vanadium oxide, by heating a precursor to 270°C. They found that this “defective” catalyst had excellent catalytic activity at temperatures as low as 100°C. At this temperature, the speed at which NOx is converted to harmless nitrogen was 10 times faster than conventional titania supported vanadium oxide catalysts. This showed good performance where conventional catalysts fell short. The improvement was attributed to the presence of “defective” vanadium oxide which creates “Lewis acid” (electron-accepting) sites, promoting the reaction of nitrogen oxide with ammonia to become nitrogen.
Beyond practical application to industrial catalysis, the team hopes that the mechanisms they have uncovered serve as a model system for further scientific studies.