Analyzing NOx in Exhaust Emissions with FT-IR Spectroscopy

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NO_x emissions and air pollution from diesel engines are a global concern because of their potential effects on human health. Researchers all over the world are developing new and improved methods to lower NO_x emissions. Infrared spectroscopy in the gas phase, which is made possible using Specac’s gas cells, plays a key role in this research.

It is estimated that every year, three million people die prematurely due to outdoor air pollution. Diseases and deaths caused by air pollution are becoming a worldwide public health concern. Due to these concerns, the World Health Organization has published guidance on limits and thresholds for main air pollutants that pose health risks. The pollutants included in the guidance are sulfur dioxide, nitrogen oxides (NO_x), ozone, and particulate matter.

NO_x Emissions are a Public Health Hazard

Diesel engines lower CO₂ emissions and provide a better fuel economy than conventional gasoline engines, and hence were once considered as the ‘greener’ alternative. However, in recent years, the image of diesel as a ‘green fuel’ has been shattered by the revelation that diesel engines give out more harmful pollutants than their gasoline counterparts.

Diesel engines work at a higher pressure and temperature than gasoline engines, which favors the production of NO_x gases. Therefore, diesel engines produce more NO_x emissions than gasoline engines. Exposure to NO_x emissions has been associated with increased deaths from lung disease, heart disease, and other respiratory illnesses. NO_x emissions also lead to the formation of environmental phenomena like acid rain and smog.

In an attempt to reduce air pollution and improve air quality, many countries are now trying to decrease the number of diesel cars on their roads. However, the uses of diesel fuels are not limited to cars alone. The higher efficiency and density of diesel fuel along with its slower combustion characteristics make diesel a better alternative to gasoline for the larger vehicles used in freight and transportation. Most buses, trucks, tractors, and military vehicles use diesel, in addition to many ships and trains. Industry, general combustion, and stationary diesel engines also considerably contribute to NO_x emissions.

As diesel engines are more prevalent, removing all diesel engines is not a realistic goal. Instead, scientists worldwide have been developing ways to decrease the NO_2x emissions produced by diesel engines.

Reducing NO_x Emissions from Diesel Engines

The production of NO_x emissions by gasoline engines are reduced using a three-way catalyst. However, these catalysts can’t be employed for diesel engines as diesel exhaust gases usually contain oxygen, which causes the three-way catalyst to be inactive. The leading method used in removal of NO_x from diesel engine exhaust gases is selective catalytic reduction (SCR). SCR reduces NO_x to water and nitrogen with the help of a reducing agent, usually ammonia, in the presence of a catalyst.

Ammonia-SCR is broadly used for industrial NO_x emissions and stationary diesel engines. SCR has several advantages, including a NO_x conversion rate of 90%. However, SCR systems are expensive, large, and can generate ammonia emissions, making them unsuitable for mobile NO_x reduction processes.

In recent days, urea-SCR has been highlighted as a potential alternative to ammonia-SCR for mobile diesel engines. In urea-SCR, the urea is broken down to form ammonia before the SCR reaction occurs. Similar to ammonia-SCR, urea-SCR can provide 90% NO_x conversion, but also provides a greater durability, wider temperature operating window, lower costs, and lower emissions. However, applications of urea-SCR still have limitations like insufficient catalyst optimization, urea dosing technology and under-developed urea infrastructures.

Studying NOx Reduction with FT-IR

Fourier transform infrared (FT-IR) spectroscopy can offer a suitable means to track the presence of gases in diesel exhaust emissions. FT-IR spectroscopy involves shining an infrared beam on a reaction or sample. Molecules absorb infrared light at specific wavelengths depending on their chemical structure, providing a molecular ‘fingerprint’. In this way, FT-IR can be used to monitor the progress of the NO_x reduction reaction and molecules present in exhaust fumes. A recent study by scientists from the Technical University of Freiberg, Germany, used FTIR to track the presence of N₂O, NO, NO₂, NH₃, HNCO, and H₂O in diesel exhaust fumes that were treated using urea and ammonia SCR.

FT-IR can also be used to give more comprehensive insights about the SCR catalyst and the progress of the SCR reaction by performing FT-IR spectroscopy on the catalyst bed during the reaction (in-situ). Molecules interact with the catalyst surface, and so the molecular fingerprint observed by FTIR is slightly changed and can give important details about how reactants, products, poisons, and intermediates interact with the catalyst. If these interactions are understood, it can be useful for intelligently designing new, improved catalysts for both urea and ammonia -SCR.

Studying NO_x reduction reactions using infrared spectroscopy needs accurate and reliable equipment. Specac provides a range of accessories for infrared spectroscopy that are perfect for studying gaseous catalysis chemistry.

Specac’s gas cells which are designed for transmission FT-IR experiments can be used to offer quantitative analysis of the SCR exit gases composition. The diffuse reflectance accessory and selector environmental chamber provide the ideal solution for studying SCR catalysts and reactions in situ. The Selector cell can be heated to 800 °C and powder or solid SCR catalyst samples can be placed onto a sampling cup within an atmospherically controllable chamber for spectroscopic analysis.

To sum up, it is a worldwide important problem to reduce NO_x emissions from diesel engines. The current research is focused on enhancing and modifying SCR reactions for mobile diesel engines. FTIR plays an important role in the development of new SCR catalysts and processes.

References and Further Reading

1. ‘The contribution of outdoor air pollution sources to premature mortality on a global scale’ J. Lelieveld, J. S. Evans, M. Fnais, D. Giannadaki, A. Pozzer, Nature, 2015.
2. ‘Air pollution and health’ http://www.who.int/airpollution/en/
3. ‘Ambient (outdoor) air quality and health’ http://www.who.int
4. ‘A Review on Selective Catalytic Reduction of NO_x by NH₃ over Mn–Based Catalysts at Low Temperatures: Catalysts, Mechanisms, Kinetics and DFT Calculations’ F Gao, X. Tang, Ho. Yi, S. Zhao, C. Li, J. Li, Y. Shi, X. Meng, Catalysts, 2017.
5. ‘NOx Storage and Reduction for Diesel Engine Exhaust Aftertreatment’ in ‘Diesel Engine - Combustion, Emissions and Condition Monitoring, B. Pereda-Ayo, J.R. González-Velasco, InTech, 2013.
6. ‘Diesel catalysts’ http://courses.washington.edu/cive494/Catalysts.pdf
7. ‘An FT-IR study of the adsorption of urea and ammonia over V₂O₅–MoO₃–TiO₂ SCR catalysts’ M.A. Larrubia, G. Ramis, G. Busca, Applied Catalysis B: Environmental, 2000.
8. ‘Urea Decomposition in Selective Catalytic Reduction on V₂O₅/WO₃/TiO₂ Catalyst in Diesel Exhaust’ M. Goldbach, A. Roppertz, P. Langenfeld, M. Wackerhagen, S. Füger, S. Kureti, Chemical Engineering & Technology, 2017.
9. ‘FTIR accessories’ https://www.specac.com/en/products/ftir-acc

This information has been sourced, reviewed and adapted from materials provided by Specac.

For more information on this source, please visit Specac.