8 Feb 2018

Reducing NOx in Diesel Emissions | Spectroscopy Solutions

Air pollution and NOx emissions from diesel engines are a global concern due to their potential impacts on human health. Researchers worldwide are developing new and improved methods to reduce NOx emissions. Infrared spectroscopy in the gas phase, which is made possible using FTIR gas cells from Specac, has an important role to play in this research.

Diesel car exhaust emitting polluting nox fumes

It is estimated that 3 million people die prematurely each year as a result of outdoor air pollution. Diseases and deaths resulting from air pollution are becoming a global public health concern. As a result of these concerns, the World Health Organization has published guidance on thresholds and limits for key air pollutants that pose health risks. The pollutants included in the guidance are particulate matter, ozone, nitrogen oxides (NOx) and sulfur dioxide.

NOx Emissions are a Public Health Hazard

Diesel engines provide a better fuel economy and lower CO2 emissions than conventional gasoline engines, and so were once seen as the ‘greener’ alternative. In recent years, however, the vision of diesel as a ‘green fuel’ has been shattered by the revelation that diesel engines produce more hazardous pollutants than their gasoline counterparts.

Diesel engines operate at a higher temperature and pressure than gasoline engines, which favors the production of NOx gases. As a result, diesel engines produce more NOx emissions than gasoline engines. Exposure to NOx emissions has been associated with increased deaths from heart disease, lung disease, and other respiratory illnesses and NOx emissions also contribute to the formation of environmental phenomena such as smog and acid rain.

In an effort to reduce air pollution and increase air quality, many countries are now trying to reduce the number of diesel cars on their roads. However, the uses of diesel fuels go far beyond cars. The higher density and efficiency of diesel fuel combined with its slower combustion characteristics make diesel a better option than gasoline for the

larger vehicles used in transportation and freight. Most trucks, buses, tractors, and military vehicles use diesel, along with many trains and ships. In addition, stationary diesel engines, industry, and general combustion are all considerable contributors to NOx emissions.

As the prevalence of diesel engines is so great, removing all diesel engines is an unrealistic goal. Instead, scientists around the world have been developing ways to reduce the NO2x emissions produced by diesel engines.

Reducing NOx Emissions from Diesel Engines

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

busy road traffic with cars emitting diesel nox fumes

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

Recently, urea-SCR has been highlighted as a potential alternative to ammonia-SCR for mobile diesel engines. In urea-SCR, the urea is decomposed to form ammonia before the SCR reaction takes place. Like ammonia-SCR, urea-SCR can provide 90% NOx conversion, but also provides a wider temperature operating window, greater durability, lower emissions, and lower costs. However, applications of urea-SCR are still limited by under-developed urea infrastructures, urea dosing technology and insufficient catalyst optimization.

Studying NOx Reduction with FTIR

Fourier transform infrared (FTIR) spectroscopy can provide a convenient means to track the presence of gases in diesel exhaust emissions. FTIR spectroscopy involves shining an infrared beam on a sample or reaction. Molecules absorb infrared light at particular wavelengths depending on the chemical structure of the molecule, providing a molecular ‘fingerprint’. In this way, FT-IR can be used to track molecules present in

A Specac FTIR Gas Transmission Cell, ideal for fume analysis.

exhaust fumes and the progress of the NOx reduction reaction. A recent study by scientists from the Technical University of Freiberg, Germany, used FTIR to track the presence of NO, N2O, NO2, H2O, NH3, and HNCO in diesel exhaust fumes that were treated using ammonia and urea SCR.

FT-IR can also be used to provide more detailed insights about the SCR catalyst and the progress of the SCR reaction by conducting FT-IR spectroscopy on the catalyst bed during the reaction (in-situ). As molecules interact with the catalyst surface, the molecular fingerprint observed by FTIR is subtly changed and can give important details about how reactants, intermediates, products, and poisons interact with the catalyst. Understanding these interactions can be useful for intelligently designing new, improved catalysts for both ammonia and urea-SCR.

Studying NOx reduction reactions using infrared spectroscopy requires reliable and accurate equipment. Specac provide a range of accessories for infrared spectroscopy that are ideal for studying gaseous catalysis chemistry.

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

In conclusion, reducing NOx emissions from diesel engines is a globally important issue. On-going research is centered on improving and modifying SCR reactions for mobile diesel engines. FTIR plays a vital role in the development of new SCR processes and catalysts.

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/mediacentre/factsheets/fs313/en/

  4. ‘A Review on Selective Catalytic Reduction of NOx by NH3 over Mn–Based Catalysts at Low Temperatures: Catalysts, Mechanisms, Kinetics and DFT