Oxidation processes in combustion engines and in the atmosphere take the same routes

IMAGE: Laboratory setup of the free jet test at TROPOS in Leipzig, which enables the investigation of the early phase of oxidation reactions under atmospheric conditions without the walls influencing the reaction … view More

Photo credits: Torsten Berndt, TROPOS

Thuwal / Helsinki / Leipzig. Alkanes, an important component of fuels for internal combustion engines and an important class of urban trace gases, react via different reaction pathways than previously assumed. These hydrocarbons, formerly known as paraffins, therefore produce large amounts of highly oxygenated compounds that can contribute to organic aerosol and thus to air pollution in cities. An international research team has now been able to prove this through laboratory experiments with the latest measurement technology at the University of Helsinki and at the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig.

The research team writes in the journal Communications Chemistry, an open journal, that the results of this interdisciplinary work provide important information on oxidation processes both in internal combustion engines and in the atmosphere – with direct effects on engine efficiency and the formation of aerosols, especially in cities. Access journal of the Springer Nature publishing group.

Oxidation processes play a major role both in the atmosphere and in combustion. A chain reaction known as auto-oxidation is made possible by high engine temperatures. However, it is also an important source of highly oxygenated compounds in the atmosphere that form organic aerosols, as researchers from Finland, Germany and the USA showed in 2014. The autoxidation is one reason for the aging processes of organic compounds through atmospheric oxygen. It contributes to the spoilage of food and wine.

This chain reaction is triggered by the formation of peroxy radicals (RO2). The tendency of organic compounds to such a multi-stage auto-oxidation determines the ignition point of fuels in engines and, on the other hand, the potential for the formation of highly volatile condensable vapors and consequently organic aerosol in the atmosphere. The extent to which a multistage autoxidation takes place depends on the molecular structure of the organic compounds and the reaction conditions. Determining the various reaction pathways of peroxy radicals, which are important intermediates in all oxidation reactions, is crucial for the formation of the various reaction products and their key properties, which can ultimately affect both human health and the climate.

Since peroxy radicals are very reactive, their chemical reactions take place very quickly and individual reaction steps have been overlooked for a long time. The discovery of highly oxygenated organic molecules (HOMs) seven years ago was only possible because of advances in measurement techniques. A special mass spectrometer (chemical ionization – interface between atmospheric pressure and time of flight (CI-APi-TOF)) was used to measure the radicals and oxidation products of alkanes, with which the very short-lived compounds can be monitored. “So far there are no studies on HOM formation from alkanes, as it was assumed that their structure is unfavorable for autoxidation,” reports Dr. Torsten Berndt from TROPOS. Methane, an important greenhouse gas, belongs to the group of alkanes. The most important fossil fuels in the world economy from crude oil and natural gas also consist of alkanes: propane, butane, pentane, hexane, heptane and octane. New findings on the oxidation behavior of this group of substances are therefore of great relevance in many areas.

In order to gain a deeper insight into the auto-oxidation of alkanes, experiments in Helsinki were also carried out in the free-jet reactor of TROPOS in Leipzig. The experimental setup is optimized in such a way that the gases do not come into contact with the walls during the reaction in order to rule out any interference with the results caused by wall processes. During the experiments almost all reactive intermediates, RO2 radicals and their reaction products could be monitored directly. The interdisciplinary collaboration between researchers from combustion chemistry and atmospheric chemistry has proven to be very useful, since processes analogous to those in the atmosphere only take place at a higher temperature in the combustion processes. “The result showed that not only isomerization reactions of RO2 radicals, but also RO radicals are responsible for the formation of more highly oxidized products. The study made it possible to identify the alkanes, the last and perhaps most surprising group of organic compounds, for whom autoxidation is important, “concludes Torsten Berndt.

Even at high nitrogen oxide concentrations, which otherwise quickly terminate auto-oxidation reactions, the alkanes apparently produce considerable amounts of highly oxidized compounds in the air. The new findings enable a deeper understanding of the autoxidation processes and lead to further investigations into isomerization reactions of RO radicals.


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