Uncovering the chemical mechanisms in soot filters can lead to innovative methods for creating synthetic fuels.
Carbon particles are a common presence in our everyday surroundings. Soot, made up of tiny carbon particles, is produced when fuels like oil or wood burn incompletely. Soot particle filters play a crucial role by eliminating these tiny carbon particles, which range from nanometer to micrometer sizes, from automobile emissions through various chemical surface reactions. Additionally, carbon particles have industrial applications, as they can be transformed into synthetic fuel precursors alongside carbon dioxide (CO2) and water at temperatures exceeding 1000 degrees Celsius. In both uses, the chemical transformations occurring on the carbon’s surface are vital; however, the specific conditions that favor particular reaction pathways are not entirely clear.
Nitrogen dioxide and oxygen degrade carbon particles
A team of scientists at the Max Planck Institute for Chemistry (MPIC) has shed light on the oxidation processes affecting carbon nanoparticles in particulate filters. They studied the behavior of these small soot particles under conditions typical of diesel engine emissions. In temperature ranges between approximately 270 to 450°C, the carbon interacts with reactive gases such as nitrogen dioxide (NO2) and oxygen (O2), leading to the oxidation and degradation of the carbon. The findings show that higher temperatures result in a quicker reduction of carbon mass. The researchers then incorporated their experimental findings into a kinetic multi-layer model referred to as KM-GAP-CARBON.
The modeling details the chemical processes: at lower temperatures, nitrogen dioxide primarily drives carbon decomposition, whereas oxygen takes over at higher temperatures. This shift in dominant reaction pathways is defined by a gradual change in the activation energy required to initiate a chemical reaction.
Origin of the chemical model in atmospheric aerosol research
“We initially developed our model to analyze the chemistry of fine-dust particles in the atmosphere, but it has proven effective for high-temperature technical applications as well,” explains Thomas Berkemeier, the lead author and research group leader at MPIC. “Our model helps clarify how temperature influences the chemical reaction pathways. It also reveals a second interesting observation: the reaction rate is highest at both the beginning and the end of the reaction.”
As reported in the recently published study in the journal Angewandte Chemie, the more reactive carbon atoms on the surface of carbon particles undergo oxidation and gasification first, leading to a buildup of less reactive atoms. This creates an initial passivation effect on the particles, which slows down the oxidation process. “Towards the end of the reaction, the large surface area comparison to the particle volume leads to a rapid increase in the volume-normalized reaction rate,” explains Berkemeier, who is eager to analyze the exact structure of the particles in upcoming research using microscopic and spectroscopic techniques. The team also plans additional studies on reaction kinetics to further investigate the impact of different oxidants and varying conditions.
Foundational research aids renewable fuel development
Ulrich Pöschl, a co-author and director at the Max Planck Institute for Chemistry, noted, “Our research not only deepens the comprehension of fundamental processes on carbon nanosurfaces, but it also paves the way for technological innovations in environmental and energy domains, such as improvements in carbon capture technologies and optimization of production conditions for synthetic fuels. Our foundational scientific research contributes to sustainable technology and societal development during the Anthropocene.”
The term Anthropocene denotes the current geological epoch marked by the significant and widespread human impact on Earth, a concept explored in scientific studies at the Max Planck Institute for Chemistry since its identification by Nobel laureate Paul Crutzen.
