Research Group Prof. Dr. G. Friedrichs

The story of NCN

July/August 2021: A review paper on "The story of NCN as a key species in prompt-NO formation" has been published in the renowned journal Progress in Energy and Combustion Science (Prog. Energy Combust. Sci. (2021) 100940/1-33 The review has been jointly written together with N. Lamoureux and Pascale Degroux (PC2A Lab, Univ. Lille, France) and Matthias Olzmann (Karlsruhe Institute of Technology, KIT, Germany) and summarizes the current understanding of the NCN spectroscopy, NCN thermochemistry and reaction kinetics, as well as its role for prompt-NO formation. The so-called prompt-NO switch, that is the highly temperature-dependent product branching ratio of the reaction NCN + H (either forming CH + N2 or HCN + N) has been highlighted as a key reaction step for accurate flame modeling. The complicated potential energy diagram of this intricate reaction is highlighted in the Figure below. Over many years, NCN radical high-temperature chemistry has been studied in the Kiel shock tube laboratory to measure the rate constants of many elementary reactions. In fact, the use of the thermal decomposition of the (highly toxic and explosive) NCN3 as a precursor for quantitative generation of NCN radicals behind shock waves is a unique feature of the Kiel experimental work, which has been mainly conducted by Johannes Dammeier, Nancy Faßheber and Sebastian Hesse in their dissertation projects.

prompt-NO switch

As is well known from the recent Diesel emission scandal, NO formation in combustion of hydrocarbon fuels is a major issue of environmental concern. Being able to describe the chemical, elementary formation steps and mechanisms of NO formation (where prompt-NO is one important pathway, thermal-NO is another), offers the possibility to optimize the combustion conditions in a way directly reducing NO emissions instead of relying on exhaust gas treatment systems (e.g. by adding Ad Blue - urea solution - for selective catalytic reduction, SCR). In this context, already in the year 2000, the paradigm that the Fenimore initiation reaction CH + N2 → HCN + N is responsible for prompt-NO formation has been challenged by a theoretical paper stating that this reaction would yield NCN + H instead of HCN + N as the main products. At that time, the high-temperature chemistry of NCN was completely unknown and it actually took tedious experimental and theoretical work to shed light on the important role of NCN in prompt-NO formation. As outlined in the review paper, it is still true that the reaction CH + N2 serves as the main initiaton step, but prompt-NO formation and the involved NCN chemistry is by far more complicated than initially believed.