Clustering at high temperatures: carbon formation in combustion

The formation of carbonaceous particles in laminar, atmospheric pressure diffusion flames is investigated using laser ionization time-of-flight mass spectroscopy and on-line analysis of nascent-particle properties while the particles are still in gas suspension at atmospheric pressure. This latter analysis includes photoelectric yield spectroscopy of the particle surface. Combustion gases from the, e.g. argon-diluted methane (CH4) flame are extracted and diluted with an inert gas prior to analysis. The molecules are ionized by two-photon photoemission inside a time-of-flight mass-spectrometer which makes it possible to observe polycyclic aromatic hydrocarbons (PAHs) at molecular masses up to ∼600 amu. The mobility diameter of the carbonaceous particles was determined in the range from 2 to 20 nm. Formation and destruction of particles and their concomitant density and surface transformations are studied together with the PAH molecules as they depend on the height in the flame. It is found that particles are formed before large PAHs appear. As opposed to previous models where the flat PAHs were generated in homogeneous gas phase reactions, we conclude that PAHs are synthesized on the particles using their surface as templates and evaporate into the gas-phase when synthesis is completed. We present a comprehensive, new mechanism for particle formation in diffusion flames.