INS-, SIMS- and XPS-investigations of diesel engine exhaust particles

Abstract

Diesel engine exhaust particles were collected under controlled conditions with and without passing a Pt/Al₂O₃ exhaust after-treatment oxidation catalyst and were studied before and after extraction treatment. The surface compositions were analyzed by means of X-ray photoelectron spectrometry (XPS). The properties of the hydrogen containing carbonaceous part of the soot were selectively characterised by combining results from inelastic neutron scattering (INS) as a bulk technique and secondary ion mass spectrometry (SIMS) as a surface and selvedge related technique. The effect of the catalyst in reducing the overall hydrogen content, the relative amounts of poorly crystalline species and of polyaromatic and aliphatic contributions at the surface was observed for the original as well as for the extracted diesel soot. Complementary, INS revealed a high integral graphiticity together with the proton dynamics of the finely divided, fluffy soot as well as the changes of the amounts of polyaromatic and aliphatic species in the surface regions. The matrix dependencies of the yields of negatively charged C/H secondary ions of the diesel soot and their time profiles were compared in SIMS experiments together with related properties of selected carbon blacks of different hydrogen content and with purified activated carbon. The C₂⁻/CH⁻ and the C₂⁻/C₂H⁻ fragment ion ratios were used as a sensitive measure to roughly differentiate between hydrogenous species associated with poorly crystalline contributions and with structures of enhanced graphiticity, respectively. The acetylene-like fragment ion C₂H₂⁻ is helpful to indicate the progression of the erosion process from the topmost atomic layers into the near-surface regions. The combination of these surface and bulk related techniques is shown to be useful in monitoring the efficiency of the oxidizing exhaust gas purification catalyst in the abatement of polyaromatic contributions from residual particulate emissions of diesel-engine passenger cars in addition to the overall lessening of soot emission. The combination deserves more common use in characterising carbons and carbonaceous emissions from other sources as well as contributing to the evaluation of the performance of new catalysts for exhaust purification processes.