Harald Klein

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

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.

Geometric and electronic structures of Pt/V2O5/TiO2 diesel engine exhaust gas purification catalysts before and after aging

In spite of their technical use and relevance the structural properties of diesel engine exhaust gas catalysts are not yet understood on an atomic scale. This study investigates structural changes during aging of Pt/V2O5/TiO2 based catalysts in order to get insight into the mutual interaction of the active constituents. X-ray photoelectron spectrometry, secondary ion mass spectrometry and X-ray absorption fine structure measurements were used to characterize the chemical composition and the electronic and geometric structure of the catalysts surface and bulk. In the freshly prepared catalyst the interaction among the active species turns out to be moderate, i.e. vanadium is present in a distorted 5+ state. TiO2, as expected, is present as in the precursor and Pt is found to form monometallic clusters of about 16 A diameter without any significant interaction with the other metal species. Particularly, no alloying is observed. Upon aging of the catalyst vanadium is reduced to 4+ and preferentially orders in a VO2 structure although other minor phases may also be present. This is accompanied by a partial transformation of TiO2 from the initial anatase to rutile. Like V, Pt undergoes a strong modification upon aging: Pt is partly oxidized but still forms metal particles with a slightly smaller size.