Frank E. Huggins1, Naresh Shah1, Artur Braun1, Gerald P. Huffman1, Kerry Kelly2, David Wagner2, and Adel. F. Sarofim2. (1) Chemical and Materials Engineering, University of Kentucky, 533 South Limestone Street, 105 S. J. Whalen Building, Lexington, KY 40506, (2) Institute for Combustion and Energy Studies, University of Utah, 1495 E 100 S Room 107, Kennecott Building, Salt Lake City, UT 84112
The addition of ferrocene, Fe(C5H5)2 in small amounts to diesel fuel is known to promote cleaner and more efficient combustion in diesel engines. In addition, it is known to change the particle-size distribution and characteristics of the diesel exhaust particles (DEP) generated during combustion. In this work, bulk spectroscopic methods (57Fe Mössbauer, sulfur XAFS and carbon 1s NEXAFS) have been used to investigate iron, sulfur and carbon forms in DEP derived from combustion of diesel fuel with and without the addition of 1000 ppm of ferrocene. Combustion was conducted in the laboratory with a two-cylinder, direct-injection Kubota model Z482B engine This engine has a displacement of 482 cc and is equipped with a Land and Sea water-break dynamometer for torque load control. The fuel, a 50:50 mixture of the Chevron/Philips reference diesel fuels, T-22 and U-15, had an average cetane number of 46.7 and a sulfur content of 64.1 ppm. Mössbauer spectroscopy showed that the iron in DEP from the ferrocene-doped fuel was largely present in the form of γ-Fe2O3. From the variation of the Mössbauer spectra as a function of temperature, it was deduced that this oxide component in DEP is of nanoparticle size (5 - 10 nm) and not strongly aggregated, presumably due to the separating presence of deposits of carbonaceous material on the particle exteriors. A second iron oxide, α-Fe2O3, which was very minor (~3% of the total iron) in DEP from the ferrocene-doped fuel, but the dominant iron phase in DEP from the undoped reference fuel, was much coarser in size and formed by a completely different mechanism. Carbon 1s NEXAFS spectroscopy showed that the ratio of the unsaturated carbon fraction (associated with “EC”, elemental carbon) relative to saturated hydrocarbon and oxygen functionalities (associated with “OC”, organic carbon) for DEP derived from the ferrocene-doped diesel fuel was smaller than that for the DEP derived from the diesel fuel without the ferrocene addition. From sulfur XAFS spectroscopy, it was shown that the addition of ferrocene promotes the formation of (i) oxidized inorganic sulfur forms (e.g. sulfate, bisulfate) relative to reduced organic sulfur forms (e.g. thiophenic sulfur), (ii) sulfate relative to bisulfate, and (iii) sulfoxide relative to thiophene forms. These observations on carbon and sulfur speciation are consistent with more efficient combustion of diesel fuel brought about by the addition of ferrocene to diesel fuel. Possible implications of these observations for differences in health effects between different DEP derived from fuels with and without ferrocene will also be briefly discussed.