Surface area, pore size distribution and microstructure of combustion engine deposits

The structural characteristics of combustion engine deposits produced from fuels with 22–44% aromatics by volume have been studied using Raman spectroscopy and gas sorption techniques. The lateral sizes of graphitic crystallites were found to increase slightly with the aromatic content of the fuel used. The surface areas of the deposits were evaluated using the BET and DR theories. Density functional theory (DFT) and mercury porosimetry were used to evaluate pore size distributions. The deposits have large internal surface areas (in the approximate range 100–300 m2/g) and their structures are highly porous. The average pore sizes of the largely microporous deposits do not depend on the aromatic content of the precursor fuel. Walls of pores with widths of about 0.5 nm are primarily responsible for the majority of the internal surface area found in the deposits. Heat treatment at temperatures above 573 K results in increased surface areas because the release of small hydrocarbon fragments from the deposits opens pores which were not accessible following heat treatment at lower temperatures.