Oxidation of Gasoline Direct Injection Engine Soot in Oxygen-Lean and Oxygen-Rich Atmospheres: A Comparative Analysis of Physicochemical Properties

For most of the time, soot oxidation in gasoline particulate filters (GPFs) occurs in oxygen-lean and high-temperature environments due to the typically stoichiometric combustion mode of gasoline direct injection (GDI). The oxygen-lean and high-temperature atmospheres inevitably change the oxidation characteristics, altering the physicochemical properties of GDI soot particles and consequently affecting the control strategies of GPF regeneration. To gain deeper insight into the GDI soot oxidation process, a detailed investigation into the changes in the physicochemical properties of GDI soot under different oxygen concentrations was performed. The morphology, nanostructure, surface functional group, and hybridized carbon state of the soot samples were analyzed using high-resolution transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). The macroscopic morphology results show that, with the conversion level increased to 80%, both GS-20 and GS-1 soot particles become more compact. The fractal dimension (<i>D</i> _f) of GS-20 and GS-1 soot particles increased by 19.5 and 26.4%, respectively, while their primary particle sizes decreased by 27.4 and 25.5%. Simultaneously, the radius of gyration (<i>R</i> _g) was reduced by 38.2 and 43.4%, respectively. A comparison between the two soot samples indicates that the oxygen-lean atmosphere tends to yield soot with a smaller <i>R</i> _g, a larger primary particle size, and <i>D</i> _f than the oxygen-rich atmosphere, thereby making the subsequent oxidation of GS-1 soot more challenging. From the nanostructural perspective, both soot samples undergo a gradual transformation into a more ordered structure, as evidenced by a similar decrease in fringe tortuosity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:mo>®</mml:mo></mml:mover> </mml:math> ) and an increase in fringe length ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow><mml:msub><mml:mi>L</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:mo>®</mml:mo></mml:mover> </mml:math> ). However, at the same conversion level, GS-1 soot demonstrates a shorter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow><mml:msub><mml:mi>L</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:mo>®</mml:mo></mml:mover> </mml:math> and higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:mo>®</mml:mo></mml:mover> </mml:math> compared to GS-20, which facilitates oxygen penetration into the primary particles and thus accelerates oxidation. Chemical analysis further reveals that under oxygen-rich conditions, the preferential oxidation of sp³-hybridized carbon leads to a lower sp²/sp³ ratio in GS-1 soot compared to GS-20 at the same level of carbon conversion. In contrast, under oxygen-lean conditions, the relatively limited availability of oxygen leads to a surface concentration of C-OH groups on GS-1 soot that is up to 9.3% lower and CO groups up to 13.8% lower than those on GS-20 at 80% conversion.