Plasma-Assisted Soot Oxidation: Ozone and NO₂ as Dual Oxidants for Efficient Diesel Particulate Filter Regeneration

This study investigates the effectiveness of an inline dielectric-barrier-discharge nonthermal plasma (DBD-NTP) system in promoting the regeneration of diesel particulate filters (DPFs) through the application of plasma-derived oxidants. Experimental analyses were performed on a diesel engine utilizing B7 fuel at an indicated mean effective pressure (IMEP) of 6 bar, during which the DBD reactor demonstrated the capacity to generate adjustable concentrations of ozone (O₃) and nitrogen dioxide (NO₂). In the context of air feed, O₃ was identified as the primary oxidant, while NO₂ was produced as a secondary byproduct via both O₃- and O-assisted pathways. Regeneration experiments indicated that the interplay between O₃ and NO₂ led to an approximate 74% reduction in pressure drop, contrasting with a 46% reduction noted under conditions employing solely O₃, thereby affirming the synergistic role of NO₂ in extending oxidation cycles. Electrical low-pressure scanning mobility particle sizer (EEPS) measurements indicated that the dual oxidant strategy of O₃ and NO₂ effectively diminished both nucleation- and accumulation-mode particulate matter, whereas high-resolution transmission electron microscopy (HRTEM) analysis implied that soot nanostructures were transformed into disordered, defect-laden graphitic layers with a slight reduction in primary particle size. Complementary thermogravimetric analysis (TGA) and kinetic assessments validated that exposure to oxidants reduced the apparent activation energy associated with soot oxidation from 101.6 to 83.9 kJ mol^-1. Collectively, the findings substantiate that DBD-NTP is capable of generating potent oxidants that are favorable for low-temperature DPF regeneration. While O₃ serves as the principal catalyst for soot oxidation, NO₂ provides a synergistic effect by lowering the energy threshold and enhancing soot reactivity. This dual-oxidant strategy represents a promising pathway for improving regeneration efficiency under realistic exhaust conditions without the need for additional modifications to the engine.