Mode transition and fragmentation behavior of ash-catalyzed soot oxidation in gasoline particulate filters

Although ash deposition is known to promote soot oxidation in gasoline particulate filters (GPFs), the underlying mechanism remains unclear. In this study, two ash surrogates (SiO₂ and Al₂O₃) were employed to investigate the evolution of soot morphology and nanostructure at different oxidation degrees under a 16.6% O₂ atmosphere at 650 °C, with a particular emphasis on oxidation mode transition and oxidation-induced fragmentation behavior. Results show that distinct hollow structures are observed in ash-free soot at late oxidation stages. Such hollow structures are largely suppressed in the presence of ash. Ash addition significantly decreases the number and size of primary particles within aggregates, with increased fractal dimension and a more disordered nanostructure. For all samples, the aggregates fragmentation rate decreases with increasing oxidation degree. However, soot oxidized with ash exhibits a higher aggregate fragmentation rate (up to 20% higher) during the initial oxidation stage than ash-free soot. In contrast, the fragmentation rate of primary particles for ash-free soot increases sharply during oxidation, reaching 0.927 in the late stage, whereas this increase is suppressed by ash addition. This indicates that ash impedes oxidant penetration into the interior of primary particles, thereby effectively suppressing internal oxidation while promoting external oxidation. Notably, SiO₂ demonstrates superior catalytic performance compared with Al₂O₃ throughout the oxidation process. This work provides new insights into ash-driven soot oxidation and offers guidance for optimizing GPF regeneration strategies.