Catalytic role of ash in gasoline direct injection (GDI) engine soot oxidation: Evolution of pore structure and surface chemistry

Ash deposition in gasoline particulate filters (GPFs) can promote soot oxidation, yet its underlying catalytic role remain unclear due to variations in soot properties under practical engine conditions. In this study, the catalytic effects of two ash surrogates (i.e., SiO₂ and Al₂O₃) on gasoline direct injection soot oxidation were investigated, with a particular emphasis on the evolution of pore structure and surface functional groups during oxidation. Results show that under the test conditions of 650 °C and 16.6% O₂ atmosphere, ash significantly accelerates soot oxidation, reducing total oxidation time and increasing the apparent rate constant by up to 2.2 times compared with ash-free soot. Ash-free soot expands during oxidation, while ash addition alters the soot oxidation mode by initiating contact oxidation which promotes the formation of mesopores (2–10 nm) and suppresses macropores (>50 nm) through pore filling and structural collapse. This shifts the pore size distribution toward smaller diameters, decreases the total pore volume, and enhances intimate soot/ash contact. Moreover, Al₂O₃ preferentially oxidizes aliphatic C–H groups at early stages due to its active hydroxyl sites, while SiO₂ sustains oxidation in later stages, enabling deeper oxidation of soot. Therefore, SiO₂ exhibits superior catalytic performance compared to Al₂O₃. This work provides mechanistic insights into ash-catalyzed soot oxidation and offer guidance for optimizing GPF regeneration strategies.