Research on the Hydrothermal Stability and Sulfur Resistance of Pt–Pd/Al2O3–SiO2 Diesel Oxidation Catalysts

Abstract

The long-term performance of conventional Pt–Pd/Al2O3 diesel oxidation catalysts (DOC) is severely compromised by two major deactivation pathways: hydrothermal sintering and sulfur poisoning. To address this dual challenge, this work presents a novel design strategy of catalyst via surface modification. A silica-modified catalyst, namely Pt–Pd/Al2O3–SiO2, was successfully fabricated using an organosilicon sol method. Comprehensive characterizations, including CO chemisorption, XRD, and HAADF-STEM, reveal that the coated SiO2 forms an amorphous, thermally stable layer on the surface of the catalyst. This layer primarily functions as a physical confinement barrier, effectively suppressing the migration and coalescence of platinum group metal (PGM) nanoparticles during harsh hydrothermal aging (650 °C, 100 h). As a result, the hydrothermally aged catalyst Pt–Pd/Al2O3–SiO2-HT retains a much smaller average PGM size (∼21 nm, 10–40 nm range) compared to the severely sintered uncoated catalyst (∼35 nm, 10–60 nm range). This structural preservation translates to markedly enhanced catalytic durability. After hydrothermal aging, the coated catalyst exhibits a significantly smaller activity loss for NO oxidation (only 7.35% decrease in NO2/NOx ratio) versus the benchmark (24.99% decrease). The silica-induced surface acidity enhances sulfur resistance by both inhibiting sulfate formation and facilitating sulfate decomposition. Following accelerated sulfur aging, the Pt–Pd/Al2O3–SiO2 shows superior performance retention, with smaller increases in the light-off temperatures for CO and C3H6 oxidation (ΔT50 = 30 and 29 °C, respectively) and a smaller decrease in the maximum NO2/NOx ratio (11.15%) than the uncoated catalyst (ΔT50 = 42 and 41 °C; 18.15% decrease in NO2/NOx). This work demonstrates that a simple SiO2-coating strategy can concurrently and effectively address the key durability issues of low-PGM DOCs, offering a practical pathway to develop more robust catalysts for real-world diesel aftertreatment systems.