Diesel oxidation catalysts (DOC) were investigated for oxidation activity, NO conversion stability, and sulfur poisoning/regeneration on Pd/Al2O3, Pt/Al2O3, and Pd–Pt/Al2O3 catalysts. The Pd/Al2O3 catalyst was more active for CO and hydrocarbon (C3H6 and C3H8) oxidation, while the Pt/Al2O3 catalyst efficiently oxidized NO. The formation of a Pd–Pt alloy in the Pd–Pt/Al2O3 catalyst maintained Pd in a more reduced phase, resulting in the superior activity of this catalyst for the oxidation of CO, C3H6, and NO in comparison with its monometallic counterparts. The Pd–Pt alloy not only provided more low-temperature activity but also retained the stability of NO oxidation. The Pd–Pt alloy also favored the spillover of SO2 to the alumina support, resulting in significantly higher adsorption capacity of the Pd–Pt/Al2O3 catalyst, extensively prolonging its lifetime. However, the stable sulfates on Pd–Pt/Al2O3 made it difficult to completely regenerate the catalyst. The bimetallic sample showed higher activity for CO, C3H8, and C3H6 after sulfur poisoning and regeneration.

Pt and Pd supported on beta zeolites, with different silica to alumina ratios (SARs), are examined in the form of diesel oxidation catalysts (DOCs) for the oxidation of CO, NO, C3H8, and C3H6. The effect of SARs on the physicochemical properties of the catalysts, their subsequent oxidation activity, and sulfur tolerance was investigated. A beta zeolite with a high SAR has low acidity and high hydrophobicity, which induces the agglomeration of either Pt or Pd in the catalysts during preparation and thermal treatment. The effect of sintering is more severe on Pt than on Pd catalysts. High-SAR zeolites retained Pt in a more metallic fraction in the obtained catalysts, whereas this effect was not significant for the Pd-based catalyst, wherein Pd exists mostly in the oxide form. High-SAR Pt/BEA and Pd/BEA catalysts exhibit better conversion of CO, NO, C3H8, and C3H6 than low-SAR catalysts. A linear relationship for turnover frequency (TOF) with SAR (and with average particle size) was found for all four oxidation reactions, namely, the higher is the SAR, the higher is the TOF. This suggests that the catalysts are tailorable by adjusting the SAR of the zeolite supports. A five-cycle test indicated that zeolite-based catalysts have greater stability than alumina-based catalysts. The activity for NO oxidation was very stable for the high-silicious Pt/BEA catalyst with time-on-stream, which was not the case for the Pt/Al2O3 catalyst. The Pd/BEA catalysts were more susceptible to sulfur poisoning than their Pt counterparts; however, they were easily regenerated. Also, the Pt/BEA catalysts were effectively regenerated, especially for the highest SAR.

Passive NOx adsorption performance was investigated for Pd-based BEA zeolite as a reference sample along with ceria, zirconium, and lanthanum as different promoters. The La-promoted sample showed favorable NOx desorption behavior regarding greater quantities of NOx release at temperatures higher than 200 °C. The study was therefore mainly focused on the La-promoted sample and Pd/BEA, since the largest effect was seen using La as a promoter. The effect of water and hydrogen pretreatments for passive NOx adsorption (PNA) processes was therefore studied for La–Pd/BEA and compared with that for pure Pd/BEA. It was shown that the presence of water reduces the NOx desorption at lower temperature. Hydrogen pretreatment decreases the NOx uptake and shifts the adsorption and desorption temperatures to higher values. Interestingly, the La–Pd/BEA sample was less sensitive to H2 pretreatment. Temperature-programmed oxidation (TPO) and X-ray photoelectron spectroscopy (XPS) results indicated the presence of more stable Pd oxide sites for La–Pd/BEA. Furthermore, Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis suggested that NO is adsorbed with stronger and more stable bonds in the form of nitrates compared with the Pd/BEA reference sample.

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