The Effect of Si/Al Ratio on the Oxidation and Sulfur Resistance of Beta Zeolite-Supported Pt and Pd as Diesel Oxidation Catalysts

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.