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

Abstract

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.

The Effect of Pt/Pd Ratio on the Oxidation Activity and Resistance to Sulfur Poisoning for Pt-Pd/Bea Diesel Oxidation Catalysts with High Siliceous Content

“…As a result, the difference in T 50 between the second and the first cycle test was +44 and − 43 • C for the Pt1 and the Pd1 catalyst, respectively. These trends for the CO conversion profiles were similar to those for the same types of catalysts (Pt/BEA and Pd/BEA) prepared by a wet impregnation route reported in our previous work [31]. In the second cycle, the Pd1 catalyst had a T 50 of 205 • C, which was lower than the T 50 for the Pt1 catalyst (215 • C).…”

“…This trend is contrasting with the CO conversion on both the Pt1 and Pd/Al 2 O 3 under the same reaction conditions [16]. However, we noted that this phenomenon was observed for the three Pd/BEA catalysts with different SiO 2 /Al 2 O 3 ratios (41, 217, and 501) [31]. Furthermore, the difference in CO conversion only occurred between the first can the second cycle while almost no difference in CO conversion was observed for all cycles from second to fifth [16,31].…”

“…However, we noted that this phenomenon was observed for the three Pd/BEA catalysts with different SiO 2 /Al 2 O 3 ratios (41, 217, and 501) [31]. Furthermore, the difference in CO conversion only occurred between the first can the second cycle while almost no difference in CO conversion was observed for all cycles from second to fifth [16,31]. Notably, the light-off temperatures of CO, NO, and C 3 H 6 were quite close to each other in the first cycle but not for the second cycle (Fig.…”

“…Zeolite Beta (BEA) and ZSM-5 have been proposed as effective supports for DOC [27]. In our previous study, we found that the use of beta zeolite as a support, in particular the one with high silica-alumina ratios, can enhance the oxidation performance of the DOC [31]. Especially, Pt/BEA was the promising catalyst in terms of the oxidation activity and sulfur regeneration, because it exhibited better performance than the benchmark Pt/Al 2 O 3 and other zeolite-based catalysts (Pt/ZSM-5 and Pt/Y) for CO, NO, C 3 H 6, and C 3 H 8 oxidation [32].…”

Advantages of High-Siliceous Zeolites in the Reactivity and Stability of Diesel Oxidation Catalysts

“…In our recent work, we investigated the effect of Si/Al ratios for Pt and Pd supported on β zeolites for the reactivity of the DOCs and found that a high Si/Al support showed better catalytic performance, in particular for the stability of NO oxidation. 26 However, to the best of our knowledge, there are no systematic studies published comparing Pt supported on different high-siliceous zeolites used as diesel oxidation catalysts. This study aimed to investigate the advantages of highsiliceous zeolites (β, Y, and ZSM-5) compared to Al 2 O 3 as supports for Pt-based diesel oxidation catalysts.…”