The Influence of Si/Al Ratios on Adsorption and Desorption Characterizations of Pd/Beta Served as Cold-Start Catalysts

• Main Authors: Ming Jiang, Jun Wang, Jianqiang Wang, Meiqing Shen
• Format: Article
• Language: English
• Published: MDPI AG 2019-03-01
• Series: Materials
• Subjects: cold start; Pd/Beta; Si/Al ratios; NO_x and C₃H₆ adsorption; low temperature desorption
• Online Access: https://www.mdpi.com/1996-1944/12/7/1045

The majority of NO_x is exhausted during the cold-start period for the low temperature of vehicle emissions, which can be solved by using Pd/zeolite catalysts to trap NO_x at low temperature and release NO_x at a high temperature that must be higher than the operating temperature of selective catalytic reduction catalysts (SCR). In this work, several Pd/Beta catalysts were prepared to identify the influence of Si/Al ratios on NO and C₃H₆ adsorption and desorption characterizations. The physicochemical properties were identified using N₂ physical adsorption, Fourier Transform Infrared Spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photo electron spectroscopy (XPS), and Na⁺ titration, while the adsorption and desorption characterizations were investigated by catalyst evaluation. The results indicated that the amount of dispersed Pd ions, the main active sites for NO and C₃H₆ adsorption, decreased with the increase of Si/Al ratios. Besides this, the intensity of Brønsted and Lewis acid decreased with the increase of Si/Al ratios, which also led to the decrease of NO and C₃H₆ adsorption amounts. Therefore, Pd dispersion and the acidic properties of Pd/Beta together determined the adsorption ability of NO and C₃H₆. Moreover, lower Si/Al ratios resulted in the formation of an additional dispersed Pd cationic species, Pd(OH)⁺, from which adsorbed NO released at a much lower temperature. Finally, an optimum Si/Al ratio of Pd/Beta was found at around 55 due to the balanced performance between the adsorption amounts and desorption temperature.