Preparation of Reduced Pt-Based Catalysts with High Dispersion and Their Catalytic Performances for NO Oxidation
Xinmei Ding,Yanli Liang,Hailong Zhang,Ming Zhao,Jianli Wang,Yaoqiang Chen.Preparation of Reduced Pt-Based Catalysts with High Dispersion and Their Catalytic Performances for NO Oxidation[J].Acta Physico-Chimica Sinica,2022,38(4):2005009.
1. Key Laboratory of Green Chemistry & Technology of the Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China;2. State Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610064, China;3. Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, China
Abstract:
Pt-based catalysts are widely used in diesel oxidation catalyst (DOC) units, primarily to oxidize the harmful HC, CO, and NO emissions. Notably, NO2 produced from NO oxidation is beneficial for low-temperature activity in NH3-SCR and promotes soot oxidation in diesel particulate filters (DPF). Thus, the conversion of NO is an important parameter for determining the performance of DOCs. Considering the increasingly stringent emission regulations and the economic effectiveness, preparation of low-cost and highly active Pt-based catalysts is indispensable. Generally, the Pt0 content is crucial as it is an active component of DOCs. Small Pt size is beneficial for improving the catalytic activity. In this study, we applied a modified alcohol reduction-impregnation (MARI) method to synthesize highly active 1% (w, mass fraction) Pt/SiO2-Al2O3 (denoted as MA-Pt/SA) catalyst. Meanwhile, using the conventional impregnation method, we prepared the Pt/SiO2-Al2O3 catalyst with the same Pt loading (denoted as C-Pt/SA) as a reference sample. X-ray photoelectron spectroscopy (XPS) and hydrogen temperature program reduction (H2-TPR) analyses proved that the MARI method could produce Pt catalysts with higher Pt0 content. Pt0 content in MA-Pt/SA was ~60.3% while that in C-Pt/SA was only ~23.1%. X-ray diffraction (XRD), CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), and transmission electron microscopy (TEM) characterization confirmed that the Pt particle size is much smaller over MA-Pt/SA as compared to that over C-Pt/SA. Performance evaluation of MA-Pt/SA and C-Pt/SA was conducted in a simulated diesel atmosphere. The results showed that the maximum NO conversion into NO2 over MA-Pt/SA is 74% and 68% in the absence and presence of H2O, respectively, which were much higher than those over C-Pt/SA (42% and 51% NO conversion with and without H2O, respectively). Furthermore, the temperature for 30% NO conversion over MA-Pt/SA (218 ℃) markedly decreased as compared to that over C-Pt/SA (248 ℃), indicating the excellent low temperature activity. After the aging treatment with reaction gas at high temperatures, aged MA-Pt/SA maintained 69% NO conversion while aged C-Pt/SA showed only 41% NO conversion. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO + O2 co-adsorption suggested that higher Pt dispersion and higher Pt0 content over MA-Pt/SA could facilitate the formation of bridging nitrates as intermediate species in NO oxidation at lower temperatures and could also facilitate their rapid decomposition (or desorption) at higher temperatures, thus imparting a high catalytic activity. Furthermore, a decrease in the Pt loading to 0.5% (w) resulted in a maximum NO conversion of 64% via the MARI method, suggesting a higher catalytic activity compared to that of C-Pt/SA with 1% (w) Pt loading. This work provides a method to prepare highly active Pt-based catalysts with low noble loading.
