Preparation of Reduced Pt-Based Catalysts with High Dispersion and Their Catalytic Performances for NO Oxidation
Preparation of Reduced Pt-Based Catalysts with High Dispersion and Their Catalytic Performances for NO Oxidation
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DOI：
10.3866/pku.whxb202005009
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发表时间：
2022-04-15
影响因子：
10.9
通讯作者：
Chen, Yaoqiang
中科院分区：
文献类型：
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作者：
Ding, Xinmei;Liang, Yanli;Chen, Yaoqiang
铂基催化剂广泛应用于柴油氧化催化剂(DOC)装置，主要用于氧化有害的HC、CO和NO排放。值得注意的是，NO氧化产生的NO2有利于NH3-SCR的低温活性，并促进柴油颗粒过滤器(DPF)的碳烟氧化。因此，NO的转化率是决定DOC性能的重要参数。考虑到日益严格的排放法规和经济效益，制备低成本、高活性的铂基催化剂势在必行。通常，铂-0含量是至关重要的，因为它是DOCS的活性成分。较小的铂颗粒有利于催化剂活性的提高。本研究采用改进的乙醇还原浸渍(MARI)法制备了高活性的1%(w，质量分数)铂/SiO_2-Al_2O_3(简称MA-铂/SA)催化剂。同时，采用常规浸渍法制备了相同铂负载量(记为C-铂/SA)的铂/SiO_2-Al_2O_3催化剂。X射线光电子能谱(XPS)和氢气程序升温还原(H-2-TPR)分析表明，MARI法制备的铂催化剂具有较高的铂氧含量。MA-PT/SA的PT-0含量接近60.3%，而C-铂/SA的PT-0含量仅接近23.1%。X射线衍射仪(XRD)、CO-漫反射红外傅里叶变换光谱(CO-DRIFTS)和透射电子显微镜(TEM)表征表明，与C-铂/SA相比，MA-PT/SA上的铂颗粒尺寸要小得多。在模拟柴油气氛中对MA-铂/SA和C-铂/SA进行了性能评价。结果表明，在无水和有水存在的情况下，MA-PT/SA催化剂上NO转化为NO2的最大转化率分别为74%和68%，远高于C-铂/SA催化剂上的42%和51%。此外，MA-铂/SA催化剂的30%NO转化温度(218℃)明显低于C-铂/SA催化剂(248℃)，表明其具有良好的低温活性。反应气体高温老化处理后，老化的MA-PT/SA催化剂保持了69%的NO转化率，而老化的C-PT/SA催化剂的NO转化率仅为41%。此外，NO+O-2共吸附的原位漫反射红外傅里叶变换光谱表明，较高的铂分散度和较高的铂含量有利于在较低温度下形成桥联硝酸盐作为NO氧化的中间物种，也有利于它们在较高温度下快速分解(或脱附)，从而具有较高的催化活性。此外，当铂负载量降至0.5%(W)时，通过MARI方法得到的最大NO转化率为%，这表明与铂负载量为1%(W)的C-铂/SA催化剂相比，该催化剂具有更高的催化活性。本工作为制备高活性、低贵重负载量的铂基催化剂提供了一种方法。
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 Pt-0 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 (H-2-TPR) analyses proved that the MARI method could produce Pt catalysts with higher Pt-0 content. Pt-0 content in MA-Pt/SA was similar to 60.3% while that in C-Pt/SA was only similar to 23.1%. X-ray di square raction (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 degrees C) markedly decreased as compared to that over C-Pt/SA (248 degrees C), 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 + O-2 co-adsorption suggested that higher Pt dispersion and higher Pt-0 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.