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直接甲醇燃料电池（DMFC）由于其能量密度高、排放低、燃料来源广泛等特点受到了广泛关注，有望成为能用于便携式电子设备和汽车的高效清洁新能源。然而，阳极催化剂对甲醇电氧化的低催化活性，易被CO等中间产物毒化导致的不稳定性，贵金属铂（Pt）基催化剂的高成本以及甲醇穿梭导致的混合电位等问题，都极大地阻碍了DMFC的商业化发展。另外，直接甲酸燃料电池（DFAFC）也有着与DMFC相似的优点和缺点。本文从催化剂各组分的协同作用入手，通过一种简单的共还原法制备了钯（Pd）-稀土元素催化剂，并分别以炭黑、活化后的多壁碳纳米管（aMWCNTs）、表面修饰的多壁碳纳米管和Ti4O7作为载体。借助、X射线衍射(XRD) 、透射电子显微镜（TEM）对所制备的催化剂的结构、形貌进行表征，并通过X射线光电子能谱（XPS）研究了催化剂表面元素的价态变化。通过循环伏安法（CV）、线性扫描法（LSV）、CO Stripping溶出法和计时电流法（CA）等测试技术对催化剂的对甲醇和甲酸电氧化的催化性能进行了深入的研究。首先，采用NaBH4还原的方法制备了不同原子比的Pd-Er/C催化剂。XRD和TEM的结果表明Pd-Er金属纳米颗粒可以很好地分散在炭黑上，仅有极少量的团聚现象。电化学测试结果表明：和其它的催化剂相比，Pd-4Er/C催化剂具有更高的甲醇电氧化催化活性和稳定性。加入的Er可以移除甲醇电氧化过程中产生的CO等有毒中间产物，所以其作用可以部分用双功能机理进行解释。XPS结果表明，加入的Er能够增加零价钯的含量，换句话说就是Er起到对钯金属化的作用。这是Er改善Pd催化剂催化性能的另一个原因。此外，还制备了Pd-Yb催化剂。发现当Yb的添加量为2%时能够达到最大的甲醇电氧化催化活性。但是由于其催化活性略低于Pd-4Er/C催化剂，所以在后续部分深入研究了Pd-Er催化剂的性能。基于Pd-4Er/C催化剂优异的催化性能，制备了Pd-4Er/aMWCNTs的催化剂。但是Pd-Er纳米颗粒在aMWCNTs载体上发生了严重的团聚现象，通过红外分析得知这可能是由于碳纳米管具有较大的疏水性引起的。电化学测试结果表明相较于Pd-4Er/C催化剂，Pd-4Er/aMWCNTs催化剂对甲醇电氧化的催化活性降低了，这是由于纳米颗粒团聚造成的。用壳聚糖（CS）功能化aCNTs以后，制备的Pd-4Er /aMWCNTs-CS的催化剂比Pd-4Er/C催化剂表现出了更好的甲醇电氧化催化性能，这是由于CS的存在改善了纳米颗粒的分散性并且增加了催化剂中Pd(0)的含量。采用NaBH4还原法将Pd-Er催化剂颗粒负载在非碳载体Ti4O7上。将Pd-4Er/Ti4O7的电化学测试结果与Pd-4Er/C催化剂对比，发现炭黑载体在碱性甲醇溶液中的作用要好于Ti4O7载体。而根据我们先前的研究，负载于Ti4O7载体上的Pd基催化剂对于甲酸直接电氧化的催化活性要高于负载于炭黑载体的Pd基催化剂。这可能和Ti4O7的晶体结构有关，因为Ti4O7是一种离子型晶体，具有较强的极性，其对甲酸的吸附能力明显好于对甲醇的吸附。为了验证Pd-Er双金属催化剂是否也符合这个猜想，用两种不同载体负载的Pd-Er催化剂催化甲酸电氧化，结果发现Ti4O7负载的催化剂催化甲酸电氧化的活性均要好于炭黑负载的催化剂。因此说明了载体的作用与电解质溶液也是密切相关的。;Direct methanol fuel cell (DMFC) has received enormous attention due to high energy density, low emissions, and abundance of methonal. It is expected to become a highly-efficient and clean future energy source for portable electronic devices and automobiles. However, challenge issues such as low catalytic activity of anode catalysts for the electrooxidation of methanol, susceptibility of the catalysts to be poisoned by CO-like intermediates formed in the methanol oxidation reaction and high cost of noble metal platinum (Pt) -based catalysts, as well as the mixed potential caused by methanol crossover have greatly hindered the commercialization of DMFC. In addition, direct formic acid fuel cell (DFAFC) also has similar strengths and weaknesses as DMFC.In this work, inspired by synergistic effect of the catalyst components, a simple simultaneous co-reduction method was used to prepare palladium (Pd)-rare earth catalysts, which were supported on carbon black, activated multi-wall carbon nanotubes (aMWCNTs), functionalized aMWCNTs and Ti4O7, respectively. The structure and morphology of the prepared catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The valence state changes of the surface elements were studied by X-ray photoelectron spectroscopy (XPS). The electrocatalytic performances of these catalysts for methanol/formic acid electrooxidation were characterized by cyclic voltammetry (CV), linear sweeping voltammetry(LSV), CO stripping experiments and chronoamperometric (CA).Firstly, Pd-Er/C catalysts with different Pd/Er ratios were prepared by a sodium borohydride reduction method. The results of XRD and TEM show that Pd-Er nanoparticles can be dispersed uniformly on carbon black with little agglomeration. Compared with other catalysts, Pd-4Er/C catalyst has higher catalytic activity and better stability for methanol electrooxidation. It can be seen that the addition of Er can remove CO-like intermediates, which can be partially explained by the bi-functional mechanism. XPS results show that the addition of Er can increase the content of Pd (0). In other words, Er has a “metallization’’ effect on Pd, which is the other reason for the improvement of catalytic performance of Pd catalysts.In addition, the Pd-Yb/C catalysts were also prepared. When the addition amount of Yb was 2%, the maximum catalytic activity of methanol electrooxidation could be achieved. However, the catalytic activity is slightly inferior to Pd-4Er/C catalyst. Thus, the performance of Pd-Er catalysts was further studied in the following part.Based on the excellent catalytic performance of the Pd-4Er/C catalyst, Pd-4Er/aMWCNTs catalyst was prepared. However, the Pd-Er particles supported on aMWCNTs have a significantly more serious degree of agglomeration, which may be due to the hydrophobicity of aMWCNTs according to FTIR analysis. The catalytic activity of Pd-4Er/aMWCNTs catalyst for methanol electrooxidation is lower than that of Pd-4Er/C catalyst, which is caused by the agglomeration of nanoparticles. After functionalized with chitosan (CS), the prepared Pd-4Er/aMWCNTs-CS catalyst exhibited better catalytic performance for methanol electrooxidation than Pd-4Er/C catalyst, which can be ascribed to the improved dispersion of the nanoparticles and the increased content of Pd (0) due to the presence of CS.The Pd-Er nanoparticles were supported on Ti4O7 by a co-reduction method. Comparing the electrochemical testing results of Pd-4Er/Ti4O7 with Pd-4Er/C, it is found that carbon black support performs better in alkaline methanol solution than Ti4O7 support. According to our previous researches, the catalytic activity of Ti4O7 supported Pd-based catalysts for direct electrooxidation of formic acid is higher than that of carbon black supported Pd-based catalysts. This may be related to the ionic crystal structure of Ti4O7 with strong polarity, and its adsorption capacity for formic acid is significantly better than that for methanol. In order to verify whether the Pd-Er bimetallic catalyst also conforms to this conjecture, the electrooxidation activity of Pd-Er catalysts supported on two different supports for formic acid was tested. It was found that the Ti4O7 supported catalysts performed better than the carbon black supported catalyst. Therefore, the effect of the support is closely related to the electrolyte solution.
|华鼎. 新型稀土-钯基催化剂对甲醇/甲酸电氧化的催化性能研究[D]. 中国科学院大学,2020.|
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