基于纳米碳材料的高效电催化剂设计及同步辐射表征

发布时间：2018-05-14 02:09

本文选题：单原子催化剂 + 纳米碳材料　；参考：《中国科学技术大学》2017年博士论文

【摘要】：电催化作为一种能够实现化学能到电能转换的手段,不仅为新能源供给及储存提供极大的便利,而且也可以有效地避免当前社会存在的环境问题,相关研究已引起了越来越多研究者的重视,特别地,设计新型催化材料是电催化研究的关键之所在。在电催化剂的设计过程中主要采取的策略有两个:增加活性位点的数量和提高反应位点的本征活性。基于此,传统合成过程中通常会采用调控纳米材料尺寸、形貌、成分以及结构等来获取更高效的电催化剂。随着单原子催化剂制备方法的日益成熟,其在电催化反应过程中也扮演着越来越重要的角色。与传统纳米颗粒相比,单原子催化剂把金属原子(尤其是贵金属)的利用率发挥到了极致,同时有着更加明显的尺寸效应、结构效应以及金属-载体界面效应。这些因素共同决定了单原子催化剂在催化反应过程中往往能够表现出高活性和高选择性。与此同时,同步辐射光源具有分辨率高、单色性好、穿透性强等优点,结合多种同步辐射表征技术能够给出吸收原子近邻配位原子的种类、距离、配位数等精细结构信息,是研究新型催化剂及其复合结构的有效手段和独特平台。在本论文中,我们以纳米碳基单原子催化剂在电催化反应中的应用为主线,从合成策略、配位结构、电子结构、金属-载体相互作用以及载体维度等角度,充分探讨了上述因素对催化性能的潜在影响。通过调控载体表面锚定位点的种类和数量、负载金属原子的种类,达到了同时增加活性位点数量,提升本征活性的目的;并依托同步辐射多种表征技术,深入地揭示了相应催化剂的电子结构、微观结构和催化性能之间的内在联系。此外,我们还初步探索了向基体材料中引入多余原子构成不饱和位点与催化剂性能的关系。相关研究内容及取得的成果总结如下:1.针对单原子催化剂中金属原子负载量过低的问题,我们创造性的提出了一种原位热解合成策略,通过简单的加热混合二氰二胺(DICY)、葡萄糖以及无机金属盐等前驱体并使其在高温惰性气氛条件下热解,成功制备了具有Metal-Nx配位结构的高负载单原子催化剂(SMAs@N-doped graphene)。DICY低温生成的g-C3N4充当了限域模板诱导芳香碳中间体在其层间聚合,同时随着温度的升高g-C3N4逐步分解以较慢的速率释放金属原子是样品成功制备的关键因素。同步辐射吸收谱和电化学测试结果表明,合成的具有Fe-Nx配位结构的Fe@N-doped graphene催化剂在碱性体系氧还原和锌空电池测试过程中都具有优异的性能,从而表现出较好的应用前景。该工作提出的这种通用的合成高密度纳米碳基单原子催化剂的方法,为含有Metal-Nx位点催化剂的高催化活性合成与表征提供了平台。2.为了验证双金属掺杂的单原子催化剂是否存在协同催化效应,我们利用低温热解法将NiFe双金属原子有效的嵌入到了 g-C3N4/CNT核壳结构中,成功地制备了 NiFe@g-C3N4/CNT催化剂。析氧反应测试发现,NiFe双金属掺杂的催化剂性能要远优于Ni/Fe单一金属掺杂的催化剂。对过渡金属(Fe/Ni) L边近边结构吸收谱(XANES)分析表明,相对于单一金属掺杂样品,双金属掺杂NiFe@g-C3N4/CNT的电子态发生了明显的变化,高氧化态的Ni有所增加,而Fe价态降低,这些是性能增加的主要原因。与此同时,前驱体中的NH4Cl促进了 g-C3N4/CNT核壳结构的形成,增加了催化剂的比表面积,从而也大大提高了 NiFe@g-C3N4/CNT催化剂的析氧性能。该工作为理解双金属掺杂的协同催化效应提供了合适的材料体系和有效的表征手段。3.为进一步研究活性位点数量和载体对催化性能的影响,我们通过水热法将不饱和硫位点引入到MoS2片层中,有效地增加了活性位点数量,激发出了面内的催化活性,其可以通过同步辐射近边吸收谱(XANES)和光电子能谱(XPS)表征结果得出。进一步地可以发现MoS2以相互交联的单壁碳纳米管束为骨架形成了独特的“树枝状”结构,使两者界面间通过C-S-Mo键形式的强化学耦合相互作用。以上两种因素协同作用加快了电化学析氢的动力学过程,从而表现出了更高的催化活性。该工作不仅提供了一种有效调控活性位点数量、强化催化剂-载体相互作用的方法,也为理解新型催化材料的微观结构-电子结构-催化性能的内在联系搭建了平台。4.单原子催化剂的性能极易受到周围配位原子环境的影响,与载体间的电荷相互作用也更加紧密。基于此,我们首先提出了不同维度的同种载体可能对催化性能也有影响,并设计制备了以零维碳纳米洋葱为载体的Pt1/OLC单原子催化剂。产氢性能测试表明其起始电位、同一电位下的电流密度数值与商业20 wt%Pt/C催化剂相当,并且远优于二维石墨烯的Pt1/Graphene催化剂。同步辐射吸收谱(XAFS和XANES)和原子分辨电镜(STEM)分析表明,零维碳纳米洋葱载体对三维空间的利用率更高有效增加了活性位点与反应物的碰撞概率。DFT计算结果进一步显示,碳纳米洋葱载体类似于一个微电容,有效的储存了电子;同时其高曲率以及Pt单原子“外凸”形成的尖端,增强了活性位点Pt附近的局域电场,进而加速了析氢动力学过程。该工作通过不同维度载体材料的对比证实了由此引起的催化性能差异,也为理性设计原子利用率和空间利用率最大化的催化剂提供了思路。
[Abstract]:Electrocatalysis, as a means to realize the conversion of chemical energy to electric energy, not only provides great convenience for the supply and storage of new energy, but also can effectively avoid the existing environmental problems in the current society. More and more researchers have paid more attention to the related research. Especially, the design of new catalytic materials is the key to the research of electrocatalysis. There are two main strategies in the design of the electrocatalyst: increasing the number of active sites and increasing the intrinsic activity of the reaction site. Based on this, the traditional synthesis process usually uses the size, morphology, composition and structure of nanomaterials to obtain more efficient electrocatalysts. With the single atom catalyst, a single atom catalyst is used. The preparation method is becoming more and more mature and plays a more and more important role in the electrocatalytic reaction. Compared with the traditional nanoparticles, the monatomic catalyst has made the use of metal atoms (especially precious metals) to the extreme, with more obvious size effect, structure effect and metal carrier interface effect. The factors jointly determine the high activity and high selectivity of the monoatomic catalyst during the catalytic reaction. At the same time, the synchrotron radiation source has the advantages of high resolution, good monochromatic and strong penetrability. Combined with a variety of synchrotron radiation characterization techniques, the type, distance and coordination number of the near coordination atoms of the absorption atom can be given. Fine structure information is an effective means and unique platform for the study of new catalysts and their composite structures. In this paper, we use the carbon based monatomic catalysts in the electrocatalytic reaction as the main line, and discuss the synthesis strategy, the coordination structure, the electronic structure, the metal carrier interaction and the carrier dimension. The potential effect of the above factors on catalytic performance. By regulating the type and quantity of the anchorage point of the carrier and the type of metal atom loaded, the number of active sites is increased and the intrinsic activity is increased, and the electronic structure and microstructure of the corresponding catalyst are deeply revealed by the multiple characterization techniques of synchrotron radiation. In addition, we have also preliminarily explored the relationship between the introduction of superfluous atoms into the matrix and the performance of the catalyst. The relevant research content and achievements are summarized as follows: 1. we creatively put forward one of the problem that the load of metal atoms in the single atom catalyst is too low. In situ pyrolysis, a high load single atomic catalyst (SMAs@ N-doped graphene).DICY with a Metal-Nx coordination structure was successfully prepared by a simple heating of two cyanamide two amine (DICY), glucose and inorganic metal salts and the pyrolysis of the precursors under the high temperature inert atmosphere. The template induces the interlayer polymerization of aromatic carbon intermediates. At the same time, the release of metal atoms at a slower rate with the increasing temperature of g-C3N4 is the key factor for the successful preparation of the samples. The synchrotron radiation absorption spectrum and the electrochemical test results show that the synthesized Fe@N-doped graphene catalyst with the Fe-Nx coordination structure is in the alkaline system. Both oxygen reduction and zinc air battery test have excellent performance, thus showing good application prospects. The general method of synthesizing high density carbon based monatomic catalysts for the synthesis and characterization of high catalytic activity containing Metal-Nx site catalyst provides a platform.2. for the verification of bimetallic doping. Whether the single atom catalyst has synergistic effect, we have successfully embedded the NiFe bimetallic atoms into the g-C3N4/CNT shell structure by low temperature pyrolysis, and successfully prepared the NiFe@g-C3N4/CNT catalyst. The oxygen evolution reaction test found that the performance of the NiFe bimetallic catalyst is much better than that of the Ni/Fe single metal doping. The analysis of the near edge structure absorption spectra (XANES) of the transition metal (Fe/Ni) L edge shows that the electronic state of the bimetallic doped NiFe@g-C3N4/CNT has been obviously changed compared to the single metal doped sample, the Ni of the high oxidation state increases and the Fe valence state decreases. These are the main reasons for the increase of the performance. At the same time, the NH4Cl in the precursor is promoted. The formation of g-C3N4/CNT core-shell structure increases the specific surface area of the catalyst, and thus greatly improves the oxygen evolution performance of the NiFe@g-C3N4/CNT catalyst. This work provides a suitable material system and an effective characterization method to understand the synergistic effect of bimetal doping,.3., as a further study of the number of active sites and the carrier pair. The influence of chemical properties is that we introduce the unsaturated sulfur site into the MoS2 layer by hydrothermal method, effectively increase the number of active sites and stimulate the catalytic activity in the surface, which can be obtained by the characterization of the synchrotron radiation near edge absorption spectrum (XANES) and photoelectron spectroscopy (XPS). Further, the single wall of cross linked MoS2 can be found. The carbon nanotube bundles form a unique "dendritic" structure, which makes the two interfaces between the two interfaces through the strong chemical coupling interaction in the form of C-S-Mo bonds. The synergistic action of the two factors accelerates the kinetic process of electrochemical hydrogen evolution, thus showing a higher catalytic activity. This work not only provides an effective regulation of active sites. Point number, the method of strengthening the interaction of catalyst carrier, also to understand the internal relationship between the microstructure, electronic structure and catalytic performance of the new catalytic material, the performance of the.4. single atom catalyst is very easy to be affected by the surrounding coordination atom environment, and the interaction with the carrier is more closely. Based on this, we first The same carrier of different dimensions may also affect the catalytic performance, and the Pt1/OLC monatomic catalyst with zero dimensional carbon nanoscale as carrier is designed and prepared. The hydrogen production performance test shows that its starting potential is equivalent to the commercial 20 wt%Pt/C catalyst at the same potential, and is far superior to the Pt of the two-dimensional graphene. 1/Graphene catalyst. Synchrotron radiation absorption spectroscopy (XAFS and XANES) and atomic resolution electron microscopy (STEM) analysis showed that the utilization ratio of zero dimensional carbon nanoscale carrier to three-dimensional space was higher and the collision probability of active sites and reactants was increased effectively. The results of.DFT calculation showed that the carbon nano onion carrier was similar to a micro capacitor and effective storage. At the same time, the high curvature and the tip formed by the "extruding" of Pt single atom enhanced the local electric field near the active site Pt, and then accelerated the process of hydrogen evolution. This work proved the difference in catalytic performance by the contrast of different dimensional carrier materials, and also designed the utilization and space utilization of the atom for rational design. The catalyst for maximizing the rate provides a way of thinking.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36;TQ127.11

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