甲醇燃料电池Pt基合金催化剂电催化甲醇氧化的理论和实验研究

发布时间：2019-06-30 23:15

【摘要】：直接甲醇燃料电池(DMFC)是直接以甲醇为燃料,将甲醇氧化时产生的化学能转变成电能的能源装置。因具有能量转化效率高,环境友好,易于储存和运输等优点而被认为是一种理想的清洁能源。贵金属铂(Pt)具有较高的催化活性,是目前应用最广泛的DMFC催化剂。但是由于Pt资源稀少、价格昂贵,制约了其商业化发展。此外,电催化甲醇氧化机理复杂,催化剂表面的活性位点容易被氧化过程中生成的CO等中间物种占据,导致催化剂中毒,催化活性降低,从而阻碍了甲醇的进一步催化氧化。因此,寻找并设计高Pt利用率、高催化活性、稳定性和抗CO中毒性强的催化剂势在必行。在本文中,我们利用密度泛函理论(DFT)研究了甲醇在PtAu和PtPd催化剂表面的催化反应机理,探索了二者抗CO中毒能力以及电催化甲醇氧化活性差异的本质原因,并通过实验方法验证理论计算结果。最后,通过软模板法制备了具有不同原子比例的PtCo合金纳米线催化剂并应用于甲醇催化氧化。我们的研究旨在为设计和优化性能更加的燃料电池催化剂提供一定的理论和实验指导,为推动燃料电池的商业化发展贡献一点绵薄之力。本论文将从以下五个部分进行研究:第一部分,主要介绍了直接甲醇燃料电池的工作原理、发展现状以及电极的反应机理,同时对催化剂的制备方法进行了简单介绍。另外,我们还阐述了本论文研究工作的意义。第二部分,详细介绍了本文研究所用到的理论计算方法:密度泛函理论(DFT)和过渡态理论(TST)。另外还对实验过程中催化剂所采用的物理表征和电化学表征手段进行了简单的描述。第三部分,基于周期性密度泛函理论(DFT)系统的研究了甲醇在PtAu(111)表面的催化反应机理。在所涉及的最稳定的中间体的吸附研究中,我们考虑的反应途径有两种:CO途径和non-CO途径。根据最稳定中间体及反应所需的过渡态,最终确定最有可能的两种途径为:CH3OH→CH2OH→CHOH→CHO→CO(CO途径)和CHO→HCOOH→COOH→CO2(non-CO途径)。对CO和non-CO途径进行比较,我们发现甲醇在PtAu(111)表面的反应主要是通过non-CO途径发生。且计算结果表明:在生成CO2之前,甲醇更倾向于生成CO,在CO途径中CHO分解生成CO的能垒仅为0.21 eV,而消除CO需要克服0.74 eV的能垒。因此,我们预测non-CO途径的发生并不能完全抑制CO的产生,在PtAu(111)表面仍然有部分CO的沉积,从而造成催化剂的中毒,使其催化活性及稳定性降低。第四部分,采用密度泛函理论(DFT)的方法对甲醇在PtPd(111)表面的分解反应进行了研究,并与甲醇在PtAu(111)表面的分解反应进行比较,探索了二者抗CO中毒能力以及催化甲醇氧化活性差异的本质原因,然后通过电化学沉积的方法合成PtAu和PtPd合金催化剂,并将其负载在单壁碳纳米管上,研究其对甲醇的电催化性能,以此来验证理论计算结果。计算结果表明,甲醇在PtPd(111)表面分解的主要途径为CH3OH→CH2OH→CHOH→CHO→CO(CO途径)。对甲醇在PtAu(111)和PtPd(111)表面催化反应机理进行比较研究,我们发现甲醇在PtPd(111)表面分解的反应速率决定步骤的能垒比其在PtAu(111)表面的低0.26 eV,表明PtPd催化剂的活性高于PtAu催化剂的活性。甲醇在PtAu(111)表面的分解主要通过non-CO途径发生,而CHO分解生成CO的能垒仅为0.21 eV,表明non-CO途径的发生并不能完全抑制CO的产生,仍然有部分CO的沉积。而甲醇在PtPd(111)表面的分解主要通过CO途径发生,且生成CO需要克服的能垒为0.26 eV,表明反应过程中会有很多的CO沉积。因此,PtAu催化剂的抗CO中毒能力优于PtPd催化剂。实验结果表明,PtPd催化剂的催化活性和稳定性优于PtAu催化剂。第五部分,通过软模板法合成了PtCo、PtCo2、PtCo3和Pt3Co合金纳米线催化剂,并将其负载在碳黑上,研究其对甲醇的电催化活性及稳定性。循环伏安测试表明PtCo2/C在甲醇催化氧化实验中表现出最高的电催化活性。基于流体动力学的方法,通过线性扫描伏安曲线分析并计算了PtCo、PtCo2、PtCo3和Pt3Co催化剂在甲醇氧化反应中速率决速步的电子转移系数(α)和电极表面溶液的扩散系数(D0)。计算结果表明,PtCo2合金纳米线催化剂的α和D0最大。因此从动力学的角度分析,PtCo2合金纳米线催化剂催化活性的提高与α和D0有关。
[Abstract]:Direct methanol fuel cell (DMFC) is an energy device that converts chemical energy generated when methanol is oxidized into electric energy directly with methanol as fuel. Has the advantages of high energy conversion efficiency, environment friendliness, easy storage and transportation, and the like, and is considered to be an ideal clean energy source. The noble metal platinum (pt) has higher catalytic activity and is the most widely used dmfc catalyst at present. However, because of the scarcity of the Pt resources, the price is expensive and the commercial development is restricted. In addition, that oxidation mechanism of the electrocatalytic methanol is complex, the active site of the catalyst surface is easily occupied by the intermediate species such as CO generated in the oxidation process, the catalyst is poisoned, the catalytic activity is reduced, and further catalytic oxidation of the methanol is hindered. Therefore, it is imperative to find and design a catalyst with high Pt utilization ratio, high catalytic activity, stability and high toxicity in CO. In this paper, we use the density functional theory (DFT) to study the catalytic reaction mechanism of methanol on the surface of PtAu and PtPd catalyst, and explore the intrinsic reason of the difference between the anti-CO poisoning ability and the oxidation activity of the electrocatalytic methanol, and verify the theoretical calculation results by the experimental method. And finally, a PtCo alloy nanowire catalyst with different atomic proportions is prepared through a soft template method and is applied to catalytic oxidation of methanol. Our research is designed to provide some theoretical and experimental guidance for the design and optimization of the fuel cell catalyst with improved performance, which can contribute to the commercialization development of the fuel cell. In the first part, the working principle of direct methanol fuel cell, the development status and the reaction mechanism of the electrode are introduced, and the preparation method of the catalyst is briefly introduced. In addition, we also set forth the significance of the research work in this paper. In the second part, the theoretical calculation method used in this study is described in detail: the density functional theory (DFT) and the transition state theory (TST). In addition, the physical characterization and electrochemical characterization of the catalyst in the course of the experiment are briefly described. In the third part, the mechanism of the catalytic reaction of methanol on the surface of PtAu (111) is studied based on the cyclic density functional theory (DFT) system. In the adsorption study of the most stable intermediates involved, there are two types of reaction pathways that we consider: the CO pathway and the non-CO pathway. The most likely two routes are: CH3OH, CH2OH, CHOH, CHO-CO (CO) and non-CO (non-CO approach), according to the transition state required by the most stable intermediate and the reaction. The CO and non-CO approaches were compared, and we found that the reaction of methanol on the surface of PtAu (111) was mainly through the non-CO approach. The results show that, before CO2 is generated, the methanol is more prone to the formation of CO, the energy barrier of the generation of CO in the CO pathway is only 0.21 eV, and the elimination of CO need to overcome the energy barrier of 0.74 eV. Therefore, we have predicted that the non-CO approach can not completely inhibit the production of CO, and the surface of PtAu (111) still has a partial CO deposition, thus causing the poisoning of the catalyst, and the catalytic activity and the stability of the catalyst are reduced. In the fourth part, the decomposition reaction of methanol on the surface of PtPd (111) was studied by the method of density functional theory (DFT) and compared with the decomposition reaction of methanol on the surface of PtAu (111). And then the PtAu and PtPd alloy catalysts are synthesized by the method of electrochemical deposition, and the PtAu and PtPd alloy catalysts are loaded on the single-wall carbon nano-tubes, and the electrocatalytic performance of the PtAu and PtPd alloy is researched, so that the theoretical calculation results are verified. The results show that the main way of the decomposition of methanol on the surface of PtPd (111) is CH3OH-CH2OH-CHOH-CHO-CO (CO approach). The mechanism of the catalytic reaction of methanol on the surface of PtAu (111) and PtPd (111) was studied. It was found that the energy base of the reaction rate determination step on the surface of PtPd (111) was lower than that of the PtAu (111) surface, indicating that the activity of the PtPd catalyst was higher than that of the PtAu catalyst. The decomposition of methanol on the surface of PtAu (111) is mainly through the non-CO approach, and the energy barrier of the CHO decomposition to generate CO is only 0.21 eV, indicating that the non-CO approach can not completely inhibit the generation of CO, and there is still some CO deposition. The decomposition of methanol on the surface of PtPd (111) is mainly through the CO approach, and the energy barrier to which CO needs to be overcome is 0.26 eV, indicating that there will be a lot of CO deposition in the reaction. Therefore, the anti-CO poisoning capability of the PtAu catalyst is superior to that of the PtPd catalyst. The results show that the catalytic activity and stability of PtPd catalyst are better than that of PtAu catalyst. In the fifth part, PtCo, PtCo2, PtCo3 and Pt3Co alloy nanowire catalysts were synthesized by soft template method, and supported on carbon black, and the electrocatalytic activity and stability of PtCo, PtCo2, PtCo3 and Pt3Co were studied. The cyclic voltammetry test showed that PtCo2/ C exhibited the highest electrocatalytic activity in the catalytic oxidation of methanol. The electron transfer coefficient and the diffusion coefficient of PtCo, PtCo2, PtCo3 and Pt3Co (PtCo, PtCo2, PtCo3, and Pt3Co catalysts in the methanol oxidation reaction and the diffusion coefficient (D0) of the electrode surface solution were calculated and calculated on the basis of the fluid dynamics. The results show that the catalyst of PtCo2 alloy is the largest and the D0 is the largest. The improvement of the catalytic activity of the PtCo2 alloy nano-wire catalyst is related to the content of D0 and D0.
【学位授予单位】：西南大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.3

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