Pt基合金催化甲醇氧化密度泛函理论研究

发布时间：2018-05-27 16:44

本文选题：欠电位沉积氢 + 含氧物种　；参考：《重庆大学》2016年硕士论文

【摘要】：直接甲醇燃料电池是一种前景可观的移动化电源,其催化剂仍是以Pt为主的贵金属催化剂,但是催化剂Pt成本高,资源稀少,阻碍了直接甲醇燃料电池的商业化进程。此外,甲醇电催化氧化机理复杂,在反应过程中生成CO等中间物种,强烈的吸附在催化剂活性位上,阻止了甲醇的进一步催化氧化,导致催化剂中毒,催化活性降低。因此寻找高Pt利用率、高催化活性、稳定性和抗中毒性强的催化剂刻不容缓。甲醇在催化剂表面的电化学氧化,不仅依赖于甲醇的脱氢步骤,还与欠电位沉积Hupd的脱附、CO的吸附、水的活化离解和CO的氧化有关。据此,为了寻找、设计高Pt利用率、高催化活性、稳定性和抗中毒性强的催化剂,本文通过密度泛函理论(DFT)计算,构建了Pt(111)、Pt3Ru(111)合金,以及Pt3Ti(111)金属间化合物三种催化剂,探索不同结构的催化剂抗CO中毒能力,以及催化甲醇氧化活性差异的量化本质原因。研究发现:(1)Pt(111)掺杂Ru和Ti形成合金后,Pt3Ru(111)与Pt3Ti(111)表面的Pt原子的d带中心均发生不同程度的负移,负移程度为:Pt3Ti(111)Pt3Ru(111)Pt(111),其主要源于Ru和Ti的d轨道与Pt的d轨道重叠,发生电子转移,以Pt原子和Ti原子间的重叠最为显著。(2)H的吸附以及外加电荷条件下水合质子的构型变化发现,Pt3Ti(111)对H的吸附最弱,且Hupd较Pt(111)和Pt3Ru(111)先发生脱附(q=1.4,E=-0.99V),利于后续反应物种甲醇的优先吸附。(3)三种催化剂对OH的吸附强弱顺序为:Pt3Ti(111)Pt3Ru(111)Pt(111),OH主要吸附在Pt3Ru(111)上Ru位,以及Pt3Ti(111)上Ti位。外加电荷下,溶剂水分子的解离证实Pt3Ti(111)更利于OHad的形成,即其在q=0(E=-0.98V)就形成OHad,而Pt3Ru(111)与Pt(111)分别在q=0.7(E=0.65V)和q=1.2(E=0.99V)才形成OHad。进一步增加外加电荷研究Pt3Ti(111)上OHad吸附构型变化发现,q=1.7(E=-0.39V)时Oad形成,表明Pt3Ti(111)表面在正电位下主要吸附Oad。(4)CO均易吸附在各个催化剂的Pt位上,Pt(111)和Pt3Ru(111)吸附CO能力差别不大,Pt3Ti(111)吸附CO能力最弱,说明Pt3Ti(111)抗CO中毒能力最强。CO与OH共吸附结果表明,Ru和Ti掺杂后,使原本Pt(111)上较易发生排斥的CO和OH,产生了吸引力,以Pt3Ti(111)更为显著,说明当CO吸附时,掺杂合金更利于OH的形成,该结论在后续计算中得到证实。即外加电荷下,CO与溶剂水分子共吸附时,其OHad形成电荷发生了正移。(5)CO的电催化氧化机理分析发现:在OHad形成的条件下,Pt(111)和Pt3Ru(111)上CO与OHad作用形成中间产物COOH,再分解为CO2。其中COOH脱氢生成CO2为速度控制步骤,Pt3Ru(111)的活化能(63.30KJ·mol-1)低于Pt(111)(49.11KJ·mol-1),说明Pt3Ru(111)活性优于Pt(111)。在Pt3Ti(111)表面,由于OHad在Ti上吸附很强,以致CO无法直接与OHad反应形成COOH,其CO的氧化主要通过CO与Oad的作用,直接氧化为CO2,其活化能为46.99KJ·mol-1,比Pt(111)和Pt3Ru(111)上COOH脱氢生成CO2步的活化能更低,说明Pt3Ti(111)更利于CO的氧化。
[Abstract]:Direct methanol fuel cell (DMFC) is a promising mobile power source, and its catalyst is still a precious metal catalyst based on Pt. However, the high cost of Pt catalyst and the scarcity of resources hinder the commercialization process of DMFC. In addition, the mechanism of methanol electrocatalytic oxidation is complex, resulting in the formation of intermediate species such as CO in the reaction process, which is strongly adsorbed on the active site of the catalyst, which prevents the further catalytic oxidation of methanol, resulting in the poisoning of the catalyst and the decrease of the catalytic activity. Therefore, it is urgent to search for catalysts with high Pt utilization, high catalytic activity, stability and high resistance to toxicity. The electrochemical oxidation of methanol on the surface of the catalyst depends not only on the dehydrogenation of methanol, but also on the adsorption of CO in the desorption of Hupd, the activation and dissociation of water and the oxidation of CO. Therefore, in order to find and design catalysts with high Pt utilization, high catalytic activity, high stability and high resistance to toxicity, three kinds of catalysts, Pt111Pt3Ru-111) alloy and Pt3TiTi111) intermetallic compound, were constructed by density functional theory (DFT) calculation. To explore the ability of different structure catalysts to resist CO poisoning, and the quantitative essential reasons of the difference of catalytic methanol oxidation activity. It is found that the d band center of Pt atoms on the surface of Pt after doping Ru and Ti into alloys has different degrees of negative shift, and the negative shift degree is: 1 / Pt3Ti1 / Pt3Ru1 / 111, which is mainly due to the overlap of d orbitals of Ru and Ti with the d orbitals of Pt, and the electron transfer takes place in the d orbitals of Ru and Ti. The adsorption of H by Pt atom and Ti atom is the most obvious. The adsorption of H is the weakest, and the change of configuration of protons in water under the condition of external charge shows that the adsorption of H is the weakest. In addition, the desorption of Hupd was prior to that of Pt3Ru1) and Pt3Ru-111). The desorption of OH-O on the three catalysts was in the order of: Pt3TiTi111Pt3Ru111Pt111Pt111OOH mainly adsorbed on the Ru site on the Pt3Ru-111) and the Ti site on the Pt3TiN111), which was favorable for the preferential adsorption of methanol in the following reaction species. The order of the adsorption of OH was: Pt3TiTi111Pt3Ru111Pt3Ru111C). Under external charge, the dissociation of solvent water proved that Pt3TiN111) was more favorable to the formation of OHad, that is, it formed OHad1 at QU0EU (-0.98V), while Pt3Ru-111) and PtC111) formed OHad at QU0.7EN (0.65V) and Q1.2EN (0.99V), respectively. Further increasing the external charge to study the OHad adsorption configuration on Pt3TiC111) it is found that the formation of Oad is observed at 1.7V ~ 0.39V). The results show that the adsorption of Oad.(4)CO on the surface of Pt3TiC111) is easy to adsorb on the Pt site of each catalyst, and the adsorption capacity of Pt3TiC111) on the surface of Pt3TiTi111) is not different from that of Pt3Ru 111). The adsorption ability of Pt3TiC111) is the weakest. The results of co-adsorption of CO and OH showed that CO and OH, which were easily repellent on Pt3TiC111), were more attractive after doping with Ru and Ti, especially in Pt3TiC111), which indicated that the doped alloy was more favorable to the formation of OH when CO was adsorbed. This conclusion is confirmed in subsequent calculations. When CO was co-adsorbed with solvent water molecule under external charge, the electrocatalytic oxidation mechanism of OHad formation charge occurred positive shift. It was found that under the condition of OHad formation, CO reacted with OHad to form an intermediate product, COOH, and then decomposed to CO 2. The activation energy of COOH dehydrogenation to produce CO2 is 63.30KJ mol-1) lower than that of Pt(111)(49.11KJ mol-1, indicating that the activity of Pt3Ruf111) is better than that of Pt111C. On the surface of Pt3TiC111), because of the strong adsorption of OHad on Ti, CO can not react directly with OHad to form COOH.The oxidation of CO is mainly through the interaction of CO with Oad, and directly oxidizes to CO2. The activation energy of COOH dehydrogenation to CO2 is lower than that of 46.99KJ mol-1 and Pt3Ru111). The results show that Pt3TiC111) is more favorable for CO oxidation.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O621.25;O641.1

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