铂表面修饰及相关电催化反应和增强红外光谱研究

【作者】 彭斌；

【导师】 蔡文斌；

【作者基本信息】 复旦大学 ， 物理化学， 2010， 博士

【摘要】 表面电化学是电化学与表面化学交叉的前沿学科,而电催化反应是表面电化学的研究热点,涵盖了燃料电池阳极毒物CO的电氧化、阳极燃料物种的电氧化、阴极氧气的电还原等。对于不同的电催化反应,催化剂的选择往往是决定催化效果的关键因素,因此对催化剂的表面改性、表征以及电催化反应机理的探索显得尤为重要。Pt是一种常见的燃料电池电催化剂,就Pt表面改性对分子吸附与反应的影响进行多层次研究,具有重要的科学意义和实际意义。电化学衰减全反射表面增强红外光谱(ATR-SEIRAS)具有表面选律简单,传质不受干扰,表面信号强,灵敏度高等特点,还可以很方便地检测出参与界面反应的共吸附水等重要信息,特别适合于实时跟踪电极表面极性较高的小物种的吸附变化。结合常规的电化学手段和现场表面增强红外光谱,可鉴别电催化反应过程的中间体物种,推断电极表面分子的取向和键合,从分子水平上探究Pt表面电催化反应机制,对催化剂表面修饰和改性提供理论依据。本论文利用ATR-SEIRAS技术,围绕Pt的表面修饰及其对电催化反应的影响展开工作。首先详细研究了氯离子特性吸附对Pt上CO氧化过程的影响,并考察了更强特性吸附的碘离子对CO氧化动力学的影响；其次,提出了一种通用的模拟欠电位沉积法(MUPD)在各种Pt表面上实现无电、可控修饰Sb(即Pt-Sb)用于甲酸的电催化氧化研究,并进一步将之拓展到Pb在Pt表面的修饰(即Pt-Pb);随后,结合SEIRAS和DFT计算研究了甲酸分子在Pt-Sb表面上的电催化氧化机理；最后,初步开展了乙醇在MUPD法制得的Pt-Sb、Pt-Pb电极上电氧化及表面红外光谱研究。论文的主要内容和结果摘要如下：1、特性吸附阴离子对Pt上CO氧化的影响CO是Pt表面电催化氧化的一个重要模型分子,另一方面,由于氯铂酸是制备阳极Pt/C催化剂的最主要的前驱体,使得氯离子在合成过程中容易被引入体系且难以在后续过程中去除,因此研究氯离子存在时Pt表面CO的氧化有着重要的意义。与此同时,氯离子在何电位下开始吸附,吸附在电极表面何处尚存在许多争议,氯离子如何影响CO的氧化过程还需要更多分子水平上的证据。本章详细研究了溶液中存在氯离子时,氢欠电位沉积区以及双层区电位下预吸附的CO在多晶Pt表面的氧化过程。电化学结果显示,氯离子的存在使CO预氧化峰及主氧化峰变宽,氧化峰电位发生正移；SEIRAS结果表明,氯离子在0.1V(vsRHE)时就开始吸附在未被CO占据的Pt空位上(包括但不仅限于缺陷位)与OH发生竞争吸附,或阻碍CO扩散到OH的吸附位发生反应,从而减缓了CO的氧化速率。低电位下氯离子的吸附对CO畴域(domain)产生一定的“压缩”效果,扰乱了吸附在CO层顶端的界面自由水排布,在光谱结果中表现为O-H伸缩振动峰积分强度的变弱。此外,当更强特性吸附的碘离子存在时CO的氧化受到了更大的阻碍,表现为碘离子不仅与表面OH物种发生竞争吸附,它还挤占了CO的表面吸附位造成CO在低电位下的非氧化脱附,并且仅当碘离子氧化后CO的主氧化才开始发生。通过比较析氢反应对CO氧化的影响,发现预吸附的CO层与其共吸附水层的结构比较稳定,其析氢后的氧化动力学并没有发生明显改变。2、一种Pt表面可控修饰Sb, Pb用于甲酸电氧化的新方法在Pt表面修饰Sb用于甲酸电氧化时,常用的表面修饰技术有不可逆吸附法(IRA)以及欠电位沉积法(UPD)两种。然而前者无法方便调控表面覆盖度达到最佳值,后者必须在电化学控制条件下完成,因此均不适于实际生产中大批量催化剂的合成和改性。在本章的工作中,提出了一种更为简捷,且可方便调控修饰Sb的覆盖度的方法来制备Sb修饰的Pt基催化剂,即模拟欠电位沉积(MUPD)法。通过将本体Pt电极或分散的Pt/C粉末浸入到酒石酸锑钾与抗坏血酸的混合溶液中,控制溶液的浓度,反应温度或时间,就可以获得最佳的覆盖度。其中抗坏血酸是一种温和的还原剂,它能显著降低体系的开路电位,使Pt表面保持还原态以利于Sb的修饰。实验发现,用MUPD法制备的Pt-Sb/C催化剂对甲酸的催化活性远高于商业化的Pt-Ru/C或传统IRA法制备的Pt-Sb/C。该方法可普遍适用于所有类型的Pt表面,特别适合大批量Pt/C的改性并用作直接甲酸燃料电池的阳极催化剂。此外,该方法还成功拓展到Pt表面Pb的修饰,相应的Pt-Pb或Pt-Pb/C对甲酸催化氧化也显示出优异的性能。3、EC-SEIRAS与DFT组合研究甲酸在Pt-Sb表面的电氧化机理迄今为止,有关甲酸在Pt表面的电催化氧化机理以及氧化过程产生的中间体甲酸根物种的作用,还存在许多争议,一种观点认为,甲酸在Pt上氧化遵循“双途径机理”,甲酸根是“直接氧化途径”的活性中间体；另一种观点认为,甲酸根对氧化电流的贡献很小,它在甲酸氧化过程中只是一个“旁观者”。也有一些DFT计算结果表明甲酸根氧化为C02需要较高的能垒,它很可能只是甲酸氧化的“助催化剂”。尽管甲酸在Pt上的氧化已有许多报道研究,但是甲酸在肤层金属原子(如Sb)修饰的Pt表面上的氧化机理目前尚无相关报道。针对于此,本章工作联用现场电化学表面增强红外光谱与周期性密度泛函理论计算,研究了甲酸在Sb修饰的Pt(即Pt-Sb)电极表面上的电化学氧化机理。电化学与红外增强光谱数据表明,与洁净的Pt相比,低电位下甲酸在Pt-Sb上的主氧化电流提升了约10倍,但同时表面甲酸根物种及CO的强度均大幅减小,说明“非甲酸根”途径是Pt-Sb电极上甲酸氧化的主要途径。基于周期性DFT计算结果,Pt表面的Sb吸附原子促进了甲酸分子以CH端向下构型吸附在Pt表面,且阻碍了甲酸O端向下构型的吸附,从动力学上更有利于甲酸完全氧化为CO2。此外,Sb的修饰还降低了CO在Pt上的吸附能,有助于减轻Pt上的CO中毒效应。4、乙醇在Pt-M(M=Sb, Pb)电极上氧化的SEIRAS研究初探乙醇作为直接醇类燃料电池的另一种可替代燃料吸引了广大科研工作者浓厚的兴趣。乙醇氧化的机理虽然已有大量研究,但还存在许多不明之处,其中最关键的问题是反应中乙醇分子C-C键如何被打断从而完全氧化生成CO2。在本章工作中,采用SEIRAS技术初步研究了乙醇在Pt及MUPD法制备的Pt-Sb、Pt-Pb电极上的电氧化过程。结果表明,相对于Pt而言,在Pt-Sb和Pt-Pb电极上乙醇的氧化峰电位负移,并且氧化峰电流大大提升；在SEIRAS谱图中,Pt-Pb及Pt-Sb电极上CO及界面自由水的峰积分强度有明显增大,意味着乙醇在这两种电极上更容易断键从而产生毒性中间体CO；另外,可能与乙酸C-O伸缩振动及O-H变形振动有关的谱峰从低电位下开始出现以及乙醛C=O振动峰的消失,意味乙醇在这两种电极表面上可能直接被氧化成乙酸。更多还原

【Abstract】 Surface electrochemistry is the foreland crossed by electrochemistry and surface chemistry, with an emphasis on the studies of electrocatalysis on metals including many fuel cell related reactions such as electrocatalytic oxidation of CO, anodic oxidation of fuel and cathodic reduction of oxygen. Catalyst design is the key to achieve higher catalytic performance, as a result, it is important to characterize the property of the catalysts upon the surface modification and to explore the mechanism of the catalysis process. Platinum is a widely used catalyst in fuel cells, therefore, it will be of great significance in both scientific and application aspects to investigate and analyze the adsorption and reaction on the modified Pt surfaces on molecular level.Electrochemical surface enhanced infrared absorption spectroscopy with Kretschmann ATR configuration (EC-ATR-SEIRAS) possesses merits of high surface sensitivity, strong surface signal and unrestricted mass transport, and facilitates the detection of coadsorbed water which may be included in the interfacial reactions. Specially, it is quite suitable for tracing the irreversible reaction of small molecules with high polarity. Furthermore, the EC-ATR-SEIRAS can effectively distinguish the intermediate during the electrocatalytic process and deduce the orientation and bonding of the molecules on the electrode surface, which provides important evidence to the electrocatalytic mechanism on molecular level and helps to design new modified catalysts with high catalytic activity.In the present thesis, investigations which are focused on the surface modification on Pt and its influence to the electrocatalysis have been carried out with ATR-SEIRAS. Firstly, the effect of the presence of specifically adsorbed chloride or iodide on the CO oxidation has been detailedly studied. Secondly, a versatile mimetic underpotential deposition (MUPD) approach to controlled modification of Sb on various Pt surface (Pt-Sb) towards efficient electrocatalysis of formic acid has been proposed and extended to the surface modification of Pb (Pt-Pb). Thirdly, the combined ATR-SEIRAS and DFT calculations has been used for the mechanistic study of formic acid electrooxidation at the Sb modified Pt electrodes. At last, the Pt-Sb and Pt-Pb electrodes prepared by MUPD have been applied to the primary investigation of ethanol electrooxidation. The main results and conclusions are summarized as follows: 1. Effect of specifically adsorbed anions on the CO electro-oxidation at Pt electrodeAs an important catalytic process, the electro-oxidation of CO has been extensively investigated on various single crystalline and polycrystalline Pt electrodes. On the other hand, high surface area Pt nanoparticles synthesized from PtCl62-ions-containing solutions, are mainly used in fuel cell-related practical catalysts in the pursuit of a better utilization of Pt. As a result, specifically adsorbed Cl- ions are likely brought into the system during the synthesis of the Pt-based catalysts and hardly removed from the Pt surfaces. Along this line, it is of fundamental and practical interest to understand the effect of Cl- on the CO electro-oxidation at Pt electrodes. Meanwhile, it is controversial that when and where the Cl" adsorbs on the surface and how it affects the CO oxidation. In this section, the electro-oxidation of CO adlayer on Pt electrode in Cl--containing 0.1 M HClO4 has been investigated with in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Two potentials were selected for predosing CO on the Pt electrode: one is in the H-UPD region, i.e.,0.1 V (vs. RHE) and the other is the double-layer region, i.e.,0.45 V (vs. RHE). The broadening of the prewave and the main peak for the CO oxidation is observed, in addition to the positively shifted oxidation potentials. The spectroelectrochemical data suggest the specific adsorption of Cl- starts at a potential as negative as 0.1 V which may compete with the adsorption of OH at CO-unoccupied sites (including but not limited to defect sites) and/or hinder the diffusion of CO to OH adsorption sites on Pt electrode, slowing down the CO oxidation. This competitive Cl- adsorption at lower potentials disrupts the interfacial free H2O molecules on the top of CO adlayer, signaled by a reduced OH stretching band intensity. Furthermore, the oxidation of CO is more interfered by the adsorption of iodides, which not only occupy the free sites but also replace partial CO molecules on the surface, and the CO main oxidation occurs only after the oxidation of iodides. On the other hand, the dynamics of CO oxidation is not obviously changed by the hydrogen evolution, indicating the structure of CO adlayer and is rather stable. 2. A new versatile approach to modify Sb or Pb on Pt towards efficient oxidation of formic acidTraditional surface modification of Sb could be achieved by either irreversible adsorption (IRA) or underpotential deposition (UPD) with subsequently external potential control to desorb partial adatoms. It is not suitable for the large scale synthesis or improvement of carbon supported catalysts in practice. In this section, a new facile approach towards developing superior Pt-based catalysts for HCOOH electrooxidation has been proposed, which is exemplified with a mimetic underpotential deposition (MUPD) of Sb on Pt surfaces to attain a favorable coverage. Suitable Sb modification was achieved simply through immersing a bulk Pt electrode or dispersing Pt/C powders in a Sb(III) solution mixed with ascorbic acid (AA). AA serves as the mild reducing agent to ensure freshly reduced Pt surfaces for Sb modification, as demonstrated by the negatively shifted open circuit potential. The catalytic activity towards HCOOH electrooxidation on the above Sb-modified Pt/C catalyst far exceeds that on commercial Pt-Ru/C or Sb-modified Pt/C through traditional irreversible adsorption. This electroless approach is generally applicable to all types of Pt surfaces, in particular suited for upgrading Pt/C for practical anode catalysts of direct formic acid fuel cells. Such an approach has also been extended to the modification of Pb on the Pt surface towards HCOOH electrooxidation.3. Combined ATR-SEIRAS and first-principles study on electrooxidation of formic acid at Sb modified Pt electrodesSo far it is still controversial about the mechanism of HCOOH oxidation on Pt surface and the effect of the formate intermediate during the process of HCOOH oxidation. The so-called "dual pathway" and "triple pathway" were proposed in which the formate was considered to be the active intermediate of’direct pathway" and the "spectator" during HCOOH oxidation, respectively. Note that the formate is also supposed to be helpful for the HCOOH oxidation on the basis of DFT calculations. However, no literatures of HCOOH oxidation on Sb-modified Pt (Pt-Sb) could be found yet. In this section, in situ electrochemical surface-enhanced infrared absorption spectroscopy (EC-SEIRAS) together with a periodic density functional theory (DFT) calculation has been initially applied to investigate the mechanism of formic acid electro-oxidation on Pt-Sb electrode. EC-SEIRAS measurement reveals that the main formic acid oxidation current on Pt-Sb electrode is ca 10-fold enhanced as compared to that on clean Pt electrode, mirrored by nearly synchronous decrease of the CO and formate surface species, suggesting a "non-formate" oxidation as the main pathway on the Pt-Sb electrode. Based on the calculations from periodic density functional theory (DFT), the catalytic role of Sb adatoms can be rationalized as a promoter for the adsorption of the CH-down configuration but an inhibitor for the adsorption of the O-down configuration of formic acid, kinetically facilitating the complete oxidation of HCOOH into CO2. In addition, Sb modification lowers the CO adsorption energy on Pt, helps to mitigate the CO poisoning effect on Pt.4. Primary investigation of the ethanol oxidation on Pt-M (M=Sb, Pb) electrodes with SEIRASEthanol is the very promising fuel in the low temperature fuel cells, and it has been intensively investigated in the last decade. The key of ethanol oxidation is how to break up the C-C bonding of ethanol molecule, and thus promotes the complete oxidation of ethanol. In this section, the primary investigation of ethanol oxidation on Pt-Sb or Pt-Pb electrodes prepared by MUPD has been carried out with SEIRAS. The results show that the Pt-M (M=Sb, Pb) not only negatively shift the peak potential but also enhance the peak current of ethanol oxidation. In the SEIRAS spectra, bands of CO and free water are clearly observed on Pt-M electrodes, which implies that it is easier for ethanol to break the C-C bonding and form CO on Pt-M electrodes than that on clean Pt. Besides that, bands at ca.1238 and 1398 cm-1 which are probably concerned to the vibration of CH3COOH, could be observed at low potentials while bands of C=O stretching of CH3CHO are hardly observed. The results imply that ethanol may be oxidized to acetic acid directly on the Pt-M electrodes.更多还原