铂钯及其合金薄膜的（电）化学沉积与电催化红外光谱研究

【作者】 王超；

【导师】 蔡文斌；

【作者基本信息】 复旦大学 ， 物理化学， 2010， 硕士

【摘要】 光谱电化学是应用各种光谱技术原位(in-situ)或非原位(ex-situ)地研究电化学体系的一门交叉学科。电化学红外反射光谱可获得电极表面吸附物种的取向、排列、覆盖度等状态信息,它是从分子水平研究电极过程的一种有效手段。其中,衰减全反射模式的表面增强红外光谱(ATR-SEIRAS),由于表面选律简单、表面信号强、传质容易以及受本体溶液干扰小的优点,特别适合实时检测电极表面动态过程,如电催化反应。铂、钯作为重要的贵金属催化剂在燃料电池中被广泛使用。由于单纯的铂、钯的局限性,有必要对其改性,以进一步提高其活性和利用率,降低其用量。常用的方法包括形成合金或者表面修饰其它金属等,通过由于金属协同效应、表面电子结构调整以及第三体效应共同作用,使多元金属组分一般具有比单一金属更为优秀的催化性质。另外,由于去合金化能够改变催化剂的元素组成、增加比表面积以及调整表面电子机构,有必要进一步探索去去合金化对合金催化活性的影响。本论文针对甲酸和CO的电氧化以及氧气的还原等体系,研究了铂基、铂基合金薄膜的制备和电催化性能,还研究了去合金化对电催化性能的影响。更重要的是,应用现场ATR-SEIRAS方法研究了部分合金、表面修饰电极的CO吸附性质以及甲酸电氧化反应。主要内容和结果如下：1.硅表面化学沉积铂钯及其合金以及表面增强红外光谱研究运用ATR-SEIRAS方法研究铂、钯及其合金表面的电催化的前提是在红外窗口硅表面沉积一层光亮、均匀且导电的纳米金属薄膜。我们采用酸性化学镀法成功地在硅表面沉积了一层具有SEIRA效应的铂、钯纳米薄膜。相比碱性专利镀铂技术,酸性化学镀法镀液不腐蚀硅基底；相比碱性镀钯技术,组分简单不使用有机络合剂(乙二胺)进而减少了镀层的污染。甚者,该方法可以通过控制前躯体金属盐的浓度沉积不同组分比的铂钯合金镀层。XPS以及ICP-AES分析测试表明,所镀铂钯合金铂在表面富集。使用CO作为探针分子进行ATR-SEIRAS测量发现：酸性化学镀法制备的铂、钯纳米薄膜具有较强的SEIRA效应,红外吸收峰峰形正常；而在铂钯合金中,随着钯组分的提高,红外谱峰逐渐减弱出现双极,并最终倒转。这种红外现象的转变可以由有效媒质理论(Effective medium theories;EMT)加以模拟解释。我们还进而研究了不同比例的铂钯合金对于甲酸电氧化性能的影响,发现Pt/Pd合金优化比例约为9/91。2.钯镍合金对于甲酸的电氧化以及去合金化效应通过合金材料部分去合金化,提高电极真实反应比表面和调节其表面电子特性,是获得各类高性能电催化材料新的重要方法。在甲酸的电氧化体系中,考虑到纯钯电极的性能以及稳定性皆差的不利因素,我们通过合金共沉积的方法制备钯镍合金薄膜电极,然后通过电位伏安扫描控制合金组分Ni的去合金化,并研究了不同程度镍的去合金化对氧化甲酸的影响。FE-SEM表征表明去合金化使得电极表面粗糙度提高,CO脱附实验进一步表明去合金化的Pd-Ni电极粗糙度提高16倍左右。XPS以及ICP-AES数据表明去合金化的Pd-Ni电极表面富集Pd。氧化甲酸测试结果发现去合金化的Pd-Ni电极的氧化甲酸电流密度与转速的平方根成正比,属于典型的扩散控制模式。3.铂表面修饰金对于甲酸的电氧化研究为提高铂在电氧化甲酸中的抗CO中毒能力,提高其氧化活性以及稳定性,我们使用欠电位沉积(UPD)的方法在Pt表面沉积单层Cu,然后将Cu置换成Au,重复两次得到亚单层的Au覆盖Pt的电极表面(Pt-Au2),研究了Pt-Au2电极的电氧化甲酸行为。电化学ATR-SEIRAS测量表明：与纯Pt电极不同,Pt-Au2电极经过CO毒性中间体路径电氧化甲酸的量大为降低,抗CO中毒能力明显提高。这为制备高性能铂基直接甲酸燃料电池阳极催化剂提供了新思路。4.铂钯铁三元合金用于氧气还原研究初步探索电沉积铂钯铁三元合金薄膜以用于氧气的电还原研究。采用恒电位合金共沉积的方法,在金电极表面制备铂钯铁三元合金。研究发现,铂钯铁三元合金表现出与铂铁合金相似的性能,性能好于纯铂。更多还原

【Abstract】 Spectro-electrochemistry is an inter-discipline with the application of spectroscopic techniques to in situ or ex situ study of electrochemical systems. Electrochemical infrared reflection absorption spectroscopy can provide the information of the orientation, arrangement, and coverage for adsorbates on electrode, and thus is regarded as a useful tool for probing electrochemical interfaces at molecular level.In particular, attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) has the advantages of simple surface selection rule, high surface sensitivity, and facile mass transport, facilitating the real-time monitoring of the dynamic electrode processes, including electrocatalytic reactions.As two important noble catalytic metals, Pt and Pd are widely used in fuel cells. However, pure Pt and Pd catalysts could not meet the performance needs. By means of alloying and surface modification with other metals, improved electrocatalysis with lower loading of Pt and Pd can be achieved owing to the synergy effect of alloys, the adjustment of surface electronic structure as well as the third-body effect. Not restricted to bimetallic alloy, ternary alloy was also usually designed to meet higher performance requirements.In addition, dealloying treatment could increase the surface area and adjust the surface electronic property, and it was also an effective approach to obtain better catalysts.Along this line, in this thesis work we not only investigated various platinum-based and platinum-based bimetallic or trimetallic films for formic acid and CO oxidation, and oxygen reduction, but also examined the dealloying effect on electrocatalytic properties for formic acid oxidation.Furthermore, in situ ATR-SEIRAS was applied to the study of the adsorption of CO or the oxidation of formic acid on Pd-Pt alloys and Au-modified Pt electrodes.The main contents and results of my research work are summarized as follows:1.Chemical deposition Pt, Pd and their alloys on silicon surface for SEIRASThe prerequisite of applying in situ ATR-SEIRAS to the electrocatalysis is to locate a facile process to deposit uniform and conductive Pt, Pd and their alloy films on the basal plane of a Si ATR element. To this end, we successfully developed a chemical plating method from simple acidic baths by using the hydrazine as the reductant. Compared to the deposition from complicated basic baths for plating Pt and Pd films in the literature, our new chemical deposition does not cause severe corrosion of Si, in addition, it is flexible and universal for fabricating Pt, Pd and their alloys of different compositions by changing the concentrations of precursor metal salts without the need of introducing organic complexing agents. XPS and ICP-AES analysis results indicated the enrichment of Pt on Pt-Pd alloys surface. ATR-SEIRAS measurements indicated that the as-deposited Pt and Pd nanofilms yield normally directed and enhanced infrared absorption for surface adsorbates. Increasing Pd component tunes the band shape, direction and intensity, which may be explained by effective medium theory (EMT).In addition, optimal electrocatalysis towards formic acid oxidations was pinpointed to the Pt9Pd91 alloy.2.Pd-Ni alloy for formic acid electro-oxidation and dealloy effectDealloyment could increase the reaction surface area and adjust the surface electronic properties, which is an important approach to obtain high-performance electrocatalytic materials.Here, we first adopted the potentiostatic method to deposit Pd-Ni thin film on Au electrode, then partly dealloyed Ni by repetitive potential scans and studied the dealloying effect on the formic acid electro-oxidation.FE-SEM, and CO stripping results showed that the roughness factor increased up to 16 with the above dealloying treatment. XPS and ICP-AES data indicated that the dealloyed Pd-Ni electrode had the Pd enrichment on its surface. The current density of the formic acid oxidation on the dealloyed electrode was found to be proportional with the square root of rotation speed, suggesting a mass-transport-controlled process.3. Au decorated Pt surfaces for formic acid electro-oxidationFor the sake of improving anti-CO poison ability and long-term stability of a Pt-based catalyst, we modified Au layer onto the platinum surface through initial Cu UPD on Pt followed by galvanic replacement of Au (and repeating the above process, the electrode thus obtained was denoted as Pt-Au2).In situ ATR-SEIRAS on the formic acid oxidation on Pt-Au2 electrode revealed that CO poisoning intermediate was significantly decreased compared to that on Pt electrode, attributable to the high efficiency and stability of Pt-Au2 towards formic acid oxidation. 4. Pt-Pd-Fe ternary alloy for oxygen reductionPt-Pd-Fe trimetallic alloys were prepared and examined for oxygen reduction. The trimetallic films on Au electrode were prepared by the potentiostatic deposition, and they showed significant electrocatalysis towards O2 reduction with a performance comparable to that of a Pt-Pd alloy film but better than that of a Pt film.更多还原