碳化钨和Vulcan XC-72炭黑载钯催化剂对甲酸氧化的电催化性能

碳基吸附催化氧化（UCAO）是一种常见的废气处理技术，用于去除有机废气中的挥发性有机物（VOCs）。

它通过两个主要的过程——吸附和催化氧化来实现废气的净化。

在UCAO过程中，废气首先通过一个活性炭床，其中的活性炭通过吸附作用将VOCs 从气相中捕集下来。

吸附是通过活性炭表面的多孔结构和吸附性能实现的。

随后，废气流经一个催化剂层，常用的催化剂包括铂、钯、银等贵金属或其氧化物。

这些催化剂能够催化氧化吸附在活性炭表面上的VOCs，将其转化为二氧化碳（CO2）和水（H2O）等无害物质。

UCAO技术的优点包括高效去除VOCs、较低的能耗和操作成本，以及适用于不同浓度和类型的有机废气。

它广泛应用于工业生产过程中产生的挥发性有机废气的治理，如印刷、油漆、溶剂使用等行业。

需要注意的是，UCAO技术的应用需要根据具体情况进行工艺设计和催化剂选择，以确保最佳的废气处理效果和经济性。

同时，在使用过程中还需要考虑废气的温度、湿度、气体成分等因素对系统性能的影响。

2008年7月第16卷第7期 工业催化I N DUSTR I A L CAT ALYSI S July 2008Vol .16　No .7无机化工与催化收稿日期:2007-12-03 基金项目:国家自然科学基金资助项目(20576033);上海市自然科学基金资助项目(05ZR14036)作者简介:俞　燕,1982年生,女,江苏省扬州市人,在读硕士研究生。

通讯联系人:赫崇衡。

E 2mail:chhe@ecust .edu .cn碳纳米管、碳化钨在直接法合成过氧化氢中的应用俞　燕,苏　勇,徐冉冉,李斯迈,赫崇衡3(华东理工大学化工学院化学工程系,上海200237)摘　要:选用新型纳米材料———碳纳米管(CNTs )作为催化剂载体,用沉积沉淀法制备负载型钯-铂合金催化剂。

同时选用过渡金属碳化物材料(碳化钨)为催化剂,其具有类似于Pt 的表面电子结构,有望替代贵金属催化剂。

探讨了Pd -Pt/CNTs 及碳化钨在氢氧直接合成过氧化氢反应中的催化性能。

关键词:催化化学;碳纳米管;碳化钨;直接法;过氧化氢;钯催化剂;铂催化剂中图分类号:T Q426.94;T Q123.6 文献标识码:A 文章编号:100821143(2008)0720039204Appli ca ti on of carbon nanotubes and tungsten carb i dei n d i rect syn thesis of hydrogen perox i deYU Yan,SU Yong,XU R anran,L I S i m ai,HE Chongheng3(Depart m ent of che m ical engineering School of Che m ical Engineering,East China Universityof Science and Technol ogy,Shanghai 200237,China )Abstract :Carbon nanotubes were used as the supports f or p reparati on of Pd 2Pt all oy catalysts f or direct synthesis of hydr ogen per oxide by depositi on 2p reci p itati on method .Tungsten carbide (WC )was als o used as the catalyst for direct synthesis of hydr ogen per oxide as a substitute for noble metal catalysts due t o their si m ilarity in surface electr onic structure .The catalytic behavi ors of WC and Pd 2Pt/carbon nanot 2bubes (CNTs )in direct synthesis of hydr ogen per oxide were investigated .Key words :catalytic che m istry;carbon nanotube (CNT );tungsten carbide;direct synthesis;hydr ogen per oxide;palladium catalyst;p latinu m catalystCLC nu mber :T Q426.94;T Q123.6 Docu ment code :A Arti cle I D :100821143(2008)0720039204 由H 2和O 2直接合成过氧化氢[1]。

钯碳氧化催化剂1. 引言钯碳氧化催化剂是一种重要的催化剂，广泛应用于有机合成、环境保护和能源转化等领域。

本文将从以下几个方面对钯碳氧化催化剂进行全面详细、完整且深入的介绍。

2. 钯碳氧化催化剂的定义与特性钯碳氧化催化剂是一种由钯、碳和氧组成的复合材料，具有优异的催化活性和选择性。

其特点包括：•高活性：钯作为一种过渡金属具有良好的电子传递能力，能够有效促进反应的进行。

•高选择性：钯碳氧化催化剂可以通过调节其表面结构和组分比例来实现对不同反应产物的选择性控制。

•可再生性：钯碳氧化催化剂具有较高的稳定性和可再生性，可以多次使用而不损失其催化活性。

3. 钯碳氧化催化剂在有机合成中的应用3.1 氢转移反应氢转移反应是有机合成中常用的一种反应类型，可以实现官能团的转化和酸碱平衡等目的。

钯碳氧化催化剂在氢转移反应中具有以下优势：•高活性：钯碳氧化催化剂能够高效催化氢转移反应，实现高产率和高选择性的转化。

•可控性：通过调节反应条件和催化剂的组分比例，可以实现对不同官能团的选择性转化。

3.2 羰基化反应羰基化反应是一类重要的有机合成反应，可以将碳氧双键转化为碳碳双键或碳氮双键。

钯碳氧化催化剂在羰基化反应中具有以下优势：•高活性：钯作为羰基还原和羰基还原偶联等重要反应中常用的催化剂元素，能够高效催化羰基化反应。

•宽底物适用性：钯碳氧化催化剂对于不同结构的羰基底物都具有较好的适用性。

4. 钯碳氧化催化剂在环境保护中的应用4.1 VOCs去除挥发性有机物（Volatile Organic Compounds，VOCs）是一类对环境和人体健康具有潜在危害的化合物。

钯碳氧化催化剂在VOCs去除中具有以下优势：•高效性：钯碳氧化催化剂能够高效催化VOCs的氧化反应，将其转化为无害的二氧化碳和水。

•选择性：通过调节反应条件和催化剂的组分比例，可以实现对不同VOCs的选择性去除。

4.2 废水处理钯碳氧化催化剂在废水处理中也具有重要应用。

高效、清洁的石墨烯负载的铂纳米团簇的合成在直接甲醇燃料电池的应用摘要石墨烯负载的铂纳米团簇是由一个高效、清洁的方法合成，使用这个方法时石墨烯氧化物和铂离子的前体会因为抗坏血酸而在一步法工艺中降低。

所得到的铂纳米团簇连接的石墨烯复合材料（PtNCs /石墨烯）通过X-射线衍射（XRD），场发射扫描电子显微镜（FE-SEM），透射电子显微镜（TEM）和能量色散X射线光谱（EDS）检测，可以直接显示，Pt纳米团簇可以成功的在石墨烯上形成,并很好地分布在石墨烯薄片的边缘和褶皱上。

进一步的电化学表征——包括循环伏安法（CV）和当前的方法表明：与普通的炭黑Vulcan XC-72和石墨负载Pt纳米团簇相比较，PtNCs /graphene具有明显更高的电催化活性和甲醇电氧化稳定性。

这将导致PtNCs/graphene进一步作为一种新的电极材料在直接甲醇燃料电池（DMFC）中的应用。

1.介绍几十年来，在直接甲醇燃料电池(DMFC)中，一直使用甲醇作为燃料而产生强化利益。

DMFC 与其他的燃料电池相比，主要的优点是它具有便携性、它所需的原料甲醇容易获得、它具有较高的能量密度——一个数量级大于压缩氢气。

然而甲醇交叉和中间物的毒效应,如一氧化碳(CO)对催化剂的毒性影响是影响DMFC在商业市场上应用的主要限制。

减少中间物产生的不良效应和提升催化剂效率是当前研究DMFC的主要问题。

在DMFC的甲醇氧化反应中主要选择铂作为催化剂，是由于它是在这个反应中催化活性最高的纯金属。

但是铂高昂的价格和自然界中的有限供应束缚了它在DMFC中的应用。

而铂不稳定的催化能力会产生具有毒效应的中间产物，是另外一个主要的障碍。

众所周知，催化剂的比活性主要取决于它们的分布和大小。

为了降低铂的负载和提高它的催化活性，可以控制铂的大小在纳米尺寸之类，这个方法也是最有效的。

由于具有很高的表面积体积比，拥有小而窄的分布特征的铂纳米粒子是高效电催化活性材料的理想当选者。

二氧化钛与碳化钨纳米复合材料制备及其对甲醇的电催化氧化活性胡仙超;陈丹;施斌斌;李国华【摘要】以市售纳米二氧化钛(TiO2)为载体,六氯化钨为钨源,将浸渍法与原位还原碳化技术相结合制备了核壳结构碳化钨(WC)/TiO2纳米复合材料; 应用X射线衍射分析、透射电子显微镜、高分辨扫描透射成像和X射线能量散射谱等手段对样品晶相、形貌、微结构和化学组成等特征进行了表征.结果表明,样品的晶相由金红石型TiO2、Ti4O7、WC、W2C和WxC构成,钨碳化物负载于钛氧化物外表面,构成比较典型的核壳结构.采用三电极体系和循环伏安法测试了样品在碱性溶液中对甲醇的电催化氧化活性,结果表明,相比于纯碳化钨和二氧化钛,复合材料的电催化活性得到了明显的提升.样品电催化活性的提升与前驱体钨钛摩尔比、还原碳化时间、核壳结构壳层的完整性和晶相组成以及核壳结构中二氧化钛和碳化钨之间的协同效应有关.这说明金红石是能够提升碳化钨电催化氧化活性的载体材料之一.%Tungsten carbide and titania nanocomposite with a core-shell structure was fabricated by combing chemical immersion with carbonization-reduction, using titania nanopowder as a support and tungsten hexachloride as a tungsten precursor. The crystal phase, morphology, microstructure, and chemical composition of the sample were characterized by X-ray diffraction, transmission electron microscopy, high resolution scanning transmission imaging, and energy dispersive spectroscopy (EDS). The results show that the crystal phase of the sample is composed of rutile, Ti4O7, WC, W2C, and WxC. The tungsten carbide particles coat onto the surface of the rutile support and thus form a core-shell structure. The electrocatalytic activity of the sample for methanol was measured by cyclic voltammetry with a three-electrode system in an alkaline solution. The results indicate that the electrocatalytic activity of the sample is higher than that of a pure titania phase and WC. The improvement in electrocatalytic activity is related to the reduction-carbonization time, the W to Ti molar ratio, the completeness of the shell layer in the core-shell structure, and the crystal phase of the sample. These factors can be correlated to a synergistic effect between titania and tungsten carbide in the nanocomposite. These imply that titania is asuitable support for the enhancement of the electrocatalytic activity of tungsten carbide.【期刊名称】《物理化学学报》【年(卷),期】2011(027)012【总页数】9页(P2863-2871)【关键词】碳化钨;二氧化钛;纳米复合材料;核壳结构;电催化活性【作者】胡仙超;陈丹;施斌斌;李国华【作者单位】浙江工业大学化学工程与材料学院,杭州310032;浙江工业大学分析测试中心,杭州310032;浙江工业大学化学工程与材料学院,杭州310032;浙江工业大学化学工程与材料学院,杭州310032;浙江工业大学化学工程与材料学院,杭州310032;浙江工业大学绿色化学合浙江工业大学纳米科学与技术研究中心,杭州310032【正文语种】中文【中图分类】O646自上世纪中叶发现碳化钨(WC)具有类铂的催化性能以来,其制备与应用研究引起了人们广泛关注.1-5因为,WC不仅在有机物的加氢脱氢反应6-8中具有明显的电催化活性,而且还对燃料电池中的氢阳极氧化9,10和甲醇的直接阳极氧化11-14具有良好的电催化活性.在实际应用过程中,人们发现WC的催化性能远不如铂15等贵金属催化剂.因此,如何提高WC的类铂催化活性,是其替代或部分替代铂等贵金属并走向实际应用的关键.正如上文所述,纯WC的电催化活性远不如铂等贵金属.因此,将WC与其它材料复合就成了提高碳化钨基电催化材料的主要技术途径之一.研究发现,在众多的复合材料载体中,贵金属与二氧化钛(TiO2)载体之间存在“强相互作用”.16这种作用是由于贵金属的被占据d轨道与Ti4+的空d轨道重叠而形成金属—金属键所致.16这一发现促使TiO2在催化领域的应用越来越广泛.受这种“强相互作用”的启发,我们意识到,既然贵金属与二氧化钛之间存在“强相互作用”,碳化钨与铂等贵金属又具有类似的催化活性,那么碳化钨与二氧化钛之间是否也存在“强相互作用”呢?或者说二氧化钛是否也可作为碳化钨的载体,以提升其电催化活性?为探索上述问题的答案,我们课题组17,18以无机材料为载体,将浸渍法与原位还原碳化技术结合制备了碳化钨基复合材料,并报道了其对对硝基苯的电催化性能;还将机械化学法与原位还原碳化技术相结合,制备了晶相组成不同的WC/TiO2纳米复合材料,并报道了其电催化活性.19虽然上述两条技术途径均成功制备了碳化钨与二氧化钛的复合材料,并证明两者之间在电催化性能方面存在协同效应,但其电催化性能仍然有待提高.因此,本文在分析总结上述技术方法的基础上,将浸渍法与原位还原碳化技术结合,首次以六氯化钨为钨源,市售纳米TiO2为载体,获得了具有核壳结构的WC/TiO2纳米复合材料,报道了影响WC/TiO2纳米复合材料电催化活性的因素.2.1 样品制备按不同的钨(W)/钛(Ti)摩尔比称取适量六氯化钨(WCl6)(中国湖南长沙市华京粉体材料科技有限公司)和二氧化钛(TiO2)(中国广东广州华力森有限公司),在室温下将TiO2分散于酒精溶液中,采用恒温磁力搅拌器(H01-1B型)(中国上海梅颖浦仪器仪表制造有限公司)以600 r·min-1的条件下搅拌2 h,直至两种物相混合均匀,再将WCl6加入到上述液体中,持续搅拌4 h,然后置于干燥箱中80°C干燥,即得前驱体.按上述方法分别制备了W/Ti摩尔比为1: 3、1:1和3:1的三种前驱体.每次取上述三种前驱体(记为TA)的一种2 g置于石英舟内,然后放置于管式炉中,通入N230 min后切换为H2(纯度≥99.999%)和CH4(纯度≥99.9%)混合气.其中,H2与CH4摩尔比为4:1;在通气体的同时,将管式炉升温到900°C,分别保温2、4、5、6和8 h后,在N2保护下冷至室温,取出后即得不同W/Ti比的复合材料样品,记为TR-a:b-n.其中,a:b为钨钛摩尔比,n为还原碳化时间(h).2.2 样品表征采用X射线衍射分析(XRD)、透射电子显微镜(TEM)、高分辨扫描透射成像[即高分辨或原子分辨原子序数Z衬度像(high resolution scanning transmission Z-contrast imaging),也叫做扫描透射电子显微镜高角环形暗场像(STEM)]和X射线能量散射谱(EDS)对样品晶相组成、形貌、微结构和化学组成进行了表征.其中TEM、STEM测试采用Philips CM 200 ultratwin高分辨电镜(附有可分析Ti、C、W和O的X射线EDS设备及液氮温度冷却台).XRD在荷兰PANalytical公司生产的XʹPert PRO型X射线衍射仪上进行,Cu Kα射线源(λ=0.1541 nm),电压40 kV,电流40 mA,步长0.033°,扫描范围10°-80°.样品的物相分析采用仪器自带的分析软件High Score.分析处理过程中,仪器本征宽化的扣除用厂家提供的仪器调试用的多晶硅标准样品.2.3 粉末微电极的制备将直径为60µm铂丝一端与导线相连,另一端与玻璃管熔封在一起,将封有铂丝的一端磨平抛光制成铂微盘电极,置于沸腾王水中腐蚀,并测量被腐蚀铂丝的深度,直到腐蚀深度为60µm左右,再将电极置于样品中轻轻挤压,即可制成粉末微电极,其结构详见文献.202.4 电化学性能测试电化学测试使用CHI 620B型电化学工作站(上海辰华仪器公司).测试过程中采用三电极电解池,工作电极为粉末微电极,辅助电极为1 cm2光亮铂电极(自制),参比电极为饱和甘汞电极(SCE).实验测试在298 K下进行.文中所给出电极电位值均相对于SCE,峰电流值已扣除背景电流.3.1 XRD结果图1为由W/Ti摩尔比为1:3的前驱体制备的样品XRD图.从图1中可看出,前驱体(TA),即反应时间为0 h的曲线中出现了15个主要衍射峰.其中,2倍衍射角(2θ)为27.47°、36.08°、41.26°、54.32°、56.68°和69.02°的6个衍射峰可归属于金红石型TiO2(PDF:73-2224)的特征衍射峰,2θ为25.32°、37.79°、48.03°、55.06°、62.76°和75.10°的6个衍射峰可归属于锐钛矿型TiO2(PDF:84-1286)的特征衍射峰,2θ为25.7°、34.2°和35.08°的3个衍射峰可归属于WO3(PDF:43-0679)的特征衍射峰.TA经过2 h还原碳化以后,锐钛矿型TiO2的三个特征衍射峰消失,在2θ为26.37°、28.97°和64.30°出现了3个新衍射峰,可归属于Ti4O7(PDF:50-0787)的特征衍射峰.这可能是由于样品在还原碳化反应过程中较薄的WO3壳层未能阻止H2对TiO2核的还原作用,使得TiO2转变成了Ti4O7.锐钛矿的衍射峰消失是由于锐钛矿为TiO2的低温稳定相,在高温条件下会转变成为TiO2的高温稳定相——金红石相.除上述衍射峰外,还可以发现三氧化钨的特征衍射峰也同时消失,取而代之的是在2θ为31.58°、35.72°和48.41°处出现的可归属于WC(PDF:51-0939)的特征衍射峰,在2θ为34.52°、38.03°和39.57°出现了可归属于W2C(PDF:35-0776)的特征衍射峰,以及在2θ为36.85°、62.03°和74.20°处出现了可归属于WxC (PDF:20-1316)的特征衍射峰.还原碳化4、5、6和8 h的样品的晶相组成与还原碳化2 h样品的晶相组成相同,即存在金红石型TiO2、Ti4O7、WC、W2C和WxC这五种晶相,但各种晶相的特征衍射峰的相对强度略有不同.这说明不同还原碳化时间下,上述五种晶相在样品中的相对含量略有不同;还说明虽然还原碳化时间不同,但其还原碳化过程基本类似;Ti4O7和WxC两种晶相的出现,说明相对于制备纯WC/TiO2复合材料而言,样品存在不同程度的还原过度.此外,对比所有还原碳化后的样品发现,在2θ为34°-45°范围内的基线均出现了不同程度的上拱现象.这种现象可能由两个方面的因素引起:一方面,碳化钨壳层包裹氧化钛核之后,两者之间必然存在一个界面.由于碳化钨与氧化钛的晶格常数有一定的差异,界面附近两种物相的晶格必然存在明显的畸变现象;另一方面,样品的氧化钛中存在三种物相,即金红石、锐钛矿和非化学计量比的Ti4O7,碳化钨中也存在三种物相,即WxC、WC和W2C,这些物相之间存在相互作用.图2为由W/Ti摩尔比为1:1的前驱体所制备的样品XRD分析结果.从图2中可看出,W/Ti摩尔比为1:1的前驱体与W/Ti摩尔比为1:3的前驱体晶相组合相同,即由金红石型TiO2、锐钛矿型TiO2和三氧化钨这三种物相构成.还原碳化2、4、5和6 h的样品由金红石型TiO2、WC和W2C三种物相组成.与W/Ti摩尔比为1:3的前驱体所制备的样品相比,不存在WxC和Ti4O7两种物相.这可能是因为随着W/ Ti摩尔比的增加,复合材料的壳层厚度增加.当还原碳化时间为2-6 h时,较厚的WO3壳层既阻止了H2对TiO2核还原作用,还保护了复合材料中TiO2核的晶相稳定性,避免了Ti4O7晶相的形成;同时,由于壳层厚度增加延长了渗碳作用的过程,使得复合材料中的壳层未能生成WxC晶相;与还原碳化2、4、5和6 h的样品相比,还原碳化8 h的样品出现了Ti4O7的晶相.这说明经过8 h的还原碳化后,W/Ti 摩尔比为1:1的前驱体的壳层未能阻止H2对TiO2核还原作用,致使Ti4O7晶相的形成.图3为W/Ti摩尔比为3:1的前驱体所制备的样品XRD分析结果.从图3中可以看出,与上述两种W/Ti的前驱体相比,晶相组成基本相同,即由金红石型TiO2、锐钛矿型TiO2和三氧化钨这三种物相构成.还原碳化2、4、5、6和8 h的样品由金红石型TiO2、WC和W2C三种物相构成.这说明W/Ti摩尔比为3:1的前驱体所制备的样品在小于8 h的还原碳化过程中,较厚的WO3壳层彻底地阻止了H2对TiO2核的还原作用,保护了TiO2核的晶相稳定性,又延长了壳层渗碳作用的过程,使得复合材料的壳层无法生成WxC物相.对比图1、图2和图3可看出,样品中物相越复杂,X射线衍射分析图谱的基线上拱越明显.这说明X射线衍射分析图谱的基线上拱不仅与样品的核壳结构有关,也与样品的晶相组合有关.3.2 TEM结果图4是由W/Ti摩尔比为1:3前驱体制备的样品TR-1:3-6的TEM照片.图4A为样品颗粒的形貌与表面结构,从中可看出样品颗粒的直径多在100 nm以下,仅有少量颗粒的直径大于100 nm,部分颗粒还具有核壳结构特征,如图4A中方框内的颗粒.图4B为样品颗粒的局部放大.从中可看出,以图中白色曲线为界,可将颗粒划分为两个部分,外围部分的灰度相对较高,构成厚薄稍有差异的壳层,并包裹内部灰度相对较浅的部分,具有明显的核壳结构特征.这可能是由于W/Ti摩尔比较小,样品中碳化钨不足,碳化钨在样品内的分布不均匀,影响样品颗粒的分散性,并致使多数颗粒的核壳结构不明显,仅部分颗粒具有核壳结构,且壳层厚度也不均匀.图5是由W/Ti摩尔比为1:1前驱体制备的样品TR-1:1-6的TEM照片.图5A为样品的颗粒大小与整体形貌,从中可看出,样品颗粒的直径均在50 nm以下,图中白色方框内的颗粒具有明显的核壳结构特征.为了进一步确认其微结构特征,将该颗粒局部放大,结果如图5B所示.从图5B中可以看出,样品颗粒依据其灰度可以分为两个部分,其中中心部分的灰度较浅,经在Philips CM 200 ultratwin高分辨电镜中测量和计算,其晶格条纹的间距为0.34 nm,与金红石(PDF:73-2224)的(110)晶面间距接近;外围壳层部分的灰度较高,经在Philips CM 200 ultratwin高分辨电镜中测量和计算,其一组晶面的晶格条纹间距为0.22 nm,与碳化钨(PDF:51-0939)的(100)晶面间距接近.上述晶面间距的实测结果与标准卡片的晶面间距数值有一定的差异.这些差异可能是系统误差和测量误差所致.结合样品的XRD分析结果,可以认为样品的核由金红石构成,壳层主要由碳化钨构成,两者构成了核壳结构纳米复合材料,且碳化钨与金红石之间存在共晶格和半晶格现象,如图5C所示.与样品TR-1:3-6相比,样品TR-1: 1-6的核壳结构更完整,壳层厚度也相对均匀.图6是由W/Ti摩尔比为3:1前驱体制备的样品TR-3:1-6的TEM照片.从图6A 中可看出,样品颗粒直径多在100 nm以下,与样品TR-1:3-6和TR-1:1-6相比,颗粒团聚严重.样品中部分颗粒也具有核壳结构特征,如图6B所示.与样品TR-1:3-6和TR-1: 1-6相比,其核壳结构特征不是那么显著.这可能是由于钨钛比过大,过多碳化钨颗粒包裹在载体二氧化钛外表面,致使碳化钨分布不均匀,并导致样品颗粒团聚. 综合比较样品TR-1:3-6、TR-1:1-6和TR-3:1-6的形貌和微结构特征可认为,WC/TiO2纳米复合材料核壳结构的完整性与前驱体的钨与钛摩尔比值相关. 3.3 STEM结果样品TR-1:1-6的EDS分析结果如图7所示.从图7中可看到,样品的EDS图谱中有O、Cu、W、Ti和C五种元素的谱峰.其中,Cu来源于分析时承载样品的铜网;C 的谱峰源自两个方面,一是分析时承载样品铜网上的碳膜,另一是样品颗粒本身的碳元素;其它三种元素均源自样品颗粒.这说明样品的化学组成由W、Ti、O和C四种元素构成.样品TR-1:1-6的STEM测试结果如图8所示.图8a和8b为采用EDS面扫时样品颗粒的形貌,图8 (c-g)为样品中元素的分布密度,图中颜色的深浅代表元素分布密度的高低,颜色越深,元素的分布密度越高,反之则越低.从图8f和8g中可看出,钨元素L和M线的密度分布轮廓与样品颗粒的形貌基本一致;从图8c中可看出,碳元素的K线密度分布较样品颗粒的形貌范围大;从图8d和8e中可看出,氧元素和钛元素的分布范围基本一致,且比样品颗粒的形貌的范围略小.结合样品的XRD分析结果,说明样品由碳化钨和氧化钛这两种化合物组成.上述结果说明碳化钨的分布范围大于氧化钛的分布范围,且碳化钨将氧化钛包裹在其内部.这与TEM的表征结果一致.其中,碳元素的分布范围大于样品颗粒形貌的范围,这是由于样品在分析过程中需要负载于铜网的碳支持膜上,受碳支持膜的影响所致.基于上述结果和分析可认为:样品TR-1:1-6的晶相由WC和TiO2构成,前者包裹于后者外表面,形成了较完整的以TiO2为核,WC为壳层的核壳结构.3.4 电催化性能不同还原碳化时间样品在碱性条件下对甲醇的循环伏安曲线如图9所示.其中,图9(A,B,C)分别是TiO2、WC和复合材料对甲醇的循环伏安曲线.由图9(A)可见,TiO2只具有一个氧化峰和一个还原峰;由图9(B)可见,WC对甲醇的电催化氧化没有明显的活性;由图9(C)可见,样品TR-1:1-2,TR-1:1-4, TR-1:1-5和TR-1:1-6在甲醇的电催化氧化过程中均具有两个氧化峰和一个还原峰,TR-1:1-8只具有一个氧化峰,还原峰不明显.这说明还原碳化2-6 h的样品对甲醇具有明显的电催化活性.为了更加直观并便于对比,将图9中各样品氧化峰和还原峰的峰电位和峰电流汇总列于表1中.由表1可看出,样品TR-1:1-2、TR-1:1-4、TR-1: 1-5、TR-1:1-6和TR-1:1-8的第一个氧化峰电位分别为-0.208、-0.188、-0.202、-0.114和-0.031 V,相应的氧化峰电流分别为13.49、15.40、9.173、3.056和4.814µA.对比所有样品发现,随着还原碳化时间的增加氧化峰电位逐渐增大.这说明在碱性溶液中样品对甲醇的电催化氧化活性随着还原碳化时间的增加而逐渐减弱,样品的电催化活性与还原碳化时间有着密切关系.样品TR-1:1-2、TR-1:1-4、TR-1:1-5和TR-1:1-6的第二个氧化峰电位分别为0.434、0.628、0.391和0.581 V,相应的氧化峰电流分别为12.00、14.13、9.708和3.247µA.对比样品第二个氧化峰的峰电位,对甲醇的电催化活性最好的为TR-1:1-5.虽然与炭载铂等贵金属催化剂21-24相比,碳化钨与氧化钛复合材料的峰电流明显较小,但其氧化峰电位明显负移.这说明若能进一步提高复合材料的导电性能,其对甲醇的电催化氧化将具有很好的应用前景.由表1还可以看出,样品TR-1:1-2、TR-1:1-4、TR-1:1-5和TR-1:1-6的第一个还原峰电位分别为0.162、0.175、0.164和0.035 V,相应的还原峰电流分别为2.336、2.137、1.900和1.644µA.对比上述还原峰电位,还原碳化时间介于2-5 h 时,电位变化不明显,还原碳化6 h后,还原峰电位明显减小.这说明样品对甲醇的电催化还原活性与还原碳化时间有关.对比表1中的氧化峰和还原峰的峰电位和峰电流,除还原碳化8 h的样品外,其它样品均具有两个氧化峰和一个还原峰.这组氧化还原峰很可能是复合材料的载体氧化钛中的钛离子对(Ti4+/Ti3+)在碱性条件下的氧化与还原可逆反应:上述反应在碱性溶液中有利向右进行而使其在循环伏安曲线中出现不对称的氧化峰和还原峰.扣除载体的一对氧化还原峰,复合材料的循环伏安曲线中的另一个氧化峰即为其对甲醇的电催化氧化峰.Frelink等25研究表明,甲醇在电催化氧化过程中在催化剂表面的主要反应是:碳化钨与二氧化钛复合材料在碱性溶液的循环伏安过程中是否按上述机理进行,有待于采用气相质谱与电化学联机,以及电化学现场红外光谱分析等方法进行深入探讨.综上所述,纯WC在碱性溶液中对甲醇没有明显的电催化氧化活性,二氧化钛在碱性溶液中对甲醇的电催化氧化活性也不强.两者复合后,复合材料在碱性溶液中对甲醇的电催化氧化活性明显高于二氧化钛对甲醇的电催化活性.这说明碳化钨与氧化钛复合后,两者对甲醇的电催化氧化活性具有显著的协同作用.图10是不同钨钛摩尔比的样品在碱性条件下对甲醇的循环伏安曲线.由图10可见,样品TR-1: 3-4、TR-1:1-4、TR-3:1-4和TR-5:1-4在甲醇的电催化过程中存在两个氧化峰和一个还原峰.这说明上述样品对甲醇具有明显的电催化活性.为了更加直观并便于对比,将图10中各样品的氧化峰和还原峰峰电位和峰电流汇总列于表2. 由表2可看出,TR-1:3-4、TR-1:1-4、TR-3:1-4和TR-5:1-4的第一个氧化峰电位分别为-0.204、-0.188、-0.201和-0.106 V,相应的氧化峰电流分别为19.96、15.40、12.81和12.95µA.比较上述四个峰电位数据发现,随着样品内钨钛摩尔比的增加,第一个氧化峰的电位变化没有明显的规律.这说明样品对甲醇的电催化氧化活性随钨钛摩尔比的增加呈现不规律变化.样品TR-1:3-4、TR-1:1-4、TR-3:1-4和TR-5:1-4的第二个氧化峰电位分别为0.611、0.628、0.674和0.637 V,相应的氧化峰电流分别为14.62、14.13、15.50和26.50µA.比较上述四个峰电位数据发现,随着样品内钨钛摩尔比的增加,第二个氧化峰的电位先升高后降低.这说明样品对甲醇的电催化氧化活性随钨钛摩尔比的增加而增强.当钨钛摩尔比达到3:1时,活性最强.这是由于当样品的钨钛摩尔比较小时,复合材料主要以TiO2/WC核壳结构的形式存在.这种结构具有优异的表面性能,使得样品具有良好的催化性能.当样品的钨钛摩尔比增大时,由于WC的团聚作用破坏了核壳结构材料的分散性,致使样品颗粒的团聚现象严重,从而使样品成为镶嵌TiO2的WC块体复合材料.这必然降低样品的比表面积,减弱了TiO2载体对催化反应的作用,降低了复合材料的催化反应活性. 由表2还可以发现,样品TR-1:3-4、TR-1:1-4、TR-3:1-4和TR-5:1-4的第一个还原峰电位分别为0.184、0.175、0.200和0.249 V,相应的还原峰电流分别为2.481、2.137、1.662和0.560µA.这说明样品对甲醇的电催化还原活性随钨钛摩尔比的增大先降低后升高,还原活性最高的样品是TR-5:1-4.将浸渍法与原位还原碳化法相结合,成功制备了核壳结构WC/TiO2纳米复合材料;样品的晶相组成以金红石型TiO2、Ti4O7、WC、W2C和WxC这五种晶相为主,各个样品的具体晶相组合不仅与其前驱体的钨钛摩尔比值有关,还与其还原碳化时间有关.不论样品的晶相组成如何,其化合物为钨碳化物和钛氧化物两种.其中,前者负载于后者的外表面,构成比较典型的核壳结构.钨碳化物在钛氧化物载体的外表面分布的均匀性,以及核壳结构中壳层的完整性和厚度与制备样品时的前驱体内的钨钛摩尔比值相关.相比于碳化钨和二氧化钛而言,复合材料对甲醇的电催化氧化活性得到了明显的提升.这与样品制备时前驱体复合材料的钨钛摩尔比、还原碳化时间和碳化钨壳层的完整性,以及核壳结构中二氧化钛和碳化钨之间的协同效应有关.【相关文献】(1) Levy,R.B.;Boudart,M.Science 1973,181,547.(2) Böhm,H.Nature 1970,227,483.(3) Xiao,T.C.;Hanif,A.;York,A.P.E.;Green,J.S.Phys.Chem. 2002,4,3522.(4) 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碳纳米管负载的Pd-Ag-Sn催化剂对甲酸的电氧化张媛媛;易清风;李广;周秀林【摘要】米用硼氢化钠还原的方法合成了碳纳米管负载的钯基纳米催化剂(Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT,Pd7Ag1Sn2/CNT,Pd7Ag2Sn2/CNT和Pd7Ag3Sn2/CNT).通过XRD,TEM和XPS对其进行了表征,结果表明,相比Pd/CNT和Pd-Ag(或Pd-Sn)催化剂的纳米颗粒,Pd-Ag-Sn催化剂展现出了更小的平均颗粒尺寸(2.3 nm).此外,还通过循环伏安(CV)和计时电流法(CA)测试了这些催化剂对甲酸氧化的电活性,在酸碱介质中,Pd-Ag-Sn/CNT对甲酸氧化都表现出了更高的电流密度.其中,Pd7Ag2Sn2/CNT催化剂在酸碱介质中的电流密度分别是108.8和211.3 mA·cm-2,相应的Pd质量电流密度高达1 364和2 640 mA·mg-1,远远高于商业Pd/C,表明Pd-Ag-Sn/CNT催化剂对甲酸氧化表现出了极好的电催化活性.%Carbon nanotube-supported Pd-based binary and ternary nanocatalysts (Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT,Pd7Ag1Sn2/CNT,Pd7Ag2Sn2/CNT and Pd7Ag3Sn2/CNT) have been fabricated by the NaBH4 reduction method.They have been characterized by using XRD,TEM and XPS techniques.The ternary Pd-Ag-Sn nanoparticles exhibit a smaller average particle size of ca.2.3 nm compared to Pd/CNT and binary Pd-Ag (or Pd-Sn) nanoparticles.The electrocatalytic activity of these catalysts towards formic acid oxidation in both H2SO4 and NaOH solutions has been investigated using cyclic voltammetry (CV) and chronoamperometry (CA).In both acidic and alkaline media,the ternary Pd-Ag-Sn/CNT catalysts present higher anodic current density for formic acid oxidation than Pd/CNT and binary PdAg/CNT or PdSn/CNTcatalysts.Among the prepared catalysts,the Pd7Ag2Sn2/CNT catalyst displays the highest HCOOH oxidation current density of 108.8 mA·cm-2 in 0.5 mol·L-1 H2SO4 solution or 211.3 mA·cm-2 in 1 mol·L-1 NaOH solution,corresponding to the Pd mass current density of 1 364 or 2 640 mA·mg-1,respectively.These currents are extremely larger than those obtained from the commercial Pd/C.Results exhibit the excellent electrocatalytic activity of the ternary Pd7Ag2Sn2/CNT catalyst towards formic acid oxidation.【期刊名称】《无机化学学报》【年(卷),期】2018(034)007【总页数】12页(P1209-1220)【关键词】钯催化剂;甲酸氧化;燃料电池;电催化【作者】张媛媛;易清风;李广;周秀林【作者单位】湖南科技大学化学化工学院,湘潭411201;湖南科技大学化学化工学院,湘潭411201;理论有机化学与功能分子教育部重点实验室,湘潭 411201;湖南科技大学化学化工学院,湘潭411201;湖南科技大学化学化工学院,湘潭411201【正文语种】中文【中图分类】O6460 IntroductionNowadays,considerable attention to seeking renewable resources has beenpaid to reduce or even stop the rapid consumption of fossil fuels and the increasing emissions of automobile exhaust[1-3].As environmental friendly electrochemical energy devices,fuel cells have received extensive attention in view of their outstanding performances.Among them,direct formic acid fuel cell （DFAFC）possesses such significant advantages as lower operating temperatures,higher theoretical open circuit voltage and energy density,safer storage and transportation and less crossover rate of formic acid through Nafion membrane compared with direct methanol fuel cell （DMFC）[4-11].There are many factors affecting the performance of DFAFC,one of which is the electroactivity of the anode catalyst.A great number of reports have demonstrated that Pt and Pt-based catalysts exhibit excellent catalytic activity for formic acid oxidation in alkaline and acidic circumstances,including Pt/C[12]and Pt-M（M=Au,Li,Sb,Ru,Ir,Bi）[13-20]and so on.Nevertheless,extremely low natural reserve and the exorbitant cost of Pt severely limit the large-scale commercial applications of Pt or Pt-based catalysts in DFAFC[21-22].A large numbers of studies have shown that pure Pd or Pdbased catalysts are found to be more promising alternative anode electrocatalysts for DFAFC owing to lower price and greater abundance of the metal Pd on the earth′s crust compared with metal Pt[23-27].Li et al.[28]synthesized phosphorus doped carbon supported Pd catalyst （Pd/P-C）by liquid reduction method,which exhibits the better electrocatalyst activity （0.8 A·mg-1）and stability for formic acid oxidation in acid medium.It is generally accepted that formic acid oxidation on Pd catalyst takes place primarily through a directpathway,leading to the direct formation of CO2.However,there is still a small amount of formic acid being oxidized by an indirectpathway,resulting in the gradual accumulation of intermediates on the surface of the catalyst during the process of formic acid oxidation.These intermediates will make the catalyst got poisoning and lead to the decline in catalyst activity and long-term stability[29-31].Inspired by this,an effective strategy has been implemented by alloying of other transition metals with Pd to enhance both the electrocatalytic activity and long-term stability of the Pd catalyst such as bimetallic Pd-Pt[32],Pd-Ag[33],Pd-Sn[34-35],Pd-Ni[36],Pd-Au[37],trimetallic Pd-Ni-Cu[29],Pd-Ni-Ag[38]and Pd-Pt-Ni[39].Typically,Lu et al.[40]have successfully synthesized nanoneedle-covered Pd-Ag nanotubes through a galvanic displacement reaction with Ag nanorods at 100 ℃ （PdAg-100）and room temperature （PdAg-25）and obtained higher catalytic activity and stability than bulk Pd.Liu and coworkers[41]have reported Pd-Sn nanoparticles supported on Vulcan XC-72 carbon by a microwave-assisted polyol process,and results indicate that Pd2Sn1/C and Pd1Sn1/C catalysts exhibit higher current density for formic acid oxidation compared with the prepared Pd/C catalyst.Carbon supported ternary PdNiCu catalyst was prepared by Hu et al.[29]and exhibit an increased electroactivity for formic acid oxidation compared to that of binary Pd-Ni and Pd-Cu catalysts.Multi-walled carbon nanotube （CNT）supported Pd1Cu1Sn1ternarymetallic nanocatalyst was also studied by Zhu et al.[42]through chemical reduction with NaBH4as a reducing agent and it reveals a higher mass activity of 534.83 mA·mg-1Pdtowards formic acid oxidation compared with bimetallic PdCu/CNTs and PdSn/CNTs.These studies have revealed that Pd-based bimetallic and ternary-metallic catalysts show a superior electrochemicalactivity and stability for formic acid oxidation compared with pure Pd catalyst in virtue of synergistic effect between metals,electronic or surfaceeffects[22,43].However,ternary-metallic catalysts are more effective than the corresponding bimetallic catalysts in tuning the electronic properties and composition of catalytic surfaces.Furthermore,ternary Pd-based catalysts are able to further improve Pd usage efficiency and enhance their electrocatalytic performances.Consequently,it is necessary to develop novel ternary Pd-based catalysts with less cost and higher performance for formic acid oxidation.In addition,the choice of suitable support for Pd-based catalyst is also significant to reduce the Pd loading and improve the dispersion of catalyst nanoparticles,such as carbon black,graphene,carbon nanotubes,conductive polymers and so on.Recently,carbon nanotubes （CNTs）as a potential carbon carrier have been reported by many researchers for Pd-based catalysts[42,44-46].As a support,CNTs possess unique structure and properties like high specific surface area,outstanding electronic conductivity and high chemical stability,which would be conducive to the dispersion and stability of Pdbased catalyst particles and further enhance their electroactivity[44,47].Therefore,carbon nanotubes are a prominent support in the development of electrocatalysts.In this study,carbon nanotubes supported Pd and Pd-based binary/ternary catalysts（Pd/CNT,PdAg/CNT,PdSn/CNT,PdAgSn/CNT）weresuccessfullysynthesized by the NaBH4reduction method.The electrochemical activities of the prepared catalysts towards formic acid oxidation in both acidic and alkaline media were evaluated by cyclic voltammetry（CV）and chronoamperometry （CA）techniques.The results demonstrate that ternary Pd-Ag-Sn catalysts exhibit much higherelectrochemicalactivityand stability towards formic acid oxidation in both acid and alkaline media.1 Experimental conditions1.1 ChemicalsPalladium chloride,stan nouschloride,silver nitrate,sodium borohydride,ethylene-glycol,formic acid,sodium hydroxide and sulfuric acid were analytical purity grade and used as received without further purification.Water was deionized water subjected to the double distillation.Before used,multi-walled carbon nanotubes （CNTs,＞90%（w/w）,outside diameter:10～20 nm,length:5～20 μm ）were added to a mixture of concentrated H2SO4and concentrated HNO3（the volume ratio was 3 ∶1）,and heated at 60 ℃ under stirring for 8 h to obtain the acidified CNTs.1.2 Catalyst synthesisCatalystswere synthesized according to our recent report[48].Typically,the ternary Pd7Ag1Sn2/CNT catalyst was prepared via the following steps:A metal precursor composed of 8.9 mg PdCl2,1.2 mg AgNO3 and 3.2 mg SnCl2was added to the mixing solvent of 12 mL ethylene glycol and 4 mL water.Then the solid salts were fully dispersed for 30 min with ultrasonication to make them be completely dissolved.Then,30 mg of theacidified carbon nanotubes was added to the resulting solution and the mixture was further treated with ultra-sonication to obtain a uniform black ink.3 mL of 50 g·L-1NaBH4dissolved in ethylene glycol was added dropwise to it under stirring to reduce the metal ions,and the mixture was stirred for 5 h.Finally,the resulting suspension was filtered,washed with water and dried at 40℃in vacuum for 10 h to obtain the Pd7Ag1Sn2/CNT catalyst.Other catalysts（Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT,Pd7Ag2Sn2/CNT andPd7Ag3Sn2/CNT）were prepared according to this procedure by adjusting the metal molar ratio in the metal precursors.For synthesis of Pd/CNT catalyst,the precursor was composed of 8.9 mg PdCl2.For Pd7Ag3/CNT catalyst,the precursor was composed of 8.9 mg PdCl2and 3.6 mgAgNO3.For Pd7Sn2/CNT catalyst,the precursor was composed of 8.9 mg PdCl2 and 3.2 mg SnCl2.For Pd7Ag2Sn2/CNT catalyst,the precursor was composed of 8.9 mg PdCl2,2.4 mg AgNO3and 3.2 mg SnCl2.ForPd7Ag3Sn2/CNT catalyst,the precursor was composed of 8.9 mg PdCl2,3.6 mg AgNO3and 3.2 mg SnCl2.1.3 CharacterizationIn order to further explore the microstructure and particle size distribution of the prepared catalysts,transmission election microscopic （TEM）images were recorded with a JEM-2100F.The X-ray diffraction（XRD）profiles of the prepared catalysts were collected to analyze the compositions of the samples in a D/MAX2500X diffractometer （Japan）operating with Cu Kα radiation generated at 40 kV and 250 mA （λ=0.15418 nm）and 2θ=20°～90°.The elemental compositions and valence statesof the samples were investigated by X-ray photoelectron spectroscopy （XPS）operated with an ESCALAB 250Xi spectrometer（VG Scientific Ltd.,England）.Inductively coupled plasma（ICP-AES-7510,Shimadzu）data of the nanoparticles were acquired to determine the Pd loading relative to the total mass of the catalyst.XRD profiles,XPS and ICP of the prepared catalysts were also investigated in our recent work[48].1.4 Electrochemical measurementsAll electrochemical measurements of the prepared catalysts for formic acid oxidation in both acid and alkaline media were conducted in a conventional three-electrode system using an AutoLab PGSTAT30/FRA electrochemical workstation（Eco Chimie,The Netherlands）.The counter electrode was a Pt sheet.A Ag/AgCl in saturated KCl solution was used as the reference electrode,and all potentials reported in this work were quoted versus the Ag/AgCl reference.The working electrode was a glassy carbon （GC）coated with a film of catalyst,which was fabricated as follows:the glassy carbon（GC,3 mm diameter,from LanLiKe,TianJing,China）was firstly polished with a 0.3 μm alumina s uspension to give a mirror surface.Then,5 mg oftheas-synthesized catalystwasdispersed ultrasonically in the mixed solution containing 0.94 mL of ethanol and 60 μL of 5%（w/w）Nafion solution to obtain a homogeneous ink.Finally,15 μL of this ink was dropped onto the top surface of the polished GC disc by a micropipette and dried at room temperature to get the working electrode.The blank CVs of the electrocatalysts were recorded in both 0.5mol·L-1 H2SO4solution and 1.0 mol·L-1NaOH solution,and the corresponding electrocatalytic activities towards formic acid oxidation were investigated both in the solution of 0.5 mol·L-1H2SO4in the presence of HCOOH and in 1.0 mol·L-1NaOH containing HCOOH.For the sake of comparison,electroactivity of the commercial Pd/C for formic acid oxidation was also examined under the same conditions.All measurements were performed at room temperature （22±2）℃）.2 Results and discussion2.1 Physical characterizationFig.1 TEM images and the corresponding size distributions of the Pd/CNT （a）,Pd7Ag3/CNT （b）,Pd7Sn2/CNT （c）and Pd7Ag2Sn2/CNT （d）samplesFig.1（a～d）show the TEM images of the preparedPd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT and Pd7Ag2Sn2/CNT catalysts as the typical samples.The corresponding inset is the particle size distribution histogram of the catalyst sample.It is evident from the images that the metallic nanoparticles have been successfully decorated on the surface of multi-walled CNTs for all prepared catalysts.Inaddition,Pd/CNT,Pd7Ag3/CNT and Pd7Sn2/CNT catalysts exhibit obvious agglomeration between the nanoparticles and some particles are even stacked together to form clumps as shown in Fig.1（a～c）,and their average particle sizes （Daverage）are 3.6,4.7 and 3.7 nm,respectively.For the ternary Pd7Ag2Sn2/CNT catalyst,however,most of the nanoparticles are well uniformly dispersed on the surface of CNTs except for a smallamount of agglomeration as indicated in Fig.1d.Furthermore,the ternary Pd-Ag-Sn catalyst exhibits a smaller average particle size of 2.3 nm compared to Pd/CNT and binary Pd-Ag（or Pd-Sn）catalysts.Results indicate that an appropriate amount of Ag and Sn additives can effectively improve the dispersion of the Pd nanoparticles in the ternary Pd-Ag-Sn catalysts.XRD patterns and XPS data of the prepared catalysts were recorded as indicated in Fig.2.Fig.2a shows that the peaks at 40.1°,46.6°,68.1°and82.1°are attributed to characteristic diffraction peaks of face-centered cubic （fcc）crystalline Pd for Pd/CNT catalyst.However,a slight negative shift is observed with regard to the angle position of the Pd diffraction peaks on the Pd7Ag3/CNT and Pd7Ag2Sn2/CNT catalysts compared to the Pd/CNT catalyst,while the Pd7Sn2/CNT catalyst does not show such a shift.This reveals that the alloy formation between Pd and Ag arises in the binary Pd7Ag3/CNT and ternary Pd-Ag-Sn catalysts.As is shown inFig.2b,the binding energies at 335.7 and 340.4 eV are ascribed toPd3d3/2and Pd3d5/2spin orbit states of zero-valent Pd[49].But the other two distinct peaks located at 337.3 and 342.7 eV are related toPd3d3/2and Pd3d5/2peaks of Pdギ,which is indexed to the Pd oxide.These results indicate that the prepared Pd-based catalysts contain the metal Pd and Pd oxide.Fig.3b shows the XPS spectra of 3d for Pd7Ag3/CNT andPd7Ag2Sn2/CNT catalysts,and the two obvious peaks centered at 367.9 and 373.9 eV are related to Ag3d5/2and Ag3d3/2respectively[50],revealing that Ag ions are reduced completely during thepreparation of the catalysts.Similarly,as indicated in Fig.3c,Sn3d XPS spectra are divided into two peaks located at 486.8 and 487.4 eV,which are associated with Sn and SnO2[51],confirming that the metal Sn in thePd7Ag2Sn2/CNT and Pd7Sn2/CNT catalysts exists in the form of Sn and SnO2.Fig.2 XRD （a）and XPS （b～d）spectra of thePd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT and Pd7Ag2Sn2/CNT samples:（b）Pd3d,（c）Ag3d,and （d）Sn3d2.2 Electrochemical performance analysisFig.3a shows CV curves of the prepared catalysts and Pd/C in 0.5 mol·L-1H2SO4solution.All catalysts reveal a similar CV curve to Pd/C in acidic solution.A well-defined hydrogen adsorption/desorption peaks around 0 V arises on all samples,and the cathode characteristic reduction peak （rp）of the Pd oxides produced during the forward potential scan is vividly observed at ca.0.48 V for all the catalysts.Also,the rppeak current density on thePd/C,Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT,Pd7Ag1Sn2/CNT,Pd7Ag2Sn2/CNT, Pd7Ag3Sn2/CNT catalysts is 6.6,6.9,10.4,8.6,11.5,15.2 and 12.2 mA·cm-2,respectively.Fig.3b shows cyclic CV curves of the prepared catalysts and Pd/C in 1.0 mol·L-1NaOH solution.Similarly,the cathode reduction peak （rn）at ca.-0.41 V is attributed to the formation of Pd oxides during the forward-going,and the rnpeak current density is14.3,17.0,20.5,24.6,20.2,31.4 and 24.0 mA·cm-2forPd/C,Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT,Pd7Ag1Sn2/CNT,Pd7Ag2Sn2/CNTPd7Ag3Sn2/CNT catalysts,respectively.Based on the charge of PdO reduction peak in each CV,the electrochemical active surface area （ECSA）of Pd for the samples can be calculated by using the methods reported in the literature and corresponding results are listed in Table 1[42,52-53].Results reveal that Pd7Ag2Sn2/CNT catalyst possesses the largest ECSA value of 9.56 m2·g-1in H2SO4solution and 15.34 m2·g-1in NaOH solution among the prepared catalysts and Pd/C,which is consistent with the results observed from TEM images.Fig.3 CV curves of the samples in 0.5 mol·L-1H2SO4 （a）and in 1.0 mol·L-1NaOH （b）at a scan rate of 50 mV·s-1Table 1 ECSA values of Pd/C and the prepared samples in both 1 mol·L-1NaOH and 0.5 mol·L-1H2SO4solutionSolution Catalyst Pd/C Pd/CNTPd7Ag3/CNT Pd7Sn2/CNT Pd7Ag1Sn2/CNT Pd7Ag2Sn2/CNTPd7Ag3Sn2/CNT ECSA/（m2·g-1）H2SO4 5.22 4.60 6.39 5.99 7.28 9.567.24 NaOH 11.13 9.73 8.75 11.83 9.96 15.34 11.08Fig.4 CV curves of the samples in 0.5 mol·L-1H2SO4containing 0.5 mol·L-1HCOOH （a）and i n 1.0 mol·L-1NaOH containing 0.5 mol·L-1HCOOH （b）at a scan rate of 50 mV·s-1Electrocatalytic activity of the prepared catalysts for formic acid oxidation was measured in 0.5 mol·L-1 H2SO4solution containing 0.5 mol·L-1formic acid by CV as indicated in Fig.4a.A characteristic anodic peaks jf1caused by formic acid oxidation is observed for all catalysts.In general,all the as-synthesized Pdbased catalysts exhibit better electrocatalytic activity for formic acid oxidation than Pd/C.Further,the ternary Pd7Ag2Sn2/CNTcatalyst shows the largest jf1 peak current density of 108.8 mA·cm-2,which is 6.7 times higher than the Pd/C catalyst.Also,the jf1peak current density on the binary Pd7Ag3/CNT and Pd7Sn2/CNT catalysts is 2.7 and 2.3 times larger than that of Pd/C catalyst respectively.This may be contributed to the synergistic effect between Pd and Ag/Sn[2,26].Furthermore,ternaryPd7Ag1Sn2/CNT and Pd7Ag2Sn2/CNT catalysts display an onset potential （OP）of ca.-0.06 V for formic acid oxidation in acidic media,which presents a negative shift compared to that of ca.-0.045 V on the other catalysts.Fig.4b shows the CV curves of all samples in 1.0 mol·L-1NaOH solution containing 0.5 mol·L-1formic pared to formic acid oxidation in acidic solution （Fig.4a）,the formic acid oxidation in alkaline solution （Fig.4b）presents a much negative onset potential of ca.-0.82 V.Fig.4b also shows that during the forward-going scan,formic acid oxidation current density displays an almost linear increment with the positive shift of the anodic potential until an anodic peak jf2at ca.0.2 V arises.The anodic current density for formic acid oxidation in alkaline medium follows the order:Pd7Ag2Sn2/CNT＞Pd7Ag1Sn2/CNT＞Pd7Ag3/CNT＞Pd7Ag3Sn2/CNT＞Pd/CNT ＞Pd7Sn2/CNT ＞Pd/C.Obviously,the ternary Pd7Ag2Sn2/CNT catalyst presents the best electrocatalytic activity for formic acid oxidation in alkaline medium among the prepared catalysts.It is generally considered that there are two possible parallel pathways for the oxidation of formic acid[42,44]:（i）a “direct pathway”in which formic acid is directly oxidized to CO2without production of anyintermediate and （ii）an “indirect pathway” which involves two steps including the dehydrogenation of formic acid and the adsorption of CO intermediate on the Pd catalyst surface.The choice of the pathway greatly depends on the properties of the catalyst used.Normally,the direct pathway prevails for formic acid oxidation on Pd-containing catalysts[54-55].The processes of formic acid oxidation in acidic solution are based on the following equations （1）～（3）[44]:During formic acid oxidation,the adsorbed HCOOad （HCOO-Pd）species are firstly formed via the adsorption of formic acid molecules on the surface of Pd-based catalysts and subsequent break of O-H bond in the adsorbed HCOOH （HCOOHad）（Equation （1）.Then,decomposition of the HCOOadspecies produces CO2by breaking C-H bond （equation （2）.In alkaline media,electro-oxidation of formic acid on Pd-based catalysts follows a similar mechanism to that in acidic media except that the adsorbed HCOOadspecies on Pd can be formed easier because of the neutralization reaction between HCOOH and NaOH,leading to the much negative onset potential of formic acid oxidation.In general,the addition of others metal or metal oxide to Pd catalyst can significantly improve its electroactivity due to the synergistic effect of different metals.It is noticed from Fig.4 that the Pd7Ag2Sn2/CNT catalyst displays the highest current density of formic acid oxidation in both acidic and alkaline media among the prepared catalysts,reflecting that the addition of proper amount of Ag or Sn isconducivetoenhancetheelectrochemical activity.It is known fromthe XRD data of the prepared catalysts that the alloying between Pd and Ag arises.Based on the so-called bifunctional mechanism therefore,the adsorption bond of intermediates like absorbed CO （COad）and COOH （COOHad）at the surface of catalysts,produced by formic oxidation during the forward scan,can be efficaciously weakened by the Pd-Ag bimetallic alloy.This makes the decomposition of formic acid to CO2go into easier.Furthermore,the presence of Pd-Ag alloy can also prominently reduce the accumulation of poisoningintermediates on the catalyst surface and release more Pd active sites.The presence of SnO2observed from the XPS data may also contribute to the removal of toxic intermediates to accelerate the adsorption and desorption of formic acid on the catalyst surface.Effect of formic acid concentration on the kinetic characterization of formic acid oxidation was further investigated.Fig.5a shows the CV curves of ternary Pd7Ag2Sn2/CNT catalyst in 0.5 mol·L-1H2SO4solution with different formic acid concentrations at 50 mV·s-1,and Fig.5b depictstherelationship between the anodic peak current density and HCOOH concentration.As can be seen from Fig.5b,the jp1peak current density exhibits a rapid rise with the formic acid concentration in the range of 0.5 to 1.8 mol·L-1,while it displays a decrease from 1.8 to 2.5 mol·L-1.In addition,the jp1peak potential shifts to more positive direction at the higher concentration of formic acid.In 1 mol·L-1NaOHsolution,dependence of the jp2peak current density upon HCOOH concentration is also studied as indicated in Fig.6（a,b）.A similarchanging trend of the jp1peak current density vs HCOOH concentration to Fig.5b is observed,revealing that the HCOOH concentration hasthe same effecton electroactivity of the ternary Pd7Ag2Sn2/CNT catalyst in both acidic and alkaline media.At high concentrations of HCOOH,the jp1peak current density for the oxidation of formic acid on the Pd7Ag2Sn2/CNT catalystdecreases.Thismay be related to the saturated adsorption of HCOOH on Pd active sites at high concentrations of HCOOH.On the other hand,high concentrations of HCCOH may result in partial decompositionof HCOOH to produce CO（equation（4）,which is absorbed on the surface of the catalyst and reduce the electroactivity of the catalyst.Fig.5 （a）CV curves of ternary Pd7Ag2Sn2/CNT catalyst in 0.5 mol·L-1H2SO4solution with different formic acid concentrations at 50 mV·s-1;（b）Relationship between the anodic peak current density and HCOOH concentration （CHCOOH）for ternary Pd7Ag2Sn2/CNT catalystFig.6 （a）CV curves of ternary Pd7Ag2Sn2/CNT catalyst in 1.0 mol·L-1NaOHsolution with different formic acid concentrations at 50 m V·s-1;（b）Relationship between the anodic peak current density and HCOOH concentration for ternary Pd7Ag2Sn2/CNT catalystFig.7（a,b）displays the CV curves for the oxidation of pre-adsorbed carbon monoxide （CO）on Pd/C,Pd/CNT and Pd7Ag2Sn2/CNT catalystsin order to investigate the anti-poisoning intermediates ability of the catalysts.It is shown from Fig.7a recorded in 0.5 mol·L-1H2SO4solution that an intense stripping peak of COadis observed on the catalysts.The peakpotential of CO stripping on Pd/C,Pd/CNT and Pd7Ag2Sn2/CNT catalysts is 0.792,0.726 and 0.722 V,respectively,reflecting that the prepared catalysts in this work have more negative CO stripping peak potential values than that of the Pd/C catalyst.The lower potential displays the weaker binding energy between Pd and COadon Pd/CNT and Pd7Ag2Sn2/CNT catalysts.Notably,the onset potential of CO oxidation for Pd7Ag2Sn2/CNT catalyst is measured at 0.67 V,showing a negative shift compared to that for Pd/C（0.72 V）and Pd/CNT （0.70 V）.Results indicate that the COadon the surface of the Pd7Ag2Sn2/CNT catalyst can be more easily removed.Furthermore,the CO stripping isalso tested forPd/C,Pd/CNT and Pd7Ag2Sn2/CNT catalysts in mol·L-1NaOH solution as indicated in Fig.7b.It is worth noting that the Pd/CNT and Pd7Ag2Sn2/CNT catalysts also exhibit a more negative CO stripping peak potential at ca.-0.19 V compared to Pd/C catalyst（ca.-0.139 V）.These results show that the ternaryPd7Ag2Sn2/CNT catalyst possesses much better resistance to COadpoisoning than the Pd/C and Pd/CNT catalysts.The long-term electrocatalytic activity of the Pd/C,Pd/CNT andPd7Ag2Sn2/CNT catalysts is evaluated by CA measurement in 0.5 mol·L-1H2SO4solution containing 0.5 mol·L-1HCOOH at different potentials as depicted in Fig.8.The current density of the studied catalysts exhibits a continuous decay in the initial stage at both 0.05 （Fig.8a）and 0.1 V （Fig.8b）.This may be attributed to the adsorption of CO-like intermediates on the surface of the catalysts,leading to the decline on the number of the active sites[22,56].However,the Pd7Ag2Sn2/CNTcatalystexhibits a significantly slower decay rate of current density than the Pd/C and Pd/CNT catalysts.Additionally,at the end of electrolysis （at 3 600 s）,the current density on the ternary Pd7Ag2Sn2/CNT catalyst is 5.8 mA·cm-2at 0.05 V or 14.2 mA·cm-2at 0.1 V,which is still the highest among the studied catalysts.Fig.9 also shows CA curves of the Pd/C,Pd/CNT and Pd7Ag2Sn2/CNT catalysts in 1 mol·L-1NaOH solution containing 0.5 mol·L-1HCOOH at the potentials of-0.75 and-0.45 V.Apparently,the current density of ternary Pd7Ag2Sn2/CNT catalyst is 13.7 mA·cm-2at-0.75 V after 3 600 s as shown in Fig.9a,which is almost 3.3 and 7.2 times larger than that of the Pd/C （4.2 mA·cm-2）and Pd/CNT （1.9 mA·cm-2）catalysts,respectively.In addition,it can be also observed from Fig.9b that ternary Pd7Ag2Sn2/CNT catalysthas the highest current density among the studied catalysts at the potential of-0.45 V after 3 600 s.The above results demonstrate that the as-synthesized ternary Pd7Ag2Sn2/CNT catalyst displays excellent electrocatalytic activity and more outstanding durability towards formic acid oxidation in both acidic and alkaline media,which is consistent with the results derived from CV analyses.Fig.7 CO stripping curves of the Pd/C,Pd/CNT and Pd7Ag2Sn2/CNT catalysts in 0.5 mol·L-1H2SO4 （a）and 1.0 mol·L-1NaOH （b）at a scan rate of 50 mV·s-1Fig.8 Chronoamperometric responses of the Pd7Ag2Sn2/CNT catalyst in 0.5 mol·L-1H2SO4comtaining 0.5 mol·L-1HCOOH at 0.05 V （a）and 0.1 V （b）Fig.9 Chronoamperometric responses of the Pd7Ag2Sn2/CNT catalyst in1.0 mol·L-1NaOH comtaining 0.5 mol·L-1HCOOH at-0.75 V （a）and-0.45 V （b）Fig.10 shows the CV profiles of the prepared catalysts where the current density is based on the mass of Pd to show the Pd usage efficiency for formic acid oxidation.It can be found from Fig.10a that during the forward-going scan the anodic peak mass current density of thePd/C,Pd/CNT,Pd7Ag3/CNT,Pd7Sn2/CNT, Pd7Ag1Sn2/CNT,Pd7Ag2Sn2/CNT and Pd7Ag3Sn2/CNT catalysts towards formic acid oxidation in 0.5 mol·L-1H2SO4solution containing 0.5 mol·L-1 HCOOH is 153,274,474,431,1 030,1 364 and 767 mA·respectively,which indicates that the ternary Pd7Ag2Sn2/CNT catalyst has the highest Pd mass current density among the prepared catalysts.Zhu et al.[42]prepared the ternary PdCuSn/CNTs catalyst with the mass current density of 534.8 mA·Binary PdCo/CFC catalyst was also synthesized with the mass current density of 1 220 mA·by Vafaei and co-workers[57].Compared with the reported catalysts,the ternary Pd7Ag2Sn2/CNT catalyst prepared in this work exhibits excellent electrocatalytic activity and higherPd usageefficiencyforformicacid oxidation.Additionally,the ternaryPd7Ag2Sn2/CNT catalyst also displays the highest Pd mass current density in 1 mol·L-1NaOH solution containing 0.5 mol·L-1HCOOH as indicated in Fig.10b,which is as high as 2 640 mA·mg-1Pd.These results reveal that the ternary Pd7Ag2Sn2/CNT catalyst can be applied to DFAFCs as a promising anodic catalyst for formic acid oxidation in both acidic and alkaline media due to the synergetic effect between Pd and Ag/Sn.。

石墨烯负载铂基催化剂的制备及其对甲醇的电催化性能路蕾蕾;杜宝中;刘杰【摘要】采用Hummers液相氧化法合成氧化石墨(GO),采用浸渍还原法一步还原氧化石墨和贵金属盐,制备了石墨烯负载铂(Pt/Gr)及不同配比的石墨烯载PtCe合金(PtCe/Gr)催化剂,对催化剂进行物理表征,用电化学方法研究了催化剂对甲醇的电催化氧化性能.TEM结果表明以石墨烯为载体制备的Pt/Gr和PtCe/Gr催化剂分散良好,催化剂粒径分别为2.2 nm和2.5 nm.与XC-72为载体制备的催化剂相比,在对甲醇电氧化的性能上Pt/Gr比Pt/XC-72的催化活性和稳定性更高.与单一金属的Pt/Gr相比,PtCe/Gr对甲醇具有更高的活性和稳定性.不同配比PtCe/Gr合金催化剂对甲醇电氧化催化活性顺序为Pt3Ce7/Gr＞Pt7Ce3/Gr＞Pt8Ce2/Gr＞Pt/Gr.由各个催化剂在甲醇溶液中的i—t曲线可知,Pt3 Ce7/Gr也是抗毒化能力最强的合金催化剂.【期刊名称】《西安理工大学学报》【年(卷),期】2013(029)004【总页数】6页(P416-420,449)【关键词】石墨烯;铂铈合金催化剂;电催化氧化;直接甲醇燃料电池【作者】路蕾蕾;杜宝中;刘杰【作者单位】西安理工大学理学院,陕西西安710054;西安理工大学理学院,陕西西安710054;西安理工大学理学院,陕西西安710054【正文语种】中文【中图分类】O646直接甲醇燃料电池(Direct Methanol Fuel Cell:DMFC)是直接氧化型燃料电池的首选类型之一［1］，近年来取得了较快发展，但阳极催化剂甲醇电催化活性低、成本高的问题仍然是制约其商业化的瓶颈之一。

研究人员尝试通过向贵金属中掺杂加速CO等氧化物质［2-4］、采用非 Pt基金属代替 Pt金属［5-7］、寻找新型催化剂载体［8-10］等途径提高阳极催化的利用率和稳定性从而提高其活性并降低贵金属用量。