CN106984307B - 一种具有Yolk-Shell结构Ag-Pt@C纳米球及其制备方法 - Google Patents
一种具有Yolk-Shell结构Ag-Pt@C纳米球及其制备方法 Download PDFInfo
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Abstract
本发明公开了一种具有Yolk‑Shell结构Ag‑Pt@C纳米球及其制备方法。所述Ag‑Pt@C纳米球的外径为300nm左右，包括厚度为50nm的碳壳，内核部分为180nm粒径的Ag‑Pt复合微球，所述Pt纳米颗料粒径为2‑5nm，晶面指数≥3(hkl)，外延生长于Ag核外围。本发明通过特殊的C层的限域作用和Ag核的诱导作用，使得Pt纳米颗粒在Ag核外延生长，大大提高了材料的稳定性和重复利用率，同时，获得高品面指数的Pt纳米颗粒，显著的提高了Pt纳米颗粒的催化活性。
Description
技术领域
本发明属于催化技术领域，尤其涉及一种具有Yolk -Shell结构 Ag-Pt@C纳米球及其制备方法。
背景技术
铂族金属纳米材料是燃料电池、石油化工等领域中广泛使用的催化剂，进一步提高其催化活性、稳定性和利用效率一直是相关领域的重大科学问题和关键工程技术问题。对模型催化剂的基础研究指出铂族金属高指数晶面由于含有高密度的台阶原子和扭结原子，催化活性和稳定性显著优于{100}，{111}等低指数晶面，因此制备表面为高指数晶面结构的铂族金属纳米粒子是显著提高催化剂性能的有效途径。但是，常规合成方法通常只能制备表面为低指数晶面的铂族金属纳米粒子。
Core-shell结构的贵金属纳米复合材料是将价格低廉、化学性质稳定的载体材料通过化学键或物理力的作用包覆在贵金属纳米粒子外部，由于C层的限域作用和Ag核的诱导作用，使获得具有Pt高指数晶面成为可能，同时C层可以有效防止贵金属成分氧化和团聚，可得到高活性、稳定的、循环利用率高的催化材料。因此，对core-shell 结构贵金属纳米催化剂的研究开发一直备受关注。但是，在core-shell 结构贵金属纳米催化剂的很多制备方法中，都会通过引入各种稳定剂来增加高指数晶面、减少贵金属纳米粒子聚集，如表面活性剂，聚合物以及不同类型的配体，这些稳定剂一般比较昂贵、难以控制，而且部分稳定剂容易吸附在贵金属纳米粒子表面，影响其催化活性。因此，探索更加简便绿色的贵金属纳米催化剂合成方法和体系，具有十分重要的意义。
因此，拓展高指数晶面Pt纳米粒子同时具有core-shell结构的纳米催化剂的开发应用也是一个十分有前景的研究方向。
发明内容
本发明的目的在于针对现有技术的不足，提供一种具有高指数晶面Pt内核的Yolk-Shell结构Ag-Pt@C纳米球及其制备方法，该方法操作简单、可重复性较高。
本发明的目的是通过以下技术方案来实现的：一种具有Yolk -Shell结构Ag-Pt@C纳米球，所述Ag-Pt@C纳米球的外径为 300nm左右，包括厚度为50nm的碳壳，内核部分为粒径180mm的 Ag-Pt复合微球，所述pt纳米颗粒粒径为2-5nm，晶面指数≥3(hkl)，外延生长于Ag核外围。
一种具有Yolk -Shell结构Ag-Pt@C纳米球的制备方法，该方法为：
(1)将2.0g葡萄糖溶解于20ml去离子水中，加入0.006g乙酰丙酮铂，磁力搅拌30min。随后加入0.5ml浓度为10-4～0.1mol/L的 AgNO3溶液，磁力搅拌30min。将所得溶液转移到水热反应釜(30ml) 中，温度180℃水热反应4h。
(2)自然冷却后，将水热反应的产物用去离子水和乙醇交替清洗3 次，60℃干燥12h，得到Yolk -Shell结构Ag-Pt@C纳米球。
本发明的有益效果是：Yolk -Shell结构Ag-Pt@C纳米球中内核部分为较小尺寸的Pt纳米颗粒包覆在Ag核周围，极大的增加了Pt 的分散度，降低成本，提高了贵金属材料的利用率；特殊的C层的限域作用和Ag核的诱导作用，使得Pt纳米颗粒在Ag核外延生长，大大提高了材料的稳定性和重复利用率，同时，获得高晶面指数的 Pt纳米颗粒({730}，{520})，显著的提高了Pt纳米颗粒的催化活性；且惰性碳壳耐酸耐碱，一方面解决了贵金属纳米颗粒易氧化、易团聚的问题，同时增加了催化剂的稳定性，提高了循环利用率，可广泛应用于对硝基苯酚催化加氢制备对氨基苯酚。
附图说明
图1是Ag-Pt@C纳米球的SEM图(A)、TEM图(B)以及HETEM 图(C)；
图2是Ag-Pt@C纳米球在Pt4f(A)、Ag3d区域的XPS图(B)；
图3是Ag-Pt@C纳米球的红外图谱(A)，Ag-Pt@C纳米球水溶性变化数码图片(B)；
图4是Ag-Pt@C纳米球核壳结构形成过程的的SEM图、TEM 图以及EDX图；
图5是Ag-Pt@C纳米球作为催化剂的催化效果图；
图6是Ag-Pt@C纳米球作为催化剂的循环利用图。
具体实施方式
下面结合具体实施例，进一步阐明本发明。应理解，这些实施例仅用于说明本发明而不用于限制本发明的范围。此外应理解，在阅读了本发明讲授的内容之后，本领域技术人员对本发明作各种改动或修改，这些等价形式同样落于本申请所附权利要求书所限定的范围。
实施例1
本实施例采用水热法制备Yolk -Shell结构Ag-Pt@C纳米球步骤如下：
量取20ml去离子水，称取2.0g葡萄糖溶解在其中，常温搅拌 10min后，葡萄糖完全溶解；称取0.006g乙酰丙酮铂粉末加入上述溶液中，磁力搅拌30min。随后加入0.5ml新鲜配制的浓度为 10-4mol/LAgNO3溶液，磁力搅拌30min。将所得溶液转移到水热反应釜(30ml)中，调节温度为180℃，反应4h。自然冷却后用去离子水和乙醇交替清洗3次，随后将所得产物置于60℃恒温干燥箱中干燥12h，得到Yolk -Shell结构Ag-Pt@C纳米球，如图1所示。
图1中SEM图(图1A)可看到由于金属与碳层衬度不同而明显表现出的Yolk -Shell结构，微球粒径为300nm左右，无明显粘连，分散性较好。TEM图(图1B)则可以明显观察到外层约为50nm左右的壳层，与SEM结果相符，其内核为180nm，插图的选取衍射图可以看到衍射斑点与衍射环同时存在，得出内核结构可能既有单晶结构又有多晶结构。图1C的HRTEM图则可以明显观察到内核的边缘为 2-5nm小颗粒聚集的树枝状纳米粒子，其中晶格间距为0.2nm，对应于Pt纳米晶体的{730}晶面，即该方法成功合成了高指数晶面的Pt 纳米粒子。
为了进一步了解Yolk -Shell结构Ag-Pt@C纳米球的元素分布，对其进行了XPS表征，分析了该结构中Ag以及Pt的存在价态。图 2(A)为该材料在Pt4f区域的XPS图，其中结合能为71.00eV和72.46eV 处的峰分别对应零价Pt的4f7/2和4f5/2，可见，该结构中Pt元素主要为零价状态，而零价Pt的活性较高，大量零价Pt的存在可有效提高该结构的催化性能。图2(B)为该材料在Ag3d区域的XPS图，其中结合能为368.4eV的峰对应零价Ag的3d3/2，结合能为374.4eV处的峰对应零价Ag的3d5/2，表明Ag在该结构中也以零价的形式存在，同样可发挥一定的催化作用。
图3(A)为Yolk -Shell结构的Ag-Pt@C纳米球的红外图谱。图中在3400、1701、1681以及2800cm-1处的吸收峰分别对应于-OH、-C ＝O、-C＝C以及-C-H等基团的振动峰。这是由于葡萄糖在参与水热反应的过程中，脱水缩合不完全，导致残留的部分氧元素和氢元素以-OH、-C＝O、-C-H等基团的形式保留下来。其中，大量的-OH可以增加Ag-Pt@C纳米球的亲水性，提高了其在溶液中的分散性和稳定性，如图3(B)中，分散在水中的Ag-Pt@C纳米球经过4h仍然分散良好。
实施例2
本实施例研究Ag-Pt@C纳米球的形成过程。
为了对Yolk -Shell结构Ag-Pt@C纳米球的内部结构以及反应历程进行探究，我们对反应时间进行了调控实验。图4为控制反应时间从1h到4h的阶段性产物所对应的SEM图、TEM图以及EDS谱图(从左到右)。图4(A)为反应1h时产物的SEM图，可以看到得到粒径约150nm左右的纳米粒子，团聚比较严重，结合图4(A2)的EDS图谱可以看到该结构中只含有Ag元素，加上TEM图中单个粒子的类六面体形状(图4A1)，基本确定反应1h的产物是银核。反应2h时由SEM 图观察到也是粒径均匀的纳米粒子(图4B)，较反应1h产物，团聚现象有所好转。图4(B1)中单个粒子的TEM图可观察到纳米粒子最外层为小颗粒聚集状，对比图4(A1)与(B1)插图中两结构的HRTEM图可以看到1h到2h的明显变化是纳米粒子粒径增大，外层由光滑表面变为较小纳米颗粒聚集状，结合图4(B2)的EDS图中出现的Pt特征峰可知，反应1h至2h，是在银纳米核周围生长Pt纳米粒子的过程。由于C层的限域作用和Ag核的诱导作用。获得具有Pt高指数晶面；图4(C)中单个粒子的TEM图可以十分直观的观察到内核外层生长有多孔碳层，但图4(C)及(C1)插图可知，这一阶段并不是全部纳米粒子都被多孔碳层包覆。到反应进行到4h，图4(D)及(D1)均可看到多孔碳层形成，EDS谱图中出现Ag、Pt、C元素，证明这一阶段Yolk -Shell 结构Ag-Pt@C纳米球基本形成，并且图4(D)中可观察到形成的纳米球分散十分均匀，没有粘连，进一步说明多孔碳层不但可以保护贵金属内核，还可以有效的防止贵金属纳米粒子的团聚，达到了最初设计制备该结构的目的。
根据以上分析，基本确定葡萄糖发挥了还原剂和多孔碳层的碳源两个作用，在Yolk -Shell结构Ag-Pt@C纳米球的制备过程中葡萄糖既还原了Ag+，也还原了Pt2+。制备过程中我们采用AgNO3和 C10H14O4Pt(乙酰丙酮铂)分别作为银源和铂源，但是由于C10H14O4Pt常温下并不溶于水，当加入葡萄糖并充分搅拌溶解后，Ag+首先被葡萄糖分子包围。反应升温后，Ag+首先被还原成为Ag核。虽然随后的高温高压环境下C10H14O4Pt溶解形成Pt2+，但由于
小于
因而Pt2+的还原一定发生在Ag+之后，即当反应溶液中的Ag+全部被还原成为银核之后Pt2+开始被还原成为Pt纳米粒子，并且是Pt纳米粒子包覆在银核周围，并不形成新的成核点。由于C层的限域作用和Ag核的诱导作用，使Pt的生长在具有不稳定的高指数晶面成为一种稳定结构；当还原反应过程结束后，剩余的葡萄糖会在高温高压条件下脱水芳构化形成多孔碳层，同时表面保留一部分亲水基团。如此，可以得出Ag-Pt@C纳米球为负载Ag、Pt 双金属的多级Yolk -Shell结构。
实施例3
本实施例将实施例1制备的Ag-Pt@C纳米球用于硝基苯酚的催化加氢，具体为：
步骤：配置1mol/L的NaBH4(还原剂，还原对硝基苯酚)溶液2ml， 5.0×10-4mol/L的对硝基苯酚(4-NP)溶液100ml，取50ml配制好的4-NP 溶液和1mlNaBH4加入到100ml烧杯中，超声均匀后，先取一次样进行紫外吸收测试，做参照物。然后加入3mgAg-Pt@C纳米球，超声混合均匀进行紫外吸收测试。每隔2min取样2ml，每次取样之后进行紫外吸收测试。
图5是Ag-Pt@C纳米催化剂催化NaBH4还原4-NP溶液时，每隔2min取样的紫外可见吸收强度随时间的变化曲线。图中400nm处的吸收峰即4-NP的特征峰位置。加入催化剂之后，400nm处的吸收峰逐渐减弱，在300nm处出现氢化还原产物4-AP的吸收峰，且强度随时间的增加而逐渐增强。随着时间的增加，对硝基苯酚的特征峰越来越弱，4-硝基苯胺的特征峰越来越强，直到反应进行15min后，曲线基本稳定，说明4-NP被彻底催化还原了。因此，本发明制备得到的高指数晶面的纳米颗粒，含有高密度的台阶原子及扭结原子，这些原子的配位数较少，化学活性高，很容易与反应物分子相互作用，破坏化学键，成为催化活性中心，大大提高了其催化活性。
重复利用率是衡量催化剂催化性能的一个重要指标，图6展示了 Ag-Pt@C纳米球作为催化剂的重复利用率。每一次重复利用后，催化剂通过离心被分离，再次用来催化反应。经过十次循环利用，对硝基苯酚的转化率仍高达92％，保持较高的催化活性，这主要归结于多孔碳层对内部贵金属的保护，一方面提供通道，同时防止贵金属氧化失活，同时，Ag可以作为载体，增加高指数晶面Pt颗粒的稳定性； Ag-Pt液相外延的轴取向转变、调控Pt的面外取向以及面内取向生长，增加其稳定性。
该实施例揭示了Ag-Pt@C纳米球对于降解有机污染，是高效的和可持续利用的，在处理工业污水中显示出突出的应用前景。
Claims (2)
1.一种具有Yolk-Shell结构Ag-Pt@C纳米球，其特征在于，所述Ag-Pt@C纳米球的外径为300nm，包括厚度为50nm的碳壳，内核部分为粒径180nm的Ag-Pt复合core-shell微球，所述Pt纳米颗粒粒径为2-5nm，具有高晶面指数{730}，{520}，外延生长于Ag核外围。
2.一种具有Yolk-Shell结构Ag-Pt@C纳米球的制备方法，其特征在于，该方法为：
(1)将2.0g葡萄糖溶解于20ml去离子水中，加入0.006g乙酰丙酮铂，磁力搅拌30min；随后加入0.5ml浓度为10-4～0.1mol/L的AgNO3溶液，磁力搅拌30min，将所得溶液转移到30ml的水热反应釜中，温度180℃水热反应4h；
(2)自然冷却后，将水热反应的产物用去离子水和乙醇交替清洗3次，60℃干燥12h，得到Yolk-Shell结构Ag-Pt@C纳米球。
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