单金属和双金属前过渡金属碳化物和氮化物纳米粒子的反相微乳液介导的合成
Summary
A “removable ceramic coating method” is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.
Abstract
反相微乳液用于封装单金属或双金属前过渡金属氧化物纳米颗粒在微孔二氧化硅壳。二氧化硅包封的金属氧化物纳米颗粒然后渗碳在甲烷/氢气气氛中在温度超过800℃，以形成二氧化硅 – 包封的早过渡金属碳化物纳米颗粒。在渗碳处理中，二氧化硅壳防止相邻碳化物纳米颗粒的烧结，同时也防止了过量表面碳的沉积。可替代地，硅石包封的金属氧化物纳米颗粒的温度下可氮化在氨气氛中超过800℃形成氧化硅 – 包封的早过渡金属氮化物纳米颗粒。通过调整反相微乳参数，二氧化硅壳的厚度，和所述渗碳/氮化的条件下，过渡金属碳化物或氮化物的纳米颗粒可以被调谐到各种尺寸，组合物，一次晶相。渗碳或氮化后，将二氧化硅壳然后使用一个室温水氟化氢铵溶液或在40-60℃的0.1至0.5M的NaOH溶液中除去。而在二氧化硅壳被溶解，一个高表面积载体，如炭黑，可以加入到这些溶液以获得支持的前过渡金属碳化物或氮化物的纳米颗粒。如果没有高表面积载体被添加，那么纳米颗粒可以被存储为一个或纳米分散体离心分离，得到纳米粉末。
Introduction
前过渡金属碳化物（差旅管理）是低成本的，表现出高的热稳定性和电化学稳定性以及特有的催化活性地球上资源丰富的材料。1-3具体地，碳化钨（WC）和碳化钼（钼2℃）有被广泛地研究其催化相似之处铂族金属（铂族金属）。4,5-由于这些有利的性质，差旅管理已被确定为候选替换昂贵的PGM催化剂在新兴的可再生能源技术，如生物质转化，燃料电池，和电解槽6,7
为了最大限度地提高催化活性，工业催化剂几乎总是配制成超小纳米颗粒（直径<10纳米）分散在高表面积载体，例如炭黑。8然而，差旅管理的合成需要温度高于〜700℃以上。这导致了nanopartic广泛烧结LES（NPS），多余的表面积炭（焦炭）和热降解的支持。这两种粒子烧结和支持的降解导致材料的表面积下降。过量表面杂质附着块活性金属位点，这已被证明能大大减少或在某些情况下，完全消除的差旅管理。9,10作为这样的催化活性，主要是在散装微粒或薄膜与进行TMC反应性的基础研究精细地控制表面，而不是在高表面积的TMC纳米材料。
许多方法已经发展到合成TMC纳米粒，但这些方法不适合于合成催化剂活性TMC纳米粒。传统的湿法浸渍技术使用浸渍在高表面积载体金属盐溶液。在热，湿热浸渍法可以暴露于破坏性的渗碳条件导致支持退化的催化剂载体。此外，烧结Ç一个仅在载体上的金属的低重量％负荷被减轻，并且它也不能使用湿法浸渍合成不受支持TMC纳米粉末。几个较新的方法包括混合的金属前体与碳前体和施加传统和非传统的加热技术11-18过量碳被用来防止烧结，但是这过量的碳会导致广泛的表面碳，使得这些材料不适合于催化用途。
由于这些合成难题，差旅管理传统上被研究作为助催化剂11为铂族金属，催化剂载体为铂族金属，19-22或支持活性铂族金属单层23-25 这里介绍的方法提供了以合成两个非烧结的能力与金属终止TMC纳米颗粒和过渡金属氮化物（TMN）纳米粒子具有可调尺寸，晶相和金属构成。26的方法提出还提供ABility获得TMC或TMN纳米分散体或沉积的TMC和TMN纳米粒上在室温高表面积的催化剂载体，从而缓解热支持降解。此方法因此适合于TMC和TMN纳米粒，先进的多金属TMC和TMN纳米粒，或需要精确地控制颗粒尺寸和表面的其他应用程序的开发独立催化应用。26
这里介绍的方法使用一个三步式协议来合成TMC和TMN纳米颗粒。在第一步骤中，一个反相微乳液（RME）被用于涂敷前过渡金属氧化物（TMO）的NP在二氧化硅纳米球。该乳液是通过使用市售非离子表面活性在非极性介质中分散的水滴制备。二氧化硅包封TMO纳米粒然后经过任渗碳或氮化热处理。这里，二氧化硅微粒防止烧结在高温下，同时允许反应气体扩散到吨他TMO纳米粒子，并将它们转换为TMC或TMN的NP。在最后的步骤中，二氧化硅壳是使用酸性或碱性处理，得到TMC或TMN纳米分散体可以在一个高表面积载体进行分散，如炭黑除去。
Protocol
Representative Results
Discussion
甲程序用于合成非烧结，金属端过渡金属碳化物和氮化物的纳米颗粒与可调谐的尺寸和结构在这里提出的方法26的关键步骤包括：使用无湿气的RBF以包含稀释金属醇盐前体，避免了碱金属在所有步骤的杂质，而不是丙酮或异丙醇，渗碳或氮化的组件，并用氟化氢铵工作时使用合适的PPE之前执行适当的泄漏检查沉淀用过量的甲醇对RME。
该方法可以在几个方面进行修改。?…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was sponsored by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, grant no. DE-FG02-12ER16352. S.T.H. thanks the National Science Foundation for financial support through the National Science Foundation Graduate Research Fellowship under Grant No. 1122374.
Materials
| n-heptane | Sigma-Aldrich | 246654 | |
| polyoxyethylene (4) lauryl ether | Sigma-Aldrich | 235989 | Brij® L4 |
| tungsten (VI) isopropoxide | Alfa Aesar | 40247 | W(VI)IPO |
| tungsten (VI) chloride | Sigma-Aldrich | 241911 | To prepare W(VI)IPO, homemade |
| tungsten (IV) chloride | Strem Chemicals | 74-2348 | To prepare W(IV)IPO, homemade |
| tantalum (V) isopropoxide | Alfa Aesar | 40038 | Ta(V)IPO |
| niobium (V) isopropoxide | Alfa Aesar | 36572 | Nb(V)IPO |
| nickel (II) methoxyethoxide | Alfa Aesar | 42377 | Ni(II)MEO |
| titanium (IV) isopropoxide | Sigma-Aldrich | 87560 | Ti(IV)IPO |
| molybdenum (V) isopropoxide | Alfa Aesar | 39159 | Mo(V)IPO |
| molybdenum (V) chloride | Sigma-Aldrich | 208353 | To prepare Mo(V)IPO, homemade |
| tetraethyl orthosilicate | Sigma-Aldrich | 333859 | TEOS |
| ammonium hydroxide | Sigma-Aldrich | 320145 | |
| methanol | Sigma-Aldrich | 34860 | |
| anhydrous isopropanol | Sigma-Aldrich | 278475 | To prepare homemade alkoxides |
| ammonium bifluoride | Sigma-Aldrich | 224820 | |
| carbon black | Cabot Corp. | Vulcan® XC72R | |
| Methane | AirGas | ME R300 | |
| Hydrogen | AirGas | HY UHP300 | |
| Ammonia | AirGas | AM AH80N705 | |
| Quartz Tube Furnace | MTI Corp. | OTF-1200X-S-UL |
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