Microwave-induced plasma promoted dielectric heating: metrology and application to the photocatalytic activation of water

人们显然需要有效地合成能够维持和发展未来技术并为现代社会奠定基础的新材料。为了增加可获得材料的多样性，有必要开发新的合成技术。拟议的研究描述了微波辐射的应用，以提供能量来驱动固体之间和/或固体与气体之间的化学反应。当受限气体暴露在微波辐射下时，会发生电离，产生微波诱导等离子体(MIP)，可用于提供热量以驱动散装固体之间的反应，并作为可对材料进行化学修饰的反应气体物种的来源。许多极性液体(如水)和一些固体确实直接与微波相互作用，导致快速加热和反应，但许多具有重要技术价值的材料在室温下对微波是透明的，因此无法用于反应。然而，直接微波(介电)加热依赖于温度，许多材料在高温下会直接与微波耦合。不幸的是，目前还没有列举大多数固体的介电加热的温度依赖关系的定量数据。我们建议使用MIP和介电加热的组合来合成新的化合物和复合材料，这将由对前体和产品材料的介电加热的温度依赖性的测量(计量学)来支持。MIP提供的热量将使许多在室温下是微波透明的固体在高温下表现出显著的介电加热。最初，将通过现场监测反应混合物的温度来确定材料的MIP促进的介质加热，其中温度的快速上升和样品温度超过等离子体温度将表明显著的介质加热。表现出强烈温度依赖性的材料将被更严格地测量，这些信息将被用来关联反应产物的结构和形态，并指导后续的合成反应。微波加热的温度依赖性将允许利用差动加热，在非均质混合物中，不同的固体可以同时加热到不同的温度。这与传统的传导/对流加热方法形成了直接对比，在传统的传导/对流加热方法中，固体混合物被均匀加热。我们将瞄准的固体是半导体催化剂，与从太阳能产生氢气的水的光催化活化有关。氢是一种清洁能源，因为燃烧产物是水，因此光催化是满足社会日益增长的能源需求的机会，也可能取代对环境有害的有限化石燃料资源。光催化剂通常由半导体金属氧化物和金属颗粒/富金属区域组成的非均相复合材料，与微波加热相比将表现出明显不同的温度依赖性，从而导致差热。因此，差热和反应性分子印迹聚合物的结合为通过分子印迹聚合物与受热组分之间的反应进行选择性修饰来合成新型材料提供了额外的机会。此外，反应性分子印迹聚合物可以用来修饰固体以改变介电性质以增加或降低介电加热的程度。分子印迹聚合物促进的介电加热代表了一种独特而新颖的合成方法来制备新材料。使用一系列显微镜和其他技术对材料进行计量学和严格的表征将为探索性合成工作奠定基础，以实现这种新方法的潜力。

There is a clear and effective need for the synthesis of new materials that can sustain and develop future technologies and underpin modern society. To increase the diversity of materials accessible it is necessary to develop new synthetic techniques. The proposed research describes the application of microwave radiation to provide energy to drive chemical reactions between solids and/or between solids and gases. A confined gas when exposed to microwave radiation can ionise giving rise to a microwave-induced plasma (MIP) that can be used to provide heat to drive reaction between bulk solids and as a source of reactive gas species that can chemically modify a material. Many polar liquids (e.g. water) and some solids do interact directly with microwaves causing rapid heating and reaction, but many technologically important materials are transparent to microwaves at room temperature thus preventing use in reactions. However, direct microwave (dielectric) heating is temperature dependent and many materials will directly couple with microwaves at elevated temperatures. Unfortunately quantitative data enumerating the temperature dependence of dielectric heating for most solids is not currently available. We propose to synthesise new compounds and composite materials using a combination of MIP and dielectric heating that will be supported by measurements (metrology) of the temperature dependence of dielectric heating of precursor and product materials. The heat provided by the MIP will cause many solids that are microwave transparent at room temperature to exhibit significant dielectric heating at elevated temperatures. Initially MIP promoted dielectric heating will be identified for materials by monitoring the temperature of a reaction mixture in situ, where rapid temperature rises and sample temperatures in excess of the plasma temperature will indicate significant dielectric heating. Materials that exhibit strong temperature dependence will then be measured more rigorously and this information used to correlate the structure and morphology of reaction products and direct subsequent synthetic reactions. The temperature dependence of microwave heating will allow differential heating to be exploited, where in a heterogeneous mixture, different solids can be simultaneously heated to different temperatures. This is in direct contrast to traditional conduction/convection heating methods where a solid mixture is heated uniformly.The solids we will target are semiconducing catalysts relevant to the photocatalytic activation of water that generate hydrogen from solar energy. Hydrogen is a clean energy resource because the combustion product is water and therefore photocatalysis represents an opportunity to meet the increasing energy demands of society and also potentially replace limited fossil fuel resources that are detrimental to the environment. Photocatalysts typically comprise a heterogeneous composite of semiconducting metal oxide and metal particles/metal rich regions that will exhibit markedly different temperature dependence with respect to microwave heating giving rise to differential heating. A combination of differential heating and reactive MIPs therefore provides additional opportunity for novel materials synthesis by selective modification from reaction between a MIP and heated component. Furthermore, reactive MIP can be used to modify a solid to alter the dielectric properties to either increase or decrease the extent of dielectric heating.MIP promoted dielectric heating represents a distinct and adventurous synthetic method for the preparation of new materials. Metrology and rigorous characterisation of materials using a range of microscopy and other techniques will underpin exploratory synthetic work to realise the potential of this novel method.