Electron Paramagnetic Resonance - Volume 21
Electron Paramagnetic Resonance - Volume 21
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电子顺磁共振 - 第 21 卷
DOI：
10.1039/b709153m
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发表时间：
2008
期刊：
影响因子：
--
通讯作者：
Murphy D
中科院分区：
文献类型：
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作者：
Murphy D
近年来，许多氧化物表面缺陷位点的研究受到了极大的关注，因为这些位点是配位不饱和的，并且在许多催化反应中经常表现出增强的活性。尽管这一重要性，氧化物表面的化学性质和它们的作用，在活化吸附基板的缺陷部位的影响是知之甚少。在许多情况下，这些缺陷是顺磁性的，因此可以通过EPR直接研究，或使用合适的探针分子间接研究。然而，这些界面现象的EPR调查并不局限于表面缺陷。瞬态自由基中间体、稳定的无机自由基、捕获的电荷载流子态以及顺磁性过渡金属离子在多相催化中都起着重要的作用。这篇综述的目的是突出和展示EPR光谱在这一研究领域的应用，并讨论如何使用EPR来识别所有上述表面物种。在我们以前的SPR评论，1我们已经集中我们的注意力奇异的表面过程和界面状态在异质氧化物表面。微孔材料（如沸石），软固体（如表面活性剂和聚合物）和散装状态不包括在内。本文将涵盖近四年来的文献，并将综述分为s-区金属氧化物、过渡金属氧化物和p-区氧化物. s区金属氧化物第2族金属氧化物，特别是MgO，在表面科学和表面化学文献中占有重要地位。这主要是由于其简单的晶体结构和良好定义的表面形态。MgO可以被认为是离子氧化物的原型，也是研究与一些科学和技术问题有关的重要方面的理想模型系统。这些范围从催化到电子微器件和防腐蚀保护。一个相关的例子是ns 1金属原子与MgO和其他碱土金属氧化物表面的相互作用。由于它们的顺磁性质，碱金属吸附原子是优良的EPR探针，这使得人们能够解开金属氧化物键合相互作用的本质和表面吸附位点的性质。因此，通过这些信息，可以间接检查这些地点的丰度和表面本身的形态。EPR谱中的超精细耦合常数反映了基体对金属物种电子性质的影响（基体效应）。例如，稀有气体和碳氢化合物中被俘获的碱金属原子的EPR谱显示，金属超精细相互作用的百分之几偏离其气相值。当碱金属原子沉积在碱性氧化物如碱土金属氧化物的表面上时，会发生截然不同的情况。特别是，吸附的K原子（I= 3/2）到高表面积氧化镁产生良好的解决EPR光谱，其特征在于由一个独特的超精细四重峰与约4 mT的分离。2该值表示金属超精细耦合常数相对于气相值的一致降低（约50%）。17 O同位素取代
The study of defective surface sites on many oxides has received considerable interest in recent years as these sites are coordinatively unsaturated and quite often exhibit enhanced activity in many catalytic reactions. Despite this importance, the influence of the defect sites on the chemical nature of the oxide surface and their role in the activation of adsorbed substrates is poorly understood. In a number of cases these defects are paramagnetic and can therefore be directly studied by EPR, or indirectly studied using suitable probe molecules. The EPR investigations of these interfacial phenomena are not however confined to the surface defects. Transient radical intermediates, stable inorganic radicals, trapped charge-carrier states and paramagnetic transition metal ions, all play an important part in the heterogeneous catalysis. The aim of this review is to highlight and demonstrate the applications of EPR spectroscopy to this field of research and discuss how EPR is used to characterise all of the aforementioned surface species. As in our previous SPR reviews, 1 we have focussed our attention singularly on surface processes and interfacial states at heterogeneous oxide surfaces. Microporous materials (such as zeolites), soft-solids (such as surfactants and polymers) and bulk states are not covered. The literature in the last four calender years will be covered and the review will be separated into the s-block metal oxides, transition metal oxides and p-block oxides.2. s-Block metal oxidesMetal oxides of group 2, and MgO in particular, feature large in the surface science and surface chemistry literature. This is primarily due to their simple crystal structure and well defined surface morphology. MgO can be considered a prototype of ionic oxides and an ideal model system to study important aspects related to a number of scientific and technological issues. These range from catalysis to electronic micro-devices and anti-corrosion protection. A relevant example is provided by the interaction of ns1 metal atoms with the surface of MgO and other alkaline earth oxides. Due to their paramagnetic nature, alkali metal adatoms are excellent EPR probes, which allow one to unravel the essence of the metal-oxide bonding interaction and the nature of the surface adsorption sites. Through this information it is therefore possible to indirectly examine the abundance of these sites and the morphology of the surface itself. The influence of the matrix on the electronic properties of the metal species (matrix effect) is reflected by the hyperfine coupling constant in the EPR spectra. For example, the EPR spectra of trapped alkali atoms in rare gases and hydrocarbons show departures of a few percent of the metal hyperfine interactions from their gas phase values. A drastically different situation occurs when alkali metal atoms are deposited on the surface of basic oxides such as alkaline-earth oxides. In particular, adsorption of K atoms (I= 3/2) onto high surface area MgO produced well resolved EPR spectra characterized by a distinct hyperfine quartet with a separation of about 4 mT. 2 This value represents a consistent reduction (about 50%) of the metal hyperfine coupling constant with respect to the gas phase value. Isotopic substitution with 17O