Drenchev Nikola L, Chakarova Kristina K, Lagunov Oleg V, Mihaylov Mihail Y, Ivanova Elena Z, Strauss Ina, Hadjiivanov Konstantin I
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences.
Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover.
J Vis Exp. 2020 Feb 1(156). doi: 10.3791/60285.
In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids with adsorbates. Vibrational spectra provide information on the chemical nature of adsorbed species and their structure. Thus, they are very useful for obtaining molecular level understanding of surface species. The IR spectrum of the sample itself gives some direct information about the material. General conclusions can be drawn concerning hydroxyl groups, some stable surface species and impurities. However, the spectrum of the sample is "blind" with respect to the presence of coordinatively unsaturated ions and gives rather poor information about the acidity of surface hydroxyls, species decisive for the adsorption and catalytic properties of the materials. Furthermore, no discrimination between bulk and surface species can be made. These problems are solved by the use of probe molecules, substances that interact specifically with the surface; the alteration of some spectral features of these molecules as a result of adsorption provides valuable information about the nature, properties, location, concentration, etc., of the surface sites. The experimental protocol for in-situ IR studies of gas/sample interaction includes preparation of a sample pellet, activation of the material, initial spectral characterization through the analysis of the background spectra, characterization by probe molecules, and study of the interaction with a particular set of gas mixtures. In this paper we investigate a zirconium terephthalate metal organic framework, Zr6O4(OH)4(BDC)6 (BDC = benzene-1,4-dicarboxylate), namely UiO-66 (UiO refers to University of Oslo). The acid sites of the UiO-66 sample are determined by using CO and CD3CN as molecular probes. Furthermore, we have demonstrated that CO2 is adsorbed on basic sites exposed on dehydroxylated UiO-66. Introduction of water to the system produces hydroxyl groups acting as additional CO2 adsorption sites. As a result, CO2 adsorption capacity of the sample is strongly enhanced.
原位红外光谱是一种廉价、高灵敏度且具有选择性的重要工具，用于研究多晶固体与吸附质之间的相互作用。振动光谱提供了有关吸附物种的化学性质及其结构的信息。因此，它们对于从分子水平理解表面物种非常有用。样品本身的红外光谱给出了有关该材料的一些直接信息。可以得出关于羟基、一些稳定的表面物种和杂质的一般结论。然而，样品的光谱对于配位不饱和离子的存在是“盲目的”，并且关于表面羟基的酸度给出的信息相当有限，而表面羟基的酸度对于材料的吸附和催化性能起着决定性作用。此外，无法区分体相物种和表面物种。通过使用探针分子（即与表面特异性相互作用的物质）可以解决这些问题；这些分子由于吸附而导致的某些光谱特征的变化提供了有关表面位点的性质、特性、位置、浓度等的有价值信息。气体/样品相互作用的原位红外研究的实验方案包括制备样品片、活化材料、通过分析背景光谱进行初始光谱表征、用探针分子进行表征以及研究与特定气体混合物的相互作用。在本文中，我们研究了一种对苯二甲酸锆金属有机骨架材料Zr6O4(OH)4(BDC)6（BDC = 苯 - 1,4 - 二羧酸酯），即UiO - 66（UiO指奥斯陆大学）。使用CO和CD3CN作为分子探针来确定UiO - 66样品的酸性位点。此外，我们已经证明CO2吸附在脱羟基的UiO - 66暴露的碱性位点上。向系统中引入水会产生羟基，这些羟基作为额外的CO2吸附位点。结果，样品的CO2吸附容量大大增强。
