Probing acid sites in solid catalysts with pyridine UV-Vis spectroscopy.
Probing acid sites in solid catalysts with pyridine UV-Vis spectroscopy.
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使用吡啶紫外-可见光谱探测固体催化剂中的酸位。
DOI：
10.1039/c8cp03991g
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
B. Weckhuysen
中科院分区：
文献类型：
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作者：
Marjolein E. Z. Velthoen;S. Nab;B. Weckhuysen
固体酸在多相催化领域有着广泛的应用。在这项工作中，我们提出吡啶紫外-可见光谱作为一种新颖且有前途的工具来研究此类固体酸的酸性特性。研究发现，在与酸性位点相互作用时，吡啶的电子性质发生显着变化。对吡啶连续吸附和解吸的监测揭示了紫外-可见光区域的吸收带特征为（a）在布朗斯台德酸位点上形成的吡啶鎓离子，（b）与路易斯酸位点配位的吡啶，（c）与表面羟基形成氢键的吡啶，以及（d）物理吸附的吡啶。经典的吡啶 FT-IR 方法也可探测不同的布朗斯台德和路易斯酸位点的存在，但对表面羟基之间的差异缺乏敏感性。相比之下，吡啶紫外-可见光谱方法被证明对于识别和区分不同表面羟基特别有用，因为紫外-可见光谱中的带位置强烈依赖于羟基的化学环境。此外，利用氮气流下的缓慢解吸程序，可以研究羟基酸性强度的差异。该方法和相关测量方案是为了研究具有不同二氧化硅/氧化铝比例的固体酸内的酸性性质而开发的，但我们认为更普遍适用于任何固体酸。
Solid acids hold widespread applications in the field of heterogeneous catalysis. In this work, we present pyridine UV-Vis spectroscopy as a novel and promising tool to study the acidic properties of such solid acids. It was found that upon interaction with acid sites, the electronic properties of pyridine change significantly. Monitoring of consecutive adsorption and desorption of pyridine revealed absorption bands in the UV-Vis region characteristic for (a) pyridinium ions formed on Brønsted acid sites, (b) pyridine coordinated to Lewis acid sites, (c) pyridine hydrogen-bonded to surface hydroxyl groups, and (d) physisorbed pyridine. The classical pyridine FT-IR method probes the presence of different Brønsted and Lewis acid sites as well, but lacks sensitivity towards the differentiation between surface hydroxyl groups. In contrast, the pyridine UV-Vis spectroscopy method proves especially useful for the identification and distinction of different surface hydroxyl groups, since the band position in the UV-Vis spectrum strongly depends on the chemical environment of the hydroxyl group. Moreover, utilizing a slow desorption procedure under N2 flow, it was possible to study the differences in acidic strength of the hydroxyl groups. This method and related measurement protocols were developed for the study of acidic properties within solid acids with different silica/alumina ratios, but are, in our opinion, more generally applicable to any solid acid.