An in situ Spectrophotometric Method for Observing the Infrared Spectra of Species at the Electrode Surface During Electrolysis.
An in situ Spectrophotometric Method for Observing the Infrared Spectra of Species at the Electrode Surface During Electrolysis.
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DOI：
10.1021/ac60233a002
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发表时间：
1966
影响因子：
7.4
通讯作者：
H. B. Mark;B. S. Pons
中科院分区：
文献类型：
--
作者：
H. B. Mark;B. S. Pons
至少，有必要对系统特性进行第二次独立但同时的测量，以便从各种可能性中找出真正的机制或过渡状态。在电子自旋共振（ESR）腔中进行电解的技术（16）已被成功地用于检测和鉴定有机电极反应中的自由径向产物和中间体（1,16）。透明导电玻璃电极（CGE）已被用于测定电解过程中某些染料的可见吸收光谱（14,is）（该技术仅测量扩散到电解溶液中产生的合理数量的物种的光谱）。上述两种技术都不够灵敏，无法单独测量存在于电极界面的物质光谱。一种光学技术，el-lipsometry，已经被用来测量电解过程中铂上氧化膜的性质（17）。然而，值得怀疑的是，这种技术能否检测到中间体、双层电场中受干扰的物质或电极表面吸附物质的光谱的轻微变化，因为在这种情况下，光路是通过大部分电解溶液的。因此，大量相似的反应物和产物对光的广泛吸收会使界面本身少量存在的物质的光谱模糊不清。由于界面上物质的性质是主要的兴趣，因此开发一种能够测量界面上第一单层（或前几层）光谱的原位光学技术是非常重要的，而不受存在于体溶液中的类似物质的干扰。抑制多重内反射（FMIR）是与单衰减全反射（ATR）光谱同时发展起来的（4,7 -10），由于FMIR是用于研究表面效应的，因此它是一种理想的技术，适合于实际电解过程中电极表面物质光谱的研究。在这种技术中，来自分光光度计的光被聚焦，以一定角度进入一个特殊设计的晶体或板（由石英、氯化钠、锗等制成，取决于所需的光谱范围）。如果晶体或电池的折射率高于与其接触的气体或液体介质，并且光束在界面处的入射角略大于临界角，则只要入射光束的频率位于第二介质（折射率较低）不吸附的区域（= 0）(4)，就可以获得光束的全反射率。什么时候?^ 0，然而，不能得到光束的全反射，如果应用频率扫描作为入射光束，则从细胞产生的出口光束的强度与波长的曲线图产生的光谱与第二介质的透射光谱非常相似（7,8）。因此，在反射过程中，光束实际上穿透了一小段距离进入第二介质。这种穿透的深度是入射角和光的波长的函数。在光谱的可见区域，每次反射的有效路径长度是几个埃深(7)，在红外区域，可以用大约50到100次反射的仪器观察到单层的光谱（19，SO）（9,22）。在这一点上，应该注意的是，感兴趣的物质的光谱可以被溶剂和支持电解质的吸收所掩盖，如果它们吸附在同一区域。因此，如果FMIR板也被制成电极，则电极表面的物质光谱…
At a minimum, a second independent but simultaneous measurement of the system properties is necessary to sort out the true mechanism or transition state from the alternate possibilities. The technique of carrying out an electrolysis in an Electron Spin Res-onance(ESR) cavity (16) has been employed successfully to detect and identify free radial products and inter-mediates in organic electrode reactions (1, 16). Transparent conducting glass electrodes (CGE) have been employed to determine the visible absorption spectra of certain dyes during electrolysis (14, IS)(this technique measures only thespectra of species generated in reasonable quantity which diffuses into the bulk of the electrolysis solution). Neither of the above techniques is suf-ficiently sensitive to measure the spectra of species present at the electrode inter-face alone. One optical technique, el-lipsometry, has been employed to measure in situ the nature of oxide films on platinum during electrolysis (17). However, it is doubtful that this technique could detect the slight variations of spectra that might be expected for intermediates, species perturbed in the electric field of the double layer, or adsorbed species ori the electrode sur-face, as the light path in this case is directed through the bulk of the electrolysis solution. Thus, the extensive absorption of the light by the similar reactants and products in the bulk would obscure the spectra of the species present in small quantity at the interface itself. As the natureof the species at the interface is of major interest, it is of extreme importance to develop an in situ optical technique which is capable of measuring the spectra of the first monolayer (or first few layers) at the interface without interference from sim-ilar species present in the bulk solution. The properties of Frustrated Multiple Internal Reflectance (FMIR) which was developed simultaneously with single Attenuated Total Reflectance (ATR) spectroscopy (4, 7-10) make it an ideal technique to adapt to the study of the spectra of species at the electrode surface during actual electrolysis as it was designed to study surfaceeffects. In this technique, light from the spectrophotometer is focused to enter a specially de-signed crystal or plate (made of quartz, NaCl, Ge, etc. depending on the spectral range desired) at an angle. If the crystal or cell is of higher refractive index than the gas or liquid media in contact with it, and this incident angle of the beam at the interface is slightly larger than the critical angle, total reflectance of the beam will be attained, provided that the incident beam frequency is in a region where the second (lower refractor index) media is nonadsorbing (= 0)(4). When?^ 0, however, total re-flection of the beam is not attained and, if a frequency scan is applied as the incident beam, a plot of the intensity of resulting exit beam from thecell vs. wavelength yields a spectrum which closely resembles the transmission spectrum of the second medium (7, 8). Thus, during the process ofreflection, the beam actually penetrates a short distance into the second medium. The depth of this penetration is a function of the incident angle as well as the wave-length of the light. In the visible region of the spectrum, the effective path length of each reflection is several angstroms deep (7) and in the infrared region, the spectra of monolayers can be observed (19, SO) with instrumentation capable of about 50 to 100 reflections (9, 22). It should be noted at this point that the spectrum of the species of in-terest can be obscured by the absorption of the solvent and supporting electrolyte if they adsorb in the same region. Thus, if the FMIR plate is also made to be an electrode, the spectra of species at the electrode surface …