Revealing the Predominant Surface Facets of Rough Cu Electrodes under Electrochemical Conditions
Revealing the Predominant Surface Facets of Rough Cu Electrodes under Electrochemical Conditions
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DOI：
10.1021/acscatal.9b05532
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发表时间：
2020-06-19
期刊：
影响因子：
12.9
通讯作者：
Waegele, Matthias M.
中科院分区：
文献类型：
--
作者：
Gunathunge, Charuni M.;Li, Jingyi;Waegele, Matthias M.
通常发现具有粗糙表面的金属电极与其光滑对应物相比以高选择性和高反应速率将CO或CO2转化为烃和含氧化合物。粗糙电极的原子级形貌可能是其相对高的催化选择性和活性的一个关键因素。然而，很少有方法能够在电催化条件下探测粗糙金属电极的原子级结构。因此，控制这些电极的催化特性的原子级表面形态的细微差别在很大程度上仍未被探索。由于原子键合CO（COatop）的C O伸缩频率取决于底层金属原子的配位，因此铜电极上该反应中间体的红外光谱原则上可以提供有关电解过程中催化表面的结构信息。然而，其他的影响，如动态偶极耦合，很容易模糊的表面形貌上的频率的依赖性。此外，在低COatop覆盖的限制下，其中耦合效应小，在Cu（111）和Cu（100）面上的COatop的C O伸缩频率几乎相同。因此，在CO伸缩频率的基础上，不能直接区分这两个普遍存在的表面小面，它们表现出截然不同的CO还原活性。在这里，我们表明，粗糙的铜电极的原子级表面形态的关键特征可以推断从COatop的C-O伸缩带的线形的电位依赖性。具体来说，我们比较了两种类型的粗糙铜薄膜电极，通常采用的上下文中的表面增强红外吸收光谱（SEIRAS）。我们发现，电化学沉积在Si支撑的Au膜（CuAu-Si）上的铜膜与化学沉积在Si晶体上的铜膜（Cu-Si）相比，对于CO还原为乙烯是较差的催化剂。如通过差示电化学质谱法（DEMS）所定量的，乙烯的起始电位类似于CuAu-Si比Cu-Si阴极化200 +/-65mV。为了揭示Cu-Si和CuAu-Si不同催化性能的起源，我们用循环伏安法（CV）和SEIRAS探测电极表面。CV表征表明CuAu-Si上的（111）面占主导地位，而（100）面在Cu-Si上更常见。SEIRAS显示，COatop的C-O伸缩的线形是由两个频带，这是由于COatop的梯田和缺陷网站。不同的表面结构表现在两种类型的电极上的COatop的C-O伸缩模式的线形的形式完全不同的电位依赖性。利用一个简单的Boltzmann模型，考虑了COatop在平台和缺陷位置上的不同吸附能，以及由此产生的平台和缺陷位置上的COatop种群，我们推断，观察到的线形状的电极特定电位依赖性与两种类型的膜上不同的主要平台位置的存在是一致的。能级形态不限于SEIRAS，也可以应用于用表面增强拉曼光谱（Sers）记录的C O伸缩带，其适合于探测宽范围的粗糙铜电极。因此，通过这项工作，我们建立了COatop的C-O伸缩带的电位依赖性，作为电化学条件下粗糙金属电极的原子级表面结构的探针。当它与互补技术相结合时，这种方法为进一步提高粗糙金属电极的反应选择性提供了必要的结构信息。
Metal electrodes with rough surfaces are often found to convert CO or CO2 to hydrocarbons and oxygenates with high selectivity and at high reaction rates in comparison with their smooth counterparts. The atomic-level morphology of a rough electrode is likely one key factor responsible for its comparatively high catalytic selectivity and activity. However, few methods are capable of probing the atomic-level structure of rough metal electrodes under electrocatalytic conditions. As a result, the nuances in the atomic-level surface morphology that control the catalytic characteristics of these electrodes have remained largely unexplored. Because the C O stretching frequency of atop-bound CO (COatop) depends on the coordination of the underlying metal atom, the IR spectrum of this reaction intermediate on the copper electrode could, in principle, provide structural information about the catalytic surface during electrolysis. However, other effects, such as dynamic dipole coupling, easily obscure the dependence of the frequency on the surface morphology. Further, in the limit of low COatop coverage, where coupling effects are small, the C O stretching frequencies of COatop on Cu(111) and Cu(100) facets are virtually identical. Therefore, on the basis of the C O stretching frequency, it is not straightforward to distinguish between these two ubiquitous surface facets, which exhibit vastly different CO reduction activities. Herein, we show that key features of the atomic-level surface morphology of rough copper electrodes can be inferred from the potential dependence of the line shape of the C O stretching band of COatop. Specifically, we compared two types of rough copper thin-film electrodes that are routinely employed in the context of surface-enhanced infrared absorption spectroscopy (SEIRAS). We found that copper films that are electrochemically deposited on Si-supported Au films (CuAu-Si) are poor catalysts for the reduction of CO to ethylene in comparison to copper films (Cu-Si) that are electrolessly deposited onto Si crystals. As quantified by differential electrochemical mass spectrometry (DEMS), the onset potential for ethylene is similar to 200 +/- 65 mV more cathodic for CuAu-Si than that for Cu-Si. To reveal the origin of the disparate catalytic properties of Cu-Si and CuAu-Si, we probed the surfaces of the electrodes with cyclic voltammetry (CV) and SEIRAS. The CV characterization suggests that the (111) surface facet predominates on CuAu-Si, whereas the (100) facet is more common on Cu-Si. SEIRAS reveals that the line shape of the C O stretching of COatop is composed of two bands that are attributable to COatop on terrace and defect sites. The different surface structures manifest themselves in the form of starkly different potential dependences of the line shape of the C O stretching mode of COatop on the two types of electrodes. With a simple Boltzmann model that considers the different adsorption energies of COatop on terrace and defect sites, and the resulting COatop populations on terrace and defect sites, we deduced that the observed electrode-specific potential dependence of the line shape is consistent with the presence of different predominant terrace sites on the two types of films.This strategy for assessing the atomic-level morphology is not restricted to SEIRAS but could also be applied to the C O stretching bands recorded with surface-enhanced Raman spectroscopy (SERS), which is suitable for probing a wide range of rough copper electrodes. Therefore, with this work, we establish the potential dependence of the C O stretching band of COatop as a probe of the atomic-level surface structure of rough metal electrodes under electrochemical conditions. When it is coupled with complementary techniques, this methodology provides essential structural information for further improvement in the reaction selectivity of rough metal electrodes.