Attard Gary A, Harris Catherine, Herrero Enrique, Feliu Juan
Department of Chemistry, Cardiff University, UK.
Faraday Discuss. 2002(121):253-66; discussion 331-64. doi: 10.1039/b110764j.
The electro-oxidation of glucose in sulfuric acid using well-defined chiral platinum single crystal electrodes has been demonstrated previously to be an enantioselective reaction with the degree of enantioselectivity being dependent on the surface density of kink sites. The chirality of the surface originates from the microstructure of the kink site whereby the sequence of the three fundamental adsorption sites [111], [100] and [110] constituting the kink may be viewed from the electrolyte phase either in a clockwise (R-enantiomer) or anti-clockwise (S-enantiomer) fashion. In the present study, this work is extended to examine the role of both kink structure and specifically adsorbed anions on the mechanism of chiral discrimination. Kinked surfaces based on [111] terraces (Pt[976],Pt[643] and Pt[531]),[100] terraces (Pt[721]) and [110] terraces (Pt[11,7,1] and Pt[841]) have been investigated and both the magnitude and potential dependence of the enantioselective electro-oxidation of glucose characterised. Additionally, the changes engendered by interchanging the character of the two steps whose confluence form the kink whilst maintaining the symmetry of the terrace has also been examined via a comparison of Pt[643] and Pt[431]. Low energy electron diffraction (LEED) was used to confirm that all surfaces when clean and thermally annealed were in their (1 x 1) state. Cyclic voltammetry (CV) confirmed this finding for flame-annealed electrodes after cooling in hydrogen. Three general points emerge from the electro-oxidation studies: (i) The highest degree of enantioselectivity is exhibited by kink sites adjacent to [111] and [110] terraces in sulfuric acid. (ii) The adsorption of specifically adsorbed anions like bisulfate/sulfate influences strongly the chiral discriminatory behaviour of all surfaces. (iii) No electro-oxidation takes place at [110] sites, as evidenced by complete overlap of the [110] step hydrogen underpotential deposition (UPD) charge in glucose and glucose-free solutions. Nonetheless it is deduced that [110] sites must play some part in the initial orienting of the glucose molecule prior to reaction. Ideas based on these findings are developed in order to rationalise in particular the influence of anion adsorption on the initial enantioselective interaction of the glucose molecule with the chiral surface.
先前已证明，在硫酸中使用明确的手性铂单晶电极对葡萄糖进行电氧化是一种对映选择性反应，对映选择性程度取决于扭折部位的表面密度。表面的手性源于扭折部位的微观结构，从电解质相来看，构成扭折的三个基本吸附位点[111]、[100]和[110]的顺序可以是顺时针（R-对映体）或逆时针（S-对映体）方式。在本研究中，这项工作得到扩展，以研究扭折结构和特异性吸附阴离子对手性识别机制的作用。研究了基于[111]平台（Pt[976]、Pt[643]和Pt[531]）、[100]平台（Pt[721]）和[110]平台（Pt[11,7,1]和Pt[841]）的扭折表面，并对葡萄糖对映选择性电氧化的程度和电位依赖性进行了表征。此外，通过比较Pt[643]和Pt[431]，还研究了在保持平台对称性的同时交换形成扭折的两个步骤的特征所产生的变化。低能电子衍射（LEED）用于确认所有表面在清洁和热退火后均处于(1×1)状态。循环伏安法（CV）证实了在氢气中冷却后的火焰退火电极的这一发现。电氧化研究得出三个一般要点：(i) 在硫酸中，与[11 ~ 1]和[110]平台相邻的扭折部位表现出最高程度的对映选择性。(ii) 硫酸氢根/硫酸根等特异性吸附阴离子的吸附强烈影响所有表面的手性识别行为。(iii) 在[110]位点没有发生电氧化，这在葡萄糖溶液和无葡萄糖溶液中[110]台阶氢欠电位沉积（UPD）电荷完全重叠中得到证明。尽管如此，可以推断[110]位点在反应前葡萄糖分子的初始定向中一定起到了某种作用。基于这些发现的观点得到了进一步发展，以便特别合理地解释阴离子吸附对葡萄糖分子与手性表面的初始对映选择性相互作用的影响。
