The influence of anions and kink structure on the enantioselective electro-oxidation of glucose

Abstract

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 × 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.