The kinetics of the electrochemical reduction of xylose on amalgamated zinc

Abstract

During the electrolysis of an aqueous solution of xylose, two parallel reactions take place on the cathode i.e. the reduction of xylose to xylitol and hydrogen evolution.

To favour the desired reaction of xylose reduction, amalgamated zinc is used to perform the reduction in this study. The catalytic amalgamated coating is formed by the electrodeposition of a zinc coating having desired properties on a copper mesh from a sulfate bath and its subsequent immersion into the HgCl₂ solution to obtain the amalgamated zinc of adequate concentration.

The present study focuses on the kinetics and mechanism of two parallel reactions of hydrogen evolution and xylose reduction on amalgamated zinc from a solution containing xylose and Na₂SO₄ as a basic electrolyte. The hydrogen evolution reaction occurs through two parallel mechanisms, either in the presence or absence of xylose. At lower overpotentials, hydrogen is evolved on zinc oxide in three consecutive elementary steps. The overall rate of the hydrogen evolution reaction within the limiting current range is governed by the heterogeneous chemical step involving a Zn-H species. At potentials more negative than -2.0 V, the reaction takes place on the metal surface through the Volmer-Heyrovsky mechanism.

Xylose is adsorbed on the metallic zinc. At potentials more negative than -1.25 V, the adsorbed xylose molecules are reduced with Zn-H to xylitol. The cathodic polarization curves, in either the presence or absence of xylose, were used to determine the cathodic current efficiency for xylitol production, which is in good agreement with the values obtained by other methods.