The electrochemical reduction mechanism of trivalent chromium in the presence of formic acid

The electrochemical reduction process of trivalent Cr in the presence of formic acid is studied. The compositions of Cr complexes in electrolytes and products on cathode are investigated using ultraviolet–visible absorption spectroscopy (UV–Vis) and X-ray photoelectron spectroscopy (XPS), respectively. The geometric structures of the original and transition state ions during the electroreduction process are optimized, using density functional theory with General Gradient Approximation/Perdew-Wang 91 (GGA/PW91) calculation. The trivalent Cr primarily exists in the form of [Cr(H2O)6]3+ in the solution. [Cr(H2O)6]3+ exhibits regular-octahedron structure which is unfavorable for center Cr3+ to contact cathodic electrons. In the presence of formic acid, formate ion promotes the formation of the reactive intermediate, [Cr(H2O)4CHOO]2+, which possesses irregular-octahedron structure with Cr3+ ion as a vertex. In this case, Cr3+ ion can contact the cathode and then obtain electrons easily.