Electrolytic reduction of carbon dioxide at mercury electrode in aqueous solution.
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Cathodic reduction of carbon dioxide at a mercury pool electrode was investigated in aqueous solutions of NaHCO<SUB>3</SUB>, NaH<SUB>2</SUB>PO<SUB>4</SUB>–Na<SUB>2</SUB>HPO<SUB>4</SUB>, NaCl, NaClO<SUB>4</SUB>, Na<SUB>2</SUB>SO<SUB>4</SUB>, LiHCO<SUB>3</SUB>, and KHCO<SUB>3</SUB> as well as their combinations. The reaction product was confirmed to be HCOO<SUP>−</SUP> by a qualitative spot test. HCOO<SUP>−</SUP> was quantitatively analyzed by a permanganate titration. The partial current densities of HCOO<SUP>−</SUP> formation (<I>i</I><SUB>c</SUB>) and H<SUB>2</SUB> evolution (<I>i</I><SUB>H</SUB>) were calculated. In the present paper, relationships between electrode behavior and solution properties, some of which have been qualitatively described in previous papers, are quantitatively clarified as follows. The electrode potential <I>E</I><SUB>c</SUB> for constant <I>i</I><SUB>c</SUB> (0.5 mA/cm<SUP>2</SUP>) remains constant irrespective of pH and anions in the pH region of 2.4 to 7.8, whereas the potential <I>E</I><SUB>H</SUB> for constant <I>i</I><SUB>H</SUB> (0.5 mA/cm<SUP>2</SUP>) varies linearly with a slope of 125 mV/pH. The present experimental results verify Eyring <I>et al.</I>'s presumption that in a relatively high overvoltage region (region 2) the cathodic reduction of CO<SUB>2</SUB> at Hg proceeds with the charge transfer to CO<SUB>2</SUB> as the rate determining step. CO<SUB>2</SUB> reduction and H<SUB>2</SUB> evolution are concluded to proceed independently at the Hg electrode. The dependence of <I>E</I><SUB>c</SUB> on electrolyte concentration is interpreted in terms of the diffuse double layer theory. The effect of cation species on <I>E</I><SUB>c</SUB> also is discussed.
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