Effect of sulfate adlayer on formic acid oxidation on Pd(111) electrode

The kinetics of formic acid oxidation (FAO) on Pd(111) in 0.1 mol/L H₂SO₄+0.1 mol/L HCOOH with and without addition of Na₂SO₄ is studied using cyclic voltammetry and potential step method, which is compared with that in 0.1 mol/L HClO₄. It is found that adsorbed sulfate has significant inhibition effect on FAO kinetics. After addition of 0.05 mol/L or 0.1 mol/L Na₂SO₄, FAO current in the negative-going scan is found to be significantly smaller than that at the same potential in the positive-going scan. We speculate that at potentials positive of the phase transition potential for the (SO4ad*)m+[(H2O)n-H3O+] or (SO4ad^∗)m+[Na+(H2O)n-H3O+] adlayer, the adlayer structure probably becomes denser and more stable with the increase of potential or with the addition of Na₂SO₄. The formation of connected adlayer network greatly enhance the stability of the adlayer, and the insertion of positive-charged H⁺ or Na⁺ into the adlayer network further reduces the electrostatic repulsion between partially charged sulfates. As a result, the destruction/desorption of compact sulfate adlayer becomes more difficult, which leaves much less free sites on the surface for FAO, and thus FAO kinetics at higher potentials and in the subsequent negative-going potential scan is significantly inhibited.