Elektrochemische oxydation von ameisensäure am platinkatalysator mit und ohne schwefelsorbat in alkalischem und saurem elektrolyten

Abstract

In the oxidation of formic acid to carbon dioxide, a platinum electrode is more strongly polarized in an acid electrolyte than it is in an alkaline electrolyte, in which carbonate is formed. Immediately after addition of the formic acid, the current is very high, but later approaches a low equilibrium value. This inhibition is due to a blockade of the platinum surface caused by the sorption of formic acid or a consecutive product. The inhibition can be suppressed by partial coverage of the platinum surface with a sulphur sorbate. At Raney-platinum electrodes in 3N H₂SO₄ a current density of approximately 200 mA/cm² is attained at a temperature of 30°C and a potential of 250 mV. At 90°C the current density even reaches a steady state value of about 2 A/cm². The large increase in the oxidation rate of formic acid by the sulphur sorbate is associated with a decrease of activation energy by about 8 kcal/ mol. The reaction rate has a maximum if the platinum surface is covered by approximately a third of a monatomic sulphur layer. The sulphur sorbate layer can easily be put on all platinum catalysts, e.g. platinum black, by a hydrogen sulfide treatment. In 3N H₂SO₄ at a temperature of 70°C the sorbate layer is resistant up to a potential of about 650 mV; only at higher potential starts the oxidation yielding sulphur dioxide. The sulphur sorbate resists also potassium hydroxide solution, however, the oxidation of formic acid is not accelerated in the alkaline solution.

The mechanism of the anodic oxidation of formic acid in alkaline and acid electrolyte will be discussed by reference to potentiodynamic potential-current density plots.