On the mechanism of electroless plating. I. Oxidation of formaldehyde at different electrode surfaces

To elucidate the mechanism of electroless plating solutions with formaldehyde as the reductant, the anodic oxidation of formaldehyde in alkaline medium was studied. The influence of electrode material, pH and potential was investigated. The experimental results can be explained by a mechanism in which methylene glycol anion (CH₂OHO^–) is dehydrogenated at the electrode surface, yielding adsorbed hydrogen atoms. The atomic hydrogen can either be oxidized to water or be desorbed as a gas. Kinetic rate laws for these two reactions are given. Electroless copper, platinum and palladium solutions behave according to the mechanism.Nomenclature E applied potential - E _a activation energy of adsorption - E _d activation energy of desorption (=–H+E _a) - E _eq equilibrium potential of the reversible hydrogen reaction at a given pH - F Faraday's constant - –H heat of adsorption - i ⁰ apparent exchange current density for the reversible hydrogen reaction - i ₀ exchange current density for the reversible hydrogen reaction - k rate constant for the desorption of hydrogen - L _s heat of atomization - R gas constant - T absolute temperature - ₇ rate of oxidation of hydrogen atoms - ₈ rate of desorption of hydrogen - transfer coefficient (0.5) - overpotential (=E–E _eq) - fraction of the surface covered by hydrogen atoms - _M work function of metal M - potential of the outer Helmholtz layer relative to the bulk of the electrolyte