Formation of methylformate during methanol oxidation revisited: The mechanism

One of the key questions for the performance of the direct methanol fuel cell is the role of intermediates and the poisoning of the surface. Here, we demonstrate that methylformate, which electrochemically is less reactive than methanol itself, is not formed in a reaction between formic acid and methanol in solution, contrary to what is usually assumed. This reaction is far too slow as shown here in a simple kinetic measurement (τ = 1.5 × 105 s). Rather, methylformate originates from a direct oxidation reaction at the surface. A corresponding reaction scheme is presented. The effect of various parameters such as convection, methanol concentration and potential on the current efficiency for CO2 and methylformate and the extent of poisoning is evaluated using a special potential step program in combination with a dual thin-layer flow-through cell which is connected directly to a mass spectrometer by a hydrophobic Teflon (PTFE) membrane. In agreement with previous results, the current efficiency with respect to CO2 is independent of flow rate; also the extent of CO poisoning does not depend on convection. Both confirm the parallel path mechanism. Under the present conditions, the formation of CO2 is solely due to the path via adsorbed CO. The current efficiency for CO2 increases with potential and decreases with methanol concentration, whereas that for methylformate shows the opposite behavior.

► Methylformate formation between formic acid and methanol in solution is extremely slow.
► During methanol oxidation, methylformate originates from a direct oxidation reaction at the surface.
► The current efficiency with respect to CO2 is independent of flow rate.
► Also the extent of CO poisoning does not depend on convection.
► The formation of CO2 is solely due to the path via adsorbed CO.