Partial Oxidation of Methanol on a Metallized Nafion Polymer Electrolyte Membrane

Abstract

The results of a study on the electro‐oxidation of methanol vapor at a platinized polymer electrolyte membrane (PEM) are reported. The PEM was Nafion 117 and serves both as a medium for proton transport and a barrier separating anodic from cathodic products. The counterelectrode was immersed in an aqueous acid solution with which the unmetallized face of the PEM was in contact. The following effects on the product distribution were studied: temperature (25 to 100°C), methanol pressure (0.5 to 54.7 kPa, at a constant total pressure of 101.3 kPa using He as a diluent), electrolyte in the counterelectrode compartment ( and 85% ), and two different electrode‐fabrication procedures. Reaction conditions were determined which produce formaldehyde, methylal, or methylformate at high selectivity. Using a PEM electrode prepared by the impregnation‐reduction (I‐R) method with a low‐water‐content electrolyte (85% ) at high temperature (100°C) and low methanol pressure (0.5 to 1 kPa) resulted in the largest selectivity for formaldehyde (∼75%). The reaction conditions for selective methylal production (∼80%) were similar except that a high methanol partial pressure was required (54.7 kPa). Selective formation of methylformate (∼75%) was found using an electrode prepared by the Takenaka‐Torikai (T‐T) method with a high‐water‐content electrolyte at low temperature (25°C) and high methanol pressure (12.9 kPa). Based upon the results, it is argued that the reaction zone in the I‐R prepared electrode is at the platinum‐membrane interface, which is of a lower water content than the reaction zone in the T‐T prepared electrode, the flooded platinum‐electrolyte interface; consequently, the former favors the two‐electron oxidation, and the latter the four‐electron oxidation which requires water to proceed.