The Catalysis of the Electrochemical Oxidation of Formaldehyde and Methanol by Molybdates

Abstract

The electrooxidation of formaldehyde and methanol on platinum has been shown to be catalyzed in sulfuric acid solution by the presence of molybdates. Decreased polarization has been shown to result from a cycle involving chemical reaction of the fuels with an oxidized molybdate species with subsequent electrooxidation of the reduced molybdate. The process requires the adsorption of molybdate at the electrode surface throughout the entire cycle.The existence of catalytic currents during the electrochemical reaction of a variety of substances is well known (1). Catalytic currents in general arise when the product of an electrochemical reaction reacts with another species in solution in a subsequent chemical step, thus to regenerate the original form. The net observed result is an increase in the effective concentration of the electroactive species near the electrode and a corresponding increase in the observed current due to its reaction. In many cases this unique feature has been found useful in improving the polarographic determination of trace constituents in solution (1, 2).An interesting variation of this principle has been investigated in the course of a recent study of the electrochemical oxidation of low molecular weight alcohol and aldehyde fuels for fuel cell use. This variation involves the catalytic reaction of HCHO or CHsOH with a molybdate species at a platinum black electrode in H2SO4. In this case the molybdate species apparently is adsorbed and remains adsorbed at the electrode surface during both the electrochemical and chemical reaction steps.This "surface redox" reaction, as it will be referred to hereafter, has been of particular interest in fuel cell studies on HCHO and CH3OH. Through its use the electrochemical oxidation of these fuels to CO2 may be carried out at a considerable decrease in polarization over that normally found on platinum based electrodes. A series of studies has been carried out to define the advantages and limitations of this system and to determine if in fact a catalytic mechanism is responsible for the observed improvements. Experimental Techniques and EquipmentAll electrochemical measurements in these systems were carried out in conventional glass "H" cells with anode and cathode compartments separated by glass frits. Voltage measurements were made against commercial saturated calomel reference electrodes through Luggin capillaries containing H2SO4 of concentration equal to that in the cell proper. All measurements were made using Keithley 610 A or 610 electrometers. Acid solutions were prepared from reagent grade 96% H2SO4 and deionized water of conductivity < 10 -6 mho/cm. Due to the practical necessity of high conductivity for fuel cell electrolytes, studies were carried out in 3.7M H2SO4, a solution exhibiting nearoptimum conductivity. Absolute methanol and sodium molybdate were reagent grade chemicals. Formaldehyde was obtained as USP Formaldehyde solution (37% in water with a small amount of CH8OH added for stabilization). Due to the higher reducing activity of HC...

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Electrocatalysis for the Direct Alcohol Fuel Cell

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