Reaction Products and the Adsorbed Intermediates

The electrocatalytic oxidation of methanol has been thoroughly investigated during the past three decades (see reviews in Refs. 21-27), particularly in regard to the possible development of DMFCs. The oxidation of methanol, the electrocatalytic reaction, consists of several steps, which also include adsorbed species. The determination of the mechanism of this reaction needs two kinds of information (1) the electrode kinetics of the formation of partially oxidized and completely oxidized products (main and side products) and (2) the nature and the distribution of intermediates adsorbed at the electrode surface. 

The quantitative analyses of reaction products due to partial or complete oxidation can be performed by different methods. This type of determination is essential to improve electrode composition. Apart from a decrease in the Coulombic efficiency (see Section 11.2), the formation of partially oxidized products can be deleterious for the DMFC application because some of these products (e.g., formic acid) may be in liquid form and are corrosive. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

In situ analysis of the reaction products can also be carried out by mass spectrometry, using the differential electrochemical mass spectrometry (DBMS) technique.This technique permits the detection of gaseous products since they are produced and captured through a porous electrode. It has been confirmed that carbon dioxide is the main reaction product. With this technique, it is also possible to determine the production of CO2 

Reaction products can also be identified by in situ infrared reflectance spectroscopy (Fourier transform infrared reflectance spectroscopy, FTIRS) used as single potential alteration infrared reflectance spectroscopy (SPAIRS). This method is suitable not only for obtaining information on adsorbed products (see below), but also for observing infrared (IR) absorption bands due to the products immediately after their formation in the vicinity of the electrode surface. It is thus easy to follow the production of CO2 versus the oxidation potential and to compare the behavior of different electrocatalysts. 

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