Anode electrocatalysis adsorption

Abstract In this chapter, we present new insights in direct alcohol fuel cell-related anode electrocatalysis based on quantitative differential electrochemical mass spectrometry (DEMS) studies. First, we review the history and development of the DEMS technique, as well as the calibration method for quantification. We then discuss some contributions of quantitative DEMS to the study of the mechanism of methanol electrooxidation on Pt and PtRu model catalysts. We also discuss quantitative DEMS studies of the mechanism of dissociative adsorption and electrooxidation of ethanol and acetaldehyde at Pt, Pt3Sn, PtRu, and PtRh nanoparticle catalysts. Finally, the mechanism of dissociative adsorption and electrooxidation of ethylene glycol and its oxidative derivatives on carbon-supported Pt, Pt3Sn, and PtRu nanoparticle catalysts are discussed, based on quantitative DEMS results. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

Electrocatalysis in DAFC anodes is complex because the reaction mechanism involves adsorption of alcohol and several elementary reaction steps including the CO oxidation. Figure 8.2 shows a possible network of reaction pathways by which the electrochemical oxidation of methanol occurs. Since more than 50 years detailed catalysis studies have attempted to analyze possible reaction pathways to find the main pathway of methanol oxidation [11, 12] (see next Section). Most studies conclude that the reaction can proceed according to multiple mechanisms and that the most significant reactions are the adsorption of the alcohol and the oxidation of CO. 

Fletcher, in his interesting book. Industrial Electrochemistry, demonstrates reasoning that can sometimes lead to a decision on the most probable reaction path when an adsorption step is involved. Electrocatalysis is of major importance industrially. Use of dimensionally stable anodes in the chlorine industry is a good example. 

In the case of electroformation of oxides, the extend of surface oxidation depends on the nature of the metals, the polarization conditions (polarization potential, current density or time, Ep, ip and tp, respectively) and the electrolyte composition and pH 1-20), The oxide formed on an electrode surface markedly affects anodic Faradaic electrode processes at the double-layer by (i) affecting the reaction energetics (ii) changing electronic properties of the metal electrode (iii) imposing a barrier to the charge transfer (iv) affecting the adsorption properties of the reaction intermediates and products 14-17), Knowledge of the chemical and electronic state of the surface oxide is of major importance in electrocatalysis since it determines the electrocatalytic properties of the surface at which various anodic Faradaic reactions take place. 

The properties of the surfaces of platinum metals can be studied by the cathodic deposition or the anodic removal of the adsorbed layer of H atoms in a simple way. Since the platinum metals and their alloys play an important role in electrocatalysis, the essential aspects of hydrogen adsorption under different experimental conditions are discussed subsequently. 

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