Oxygen reduction reaction catalysts pathways

In the case of iron phthalocyanine catalyst dispersed on carbon, infrared spectroscopy measurements enabled to confirm the series mechanistic pathway for the ORR in the high overpotential domain (below 0.7 V vs RHE). Two intermediates species, 02 and H02 and two products, water (majority product) and hydrogen peroxide could be identified. However, since only few studies are led on the oxygen reduction reaction by infrared spectroscopy, a confident attribntion of the different vibrational transitions observed to a particnlar intermediate remains difficult. 

Let us consider the oxygen reduction reaction (ORR) that occurs in the cathode of the polymer electrolyte membrane fuel cell (PEMFC), in an acidic environment. Although a variety of ORR mechanisms have been proposed, the four-electron pathway is primarily used to characterize the behavior of this reaction at a platinum electrode or a glassy carbon electrode coated with a platinum-based catalyst. The overall reaction is given by... 

Fig. 14.9 Potential oxygen reduction reaction pathways on metallic catalysts [47]...

Non-noble metal catalysts Catalysts prepared without any noble metals. As a result, the fabrication costs of such materials should be essentially lower as all basic-components are cheap. Oxygen Reduction Reaction. In this contribution, it stands for the electrochemical reduction of oxygen. The favored pathway is the... 

Studies have revealed that the type of interaction between catalyst and oxygen dictates the oxygen reduction reaction pathway and end products. The direct four-electron pathway requires dissociation of oxygen prior to the transfer of the first electron [25]. Research has demonstrated that both side-on and bridge interactions weaken the 0-0 bond to such an extent that a break is inevitable. Therefore, four-electron reduction will follow and the final product will be water. On the other hand, catalysts that cannot effectively stretch the 0-0 bond will only end up with the two-electron reduction product H2O2, which makes the catalyst unstable and degrades the fiiel cell system [31]. 

The oxygen reduction reaction (ORR) at the PEM fuel cell cathode is a multielectron, multistep reaction with a sluggish kinetics thus, a catalyst is generally required to accelerate the reaction. At present, platinum (Pt)-based catalysts are the most practical catalysts for the ORR in PEM fuel cells. The mechanism of the Pt-catalyzed ORR has been an active research area for about the past 40 years [21-24]. Yet, despite numerous studies, the detailed mechanism remains elusive. Figure 6.2 illustrates the simplified mechanism [24]. On Pt, the oxygen reduction reaction can proceed along several pathways for example, a "direct" four-electron reduchon to water, a two-electron pathway to hydrogen peroxide, and a "series" pathway with two- and four-electron reduction to water. 

The oxygen reduction reaction (ORR) is the primary electrochemical reaction occurring at the cathode of a PEMFC, and is central to this promising technology for efficient and clean energy generation. The ORR is a multi-electron reaction that follows the direct four-electron mechanism on platinum-based electro-catalysts. It appears to occur in two pathways in acid electrolytes (Adzic and Lima, 2009) ... 

The KOCAT Society (no patent reference known by authors), in South Korea, solved the DeNO problem of big burners, by directly injecting oxygenates on the catalyst at the outlet of the burner. This process involves the third function of our model. This example shows that only one model (the present one) for DeNO reaction can be used for either mobile or stationary sources. Pathways are the same what is changing is the nature of the reductant, which has to be activated, through its partial oxidation, at the temperature when N—O bonds (dinitrosyl species) are broken. 

This second pathway, which is indirect oxygen reduction pathway, is commonly designated as the series pathway. The total number of exchanged electrons (nO involved for catalysts which performs the ORR following the series pathway can vary from two to four depending on the amount of water peroxide present in the reaction products (Eq. 45) ... 

However, due to a different experimental setup, the assignation of these species could not be confirmed from the infrared spectroscopy measurements carried out on a gold catalyst in alkaline medium, " which was found to present an exclusive series of oxygen reduction pathway. " The presence of these species as intermediates in the ORR mechanism was observed by Raman spectroscopy coupled with electrochemistiy in acidic medium on a gold electrode with bismuth adatoms. " Later, this mechanistic study of ORR by Raman spectroscopy was extended to a gold electrode in alkaline medium "" with an evidence for O2 formation as a reaction intermediate. H02 was also identified, but as a product of the ORR in alkaline medium. 

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