The electrooxidation of carbon monoxide

A purely bifunctional mechanism was assumed to be operative in the CO oxidation on Pt3Sn surfaces as well as on other Pt alloys with oxophilic transition metals [157-159]. Here, the oxophilic Sn surface atoms are believed to provide nucleation sites for water and, following stepwise hydrogen abstraction, for its subsequent oxygenated surface products OH and O. CO oxidation on Sn atoms is unlikely [131,160] such that no competition for Sn sites occurs between water and CO molecules. All Pt atoms are covered with CO. 

Liu and co-workers [160,172] reported a detailed computational study on the CO and CO/H2 electrooxidation reactions, in which they clarified the mechanistic role of the alloy metals in the promotion of electrocatalytic activity. First, they presented comparative DFT calculations of adsorption energies of CO, H2, and OH on surface slabs representing Pt(lll), a Ru(0001), a Pt monolayer on Ru 

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Gomez R, Orts JM, Feliu JM, Clavilier J, Klein LH. 1997. The role of surface crystalline heterogeneities in the electrooxidation of carbon monoxide adsorbed on Rh(lll) electrodes in sulphuric acid solutions. J Electroanal Chem 432 1 -5. 

Narayanasamy, J., Anderson, A. (2003). Mechanism for the electrooxidation of carbon monoxide on platinum by HjO. Density functional theory calculation. /. Electroanalytical Chem. 554-555,35-40. 

Davies JC, Hayden BE, Pegg DJ, Rendall ME. The electrooxidation of carbon monoxide on ruthenium modified Pt(l 11). Surf Sci 2002 496 110-20. 

The electrooxidation of carbon monoxide is dependent on the coverage of CO and the anode overpotential and is given by ... 

Antolini E., E. R. Gonzalez, The electrooxidation of carbon monoxide, hydrogen/carbon monoxide, methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells a comparative effect of Pt-Sn structural characteristics, Electrochim. Acta, 56, 1 (2010). 

Chang SC, Hamelin A, Weaver MJ. 1990. Reactive and inhibiting adsorbates for the catal34ic electrooxidation of carbon-monoxide on gold (210) as characterized by surface infrared-spectroscopy. Surf Sci 239 L543-L547. 

Two examples of the application of SERS and potential-difference IRRAS methods to the identification of adsorbed intermediates and reaction mechanism elucidation are also described, involving the catalytic electrooxidation of carbon monoxide and small organic molecules on transition-metal surfaces. 

Electrooxidation of carbon monoxide to carbon dioxide at platinum has been extensively studied mainly not least because of the technological importance of its role in methanol oxidation in fuel cells [5] and in poisoning hydrogen fuel cells [6]. Enhancing anodic oxidation of CO is critical, and platinum surfaces modified with ruthenium or tin, which favor oxygen atom adsorption and transfer to bound CO, can achieve this [7, 8]. 

CO adsorption and oxidation have been studied for many years, but a greater understanding was achieved by the development of ex situ and in situ spectroscopic and microscopic methods for application in electrochemistry [9, 143-146], together with the use of well-defined nanocrystalline electrode surfaces [147]. The opportunity to study in situ electrooxidation of carbon monoxide [148-157] under fuel cell reaction conditions has brought significant progress in understanding interfacial electrochemistry on metallic surfaces, hi combination with conventional electrochemical methods these techniques have been used to find connections between the microscopic surface structures and the macroscopic kinetic rates of the reactions. 

Numerical modeling relies directly on the physical problem at hand. Before simulating the physical problem, it is necessary to establish the mathematical model with all the governing equations. In the case of carbon monoxide poisoning, the electrochemical kinetics of this phenomenon should first be established. In this section, the adsorption, desorption, and the electrooxidation of hydrogen and carbon monoxide are analyzed. 

This method is also known as micelle encapsulation. The cluster size is varied by changing only the length of the block copolymer head. For particles with diameters between approximately 1 and 6 nm, the particle size and the support were found to strongly influence the oxygen reactivity, the formation and stabilization of a metal-oxide, and the catalytic activity for electrooxidation of carbon monoxide. The smallest particles studied (1.5 nm) were the most active for electrooxidation of CO and had the largest fraction of oxygen associated with gold at the surface as measured by the Au 7Au° X-ray photoemission intensities. 

Chang SC, Hamelin A, Weaver MJ. 1991. Dependence of the electrooxidation rates of carbon monoxide at gold on the surface crystallographic orientation A combined kinetic-surface infrared spectroscopy study. J Phys Chem 95 5560-5567. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

Other electrocatalysts were considered for the electrooxidation of ethanol, such as rhodium, iridium or gold, " " leading to similar results in acid medium. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and carbon monoxide. Two types of adsorbed CO are formed, i.e., linearly-bonded and bridge-bonded, in a similar amount, at relatively low potentials, then leading rapidly to carbon dioxide when the rhodium surface begins to oxidize, at 0.5-0.7 V/RHE. On gold in acid medium the oxidation reaction leads mainly to the formation of acetaldehyde. " " ... 

On the basis of the scheme described by Eq (24), Eqs. (30) and (31) represent the steady-state catalyst surface balance of adsorption, desorption, and electrooxidation fluxes of carbon monoxide and of hydrogen. If the intermediate hydrogen step is second order in catalyst sites, as was assumed on the basis of the nature of process (25) [66], then, in Eq. (31), n = 2. Equations (30) and (31) determine 0co, the fraction of catalyst sites with adsorbed CO,... 

Several cases involve the adsorption of reactants and products (shielding or blocking effects), but one interesting situation in electrocatalysis is that of the electrooxidation of an organic fuel such as methanol. It involves the formation of a carbon monoxide adsorbed residue that follows a progressive oxidation. We can first state that the reaction occurs without an electrocatalytic influence of the... 

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. 

McIntyre DR, Burstein GT, Vossen A (2002) Effect of carbon monoxide on the electrooxidation of hydrogen by tungsten carbide. J Power Sources 107(l) 67-73... 

In a comprehensive review, Antolini and Gonzalez (2010) present an overview of the relationship between structural characteristics of Pt-Sn catalysts (catalyst composition, degree of alloying, presence of oxides) and their electrocat-alytic activity for the electrooxidation of various fuels (i.e., carbon monoxide, hydrogen-carbon monoxide, methanol) in acid media. 

The pH of the electrolyte is effective on the reaction kinetics at the individual electrodes and the electrode potential at which oxidation or reduction takes place [26]. Electrolyte is typically a strong acid or a strong base, such as sulfuric acid or potassium hydroxide, which include highly mobile hydronium or hydroxide ions, respectively [20]. Typically, operation of fuel cell in alkaline media can develop the electrooxidation of the catalyst-poisoning carbon monoxide species on the anode and the kinetics of ORR is improved at the cathode [26]. However, in membrane-based fuel cells, due to the potential of carbonate formation resulting in clogging the membrane, the long-term stability is restricted and limits the use of these alkali-compatible membranes for liquid fuel cell operations [26]. 

Vqq, of adsorbed carbon monoxide against electrode potential at gold electrodes, for CO-saturated (ca. ImM) solutions. Electrolytes were ( , ) 0.1 M HCIO, ( ) 0.1 M NaClO, and (A) 0.1 M KF. The arrowed dashed curves represent the sequential peak frequencies obtained upon potential excursions from 100 mV to 500 mV and return, into the region where CO electrooxidation, occurs. The solid straight line, drawn through the points obtained at potentials (< 100 mV) where adsorbed CO is stable towards electrooxidation, has a slope of about 50 cm l V. ... 

Different electron-conducting polymers (polyaniline, polypyrrole, polythiophene) are considered as convenient substrates for the electrodeposition of highly dispersed metal electrocatalysts. The preparation and the characterization of electronconducting polymers modified by noble metal nanoparticles are first discussed. Then, their catalytic activities are presented for many important electrochemical reactions related to fuel cells oxygen reduction, hydrogen oxidation, oxidation of Cl molecules (formic acid, formaldehyde, methanol, carbon monoxide), and electrooxidation of alcohols and polyols. 

Carbon monoxide is a key molecule in the electro-oxidation of Cl compounds and of many alcohols, since it is always produced by the dissociative chemisorption of the molecule, and since it may block the active catalytic sites. Therefore, its electrooxidation on platinum-based metals dispersed in an electron-conducting polymer, such as PAni, was investigated for a long time in our laboratory [8,28,34]. 

Carbon monoxide preferably adsorbs on the Pt surface and inhibits the dissociative adsorption of H2, which may be explained by the strong bond strength of Pt-CO compared to that of Pt-H. The last two reactions explain CO removal from the surface via electrooxidation at high anode overpotentials. Alloying Pt with Ru improves CO tolerance through the facile removal of Pt-(CO)ads... 

Leung, L.-W.H. and Weaver, M.J. (1987) Extending surface-enhanced Raman spectroscopy to transition-metal surfaces carbon monoxide adsorption and electrooxidation on platinum- and palladium-coated gold electrodes. Journal of the American Chemical Society, 109, 5113-5119. 

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