Mechanism of the Hydrogen Oxidation Reaction

The overall reaetions of anodie hydrogen oxidation in aeidie and alkaline mediums ean be expressed as Equations 3.1 and 3.2, respeetively  

The hydrogen oxidation reaetion may oeeur by the following three sequential steps  

1) Adsorption step the hydrogen moleeule diffuses from the eleetrolyte to the eleetrode, then adsorbs on the eleetrode surfaee to form surfaee speeies (H2,ad)  

2) Hydration/ionisation step the adsorbed hydrogen forms adsorbed H atoms (Had) through proeess (a) or (b)  

Had + OH H2O + e (Volmer reaction in alkaline medium). (3.6) (b) Heyrovsky-Volmer route  

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However, the complete reaction mechanism of the hydrogen oxidation reaction is much more complex, both in its number of reaction steps, number of intermediates (OOH and H2O2), and observed behavior. A mixture of H2 and O2 can sit in a diy bulb for many years with absolutely no H2O detected. However, if water is initially present, the reaction will begin, and if a spark is ignited or a grain of platinum is added to the mixture at room temperature, the reaction wiU occur instandy and explosively. 

Camara, G.A. et al.. The CO poisoning mechanism of the hydrogen oxidation reaction in proton exchange membrane fuel cells, J. Electrochem. Soc., 149, A748, 2002. 

The kinetics of the hydrogen oxidation reaction under excess 02 satisfactorily obeyed the following kinetic equation, typical of a two-step redox mechanism ... 

An oxidation of alkanes by molecular oxygen, which is catalyzed by N-hydroxyphthalimide (NHPI) combined with Co(acac) ( =2,3) or transition metal salts, has been described by Ishii and coworkers (see a review [33] and original papers [34] in some cases NHPI also efficiently catalyzes the oxidation in the absence of metal derivatives [35]). For example, the oxidation of isobutane in the presence of catalytic amounts of NHPI and Co(acac)2 under an air atmosphere in benzonitrile at I(X) °C gave terf-butyl alcohol (yield 84%) as well as acetone (13%) [34d], The co-catalytic effects of various metal compounds are compared in Table IX.2. The proposed mechanism of the Ishii oxidation reaction involves hydrogen abstraction from isobutane (or any other compound with reactive C-H bonds) by the phthalimidooxyl radical (PINO) (Scheme IX. 4) [34d]. 

Mechanism and reaction intermediates of the hydrogen oxidation reaction on platinum have been studied with SEIRAS [324]. A band observed around 2090 cm was assigned to adsorbed hydrogen species and the band height dependency on electrode potential and hydrogen overpressure was found to match values predicted on the basis of the Volmer-Tafel mechanism. 

The adsorption of hydrogen on metal electrodes such as platinum has been studied extensively in electrochemical systems over the last several decades. The mechanism for the hydrogen oxidation reaction on a Pt electrode in an acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrosky-Volmer, both of which involve the adsorption of molecular hydrogen followed by a... 

Along with the identity of the intermediate in the methanol oxidation reaction, the mechanism of the hydrogen evolution reaction, (HER), at Pt group metals has remained relatively ill understood. The key problem lies in reconciling the observed kinetic data with the known coverage of, for example, platinum electrodes by adsorbed hydride at the onset of hydrogen evolution. The HER may be most simply represented by ... 

Mechanism of the Hydrogen Electrode Reaction as Studied by Means of Deuterium as a Tracer The Mechanism of Oxidation of Organic Fuels... 

Studies of the reaction mechanism of the catalytic oxidation suggest that a tit-hydroxyethylene—palladium 7t-complex is formed initially, followed by an intramolecular exchange of hydrogen and palladium to give a i yW-hydtoxyethylpalladium species that leads to acetaldehyde and metallic palladium (88-90). 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4],... 

The mechanism of anodic hydrogen oxidation is much simpler than that of oxygen reduction. Reaction pathways would be the reverse of the Volmer-Heyrovsky or the Volmer-Tafel mechanism, that is, with anodic adsorption... 

The hydrogen oxidation within a fuel cell occurs partly at the anode and the cathode. Different models were supposed for the detailed reaction mechanisms of the hydrogen at Ni-YSZ (yttria stabilised zirconia) cermet anodes. The major differences of the models were found with regard to the location where the chemical and electrochemical reactions occur at the TPB (three-phase boundary of the gaseous phase, the electrode and the electrolyte). However, it is assumed that the hydrogen is adsorbed at the anode, ionised and the electrons are used within an external electrical circuit to convert the electrical potential between the anode and the cathode into work. Oxygen is adsorbed at the cathode and ionised by the electrons of the load. The electrolyte leads the oxide ion from the cathode to the anode. The hydrogen ions (protons) and the oxide ion form a molecule of water. The anodic reaction is... 

Eley-Rideal) mechanism, one of the reactants comes directly from the fluid phase to react with the other, which is already chemisorbed. This procedure was devised to explain the kinetics of the hydrogen-deuterium reaction on certain metals (see Section 9.2), but has also been suggested for other reactions. The Mars-van Krevelen mechanism applies to oxidations catalysed by oxides that are easily reducible, and are therefore able to release their lattice oxide ions for the purpose of oxidising the other reactant they are then replaced by the dissociation of molecular oxygen. With gold catalysts supported on such oxides, it is sometimes proposed that this mechanism plays a part in the total process. 

In this chapter we discnssed the reaction mechanisms of the most important electrochemical reactions involved in low temperature fuel cells. Apart from the hydrogen oxidation reaction, which is relatively easy, and does not lead to a large overpotential even at high current densities, the oxidation of low weight alcohols... 

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