Hydrogen-oxidation reaction

The adsorption of hydrogen on metal electrodes such as platinum has been studied extensively in electrochemical systems over the last several decades (Markovic et al, 2002). Non-noble metals also develop high activity for hydrogen oxidation reactions (HORs), but in acidic electrolytes, noble metals show the greatest stability towards corrosion or passivation. 

The mechanism for the hydrogen oxidation reaction on a Pt electrode in an acid electrolyte can proceed through two main pathways, Tafel-Vohner and Heyrosky-Vohner, both of which involve the adsorption of molecular hydrogen (Had), followed by a charge transfer step  

The major effort in HOR electro-catalysis has been focused on understanding the rate dependency on the atomic-scale morphology of a platinum single-crystal surface. Recently, catalysis studies on well-defined Pt single-crystal electrodes clearly demonstrated that the delivered current during the HOR on Pt (hkl) varies with the crystal face symmetry, i.e., it is a structure sensitivity reaction (Markovic, 2003 Markovic etal., 2002). The facets of a well formed crystal, or internal planes through a crystal structure, or a lattice, are specified in terms of miller indices, h, k and 1. These indices (hkl) represent the set of all parallel planes. 

Therefore, CO adsorbed on platinum sites diffuses and reacts to form carbon dioxide at Ru sites. A second mechanism concerning an electronic effect between the interaction of RU and Pt was also proposed. This mechanism is widely recognized, especially after the in situ FTIR work of the Iwasita-Vielstich team in the early 1990s that is associated with an energy shift of the Ptd electronic states caused by the second element, and resulted in a weakening of the Pt-CO bond (Iwasita, 2002 2003). 

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A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

Wang JX, Springer TE, Adzic RR. 2006. Dual-pathway kinetic equation for the hydrogen oxidation reaction on Pt electrodes. J Electrochem Soc 153 A1732-A1740. 

In the electron transfer theories discussed so far, the metal has been treated as a structureless donor or acceptor of electrons—its electronic structure has not been considered. Mathematically, this view is expressed in the wide band approximation, in which A is considered as independent of the electronic energy e. For the. sp-metals, which near the Fermi level have just a wide, stmctureless band composed of. s- and p-states, this approximation is justified. However, these metals are generally bad catalysts for example, the hydrogen oxidation reaction proceeds very slowly on all. sp-metals, but rapidly on transition metals such as platinum and palladium [Trasatti, 1977]. Therefore, a theory of electrocatalysis must abandon the wide band approximation, and take account of the details of the electronic structure of the metal near the Fermi level [Santos and Schmickler, 2007a, b, c Santos and Schmickler, 2006]. 

In order to establish a clear strategy, we have examined the properties of Pt-based catalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) systematically and comprehensively using various techniques, the results of which complement each other. We have also developed a standard method to evaluate the real activity. In this chapter, we summarize our recent research results. 

When pure hydrogen is used as the fuel, the overpotential for the hydrogen oxidation reaction (HOR) at the Pt anode is negligibly small. 

Uchida H, Izumi K, Watanabe M. 2006. Temperature dependence of CO-tolerant hydrogen oxidation reaction activity at Pt, Pt-Co, and Pt-Ru electrodes. J Phys Chem B 110 21924-21930. 

Quaino PM, JL Femkidez, Chialvo MRG, Chialvo AC. 2006. Hydrogen oxidation reaction on microelectrodes Analysis of the contribution of the kinetic routes. J Mol Catal A. 252 156-162. 

To use a real example, consider a hydrogen fuel cell. The reaction at electrode A is the hydrogen oxidation reaction,... 

In addition to the hydrogen partial pressure in the feed, the 02/C0 stoichiometric ratio also influences the selectivity of the catalyst. If excess oxygen is present in the reactor feed (1 = 2.5), more oxygen is available for the hydrogen oxidation reaction to form water as compared to the case when these two reactants are present in the stoichiometric ratio equal to 1 (/. = 1), which is clear if we compare full and open circles as well as full and open squares in Figure 7.11a. 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

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. 

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

Indeed, tungsten carbide (WC) was the first transition metal carbide shown to be active for the hydrogen oxidation reaction,1 and this was followed by numerous reports of the reactivity of carbides in other oxidation reactions (for reviews, see References 2-4). 

The possibility of hydrogen activation on the surface of transition metal carbides has been supported by experiments in which WC was used as a promoter for the hydrogen oxidation reaction over the oxide catalyst V205.1 WC additions to V2O5 were shown to critically accelerate the reaction of H2 + 02 (so that under the same conditions the activities of V205 and WC were separately much lower). Moreover, WC additions to V205 boosted the reduction of vanadium oxide by hydrogen. Qualitatively,... 

This method is well suited for slightly soluble electroactive materials. It has been widely used for the study of oxygen reduction reaction and hydrogen oxidation reaction, the two main reactions occurring in fuel cells. 

The advantage of AFCs over the other systems lies in the fact that the reduction of oxygen to OH- is much faster than the acidic equivalent of oxygen to H20 due to a better kinetics, which makes the AFC a more efficient system [15]. The hydrogen oxidation reaction in alkaline medium, however, is slower. 

The capability of SECM to detect and to image regions with different catalytic activities is well known [120-122]. So far, this technique has been applied to studies of mainly two electrocatalytic reactions, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), which have important implications for fuel cells. Unlike reversible redox mediators usually employed in SECM experiments, the kinetics of oxygen and hydrogen reactions are strongly dependent on the catalytic activity of the substrate surface. 

Jayaraman, S. Hillier, A. C. Screening the Reactivity of PtxRuy and PtxRuyMoz Catalysts toward the Hydrogen Oxidation Reaction with the Scanning Electrochemical Microscope. Journal of Physical Chemistry B 2003 107(22) 5221-5230. 

Shim, J. Lee, H.-K. Improved performance of Raney nickel electrode by the addition of electrically conductive materials for hydrogen oxidation reaction. Materials Chemistry and Physics 2001 69(1-3) 72-76. 

In alkaline conditions, the oxygen reduction reaction is much faster in AFCs while the hydrogen oxidation reaction (HOR) is slower, compared with acidic fuel cells (H2/air and direct methanol fuel cells). 

The parameters in this equation have similar meanings to those for the hydrogen oxidation reaction, as apphed to the cathode. 

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