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Abstract: Fundamental electrochemical relations predict that the kinetic properties of an electrochemical charge-transfer reaction depend on reactant and product concentrations due to electrical equilibrium-potential (Nernst potential) effects. This paper discusses the consequences for the interpretation of observed reaction rates and orders of the electrochemical hydrogen oxidation at solid oxide fuel cell (SOFC) Ni/YSZ anodes. A thermodynamic model of the three-phase boundary is developed that describes the coupling of electroactive intermediates with global gas-phase reactants and products. The model is used to study the behavior of various reaction pathways proposed before, including hydrogen spillover, oxygen spillover, and interstitial hydrogen transfer. The results are compared with literature experimental data. The well-established activating effect of water on the SOFC anode kinetics can be explained by equilibrium-potential effects alone, without the necessity of assuming any additional kinetic or catalytic effect. [Copyright &y& Elsevier]

Abstract: The high-temperature catalytic partial oxidation of methane (CPOM) over a platinum gauze reactor was modeled by integrating 3D numerical simulations of the flow field coupled with heat transport as well as detailed micro-kinetic models including gas-phase and surface reaction mechanisms. Model results describe well CPO experiments over Pt-gauzes in the literature. The conversions of CH4 and O2 increase with an increased contact time and were constant in the temperature range of 1000–1200K. The selectivity to CO linearly increases with temperature. H2 was only observed above 1200K, below this temperature H2O was the only hydrogen-containing product. The contribution of heterogeneous steps in the overall process is prominent, but in the later stages of the reactor, gas-phase reactions become important at certain conditions of temperature, pressure and residence time. Simulations predicted significant gas-phase production of ethane and ethylene via methane oxidative coupling upon increasing the total pressure and residence time. Consequently, homogeneous and heterogeneous processes should be simultaneously implemented in order to accomplish a solid reactor modeling. [Copyright &y& Elsevier]

Abstract: The high-temperature catalytic partial oxidation (CPO) of methane over a platinum gauze reactor was modeled by three-dimensional numerical simulations of the flow field coupled with heat transport as well as detailed gas-phase and surface reaction mechanisms. Model results agree well with data of CPO experiments over Pt-gauzes in the literature, confirming the presence of strong mass and heat-transport limitations. The conversions of CH4 and O2 increase with an increased contact time and were practically constant in the temperature range of 1000–1200K. The selectivity to CO linearly increases with temperature. H2 was only observed above 1200K, below this temperature H2O was the only hydrogen-containing product. The contribution of heterogeneous steps in the overall process is prominent, but in the later stages of the reactor, gas-phase reactions become significant at certain conditions of temperature, pressure, and residence time. For example, simulations predicted some gas-phase production of ethane and ethylene via methane oxidative coupling at elevated pressure and residence time. The study shows that today''s CFD tools allow the implementation of detailed homogeneous and heterogeneous reaction schemes even for complex catalyst geometries. [Copyright &y& Elsevier]

In this study, density functional theory calculations were performed to investigate the influence of oxygen preoccupation on the nitrogen oxide decomposition on rhodium. Besides gauging the coverage dependence of the adsorption energy of NO on the (111) rhodium facet, the influence of the initial oxygen coverage on the kinetics and thermodynamics of the nitrogen oxide decomposition reaction was also studied. The results are discussed with respect to a novel NOx decomposition catalyst. Furthermore, the influence of spin effects on the adsorption geometry as well as the adsorption energy is examined. It will be addressed why spin effects only have a minor influence on the behavior of NO on a rhodium surface. [ABSTRACT FROM AUTHOR]

A systematic study of oxygen adsorption, decomposition and diffusion on Rh(111) and its dependence on coadsorbed oxygen molecules has been performed using density functional theory calculations. First, the bonding strength between metal surface and adsorbed oxygen molecules has been studied as a function of initial oxygen coverage. The bonding strength decreases with increasing oxygen coverage, which points towards a self-inhibition of the adsorption process. The potential energy hypersurface (PES) for the dissociation of oxygen molecules adsorbed on a threefold fcc position perpendicular to the surface was calculated using a combined linear/quadratic synchronous transit method with conjugate gradient refinements. The results indicate that a minor amount of oxygen on the surface enhances the decomposition of further oxygen molecules, while this process is inhibited at higher coverage. Moreover, PES calculations of a single site jump of atomic oxygen on rhodium (111) indicate that the activation energy increases as well with increasing oxygen coverage. All results are discussed with respect to a rhodium based catalytic NOx reduction/decomposition system proposed by Nakatsuji, which decomposes nitrogen oxides in oxygen excess. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]