This research is focused on the analysis of adsorbed bare and oxidized Pd(9) nanoparticles supported on γ-alumina. From first-principle density functional theory calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Studies of Pd/PdO nanoparticles prove that they are stable at γ-alumina supports. It is shown that the Pd(9) nanoparticle favors dissociative adsorption of oxygen molecules. The most energetically preferable sites for adsorption are close to the contact between the cluster and the support, where one oxygen atom interacts with a 5-coordinated aluminium atom, and the remaining oxygen is in contact with the closest palladium atom. After first dissociation, one oxygen atom creates a bridge between the palladium atom and the 5-coordinated aluminium atom and the second oxygen atom moves to the top of the Pd(9) cluster, making a bridge between two palladium atoms. Subsequent dissociations arise analogously, with the difference that oxygen atoms in the second layer of the palladium cluster occupy hollow sides of the cluster. Investigation of the charge distribution in each oxidation step reveals that charge transfer increases towards the Pd/PdO nanoclusters. The electronic density of states indicates that gradual oxygen molecule adsorption and dissociation shift the highest states of the Pd/PdO nanoparticles in different ways. The overall investigation is found to be beneficial for studying methane oxidation.
For the further advancement of a filter vent, the influence of silver zeolites on the hydrogen recombining reaction has been studied. It is confirmed that the AgX is active and plays an auxiliary role of hydrogen recombiner in the hydrogen catalyst reaction. On the other hand, it has been confirmed that AgX can also remove radioactive iodine in the atmosphere containing hydrogen gas. For AgR, it is proved that it is a kind of special silver zeolite which does not almost have hydrogen catalyst reaction. Owing to these characteristics, AgX and AgR are expected to be utilized under the severe conditions in a containment vessel. In this paper, the development status of these silver zeolites will be introduced, and some possible applications will be proposed.
transport in gas diffusion layers (GDLs) Polymer electrolyte fuel cells (PEFCs) Channel cross-flows in PEFCs Impact of dry and wet GDLs on cross-flows X-ray tomographic microscopy a b s t r a c t Three-dimensional direct numerical simulations were performed for investigating the flow in a serpentine channel and the under-laying porous gas diffusion layer (GDL) of a micro polymer electrolyte fuel cell (PEFC).The flow field comprised three straight sections and two U-turns.The geometry was acquired with high-resolution (2.9 μm) in situ X-ray tomographic measurements on an operating cell.Simulations considered the GDL under dry and partially saturated conditions, whereby saturation was established via electrochemically produced water.A lattice Boltzmann (LB) methodology was adopted for simulating the single-phase (gas) transport in the actual 3D channel and porous GDL geometry.The global pressure drop in the dry GDL was dominated by the turns in the gas channel, while the pressure drops were quite small along the straight channel sections.In the wet GDL case, however, the pressure drop was mainly dictated by the neck-shaped passages created by the large water clusters inside the channel.Owing to the water blockage, the local accumulated cross-flows along the serpentine channel length, when normalized by the inlet channel flow, were substantially higher in the wet GDL, reaching local values up to 45% compared to 18% for the dry GDL.The implications are that in an electrochemically operating cell, the GDL under the rib would receive more gas (and thus O 2 ).The creation of cross-flows through the porous GDL would enhance cell performance under the ribs since diffusion will not be the main driving mechanism for oxygen transport and water evaporation.The analysis indicated that the flow field, although designed as serpentine, behaved like half-interdigitated (with a rib of 1.5 mm, half-serpentine flow field, depending on the state of channel flooding).
The catalytic combustion of syngas/air mixtures over Pt has been investigated numerically in a channel-flow configuration using 2D steady and transient computer codes with detailed hetero-/homogeneous chemistry, transport, and heat transfer mechanisms in the solid. Simulations were carried out for syngas compositions with varying H2 and CO contents, pressures of 1 to 15 bar, and linear velocities relevant to power generation systems. It is shown that the homogeneous (gas-phase) chemistry of both H2 and CO cannot be neglected at elevated pressures, even at the very large geometrical confinements relevant to practical catalytic reactors. The diffusional imbalance of hydrogen can lead, depending on its content in the syngas, to superadiabatic surface temperatures that may endanger the catalyst and reactor integrity. On the other hand, the presence of gas-phase H2 combustion moderates the superadiabatic wall temperatures by shielding the catalyst from the hydrogen-rich channel core. Above a transition temperature of about 700 K, which is roughly independent of pressure and syngas composition, the heterogeneous (catalytic) pathways of CO and H2 are decoupled, while the chemical interactions between the heterogeneous pathway of each individual fuel component with the homogeneous pathway of the other are minimal. Below the aforementioned transition temperature the catalyst is covered predominantly by CO, which in turn inhibits the catalytic conversion of both fuel components. While the addition of carbon monoxide in hydrogen hinders the catalytic ignition of the latter, there is no clear improvement in the ignition characteristics of CO by adding H2. Strategies for reactor thermal management are finally outlined in light of the attained superadiabatic surface temperatures of hydrogen-rich syngas fuels.
A lattice Boltzmann model for thermal gas mixtures is derived. The kinetic model is designed in a way that combines properties of two previous literature models, namely, (a) a single-component thermal model and (b) a multicomponent isothermal model. A comprehensive platform for the study of various practical systems involving multicomponent mixture flows with large temperature differences is constructed. The governing thermohydrodynamic equations include the mass, momentum, energy conservation equations, and the multicomponent diffusion equation. The present model is able to simulate mixtures with adjustable Prandtl and Schmidt numbers. Validation in several flow configurations with temperature and species concentration ratios up to nine is presented.
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