Heterogeneous and Combined Heterogeneous/Homogeneous Combustion Modeling of SOFC Off-Gases.

The heterogeneous (catalytic) and the hetero-/homogeneous (catalytic and gas-phase) combustion processes of solid oxide fuel cell (SOFC) off-gases with compositions typical of a high cell utilization rate are investigated with high-fidelity 2D simulations in a platinum-coated planar channel using detailed hetero-/homogeneous chemistry. The pressures are 1-8 bar; the reactant streams have volumetric H2 and CO contents 0.7-1.5 and 5.3-9.7%, respectively; H2O and CO2 dilutions are ∼40 and ∼50%, respectively; and the global fuel/air equivalence ratio is 0.90. Water inhibits chemically the catalytic oxidation of H2, as it leads to high H(s) surface coverage that favors the recombinative desorption of H(s) to H2. On the other hand, H2O promotes chemically the catalytic oxidation of CO by creating high OH(s) coverage that in turn accelerates the CO consumption. Strong flames are established at the highest H2 content cases and for pressures p ≥ 3 bar. For all cases with vigorous homogeneous combustion, the catalytic and gas-phase reaction pathways coexist and compete with each other for the consumption of H2 and CO. The large H2O content leads to gas-phase production of H2 via the reaction H2O + H = H2 + OH. However, the gas-phase produced H2 is subsequently consumed by the catalytic pathway, such that nearly complete H2 conversion is attained at the reactor outlet. Gaseous chemistry does not affect the reactor lengths required for complete H2 conversion but substantially reduces the corresponding lengths for CO conversion. The H2 emissions decrease with increasing pressure and are in the range 8-110 ppmv, while the CO emissions increase with rising pressure and span the range 0.3-52 ppmv, thus leading to corrected CO emissions (at 15% O2) of less than 15 ppmv. Finally, the peak wall temperatures are largely acceptable in terms of catalyst thermal stability.

Lattice Boltzmann modeling and simulation of velocity and concentration slip effects on the catalytic reaction rate of strongly nonequimolar reactions in microflows.

A lattice Boltzmann (LB) interfacial gas-solid three-dimensional model is developed for isothermal multicomponent flows with strongly nonequimolar catalytic reactions, further accounting for the presence of velocity slips and concentration jumps. The model includes diffusion coefficients of all reactive species in the calculation of the catalytic reaction rates as well as an updated velocity at the reactive boundary node. Lattice Boltzmann simulations are performed in a catalytic channel-flow geometry under a wide range of Knudsen (Kn) and surface Damköhler (Da_{s}) numbers. Comparisons with simulations from a computational fluid dynamics (CFD) Navier-Stokes solver show good agreement in the continuum regime (Kn<0.01) in terms of flow velocity and reactive species distributions, while comparisons with direct simulation Monte Carlo results from the literature attest to the model's applicability in capturing the correct slip velocity at Kn as high as 0.1, even with a significantly reduced number of grid points (N=10) in the cross-flow direction. Theoretical and numerical results demonstrate that the term Da_{s}×Kn×A_{2} (where A_{2} is a function of the mass accommodation coefficient) determines the significance of the concentration jump on the catalytic reaction rate. The developed model is applicable for many catalytic microflow systems with complex geometries (such as reactors with porous networks) and large velocity/concentration slips (such as catalytic microthrusters for space applications).

Lattice Boltzmann model with generalized wall boundary conditions for arbitrary catalytic reactivity.

A lattice Boltzmann model for multispecies flows with catalytic reactions is developed, which is valid from very low to very high surface Damköhler numbers (Da_{s}). The previously proposed model for catalytic reactions [S. Arcidiacono, J. Mantzaras, and I. V. Karlin, Phys. Rev. E 78, 046711 (2008)PLEEE81539-375510.1103/PhysRevE.78.046711], which is applicable for low-to-moderate Da_{s} and encompasses part of the mixed kinetics and transport-controlled regime, is revisited and extended for the simulation of arbitrary kinetics-to-transport rate ratios, including strongly transport-controlled conditions (Da_{s}→∞). The catalytic boundary condition is modified by bringing nonlocal information on the wall reactive nodes, allowing accurate evaluation of chemical rates even when the concentration of the deficient reactant at the wall becomes vanishingly small. The developed model is validated against a finite volume Navier-Stokes CFD (Computational Fluid Dynamics) solver for the total oxidation of methane in an isothermal channel-flow configuration. CFD simulations and lattice Boltzmann simulations with the old and new catalytic reaction models are compared against each other. The new model demonstrates a second order accuracy in space and time and provides accurate results at very high Da_{s} (∼10^{9}) where the old model fails. Moreover, to achieve the same accuracy at moderate-to-high Da_{s} of O(1), the new model requires ∼2^{d}×N coarser grid than the original model, where d is the spatial dimension and N the number of species.

Thermal multicomponent lattice Boltzmann model for catalytic reactive flows.

Catalytic reactions are of great interest in many applications related to power generation, fuel reforming and pollutant abatement, as well as in various biochemical processes. A recently proposed lattice Boltzmann model for thermal binary-mixture gas flows [J. Kang, N. I. Prasianakis, and J. Mantzaras, Phys. Rev. E. 87, 053304 (2013)] is revisited and extended for the simulation of multispecies flows with catalytic reactions. The resulting model can handle flows with large temperature and concentration gradients. The developed model is presented in detail and validated against a finite volume Navier-Stokes solver in the case of channel-flow methane catalytic combustion. The surface chemistry is treated with a one-step global reaction for the catalytic total oxidation of methane on platinum. In order to take into account thermal effects, the catalytic boundary condition of S. Arcidiacono, J. Mantzaras, and I. V. Karlin [Phys. Rev. E 78, 046711 (2008)] is adapted to account for temperature variations. Speed of sound simulations further demonstrate the physical integrity and unique features of the model.

Methane catalytic combustion on Pd9/gamma-Al2O3 with different degrees of Pd oxidation.

This research is focused on the analysis of adsorbed CH4 intermediates at oxidized Pd9 nanoparticles supported on gamma-alumina. From first-principle density functional theory (DFT) calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Furthermore methane oxidation cycles have been investigated on Pd nanoparticles with different degrees of oxidation. In case of low oxidized Pd nanoparticles, activation of methane is observed, whereby hydrogen from methane is adsorbed at one oxygen atom. This reaction is exothermic. In a subsequent step, two water molecules desorb. Additionally, a very interesting structural effect becomes evident; Pd-carbide formation, which is also an exothermic reaction. Furthermore, oxidation of such carbidized Pd-nanoparticles leads to CO2 formation, which is an endothermic reaction. One important result is that the support is involved in the CO2 formation. A different mechanism of methane oxidation was discovered for highly oxidized Pd nanoparticles. When the Pd nanoparticle is more strongly exposed to oxidative conditions, adsorption of methane is also possible, but it leads to carbonic acid production at the interface between the Pd nanoparticles and support. This process is endothermic.

CH4 combustion cycles at Pd/Al2O3--important role of support and oxygen access.

This research is focused on the analysis of adsorbed CH4 intermediates at oxidized Pd9 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. Methane oxidation cycles have been considered as a further step at differently oxidized Pd nanoparticles. For low oxidized Pd nanoparticles, activation of methane is observed, whereby hydrogen from methane is adsorbed at one oxygen atom. This reaction is exothermic with adsorption energy equal to -0.38 eV. In a subsequent step, desorption of two water molecules is observed. Additionally, a very interesting structural effect is evident, mainly Pd-carbide formation, which is also an exothermic reaction with an energy of -0.65 eV. Furthermore, oxidation of such carbidized Pd nanoparticles leads to CO2 formation, which is an endothermic reaction. Important result is that the support is involved in CO2 formation. A different mechanism of methane oxidation has been found for highly oxidized Pd nanoparticles. When the Pd nanoparticle is more strongly exposed to oxidative conditions, adsorption of methane is also possible, but it will proceed with carbonic acid production at the interface between Pd nanoparticles and support. However, this step is endothermic.

Lattice Boltzmann model for thermal binary-mixture gas flows.

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.

DFT studies of oxidation routes for Pd9 clusters supported on γ-alumina.

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.

Electronic structure and oxygen vacancies in PdO and ZnO: validation of DFT models.

PdO is one of the most important catalytic materials currently used in the industry. In redox catalytic reactions involving PdO, the bulk phase is an additional source of oxygen. This leads to strong transformations not only at the surface of PdO but also in the near sub-surface and bulk regions. The redox process is, therefore, governed not only by the extent of PdO d-band filling, but also depends on the material properties of the PdO crystal--the ease with which its structure can be deformed. Methane oxidation is of key industrial interest, and therein the rate of CH(4) conversion depends strongly on the reversible oxygen defects formation on the surface and in the bulk of the catalyst. The present study gives a first insight into these complex phenomena at the atomistic level. Comparison of different density functional theory (DFT) approaches and their capacity to reproduce experimental values of the heat of formation as well as the band gap of the PdO are discussed in detail. Results from DFT calculations for an oxygen vacancy creation in the bulk and on the surface of PdO are presented and compared at the level of accuracy of the implemented approaches with defect calculations for ZnO. Many different modeling approaches based on functionals and pseudopotentials (non-modified PP and empirically tuned) have been evaluated in their aptness to capture key PdO properties. It was shown that simulations with the PP-115 pseudopotential gave the closest possible agreement to the relevant PdO thermodynamic data and energy of oxygen vacancy formation.