Surface reaction mechanism development for platinum-catalyzed oxidation of methane
01 Jan 2002-Vol. 29, Iss: 1, pp 997-1004
TL;DR: In this paper, a new C1 reaction mechanism for methane oxidation on Pt is presented, consisting of 31 reversible, elementary, thermodynamically consistent steps, and activation energies are calculated using the semi-empirical bond order conservation (BOC) technique.
Abstract: Previously published mechanisms for methane oxidation on platinum have focused on fuel-lean or fuelrich conditions and were often limited to a narrow range of operating conditions. A new C1 reaction mechanism for methane oxidation on Pt is presented, consisting of 31 reversible, elementary, thermodynamically consistent steps. Preexponential factors are estimated using transition state theory, and activation energies are calculated using the semiempirical bond order conservation (BOC) technique. Recently optimized reaction subsets for hydrogen and carbon monoxide oxidation on platinum are also used. The effect of adsorbate-adsorbate interactions on the activation energies of the surface reactions is also included through the BOC framework. Using this mechanism, ignition-extinction for fuel-lean and fuel-rich mixtures is studied. Reaction path and sensitivity analyses are carried out to capture the underlying physics in methane oxidation on Pt. For example, it is found for the first time that the dominant path for methane decomposition changes from oxygen and hydroxyl assisted, prior to ignition, to pyrolytic at high temperatures and for relatively fuel-rich mixtures. Finally, comparison of model predictions to experimental conversion and selectivity data for fuel-rich mixtures and laser-induced fluorescence data for fuel-lean mixtures is presented, showing good performance of the new mechanism. Overall, this mechanism captures the underlying physics of methane oxidation on Pt and overcomes most of the limitations of previously published mechanisms.
Citations
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TL;DR: In this article, the authors present three examples of ethylene hydrogenation, ammonia synthesis, and hydrogen oxidation to assess the thermodynamic validity of literature mechanisms and demonstrate various methods to ensure thermodynamic consistency.
Abstract: The foundation of microkinetic analysis over 10 years ago has drastically changed the way of parametrization of rates of surface-catalyzed reactions. In the initial stages of development, some parameters arose from experimental data whereas others were fitted to experimental data. Semiempirical and first principles quantum mechanical and statistical mechanics simulations nowadays are powerful tools in estimating kinetic parameters. However, even in best cases, some tuning of parameters is typically necessary for quantitative model predictions. During this process, parameters of reaction mechanisms may violate thermodynamics. Here we review thermodynamic constraints of reaction networks and derive expressions applicable to surface reactions. We present three examples of ethylene hydrogenation, ammonia synthesis, and hydrogen oxidation to assess the thermodynamic validity of literature mechanisms. Various methods to ensure thermodynamic consistency are discussed and demonstrated with a specific example of H...
156 citations
TL;DR: In this paper, the catalytic combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in the pressure range 4 to 16 bar in an optically accessible, laminar channel-flow catalytic reactor.
140 citations
TL;DR: In this article, the hydrogen assisted catalytic combustion of preheated mixtures of methane-hydrogen-air in a micro-combustor was modeled by a two-dimensional model including an elementary-step surface reaction mechanism.
69 citations
01 Jan 2009
TL;DR: In this paper, the authors investigated the heterogeneous combustion of fuel-lean hydrogen/air premixtures over platinum in an optically accessible channel-flow catalytic reactor and included planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition, and 1-D Raman measurements of major gas-phase species concentrations for the evaluation of the catalytic processes.
Abstract: The hetero-/homogeneous combustion of fuel-lean hydrogen/air premixtures over platinum was investigated experimentally and numerically in the pressure range 1 bar ⩽ p ⩽ 10 bar. Experiments were carried out in an optically accessible channel-flow catalytic reactor and included planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition, and 1-D Raman measurements of major gas-phase species concentrations for the evaluation of the heterogeneous (catalytic) processes. Simulations were performed with a full-elliptic 2-D model that included detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduced the measured catalytic hydrogen consumption, the onset of homogeneous ignition at pressures of up to 3 bar and the diminishing gas-phase combustion at p ⩾ 4 bar. The suppression of gaseous combustion at elevated pressures bears the combined effects of the intrinsic homogeneous hydrogen kinetics and of the hetero/homogeneous chemistry coupling via the catalytically produced water over the gaseous induction zone. Transport effects, associated with the large diffusivity of hydrogen, have a smaller impact on the limiting pressure above which gaseous combustion is suppressed. It is shown that for practical reactor geometrical confinements, homogeneous combustion is still largely suppressed at p ⩾ 4 bar even for inlet and wall temperatures as high as 723 and 1250 K, respectively. The lack of appreciable gaseous combustion at elevated pressures is of concern for the reactor thermal management since homogeneous combustion moderates the superadiabatic surface temperatures attained during the heterogeneous combustion of hydrogen.
63 citations
TL;DR: In this article, the authors describe physics-aided methods (sensitivity, partial equilibrium, and most abundant reactive intermediate analyses) and statistics-based methods (A, D, and E optimal designs) for the design of experiments.
Abstract: Fundamental multiscale models are increasingly being used to describe complex systems Microkinetic models, which consider a detailed surface reaction mechanism containing all relevant reactions, are a prototypical multiscale model example The computational effort in calculating all parameters of a multiscale model for real systems from first principles is prohibitive, and parameter uncertainty still limits full quantitative capabilities of these models This motivates the development of rational model-based techniques in order to refine uncertain parameters and assess the global (in the entire experimental parameter space) model robustness Herein we describe physics-aided methods (sensitivity, partial equilibrium, and most abundant reactive intermediate analyses) and statistics-based methods (A, D, and E optimal designs) for the design of experiments While our methods are fairly general, we demonstrate them for the catalytic decomposition of ammonia on ruthenium to produce hydrogen A global Monte Car
54 citations
References
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TL;DR: In this article, the surface ignition of methane/oxygen/nitrogen mixtures over a platinum foil is modeled at atmospheric pressure with a revised detailed surface kinetics mechanism including 20 surface reactions and seven surface species, detailed gas-phase kinetics and simplified multicomponent transport.
41 citations
01 Jan 2000
TL;DR: In this article, a detailed surface reaction mechanism for oxidation of CO on polycrystalline Pt surfaces, capable of predicting various available experimental features, has been developed using a multistep methodology.
Abstract: A detailed surface reaction mechanism for oxidation of CO on polycrystalline Pt surfaces, capable of predicting various available experimental features, has been developed using a multistep methodology. First, thermodynamically consistent, coverage-dependent activation energies and heats of reactions were derived from the application of the unity bond index-quadratic exponential potential formulation. Next, initial estimates of pre-exponentials were obtained from transition state theory or available experiments. Important feature identification analysis was performed to determine key reaction parameters for each experiment. Model responses were then parameterized in terms of these important parameters by simple polynomials and factorial design techniques and subsequently used in simultaneous optimization through simulated annealing against different sets of literature and new experimental data from our laboratory. Model validation with independent experiments shows that the proposed surface reaction mechanism performs very well. The potential of our approach for developing surface reaction mechanisms for catalytic combustion of more complex fuels is also discussed.
37 citations
TL;DR: In this paper, surface ignition and extinction of hydrogen/air mixtures on platinum surfaces are modeled using a detailed surface kinetic mechanism and transport phenomena, and it is shown that the platinum surface can be poisoned by different adsorbates, and the dynamic process of catalytic ignition is associated with a phase transition from one poisoning species to another.
31 citations
TL;DR: In this paper, the contributions of homogeneous and heterogeneous reactions to high-temperature catalytic methane oxidation were studied over three different gauze catalysts (Pt, Pt-10%Rh, and Ni) using laser-induced fluorescence (LIF) spectroscopy to measure OH · concentrations in the boundary layers downstream of the gauzes.
31 citations