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...

Thermodynamic consistency in microkinetic development of surface reaction mechanisms

High-pressure catalytic combustion of methane over platinum: In situ experiments and detailed numerical predictions

Numerical simulation of the effect of hydrogen addition fraction on catalytic micro-combustion characteristics of methane-air

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.

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Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum at pressures up to 10 bar

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

Multiscale Model and Informatics-Based Optimal Design of Experiments : Application to the Catalytic Decomposition of Ammonia on Ruthenium

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.

V. Papavassiliou

Papers

Surface reaction mechanism development for platinum-catalyzed oxidation of methane