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Journal ArticleDOI

Surface reaction mechanism development for platinum-catalyzed oxidation of methane

01 Jan 2002-Vol. 29, Iss: 1, pp 997-1004

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.
Topics: Oxidative coupling of methane (60%), Anaerobic oxidation of methane (57%), Methane (54%), Platinum (52%), Reaction mechanism (52%)
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Journal ArticleDOI
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...

144 citations

Journal ArticleDOI
Michael Reinke1, John Mantzaras1, Rolf Schaeren1, Rolf Bombach1  +2 moreInstitutions (1)
Abstract: The catalytic combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in the pressure range 4 to 16 bar. Experiments were performed in an optically accessible, laminar channel-flow catalytic reactor. In situ, spatially resolved Raman measurements of major species and temperature over the reactor boundary layer were used to assess the heterogeneous (catalytic) reactivity and planar laser-induced fluorescence (LIF) of the OH radical confirmed the absence of homogeneous (gas-phase) ignition. Numerical predictions were carried out with a two-dimensional elliptic code that included elementary heterogeneous and homogeneous chemical reaction schemes. Comparisons between measurements and numerical predictions have led to the assessment of the high-pressure validity of two different elementary heterogeneous chemical reaction schemes for the complete oxidation of methane over platinum. It was shown that the catalytic reactivity increased with increasing pressure and that crucial in the performance of the heterogeneous reaction schemes was the capture of the decrease in surface free-site availability with increasing pressure. Even in the absence of homogeneous ignition, the contribution of the gaseous reaction pathway to the conversion of methane could not be ignored at high pressures. The delineation of the regimes of significance for both heterogeneous and homogeneous pathways has exemplified the importance of the preignition gaseous chemistry in many practical high-pressure catalytic combustion systems. Sensitivity and reaction flux analyses were carried out on a validated elementary heterogeneous reaction scheme and led to the construction of reduced catalytic reaction schemes capable of reproducing accurately the catalytic methane conversion in the channel-flow configuration as well as in a surface perfectly stirred reactor (SPSR) and, when coupled to a homogeneous reaction scheme, the combined heterogeneous and homogeneous methane conversions. A global catalytic step could not reproduce the measured catalytic reactivity over the entire pressure range 4 to 16 bar.

133 citations

Journal ArticleDOI
John Mantzaras1, Rolf Bombach1, Rolf Schaeren1Institutions (1)
01 Jan 2009-
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.

59 citations

Journal ArticleDOI
Yunfei Yan1, Weimin Tang1, Li Zhang1, Wenli Pan1  +3 moreInstitutions (1)
Abstract: Understanding of micro-scale combustion mechanism is very essential to the development of micro-power devices, so hydrogen assisted catalytic combustion of methane on platinum was studied in this paper. The 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. It was demonstrated that the model could predict the effects of changes of hydrogen fraction. It was shown that the mole fraction of H, OH and C(s) increase and ignition time decreases with hydrogen addition. It was also shown that the improving effect of hydrogen on the ignition temperature of the fuel and O(s) coverage is particularly evident at relatively low hydrogen fraction. The promotion of the combustion stability is due to the decrease of coefficient of variation with hydrogen addition. The methane combustion will move toward the more stabilized reaction and there is a great potential to reduce the pressure fluctuation.

58 citations

Journal ArticleDOI
Vinay Prasad1, Dionisios G. Vlachos1Institutions (1)
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50 citations

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01 Jan 1991-
Abstract: This second edition is aims to reflect new developments in theory and in commercial application. The guide focuses on the use of solid catalysts and catalytic reactions in large-scale industrial operations. The author provides explanations of the chemistry, kinetics and engineering of heterogeneous catalysis, listing applications to procedures and processes in the lab, pilot plant and commercial setting. Throughout, the emphasis is on workable solutions to real problems.

924 citations

Journal ArticleDOI
Daniel A. Hickman1, Lanny D. Schmidt1Institutions (1)
15 Jan 1993-Science
TL;DR: Rhodium was considerably superior to platinum in producing more H2 and less H2O, which can be explained by the known chemistry and kinetics of reactants, intermediates, and products on these surfaces.
Abstract: The reaction between methane and oxygen over platinum and rhodium surfaces in metalcoated ceramic monoliths can be made to produce mostly hydrogen and carbon monoxide (greater than 90% selectivity for both) with almost complete conversion of methane and oxygen at reaction times as short as 10–3 seconds. This process has great promise for conversion of abundant natural gas into liquid products such as methanol and hydrocarbons, which can be easily transported from remote locations. Rhodium was considerably superior to platinum in producing more H2 and less H2O, which can be explained by the known chemistry and kinetics of reactants, intermediates, and products on these surfaces.

880 citations

Journal ArticleDOI
Harald Brune1Institutions (1)
Abstract: Thin films are often grown away from thermodynamic equilibrium and their morphology becomes determined by kinetics. The final structure of the epitaxial film is decided in the very early stage of submonolayer nucleation and island growth. Recent experiments with scanning tunneling microscopy opened up an unprecedented view of this early stage of epitaxial growth. Variable sample temperatures enabled quantification of the rates of the most important atomic diffusion events and tracing back their interplay yielding the final submonolayer morphology. The present understanding of nucleation and aggregation in light of these new experimental results is reviewed for the case of metals. Examples are given how the growth kinetics can be employed to create well-defined island morphologies and island arrays in a self-organization process.

828 citations

Journal ArticleDOI
TL;DR: Test results indicate that many problems can be solved more efficiently using this scheme than with a single class of methods, and that the overhead of choosing the most efficient methods is relatively small.
Abstract: This paper describes a scheme for automatically determining whether a problem can be solved more efficiently using a class of methods suited for nonstiff problems or a class of methods designed for stiff problems. The technique uses information that is available at the end of each step in the integration for making the decision between the two types of methods. If a problem changes character in the interval of integration, the solver automatically switches to the class of methods which is likely to be most efficient for that part of the problem. Test results, using a modified version of the LSODE package, indicate that many problems can be solved more efficiently using this scheme than with a single class of methods, and that the overhead of choosing the most efficient methods is relatively small.

777 citations

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