Diffusion and Heterogeneous Reaction. III. Atom Recombination at a Catalytic Boundary

Motz, H.; Wise, Henry

doi:10.1063/1.1731060

Cited by 109 publications

(51 citation statements)

References 3 publications

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“…The additive constants −0.38 and −0.47 on the right hand side of Eqs. (24) and (25), respectively, correspond to the correction term −1/2 in traditional resistor model formulations for the effect of gas phase diffusion in the continuum regime (Danckwerts, 1951;Finlayson-Pitts and Pitts, 2000;and references therein), which have been attributed to an effective doubling of the mean molecular velocity component directed towards the surface in case of high net uptake (distortion of Maxwellian flow; Motz and Wise, 1960). If, however, Kn Xi is indeed more than an order of magnitude below unity, the additive constants contribute less than ∼5 % to the gas phase diffusion resistances and can be omitted from Eqs.…”

Section: Gas Kinetic Fluxes and Uptake Coefficientsmentioning

confidence: 99%

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

2007

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Abstract. Aerosols and clouds play central roles in atmospheric chemistry and physics, climate, air pollution, and public health. The mechanistic understanding and predictability of aerosol and cloud properties, interactions, transformations, and effects are, however, still very limited. This is due not only to the limited availability of measurement data, but also to the limited applicability and compatibility of model formalisms used for the analysis, interpretation, and description of heterogeneous and multiphase processes. To support the investigation and elucidation of atmospheric aerosol and cloud surface chemistry and gasparticle interactions, we present a comprehensive kinetic model framework with consistent and unambiguous terminology and universally applicable rate equations and parameters. It enables a detailed description of mass transport and chemical reactions at the gas-particle interface, and it allows linking aerosol and cloud surface processes with gas phase and particle bulk processes in systems with multiple chemical components and competing physicochemical processes.The key elements and essential aspects of the presented framework are: a simple and descriptive double-layer surface model (sorption layer and quasi-static layer); straightforward flux-based mass balance and rate equations; clear separation of mass transport and chemical reactions; well-defined and consistent rate parameters (uptake and accommodation coefficients, reaction and transport rate coefficients); clear distinction between gas phase, gas-surface, and surfacebulk transport (gas phase diffusion, surface and bulk accommodation); clear distinction between gas-surface, surface layer, and surface-bulk reactions (Langmuir-Hinshelwood and Eley-Rideal mechanisms); mechanistic description of The outlined double-layer surface concept and formalisms represent a minimum of model complexity required for a consistent description of the non-linear concentration and time dependences observed in experimental studies of atmospheric multiphase processes (competitive co-adsorption and surface saturation effects, etc.). Exemplary practical applications and model calculations illustrating the relevance of the above aspects are presented in a companion paper (Ammann and Pöschl, 2007).We expect that the presented model framework will serve as a useful tool and basis for experimental and theoretical studies investigating and describing atmospheric aerosol and cloud surface chemistry and gas-particle interactions. It shall help to end the "Babylonian confusion" that seems to inhibit scientific progress in the understanding of heterogeneous chemical reactions and other multiphase processes in aerosols and clouds. In particular, it shall support the planning and design of laboratory experiments for the elucidation and determination of fundamental kinetic parameters; the establishment, evaluation, and quality assurance of comprehensive and self-consistent collections of rate parameters; and the development of detailed master mechanisms for process mo...

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Section: Gas Kinetic Fluxes and Uptake Coefficientsmentioning

confidence: 99%

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

2007

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“…When the sticking coefficient is large, however, the molecules have a high probability of attaching to the surface, resulting in a non-Maxwellian velocity distribution which alters the species flux to the surface. Motz and Wise [21] examined this effect for a surface covered only with free sites and derived the correction factor 1/(1 Ϫ ␥ k / 2) appearing henceforth in the brackets of Eq. 13.…”

Section: Surface Chemistry Modellingmentioning

confidence: 99%

Two-dimensional modelling for catalytically stabilized combustion of a lean methane-air mixture with elementary homogeneous and heterogeneous chemical reactions

Dogwiler

Benz

Mantzaras

1999

Combustion and Flame

114

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The catalytically stabilized combustion (CST) of a lean (equivalence ratio ⌽ ϭ 0.4) methane-air mixture was investigated numerically in a laminar channel flow configuration established between two platinum-coated parallel plates 50 mm long and 2 mm apart. A two-dimensional elliptic fluid mechanical model was used, which included elementary reactions for both gaseous and surface chemistry. Heat conduction in the solid plates and radiative heat transfer from the hot catalytic surfaces were accounted for in the model. Heterogeneous ignition occurs just downstream of the channel entrance, at a streamwise distance ( x) of 4 mm. Sensitivity analysis shows that key surface reactions influencing heterogeneous ignition are the adsorption of CH 4 and O 2 and the recombinative desorption of surface-bound O radicals; the adsorption or desorption of radicals other than O has no effect on the heterogeneous ignition location and the concentrations of major species around it. Homogeneous ignition takes place at x ϭ 41 mm. Sensitivity analysis shows that key surface reactions controlling homogeneous ignition are the adsorption/desorption of the OH radical and the adsorption/ desorption of H 2 O, the latter due to its direct influence on the OH production path. In addition, the slope of the OH lateral wall gradient changes from negative (net-desorptive) to positive (net-adsorptive) well before homogeneous ignition ( x ϭ 30 mm), thus exemplifying the importance of a detailed surface chemistry scheme in accurately predicting the homogeneous ignition location. The effect of product formation on homogeneous ignition was studied by varying the third body efficiency of H 2 O. Product formation promotes homogeneous ignition due to a shift in the relative importance of the reactions H ϩ O 2 ϩ M 3 HO 2 ϩ M and HCO ϩ M 3 CO ϩ H ϩ M.

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“…r is the gas density, Y i the mass fraction of the i-th species, W the axial velocity component, V i the diffusion velocity of the i-th species, and R i the surface reaction rate of the i-th species, which is calculated by the widely used Motz-Wise formula, [19] given by Equation 4.…”

Section: Slip Boundary Conditionsmentioning

confidence: 99%

CVD in Weakly Rarefied Rotating Disk Flows

Zhang

Law

et al. 2009

Chemical Vapor Deposition

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CVD in a weakly rarefied rotating disk flow is numerically investigated using the SPIN code modified by including slip boundary conditions for velocity, concentration, and temperature, and thereby extending the capability of the code to describe weakly rarefied flows. A model reaction mechanism for silicon deposition, including the gas-phase decomposition of silane to silylene and the surface reactions of silane and silylene, is used in order to demonstrate rarefied gas effects. Results show that, by taking into account the temperature slip, the deposition rate decreases from its value based on the continuum model, mainly due to either the decrease of the concentration of silylene, which is the dominant deposition species at high temperatures, or the decrease of the sticking coefficient of silane, which is the dominant deposition species at low temperatures. Compared to that of the temperature slip, the influence on the deposition rate of the velocity and concentration slips is negligible.

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Diffusion and Heterogeneous Reaction. III. Atom Recombination at a Catalytic Boundary

Cited by 109 publications

References 3 publications

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

Two-dimensional modelling for catalytically stabilized combustion of a lean methane-air mixture with elementary homogeneous and heterogeneous chemical reactions

CVD in Weakly Rarefied Rotating Disk Flows

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