Abstract
A general approach for constructing finite rate surface chemistry models using time-of-flight (TOF) distribution data acquired from pulsed hyperthermal beam experiments is presented. First, a detailed study is performed with direct simulation Monte Carlo (DSMC) to analyze the TOF distributions corresponding to several types of reaction mechanisms occurring over a wide temperature range. This information is used to identify and isolate the products formed through different reaction mechanisms from TOF and angular distributions. Next, a procedure to accurately calculate the product fluxes from the TOF and angular distributions is outlined. Finally, in order to derive the rate constant of the reactions within the system, the inherent transient characteristic of the experimental pulsed beam set up must be considered. An analysis of the steady-state approximation commonly used for deriving the rate constants reveals significant differences in terms of the total product composition. To overcome this issue, we present a general methodology to derive the reaction rate constants, which takes into account the pulsed setup of the beam. Within this methodology, a systematic search is performed through the rate constant parameter space to obtain the values that provide the best agreement with experimentally observed product compositions. This procedure also quantifies the surface coverage that corresponds to the rates of product formation. This approach is applied to a sample system: oxidation reaction on vitreous carbon surfaces to develop a finite-rate surface chemistry model. Excellent agreement is observed between the developed model and the experimental data, thus showcasing the validity of the proposed methodologies.
Highlights
Developing accurate surface chemistry models is important for a number of applications such as chemical processes for manufacturing, TPS design, as well as material processing for semiconductors, and technologies for medical sciences, corrosion protection, and lubrication
Notice that there are two different types of CO being produced from the surface labeled as CO{a} and CO{b}
We have presented a general approach for constructing finite rate surface chemistry models using pulsed hyperthermal beam experimental data, which is of great interest in a broad range of fields
Summary
Developing accurate surface chemistry models is important for a number of applications such as chemical processes for manufacturing, TPS (thermal protection systems) design, as well as material processing for semiconductors, and technologies for medical sciences, corrosion protection, and lubrication. Surface chemistry models are often developed with the use of macroscopic experimental data like total product fluxes, heat flux measurements, material recession, and radiative signatures.3–20 These quantities are used to infer the concentration of the various products near the surface, which are utilized for obtaining the corresponding reaction rates. Often one or more reaction mechanisms are assumed for each product in order to fit the rate constants to an Arrhenius form These experimentally measured macroscopic quantities are characterized by highly coupled processes on and near the surface, which are almost impossible to isolate.. These experimentally measured macroscopic quantities are characterized by highly coupled processes on and near the surface, which are almost impossible to isolate.3,4 This limits the extrapolation of these finite-rate surface chemistry models to conditions different from those of the experiments. Experimental data elucidating the molecular level details of the gas-surface interactions is of paramount importance in constructing a general, physically accurate surface chemistry model
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