Surface (Heterogeneous) Reaction#

Surface reactions transform gas-phase species into gas-phase products according to a rate that is calculated based on the total exposed surface area of a condensed phase.

For surface reactions of the gas-phase species X, the reaction rate is calculated assuming large particles (continuum regime) as:

\[r_{surface} = k_{surface}[X]\]

where [X] is the gas-phase concentration of species X \([\mathrm{molecule}\,\mathrm{cm}^{-3}]\), and the rate constant \((k_{\mathrm{surface}}) [\mathrm{s}^{-1}]\) is calculated as:

\[k_{surface} = \frac{4N_a \pi r_e^2}{\left(\frac{r_e}{D_g} + \frac{4}{v(T)\gamma}\right)}\]

where N_a is the number concentration of particles \([\mathrm{particles}\,\mathrm{m}^{-3}]\), \(r_e\) is the effective particle radius \([\mathrm{m}]\), D_g is the gas-phase diffusion coefficient of the reactant \([\mathrm{m}^2\mathrm{s}^{-1}]\), gamma is the reaction probability [unitless], and v is the mean free speed of the gas-phase reactant:

\[v = \sqrt{\frac{8RT}{\pi MW}}\]

where R is the ideal gas constant \([\mathrm{J} \mathrm{K}^{-1} \mathrm{mol}^{-1}]\), T is temperature \([\mathrm{K}]\), and MW is the molecular weight of the gas-phase reactant \([\mathrm{kg}\ \mathrm{mol}^{-1}]\).

A conforming implementation of version v1.0.0 of the configuration will provide ways to specify the particle effective radius and number concentration.

Input data for surface reactions have the following format:

type : SURFACE
gas-phase reactant : my gas species
reaction probability : 0.2
gas-phase products :
   my other gas species :
   another gas species : { "yield" : 0.3 }

{
  "type" : "SURFACE",
  "gas-phase reactant" : "my gas species",
  "reaction probability" : 0.2,
  "gas-phase products" : {
     "my other gas species" : { },
     "another gas species" : { "yield" : 0.3 }
  }
}

The key-value pairs gas-phase reactant and reaction probability are required. Only one gas-phase reactant is allowed, but multiple products can be present. The key-value pair yield for product species is optional and defaults to 1.0.

The gas-phase reactant species must include properties diffusion coeff [m^2 s-1], which specifies the diffusion coefficient in \(\mathrm{m}^2\ \mathrm{s}^{-1}\), and molecular weight [kg mol-1], which specifies the molecular weight of the species in \(\mathrm{kg}\ \mathrm{mol}^{-1}\).

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