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 \([\mathrm{mol}\,\mathrm{m}^{-3}]\), 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 1.0 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
name : foo-surface
gas phase : gas
gas-phase species: foo
gas-phase products :
   - species name: bar
     coefficient: 0.5
   - species name: baz
     coefficient: 0.3
reaction probability : 0.2

{
   "type": "SURFACE",
   "name": "foo-surface",
   "gas phase": "gas",
   "gas-phase species": "foo",
   "gas-phase products": [
      {
         "species name": "bar",
         "coefficient": 0.5
      },
      {
         "species name": "baz",
         "coefficient": 0.3
      }
   ],
   "reaction probability": 0.2
}

The gas-phase species is required and should be name of the species colliding with the surface.

The gas-phase products is a list of gas-phase species that are produced by the surface reaction. Each product must have a species name. The the coefficient is not specified, it defaults to 1.0.

The gas phase key is required and must be set to the name of the phase the reaction takes place in. The gas-phase species and each gas-phase product must be present in the specified phase.

The gas-phase reactant species must include the property molecular weight [kg mol-1], which specifies the molecular weight of the species in \(\mathrm{kg}\ \mathrm{mol}^{-1}\).

The phase configuration must include the property diffusion coeff [m^2 s-1] for the gas-phase species, which specifies the diffusion coefficient for the reacting species in that phase in \(\mathrm{m}^2\ \mathrm{s}^{-1}\).

Rate constants are in units of \(\mathrm{s}^{-1}\).

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