The surface reaction mechanism may consist of any number of chemical reactions involving the solid species named in the site and bulk data, as well as the gas-phase species declared in the Gas-phase Kinetics Pre-processor. A reaction may be reversible or irreversible.
Reaction data must start with the word REACTIONS (or
REAC). On the same line the user may specify units of the Arrhenius rate
coefficients to follow by including the words CAL/MOLE,
KCAL/MOLE, JOULES/MOLE,
KJOULES/MOLE, EVOLTS, or KELVIN
to indicate the units for all rate parameters that have energy units, for example, the
activation energy 
of Equation 4–47.
Note: Even if the default energy units are changed by giving one of the REACTION line keywords,
the temperature appearing in the Arrhenius expression of Equation 4–47 , that is, in 
raised to the 
power and in the denominator of the activation energy term, is always in
degrees Kelvin.
The special words MOLES or MOLECULES can appear
on the REACTIONS line to indicate the units for 
(see Equation 4–61). If
MOLECULES is specified, then the units for 
are based on cm, molecules, sec, or K.
Note: If units are not specified, 
and 
are assumed to be in cm, mole, sec, K, and cal/mole, respectively.
In addition to controlling the units options of the rate coefficients, the user may also
include a keyword on the REACTIONS line to control the way that sticking
coefficients are converted to reaction-rate coefficients. In particular, the user may enable or
disable the Motz-Wise [18] correction factor (see
Equation 4–61), which is off by default. The
keyword MWON can be included on the REACTIONS line to
turn on this correction for all reactions that are specified as sticking-coefficient reactions.
Alternatively, the user may include MWON as an auxiliary keyword following
an individual reaction to turn on the Motz-Wise correction for that reaction only, as discussed
in section Motz-Wise Correction . Conversely, if
the MWON parameter was given on the REACTIONS line,
the user can include the MWOFF auxiliary keyword following an individual
reaction, specifying that the Motz-Wise correction is to be used for that reaction only.
MWOFF is the default.
Note: Ansys recommends that the user leave the Motz-Wise [18] correction off in the conversion between a sticking coefficient and a rate constant, which is the default.
Finally, on the REACTIONS line, the user may specify
USRPROD to indicate that the user will provide a user-written rate routine
that will supply all of the species net rates of production, overriding any other reaction
input in the Surface Kinetics input file. The USRPROD input optionally
includes a slash(/)-delimited integer parameter, which allows the user to
distinguish one reaction model from another within their user routine. When the
USRPROD parameter is included, KINetics will call a user-supplied
subroutine, SKUPROD wherever net species production rates are required. A
template of SKUPROD can be obtained from Ansys as well as information on
how to compile and link user routines into KINetics.
Note: Ansys Chemkin does not support user-written programs, so you are cautioned to use this feature at your own risk.
The lines following the REACTION line contain reaction descriptions
together with their Arrhenius rate coefficients. The reaction description is composed of
reaction data and optional auxiliary information data.
Each reaction "line" is divided into two fields, where a "line" may take up two or more
physical lines if it is more than 80 characters long. A reaction data entry is continued
on the next line using the special character "&" at the end of the line; any information
following the & symbol on the same line is ignored. The first field in the reaction entry
contains the symbolic description of the reaction, while the second contains the
Arrhenius rate coefficients. Both fields are format free, and blank spaces are ignored.
All characters on a line following an exclamation mark (!) are considered comments
and are ignored. Blank lines are also ignored.
The reaction description, given in the first field, must be composed of the species symbols, coefficients, and delimiters as summarized below.
Table 3.10: Surface reaction data criteria
| Species Symbols | |
|
Each species in a reaction is described with the unique sequence of characters as they appear in the species data and the thermodynamic data. | |
| Coefficients | |
|
A species symbol may be preceded by an integer or real coefficient. The coefficient
has the meaning that there are that many moles of the particular species present as either
reactants or products; for example, | |
| Delimiters | |
| + |
A plus sign is the delimiter between each reactant species and each product species. |
|
= |
An equality sign is the delimiter between the last reactant and the first product in a reversible reaction. |
|
<=> |
An equality sign enclosed by angle brackets can also be used as the delimiter between the last reactant and the first product in a reversible reaction. |
|
=> |
An equality sign with an angle bracket on the right is the delimiter between the last reactant and the first product in an irreversible reaction. |
The second field of the reaction line is used to define the Arrhenius rate coefficients
, 
, and 
in that order, as given by Equation 4–47. At least one blank space must separate the last species name in
the reaction and first number. The three numbers must be separated by at least one blank space,
be stated in either integer, floating point, or “E” format (for example,
123 or 123.0 or 12.3E1), and
have units associated with them (although the units do not appear on the input line).
Note: Unless modified by the REACTIONS line or by the UNITS auxiliary keyword, the default
units for 
are cgs (cm, sec, K, mole), the exact units depending on the order of the
reaction. The factor 
is dimensionless. The default units for the activation energies are
cal/mole.
The second field of the REACTIONS line may optionally be used to specify
the coefficients 
, 
, and 
of Equation 4–59 for a sticking
coefficient. In order for the second field to apply to sticking coefficient parameters, the
next line of input must contain the auxiliary keyword STICK.
Examples of some reaction data are shown in Example 3.6: Examples of Surface reaction data. Table 3.11: Summary of the rules for Surface reaction data summarizes the reaction data rules.
Example 3.6: Examples of Surface reaction data
REACTIONS KCAL/MOLE ASH3 + AS(P) <=> ASH3(P) + AS(D) 4.0E11 0 25 ! Ref. 21 ! ASH3 + AS(P) <=> ASH3(P) + AS(D) 4.0E11 0 0 ! same as previous ASH <=> AS(D) + H(S) 1.0 0 0 STICK GA(CH3)3(L) + GA2AS(A) <=> AS + GA(CH3)(L) + 2 GAME & ! continued on next line 1.0E13 0 4000.
Table 3.11: Summary of the rules for Surface reaction data
| Rule | Description |
|---|---|
| 1 |
The first reaction line must start with the word |
| 2 |
The word |
| 3 |
Valid unit declarations are |
| 4 |
The reaction description can begin anywhere on this line. All blank spaces, except those separating the Arrhenius coefficients, are ignored. |
| 5 |
Each reaction description must have =,
|
| 6 |
Each species in a reaction is described with a unique sequence of characters (name)
as they appear in the species data and the thermodynamic data. However, if a species name
is not unique (because it is duplicated in another phase), the name must be modified by
appending its slash-delimited phase name, such as |
| 7 |
Stoichiometric coefficients are represented by an integer or real number preceding a
species name. The default is to assume a stoichiometric coefficient of
|
| 8 |
A reaction description may be contained on more than one line. If a line contains the
symbol |
| 9 |
Three Arrhenius coefficients must appear in order ( |
| 10 |
There cannot be more than six reactants or six products in a reaction. |
| 11 |
To specify a sticking coefficient rather than a rate constant the three numbers after
the reaction description have the meaning |
| 12 |
All characters on a line following an exclamation mark are considered comments and are ignored. |
Auxiliary information data appears on one or more separate lines after the reaction data line
is read, and serves to modify or give additional parameters needed to evaluate that reaction's
rate expression. The format in an auxiliary information line is a character string keyword
followed by a slash-delimited (/) field containing an appropriate number
of parameters (either integer, floating point, "E" format, or character). Any number of
auxiliary information lines may follow a reaction line, in any order, and any number of
keywords may appear on an auxiliary information line; however, an auxiliary keyword and its
parameter(s) must appear on the same line.
Examples of the auxiliary information are shown in Example 3.7: Examples of auxiliary surface reaction data. The rules are summarized in Table 3.11: Summary of the rules for Surface reaction data.
Reverse Reaction Parameters
For a reversible reaction, auxiliary information data may follow the reaction to specify
Arrhenius parameters for the reverse-rate expression. Here, the three Arrhenius parameters (
, 
, and 
) for the reverse rate must follow the auxiliary keyword
REV. Using this option overrides the reverse rates that would normally be
computed through the equilibrium constant, Equation 4–48.
Duplicate Reactions
It sometimes happens that two or more reactions can involve the same set of reactants and products, but
proceed through distinctly different reaction channels. In these cases it may be appropriate to state a reaction
mechanism that has two or more reactions with identical reactants and products, but have different rate parameters.
However, duplicate reactions are normally considered errors by the Surface Kinetics
Pre-processor; if the user requires duplication (for example, the same reactants and products with different Arrhenius
parameters), an auxiliary information statement containing the auxiliary keyword DUP (with
no parameters) must follow the reaction line of each duplicate reaction (including the first occurrence of the
reaction that is duplicated.) For example, to specify different rate expressions for each of three identical
reactions, there must be three occurrences of the keyword DUP, one following each of the reactions.
Sticking Coefficients
If the three coefficients given in the second field of the reaction line are to be interpreted
as the parameters 
, 
, and 
of Equation 4–59 for a
sticking coefficient, then the keyword STICK (with no parameters) must
follow the reaction line as auxiliary information. There can be only one gas-phase reactant
species in a sticking-coefficient reaction; moreover, its stoichiometric coefficient must be
1.
Motz-Wise Correction
By default the Motz-Wise [18] correction of
Equation 4–61 will not be applied to any
reactions using sticking coefficients unless the MWON keyword is given on
the REACTIONS line (discussed earlier), in which the new default will be
to apply this term. Whichever of these choices has been set as the default can be overridden
for an individual sticking-coefficient reaction by including the auxiliary keyword
MWOFF or MWON following the reaction line.
Arbitrary Reaction Orders
The reaction order, that is, the dependence of the reaction's rate-of-progress upon the
concentration of chemical species, can be changed via the FORD or
RORD auxiliary keywords for the forward or reverse reaction, respectively.
Each occurrence of these keywords must be followed by the species name and the new reaction
order, for example, FORD/SI(S) 0.5/. This option overrides the values of
and 
in Equation 4–44
pertaining to the particular species named on the line. The reaction order for all other
species maintain their default values of 
and 
. Multiple occurrences of the FORD and
RORD construct may appear on the auxiliary line. A
FORD or RORD keyword can be specified even for
species that do not appear as a reactant or product in the reaction, although one might
reasonably wonder how such a functional dependence could occur.
Coverage Dependent Reactions
To modify the expression for the forward rate constant by optional coverage parameters (see
Equation 4–56), one uses the auxiliary
keyword COV followed by (slash delimited) surface
species name and the three parameters 
, 
and 
. More than one set of COV data can appear for a given
reaction, and these would be applied multiplicatively as in Equation 4–56.
Unit Specifications for Reaction Parameters
The default units for specifying the reaction rate parameters (either the global defaults set
up by the Surface Kinetics Pre-processor, or the set of units requested by the user on the
REACTIONS line) can be overridden for an individual reaction through use
of the UNITS auxiliary keyword. Following the reaction, one activates this
option with UNITS/string/, where
string is one of the unit specifiers EVOLTS,
KELVIN, CAL/MOLE, KCAL/MOLE,
JOULES/MOLE, or KJOULES/MOLE (to change the units for
parameters that have energy units), or MOLES or
MOLECULES (to change the units of the pre-exponential 
). The UNITS auxiliary keyword allows only one
string parameter, but the user can repeat the
UNITS/string/ option as many
times as needed for a given reaction.
Note: Even if the default energy units are changed by giving one of the
UNITS keywords, the temperature appearing in the Arrhenius expression of
Equation 4–47, that is, in 
raised to the 
power and in the denominator of the activation energy term, is still in
Kelvin.
Langmuir Hinshelwood and Eley-Rideal Reaction Parameters
In the case of a Langmuir-Hinshelwood reaction, the numbers on the primary reaction
description line give 
, 
and 
for the rate constant. By default, the equilibrium constants in the numerator
are included in the rate constant (
in Equation 4–65 is used,
see Langmuir-Hinshelwood and Eley-Rideal Reactions). The default
stoichiometric coefficients for the chemical species concentrations in the numerator can be
overridden using the FORD keyword. The units of the rate parameters should
follow those used in the reaction mechanism description as a whole, either the defaults of
moles, cm, and sec, or whatever is declared in the REACTIONS line. The
dimensions of the rate constant need to be matched to the order of the reaction, either the
default from the law of mass-action, or whatever has been specified via
FORD statements.
The auxiliary keyword, LANG, on a supplementary line, indicates the use
of the Langmuir-Hinshelwood rate expression. It can only be used for an irreversible reaction,
and one auxiliary line should be supplied for each species appearing in the denominator of
Equation 4–65 . The keyword is followed, in
slash delimited format, by the species name, the pre-exponential, temperature factor, and
enthalpy for the equilibrium constant, and the reaction order for that species (usually one).
The equilibrium constant is defined as 
, in parallel with the standard expression for rate constants.
An additional auxiliary keyword, LHDE, allows the default value of 2 for the overall
exponent for the denominator (
) to be overridden. To specify an Eley-Rideal reaction, this keyword would be used to set 
to 1. The use of any positive number is permitted, including real or fractional numbers.
The auxiliary keyword LHNU allows the explicit inclusion of equilibrium
constants in the numerator of the LH rate expression (the use of 
rather than 
, see Langmuir-Hinshelwood and Eley-Rideal Reactions).
This keyword is followed by a slash delimited list of species names. For each species in the
list, a multiplier of 
will be applied to the rate constant. Each species listed in a
LHNU statement must have a LANG statement. In the
rare case that the desired rate expression does not have matching values for 
and 
(see Equation 4–65) for a
given species, the LHNU option should not be used because the units will
not be correct. In such cases, the user should manually lump the 
values with 
to yield 
. If LANG is used with a complete set of
LHNU statements, the rate constant should be provided by the user in moles
cm-2sec-1, or whatever units have been
specified on the REACTIONS line.
An additional auxiliary keyword LHPR indicates that the equilibrium
constants are given in pressure units. The keyword will affect the equilibrium constants for
the specified reaction only; the reaction rate will remain in whatever units are given on the
REACTION line, or in moles, cm, and sec. The keyword is followed by the
name of the pressure unit being used: atm, bar, torr, pasc (for Pascal), or dyne (for dyne per
square cm), where the names are not case sensitive.
Optional User Rate Subroutine SKUPROG
The auxiliary keyword USRPROG, optionally appended
by a slash(/)-delimited integer parameter, implies that the net
rate-of-progress for the reaction will be obtained by calling a user-supplied subroutine,
SKUPROG. The optional parameter allows the user to select from more than
one type of rate formulation. Wherever the net reaction rate is required, it will be obtained
by calling the user-written subroutine. USRPROG applies only to
irreversible reactions, and cannot be used in conjunction with USRPROD
(entered on the REACTIONS line). A template of SKUPROD is
provided in the Ansys Chemkin installation, in the file sklib_user_routines.f located in the directory user_subroutines. Contact Ansys Support if that file is not available.
Summary of Surface Auxiliary Reaction Data
Any number of auxiliary information lines may follow a reaction line, in any order, and any number of keywords may appear on an auxiliary information line; however, an auxiliary keyword and its parameter(s) must appear on the same line.
The above rules are summarized in Table 3.12: Summary of the rules for Surface auxiliary data. Examples of auxiliary information are shown in Example 3.7: Examples of auxiliary surface reaction data.
Table 3.12: Summary of the rules for Surface auxiliary data
| Rule | Description |
|---|---|
| 1 | Auxiliary information lines may follow a reversible reaction to specify the reverse rate parameters explicitly; auxiliary information must follow any reactions that are duplicated. |
| 2 | Auxiliary keyword declarations may appear anywhere on the line, in any order. |
| 3 | Any number of auxiliary keywords may appear on a line, and more than one line may be used, but a keyword and its parameter(s) must appear on the same line. |
| 4 | Multiple keywords appearing on the same line must be separated by at least one blank space. |
| 5 | Any blank spaces between a keyword and the first slash are ignored and any blanks between the slashes and parameter(s) are also ignored. However, no blank spaces are allowed within a keyword or parameter. |
| 6 | The keyword REV followed by three slash-delimited Arrhenius
coefficients may be used to specify the reverse rate parameters. |
| 7 | The auxiliary keyword DUPLICATE (or DUP)
must follow every occurrence of a duplicated reaction. |
| 8 | The keyword STICK indicates that the three coefficients on the
reaction line are to be interpreted as the parameters  ,  , and  in Equation 4–59.
There must be exactly one gas-phase reactant species; its stoichiometric coefficient must
be 1. |
| 9 | The keyword COV is used to modify the forward rate constant by
the expression in Equation 4–56. The
word COV is followed by a surface species name and the three coverage
parameters  ,  and  . The four entries after the word COV are
slash-delimited. |
| 10 | The FORD or RORD auxiliary keywords can be
used to change the reaction order (with respect to species concentration) of the forward or
reverse reaction, respectively, for any species in the mechanism, regardless of whether the
species appears as a reactant or a product in the reaction. The species name and the new
reaction order (slash-delimited) follow the keyword. |
| 11 | The UNITS auxiliary keyword can be used to override the current
default units for parameters with energy units or the pre-exponential for a given reaction.
The usage is UNITS/string/,
where string is one of the following: EVOLTS,
KELVIN, CAL/MOLE, KCAL/MOLE,
JOULES/MOLE, or KJOULES/MOLE (for parameters with
energy units), or MOLES or MOLECULES (for
pre-exponential). |
| 12 | The string MWOFF can be used to turn off the Motz-Wise
[18] correction of Equation 4–61 or the string MWON can
be used to turn on this correction for a sticking coefficient reaction. Using the
MWOFF or MWON auxiliary keyword overrides the
default option set up on the REACTION line or the default supplied by
the Surface Kinetics Pre-processor (which is MWON). |
| 13 | All characters on a line following an exclamation mark are considered comments and are ignored. |
| 14 | The LANG keyword can be used to input a Langmuir-Hinshelwood
rate expression. The keyword is followed by a species name, three parameters giving the
equilibrium constant, and a fourth parameter giving the order of that species in the
reaction. Additional auxiliary keywords LHDE,
LHNU, and LHPR provide more flexibility in the
form of the Langmuir-Hinshelwood or Eley-Rideal rate expressions. |
Example 3.7: Examples of auxiliary surface reaction data
! THE FOLLOWING ARE *CONTRIVED* EXAMPLES OF AUXILIARY KEYWORD USAGE
SICL(S) <=> CL + SI(S) 1.0E-3 0.0 2.
REV/1.0E13 0.0 37./
CL + SICL(S) <=> CL2 + SI(S) 0.1 1.1 20.
DUPLICATE STICK
RORD /SI(S) 0/
CL + SICL(S) <=> CL2 + SI(S) 1.4E11 0.0 15.
DUPLICATE COV/SICL(S) -1.2 0.5 32./ FORD/CL+ 1.0/
CL* => CL 1.0 0.0 0.
STICK MWOFF
E + CL+ + SICL3(S) + SI(B) => SICL4 + SI(S) 0.50 0.0 0.
BOHM
ENRGDEP/1. 0.5 1.0/ UNITS/EVOLT/
E + CL+ + #SICL3(S) + #SI(B) + SICL(S) &
=> SICL2(S) + #SICL2 + #SICL(S) 0.50 0.0 0.0
BOHM
YIELD/0.0712 1.21 0.5 1.0/ UNITS/EVOLT/
E + CL2+ + SICL3(S) + SI(B) => SICL4 + SICL(S) 0.50 0.0 0.0
FORD/ CL2+ 2.43/
A + B => C + D 7.9E13 0. 56000.
LANG /A 3.6308E+36 0.0 -7.36 2./
LHDE /2/
LHNU /A C/
LHNU /A A/
LHPR /atm/ The Pre-processor provides the option of specifying units of the Arrhenius parameters. The parameters are always stored internally in the same way, that is, activation energies in Kelvin, dimensionless temperature exponents, and pre-exponential factors consistent with cm, molecules, sec, and K. The program converts the user input activation energies accordingly.
However, it is worthwhile to state explicitly the conversion for the Arrhenius pre-exponential constant. In converting from “molecules” to “moles”
 | (3–4) |
where
is the Avogadro number, and
 | (3–5) |
Note that the 
are the stoichiometric coefficients for the gas-phase and surface
ki reactant species (not product species or bulk-phase species).