problems involving complex heterogeneous reactions using ...surface chemkin and its history! a...

Sandia National LaboratoriesCombustion Research Facility

Using SURFACE CHEMKIN to Facilitate the Solution ofProblems Involving Complex Heterogeneous Reactions

Anthony McDanielCombustion Research Facility

Sandia National Laboratories, Livermore, CA

CLEERS workshop, May 8, 2001


SURFACE CHEMKIN and its History

! A collection of subroutines designed to facilitate the solution of problemsinvolving large systems of chemical reactions on surfaces– Developed by Mike Coltrin, Robert Kee, and Fran Rupley (1990, 1996)

– Used in conjunction with its predecessor CHEMKIN (gas-phase analog)

– More than 125 journal citations since 1990

chemical vapor deposition, plasma synthesis (deposition and etching), catalysis,particulate chemistry in flames, multi-phase atmospheric chemistry, andheterogeneous combustion

! 67 Fortran subroutines manage computation and provide information

– Species (composition, concentration, charge, site occupancy, etc.)

– Surface reactions (kinetic parameters, stoichiometric coefficients, etc.)

– Chemical production rates, thermodynamic properties, other utilities


Basic Structure

! General constructs

– Symbolic representation of elements, species,and reactions (alphanumeric characters)

– Define three types of species

gas, surface, and bulk

– Surface species reside in top-most layer of solid

adsorption, desorption, surface reaction

species can occupy one or more sitesany number of sites allowed

– Bulk species reside below surface layer

deposition, mixingany number of pure bulk species allowed

any number of bulk mixtures allowed


Thermodynamic Formalism

! Assume standard-state thermodynamic properties given by polynomialfits to specific heats at constant pressure– Conform to NASA chemical equilibrium code (Gordon and McBride)

– Enthalpy and entropy calculated from integrals of specific heat

– Internal energy, Gibbs free energy, and Helmholtz free energy calculated fromT,


Kinetic Formalism

! Chemical rate expressions governed by laws of mass action

! Generalized expression for I surface reactions involving K species

! With net production rate and rate of progress variables defined as

! Given the following definition of surface concentration

′ + ′ ⇔ ′′ + ′′υ χ υ χ υ χ υ χa a b b c c d d


Specification of Mass-Action Kinetic Rate Constants

! Forward and reverse rates from modified Arrhenius expression

– Allow for coverage-dependent reaction rates

… Or calculate reverse rates from equilibrium constant

! Can also specify coverage dependent sticking coefficients (γi )

– Converted to mass-action rate constants internally

! Include provisions for ion-energy dependent rate expressions, andelectron-neutral and ion-neutral interactions for modeling plasmas


Additional Bells, Whistles and Considerations

! General SURFACE CHEMKIN formalisms allow for but do not require anelementary (micro-kinetic) description of the reacting system– Global reactions (maximum of 6 reactant and 6 product species)

– Non-integer stoichiometric coefficients

– Specify arbitrary reaction orders (hardwire order dependence)

– User must exercise caution that reactions may no longer satisfy microscopicreversibility or exhibit proper equilibrium behavior

! Conventions of catalysis community compatible with SURFACE CHEMKIN– Site coverage or site fractions

– TOF or production rate

– Open site or atomic site formalism n

nknk

nk

nknk

sZ

Γ=Ω= )(

)()(

)()( ,

&

σθ


Program Structure

! Gas-phase chemistry, surfacechemistry, and transport librariescompiled with application code

! Input files specify gas, surface, andtransport processes

– Databases and reaction mechanisms

– Interpreters generate linking filesthat communicate with applicationcode

! Subroutine calls to CHEMKINlibraries simplify program structure


Typical Example Problem

! Evaluate boundary condition in application code

! Subroutines and outputs– SKWT returns array of molecular weight for species, WT(K)

– SKHMS returns array of enthalpies in mass units, HMS(K)

– SKRAT returns production rates for species and sites, SDOT(K)

! Example code


Partial Oxidation of Ethane to Ethylene in Stagnation Flow

! Catalytic Autothermal Oxidation of C2H6 over platinum in short contact-time reactors may replace steam cracking as the primary route to C2H4 inthe chemicals industry

! Investigate reaction chemistry with Raman spectroscopy and massspectrometry

SPIN code, 1-D solution tostagnation flow problem


Reaction Mechanism and Excerpt from Input File

! Reaction mechanism andthermochemistry assembledfrom various papers– 25 gas-phase species, 131

reactions

– 19 surface species, 82surface reactions

! Surface reactions arethermodynamicallyconsistent

! Reactions are enteredsymbolically

! Solution time less than 2 minon PIII laptop at 800 MHz

!SURFACE MECHANISM OF THE C2H6-O2 REACTION!SITE/PLATINUM/ SDEN/2.72E-9/PT(s) O(s) H(s) OH(s) H2O(s) C(s) CO(s) CO2(s) CH(s) CH2(s) CH3(s) C2H3(1s)C2H4(1s) C2H4(2s)/2/ C2H5(s) C2H3(2s)/2/ C2H2(1s) C2H2(3s)/3/ C2H6(s)END BULKEND THERMO!Sandia thermo (C&F, Warnatz et al.)O(s) 92491O 1PT 1 I 300.00 3000.00 1000.00 1 0.19454180E+01 0.91761647E-03-0.11226719E-06-0.99099624E-10 0.24307699E-13 2-0.14005187E+05-0.11531663E+02-0.94986904E+00 0.74042305E-02-0.10451424E-05 3-0.61120420E-08 0.33787992E-11-0.13209912E+05 0.36137905E+01 4O2(s) 92491O 2PT 1 I 300.00 3000.00 1000.00 1 0.35989249E+01 0.20437732E-02-0.23878221E-06-0.22041054E-09 0.53299430E-13 2-0.41095444E+04-0.21604582E+02-0.20174649E+01 0.14146218E-01-0.16376665E-05 3-0.11264421E-07 0.60101386E-11-0.25084473E+04 0.79811935E+01 4(continued)END REACTIONS KJOULES/MOLEH2 +PT(s) +PT(s) =>H(s) +H(s) 0.046E-00 0.0 0.0 STICK COV/PT(s) 0.0 -1.0 0.0/O2 +PT(s) +PT(s) =>O(s) +O(s) 1.891E+21 -0.5 0.0CH4 +PT(s) +PT(s) =>CH3(s) +H(s) 9.000E-04 0.0 72.0 STICKCH4 +C(s) =>C2H4(2s) 7.000E-09 0.0 23.0STICK COV/C(s) 0.0 0.0 47.5/ (continued)END

Zerkle et. al., J. Cat., 196, p. 18, 2000.


Comparison Between Experimental and Numerical Result

! C2H6 concentration on centerline probed with Raman spectroscopy

– Sensitivity analysis indicates C2H6 adsorption determines surface flux

– Profile suggest C2H6 sticking coefficient closer to unity

! Zerkle mechanism cannot accurately predict the effects of CH4 on richH2:O2 chemistry

0.10

0.05

0.00

Mol

e F

ract

ion

0.200.150.100.050.00

Inlet Mole Fraction CH4

THIGH

γa

γb

γa

γb

γa

γb

, O2, H2O, CO (x10)


! General formalism applicable to most descriptions of surface chemistry

– Convenient symbolic representation of species and reactions

– Site dependent kinetics

multi-site expressions can describe storage phenomenaΓn(t) need not be conserved (ageing studies)

! Streamline code development– Code writers no longer concerned with managing detailed chemistry

– Rate equations decoupled from code

! A truly elementary description of a surface mediated process requires agreat deal of information– Computational chemistry may provide requisite information (heats of formation

or adsorption, other kinetic parameters)

– No reason no to try given current and future computational speeds

The Good and The Bad Challenging

problems involving complex heterogeneous reactions using ...surface chemkin and its history! a...

Documents