F. H. Ribeiro, J. M. Caruthers, W. N. Delgass, K. T. Thomson, V. Venkatasubramanian Dept. of...
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Transcript of F. H. Ribeiro, J. M. Caruthers, W. N. Delgass, K. T. Thomson, V. Venkatasubramanian Dept. of...
F. H. Ribeiro, J. M. Caruthers, W. N. Delgass, K. T. Thomson, V. Venkatasubramanian
Dept. of Chemical Engineering, Purdue University
NSF Workshop, Washington, April 19-21, 2004
Constructing a Kinetics Database in Heterogeneous Catalysis
Outline
• Analogy with homogeneous reactions
• Definitions and concepts in heterogeneous catalysis
• Challenges in setting a data base
• Suggestions
Catalysis is a kinetic phenomenon
How to study kinetics?H2 + Br2 2 HBr
Method of study:1-Determine the rate equation experimentally
}
][{1
2
2/122
Br
HBrbBrHa
r
(1907)
2-Propose reaction steps
(1) Br2 2 Br(2) Br + H2 = HBr + H(3) H + Br2 HBr + Br(-1) 2Br Br2
Same general principle applied to Langmuir-Hinshelwood kinetics
3-Use QSSA. Derive rate equation from step 2 compare with 1
a = k2(k1/k-1)1/2, b = k-2/k3
1
2
3
2
o 1
k 62 1
k 32 2
k 102 3
k2
At 251.4 C
Br 2Br k 1.51x10 L/mol/s
Br+H HBr H k 2.12x10 L/mol/s
H+Br HBr Br k 5.08x10 L/mol/s
H+HBr H
1
9-2
k 9 2 22 -1
Br k 6.04x10 L/mol/s
2Br Br k 3.69x10 L / mol / s
1Campbell and Fristrom, Chem. Rev., 58, 173 (1958)
Rate constants from NIST database 17
0 50 100 150 200 250 300
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
HB
r P
rodu
ctio
n R
ate/
10-5m
ol L
-1 s
-1
Time/min
Numerical Solution QSSA Rate
Numerical solution is easy to implement, provides perfect fitting
Reaction rate simulation
Kinetic modeling
• Can calculate rates, coverages, selectivities at any condition, even if the rds changes
• There is no equation
But it needs the rate constants!
Help needed from Surface science
Theoretical methods
The key tool for analysis: TURNOVER RATES
TOR = (Number of molecules reacted)
(Number of sites) x (time)
Allows for comparison of data in different catalysts in different laboratories
Turnover frequency Turnover rate
Problems with defining a site*
• What is a site?
• How do we count them?
• If they are not the same, what is the value of reporting a TOR (Langmuirian kinetics)?
• Need to be reported under reaction conditions.
*Boudart, M. Turnover Rates in Heterogeneous Catalysis. Chem. Rev. (1995), 95(3), 661-6.
Reasons for using a TOR
• A method to compare catalyst performance, including the ones from different laboratories
• Study the influence of the arrangement of atoms on rates (structure sensitivity)
• It is a method to quantify heterogeneous catalysis (not perfect)
Issues on developing a database in heterogeneous catalysis
Reaction mechanism changes with nature of surface
Variation of rates on ammonia synthesis
Ozaki, A. and K. Aika, Catalytic Activation of Dinitrogen, in Catalysis: Science and Technology, J.R. Anderson and M. Boudart, Editors. 1981, Springer Verlag: New York. p. 87-158.
d-band occupancy/ %
NH
3 ac
tivi
ty/
arb
itra
ry u
nit
s
Issues on developing a database in heterogeneous catalysis
Reaction mechanism changes with nature of surface
– Not only with the nature of metal but also surface arrangement
The rate is dependent on
surface structure
0
2
4
6
8
10
12
14
16
111 211 100 210 110
Surface Orientation
mo
les
NH
3/c
m2-s
ec
x 1
0-9
T = 673 K20 atm 3:1 H2:N2
0
2
4
6
8
10
12
14
16
111 211 100 210 110
Surface Orientation
mo
les
NH
3/c
m2-s
ec
x 1
0-9
T = 673 K20 atm 3:1 H2:N2
0
2
4
6
8
10
12
14
16
111 211 100 210 110
Surface Orientation
mo
les
NH
3/c
m2-s
ec
x 1
0-9
T = 673 K20 atm 3:1 H2:N2
Strongin, D.R., Carrazza, J., Bare, S.R., Somorjai, G.A., J. Catal. 103, 213 (1987).
Profile view images of Rh/SiO2. Particles are nearly cubo octahedral and exhibit (111) and (100) facets (from A. K. Datye U. New Mexico )
(111)
(100)
SiO2 support
Rh2 nm
Practical catalysts are composed of nanometer size particles
Issues on developing a database in heterogeneous catalysis
Reaction mechanism changes with surface nature
– Not only with the nature of metal but surface arrangement – Rate constants are a function of coverage
Rate constant value depends on coverage
Heat of adsorption of CO on Pt(111)
Yeo, Y.Y. , Vattuone, L. and King, D.A., J. Chem. Phys. 106 (1), 392, 1997
Issues on developing a database in heterogeneous catalysis
Reaction mechanism changes with surface nature
– Not only with the nature of metal but surface arrangement– Rate constants are a function of coverage
The situation is not hopeless
Surfaces are non-uniform but in practice surface coverage varies in a limited range. What is needed?
•In many examples a simple power rate law
•In more involved cases, a full kinetic analysis
•In rare cases, Monte Carlo analysis
What to do for a tabulation?
1.Tabulate turnover rates and reaction orders
Useful and easy to use
Problems:• Limited approach. Would have to tabulate reactions at all conditions• Data scatter
Some Problems with TOR Measurements on Metals
Turnover rate for the hydrogenolysis of ethane on Pt at 473 K
"Reproducibility of Turnover Rates in Heterogeneous Metal Catalysis: Compilation of Data and Guidelines for Data Analysis" F.H. Ribeiro, A.E. Schach von Wittenau, C.H. Bartholomew, and G.A. Somorjai, Catal. Reviews - Sci. and Eng. , 39, 49, 1997
The data are scattered!
2. Tabulate rate constants
Compact and thus can solve more problems
Problems:
Availability of data
Kinetic analysis suite not available
What to do for a tabulation?
What to do for a tabulation?
3. Compute the rate constants
Tabulate the rates constants and also the output from the ab initio calculations
How to store the data?
O
MO
CH3
R1
R2
R3
R2
R1
CH2
CH2
Chemical Structure
Molecular Conformation
HOMO
LUMO
Quantum Chemistry
Descriptor
Property DeterminationFrom Data
Kinetic Data
• • •
A
B
C
time
time
time
{ ki }
{ D }
Attributes
• • • A + B C
ReactionNetwork
• • •
C + D EB + E F
Catalyst Chemistry Model
Recommendations
• Make available to the community a robust software that will allow for kinetic modeling with rate constants
• Encourage the archival on a national database of:– Turnover rates– Rate constants experimentally determined– Rate constant calculations and wave
functions