f. h. ribeiro, j. m. caruthers, w. n. delgass, k. t. thomson, v. venkatasubramanian dept. of...
TRANSCRIPT
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