Constructing a Kinetics Database
in Heterogeneous Catalysis
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
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
2-Propose reaction steps
(1) Br2  2 Br
(2) Br + H2 = HBr + H
(3) H + Br2  HBr + Br
(-1) 2Br  Br2
aH 2 Br2 1 / 2
r
(1907)
b[ HBr ]
1{
}
Br2
3-Use QSSA. Derive rate equation from step 2 compare with 1
a = k2(k1/k-1)1/2, b = k-2/k3
Same general principle applied to Langmuir-Hinshelwood kinetics
Rate constants from NIST database 17
At 251.4 oC 1
6
Br2 
 2Br
k1  1.51x10
k2
Br+H 2 
 HBr  H
k 2  2.12x103 L/mol/s
k1
L/mol/s
H+Br2  HBr  Br k 3  5.08x10 L/mol/s
k3
10
k 2
H+HBr 
H 2  Br k -2  6.04x109 L/mol/s
k 1
2Br 
 Br2
1
k -1  3.69x109 L2 / mol 2 / s
Campbell and Fristrom, Chem. Rev., 58, 173 (1958)
HBr Production Rate/10-5mol L-1 s-1
Reaction rate simulation
2.4
Numerical Solution
QSSA Rate
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0
50
100
150
200
250
300
Time/min
Numerical solution is easy to implement, provides perfect fitting
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
Turnover frequency  Turnover rate
TOR = (Number of molecules reacted)
(Number of sites) x (time)
Allows for comparison of data in different
catalysts in different laboratories
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.
95(3), 661-6.
Chem. Rev. (1995),
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
NH3 activity/ arbitrary units
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/ %
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
16
T = 673 K
20 atm 3:1 H2:N2
12
10
2
moles NH3/cm -sec x 10
-9
14
The rate is
dependent on
surface structure
8
6
4
2
0
111
211
100
210
Surface Orientation
110
Strongin, D.R., Carrazza,
J., Bare, S.R., Somorjai,
G.A., J. Catal. 103, 213
(1987).
Practical catalysts are composed of
nanometer size particles
Rh
(111)
2 nm
(100)
SiO2 support
Profile view images of Rh/SiO2. Particles are nearly cubo octahedral and exhibit
(111) and (100) facets (from A. K. Datye U. New Mexico )
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
Ethylene Hydrogenation
Ribeiro et al. , Catal. Rev. - Sci. Eng.,39, 49-76, 1997
Some Problems with TOR
Measurements on Metals
Turnover rate for the
hydrogenolysis of ethane
on Pt at 473 K
The data are scattered!
"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
What to do for a tabulation?
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?
3. Compute the rate constants
Tabulate the rates constants and also the
output from the ab initio calculations
How to store the data?
R2
R3
R1
R2
O
R1
M
Kinetic Data
H2C
CH2
O
A
CH3
Chemical Structure
Catalyst Chemistry Model
time
B
Attributes
Molecular Conformation
••
•
{ ki }
Reaction
Network
A+B
C+D
B+E
••
•
HOMO
time
C
C
E
F
••
•
{D}
LUMO
Quantum Chemistry
Descriptor
Property Determination
From Data
time
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
Acknowledgments
• Department of Energy
Office of Science