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

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

"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

Acknowledgments• Department of Energy

Office of Science