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? H 2 + Br 2  2 HBr Method of study: 1-Determine the rate equation experimentally (1907) 2-Propose reaction steps (1) Br 2  2 Br (2) Br + H 2 = HBr + H (3) H + Br 2  HBr + Br (-1) 2Br  Br 2 Same general principle applied to Langmuir-Hinshelwood kinetics 3-Use QSSA. Derive rate equation from step 2 compare with 1 a = k 2 (k 1 /k -1 ) 1/2, b = k -2 /k 3

Rate constants from NIST database 17

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),

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 d-band occupancy/ % NH 3 activity/ arbitrary units

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 Strongin, D.R., Carrazza, J., Bare, S.R., Somorjai, G.A., J. Catal. 103, 213 (1987).

Profile view images of Rh/SiO 2. Particles are nearly cubo octahedral and exhibit (111) and (100) facets (from A. K. Datye U. New Mexico ) (111) (100) SiO 2 support Rh 2 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?

3. Compute the rate constants Tabulate the rates constants and also the output from the ab initio calculations How to store the data?

Chemical Structure Molecular Conformation HOMO LUMO Quantum Chemistry Descriptor Property Determination From Data Kinetic Data A B C time { k i } { D } Attributes A + B C Reaction Network C + D E B + 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