1. Degree of rate control:
How Much the Energies of Intermediates and
Transition States Control Rates
Carsten Stegelmann, Anders Andreasen and Charles T. Campbell
J. Am. Chem. Soc., 2009, 131 (23), pp 8077–8082
DOI: 10.1021/ja9000097
Copyright © 2009 American Chemical Society
Presentation
by Anders Andreasen
Department of Chemistry and Applied Engineering Science, Aalborg University, Denmark, Materials Research Department,
National Laboratory for Sustainable Energy, Risø DTU, Denmark, and Department of Chemistry, University of
Washington, Seattle, Washington 98195-1700

2. Degree of rate control
 In (micro) kinetic modelling a detailed reaction mechanism is used to
model the conversion from reactants to products
 However, a vast amount of parameters need to be estimated (Surface
science, rate theory, DFT, reactor measurements)
 Often only a few reaction steps control the overall rate of reaction
 How do we pinpoint the important parameters and ways to
increase the reaction rate?
Andreasen et al., Surf Sci, 544 (2003) 5-23
Wachs, Surf Sci, 544 (2003) 1-4
Andreasen et al., Appl Catal, 289 (2005) 267-273
Lynggaard et al., Prog Surf Sci, 77 (2004) 71-137

3. Degree of rate control
 Campbell proposed a so-called degree of rate control [1,2]
 Dumesic proposed a so-called sensitivity, similar to Campbell's
definition – although very different at the same time [3]
 Campbell's definition corresponds to changing the energy of the
transition state – Dumesic's definition somewhat unclear
 We needed a clear-cut definition of rate control also including
the thermodynamics of all reaction intermediates (not only TS).
[1] Campbell, C. T. Top. Catal. 1 (1994) 353
[2] Campbell, C. T. J. Catal. 204 (2001) 520
[3] R. D. Cortright, J. A. Dumesic, Adv. Catal. 46 (2001) 161

4. Degree of rate control
 Example PES
k fwd ,i k
 Principle of microscopic reversibility: k rev ,i = ⇔ K i= fwd ,i
Ki k rev , i
 Overall equilibrium constant: K tot = ∏ K i

5. Degree of rate control
 Introducing the degree of thermodynamic rate control
 Analogy with Campbell's degree of rate control
 Simple relation to surface coverage of intermediates
 Catalyst improvement: stabilise key transition states without
stabilising key intermediateds too much or vice versa in agreement
with the priciple of Sabatier
 Improvements are difficult due to the BEP [1] effect but possible [2]
 Parameter estimation: look for intermediates with non-zero coverage
[1] Nørskov et al., J. Catal. 209 (2002) 275. [2] Greeley & Mavrikakis, Nat. Mater. 3 (2004) 810.

6. Summary
 A general ”degree of rate control” has been proposed
probing the entire PES
 Thermodynamic consistensy is achieved
 General applicability not only to heterogeneous catalysis
 Extendable to ”degree of selectivity control” [1]
 Many more examples will be given [1]
[1] Stegelmann & Andreasen, In prep., 2009