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University of Virginia
Department of Chemistry
409 McCormick Road
P.O. Box 400319
Charlottesville, VA 22904-4319
Phone: (434)924-3639/3654
Fax: (434)924-3966

email : Prof. Ian Harrison

 

   Microcanonical Unimolecular Rate Theory (MURT)
Graduate Students: Scott Donald
Research Overview

We have developed a “local hot spot” model of gas-surface reactivity based on RRKM (Rice-Ramsperger-Kassel-Marcus) transition state theory. The model handles both equilibrium and non-equilibrium (e.g., molecular beam or photochemically energized) reactions. MURT makes the following key assumptions:

  1. Surface chemistry is a local phenomenon.
  2. The energy of the adsorbate plus “ s ” surface oscillators (i.e., the “physisorbed complex” [PC]) suffices to fix RRKM rate constants for reaction and desorption.
  3. Energy exchange between the physisorbed complex and the bulk may be accounted for using a Master Equation approach.
We treat the system adiabatically in the simplest physisorbed complex version of the theory (PC-MURT) because thermalization between a PC and the surrounding surface is relatively slow in comparison to desorption rates at reactive energies. Sticking coefficients are calculated by averaging the microcanonical sticking coefficient,
where ki are the RRKM rate constants and Wi‡ are the sum of states for reaction (R) and desorption (D), over the flux distribution for creating a PC at total energy E * ,
Typically, just three adjustable parameters must be determined by iteratively simulating a particular experimental data set and optimizing the average relative discrepancy (ARD) between theory and experiment.
 

 

FIG 1. Schematic depiction of the kinetics and energetics of activated dissociative chemisorption. Zero point energies are implicitly included within the potential energy curve along the reaction coordinate

The model for surface reactivity was developed to handle both equilibrium and non-equilibrium reactions (e.g., molecular beam studies of activated dissociative chemisorption or surface photochemistry). Our initial papers numerically implemented this statistical model with several approximations in the interests of timely computation:

V.A. Ukraintsev and I. Harrison, "A Statistical Model of Activated Dissociative Adsorption: Application to Methane Dissociation on Pt(111)", J. Chem. Phys. 101, 1564-1581 (1994) [2.2 Mb pdf]

I. Harrison, "Photochemical Exploration of Reaction Dynamics on Catalytic Metal Surfaces: From Ballistics to Statistics", Acc. Chem. Res. 31, 631-639 (1998) [144 Kb pdf]

More recently, we've refined the model, implemented it in numerically exact fashion, introduced the possibility of vibrational energy exchange with the surface bulk through a Master Equation approach, and tested the model against several important experimental and theoretical benchmark systems:

A. Bukoski, D. Blumling, and I. Harrison, "Microcanonical Unimolecular Rate Theory at Surfaces. I. Dissociative Chemisorption of Methane on Pt(111)", J. Chem. Phys. 118, 843-872, (2003) [1.3 Mb pdf]

A. Bukoski, and I. Harrison, "Assessing a Microcanonical Theory of Gas-Surface Reactivity: Applicability to Thermal Equilibrium, Non-Equilibrium, and Eigenstate-Resolved Dissociation of Methane on Ni(100)", J. Chem. Phys. 118, 9762-9768 (2003) [180 Kb pdf]

H.L. Abbott, A. Bukoski, D.F. Kavulak, and I. Harrison, "Dissociative Chemisorption of Methane on Ni(100): Threshold Energy from CH4(2v3) Eigenstate-Resolved Sticking Measurements", J. Chem. Phys. 119, 6407-6410 (2003) [91 Kb pdf]

H.L. Abbott, A. Bukoski, and I. Harrison, "Microcanonical Unimolecular Rate Theory at Surfaces. II. Vibrational State Resolved Dissociative Chemisorption of Methane on Ni(100)", J. Chem. Phys. 121, 3792-3810 (2004) [317 Kb pdf]

D.F Kavulak, H.L. Abbott, and I. Harrison, "Nonequilibrium Activated Dissociative Chemisorption: SiH4 on Si(100) ", J. Phys. Chem. B 109, 685-688 (2005) [82 Kb pdf]

H. L. Abbott and I. Harrison, "Dissociative Chemisorption and Energy Transfer for Methane on Ir(111)," J. Phys. Chem. B 109, 10371-10380 (2005) [214 kB pdf]

A. Bukoski, H. L. Abbott, and I. Harrison, "Microcanonical Unimolecular Rate Theory at Surfaces. III. Thermal Dissociative Chemisorption of Methane on Pt(111) and Detailed Balance", J. Chem. Phys. 123, 094707 [18 pgs] (2005) [331 kB pdf ]

H. L. Abbott and I. Harrison, "Seven-dimensional microcanonical treatment of hydrogen dissociation dynamics on Cu(111): Clarifying the essential role of surface phonons," J. Chem. Phys. 125, 024704 [14 pgs] (2006) [1.3 MB pdf ]

H. L. Abbott and I. Harrison, "Activated Dissociation of CO2 on Rh(111) and CO Oxidation Dynamics," J. Phys. Chem. C. 111, 13137-13148 (2007) [311 kB pdf ]

H. L. Abbott and I. Harrison, "Microcanonical Transition State Theory for Activated Gas-Surface Reaction Dynamics: Application to H2/Cu(111) with Rotation as a Spectator," J. Phys. Chem. A. 111, 9871-9883 (2007) [624 kB pdf ]

H. L. Abbott and I. Harrison, "Methane Dissociative Chemisorption on Ru(0001) and Comparison to Metal Nanocatalysts" J. Catal. 254, 27-38 (2008) [link; 358 kB pdf]

New experimental techniques designed to extract transition state and energy transfer characteristics for activated gas-surface reactions based on MURT analysis are being developed, e.g.:

K. M. DeWitt, L. Valadez, H. L. Abbott, K. W. Kolasinski, I. Harrison, "Using Effusive Molecular Beams and Microcanonical Unimolecular Rate Theory to Characterize CH4 Dissociation on Pt(111)," J. Phys. Chem. B. 110, 6705-6713 (2006) [189 kB pdf]

K. M. DeWitt, L. Valadez, H. L. Abbott, K. W. Kolasinski, I. Harrison, "Effusive Molecular Beam Study of C2H6 Dissociation on Pt(111)," J. Phys. Chem. B. 110, 6714-6720 (2006) [119 kB pdf]

Some Recent Presentations:

L’Université Pierre et Marie Curie, Laboratoire de Réactivité de Surface, “Connecting Small Molecule Reaction Dynamics to Catalysis”, Paris, France, November 22, 2007 [1.8 Mb ppt]

 
This material is based upon work supported by the National Science Foundation, the Department of Energy, and the American Chemical Society's Petroleum Research Foundation.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF, DOE, or ACS PRF.
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