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Pre-exponential factors

Using single-crystal adsorption calorimetry (SCAC), pre-exponential factors for first-order desorption, ν, can directly be evaluated at the steady state regime by equating the rate of adsorption to the rate of desorption [1]. Gas-surface systems include CO, O2, NO and hydrocarbons on flat and stepped Pt, Ni, Pd, Rh and Fe.

Within the conventional transition state theory (CTST) framework the pre-exponential is related to the activation entropy of desorption, i.e. the difference in entropy between the adsorbed state and the transition state. The figure below reports values of ν and entropy change as a function of the adsorption heat for room temperature molecular desorption at high coverages.

One can clearly see that values of ν show considerably more variation than always assumed: depending on the gas-surface system, we find values spanning from 1x1011 to 1x1022 s-1. In this respect, evaluations of the activation energy for first-order desorption using temperature-programmed desorption analyses may be severely flawed if a fixed value of ν, usually 1013 s-1, is chosen.

We interpret these unexpectedly high values of the pre-exponential factors in terms of desorption from highly localized initial states. The consequence is that desorption is accompanied by a large change in entropy. For more details the reader is referred to the article below.

Plot of log10 ν and the change in entropy ΔS# against the heat of adsorption, qd, for number of gas-surface systems studied by SCAC; data are all obtained at 300 K.

Related publications

"Surface Thermodynamics: Small Molecule Adsorption Calorimetry on Metal Single Crystals "
Vittorio Fiorin, David Borthwick, and David A. King
in "Model Systems in Catalysis: Single Crystals to Supported Enzyme Mimics", Edited by R. Rioux, Springer (2009)

"Microcalorimetry of O2 and NO on flat and stepped platinum surfaces"
V. Fiorin, D. Borthwick and D.A. King
Surf. Sci. 603 (2009) 1360

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Last updated 4/3/2010 by sjj24 -at-