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, O_{2}, 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 1x10^{11} to 1x10^{22} 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 10^{13} 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 log_{10} ν and the change in entropy ΔS^{#} against the heat of adsorption, q_{d}, 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 O_{2} and NO on flat and stepped platinum surfaces"
V. Fiorin, D. Borthwick and D.A. King
Surf. Sci. 603 (2009) 1360
Last updated 4/3/2010 by sjj24 -at- cam.ac.uk