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VlachosGroup / pMuTT

Python Multiscale Thermochemistry Toolbox (pMuTT)
https://vlachosgroup.github.io/pMuTT/
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Using G/RT vs. H/RT for adsorption reactions #146

Closed jonlym closed 4 years ago

jonlym commented 4 years ago

Currently, we recommend using the Gibbs energy of activation, ΔG_act, in write_EA and write_surf. This choice was based on book-keeping where the entropic effects from the pre-exponential were lumped into the activation energy. I believe this approach is fine for surface reactions.

However, we should probably use the enthalpy of activation, ΔH_act for adsorption reactions. We have observed adsorption reactions tend to be endergonic (ΔG > 0) due to the entropy difference between gas species and surface species. Since we tend not to specify a barrier, ΔG_act is set to ΔG.

This is problematic as increases in temperature stabilize the gas species more than surface species (due to larger entropy), resulting in larger ΔG and consequently larger ΔG_act. Physically, one would expect the rate of adsorption to increase with temperature as gases are colliding with the surface more frequently and with more energy. However, using ΔG_act results in a net decrease in the rate.

g_prof

This issue could be solved by using ΔH_act as adsorption is usually exothermic (ΔH < 0) so ΔH_act is set to 0.

jonlym commented 4 years ago

Implemented in commit 73a9dc8. The argument, ads_act_method, allows users to choose whether to use enthalpy of activation or Gibbs energy of activation.

jonlym commented 4 years ago

Reopening this issue since there is interest to implement this feature in pmutt.io.chemkin.write_surf