A New Approach of Kinetic Modeling: Kinetically Consistent Energy Profile and Rate Expression Analysis

Abstract

We report a scaling relation-based L-H kinetic model to assess reaction rate, where general principles for making a kinetically consistent free energy profile are suggested. Water gas shift, Fischer-Tropsch synthesis, and steam methane reforming are employed as three model reactions to illustrate this approach. Herein, potential energies and free energies are calculated by improved scaling relations. The different methods to construct energy profiles are compared and free energy diagrams rather than potential energy diagrams are more favorable to be employed in the field of reaction mechanism discriminations and kinetics investigations due to consisting of thermodynamic information. The sequence of various elementary steps is important to generate appropriate rate determining step and rate expression. It is suggested that the sequence of the elementary step follows a principle that reaction step is set just before it is required, and desorption occurs just after the formation of the intermediates. Based on the procedure elucidated here, it is easy to get turnover frequency straightforwardly from the free energy diagram of reaction, and the surface site coverages from free energy diagram of intermediates. The scaling relation-based L-H kinetic model exhibits a similar accuracy for predicting the surface coverages and reaction rates with a full microkinetic modeling toward all the three model reactions. It demonstrates that the approach proposed herein greatly simplifies the calculation but maintains the high accuracy as compared to a full microkinetic modeling. This approach can be widely applied for kinetic modeling to elucidate reaction mechanism and kinetics for chemical processes of interest.

Keywords: Scaling relation-based L-H kinetic model, Water gas shift, Fischer-Tropsch synthesis, Steam methane reforming, Free energy diagram, Reaction rate