Accelerated Molecular Dynamics Simulation of Thermal Desorption

Open Access
Author:
Becker, Kelly E.
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 07, 2008
Committee Members:
  • Kristen Ann Fichthorn, Committee Chair
  • Seong H Kim, Committee Member
  • Janna Kay Maranas, Committee Member
  • Milton Walter Cole, Committee Member
Keywords:
  • accelerated molecular dynamics
  • temperature programmed desorption
  • simulation
  • desorption
  • alkanes
Abstract:
Desorption is a process ubiquitous in phenomena involving surfaces. However, it has rarely been simulated on the molecular level. Molecular dynamics simulation can provide the atomic-level detail necessary to study desorption on the molecular level but has limitations on the time scales that can be simulated. We present a set of accelerated molecular dynamics methods that can be used to simulate desorption of alkanes from the basal plane of graphite (a process that occurs over the time scale of seconds or minutes) with atomic level detail. Single molecule simulations, accelerated by increasing the temperature of the simulation, shed light on conformational changes that occur in alkane molecules on the graphite surface. These temperature accelerated simulations are compared to a compensating potential method for the single molecule so that the simulations can be extended to finite coverage. We present finite coverage simulations for the desorption of pentane from the basal plane of graphite. Desorption environments are characterized, and the presence of entropy-driven desorption is discovered. Ultimately, the accelerated simulations are extended to simulate temperature-programmed desorption experiments. We find that analyzing the results as in experimental literature fails to predict some of the features in the temperature programmed desorption profiles. Suggestions are made for further characterization of the desorption process.