(754b) Surface Reactivity From First Principles: Modeling Temperature Programmed Desorption of Oxygen From Platinum Surfaces

Adsorbate coverage is known to affect the binding strength of adsorbates and their subsequent reactivity on surfaces.  Surface reactivity is a macroscopically observable property stemming from an ensemble average of numerous microscopic reaction events, where each individual reaction and activation energy depend strongly on the local environment surrounding the reaction site.  We are developing a unique computational approach¹ incorporating the effects of local adsorbate configurations, coverage, and temperature to more reliably predict desorption rates as observed in experiments such as temperature programmed desorption (TPD).  Here we apply this approach to predicting O₂ desorption rates from Pt surfaces and the corresponding TPD spectra.

We capture coverage effects originating from interactions between nearby adsorbates on the surface by using DFT simulations to parameterize Ising-type cluster expansion (CE) models of O on the Pt surface.  These adsorbate-adsorbate interactions influence the availability and reactivity of “reaction sites”, and we are able to apply the CE with grand canonical Monte Carlo (GCMC) in novel ways to predict reaction rates.  Conducting simulations under various temperature and coverage conditions ultimately allows us to generate predictions of TPD spectra.

We demonstrate the capabilities of our reactivity model by comparing predicted TPD spectra with experimental results² of oxygen desorption from Pt surfaces.  These results represent major progress in reliably predicting macroscopic observables from atomic-scale simulations, reproducing the major features of the observed TPD.  We are also able to identify key physical phenomena that correspond to different spectral features and thereby increase the fundamental understanding of desorption processes.

References:

(1)   Wu, C.; Schmidt, D. J.; Schneider, W. F. J. Catal. 2012, 286, 88.

(2)   Parker, D. H.; Bartram, M. E.; Koel, B. E. Surf. Sci. 1989, 217, 489.