Imaging the way molecules desorb from catalysts
We have investigated oxygen on Ag(111) combining structure sensitive electron microscopy with TPD into in the low-energy electron microscope (LEEM). Our approach allowed us to correlate the TPD spectrum to the evolution of the O-adlayer morphology, formulating a rate law to quantitatively reproduce the observed desorption kinetics. S. Günther, et al. Nat. Comm. 5, 3853 (2014);
Unlike surface catalytic reactions, desorption has been thought as a relatively simple process consisting of a series of statistically independent events randomly and uniformly occurring over the surface: in this picture adsorbates take off as soon as their thermal energy exceeds the binding energy. Following Irving Langmuir’s mean-field treatment, the rate of recombinative desorption of adsorbed particles is thought to be proportional to the square of the coverage. However, several measurements contradicted this model. As a matter of fact, temperature-programmed desorption (TPD), the standard method for determining desorption kinetics, hardly ever shows pure Langmuir behavior. The exponent m in the desorption rate law, referred to as desorption order, is often a fractional number rather than an integer (m = 1 or 2); further, the desorption peaks in TPD exhibit unexpected widths or symmetries or are split into multiple peaks. |
In such cases, the desorption mechanism cannot be uniquely understood from TPD data. Instead, a detailed characterization of the complex microscopic mechanisms occurring into the adlayer is instead needed. In order to shed light on the microscopic origin of desorption, we have investigated oxygen on Ag(111) combining structure sensitive electron microscopy with TPD. Retrieve article
Desorption kinetics from a surface derived from direct imaging of the adsorbate layer; |