Chapter 19: Processes at Solid Surfaces

Modern analytical techniques including Auger electron spectroscopy and scanning tunneling microscopy enable direct visualization and compositional analysis of these surfaces at the atomic level. The chapter then explores adsorption, the spontaneous accumulation of molecules onto solid surfaces, describing this process through quantitative adsorption isotherms that relate surface coverage to pressure or concentration. The Langmuir isotherm provides a theoretical framework assuming uniform surface sites and the possibility of dissociative adsorption, while empirical models such as the Freundlich and Temkin isotherms accommodate real-world deviations from ideal behavior. The sticking coefficient quantifies the probability that an impinging molecule will remain bound to the surface rather than scatter away. A major application of adsorption principles is heterogeneous catalysis, where reactants bind strongly to the catalyst surface through chemisorption, which lowers the activation energy and accelerates the reaction. Surface reaction mechanisms are classified into two categories: the Langmuir-Hinshelwood pathway where both reactants adsorb before reacting, and the Eley-Rideal mechanism where one reactant adsorbs while the other approaches from the gas or solution phase. The final section addresses electrochemistry at solid electrode surfaces, treating electron transfer as an activated process governed by both thermodynamic and kinetic factors. The Butler-Volmer equation provides the quantitative relationship between electrode overpotential and current density, explaining how electrodes deviate from thermodynamic equilibrium during operation. Tafel analysis, based on graphical plots of current versus potential, extracts kinetic parameters and reveals reaction order and transfer coefficients. These principles directly underpin technologies including batteries, fuel cells, and photoelectrochemical solar devices.