This chapter explores the fundamental processes that occur at surfaces—adsorption, desorption, vibration, and reaction—understanding them as ongoing events taking place during the passage of time, as opposed to historical facts to be explained after time has elapsed. In so doing, it addresses both kinetics (variation of macroscopic system properties) and dynamics (microscopic motion of atoms and molecules) to present a 'moving picture' of the surface as a place of constant agitation and continual change. A full description of adsorption dynamics should be multidimensional, but one- and two-dimensional approximations may yet provide useful insights if treated with caution. Meanwhile, the kinetic order of desorption may be established by analysis of peaks obtained in a temperature-programmed desorption (TPD) experiment. The chapter also considers the Brønsted–Evans–Polanyi relation and looks at how quantised surface-localised vibrations (surface phonons) of various types may be observed at frequencies disallowed in the bulk.
Chapter
Kinetics and Dynamics
Chapter
Adsorption and desorption
This chapter examines adsorption and desorption. There are several possible outcomes when an atom or a molecule hits a surface, including elastic and inelastic scattering. However, the outcome of an atomic or a molecular collision that results in the retention of the molecule on the surface is of far greater importance to the study of surface chemistry. There are two types of interactions that can occur: physical adsorption (or physisorption) and chemisorption. In each case, the atom or molecule being adsorbed on the surface is usually described as the adsorbate; the adsorbing surface is usually termed the adsorbent or substrate. The chapter then looks at adsorption isotherms, before considering the measurement of heats of adsorption, isosteres, and desorption rates. It also discusses adsorption sites and geometries. Finally, the chapter highlights two methods which can provide information on surface chemical composition: auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS).
Book
Elaine M. McCash
Surface Chemistry conveys the fundamental concepts of surface chemistry. It describes solid surfaces, their properties at macroscopic and microscopic levels and their interrelation, and reflects the striking advances made in recent years through the study of well-defined single crystal surfaces. It begins with a discussion of the clean surface, its electronic and structural properties and goes on to describe adsorption, desorption, reactions, and reactivity at the surface. In the final section, the growth and properties of ultrathin films is introduced. Starting with the established concepts in terms of kinetics and thermodynamics, the book develops to look at phenomena such as surface dynamics and photochemistry. Important techniques which are applied to surfaces are also covered; this is a concept-driven rather than technique-driven approach.
