This chapter looks at the extent to which molecules attach themselves to a surface,
which is crucial to understanding the way in which a surface influences chemical
processes. It discusses the extent of adsorption that can be explored with the aid
of some simple models that allow quantitative predictions to be made about how the
extent of surface coverage varies with both pressure and temperature. It also
demonstrates how surfaces can affect the rates of chemical reactions by assessing
the extent of surface coverage and the factors that determine the rates at which
molecules attach to and detach from solid surfaces. The chapter outlines the extent
of surface coverage that can be expressed in terms of isotherms derived on the basis
of dynamic equilibria between adsorbed and free molecules.
Chapter
Adsorption and desorption
Chapter
Insoluble monolayers and Langmuir-Blodgett films
This chapter reviews the adsorption at the gas-liquid interface of amphiphiles whose hydrophilic-lipophilic balance made them soluble in water. It discusses adsorbed films substances that have significant dynamic and equilibrium relationships with one of the bulk phases, noting that much information about the adsorbed films can be obtained from these relationships and from measurements on the bulk phase. It also examines amphiphiles with a stronger lipophilic or hydrophobic moiety that are less soluble in water and in the extreme are practically insoluble. The chapter cites water-insoluble amphiphiles that have special properties and are able to form insoluble films at the air-water interface. It analyses substances that may form monolayers that consist of molecules that are amphiphilic.
Book
The Basis and Applications of Heterogeneous Catalysis covers a wide range of topics within this field. Catalysis is one of the most important technologies in our modern world. We depend on it to produce materials, such as plastics, from oil; we depend on it to produce fuel to power our cars; we depend on it to remove the pollutants emitted from the engines of those cars; we even depend on it for the functioning and growth of our own bodies. It is therefore highly important that we ask ourselves the question, 'What is catalysis?' This book does exactly that, concentrating on the most important type of catalysis for industry, namely heterogeneous catalysis. The book is split into three sections, dealing with the fundamentals of adsorption and reaction at surfaces, the nature of heterogeneous catalysts and their synthesis, and the applications of this technology in the modern world.
Chapter
Thermodynamics
This chapter discusses the concept of surface free energy—the property which minimisation may be regarded as the prime mover of all surface processes—and the related concepts of surface tension (liquid surfaces) and surface stress (solid surfaces). In a liquid, surface tension takes the same value as the specific surface energy and is isotropic. In a solid, surface, stress may be anisotropic, and its components generally differ in value from the specific surface energy (i.e., the Shuttleworth equation). The chapter then explores the consequences of surface curvature, before considering the role of surfactants in modifying surface tension. Finally, the chapter addresses adsorption, discussing a number of common gas/solid isotherms and assessing how the heat of adsorption may be affected by interadsorbate lateral interactions. It explains Young's equation; the Young–Laplace equation; the Kelvin equation; Gibbs isotherm; Langmuir, Kisliuk, Brunauer–Emmett–Teller isotherms; and the Clausius—Clapeyron equation.
Chapter
Kinetics and Dynamics
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
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).
Chapter
The reactive interface
This chapter provides a background of catalysis, which is an extremely important phenomenon for the modern industrial economy. It is a process whereby a reaction occurs faster than the uncatalysed reaction, the reaction being accelerated by the presence of a catalyst. Industrial catalysis can be divided into two broad types: heterogeneous and homogeneous. Most large-scale, industrially catalysed processes are of the former type, and the widespread recent application of catalysis to car emission control uses such solid catalysts in contact with the gas phase exhaust stream. For these kinds of reaction, the nature of the interface is crucial for the efficiency of the process. The nature of the top layer of atoms determines how fast a catalytic reaction takes place and small amounts of additives can reduce or enhance the reaction. The chapter then details the catalytic cycle and the adsorption on surfaces.
Chapter
The effect of surface structure on reactivity
This chapter discusses the effect of the surface structure on reactivity. Since the nature of the surface involved in heterogeneous catalysis is crucial to performance, it is important to consider the structure as a starting point for the discussion of catalytic properties. Generally speaking, surfaces with lower coordination surface atoms have the highest surface free energy, the highest reactivity for adsorption, and the strongest binding for the adsorbate (high adsorption heat). Because the surface is a region of high energy, thermodynamics dictates that there will be a tendency to minimise this energy. In nature, this occurs in several ways, including surface relaxation, surface reconstruction, sintering, and adsorption. The chapter then considers the surface structure of catalysts, as well as the structure dependence and independence of catalytic reactions. The best evidence for the influence of structure on a reaction comes from studies using model catalysts, namely single crystals.
Chapter
Catalytic activity and selectivity
This chapter focuses on catalytic activity and selectivity. The most important feature of a catalyst is how well it does the job of converting reactants into products, and this is measured by two parameters: activity and selectivity. The activity of a catalyst is defined as the rate of consumption of reactant, whereas selectivity is the fraction of the total products which a particular substance represents. The chapter then looks at the catalytic parameters, including adsorption, simple reaction, bimolecular reactions, and temperature dependence of the rate. In general, more than one product is formed from any particular reaction, and only one of these is the desired product. The objective of the catalytic scientist, then, is to optimise the reaction for that desired product through the appropriate catalytic design. The chapter also examines forces on surfaces; weakly held states and surface diffusion; and surface dependence of reaction rates and the volcano principle.
Chapter
Adsorption and the thermodynamics of surfaces
This chapter provides a loose interpretation of adsorption, which is the tendency for one component of a system to have a higher or lower concentration at the interface than it has in the adjacent bulk phases. It highlights an example wherein it observes a decrease in pressure when a gas is adsorbed onto a solid, or a decrease in solute concentration when a solution is shaken with a solid powder. It also points out how adsorption can be inferred from other information, such as a change in the surface tension of the solution. The chapter talks about the three regions of a system containing an interface: bulk phase α, bulk phase β, and the interface σ. It looks at the surface phase approach wherein a distinct surface phase is defined by two mathematical boundaries, one on either side of and parallel to the interface.
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.
Chapter
Heterogeneous catalysis
This chapter studies heterogeneous catalysis, in which the catalyst and the reagents are
in different phases. A common example is a solid that increases the rate of a gas-phase
reaction. The solid provides a surface to which the reactants bind and so facilitates
encounters between reactants. In general, heterogeneous catalysts are highly selective and
to find an appropriate catalyst each reaction must be investigated individually. Two
techniques have revolutionized the study of surfaces in recent years: scanning tunnelling
microscopy (STM) and atomic force microscopy (AFM). Ultimately, the key to the operation of
a heterogeneous catalyst is the attachment of molecules to a surface by the process called
adsorption. The chapter then differentiates between physisorption and chemisorption, before
looking at adsorption isotherms. It also considers the mechanisms of surface-catalysed
reactions, including the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism.
