Surface chemistry

Type of physical science: Chemistry

Field of study: Chemical reactions

Surface chemistry is the study of chemical processes that occur in the vicinity of surfaces. The majority of both man-made and naturally occurring objects that exist in either the liquid or solid phase have surfaces and, thus, such studies have an unlimited number of applications.

Overview

Surface chemistry is the study of those chemical processes that take place on or in the neighborhood of a surface, which is usually also an interface between two phases (for example, a solid and a liquid). In order to realize the importance of surface processes, one has only to consider the human brain. The brain of a human has a volume approximately seven times that of an ape, whereas the surface area is approximately ten times greater. A good argument can be presented that supports the idea that it is the processes that occur at the surfaces of the brain that allow humans to consider themselves superior to the ape.

Numerous objects exist in nature in which the surface-to-volume ratio is large. (A value of one is the largest possible value and corresponds to the case in which all molecules are at the surface.) Biological systems provide many examples. Membranes have large surface-to-volume ratios, and processes occurring at the surfaces of membranes are fundamental in regard to the function of, for example, cells and organs. The discussion presented here is focused more on the chemical reaction processes that occur at solid surfaces. Chemical researchers and industry have learned to use such surfaces to speed up chemical reactions so that they occur at relatively faster rates.

It is required that either a solid or a liquid be present in a system under study in order to have a surface. The forces acting between the molecules (or atoms) of the adjacent phase and the molecules in the surface (for example, between molecules in a gas and molecules in a solid surface) determine the way in which the surface and the adjacent phase interact. As in most molecular interactions, it is the shape and character of the electron clouds around the different molecules that determine the forces between them. In general, surface chemistry can be divided into five areas: solid-gas, solid-liquid, liquid-liquid, liquid-gas, and solid-solid interfaces. The discussion presented here, however, is related more to the surface chemistry of solid-liquid interfaces and solid-gas interfaces. This does not mean that the chemistry associated with the other interfaces is unimportant. For example, the chemistry of solid-solid interfaces determines the mechanical properties of solids, and such interfaces are important in the manufacture of solid-state devices.

An important chemical surface process occurs in the catalytic converter of an automobile. The exhaust that moves through the pipe that extends from the exhaust manifold, which is located near the automobile's engine, contains carbon monoxide, nitric oxide, and oxygen. This exhaust enters the catalytic converter, where chemical reactions occur on the surface of beads that convert much of the exhaust into carbon dioxide and nitrogen gas.

Eventually, the treated exhaust exits through the tail pipe. It is important to note that the molecules and atoms that comprise the surface of the beads remain there and do not become part of the products of the reaction.

In order to expand on the fundamental concept that led to the design of catalytic converters, some startling observations are reviewed from experiments. To do this, consider a closed vessel that contains a two-to-one mixture of hydrogen gas (H₂) and oxygen gas (O₂); that is, there are twice as many hydrogen gas molecules as there are oxygen gas molecules. The potential for the gases to react and form water is high. In fact, it can be shown that the state obtained after the reaction in which only water is present in the vessel is much more stable than the state in which the vessel contains only oxygen and hydrogen gases. Nevertheless, the vessel containing the two-to-one mixture of gases will remain as an apparent stable state for an indefinitely long time. On the other hand, if a platinum surface is located in the vessel, the reaction producing water occurs rapidly. At the end of the reaction, the platinum surface essentially remains unchanged. The results of such experiments represent a fascinating component of chemistry. Yet, the results are not at all magical. The platinum surface acts as a catalyst for the reaction; a catalyst is a substance that increases the speed at which a reaction occurs without being consumed by the reaction. More specifically, the platinum surface is classified as a heterogeneous catalyst because it is in a phase (a solid) different from the reactants (gases). The reaction between oxygen and hydrogen provides an example of why a catalyst is necessary and gives an insight into how a catalyst works. First of all, it is not very surprising that the mixture of oxygen gas and hydrogen gas apparently remains stable. The chemical bonds in hydrogen gas and oxygen gas are strong. Although the transformation to water through a chemical reaction is favored, these chemical bonds must be broken in order for the reaction to proceed. The breaking of these bonds in the absence of the platinum surface takes so long that, from a practical point of view, the reaction does not proceed at all. When a platinum surface is present, hydrogen and oxygen gas molecules stick to the surface. The interaction with the platinum atoms breaks the bonds in the gas molecules, producing hydrogen and oxygen atoms.

These atoms are much more reactive; they can react with the gas molecules and with other atoms.

The role of the platinum surface is to provide additional reaction pathways.

How does the platinum surface provide additional reaction pathways? In order to obtain some insight into this question, properties of surfaces should be analyzed. One might be tempted to think that, when a gas or liquid is in the presence of a solid surface, the only interactions between the two phases involve molecules (or atoms) approaching the surface and the deflection of the molecules by the surface through collisions. Nevertheless, there is always a force of attraction between an approaching molecule and the solid surface. Consequently, there is a finite probability that the molecules will become trapped on the surface. The process by which an atom or molecule becomes trapped on a surface is called adsorption. Heat is released through this process, and thus it is known that the system, after the molecules are adsorbed, is in a lower energy state. The amount of heat released, which can be measured and is called the heat of adsorption, reveals the relative strength of the attractive force. The fact that molecules are adsorbed is witness to the fact that the driving force that pushes the system to a lower energy state is usually very strong because of the forces of attraction between the atoms or molecules of the solid surface and the molecules in the gas or liquid phase.

An atom or molecule adsorbed on the surface is in a microscopic state that is different from the state it was in when it was a surrounding gas or liquid phase; that is, the electron cloud of the atom or molecule can be drastically different. Consequently, the interaction between other species in the phase surrounding the surface is also quite different, and this new interaction can have a considerably higher probability of leading to a chemical reaction.

Additional properties of surfaces that are helpful in the understanding of adsorption are roughness and porosity. If one uses the eye to judge roughness, a surface may look very smooth.

Yet, after magnification to the molecular level, a surface will appear very rough even though it appears very smooth at the macroscopic level. An atomic model of a surface includes the presence of several different types of sites. The surface contains terraces, flat areas, and steps that lead from one terrace to either a lower or a higher terrace. The steps very seldom form straight lines, but rather have indentations called kinks. Other sites exist on a surface called detects, which include adatoms (atoms lying on top of a terrace) and vacancies (atoms missing from the flat area of a terrace). The environment of each type of site is unique. For example, a molecule (or atom) contained in a terrace will have the most molecules surrounding it, a molecule located in a step will have fewer neighbors, but more than a molecule in a kink. An adatom will ordinarily have the least number of neighbors. The different sites will interact differently with molecules of a different phase in the vicinity of the surface. The interaction with molecules of the surrounding phase might be strong enough that chemical reactions occur at some sites, but not others. Chemical products from reaction at one site may react at a different site.

Even if a surface appears to be smooth and dense, it is, to some degree, porous. For example, hydrogen atoms are known to penetrate the surfaces of palladium and platinum. When this occurs, the process is called adsorption. This phenomenon is known to affect catalytic properties of the surface and, hence, the outcome of catalytic reactions, especially in the case of platinum and palladium surfaces, reactions in which hydrogen is either added to or subtracted from a molecule.

Two kinds of adsorption can occur. One kind is called physical adsorption and the other is called chemisorption. In physical adsorption, the interaction between the molecules that are adsorbed and the molecules of the surface is relatively weak; that is, no strong chemical bonds are formed. Physical adsorption is closely related to condensation and, in fact, a special type of physical adsorption occurs when gas molecules fill tiny capillaries on the "surface" of a solid and condense to form liquid. Only one layer of atoms or molecules may result as a consequence of physical adsorption. Such a layer is called a monolayer. Physical adsorption may also involve several layers. The number of layers and character of the adsorption can change with respect to changes in conditions such as temperature and pressure. Adsorption can be characterized by what are called isotherms, plots of the amount of substance adsorbed on a surface versus the density (or pressure) of the substance. Physical adsorption involving only a monolayer allows the implementation of a method for accurately measuring the surface area of a solid. By assigning a value to the area covered by one adsorbed molecule (this can usually be calculated), the area of a surface can be calculated (for example, when a monolayer completely covers the surface), by counting the total number of molecules adsorbed; nitrogen gas is often used for this purpose.

Chemisorption is relatively strong and specific. It involves chemical reactions and as a result of the reactions, chemical bonds are formed between the molecules of the surface and the adsorbed molecules. Usually, chemisorption can be distinguished from physical adsorption. The heat of adsorption in physical adsorption has a value that is on the same order of magnitude as the heat released (per mole) when a gas is liquefied, whereas in chemisorption, the value is much larger and is in the same range as typical heats of reactions (heat released or adsorbed during chemical reactions). The two types of adsorption can be differentiated by examining the effect of temperature and pressure. Physical adsorption, in general, decreases rapidly with increases in temperature, whereas chemisorption first increases and then decreases with increases in temperature. (Unfortunately, there are many exceptions to the "rule.") Physical adsorption of gases increases gradually with pressure and reaches a limiting value only at very high pressures.

Generally speaking, chemisorption increases rapidly with increasing pressure at low pressures and soon reaches a maximum. Under certain conditions, however, the two types of adsorption processes will occur simultaneously, and it is often difficult to differentiate between the two.

Chemisorption occurs quite readily at certain temperatures when gases, such as hydrogen, oxygen, and carbon monoxide, are in the presence of a solid surface. Examples include the formation of a film of oxygen on a hot filament of carbon and the bonding of carbon monoxide to a surface of tungsten. Chemisorption also occurs readily when a liquid is in the presence of a solid surface; however, the conditions for optimal chemisorption are more varied.

For example, the concentration of solute and the type of solvent play determining roles. As in physical adsorption, chemisorption can often, but not always, be characterized by plotting isotherms. Chemisorption is also divided into two types, molecular chemisorption and dissociative chemisorption. In molecular chemisorption, the whole molecule that approaches the surface bonds to the surface, whereas in dissociative chemisorption, the chemical reaction results in only part of the molecule bonding to the surface; the other part or parts dissociate during the reaction.

Chemisorption is important primarily because of the use of surfaces as catalysts. Much of the past research on catalytic reactions at surfaces has been empirical. It is, however, generally agreed that the forces involved are of the same type as those that are active in ordinary chemical reactions. Solid catalysts almost always combine chemically at the surface with one or more reactants. Yet, there are large differences between the activity of different catalysts and the type of activity. In some catalytic reactions on surfaces, it seems there is a uniform interaction over the whole surface between the reactants and surface molecules. Other reactions occur only at certain sites on the surface; these are called active sites.

The details of the reaction can be very complicated. The oxidation of small organic reactions at a platinum surface usually involves breaking the chemical bonds in the organic molecules that connect hydrogen atoms with carbon atoms. Chemical bonds form between hydrogen atoms and surface platinum atoms, and also between the surface atoms and the carbon atoms of the dehydrogenated organic molecules. Subsequent reactions depend on the constraints of the system and the phase (liquid or gas) in which the organic molecules originate. In both liquids and gases, pairs of neighboring hydrogen atoms combine to form hydrogen gas. In liquid solutions, if the platinum surface is charged, the hydrogen atoms can be transformed into protons. (A hydrogen atom minus an electron is a proton.) If the organic compound contains one or more oxygen atoms, or if the fluid (liquid or gas) contains oxygen or water, carbon dioxide can be a final product. Under different conditions, the reverse reaction can take place; using carbon dioxide as a reactant, organic compounds can be synthesized on platinum. Much research has been conducted on the latter reaction because this is how petroleum can be made. Both the forward and reverse reaction have been found to be affected by the crystal structure of the platinum, metal adatoms placed on the platinum surface, and the roughness of the surface. Many desired organic reactions do not proceed on a bare platinum surface. An oxide layer can be added to the surface by exposing the surface to oxygen gas or exposing water to a charged surface (reactions occur in which oxygen atoms and oxygen gas molecules form chemical bonds with the surface platinum atoms), and it has been found that additional organic reactions are catalyzed by the platinum-oxide surface.

In order for reactions that are catalyzed by solid surfaces to proceed to completion, there must be desorption of the products. Nevertheless, some reactions produce intermediates that remain on the surface and cannot be removed. These intermediates poison the catalysts, and either some method is required to remove the poison or the surface has to be replaced. On the other hand, chemically bonded species on a metal surface can protect that surface from unwanted chemical reactions. For example, oxide layers on aluminum protect it against corrosion.

Applications

In addition to looking at chemical processes at surfaces from a microscopic view--that is, the interaction of molecules--one can look at certain aspects of these processes from a macroscopic point of view--that is, the collective interaction of a tremendously large number of molecules. Surface processes, as a macroscopic process, were originally looked upon in this way, and many applications are still considered from this point of view. One important macroscopic quantity is surface energy; the corresponding force is called surface tension. A molecule in the bulk phase (for example, in the middle of a solid) interacts with molecules from all sides. At a surface, however, a molecule has fewer neighbors. Because of less molecular interaction, surface molecules have a higher energy. There is a tendency for the molecules at surfaces to behave collectively in such a way as to minimize the surface energy. This is especially noticeable at liquid surfaces because this process leads to surfaces with curvature.

The macroscopic view of some adsorption processes of liquids by solid surfaces is called wetting. Here, surface energy plays an important role. If a liquid is brought into contact with a surface that adsorbs it and if the liquid forms a film over the surface, the liquid is said to wet the solid. Surface forces determine whether a liquid substance spreads over a solid surface as well as into crevices and pores. Wetting usually means that a liquid spreads over a solid suface easily, whereas nonwetting usually means that the liquid tends to form a ball that runs off the surface easily. In order for a liquid to spread, it must increase its surface area, and hence, the adsorption must lower the energy more than the lowering of energy obtained when the liquid minimizes its surface by forming, say, a ball. Additives, called surfactants, are often necessary so that the liquid will possess a satisfactory degree of wetting. These additives consist of molecules with a polar and nonpolar part. The polar part (which can be thought of as an electrically charged part) sticks to the surface, whereas the nonpolar part protrudes away from the surface into the liquid. The nonpolar part must be sufficiently inert with respect to interactions with other molecules so that it does not interfere with the flow of the liquid.

The processes of adsorption and wetting of a solid by a liquid are important in the lubrication of surfaces and sprays for plants. Lubrication requires maintaining an adsorbed layer of a deformable material on each of two solid surfaces so that they do not come into contact.

Liquids used as horticultural sprays for insects must have the property of wetting. Thus, they must be easily adsorbed and have low surface energy.

Many applications are described best by the microscopic point of view, especially adsorption of gases. Because of the fact that different compounds are adsorbed to a different degree, both physical and chemical adsorption can be used to separate species from gas mixtures.

At 1,000 degrees Celsius, charcoal adsorbs oxygen, nitrogen, and argon gases from air, but does not adsorb helium and neon. Close to the temperature that air becomes a liquid, however, charcoal adsorbs almost all the neon gas, but not the helium. Charcoal was also employed in the gas masks of World War I to adsorb toxic gases. Charcoal is also used by industry to adsorb, and thus save, valuable volatile solvents during processes in which rapid evaporation of the solvent is necessary.

Almost all chemical technologies use surface catalysis. An important catalytic industrial process is the hardening of vegetable oils. The process causes the substance to change from a liquid oil to a solid. For example, margarine is produced using this process. The product is more stable against malodorous chemical derivatives that can form through chemical reactions that occur when vegetable oil is exposed to air. The process is carried out on a nickle surface.

The catalytic properties of the surface enable chemical reactions, in which additional hydrogen atoms are added to the molecules that make up the vegetable oils, to occur at a sufficiently rapid rate to make the process economically feasible.

Surfaces with special molecular properties can be prepared for use as catalysts in specific processes. One such catalyst is employed in the production of linear polyethylene, which is used for making plastics. The catalyst is prepared by sticking a salt containing chromium into a glasslike silica gel. The mixture is heated and then cooled. The resulting surface, which is rather rough, consists of chromate (a chromium atom surrounded by four oxygen atoms) dispersed on a layer of silica. Isolated chromium centers provide sites at which, beginning with a solution containing ethylene, polymers (very long molecules) grow by way of chemical reactions. This is also an example of where the catalyst changes structure during the reaction; the particles of the catalyst become fragmented during the polymerization process. The polymer chains grow outward from the sites on the catalyst, and these sites may be within pores. Hence, particles on the surface can be shattered. In typical industrial reactors, the polymers precipitate out of solution and become separated from the catalytic particles.

Context

Many aspects of surface chemistry have been known since the earliest development of the physical sciences. One can follow some parts of the historical development of surface chemistry by noting the era in which applications were initiated. For example, it apparently was known in the fifteenth century that the carbon obtained by heating wood in a closed container removes coloring from solution. In 1791, this knowledge led to the process in which charcoal was used commercially to remove colored matter from sugar solutions.

In the first part of the nineteenth century, a patent was obtained for employing spongy platinum as a catalyst for the reaction in which sulfur dioxide is transformed to sulfur trioxide.

(At that time, the process failed because the catalyst was easily poisoned.) Techniques for liquefaction of gases through physical adsorption on solids were developed around 1884. At approximately the same time, the procedure of using adsorbent carbon to remove gases in order to obtain a vacuum was developed.

One of the more famous chemical reactions, the synthesis of ammonia from nitrogen gas and hydrogen gas, was realized shortly before World War I by the chemist Fritz Haber. An iron catalyst is used in the process and today this process is so developed that ammonia is quite inexpensive. The development of the Haber process also calmed the fear that a world disaster would occur in the middle of the twentieth century because of a predicted shortage in the amount of fixed nitrogen (nitrogen is an atom in ammonia).

Measurements of macroscopic quantities, such as surface tension and the amounts of gases adsorbed by a solid surface, were possible around the middle of the nineteenth century.

Experimental measurements could be related to macroscopic theoretical parameters; most of the theory was developed by Josiah Willard Gibbs. Microscopic measurements of chemical surface processes could not keep pace with the development of such measurements in other branches of chemistry until well into the 1950's. At that time, space exploration promoted the development of methods for the preparation of clean surfaces and reproducible surface studies. Important advances for microscopic investigations of surface chemistry are the developments of surface spectroscopies. Surface spectroscopies have enabled chemists to determine the elemental composition and structure of the surface layer, the state of surface adsorbates, and the determination of intermediate species of surface catalyzed reactions. Much research remains to be done on determining the mechanism of surface reactions, and it is expected that spectroscopy will play a key role in this endeavor. Research will also continue on the development of modified surfaces that will increase the efficiency of, for example, nuclear energy conversion, and both fossil-fuel conversion and generation.

Principal terms

ADSORPTION: a process in which a molecule or an atom becomes trapped on a surface

CATALYSIS: a process whereby a substance called a catalyst increases the rate of a chemical reaction; the catalyst does not become part of the products of the reaction

CHEMICAL BOND: the sharing of electrons to link atoms together to form molecules

ELECTRON CLOUD: the orbiting electrons that surround the positively charged nucleus of every atom; each chemical element is defined by the number of electrons (and their energy state); when an atom is part of a molecule, the local electron cloud surrounding that atom is different from the electron cloud surrounding that atom when it is isolated

HETEROGENEOUS CATALYST: a catalyst that is not of the same phase as the reactants

SPECTROSCOPY: a field in which the absorption and emission of different frequencies of radiation by molecular species are studied

Bibliography

Adamson, Arthur W. PHYSICAL CHEMISTRY OF SURFACES. New York: John Wiley & Sons, 1976. A somewhat technical account of surface chemistry written at the level of an undergraduate junior or senior. Yet, many of the descriptive parts are accessible to those with less background but who have a good intuitive approach to the physical sciences. A good starting point for those who want to examine a particular aspect of surface chemistry in detail.

Campbell, Ian M. CATALYSIS AT SURFACES. New York: Chapman and Hall, 1988. This book almost exclusively focuses on catalytic surface reactions. Details are presented on the mechanism of such reactions. Campbell presents an excellent account of applications, which include catalytic cracking of crude oils (the breakdown of crude oil to small hydrocarbons, which are then used as fuel), synthetic gasoline production, and the hardening of vegetable oils.

Somorjai, Gabor A. CHEMISTRY IN TWO DIMENSIONS: SURFACES. Ithaca, N.Y.: Cornell University Press, 1981. This is a detailed descriptive account that covers almost all topics of surface chemistry. Although there are mathematical explanations of various aspects of surface chemistry, these are mainly useful formulas that can often be skipped without losing the basic points. Chapters on adsorption and the surface chemical bond are particularly well written and are enjoyable to read.

Soriaga, Manuel P. ELECTROCHEMICAL SURFACE SCIENCE: MOLECULAR PHENOMENA AT ELECTRODE SURFACES. Washington, D.C.: American Chemical Society, 1988. Contains many articles on the surface chemistry of electrode-electrolyte interfaces. The majority of articles describe applications of surface spectroscopies. For example, chapter 24 presents the results of an investigation using surface spectroscopy of the process in which a current is produced when carbon dioxide is obtained after methanol reacts at a platinum surface.

Weiser, Harry B. A TEXTBOOK OF COLLOID CHEMISTRY. New York: John Wiley & Sons, 1949.

Hauser, Ernst A. COLLOIDAL PHENOMENA: AN INTRODUCTION TO THE SCIENCE OF COLLOIDS. New York: McGraw-Hill, 1939. Although dated, these books are good sources on colloid chemistry, which is considered by many colloid chemists as equivalent to surface chemistry. Highly descriptive. One might think that later developments would prove some of the theory discussed in these books as false. It is incredible, however, how much of a somewhat intuitive approach to the subject in these books was later proved correct.

Absorption vs. adsorption