Catalysis, heterogeneous

A heterogeneous catalyst is one that exists in a phase different from the phase of the reactant molecules, usually as a solid in contact with either gaseous reactants or with reactants in a liquid solution. Many industrially important reactions are catalyzed by the surfaces of solids. For example, hydrocarbon molecules are rearranged to form gasoline with the aid of what are called cracking catalysts. Heterogeneous catalysts are often composed of metals or metal oxides. Because the catalyzed reaction occurs on the surface, special methods are often used to prepare catalysts so that they have very large surface areas. 

The initial step in heterogeneous catalysis is usually adsorption of reactants. Adsorption refers to the binding of molecules to a surface, whereas absorption refers to the uptake of molecules into the interior of a substance. — (Section 13.6) Adsorption occurs because the atoms or ions at the surface of a solid are extremely reactive. Unlike their counterparts in the interior of the substance, surface atoms and ions have unused bonding capacity that can be used to bond molecules from the gas or solution phase to the surface of the solid. 

After H—H bond breaks, H atoms migrate along metal surface 

Ethane, C2H6, desorbs from metal surface 

The reaction of hydrogen gas with ethylene gas to form ethane gas provides an example of heterogeneous catalysis  

Heterogeneous Catalysis Aim of Catalyst Characterization Spectroscopic Techniques Research Strategies 

Catalysis plays a prominent role in our society. The majority of all chemicals and fuels produced in the chemical industry have been in contact with one or more catalysts. Catalysis has become indispensable in environmental pollution control selective catalytic routes are replacing stoichiometric processes that generate waste problems. The three-way catalyst effectively reduces pollution from car engines. Catalytic processes to clean industrial exhaust gases have been developed and installed. In short, catalysis is vitally important for our economy now, and it will be even more important in the future. 

A heterogeneous catalytic reaction begins with the adsorption of the reacting gases on the surface of the catalyst, where intramolecular bonds are broken or weakened. The Appendix explains how this happens on metals in terms of simplified molecular orbital theory. Next, the adsorbed species react on the surface, often in 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Catalysts may be metals, oxides, zeolites, sulfides, carbides, organometallic complexes, enzymes, etc. The principal properties of a catalyst are its activity, selectivity, and stability. Chemical promoters may be added to optimize the quality of a catalyst, while structural promoters improve the mechanical properties and stabilize the particles against sintering. As a result, catalysts may be quite complex. Moreover, the state of the catalytic surface often depends on the conditions under which it is used. Spectroscopy, microscopy, diffraction and reaction techniques offer tools to investigate what the active catalyst looks like. 

Heterogeneous Catalysis Approximately 90% of the reactions that are practiced commercially in fields such as petroleum refining, chemicals and pharmaceuticals manufacture, and pollution abatement involve solid, heterogeneous catalysts. The reaction takes place on 

The amount of catalyst may he expressed in several vaUd ways, e.g., weight, volume, and surface area. The choice between these measures of catalyst quantity is one of convenience. However, weight is frequently used in engineering applications. For this choice. 

In fundamental catalyst research, an attempt usually is made to relate the reaction rate to the number of atoms of the catalytic component that are in contact with the fluid. For example, if the decomposition of hydrogen peroxide (H2O2) is catalyzed by palladium metal, the rate of disappearance of H2O2 might be defined as. 

Expressed in this manner, —rH202 units of inverse time and is called a turnover 

Unfortunately, except in special cases, the symbol that is used to denote reaction rate is not constructed to tell the user what basis was used to make the rate intensive. This task usually is left to the units of the reaction rate. 

The first example of a heterogeneously catalyzed hydroamination of an alkene appeared in a 1929 patent in which it is claimed that NHj reacts with ethylene (450°C, 20 bar) over a reduced ammonium molybdate to give EtNH2 [24]. An intriguing reaction was also reported by Bersworth, who reacted oleic acid with NH3 in the presence of catalysts like palladium or platinum black or copper chromite to give the hydroamination product in quantitative yields [25]. However, this result could not be reproduced [26]. 

Teter et al. filed a series of patents aimed at the production of organic compounds containing nitrogerf or the production of nitriles and amines from ammonia and olefins by passing mixtures of olefin and NH3 over transition metals, mainly cobalt deposited on various supports at 250-370°C and 100-200 bar [27- 3]. With cobalt on asbestos, a mixture of amine, nitrile, olefin hydrogenation product, polymers, and cracking products is obtained (Eq. 4.1) [31]. 

At higher temperatures, propene and NH, react over basic catalysts to afford a mixture of nitriles (Eq. 4.2) [42]. 

It was thought that propionitrile came from dehydrogenation of the anti-Markovnikov hydroamination product, w-PrNHj. Propionitrile can break down to ethylene and HCN, the former reacting with NH3 to generate acetonitrile via ethyl-amine, the latter adding to propene to form the butyronitriles [26, 37]. 

PhNH2 reacts with ethylene in the presence of alkali metals, e.g., sodium deposited on alumina, to afford the hydroamination product in good yield but with a low turnover frequency (TOP = mol of product synthesized per mol of catalyst in 1 h) (Bq. 4.3) [44]. 

Although heterogeneous catalysis has been recognized for over 150 years and practical applications increased dramatically from the beginning of the present century, mechanisms were poorly understood and selection was based on trial-and-error or empirical rules. In the last 25 years or so many new techniques have been applied to characterize the materials and throw light on mechanistic aspects, though no overall theory has found universal acceptance. 

We will start, therefore, with some of the more pragmatic aspects of heterogeneous catalysis, for which little theoretical background knowledge is required. 

The special nature of gold chemistry and gold catalysis is now known and many applications can be based on the low-temperature activity of supported gold compared to that of other metals [196]. 

In the same way as the synthesis of new organometallic derivatives or new applications of catalytic systems are sought in the field of homogeneous catalysis, in heterogeneous catalysis there are several aspects to bear in mind and sometimes the significance of these aspects becomes apparent when gold is concerned [197— 199]  

There is also growing interest today in the development of gold nanoparticle-based systems that are providing new sources of substrates for study in the field of catalysis. 

One of the most important examples of heterogeneous catalysis is the addition of H2 to the C=C bonds of organic compounds to form C—C bonds. The petroleum, plastics, and food industries frequently use catalytic hydrogenation. The conversion of vegetable oil into margarine is one example. 

H — H(g) + 2catM(j)--- 2catM—H (H atoms bound to metal surface) 

C2H4 adsorbs and reacts with two H atoms, one at a time, to form C2H6. The H—H bond breakage is the rate-determining step in the overall process, and interaction with the catalyst s surface provides the low-fa step as part of an alternative reaction mechanism. 

Unlike the industrial examples we just discussed, catalytic processes occur in natural settings as well, and a brief description of two important systems follows. The first concerns the remarkable abilities of catalysts inside you, and the second focuses on how catalysis operates in the stratosphere. 

Cellular Catalysis The Function of Enzymes Within every living cell, thousands of individual reactions occur in dilute solution at ordinary temperatures and pressures. The rates of these reactions respond smoothly to various factors, including concentration changes, signals from other cells, and environmental stresses. Virtually every cell reaction is catalyzed by its own specific enzyme, a protein whose complex three-dimensional shape—and thus its function—has been perfected through natural selection (Section 15.6). 

Transition metals play an important role in heterogeneous catalysis where reactions occur on the surfaces of metal or oxide crystals. Typical of these metals are V or Mo which exist in oxides with tetrahedral, tetragonal pyramidal, or octahedral coordination and which can change their oxidation states with minimal changes in their coordination environment. As in the case of soil minerals (Section 13.4.1), bond valences can be used to determine the bonding strength of the anions on the surface, by noting how far the valence sums around the surface ions fall short of 2.00 vu. 

A somewhat different kind of catalytic surface, a monolayer of vanadium grown on both the rutile and anatase forms of Ti02 (202240, 202242), has been modelled by Depero (1993) who uses bond valences to show which sites will be 

Schematic energy diagram for the oxidation of CO and a Pt catalyst. (From data presented by G. Ertl in Catalysis Science and Technology, J. R. Anderson and M. Boudart, Eds., vol. 4, Springer-Verlag, Berlin, 1983, p. 245.) All energies are given in kJ mol. For comparison, the heavy dashed lines show a noncatalytic route. 

Body-centered cubic Face-centered cubic  

Estimation of the number of exposed metal atoms is rather straightforward in the case of a single crystal of metal since the geometric surface area can be measured and the number density of surface atoms can be found from the crystal stracture. 

Selective chemisorption uses a probe molecule that does not interact significantly with the support material but forms a strong chemical bond to the surface metal atoms of the supported crystallites. Chemisorption will be discussed in more detail in Section 5.2. Dihydrogen is perhaps the most common probe molecule to measure the fraction of exposed metal atoms. An example of H2 chemisorption on Pt is shown below  

The exact stoichiometry of the Pt-H surface complex is still a matter of debate since it depends on the size of the metal particle. For many supported Pt catalysts, an assumption of 1 H atom adsorbing for every 1 Pt surface atom is often a good one. Results from chemisorption can be used to calculate the dispersion of Pt, or the fraction of exposed metal atoms, according to  

Heterogeneous catalyst and reactants of a catalyzed reaction remain in two different phases. Heterogeneous catalysis affects the rate of a reaction, which occurs at the interface of two distinct phases — for example, the hydrogenation of alkenes in the presence of solid catalysts such as Pt C, Pd/C, and Ni. The details of the mechanisms of catalytic hydrogenation and related reaction are not yet well understood. 

Tiltscher, Wolf, and Schelchshorn (1984) describe the influence of an SCF reaction medium on the activity of a heterogeneous catalyst used in a 

In another test they deactivate the catalyst by introducing a small amount of a finely dispersed catalyst-fouling substance (M0S2) into the reactor under 

In the final deactivation mode reported by the authors, the active acidic sites of the catalyst are poisoned (7 = 145°C, P = 50 bar) by continuous addition of a very dilute solution of pyridine to the reacting mixture over a period of 12 h (see figure 11.10). The catalyst can be reactivated by heating and compressing the reaction mixture to conditions well within the mixture critical region (7 = 250°C, P = 500 bar). Tiltscher and coworkers report that the catalyst poison is precipitated from the product solution as pyridinium chloride. Presumably only a very small amount of pyridinium chloride is needed to deactivate the catalyst since supercritical hexene probably would not be able to solubilize much of this salt. It is surprising, however, that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride. 

It is well known that solvent viscosity can have an effect on the product distribution for certain reactions (Saltiel and Charlton, 1980). The effect of solvent viscosity can be studied with an SCF reaction medium since a wide range of viscosities can be obtained with a single SCF solvent if the system is operated in the vicinity of the critical point of the solvent. 

Squires, Venier, and Aida (1983) describe an experimental technique they use to study the effect of solvent viscosity on the cisitrans ratio of stilbene irradiated in supercritical CO2. They use a dynamic flow technique similar to that described in chapter 4. In their system trau5-stilbene is coated onto glass beads, which are then packed into a high-pressure column. Supercritical CO2 flows through the column and solubilizes some of the trans-stilbene. The C02-stilbene phase is continuously irradiated with ultraviolet light as it flows through a quartz photoreactor at a fixed temperature and pressure. As the solvent viscosity increases, the photoisomerization of the cis isomer is inhibited while that of the trans isomer is facilitated. We should expect to see the cisitrans ratio of stilbene vary as the density of CO2 varies. This viscosity effect is clearly shown in figure 11.11. While there is a small effect of pressure on the 

In many cases, homogeneous catalysts are attached to organic or inorganic supports such as polymer, silica, and layered clay to form heterogeneous catalysts. The major advantages of heterogenized catalysts are practical and economical since they may be readily recovered from reaction mixtures and reused multiple times without loss of catalytic activity. 

It was found that dehydrative esterification in water was effectively catalyzed by hydrophobic polystyrene-supported sulfonic acids as recoverable and reusable catalysts. As shown in 

It should be mentioned that, imder the reaction conditions, opposite types of reactions (hydrolysis and dehydration) occur in the same pot. Thus, this system provides an effective method for conversion of thioesters to benzylic thioethers without isolating the thiol 

Catalyst PS-SO3H (1.55 mmol/g) 41% yield PS-SO3H (0.46 mmol/g) 77% yield 

There are three general types of catalysis, depending on the nature of the rate-increasing substance heterogeneous catalysis, homogeneous catalysis, and enzyme catalysis. 

In heterogeneous catalysis, the reactants and the catalyst are in different phases. The catalyst is usually a solid, and the reactants are either gases or liquids. Heterogeneous catalysis is by far the most important type of catalysis in industrial chemistry, especially in the synthesis of many important chemicals. Heterogeneous catalysis is also used in the catalytic converters in automobiles. 

At high temperatures inside a car s engine, nitrogen and oxygen gases react to form nitric oxide  

When released into the atmosphere, NO rapidly combines with O2 to form NO2. Nitrogen dioxide and other gases emitted by automobiles, such as carbon monoxide (CO) and various unbumed hydrocarbons, make automobile exhaust a major source of air pollution. 

For other carbon materials, and especially for the activated carbons, it has been known for long that they are serviceable supports for heterogeneous catalysts. Counting among the reasons for this is their large specific surface. Carbon nanotubes are suitable catalyst supports as well. Apart from a better control over 

The work of Perry [121] on the exothermic CO oxidation on Pt/Al203 and Pd/ AI2O3 catalysts proved there are only minimal measurable differences between catalysis under the action of microwaves and under classical conditions. Because of imprecise temperature measurement, these differences diminish even further if experimental conditions are improved. For exothermic reactions, formation of hot spots can therefore be excluded for metal-containing supported catalysts in continuous microwave fields at low frequencies [139]. Mingos et al. [140] found hot spots of diameter 0.09-1 mm on a 20% MoS2/y-Al203 catalyst used for endothermic decomposition of H2S. Resulting differences between catalyst bed temperatures was approx. 100-200 K. 

The appropriate catalyst to choose for heterogeneous (or immobilized homogeneous) gas-phase catalysis under the action of microwaves unclear. The choice is difficult because catalysts must have appropriate catalytic properties and suitable 

An apparatus suitable for gas phase catalysis has been developed and successfully tested in Jena (Figs. 2.24 and 2.25). A Panasonic NE-1846-type microwave oven was modified by Fricke and Mallah Microwave Technology (Hannover) [95, 96]. 

In this chapter we shall consider first the general characteristics of heterogeneous catalysis and adsdilptiQnlCphy and chemic and then physical properties of solid catalysts and methods of preparation. Kinetics 

From a theoretical point of view the situation is eonsiderably more complex when the chemical reactions involve signifieantly different length scales. This includes heterogenous catalysis where moleeules interaet on the surface of some solid. The solid may be idealized as being infinite and periodic, but the periodicity is broken in the direction perpendicular to the smface at the surface where the chemical reactions are taking place. Moreover, it is almost never the case that the adsorbed molecules form some regular periodic structure on the surface so also in the directions parallel to the smface the periodicity is broken. 

In order to overcome these problems, simplified systems are studied theoreti-eally. One such corresponds to approximating the semi-infinite solid through a finite cluster and then studying the interactions between this and the reactants. In this approximation a number of bonds that are present in the infinite solid have been cut and the resulting dangling bonds have therefore to be saturated through, e.g. hydrogen atoms. Nevertheless, finite-size effects as well as effects due to the saturated bonds may obscure the results of such calculations. 

Some of these problems may be removed by studying infinite, periodic structures. Then, the semi-infinite crystal is modelled as a film or slab of finite thickness that is periodic in the two directions parallel to the film. Periodicity may also be introduced in the third direction, perpendicular to the film, although one then has to be very careful avoiding interaetion between the different films. Reaeting moleeules or atoms will then have to be introdueed periodieally, too, and this may lead to the further complieation that different, but equivalent, moleeules may interact if the size of the repeated unit is not chosen sufficiently large. 

In the study of the CO -I- 0 — CO2 reaction on the Pt(l 11) surface the reaction path was identified, and it was found that one CO molecules moves in the direction of an 0 atom on an essentially flat surface. In reality the situation may be somewhat more complicated, including that the reactants move more or less randomly on the surface before reacting, that the surface may contain steps or other types of defects, that also other types of molecules may be present on the surface, and that also the surface itself may not be inert. Examples of theoretical studies where these issues have been addressed shall now be briefly mentioned. 

Also the presence of other molecules on the surface may influence the chemical reactions on the surface. In another study. Hammer studied how the presence of N, O, or H on the Ru(OOOl) surface would influence the dissociation of N2 on the 

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

In heterogeneous catalyzed reaction, the surface of a solid serves as a catalyst while the reactants are in gaseous or liquid phase. One or more reactants get adsorbed on the surface of the solid. This phenomenon is similar to the formation of intermediate complex in homogeneous catalysis. In many cases, heat of adsorption leads to the activation of reactants and the reaction takes place easily. The surface thus provides an alternative path for the reaction to proceed with an accelerated rate. The reaction consists of the following four consecutive steps. 

Steps (i) and (iv) are generally very fast and do not play any part in determination of rate of the reaction. The adsorption and desorption equilibria are easily attained. The concentration of reactant molecules on the surface is an important factor because the molecules which are adsorbed on the surface will undergo the chemical transformation. The concentration of the adsorbed molecules on the surface at any moment is proportional to the fraction of the surface (say 0) covered. Therefore, the rate of reaction will also then be proportional to the covered portion of the surface, i.e. 

In Langmuir adsorption isotherm, the fraction of the surface 0 covered by adsorption has been related to the pressure P of the gas by relation 

When there is a single reactant, it is first adsorbed on the surface, activated and subsequently dissociates into products. 

When adsorption is low either due to very low pressure or due to low adsorption capacity of the surface and aP is negligible compared to unity, then 

Fleterogeneous catalytic activity was one of the first proposed and demonstrated host-guest properties of MOFs, with subsequent research providing a range of different catalytic activities across a diverse array of framework systems. Most notable among these systems are cases in which catalysis arises due to chemical activation at specific surface binding sites. 

Framework structure of [Zn2(bpdc)2L]- (guest). (c) Plots of total turnover number (TON) vs time for the enantioselective epoxidation of 2,2-dimethyl-2H-chromene catalysed by [Zn2(bpdc)2L]- (guest) (squares) and the free ligand L (circles). Reprinted with S.H. Cho, B.Q. Ma, S.T. Nguyen, J.T. Hupp and T.E. Albrecht-Schmitt, Chem. Commun., 2563-2565. Copyright (2006) Royal Society of Chemistry 

Post-synthetic incorporation of metal sites into MOFs has proven to be a particularly powerful technique for generating reactive surfaces that would otherwise be inaccessible. Of particular note here is 

Finally, in an approach analogous to that used for porous carbons and zeolites, highly robust MOFs have been used as surface supports for metal atoms and clusters. An example here is the chemical vapour deposition of various metals into MOF-5, yielding materials classified as metal MOF-5 for which the nature of metal inclusion and the extent of exogenous loading is currently unknown. Of these, Cu MOF-5 is active in the synthesis of methanol from syngas and Pd MOF-5 catalyses the reduction of cyclooctene by hydrogen.  

Whilst metal centres have provided the majority of known catalytic sites in MOFs, organic units have also provided a number of compelling examples. The size-selective catalytic activity of POST-1, described in Sections 1.2.1.1.2 and 1.2.3.2, in the transesterification of alcohols is attributed to the presence of unprotonated pyridyl groups that project into the channels and which likely assist in the deprotonation of the alcohol reactants. Catalytic yields in excess of 77 % were achieved with an ee = 8 % using this homochiral system. 

The type of bond between the surface of the solid and adsorbate molecules determines the kind of surface processes that can take place crystal growth, growth inhibition, nucleation, corrosion, catalytic activity, and chemical passivation. Sometimes there are two types of surfaces involved in the reaction metallic and ionic (many heterogeneous catalysts consist of very small metal particles on oxidic carriers). 

The reactions with heterogeneous catalysts can be categorized according to the nature of the surface bonds. Thus there are reactions on metal surfaces, reactions on ionic surfaces, and reactions on covalent solids. The observed kinetics are specific and proposed reaction mechanisms vary widely. Reaction mechanisms come and go, and their ephemeral existence is often disconcerting. By contrast, the results of good chemical kinetics remain unchanged, whatever may be the future revisions of their underlying mechanism.  

The morphological state of a surface is usually decisive for its reactivity as can be seen in the following examples  

Oxidation of carbon monoxide on platinum The adsorption reaction of oxygen on platinum is 

The models of these surface reactions (more details are given in Section 6.7) imply a number of assumptions (I) there is equilibrium in adsorption and desorption (2) there is only one rate-determining intermediate reaction between adsorbed species (3) the species on the surface are well mixed and (4) there is a thermal probability of a transition of the physisorbed to the chemisorbed state before subsequent reaction and diffusion. These assumptions are not independently proved but are justified by the degree of success of the models in predicting the kinetics. 

The fact that the normal components of the vectorial fluxes are scalar and couple to a chemical reaction at an interface cf. eq 14.40 and 14.42), have implications for descriptions of heterogeneous catalysis. We demonstrate here the application of eqs 14.40 to a simple example. 

Consider, as a specific example, a flat catalytic surface at x = 0, with an adjacent film layer. A chemical reaction takes place at this surface at a rate r. The surface is a good heat conductor and has the temperature T°. A diffusion layer develops in front of the surface with thickness d. The reactants enter with temperature T. The concentrations of the reactants and products as well as the temperature in x = —d are known. We consider a stationary state in which the total heat flux fq, as well as the mass fluxes Jj, are independent of position and directed in the x-direction. Other variables depend on x only. On the o-side of the catalyst the mass fluxes are zero. It follows from eq 14.31 that fg = J = J g in a stationary state. The mass fluxes become proportional to the reaction rate, eq 14.32, giving J)= — n/. All chemical potential gradient terms in eq 14.15 can then be contracted, and we obtain for the entropy production in the i-phase  

With constant fluxes and constant resistivities, we integrate and obtain  

For the surface we obtain the excess entropy production from eq 14.34, 

The last term comes from the chemical reaction at the surface. This gives as linear force-flux relations 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be 

Contact with a mineral surface can in many cases allow a redox reaction to proceed at a rate considerably greater than attainable within an aqueous solution itself. The catalyzing mineral sorbs the electron donating and accepting species, then, within its structure or along its surface, conducts electrons from one to the other. Where electron transfer by this pathway proceeds more rapidly than via a direct transfer in solution between colliding molecules, the redox reaction proceeds preferentially by heterogeneous catalysis. 

Catalysis by such a mechanism can be accounted for in a kinetic rate equation by including as a factor the catalyzing mineral s surface area. Sung and Morgan (1981), for example, in studying the oxidation on Mn11 by dissolved dioxygen, 

Following development in the previous section, rate laws of this type can be written in a general form, 

To incorporate into a geochemical model a rate law of this form, it is common practice to specify the specific surface area (cm2 g-1) of the catalyzing mineral. The mineral s surface area As, then, is determined over the course of the simulation as the product of the specific surface area and the mineral s mass. 

A more robust way to write a rate law for a catalytically promoted reaction is to include the concentrations of one or more surface complexes, in place of the surface area As. In this case, the simulation can account not only for the catalyzing surface area, since the mass of a surface complex varies with the area of the sorbing surface, but the effects of pH, competing ions, and so on. 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Center for Atomic-scale Materials Design, Department of Physics, NanoDTU, Technical University of Denmark, DK-2800 Lyngby, Denmark 

A solid surface has three closely coupled functions when it works as a catalyst for a chemical reaction. First, it adsorbs the reactants and cleaves the required bonds. Next it holds the reactants in close proximity so that they can react, and finally the surface lets the products desorb back into the surrounding phase. Understanding the adsorption bond is therefore crucial to the understanding of the way surfaces act as catalysts. If we can understand, which factors determine if a surface is a good catalyst for a given chemical reaction, we will have the concepts needed to guide us to better and more efficient catalysts. 

In the following, we will develop a set of simple concepts that allow us to understand variations in the reactivity from one surface to the next. These variations 

Factors determining the reactivity of a transition metal surface 

The description of bonding at transition metal surfaces presented here has been based on a combination of detailed experiments and quantitative theoretical treatments. Adsorption of simple molecules on transition metal surfaces has been extremely well characterized experimentally both in terms of geometrical structure, vibrational properties, electronic structure, kinetics, and thermo-chemistry [1-3]. The wealth of high-quality experimental data forms a unique basis for the testing of theoretical methods, and it has become clear that density functional theory calculations, using a semi-local description of exchange and correlation effects, can provide a semi-quantitative description of surface adsorption phenomena [4-6]. Given that the DFT calculations describe reality semi-quantitatively, we can use them as a basis for the analysis of catalytic processes at surfaces. 

FT-IR studies indicate that the lanthanide species supported on oxidic materials can be assigned to an amide structure according to Eq. (28) [300, 301], 

The hybridic systems exhibit selective hydrogenation of conjugated and non-conjugated double bonds. Polyvinyl alcohol, PVA, and HOSiPh3 were used as model compounds [301]. 

Catalytic activity in zeolitic materials is strongly influenced by the type of alkali metal cations, and maximum catalytic activity, e.g, in isomerization reactions, is explained by the formation of an imide species EuNH [305]. Synergetic effects were observed in bimetallic supported Si02 which showed considerable hydrogen uptake during hydrogenation reactions [307]. The formation of Ln-NH2, -NH, -N species seemed to be suppressed in the presence of transition metal powders and precipitation of elemental lanthanides is favored [309]. Lanthanide imides were favored as active species in the Ln/AC-mediated cyclization of ethyne and propyne [310]. 

Vanadium catalysts for oxidation of heterocyclic compounds 90MI3. 

A CATALYST is a substance that increases the rate at which a chemical reaction approaches equilibrium, while not being consumed in the process. Thus, a catalyst affects the kinetics of a reaction, through provision of an alternative reaction mechanism of lower activation energy, but cannot influence the thermodynamic constraints governing its equilibrium. 

Although the general definition of a catalyst given above emphasizes the acceleration of the approach to equilibrium, the selectivity of a catalyst is often of more importance than its overall catalytic activity. An unselec-tive catalyst may accelerate undesirable reaction pathways as well as, or more than, the desired one. A commercially important example of selective 

The catalytic mechanism of reaction on solids can be broken down into five consecutive steps  

In the case of a solid catalyst operating in a liquid phase reaction system the problems of diffusion and concentration gradients can be particularly severe. Substrate diffusion can be further broken down into two steps, external diffusion and internal diffusion. The former is controlled by the flow of substrate molecules through the layer of molecules surrounding catalyst particles and is proportional to the concentration gradient in the bulk liquid, i.e. the difference in the concentrations of the substrate in the bulk medium and at the catalyst surface. The thickness of the external layer in a liquid medium is dependent on the flowing fluid and on the agitation within the reaction system typically it is 0.1-0.01 mm thick. Internal diffusion of substrate molecules is a complex process determined not only by the resistance to flow due to the 

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. 

This chapter reviews the field of heterogeneous catalytic reactions in SCFs [5-8]. By exploiting the unique solvent properties of SCFs, it may be possible to enhance reaction rates while maintaining or improving selectivity. The following benefits can be expected. 

Gaseous reactants such as hydrogen are able to mix well with an SCF at high concentrations. Reaction rates dependent on gas concentration will be enhanced. 

Organic compounds become soluble in SCFs to form a homogeneous reaction phase, leading to high reactivity and selectivity. 

Through pressure control, separations between the catalyst and reactant/ product are easily realized. This property has a strong effect on reaction equilibrium shift, suppresses side reactions and assists in the extraction of precursors to catalyst poisons. 

The generally accepted mechanism for photocatalytic transformations in aqneons media is the attack of OH on the organic moiety. Bhatkhande et al. (2002) have discussed the various mechanisms proposed for photocatalytic pathways. In the case of aromatic componnds, it has been shown that hydroxy aromatic componnds are formed through the mediation of OH. It has also been shown that a maximum of three hydroxyl gronps can be attached after which the compound becomes highly unstable and decomposes to CO and water. This is evident because no ahphatic compounds are formed. This mechanism can be nsed to obtain di- and trihydroxy compounds (Brezova et al. 1991 Centi and Misono 1998 Ye and Ln 2013). Other hydroxylated componnds such as o (salicylic acid) and p (-hydroxy benzoic acids) 

Pahnisano et al. (2007) have reviewed pertinent literature up to 2007. Fagnoni et al. (2007) have presented an excellent review of the literature on photocatalysis for the formation of the C-C bond. Table 1.1 summarizes some recent investigations besides those covered by Pahnisano et al. 

TABLE 1.1 Some Important Investigations on Selective Photocatalytic Oxidation 

Substrate Main products Photocatalyst employed References 

Ethane Acetaldehyde, ethanol, MoOj/SiO, Wada etal. (1991)  

In addition to varying pH, enzyme kinetics can be studied by adding other substances which react with the enzyme in a variety of ways. The use of inhibitors is often an aid to understanding the nature of the enzymatic activity. In competitive inhibition the inhibitor complexes the enzyme and renders it inactive. Assuming the dissociation equilibrium 

Comparison with (5.53) indicates that the dependence of reaction rate on substrate concentration is unaltered however, the apparent Michaelis constant is now a function of inhibitor concentration. 

In uncompetitive inhibition the inhibitor is presumed to complex the reaction intermediate ES, 

Again the dependence on [S] is unaffected however, the limiting velocity at high substrate concentration is now a function of inhibitor concentration. The rate laws (5.55) and (5.56) are kinetically distinguishable and permit differentiation between these possible modes of inhibition. Another important, distinguishable process is noncompetitive inhibition (Problems 5.13 and 5.14). 

When metal catalysts are used to accelerate gaseous reactions the mechanism involves five steps  

We have considered a variety of precedents for radical/ionic addition polymerization of unsaturated monomers to yield polymeric materials. However, ntme of the aforementioned techniques offer stereoselective control over the growing polymer chain, resulting in purely atactic polymers. In order to introduce such control, it is necessary to spatially confine the reactive site to control the direction of incoming monomer/growing polymer. The most common method used to control the tacticity of the resulting polymer is Ziegler-Natta polymerization. 

If TiCls is used as the catalyst surface, an isotactic polymer is formed. However, changing the surface to VCI3 yields a syndiotactic product. This difference may be explained by looking at the relative sizes of Ti and Due to an increase in the effective nuclear charge (Zeff) as one moves from left to right of the Periodic Table, the center is smaller which creates more steric hindrance among the coordinated 

This is a reaction between reactants leading to products that usually belong to the same phase (liquid or gas) and utilize the surface of a solid phase in then-mechanism. 

The speed of a heterogeneous catalysis reaction generally depends on three categories of variables  

Heterogeneous catalysis causes a reaction to take place at the surface of a solid through to the adsorption phenomenon. 

The full mechanism of heterogeneous catalysis involves five steps  

The catalytic act, which is the purely chemical part of heterogeneous catalysis, can occur according to two mechanisms  

Gaseous mixtures of styrene and NH3 have been daimed to give hydroaminated products over a ternary K/graphite/Al203 catalyst [45]. 

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Heterogeneous catalysts. In heterogeneous catalysis, the catalyst is in a different phase from the reacting species. Most often, the... 

Adsorption is of technical importance in processes such as the purification of materials, drying of gases, control of factory effluents, production of high vacua, etc. Adsorption phenomena are the basis of heterogeneous catalysis and colloidal and emulsification behaviour. 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

This chapter concludes our discussion of applications of surface chemistry with the possible exception of some of the materials on heterogeneous catalysis in Chapter XVIII. The subjects touched on here are a continuation of Chapter IV on surface films on liquid substrates. There has been an explosion of research in this subject area, and, again, we are limited to providing just an overview of the more fundamental topics. 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

The plan of this chapter is as follows. We discuss chemisorption as a distinct topic, first from the molecular and then from the phenomenological points of view. Heterogeneous catalysis is then taken up, but now first from the phenomenological (and technologically important) viewpoint and then in terms of current knowledge about surface structures at the molecular level. Section XVIII-9F takes note of the current interest in photodriven surface processes. 

Influence of the Adsorption Isotherm on the Kinetics of Heterogeneous Catalysis... 

D. A. King and D. P. Woodruff, eds.. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1982. 

The concluding chapters, Chapters XVI through XVIII, take up the important subjects of physical and chemical adsorption of vapors and gases, and heterogeneous catalysis. As with the earlier chapters, the approach is relatively quantitative and problem assignments regain importance. 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

Somor]ai G A 1998 Molecular concepts of heterogeneous catalysis J. Moi. Struct (Theochem) 424 101... 

Thomas J M and Thomas W J 1996 Principles and Practice of Heterogeneous Catalysis (Weinheim VCH)... 

Jacobs P W and Somoqai G A 1997 Conversion of heterogeneous catalysis from art to science the surface science of heterogeneous catalysis J. Mol. Catal. A 115 389... 

ErtI G 1990 Elementary steps in heterogeneous catalysis Agnew. Chem., Int. Ed. Engl. 29 1219... 

Imbihl R and ErtI G 1995 Oscillatory kinetics in heterogeneous catalysis Chem. Rev. 95 697-733... 

Brey W S 1983 Applications of magnetic resonance in catalytic research Heterogeneous Catalysis Selected American Stories ed B FI Davis and W P Flettinger Jr (Washington American Chemical Society)... 

Delgass W N, Haller G L, Kellerman R and Lunsford J H 1979 Spectroscopy in Heterogeneous Catalysis (New York Academic)... 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Satterfield C N 1991 Heterogeneous Catalysis in Industrial Practice (New York MoGraw-Hill)... 

N. Satterfield, Mass Transfer in Heterogeneous Catalysis, M.I.T. Press, 1970. 

Boero M, M Parrinello and K Terakura 1999. Ziegler-Natta Heterogeneous Catalysis by First Piincip Computer Experiments. Surface Science 438 1-8. 

FIGURE 6 1 A mechanism for heterogeneous catalysis in the hydrogenation of alkenes... 

It would be difficult to over-estimate the extent to which the BET method has contributed to the development of those branches of physical chemistry such as heterogeneous catalysis, adsorption or particle size estimation, which involve finely divided or porous solids in all of these fields the BET surface area is a household phrase. But it is perhaps the very breadth of its scope which has led to a somewhat uncritical application of the method as a kind of infallible yardstick, and to a lack of appreciation of the nature of its basic assumptions or of the circumstances under which it may, or may not, be expected to yield a reliable result. This is particularly true of those solids which contain very fine pores and give rise to Langmuir-type isotherms, for the BET procedure may then give quite erroneous values for the surface area. If the pores are rather larger—tens to hundreds of Angstroms in width—the pore size distribution may be calculated from the adsorption isotherm of a vapour with the aid of the Kelvin equation, and within recent years a number of detailed procedures for carrying out the calculation have been put forward but all too often the limitations on the validity of the results, and the difficulty of interpretation in terms of the actual solid, tend to be insufficiently stressed or even entirely overlooked. And in the time-honoured method for the estimation of surface area from measurements of adsorption from solution, the complications introduced by... 

Heterocoagulation Heterocyclic Heterocyclic amines Heterocyclic azo dyes Heterocyclic compounds Heterocyclic dyes Heterocyclic polymers Heterocyclic thiophenes Heteroepitaxy Heterogeneous catalysis Heterogemte Heteroglycan Heterojunction... 

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