Heterogeneous Catalytic Chemistry by Example of IndustrialApplicationsJosef Heveling*

Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa

ABSTRACT: Worldwide, more than 85% of all chemical products are manufactured with thehelp of catalysts. Virtually all transition metals of the periodic table are active as catalysts orcatalyst promoters. Catalysts are divided into homogeneous catalysts, which are soluble in thereaction medium, and heterogeneous catalysts, which remain in the solid state. A heterogeneousmetal catalyst typically consists of the active metal component, promoters, and a supportmaterial. In some cases, the metallic state itself forms the active ingredient. However, thissituation is largely restricted to precious metal catalysts and to some base metals used underreducing conditions. In most cases and especially in homogeneous catalysis, it is a metalcompound or a complex that forms the active catalyst. Catalysis can be rather puzzling as a givenmetal can catalyze a variety of different chemical transformations, while the same substrate,passed over different catalysts, can give different products. It is therefore helpful to be familiarwith the fundamentals of catalytic science before being exposed to the uncountable applications,which form the backbone of industrial chemistry. Examples of practical importance are used inthis paper to highlight important principles of catalysis.

KEYWORDS: General Public, Public Understanding, Upper-Division Undergraduate, Curriculum, Inorganic Chemistry, Catalysis,Green Chemistry, Industrial Chemistry

A catalyst is a substance that accelerates the rate of achemical reaction. The catalyst is not consumed and

therefore does not appear in the overall reaction equation. Acatalyst promoter is an additive that improves the performanceof a catalyst, but has no catalytic activity for a given chemicalconversion. Often the catalyst is written above the reactionarrow in square brackets. This indicates that a catalyst is neededfor the reaction to attain equilibrium within a reasonable time.For example, the esterification of carboxylic acids with alcoholstakes place in the presence of acids or bases, as does the reversereaction, the hydrolysis of esters. In the equation below(Scheme 1), the acid shows above the reaction arrow toindicate that this reaction is acid catalyzed.

■ ECONOMIC BACKGROUNDThe economic importance of catalysis reflects in the followingnumbers:

• More than 85% of all chemical products are manufac-tured with the help of catalysts.1

• 15−20% of the economic activities in industrializedcountries depend directly on catalysis.2

• The commercial value of the catalysts produced annuallyamounts to ∼14 billion US$.2

• In 2005 the value of the goods produced with the help ofcatalysts amounted to ∼900 billion US$.3

Looking at these figures, one has to keep in mind that catalystsemployed for environmental abatement processes, such asautomotive exhaust catalysts, do not produce any goods ofeconomic value. A breakdown of catalyst usage by industrysectors indicates that there is an almost even distribution acrossfour different sectors, namely, (i) the polymer industry (21%);(ii) coal, oil, and gas refining (22%); (iii) manufacturing ofchemicals (27%); and (iv) environmental applications (30%).2

■ INDUSTRIAL CATALYTIC CONVERTERSVarious reactor types have been designed to facilitate catalyticconversions on an industrial scale. Four examples are given inFigure 1.4 Reactor A is a stirred tank reactor operatedbatchwise. The catalyst is dissolved in the liquid phase, and itcould be an acid or a base, a metal salt, or a metal−organiccomplex. Shown is a ruthenium alkylidene complex, which isactive for the olefin metathesis reaction.5 Catalysis in solution isreferred to as homogeneous catalysis. Homogeneous catalysts areusually more active and selective than heterogeneous catalysts.They are also easier to tailor for specific purposes, as thereaction mechanism is often well understood. The majordisadvantage of homogeneous catalysis lies in the fact that it isdifficult to separate the products from the catalyst, as thecatalyst is present in the same phase. Also, homogeneouscatalysts do not lend themselves easily for continuousoperations. However, these problems can be overcome by

Published: October 9, 2012

Scheme 1. Acid-Catalyzed Esterification

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immobilizing homogeneous catalysts onto heterogeneoussupports. For a successful heterogenization of a transition-metal complex, the reaction mechanism should predict that themetal ligand chosen for bonding to the support remains firmlyattached to the metal. The cyclopentadienyl ligand is anexample for such a ligand.6

Reactors B, C, and D (Figure 1) are used for heterogeneouscatalytic processes. Example B represents a sparged stirred tankreactor for gas−liquid reactions. The catalyst is suspended inpowder form. Shown below the reactor is a precious-metalcatalyst supported on activated carbon. The catalyst can readilybe filtered off and reused until it is no longer of sufficientactivity. Spend metal catalysts are normally returned to thecatalyst manufacturer for metal recovery. Examples C and Dshow fixed-bed reactors. These operate in a continuous mode.The solid catalyst is stationary and the gaseous or liquid feed ispassed over the catalyst bed. Multitube reactors (D) allow forefficient heat removal in the case of exothermic reactions.Heterogeneous catalysts for fixed-bed operations come in manydifferent forms, as determined by the macro- and themicrokinetics of the process. The artistic composition ofexamples shown below the two reactors was taken from aClariant, formerly, Sud-Chemie catalogue, with permission.Clariant is a catalyst manufacturer. Other companies producingcatalysts for the chemical industry and other applicationsinclude BASF, Evonic, Johnson Matthey, Heraeus and Haldor-Topsoe. Reactor types used for catalytic conversions are notlimited to those shown in Figure 1. See Henkel4 for a morecomprehensive selection.

■ ENVIRONMENTAL IMPACT OF CATALYSISCatalysis is inherently a green technology. This is clearlydemonstrated by the influence catalysis has on the environ-mental factor (E-factor).7 The E-factor is defined as mass ofwaste produced per mass of desired product.

=Ekg of waste

kg of desired product

In the oil refining industry, almost all chemical conversions arecatalyzed, and the E-factor is less than 0.1. However, whenascending the value creation chain in the chemical industry, theimpact of catalysis decreases. The synthesis of pharmaceuticals,for example, is clearly dominated by traditional, multistep,stoichiometric organic chemistry, and burdened with a high E-factor; the quantity of waste produced can exceed the quantityof the targeted active pharmaceutical ingredient by a factorhigher than 25 (see Figure 2).

Another convenient measure for the environmental impact ofchemical conversions is the atom utilization.7,8 Calculation ofthe atom utilization (AU) is based on molecular weights (MW)and defined as follows:

=AUMW of desired product

Sum of MWs of all products formed in the stoichiometric equation

The usefulness of the atom utilization for the comparison ofdifferent chemical processes is demonstrated by example of tworoutes to niacin or niacinamide. Niacin or nicotinic acid is avitamin of the B group. In vivo, the acid is converted into theamide and both can be used as nutritional supplements. LonzaAG (Switzerland) is a major supplier and has developed tworoutes. The classical route is used in Switzerland, while a newcatalytic route is operated by Lonza Guangzhou in China. Theclassical route starts with acetaldehyde and ammonia (Scheme2). These are converted into 5-ethyl-2-methylpyridine by a

Tschitschibabin pyridine synthesis,9 followed by oxidation withnitric acid.10 As Scheme 2 shows, the environmental impact ofthis route is contained by recycling of the nitrous oxide, but twocarbons are lost as carbon dioxide. The overall atom utilizationis 37%. It is important to keep in mind that the atom utilization(in contrast to the E-factor) does not account for losses causedby low selectivities, as these are not reflected in the overallreaction equation. If a reactant A undergoes a variety of parallel

Figure 1. Some catalytic reactors (a, gaseous feed; b, gaseous or liquidproduct; c, liquid feed; d, liquid product; e, off-gas; f, catalyst; g,coolant). Examples for the catalysts used in each reactor type arepictured at the bottom of the figure. See the text for a detailedexplanation.

Figure 2. The impact of catalysis on the environmental E-factor.

Scheme 2. Classical Route to Nicotinic Acid (Lonza AG,Switzerland)

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or consecutive reactions and B is the desired product, theselectivity S to B (in %) is defined as

=SNumber of moles of A converted to BTotal number of moles of A converted

100%B

The catalytic route (Scheme 3) starts with 2-methylpenta-nediamine, which is cyclized to 3-methylpiperidine over an

acidic catalyst. 3-Methylpiperidine is dehydrogenated to 3-picoline over palladium.11 It follows an ammonoxidationreaction to 3-cyanopyridine using a vanadium oxide containingcatalyst. Finally, selective enzymatic nitrile hydrolysis leads topharma-grade niacinamide.12 Notably, the ammonia set free inthe first step is re-incorporated in the third step. The onlybyproducts are hydrogen and water, and the overall atomutilization is 75%. Hydrogen can be recovered for further use,and water is environmentally beneficial. The process shown istherefore a prime example of modern green chemistry. Thestarting material, 2-methylpentanediamine, is marketed byInvista (previously DuPont) under the trade name Dytek A.It is obtained by hydrogenation of 2-methylglutaronitrile, whichis a structural isomer of adipodinitrile and, therefore, abyproduct of the Nylon 6.6 manufacturing route.

■ THERMODYNAMIC AND KINETIC ASPECTS OFCATALYSIS

The thermodynamic and kinetic implications of catalysis areschematically demonstrated in Figure 3. A catalyst provides anew route with a lower Gibbs energy of activation (ΔG⧧) forthe product formation from given starting materials (reac-tants).13 In this way, a catalyst accelerates the reaction. TheGibbs energy for the reaction (ΔG⊖) remains unchanged. Thismeans a reaction that is thermodynamically unfavorable cannotbe made favorable with the help of a catalyst. (Catalysis is akinetic, not a thermodynamic, phenomenon.) All the catalyticintermediates of the reaction must be less stable than theproduct. If an “intermediate” is more stable than the desiredproduct, the reaction will stop there (dotted line in Figure 3),and this intermediate will be the final product.The addition of hydrogen to the double bond of ethylene, for

example, is thermodynamically favorable, but does not proceedat room temperature at any appreciable rate. At the turn of the19th century, Paul Sabatier discovered that such reactions couldbe greatly accelerated by the addition of finely divided nickel.14

However, he maintained that this would only be possible with

substrates in the gas phase. Wilhelm Normann proved himwrong; in 1902, he patented the catalytic hydrogenation of fatsand oils in the liquid phase.15 By 1914 not less than 25 plantsfor the saturation of fats and oils (fat hardening) were runningworldwide, mainly for the production of margarine andshortening.16,17

■ MECHANISTIC ASPECTS OF CATALYSIS ON METALSURFACES

Today’s understanding of double-bond hydrogenation is basedon the mechanism originally proposed by Horiuti and Polanyi18

for the hydrogenation of ethylene on a Pt(111) surface (Figure4). This mechanism is instructive, as the principle reaction steps

involved offer a good starting point for the understanding ofmany other reactions taking place on metal surfaces.The following steps describe the mechanism:

• Physisorption of the reactants on the metal surface.(Physisorption takes place via van der Waals forces.)

• Chemisorption of the reactants. (Chemisorption involvesthe formation of chemical bonds with the catalystsurface.) Ethylene forms an equilibrium between the π-bonded and the di-σ-bonded form, and the H−H bondof hydrogen is cleaved. According to Somorjai,19 it is

Scheme 3. Modern Catalytic Route to Niacinamide (LonzaGuangzhou)

Figure 3. Schematic comparison of the thermodynamics and kineticsof a catalyzed versus an uncatalyzed reaction.

Figure 4. Schematic presentation of the mechanism for thehydrogenation of ethylene on a platinum surface.

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mainly π-bonded ethylene that, in the following step,reacts further to the ethyl intermediate.

• Formation of the ethyl intermediate. This step must bereversible, as the complete range of deuterated ethanes(and not only DH2C−CH2D) is observed in case D2 isused instead of H2.

20

• Formation of the second C−H bond.• Desorption of the product.

The key step is the activation of the reactants by chemisorptionto the platinum surface. This facilitates the catalytic route oflower activation energy for the formation of the products.Sabatier postulated that a good catalyst provides an optimumstrength of bonding between the reactants and the catalystsurface (Sabatier’s principle).21 Both too weak and too strongbonds will slow down or prevent the reaction. This principlewas convincingly demonstrated by Rootsaert and Sachtler,22 asillustrated in Figure 5. The model reaction is the decomposition

of formic acid to carbon dioxide and hydrogen over variousmetals. It is reasonable to assume that the metal formate shownis the reaction intermediate. The strength of the bond betweenthe formate and the catalyst surface can be estimated by theenthalpy of formation (ΔfH

⊖) of the corresponding metalformate salt. ΔfH

⊖ is plotted on the x axis of the graph shownin Figure 5. The reaction temperature required to reach aprefixed reaction rate (log r = 0.8) for the decomposition offormic acid over the corresponding metal catalysts appears onthe y axis. The result is a volcano curve. The most active metalsappear at the top of the volcano curve at the lowesttemperatures required to achieve the preset reaction rate. Tothe left (Au and Ag), bond formation between the catalystsurface and the formate is too weak, as indicated by thecorresponding ΔfH

⊖ values; to the right (W, Fe, Co, Ni, andCu), the enthalpy of formation is highly negative and bondformation is too strong. In the first case, the intermediate doesnot form at a sufficiently high rate; in the second case, theintermediate is too stable and does not decompose at asufficiently high rate. The platinum group metals in the middle

strike the correct balance and provide the optimum strength ofbonding between the reactant and the catalyst surface for thereaction to occur.For ethylene hydrogenation the situation translates into the

picture shown in Figure 6.23,24 First-row transition metals and

second- and third-row transition metals appear on separatevolcano curves. The group VIII metals (periodic groups 8−10)are most active and among these the platinum group metals.It is well-known from practical applications that base metals

are less active hydrogenation catalysts than the more expensiveplatinum group metals (PGMs). Base metals generally requiremore stringent reaction conditions than PGMs to achieveacceptable results. However, for bulk chemical applications, thecatalyst price is a determining factor, and for this reason, nickeland cobalt are often preferred to precious metal catalysts (Ru,Rh, Pd, and Pt).It should be mentioned that the exact chemical nature of the

chemisorbed species that play an active role in a given catalyticconversion is often difficult to establish. The precise molecularstructure of the intermediates depends on many factors. Theseinclude

• Composition of the catalytically active metal or alloy.• Crystallite size of the metal or alloy.• Exact metal surface structure (e.g., type of exposed

crystal phase, presence of kinks and steps).• Adsorbate induced restructuring.19

• Presence of other chemisorbed species.• Nature of the catalyst support.• Presence of catalyst promoters.• Temperature and pressure.

A catalyst surface must be seen as a flexible, dynamic systemthat is able to accommodate a sequence of catalyticintermediates. The multiplicity of possible surface sites makesit difficult to determine the exact chemical environment of eachintermediate that contributes to the catalytic cycle. For anadvanced treatment of this important aspect of heterogeneouscatalysis, see van Santen and Neurock.25

■ CONVERSIONS BY METALLIC-STATE CATALYSISIt is necessary to clearly distinguish between catalysis by metalsand catalysis in the metallic state. The general term “metalcatalysis” is usually understood to include catalysis by metalcompounds, such as metal oxides, salts, and organometalliccomplexes. Homogeneous transition-metal catalysis is alwaysfacilitated by metal compounds. A border case between

Figure 5. Demonstration of Sabatier’s principle by example of thedehydrogenation of formic acid. The plot shows the reactiontemperature required to achieve a given rate in the metal-catalyzeddehydrogenation reaction versus the enthalpy of formation of themetal formates (adapted from ref 22).

Figure 6. Rate of ethylene hydrogenation over various metals relativeto rhodium (Rh = 1; adapted from ref 24).

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homogeneous and heterogeneous catalysis would be thecatalysis induced by colloidal nanoparticles kept in suspen-sion.26 Only groups VIII and IB of the periodic table (all themetals shown in Figure 7) display catalytic activity of practical

significance in the metallic state, especially for hydrogenationand oxidation reactions. Other transition elements are toodifficult to reduce and to maintain in the metallic state.24 In themetallic state, iron, cobalt, nickel, copper, and rutheniumcatalyze hydrogenation reactions, including the hydrogenationor hydrogenolysis of carbon monoxide. (Hydrogenolysis, incontrast to hydrogenation, involves the cleavage of aninteratomic connection in the substrate.) Iron and rutheniumare unique in their capability of converting nitrogen toammonia, a reaction that can be regarded as the hydrogenolysisof N2. Silver is used for some large-scale selective oxidationreactions, such as the conversion of ethylene to ethylene oxideand the oxidation of methanol and ethanol to thecorresponding aldehydes. In contrast, oxidations over Rh, Pd,Ir, and Pt are prone to result in deep oxidation, which is thetotal oxidation to CO2 and water. These four metals are alsoextremely useful for many conversions involving hydrogen,such as hydrogenations, dehydrogenations, hydrogenolyses, andnaphtha reforming. The catalytic activity of gold appears to berestricted to the nanostate,27 as confirmed by numerous morerecent reports.28

Metallic-state catalysis over group VIII and IB elementsincludes many important large-scale processes. Examples areammonia synthesis (Fe), ammonia oxidation (Pt−Rh),Fischer−Tropsch synthesis (Fe, Co), methanol synthesis(Cu), and refinery processes such as platforming.

■ METAL-DEPENDENT PRODUCT FORMATIONFROM SYNTHESIS GAS

The conversion of synthesis gas (mixtures of CO/H2 indifferent ratios) provides an interesting case concerning thechange in product selectivity depending on the catalyticelement used. This is demonstrated in Figure 7. The productselectivity changes along the first transition-metal series fromleft to right. Iron and cobalt (and also ruthenium) convertsynthesis gas into a mixture of hydrocarbons by a processknown as Fischer−Tropsch synthesis. Nickel is a selectivemethanation catalyst, and copper is used for methanol

synthesis. As indicated in Figure 7, a line drawn across theperiodic table separates copper and nickel, separates the basemetals from the PGMs up to ruthenium, divides rutheniumfrom rhodium and osmium, and finally crosses down betweenosmium and rhenium. Under normal catalytic reactionconditions, metals to the left of this line dissociativelychemisorb carbon monoxide, whereas metals to the right willchemisorb CO molecularly (without breaking the carbon−oxygen bond).29 In other words, on iron, cobalt, and nickel COdissociates, whereas it remains undissociated on the coinagemetals (Cu, Ag, and Au) and the PGMs (except Ru).Regardless of the detailed mechanisms involved,29 the fullhydrogenation of undissociated CO over a copper catalyst leadsto methanol, whereas the hydrogenation over the base metalsiron, cobalt, and nickel leads to hydrocarbons and water. Thefact that commercial nickel catalysts can produce methane witha selectivity of up to 96% is explained by the high abundance ofchemisorbed hydrogen relative to chemisorbed carbon(carbidic surface carbon) on a nickel surface. As a result,almost all the surface carbon is rapidly hydrogenated tomethane.29 This is in contrast to the situation found on iron(and cobalt). The coverage by carbidic carbon is extensive andthe reduced availability of hydrogen favors hydrocarbon chaingrowth to give alkanes, and unsaturated compounds (olefins)form as byproducts. In terms of Sabatier’s principle iron andcobalt provide the optimum bond strength for the chemisorbedspecies to form Fischer−Tropsch products, whereas nickelprovides the optimum bond strength for the formation ofmethane.The reverse reaction of methanation (double arrow in Figure

7) is known as steam reforming and is of even greater industrialimportance. Most of the synthesis gas used for processes suchas methanol synthesis, Fischer−Tropsch synthesis, and hydro-formylation (the conversion of olefins into aldehydes) isproduced by nickel-catalyzed steam reforming.

■ AUTOMOTIVE EXHAUST CATALYSIS

As indicated above, oxidations over Rh, Pd, Ir, and Pt are proneto result in deep oxidation. Environmental abatement catalystsmake use of this. The most important single application isprobably automotive exhaust catalysis. It is a remarkableachievement that the deep oxidation of hydrocarbons andcarbon monoxide and the reduction of nitrous oxides can bedone in a single catalytic converter.30 This is possible by the useof platinum or palladium or both combined with rhodium. Aschematic presentation is given in Figure 8. Rhodium is able todissociatively adsorb NO in preference to O2, whereaspalladium and platinum are responsible for the splitting ofoxygen.31 The chemisorbed nitrogen atoms combine to N2, andthe chemisorbed oxygen atoms oxidize carbon monoxide andhydrocarbons (HC) to CO2 and water.

Figure 7. Synthesis gas (CO/H2) products obtained over variousmetals depending on the mode of CO chemisorption (associative ordissociative; chemisorption of hydrogen not shown).

Figure 8. Schematic presentation of the key aspects of an automotivecatalytic converter (HC = hydrocarbons; adsorption of CO and HCnot explicitly shown).

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The introduction of the automotive exhaust catalyst showsthat legislative pressure can trigger innovative solutions andcreate business opportunities on a large scale (even for metalbrokers and speculators). To understand some of the pricefluctuation seen for rhodium over the last four decades,32 it hasto be kept in mind that the annual mining output of rhodium isonly about 25 tons. This constitutes ca. 1% of the annual goldproduction. The annual production of platinum amounts toapproximately 130 tons. In 2008, more than 75% of the totalrhodium output was used for the manufacturing ofautocatalysts.33

■ SELECTIVE OXIDATION CATALYSIS UNDER MILDCONDITIONS

Deep oxidation, or over oxidation, is likely to occur with PGMsunder oxidative conditions, unless special precautions are taken.However, for selective oxidation reactions, high-oxidation-statebase-metal oxides can be used as catalysts. These must be ableto switch between two oxidation states (redox catalysis).Examples are MoO3, V2O5, and Sb2O5. The reactions take placeat elevated temperatures in the gas phase. These conditions canbe prohibitive for fine chemical applications, due to thetemperature sensitivity of complex organic molecules. However,for applications in the fine chemicals industry, palladium andplatinum catalysts can be modified by incorporation of bismuthor lead, resulting in improved selectivity for partial oxidations.Although the exact role of bismuth and lead is still a matter ofspeculation, Besson et al.34 provided an instructive suggestionby example of the selective oxidation of an hydroxyl group (seeScheme 4).

According to this proposal, dehydrogenation of the alcoholto the aldehyde takes place on the noble metal surface. Thechemisorbed hydrogen reacts with a higher-oxidation-statebismuth oxide species, giving water and a lower-oxidation-statebismuth species. The lower-oxidation-state bismuth is re-oxidized by the oxidizing agent used for the overall reaction.The addition of bismuth keeps the platinum surface free ofexcess oxygen, thus preventing over oxidation of the substrate.The left part of the catalytic cycle exemplifies the action of acommon base-metal oxidation catalyst: bismuth switchesbetween two oxidation states and (in the case of Scheme 4)oxidizes hydrogen to water. Depending on the substrate, Pt−Bicatalysts allow for very high selectivities under mild reactionconditions. Instead of air, hydrogen peroxide can also be usedas the oxidizing agent. Examples for the oxidation ofhydroxymethylimidazoles to formylimidazoles are shown inScheme 5.35,36 Formylimidazoles are important intermediates inthe synthesis of pharmaceuticals. Further examples for thesuccessful application of Pt−Bi oxidation catalysts werereported by Anderson et al.37

■ PEDAGOGICAL ASPECTSThe content of this article can be used as the basis for anintroduction to heterogeneous catalysis. For example, it couldlay the foundation for a course on descriptive heterogeneouscatalysis or industrial process chemistry. It also could serve asan amendment to a course on homogeneous catalysis. Studentsare provided with a background on economic and ecologicalaspects of catalysis, types of catalysts used in industrial practice,modification of catalysts (to improve selectivity), theimportance of surface chemistry for the understanding ofcatalytic reactions at a molecular level, and thermodynamic andkinetic implications. Examples of industrial importance are usedfor demonstration purposes. After they have been introduced tocatalysis using the concept of this paper, learners should be ableto answer the following question: Why do certain metalsselectively catalyze certain transformations?Parts of this paper could be given as exercises. Depending on

their level, students could be asked to match catalyst types togiven catalytic converters (Figure 1). After Scheme 2 has beenexplained, students should be able to calculate the atomutilization for the reaction of Scheme 3 and compare the twoprocesses. After they have been introduced to one of theFigures 5 and 6, they could be asked to find an interpretationfor the remaining figure. On the basis of the informationcontained in Figure 7, students should be able to proposepossible catalysts for other reactions of carbon monoxide.Examples for other reactions, including those of CO, are foundelsewhere.38

■ CONCLUSIONCatalysis, and heterogeneous catalysis in particular, is a maturefield of science. Catalysis grows and diversifies further as theworld economy grows and diversifies. It contributes signifi-cantly to value creation in the real economy. From anenvironmental point of view, it impacts positively on theviability of many human activities, not only those of a purechemical nature. To continue educating the catalysis experts oftomorrow at schools and universities and to raise publicawareness for the importance of catalysis is therefore of greatsignificance. To the lay person and to many undergraduatestudents, catalysis appears as a rather complex field, not theleast because of the huge number of different catalytic materialsand the many different catalytic processes that go with them.Often students are exposed to specific applications of catalysis(e.g., descriptive heterogeneous catalysis) before they havebeen exposed to the basic principles of the relevant surfacechemistry. This often contributes to the perception thatcatalysis is a “magic” art, rather than a science. Teachingimportant fundamentals of catalysis at an early stage can beexpected to contribute to a deeper understanding. This, in turn,will allow the student to put forthcoming information intoperspective. In the paper at hand, an attempt was made toexplain some important concepts of catalysis in a brief, but

Scheme 4. Speculative Mechanism for the Pt−Bi CatalyzedOxidation of Alcohols (adapted from ref 34)

Scheme 5. Selective Oxidation of Hydroxymethylimidazolesto Formylimidazoles

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understandable manner. Applications of industrial and generalimportance were used for demonstration purposes.

■ AUTHOR INFORMATION

Corresponding Author

*E-mail: hevelingj@tut.ac.za.

Notes

The author declares no competing financial interest.

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dx.doi.org/10.1021/ed200816g | J. Chem. Educ. 2012, 89, 1530−15361536