# Heterogeneous catalysis

Monolithic low temperature oxidation catalyst

Heterogeneous catalysis is a form of catalysis in which the catalyst and the reacting substances of a chemical reaction are present in different phases , for example as gas and solid or as immiscible liquids. In industrial processes, the heterogeneous catalyst is usually in the form of a solid, a so-called contact, and the reaction partners are gases or liquids. The advantages of heterogeneous over homogeneous catalysis are the easy separation of the catalyst from reactants and products as well as the possibility to work up deactivated catalysts easily.

Heterogeneous catalytic processes for the production of basic chemicals , such as the contact process for sulfuric acid production , the Haber-Bosch process for ammonia synthesis or the production of methanol , play a decisive role in the commercial success of the chemical industry. Synthetic XtL fuels and olefinic raw materials for the chemical industry can be obtained by Fischer-Tropsch synthesis of synthesis gas from biomass , coal or natural gas .

In petroleum refineries , heterogeneous catalytic processes make a significant contribution to the production of high-octane petrol and other valuable hydrocarbon components . In environmental technology , heterogeneous catalysis is used to keep the air clean by reducing and removing pollutants from power plant emissions . The development and use of the three-way catalytic converter significantly reduced the pollutant emissions of vehicles with gasoline engines when operated with an optimal combustion air ratio.

## History of Heterogeneous Catalysis

Jöns Jakob Berzelius

In 1783 Joseph Priestley discovered the decomposition of ethanol into ethylene and water on alumina  - one of the first examples of heterogeneous catalysis. In 1796, Martinus van Marum described the catalytic dehydrogenation of alcohols to aldehydes on glowing metals such as copper . After further discoveries such as the decomposition of ammonia into nitrogen and hydrogen on iron catalysts by Claude-Louis Berthollet and the decomposition of hydrogen peroxide on silver , silver oxide and manganese dioxide by Louis Jacques Thénard , the ignition of hydrogen on platinum by Johann Wolfgang Döbereiner, found in 1823, led to the development of the Döbereiner Lighter . This lighter, produced in relatively large numbers, was used until the middle of the 19th century.

In 1835, Jöns Jakob Berzelius recognized the commonality in these reactions that, in addition to the reactants and products, another substance was always necessary in the reaction that was apparently not used up. To this end, he coined the term catalysis.

In 1894, Wilhelm Ostwald defined the process of catalysis:

Catalysis is the acceleration of a slow chemical process through the presence of a foreign substance. "

A catalyst is a substance that increases the speed of a chemical reaction without being consumed itself and without changing the final position of the thermodynamic equilibrium of this reaction. "

- Wilhelm Ostwald

In recognition of his work on catalysis, the Nobel Prize Committee awarded Ostwald the Nobel Prize in Chemistry in 1909 .

In the chemical industry, heterogeneous catalysis found its first large-scale application in sulfuric acid production using the contact process . As early as 1901, Wilhelm Normann developed fat hardening through the catalytic hydrogenation of oleic acid to stearic acid with hydrogen on finely divided nickel . It has been the basis of large-scale margarine production since 1909 .

In the early 20th century, a number of processes began to be developed that are still among the most important in the chemical industry today. Fritz Haber , Carl Bosch and Alwin Mittasch developed the synthesis of ammonia from the elements nitrogen and hydrogen on heterogeneous iron contacts at BASF in 1910, the Haber-Bosch process . Wilhelm Ostwald developed the Ostwald process of ammonia oxidation on platinum networks to nitric acid , which made the previously scarce nitrate fertilizer available on a large scale. Also at BASF in 1923, Matthias Pier developed a high-pressure catalytic process for the synthesis of methanol from synthesis gas on heterogeneous zinc oxide-chromium oxide catalysts.

In the field of refinery technology, chemists and engineers developed further heterogeneous catalytic processes. The catalytic reforming of low-octane alkanes on heterogeneous platinum- tin or platinum- rhenium on aluminum oxide contacts resulted in high-octane, aromatic and isoalkane-rich gasolines. According to this process, several million liters of high-octane gasoline are produced every day.

Gerhard Ertl

In the 1920s and 1930s, the studies of Irving Langmuir (Nobel Prize 1932) on the adsorption of gases and later the kinetic studies of Cyril Norman Hinshelwood (Nobel Prize 1956) led to a better understanding of the mechanisms of heterogeneously catalyzed reactions of two co-adsorbed reactants (Langmuir -Hinshelwood mechanism). In 1943 Eley and Rideal developed a mechanism for reactions in which one of the reactants is adsorbed and the second reacts from the gas phase.

In environmental technology , Engelhard Corporation developed the selective catalytic reduction (SCR) process of nitrogen oxides using ammonia as the reducing agent for the denitrification of power plant emissions in 1957 . A decisive contribution to keeping the air clean was the development of the three-way catalytic converter by Carl D. Keith and John J. Mooney in 1973 .

Scientists such as Gerhard Ertl (Nobel Prize for Chemistry 2007) and Gábor A. Somorjai have been investigating the elementary steps of surface reactions and the structure of adsorbates on model catalysts such as single crystal surfaces using modern surface analytical methods since the 1960s. In this way, Ertl clarified the mechanism of ammonia synthesis in the 1970s.

In the 1970s, researchers developed a number of novel zeolites with properties that were previously completely unknown. In 1972, Mobil Oil employees succeeded in laying the foundation for a whole series of new zeolites known as pentasils. The most important representative of the Pentasile is the "ZSM-5", which is used in petrochemical processes such as methanol to gasoline .

## Construction of solid catalysts

The most common physical state of heterogeneous catalysts is the solid form. In this case, the catalyst or contact either consists entirely of the active component, so-called unsupported catalysts, or the active active component is applied to a carrier material. The process parameters of the chemical reaction taking place place a number of requirements on the physical and chemical properties of the contacts. Catalyst development must therefore provide an expedient manufacturing method that provides the desired chemical and physical properties of the contacts. Solid catalysts can be roughly divided into powder, shaped, monolith or network catalysts.

### Shaped body catalysts

The shaped body catalysts usually consist of ceramic particles coated with an active component . Typical dimensions here are, for example, 3 × 3 mm to 6 × 6 mm cylinders or balls with diameters of 2 to 6 mm. The ceramic materials are often alumina , zeolites and silica . These catalysts are mostly used in so-called fixed bed reactors , in which the starting materials are continuously fed in and the resulting products are discharged. In addition to the catalytic effectiveness, important parameters here are the bulk density and pressure loss properties of the material.

### Monolith catalysts

Monolith honeycomb and ruler with centimeter scale for size comparison

In the case of monolith catalytic converters, a honeycomb body is coated with a so-called washcoat . The base body consists mostly of mineral ceramic such as cordierite or of metal . The washcoat is a powder suspension , for example a mixture of aluminum oxide , silicon dioxide and other metal oxides . This mostly aqueous powder suspension is applied to the honeycomb and dried on , for example, in three-way catalysts . It is then impregnated with a metal component, for example platinum , rhodium or palladium in the form of their aqueous acid or salt solutions, and then activated by calcination . So-called washcoating is used because the pure carrier does not have enough internal surface and therefore cannot absorb enough active metal. In addition to the catalytic effectiveness, important parameters are the cell density, i.e. the number of channels per inflow surface unit, and the pressure loss of the monolith.

### Powder catalysts

Powder catalysts, on the other hand, are mostly used in non-continuous or only semi-continuous stirred tank reactors or fluidized bed reactors . The catalyst powder is prepared by adsorbing or impregnating a carrier made of, for example, activated carbon , alumina, silica and others with a metal salt solution containing the active component. Important parameters here are, in addition to the catalytic effectiveness , the filterability , the abrasion resistance and the density of the material.

### Network catalysts

The Ostwald process for the catalytic combustion of ammonia to nitrogen oxides uses network catalysts made of a platinum-rhodium alloy. The activity of the network catalysts is determined by the wire size , the mesh size and the surface structure. Oxidation losses and the evaporation of the active catalyst component lead to a loss of catalytic activity.

## Manufacturing methods for heterogeneous catalysts

In conjunction with the reaction engineering conditions , an attempt must be made to optimize the activity , selectivity and space-time yield of the contact. In addition to the chemically important properties, the contact must meet the process requirements, that is, properties such as regenerability, abrasion resistance, reproducibility of production must be adjustable and production must take place at the lowest possible cost.

### precipitation

The precipitation , precipitation or co-precipitation is a common method for the preparation of solid catalysts. One example is the production of copper - zinc oxide- alumina catalysts for methanol production . The steps of washing, drying, calcining and activating the catalyst follow after the precipitation. The activity and selectivity of the finished catalyst can be influenced by the chemical and physical parameters of the manufacturing steps.

The choice of metal salts can already have an influence on the subsequent properties of the contact. In industrial practice, the common inorganic metal salts such as hydroxides , nitrates or carbonates are often used for production. The precipitation can be triggered by various methods such as pH adjustment, supersaturation of the solution or by adding special precipitating agents.

Washing removes foreign matter. The drying often takes place in a gas stream at temperatures that avoid decomposition of the metal salts. The calcination is carried out at temperatures between 300 and 800 ° C and converts the metal salts used into the oxides . In addition, any organic additives required for the precipitation, such as citric acid for buffering, are removed. Activation, for example by reduction with hydrogen , often takes place in situ under reaction conditions .

In addition to setting the desired chemical composition, attempts may be made to use the manufacturing conditions to produce, for example, lattice defects that have a higher catalytic activity. This can be done, for example, by inserting heteroatoms into the active phase.

### impregnation

Active component profiles of heterogeneous catalysts

Impregnation is an important method for the production of industrial supported catalysts. Here, porous support materials with a large internal surface are mixed with an aqueous solution of a metal salt. These techniques include diffusion-controlled and dry impregnation. In the following steps, the impregnated catalyst is dried and finally calcined , it being possible for the active component to be converted into the metal or the metal oxide.

By suitable choice of the impregnation conditions, profiles of the active component can be formed which are advantageous for carrying out the chemical reaction. Catalysts with a uniform active component profile are widespread: coated catalysts in which the active component is only applied in the outer area, or catalysts in which the active component is only in the interior of the catalyst. Shell catalysts are used in reactions in which pore diffusion is a limiting factor. If the catalyst is subject to mechanical abrasion during the process, it is advantageous if expensive noble metal is only present in the interior of the contact.

#### Diffusion-controlled impregnation

In the first step of diffusion-controlled impregnation, the contact is wetted in excess with pure solvent. A metal salt is then added to the mostly aqueous solution. The metal ions migrate into the pores of the contact by diffusion and are adsorbed there. The process of diffusion-controlled impregnation often leads to a uniform distribution of the active component over the catalyst grain. Here, however, the time required is greater than with dry impregnation. If the metal salt used in the impregnation is only weakly absorbed, the metal profile may be significantly influenced by the drying conditions.

#### Dry impregnation

With dry impregnation or capillary impregnation, the contact is mixed with a solvent that already contains the dissolved active component. The amount of solvent is at most equal to the total pore volume of the contact mass, so the contact appears dry after impregnation. The driving force behind the impregnation in this case is the capillary force. They and the resulting heat of adsorption mean that some of the gases trapped in contact cannot escape, which can lead to the catalyst grain bursting during impregnation. In order to prevent this, the contact is partially placed under vacuum before the impregnation.

### Melt

Various heterogeneous catalysts can be obtained by melting suitable precursor components or metals. Examples are platinum-rhodium alloys, which are obtained by melting and which are used for platinum-rhodium networks in the oxidation of ammonia to nitrogen oxides.

### Sol-gel process

Sol, gel, xerogel and airgel

The sol-gel process can be used to produce inorganic catalysts from colloidal dispersions, the so-called sols. The starting materials or precursors used are preferably organometallic compounds such as aluminum (2-propylate) , aluminum (2-butylate) , zirconium propylate , titanium ethylate or titanium (2-propylate) , which are first hydrolyzed . By crosslinking and drying, powders, fibers, layers or aerogels of high purity and with a defined pore size distribution can be produced.

### Chemical vapor deposition

In addition to applying the active component from the liquid phase, this can be done by adsorbing volatile inorganic or organometallic compounds from the gas phase. After the adsorption, the chemical or thermal decomposition of the precursor takes place , releasing the ligands and fixing the metal at the adsorption site.

The process is usually carried out at higher temperatures in a vacuum. In addition to the parameters of temperature and pressure, the selection of the precursor, especially its volatility and thermal stability under the selected process conditions, is decisive for the implementation.

### Heterogenization of homogeneous catalysts

Water soluble tri- (sodium-meta-sulfonatophenyl) -phosphine (TPPTS)

By heterogenizing homogeneous catalysts, an attempt is made to combine the advantages of homogeneous catalysis, such as high selectivity and complete availability of the catalytically active species, with the advantages of heterogeneous catalysis, such as the ease with which the catalyst and reactants can be separated.

The heterogenization of homogeneous transition metal complexes usually takes place by fixing the soluble complexes on a solid support. The fixation can take place covalently by modifying the complex ligands, but ionic or adsorptive fixation is also possible. In the case of porous solids, physical incorporation into the pore structure of the solid is possible. Examples show that too strong a fixation leads to a decrease in the catalytic activity, while too weak a fixation leads to "bleeding" or " leaching " of the complex from the solid.

Another method is heterogenization in liquid-liquid systems, where the metal complex catalyst becomes water-soluble through ligand modification and can thus be easily separated from the organic phase that is formed. One example is hydroformylation according to the Ruhrchemie / Rhône-Poulenc process, where rhodium is complexed with tri- (sodium-meta-sulfonatophenyl) -phosphine , which has hydrophilic properties due to the ligand substitution with sulfonate groups. The reaction takes place in the aqueous phase. The organic product phase is insoluble in water and is separated by means of phase separation, the aqueous catalyst phase is fed back into the reactor. The starting materials used are propene and synthesis gas, which consists of hydrogen and carbon monoxide in a ratio of 1.1: 1. The Ruhrchemie / Rhône-Poulenc process is one of the first commercialized two-phase systems in which the catalyst is in an aqueous phase.

## Catalyst deactivation and regeneration

The mechanisms of catalyst deactivation are diverse. The deactivation can be roughly divided into mechanical, for example by abrasion or disintegration, thermal such as for example by sintering, physical such as coking or the physical blockage of active centers, and chemical deactivation by the formation of inactive metal components such as sulfides.

In heterogeneous catalysis, coking, sintering of the active surface or the disintegration of the catalyst due to mechanical abrasion, for example in fluid bed processes, are known in refinery processes. Aging processes can reduce the catalytically active surface or clog pores, for example in zeolites. Deactivation through phase changes is often irreversible. It is observed, for example, in zinc-aluminum oxide catalysts for the synthesis of methanol that have been exposed to excessively high temperatures. The formation of a spinel phase deactivates the catalyst and cannot be regenerated.

The regeneration processes include, for example, the burning off of coke from contacts that are used in cracking processes or catalytic reforming , or oxychlorination to restore acidic centers. If the catalyst is deactivated to such an extent that regeneration no longer makes sense, the catalyst is discharged from the process. In the case of noble metal catalysts , the supports are melted and the noble metal is recovered through smelting and electrochemical processes.

## Analytical methods for the characterization of heterogeneous catalysts

Heterogeneous catalysts can be analyzed using the conventional methods for analyzing solids. The chemical composition can be determined using methods such as atomic absorption spectroscopy or X-ray fluorescence analysis. In addition, scientists developed special methods for catalyst characterization. The selective adsorption of hydrogen or carbon monoxide on metal centers is used to determine the dispersion of metals. In addition to the chemical composition, the type of phases present in contact, the pore structure, the inner surface, the characterization of acidic centers and the surface properties are of interest.

### Surface sensitive methods

Measuring system for X-ray photoelectron spectroscopy (XPS) with hemispherical analyzer , X-ray tubes and various preparation methods

Special methods of surface analysis are used to examine structures and processes on surfaces . They only detect processes and structures that differ from those of the solid. For this purpose, the interactions of electrons, photons, neutral particles, ions or heat with the surfaces are used. A multitude of microscopic, spectroscopic, thermoanalytical adsorption and diffraction methods have been developed for various questions.

A prerequisite for the surface sensitivity of a method is often that the interacting or the detected particle has a short mean free path in the matter. Therefore, an ultra-high vacuum is necessary for many methods . Since many reactions take place under higher pressure, it is often difficult to transfer the knowledge gained with surface-sensitive methods into industrial practice.

### Determination of the inner surface

The BET measurement, named after the last names of the developers of the BET model Stephen Brunauer , Paul Hugh Emmett and Edward Teller , is an analytical method for determining the inner surface of porous solids by means of gas adsorption . By determining an adsorption-desorption isotherm of nitrogen in certain pressure ranges, the measured amount of adsorbed or released nitrogen is proportional to the inner surface. The BET surface area is given in m 2 · g −1 .

### Determination of acidic centers

One method for determining the acidity of heterogeneous catalysts is the temperature-programmed desorption of ammonia . A distinction can be made between Lewis and Brønsted centers using infrared spectroscopy or the desorption temperature.

For verification purposes, the catalyst sample is first saturated with ammonia and then heated, for example in a temperature-programmed manner, in an inert gas stream, and the desorbed ammonia is recorded quantitatively. Physisorbed ammonia without accumulation on acidic centers desorbs at temperatures between 150 and 200 ° C. Ammonia, which desorbs in the range between 200 and 400 ° C, is attributed to Lewis acidic centers. Ammonia that desorbs at higher temperatures of 400 to 600 ° C comes from the decomposition of ammonium ions formed by the reaction of ammonia with Brønsted centers.

### Determination of the metal dispersion

The activity of supported catalysts is determined by the dispersion of the metal on the support material, since only the surface atoms of the metal can take part in the chemical reaction. A method that is often used to measure the dispersion of metals is the chemisorption of hydrogen or carbon monoxide. It is assumed that the stoichiometric metal / hydrogen ratio is 1: 1, i.e. that the hydrogen is dissociatively chemisorbed. If the hydrogen is soluble in the metal, as is the case with palladium catalysts, for example , the result of the hydrogen chemisorption measurement may be faulty.

The amount of chemisorbed hydrogen can be determined gravimetrically or volumetrically. Alternatively, the chemisorbed hydrogen can be titrated with oxygen and the resulting water detected.

### Determination of the pore size distribution

According to IUPAC, there are three size ranges for pores:

• Micropores with a diameter of less than 2 nm
• Mesopores with a diameter of 2 nm to 50 nm
• Macropores with a diameter greater than 50 nm

The pore size distribution of a heterogeneous catalyst can be determined by means of mercury porosimetry . The property of mercury to behave like a non-wetting liquid is the basis of this method. For this purpose, mercury is pressed into pores of different sizes at pressures of up to 4000 bar. Initially the large pores and, at higher pressures, the smaller pores are recorded. Statements about the nature, shape, distribution and size of the pores can be made via the dependence of the amount of mercury on the pressure applied. Relatively large pore area distributions can be determined using mercury porosimetry.

### Determination of the particle size

To determine the particle size of metal particles dispersed on supports, both direct imaging methods such as transmission electron microscopy and scattering methods can be used. Direct imaging methods require more effort for sample preparation and evaluation for a statistically reliable statement about the particle size.

Small-angle X-ray scattering (SAXS) can be used to determine the particle size of metals dispersed on carrier materials in the range from approx. 1 to 100 nm . For this purpose, the catalyst sample is exposed to monoenergetic X-rays and the intensity of the scattered X-rays is determined at small scattering angles.

## Kinetics of Heterogeneous Catalysis

The seven steps of heterogeneous catalysis on porous catalysts

The heterogeneous catalysis in a porous catalyst grain can be divided into seven sub-steps, each of which can be rate-determining. The first step is the diffusion of the reactants to the surface of the contact through the stationary boundary layer. The thickness of the boundary layer changes with the flow velocity. The second step is the diffusion of the reactants in the pores of the contact to the catalytically active center. In the third step, the reactants are adsorbed on the active center. The fourth step is the reaction of the reactants on the surface. One possible step in heterogeneous catalysis is spillover . An activated species diffuses from one catalytically active center to another center that is chemically different from the first center.

The products are now transported away in reverse order: In the fifth step, the products desorb from the active center. The products then diffuse through the pore system of the contact as a sixth step. In the seventh step, the products diffuse through the boundary layer into the main gas flow and are transported away.

As with all consecutive reactions, only the slowest elementary step determines the rate. Hougen and Watson developed a general approach to determining the gross reaction rate for heterogeneously catalyzed reactions: ${\ displaystyle r _ {\ rm {b}}}$

${\ displaystyle r _ {\ rm {b}} = {\ frac {\ rm {{kinetic \ term} \ \ times \ {\ rm {potential term}}}} {\ rm {{adsorption term} ^ {n}}} }}$

The kinetic term comprises the rate constants of the rate-determining elementary step, the potential term comprises the concentration terms and the reaction order, and the adsorption or inhibition term comprises the occupancy of the catalytically active centers. The exponent stands for the number of centers involved in the elementary reaction. ${\ displaystyle n}$

### Langmuir-Hinshelwood mechanism

Irving Langmuir

The Langmuir-Hinshelwood mechanism describes a method for representing the reaction rate of a two- or multi-component heterogeneously catalyzed reaction as a function of partial pressure or concentration with adsorption of all reactants .

Irving Langmuir studied in 1915 the wear of the tungsten - filament by oxygen ; it led to the development of a theory about the adsorption of gases as a function of partial pressure. In connection with the theory of the active centers by Hugh Stott Taylor Langmuir succeeded in 1927 formulating the Langmuir-Hinshelwood kinetics. Up until the development of surface-sensitive analysis methods in the second half of the twentieth century, kinetic analyzes were an important instrument for studying heterogeneously catalyzed processes.

The adsorption of a molecule A on the surface S of a catalyst can be described by the following reaction equation (1):

${\ displaystyle {\ rm {(1) \ quad A + S \ to \ AS}}}$

The law of mass action (2) is:

${\ displaystyle {\ rm {(2) \ quad K_ {A} = C_ {AS} / (C_ {S} P_ {A})}}}$

with the equilibrium constant K A . For a unimolecular heterogeneously catalyzed reaction of a substance A it is assumed that the reaction rate is proportional to the degree of coverage . This is defined as the concentration of chemisorbed A per unit mass of the catalyst according to equation (3): ${\ displaystyle \ theta _ {A}}$${\ displaystyle C_ {AS}}$

${\ displaystyle {\ rm {(3) \ quad \ theta _ {A} = C_ {AS}}}}$

The concentration of the free active surface sites is thus defined by equation (4): ${\ displaystyle C_ {S}}$

${\ displaystyle {\ rm {(4) \ quad C_ {S} = (1- \ theta _ {A})}}}$

Substituting into equation (2) and rearranging according to , one obtains ${\ displaystyle \ theta _ {A}}$

${\ displaystyle {\ rm {{(5) \ quad \ theta _ {A} = K_ {A} P_ {A} / (1 + K_ {A} P_ {A})},}}}$

where K A is the equilibrium constant of adsorption and P A is the partial pressure of component A.

The reaction rate r is equal to the product of the degree of coverage and the rate constant k:

${\ displaystyle {\ rm {(6) \ quad r = k \ theta _ {A} = kK_ {A} P_ {A} / (1 + K_ {A} P_ {A})}}}$

For very small values ​​of K A P A (K A P A << 1) results

${\ displaystyle {\ rm {{(7) \ quad r = kK_ {A} P_ {A}},}}}$

that is, the reaction is first order with respect to P A . If K A P A is very large (K A P A ≫ 1), this results

${\ displaystyle {\ rm {{(8) \ quad r = k},}}}$

that is, the reaction is zero order with respect to P A .

For reactions between two molecules, the mechanism is based on the assumption that two reactants  A and B are adsorbed on the surface  S of a heterogeneous catalyst and that a bimolecular reaction takes place in the adsorbed state:

${\ displaystyle {\ rm {\ \ (9) \ quad A + S \ rightleftarrows \ AS}}}$
${\ displaystyle {\ rm {(10) \ quad B + S \ rightleftarrows \ BS}}}$
${\ displaystyle {\ rm {(11) \ quad AS + BS \ {\ xrightarrow {k}} \ AB + 2 \ S}}}$

The rate constant for the reaction of A and B is . ${\ displaystyle k}$

The concentration C of the surface sites AS and BS occupied by A and B is defined as and , the degree of coverage as and the number of active centers as . With the definition ${\ displaystyle C_ {AS}}$${\ displaystyle C_ {BS}}$${\ displaystyle \ theta}$${\ displaystyle C_ {S}}$

${\ displaystyle {\ rm {\ theta _ {A} = K_ {A} P_ {A} / (1 + K_ {A} P_ {A} + K_ {B} P_ {B})}}}$
${\ displaystyle {\ rm {\ theta _ {B} = K_ {B} P_ {B} / (1 + K_ {A} P_ {A} + K_ {B} P_ {B})}}}$

the degree of coverage and the law of speed reads: ${\ displaystyle \ theta _ {A}}$${\ displaystyle \ theta _ {B}}$

${\ displaystyle {\ rm {r = k \ theta _ {A} \ theta _ {B} C_ {S} ^ {2}}}}$

When two molecules compete for the same adsorption site, the reaction rate is greatest for

${\ displaystyle {\ rm {{\ theta _ {A} = \ theta _ {B}}.}}}$

At a constant partial pressure of B, the reaction rate as a function of the partial pressure of A passes through a maximum. In the oxidation of carbon monoxide on palladium catalysts , for example, the phenomenon occurs that when a certain carbon monoxide partial pressure is exceeded, the free surface spaces of the catalyst are invariably occupied by carbon monoxide, so that the oxidation reaction comes to a standstill.

### Eley-Rideal mechanism

In the Eley-Rideal mechanism proposed by DD Eley and EK Rideal in 1938, reactant A initially adsorbs on the catalyst surface:

${\ displaystyle {\ rm {A_ {g} \ rightarrow A_ {ads}}}}$

The adsorbed educt then reacts with another educt B from the gas phase to form product C:

${\ displaystyle {\ rm {A_ {ads} + B_ {g} \ rightarrow C_ {ads}}}}$

In the last step, product C desorbs :

${\ displaystyle {\ rm {C_ {ads} \ rightarrow C_ {g}}}}$

The Eley-Rideal mechanism results from the Langmuir-Hinshelwood mechanism as a limiting case when the adsorption coefficient of B approaches zero. In this case, the reaction rate is proportional to the degree of coverage of A and the partial pressure of B:

${\ displaystyle {\ rm {r = k \ theta _ {A} P_ {B}}}}$

Reactions that proceed according to a pure Eley-Rideal mechanism are relatively rare and so far only proven for a few reactions, for example for the hydrogen- deuterium exchange. Often weakly bound surface species are involved in the mechanism. The reason for this is the response times. A gas-surface collision lasts only a few picoseconds , while surface-bound species can have a lifetime of a few microseconds .

## Reaction kinetic investigation methods

Heterogeneous Catalysis Regimes

The aim of the kinetic investigations is, among other things, to determine the rate-determining step of the heterogeneously catalyzed reaction. A distinction is made between different regimes. The regime describes the transport or chemical process that determines the overall kinetics of the reaction. In addition to the chemical reaction, diffusion-controlled steps such as diffusion in the boundary layer or pore diffusion occur as rate-determining steps.

Kinetic investigations of gas-solid reactions are carried out experimentally in devices such as the integral tapping point reactor or the differential circulating gas reactor . Both types of apparatus have different methodological and apparatus-related advantages and disadvantages and must be designed for the respective investigation, the simplest form being the fixed bed reactor with different tapping points.

### Integral reactor

The integral reactor is often designed as an easy-to-build fixed-bed tubular reactor with taps. For this purpose, taps are installed at regular intervals along the reactor, which allow sampling and thus the determination of a concentration profile over the length of the reactor. The design often resembles a small-scale industrial reactor. The dimensions should be chosen so that the use of industrially used catalysts is possible and thus transport processes roughly correspond to those in industrial practice.

A significant disadvantage is the occurrence of concentration and possibly temperature gradients which prevent a direct measurement of the reaction rate. The processing of the measurement data usually leads to differential equations that can only be solved numerically.

### Differential reactor

Berty reactors are used as differential reactors . It is a circulating gas reactor in which a turbine generates a circulation stream, whereby the starting materials and products are mixed. Typical speeds are on the order of 4,000 to 10,000 revolutions per minute.

The amount of catalyst used is small in this reactor. If the circulation volume flow compared to the input volume, the so-called circulation or return ratio, is greater than 10, conditions are achieved which correspond to the ideally mixed stirred tank. Concentration or temperature gradients do not occur. The isothermal energy enables the temperature dependency of the conversion to be determined precisely. A disadvantage are the relatively large wall surfaces and dead spaces, which can lead to blind reactions and thus falsify the measurement results.

## Parameters and figures

An important parameter of heterogeneous catalytic processes is the space velocity (English: gas hourly space velocity ; GHSV; for liquids: liquid hourly space velocity ; LHSV). The GHSV is the quotient of the gas volume flow and the catalyst volume. The GHSV is required if the amount of catalyst has to be calculated that is needed for a given residence time of the components in the catalyst volume.

E. W. Thiele introduced the Thiele module in 1939 in order to describe the relationship between diffusion and reaction rates in porous catalyst pellets , in which diffusion through the outer boundary layer does not determine the rate . For the sake of simplicity, it is assumed that the reaction takes place in the pores in the interior of the pellet and, for simplification, that the pore is represented by a cylinder with the length L , at the end of which the reaction is terminated. The Thiele module for first-order reactions is thus defined as the root of the quotient of the reaction and diffusion speed, multiplied by the pore length.

The Thiele module can be used to determine the catalyst efficiency, the ratio of the effective reaction rate to the reaction rate without pore diffusion inhibition.

The Weisz-Prater criterion or the Weisz module is used to assess the influence of pore diffusion on the reaction rate in heterogeneous catalytic reactions . The Weisz module is linked to the Thiele module via the catalyst efficiency.

## Large-scale application examples

The table lists a selection of important large-scale applications of heterogeneously catalyzed processes.

process Educts Products catalyst annotation
Sulfuric acid production ( contact process ) SO 2 + O 2 SO 3 , H 2 SO 4 Vanadium oxide
Ammonia synthesis ( Haber-Bosch process ) N 2 + H 2 NH 3 Iron oxides on aluminum oxide requires 1% of global energy consumption
Nitric acid production ( Ostwald process ) NH 3 + O 2 ENT 3 Pt-Rh networks direct production from N 2 does not make economic sense
Hydrogen production in steam reforming CH 4 + H 2 O H 2 + CO 2 Nickel on K 2 O other production routes directly from water are being investigated intensively
Hydrocarbon mixtures according to the Fischer-Tropsch process CO + H 2 C n H m + H 2 O Cobalt, iron, nickel, ruthenium on carriers with large specific surfaces wide range of catalysts, pressures and temperatures with different target products
Ethylene oxide synthesis C 2 H 4 + O 2 C 2 H 4 O Silver on aluminum oxide , with many promoters not applicable to other olefins
Hydrogen cyanide synthesis ( Andrussow process ) NH 3 + O 2 + CH 4 HCN Pt-Rh the related process of ammoxidation converts hydrocarbons into nitriles
Olefin polymerization ( Ziegler-Natta process ) Ethene , propene Polyethylene , polypropylene TiCl 3 on MgCl 2 there are many process variants
Desulfurization of petroleum ( hydrodesulfurization ) H 2 + R 2 S RH + H 2 S Mo - Co on alumina produces low-sulfur hydrocarbons, the sulfur is recovered in the Claus process
Fat hardening ( hydrogenation ) H 2 + unsaturated fatty acid esters saturated fatty acid esters Raney nickel , nickel on carrier used in margarine production
Catcracking ( Fluid Catalytic Cracking ) long chain hydrocarbons short chain hydrocarbons Zeolite Y and other zeolites produces short-chain hydrocarbons

## literature

Commons : Heterogeneous Catalysis  - Collection of Images, Videos and Audio Files
Wiktionary: Catalysis  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

1. a b c d Dirk Steinborn : Fundamentals of organometallic complex catalysis. Verlag Teubner BG GmbH, 2007, ISBN 3-8351-0088-2 , p. 2.
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 This version was added to the list of articles worth reading on October 10, 2013 .