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Heterogeneous catalysis (I)Heterogeneous catalysis (I)

Francisco JosFrancisco Jos MaldonadoMaldonado HHdardarDpt. of Inorganic ChemistryDpt. of Inorganic Chemistry

University of Granada. Spain.University of Granada. Spain.Email:Email: [email protected]@ugr.esPhone: +34 958240444Phone: +34 958240444

Advanced Catalysis and Organometallic ChemistryIntensive Programs (IP)

Lifelong Learning Program ErasmusCamerino (Italy) 16-28 August 2009

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Contents

1. Introduction: the importance of catalysis and some historical data.

2. Concepts:

- Catalyst

- Activation energy- The catalytic cycle

3. The catalyst selection

4. Heterogeneous catalysis and surface science

- Structural consideration

- Surface and adsorption


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Contents

5. About the active sites of surfaces

- Relationships of crystal defects and catalytic activity

- The nature of the crystallographic layer

- The geometric factor

- Bi - functional catalysts

- Acid sites in zeolites

- Electronic properties

6. Catalysts preparation:

- Bulk catalysts

- Supported catalyst

7. The catalysts deactivation / regeneration

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IntroductionIntroduction


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The importance of catalysisThe importance of catalysis

More than 90% of the industrial processes areMore than 90% of the industrial processes arecatalyzed (chemical, pharmaceutics, materials,catalyzed (chemical, pharmaceutics, materials,polymers, energy, etc)polymers, energy, etc)

Raw materials Catalyzed processes Valuable products

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The importance of catalysisThe importance of catalysis

EnvironmentalCatalysis


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There is no a single theory of heterogeneous catalysis, but onlyThere is no a single theory of heterogeneous catalysis, but only aaseries of principles to interpret and predict the processes thatseries of principles to interpret and predict the processes thatoccur, mainly in the gasoccur, mainly in the gas solid interface.solid interface.

What to know ?What to know ?

Catalysis is an inter-disciplinal science!!!

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Some historical data

Fermentation is the oldest type of catalytic processes. Several thousand years BCbeer, bread and winemaking were typical fermentation processes. Today,thousands of different catalysts (enzymes) are available to perform thesetransformations, which has been classified by six major groups of reactions:

Oxidation and reduction OxidoreductasesTransfer of functional groups TransferasesHydrolysis Hydrolases

Addition and elimination LyasesIsomerisation IsomerasesCarbon bond formation Ligases


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Among the first industrial catalytic processes are a few inorganic oxidation processes,as the Deacon process (HCl into Cl2) and the production of sulfuric acid. Theproduction of sulfuric acid was commercialized in the mid-18th century. In the so-called lead chamber process the oxidation of SO2 into SO3 was catalyzed by NO.

Industrial catalyzed processes

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Many reasons underlie the development of the science and technologyof catalysis.

One of the main driving forces is the availability of the raw materials.As an example, in the energy field:

Biomass was originally predominant.Later coal became the most important industrial feedstock.

Coke oven gas components played the role of base chemicals.Subsequently, oil took over the place of coal and the technological scenechanged profoundly (petrochemical industry)More recently, natural gas resources appear to have become much moreimportant and a lot of research in catalysis is aimed at processes based onnatural gas.It is also clear that biomass is experiencing a revival as an environmentalcare alternative to oil (biodiesel, syngas and Fischer Tropsch synthesis,etc.

In recent years environmental considerations have been also the majordriving force for novel catalytic processes.


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ConceptsConcepts

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The name 'catalysis' was coined by Berzelius in 1836. He concluded that besides'affinity' a new force is operative, the 'Catalytic Force'. With this, try to focus thebehavior of some substances that permit initiate reaction of decomposition orsynthesis. At that time no understanding existed, on a molecular level, ofreaction rates.

Definition of catalyst

According to the IUPAC (1976) a catalyst is a substance that, being present insmall proportions, increases the rate of attainment of chemical equilibrium,without itself undergoing chemical change.

Comments

- This means that catalyst is not a reactant (is not consumed by the reaction)

- In fact, catalyst undergoes transitory changes to catalyze the reaction, andpermanent changes which typically leads to the catalyst deactivation

What is a catalyst?


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Types of catalysts:

Catalytic processes can be divided in three main types: heterogeneoushomogeneous and enzymatic processes.

In a heterogeneous reaction, the catalyst is in a different phase fromthe reactants. Normally, the catalyst is a solid and reactants are fluids (liquids orgases). It is characterized by the presence of active sites on the catalyst surface.

In a homogeneous reaction, the catalyst is in the same phase as thereactants (as the hydrolysis of esters by acid catalysts, all reactants and catalystare dissolved in water:

H+

CH3CO2CH3(aq) + H2O(l) CH3CO2H(aq) + CH3OH(aq)

The enzymatic catalysis (biocatalysis) have an intermediate characterbetween homogeneous and heterogeneous processes, because although theenzymes and reactants are in the same phase (solution), they have active sites in their structures.

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Kinetic Vs. Thermodynamic:

the necessity of catalysts

A reaction may have negativeA reaction may have negative G, but the reaction rateG, but the reaction rate

may be so slow that there is no evidence of it is occurring.may be so slow that there is no evidence of it is occurring.

Conversion of diamond to graphite is a

thermodynamic favor process (G


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A catalyzed reaction is therefore a sequence of elemental steps,continuously repeated, where the catalyst is regenerated after eachreaction cycle.

R

RC

P

C

R + C RC

RC P + C

Thus, for a given reaction:

R P (R = reactant, P = Product)

where the reaction rate is null or negligible, the use of a catalyst (C)provide a new reaction pathway trough the formation of anintermediate specie on the catalyst surface

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Activation Energy : The energy required to overcome the reactionbarrier. Usually given a symbol Ea or G

The Activation Energy (Ea) determines how fast a reaction occurs.The lower the Activation barrier, the faster the reaction

Activation Energy


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Nevertheless, taking into account thatthe initial and final states are thesame by both un-catalyzed andcatalyzed process, the overall freeGibbs energy, G0 is thereforeidentical.

This means that all thethermodynamic parameters, includingthe equilibrium constant

K = exp (- G

0

/RT)were not affected and catalysts onlypermit to attain the equilibrium morequickly.

G0

equilibrium

without catalyst

with catalyst

Time

co

n

ve

rsio

n

Activation Energy

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The progress of a heterogeneous catalytic reactions can be resolved into at leastfive distinct steps:

(i) Diffusion of the reactants to the catalyst(ii) Adsorption of reactants (reactant-surface)(iii) The chemical changes on the surface, formation of the adsorbed complex(iv) Decomposition of the adsorbed complex (product-surface)(v) Desorption and diffusion of the reaction products from the catalyst

diffusion of the reactantsto the catalyst (i)

Diffusion and reaction (iii, iv)

Desorption of reactionproducts (v)

adsorption (ii)


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Each step of the heterogeneous catalytic cycle involves acharacteristic energy.

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(i, ii) diffusion and adsorption

If no diffusion restriction occurs, one or more of the reactants areadsorbed on to the surface of the catalyst

Be careful!

Adsorption is where something sticks to a surface. It isn't the same as

Absorption where one substance is taken up within the structure ofanother.

Differences between physisorption and chemisorption will be analyzedbefore.

Steps of reaction

Heterogeneous Catalysis Must Always

be Preceded by Adsorption!!!


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Langmuir-Hinshelwood mechanisms

Example: The Reaction A2 + 2 B = 2AB

A2 + * = A2*

A2* + * = 2A*

B + * = B*A* + B* = AB* + *

AB* = AB + *

Eley-Rideal mechanism:

A2 + * = A2*

A2* + * = 2A*

A* + B = AB + *

How adsorption is produced on the catalyst surfacedetermine the reaction mechanism:

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Eley-Rideal or Langmuir-Hinshelwood?

For the Eley-Rideal mechanism:

the rate will increase with increasing coverage until the

surface is completely covered by A*.

For the Langmuir-Hinshelwood mechanism:

the rate will go through a maximum and end up at zero,

when the surface is completely covered by A*.

This happens because the step B + * = B*

cannot proceed when A* blocks all sites.

The trick is that the step B + * = B*

requires a free site.


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(iii, iv) The adsorbed species can move, interacting between them and

with the catalyst surface and reaction happens through the formation anddecomposition of an intermediate adsorbed specie.

At this stage, both of the reactant molecules might be attached to the surface,or one might be attached and hit by the other one moving freely in the gas orliquid.

The mobility of chemical species on the catalyst surface is also noticeable,phenomena such the spillover of hydrogen atoms play an important role inhydrogenation reactions.

The apparition of bonds or interactions with the catalyst surface debilitates ordestroys the intra-molecular bonds, inducing the reactions.

(v) The product from the decomposition of the intermediate specie shouldbe desorbed and diffuse in the flow.

Desorption simply means that the product molecules break away. This leavesthe active site available for a new set of molecules to attach to and react.

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Example:

The hydrogenation of a carbon-carbon double bond

The simplest example of this is the reaction between ethene and

hydrogen in the presence of a nickel catalyst.

In practice, this is a pointless reaction, because you are converting theextremely useful ethene into the relatively useless ethane.

Ethene hydrogenation is widely used asreaction test of new hydrogenation catalysts

CH2 = CH2 + H2 CH3-CH3Ni


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Ethene molecules are adsorbed on the surface of the nickel. The doublebond between the carbon atoms breaks and the electrons are used to bond itto the nickel surface

Hydrogen molecules are also adsorbed on to the surface of the nickel. Whenthis happens, the hydrogen molecules are broken into atoms. These can movearound on the surface of the nickel.

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If a hydrogen atom diffuses close to one of the bonded carbons, thebond between the carbon and the nickel is replaced by one betweenthe carbon and hydrogen

As before, one of the hydrogen atoms forms a bond with the carbon, and that endalso breaks free. There is now space on the surface of the nickel for new reactantmolecules to go through the whole process again.


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The catalyst selectionThe catalyst selection

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Criteria for a good catalyst

Chemical related aspect

Activity

Selectivity

Stability (thermal properties)

Non-chemical

Morphology

Mechanical strength

Cost


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About the catalyst selection

Example: Transformations of syngas obtained by gasification on steam ofcoal or biomass, or by partial oxidation of natural gas. Depending on thecatalyst used, a wide diversity of products can be generated.

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Definitions of conversion and selectivity

Conversion is defined as the fraction of reactant transformed:

Conversion (%) = XT = [Ri] [Rf] / [Ri] * 100

While selectivity is defined for each product on the basis of it formation:

Selectivity (% Px) = [Px] / [P] = [P1] / [Ri] [Rf] *100

The reaction yield (or partial conversion) for a given product is therefore:

Y (% i) = XT Si = [P1] / [Ri]

RiRf+ P1 + P2

T, P, flow


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RiRf+ P1 + P2

T, P,

flow

Be careful with the meaning of the curves X vs time!!

Continuous flow reactors

Conversion(%

Time on stream

T1

T2

Curves of conversion vs reaction times are often used todetermine the catalyst stability. In continuous flow reactorsconversion tends to decrease until reaching the stationarystate at each experimental conditions. The longer time atconstant conversion values, the more stable catalysts.

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R1 R2

Products

Semicontinuous reactors

Conversion(%)

Time on stream

T1

T2

In semicontinous reactors where reactants are loaded initially,conversion vs time curves indicates the conversion of this chargeuntil the equilibrium is reached, then, the reactor is discharged.

Obviously, catalysts are not activated during reaction.

Be careful with the meaning of the curves X vs time!!


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The catalytic activityThe catalytic activity

In general, the rate of any gas-solid or liquid-solid-catalyzed reaction can be

expressed as the product of the apparent rate coefficient k and a pressure- (orconcentration-) dependent term:

r = K PiK, apparent rate coefficientPi, Partial pressure of reactant i

this rate coefficient (K) will change as the prevailing conditions of the reactionvary, and it is convenient to use the Arrhenius equation:

where A' is a temperature-independent pre-exponential factor and E' is theapparent activation energy of the catalytic reaction. E' cannot be expected tobe the true activation energy because the concentration of reactant at thecatalyst surface will be temperature-dependent.

Thus, the catalytic activity can not be defined

in terms of apparent activation energy.

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Turnover frequencyTurnover frequencyFar more convenient is the use of the concept of turnover frequency or turnovernumber. The turnover frequency (often designated TOF) is simply the numberof times n that the overall catalytic reaction in question takes place per catalyticsite per unit time for a fixed set of reaction conditions (temperature, pressure orconcentration, reactant ratio, extent of reaction).

where S is the number of active sites.

Note that TOF is a rate, not a rate coefficient, so that it is necessary to specifyall the prevailing conditions of the catalytic reaction.

It is sometimes difficult todetermine the number of activesites. For such situations, S isoften replaced by the total,readily measurable, area A ofthe exposed catalyst.


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RiRf+ P1 + P2

T, P, flow

About the catalyst selection

A good catalyst must possess both high activity and long-termstability. But its single more important attribute is selectivity.

Selectivity is more important than activity:

The unconverted reactant can be re-circulated.

It is not interesting the transformations of reactants in valueless products.

The selective transformation of reactants facilitate the final separation process.

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Heterogeneous catalysisand Surface Science


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Heterogeneous Catalysts is a

Surface Science!!!

The catalytic processes develops ONLY on the catalyst surface.

Therefore, the catalyst surface should be as large as possible,but moreover, surface must be accessible to the reactants.

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Reactants will be chemisorbed preferentially in the more energetic sites

Surface heterogeneity


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Surface energy of solids

The coordination number is 12 for fcc and hcp and 8 for bcc, but, it is clear thesmaller coordination of surface atoms (in the plane 4 for bcc and 6 for fcc and hcp).

The resulting force unbalance at the surface is responsible of the interactions of thesolid with the environment (adsorption).

Schematic representation of thesurface energy of solids.

A surface site Bj has j nearest neighbors.Three distinct sites at the surface of a bccmetals are showed here.

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The adsorption process

Adsorption is brought about by the interactions between the solid and themolecules in the fluid phase. Two kinds of forces are involved, which give rise toeither physisorption or chemisorption.

Physisorption forces are mainly dispersion and van der Waals attractive forces,which do not depend on the polar nature of the adsorbent or adsorptive and aretherefore regarded as non-specific, whereas chemisorption interactions areessentially responsible for the formation of chemical compounds.


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Adsorption is an spontaneous process, G < 0

G = H T S

Because S decrease with adsorption (the adsorbed molecules moves on thecatalysts surface, but not freely as in gas phase) S < 0 and consequently H < 0,adsorption is an exothermic process, and the adsorption capacity thereforedecrease when the temperature increase ( Le Chatelier-vant Hoff Principle)

distance

Heat of physisorption

Heat of chemisorption

Ea forchemisorption

Energy of X2dissociation

Ep

Physisorption Chemisorption

The Ea for chemisorption leads, ingeneral, to small and slowchemisorption at low temperature

The adsorption process

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High (only on someHigh (only on some

specific sites)specific sites)None (all theNone (all the

surface)surface)SpecificitySpecificity

LowLowHighHighInfluence of P o the amountInfluence of P o the amount

adsorbed (high P)adsorbed (high P)

HighHighLowLowAmount (%) adsorbed at lowAmount (%) adsorbed at low

pressurepressure

LowLowHighHighAmount adsorbedAmount adsorbed

HighHighLowLowTemperatureTemperature

Some gasesSome gasesAll vaporsAll vaporsAdsorbateAdsorbate

Some solidsSome solidsAll solidsAll solidsAdsorbentAdsorbent

ChemisorptionPhysisorptionParameterParameter

monolayermonolayermultilayermultilayerSurface coverageSurface coverage

High (5High (5 100 Kcal/mol)100 Kcal/mol)Low (0.5Low (0.5--55

Kcal/mol)Kcal/mol)Heat of adsorptionHeat of adsorption

irreversibleirreversibleReversibleReversibleReversibilityReversibility

Comparison between physical and chemical adsorption


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Nevertheless, the strongest adsorptionsites are not necessary the best

catalytic sites. A good catalyst needsto adsorb the reactant moleculesstrongly enough for them to react, butnot so strongly that the productmolecules stick more or lesspermanently to the surface.

Gold, isn't a good catalyst because itdoesn't form strong enough attachmentswith reactant molecules.

Tungsten, isn't a good catalyst because itadsorbs too strongly.

Metals like platinum or iridium make goodcatalysts because they adsorb stronglyenough to hold and activate the reactants,but not so strongly that the products can'tbreak away.

The Sabatier principle

Volcano plot for the decompositionof formic acid on transition metals.

T corresponds to the T needed toobtain a lg r = -0.8, and Hf is theheat of formation of the metalformate salt (intermediate Metal-acidformic), that is to say, the strength ofthe M-reactant interaction.

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On the basis of their chemical nature, heterogeneous catalyst can be classified as:

SiOSiO22, Al, Al22OO33,, zeoliteszeolitesPolymerization,Polymerization,Isomerization, cracking,Isomerization, cracking,alkylationalkylation

StoichiometricStoichiometricoxidesoxides

NiONiO,, ZnOZnO, MnO, MnO22,WS2, MoS2

Oxidations,Oxidations, desulfurationsdesulfurationsNonNon -- stoichiometricstoichiometricoxides andoxides and sulphurssulphurs

Fe, Ni, Pd, Pt, Ag,Fe, Ni, Pd, Pt, Ag,Au, CuAu, Cu

HydrogenationHydrogenation/dehydrogenation,/dehydrogenation,

oxidations, hydrogenolysisoxidations, hydrogenolysis

MetalsMetals

Catalytic phasesCatalytic phasesReactionsReactionsCatalyst typeCatalyst type

The catalytic surface: Structural considerations

The crystalline structure of the catalyst will define the catalytic active surface


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Structure of metals

Metal present only three main structures. Most of them (71%) adopt a packingarrangement which maximizes the occupation of space: ccp or hcp.Alternatively, alkali metals and some transitions metals present bcc structure.

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Structure of metals

The compact layers are packed with asequence ABAB leading to thehexagonal (hcp) structure, orABCABC, leading to the cubic (ccp orfcc) structure.

hcp

ccp

The coordination number is inboth cases 12 and the fractionof occupied volume of 74%


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N atoms = N oct holes = 2N td holes

Two distinct types of interstices appears: tetrahedral and octahedral.

Hole distributions in close packed structures

D = 0.225r

D = 0.414r

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Hole distributionHole distribution

The interstices also form layers between the atom layers, located at dTd+ d Oct and d Td-. The occupation of these holes willdetermine the structure of metal compounds.

Analysis of the FCC structure: a) primitive sites b) octahedralholes c) tetrahedral T+ and T- holes


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BCC

Structure of metals

The BCC structure is not a close

packing structure, the coordination

number decrease from 12 to 8 and

the fraction of the occupied volume

from 74 to 68%.

Metal atoms are in contact only

along the diagonals of the cube.

BCC structure

hcp, C.N =12 ccp, C.N =12 bcc, C.N = 8

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Some oxides of interest in heterogeneous catalysis

Many oxides are of interest in catalysis, they are widely used in oxidations orhydrogenation / dehydrogenation processes.

In contrast to the simplicity of the structural characteristics of metals, metaloxides and sulfurs present a wide variety of structures, and most of them presentpolymorphism.


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Structural consideration of metal oxides

By simple we mean oxides formed from a single metallic element andwhere the degree of departure from exact stoichiometry (MO, MO2) maybe quite small.

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All the crystallographic forms are based on close packed O2- ions with Al+3 in Oand T sites, the differences arise when O2- layers are superposed in differentsequences: - Al2O3 (corindon) ABAB, -series (- phases) ABACABAC, -Al2O3ABCABC.

Thermal dehydration process in air of Al(OH)3

Alumina (Al2O3) have considerable prominence in heterogeneous catalysis.


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It is noteworthy that designed catalysts, possessing desired diameters oftunnels and requisite composition of their inner walls can be obtained. Thecatalyst characteristic can be tailored knowing the structure transformations.

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Schematic diagram of product shape selectivity: Para-xylene

diffuses preferentially out of the zeolite channels

Structural effects


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In some oxide, the degree of non-stoichoimetry may be gross. For example, ferrousoxide retains the rock salt structure over the composition range Fe0.82 O to Fe0.96O

Careful thermodynamicmeasurements leave nodoubt about the existenceof numerous non-stoichoimetric materials ofgeneral formula TinO2n-1and VnO2n-1

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Miller indices Planes in a crystallographicstructure are labeled usingMiller indices

Calculations:

i) Measure the interception ofthe plane with the axis(abc, xyz)

ii) Take reciprocal of theindices

iii) Rationalize the dividends

iv) Place the rationalizeddividend in parenthesis

Example: for a plane withintercepts (3a, 2b, c), thereciprocal are (1/3, , 1/ )and clearing the fractions yield(2,3,0)

The higher the value of (hkl) indices,the smaller the inter-planar distancebetween the planes in that direction.


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Planes with different Miller

indices in cubic crystals

Dense crystallographic planes

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1. point defects, which are places where an atom is missing orirregularly placed in the lattice structure. Point defects includelattice vacancies, self-interstitial atoms, substitution impurityatoms, and interstitial impurity atoms

2. linear defects, which are groups of atoms in irregular positions.Linear defects are commonly called dislocations.

3. planar defects, which are interfaces between homogeneousregions of the material. Planar defects include grain boundaries,stacking faults and external surfaces.

Classification of crystal imperfections


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Point defect: Frenkel and Schottky defects

A Schottky defect is simply an unoccupied lattice site. In ionic solids defects inthe cation lattice balances out the number in the anion lattice. The Frenkeldefect also implies the combination of a vacancy with interstitial defects. Pointdefects may combined or associated, forming clusters.

The number of vacanciespresent in a materialincreases exponentially asthe temperature increases

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Substitutional defects are produced when oneatom is replaced by a different type of atom.

If the substitutional atom is smaller than the

original atom then the lattice is in tension.

If the substitutional atom is larger than theoriginal atom then the lattice is in compression.

Substitutional defects


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Substitutional defects

But substitional defect also produce strong changes in the conductivity of the solids:

When Ge structure is doped with P or As, which have five valence electrons,four of them are shared with germanium atoms forming covalent bonds,while the fifth becomes quasi-free. This type of impurities is known as adonor impurity and it confers n-type semiconductivity.

On the contrary, when Ge is doped with B, Ga or In, each trivalent impurityatom accommodated, there would be one positive hole formed into thevalence band. This type of impurity is known as an acceptor impurity and itconfers p-type semiconductivity.

P+e

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Screw dislocations

Edge dislocations

linear defects

Edge dislocations are caused by the terminationof a plane of atoms in the middle of a crystal

The screw dislocation is more difficult to visualize, but basically comprises astructure in which a helical path is traced around the linear defect (dislocationline) by the atomic planes of atoms in the crystal lattice


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The crystalline structure of each grain isidentical but there orientations are not

The grain boundaries is a narrow zone wherethe atoms are not properly spaced

The misorientation angle is generally < 10.

Polycrystalline catalysts: Grain Boundaries

Boundaries are regions of high energy, and consequently, with strong interactionswith the environment.

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Relationship of crystal defects and catalytic activity

The overall decomposition of nitrous oxide is:

2N2O == 2N2 + O2

And the reaction mechanism may be written:

N2O + e (from catalyst surface) == N2 + O- (ads)

Followed by

2 O- == O2 + 2 e (to catalyst)

Or by

O- (ads) + N2O == N2 + O2 + e


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The activity of different metal oxide in this reaction can be divided into threegroup: p-type semiconductor, insulator, n type semiconductor. The mostactive catalyst are p-type semiconductor.

n type semiconductorinsulatorp type semiconductor

It is assumed that the slow step is:

2 O- == O2 + 2 e (to catalyst)

If this transfer of electrons is the slow step, thehigher activity of p-semiconductors is relatedwith their lower lying energy levels.

Activity

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Oxidation of CO on NiO: the doping of NiO catalyst with Li+ results in anincrease of positive holes enhancing the electrical conductivity, this favourthe catalytic activity because the Ea for the CO oxidation simultaneouslydecreases . The contrary effect is obtained by doping with Cr+3.


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Looking for the active site natureLooking for the active site nature

Now a break before.

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The catalyst surface is heterogeneous. Not all the surface will participate on thereactions, but reactions will be developed on the active sites or activecenters

These terms are widely used, but with wider meanings related to the activation/ deactivation of the catalysts.

The crystal defects are the genesis of the active sites concept, associated tochemisorption sites, but remember that strong chemisorption avoid the

reactions and deactivates the catalysts by blocking the active sites.

Hf(Kcal/mol)

catalytic

activity

weak

adsorption

strong

adsorption

Volcano plots

The active sites and chemisorption


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Taylor attributed special activity to those atoms which,because of the uneven geometry of the surface (Fig. 9), havelow coordination to other catalyst atoms (unsaturation), andthese are the atoms that were attributed to provide the seatof most catalytic conversions.

This view focused attention onthe heterogeneity of the

surface of almost all catalysts,and the fact that the totalsurface would not be equallyactive in effecting chemicalreactions.

The active sites: unsaturation

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The nature and characteristics of the catalyst surface, and consequently thenature and strength of the interactions with the environment, stronglydepends on the crystallographic plane where reactions will occurs

The active sites: crystallographic layer

Boudart divides the catalytic reaction in two classes: Structure sensitiveand structure insensitive, if reaction rate changes markedly or not as theparticle size of the catalyst is changed, or as the crystallographic face ofthe catalyst surface is altered.


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The catalytic activity depends on the presence on surface of correctly spacedgroups of atoms to accommodate the reactant molecules.

Balandin in the mid-1940s predict that metals with interatomic distancesranging from 2.48-2.77 should exhibit catalytic activity for the hydrogenationof benzene an the dehydrogenation of cyclohexane, since, for thesereactions, the metal spacing match the interatomic distances in the cyclicmolecules.

Schematic illustration of the Balandinconcept of multiplets involved in themolecular bonding. Sextet complex

The active sites: Geometric factor

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The active sites: the oxide surface

Numerous surface entities may be present on reacting catalysts but this entitiesmustnt to be always the active sites for different reactions.

Certain oxidations catalyzed by oxides occurs in two steps, according to the so-called Mars-van Krevelen mechanism: first, a reaction between the oxidecatalysts and the hydrocarbon, in which the later is oxidized and the formerreduced, followed by the reaction of the reduced oxide with O2 to restore theinitial state.

Mars-van Krevelen mechanism:

Conversion of propylene to acroleineby bismuth molybdate catalyst

The redox cycle undergoes by the catalystsurface determine the activity and selectivityof the reaction. Easily reducible oxides arequite active, but poorly selective, if reductionis not facile enough, catalyst is poorlyactive, if reduction is too easy, the activityincrease but probably with formation ofundesired compounds, and if re-oxidation isnot appropriate, catalyst is quicklydeactivated. The catalytic behavior dependson the mobility of ions and electrons.


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The acid-base character of the catalyst surface must be also take into account.The catalytic behavior of many solids is associated to this character.

The active sites: acidity in zeolites

There is one acid hydrogen for every tetrahedrally bonded aluminium. Theseactive sites are distributed uniformly throughout the bulk and are bridging hydroxylgroups. These are the classic Bronsted acid sites, the intrinsic strength of which isa function both of the particular local environment and also the Si/Al ratio.

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Acidity can be introduced in four ways:

i) Ion exchange with NH4+ followed by

thermal decomposition.

ii) Hydrolysis of ion-exchangedpolivalent cations followed by partialdehydration

iii) Direct proton exchange

iv) Reduction of exchanged metal ionsto a lower valence state:

Mn+Z- + H2 M(n-1)+Z- + H+Z-

The active sitesThe active sites: acidity in zeolites

Some acid catalyzed reactions


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The idea that catalytic activity could be related to the bulk electronicproperties of the catalyst (electronic band structure) leads to divide thecatalyst into metals, semiconductors and insulators (Dowden, 1950s)

The active sites: conductivity

Donor (n-semiconductors) or acceptor levels (p-semiconductors) can begenerated by doping catalysts, as previously showed. This means that theelectronic properties of the catalyst can be also tailored.

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The active sites: electronic properties

When chemisorption occurs on a semiconductor surface, the resulting changein electrical conductance of the solid yields unambiguous informationconcerning the type of electronic rearrangement, or the direction of the chargetransfer at the surface.

A fall in the conductivity of n-type semiconductor as a result of chemisorption,signifies an electron transfer from the conduction band of the catalyst to theadsorbed species (the concentration of electrons from the donor levels

decreases and anionic species are formed on surface).


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A fall in conductivity of a p-type semiconductor signifies an electron transferfrom the chemisorbed gas to the solid leading to a fall in the surfaceconcentration of positive holes in the full band due to the entry of electronsfrom the adsorbate.

In both cases, chemisorption leads to a depletion of carriers insemiconductors, and both are examples of deplective chemisorption. Thereverse process, namely cumulative adsorption, distinguished by a rise inconductivity because the surface concentration of carriers has increased.

The active sites: electronic properties

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The active sites

The concept of active site is therefore very wid. Some examples of adsorbedsurface complex are showed, you can observe how reactants interacts with thecatalysts surface depending on the nature and distribution of the active sites.

A. NH3 (Lewis base) coordinately linked to Al+3 ions (Lewis acid) on Al2O3 surface. B and

C. Linear and bridge adsorption of CO on Pt. D and E. Dissociative adsorption on Pt of H2or alkanes. F. Dissociative adsorption of N2 on Fe. G. Heterolitic dissociative adsorption ofH2 on the ZnO surface. H.Adsorbed complex with charge transference. I.Adsorption of iso-butene on silica alumina where the acid surface proton (-OH) was transferred to theisobutene. J and K. Possibilities of ethylene adsorption on Pt. L.Adsorption of O2 on metaloxides with charge transference. M. Dissociative adsorption of O2. N. Heterolitic dissociativeadsorption of propylene on ZnO.


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The active sites: bifunctional catalysts

Another question is the notion of bifunctional or multifunctional catalysts,namely in the case of supported catalyst.

Example: the Al2O3/Pt catalysts used in the hydroprocessing ofpetrochemicals, the metal serves to dissociate H2, while the acid supportserves to catalyse the build-up of vital carbonium ion intermediates.

In this case catalyst, electron transfers between both phases canalso modify the overall reaction.

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Preparation of catalysts

Types of solid catalysts


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The development of a solid catalyst requires knowledge of the parameterswhich have the greatest influence on catalyst performance.

The main objective of catalyst development is optimization of the variousdifferent catalyst properties, an essential precondition for achieving such anobjective is a close cooperation between experts working in very different fieldsof scientific research.

Preparation of solids catalysts

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Currently, two principal routes exist for the production of technical catalysts:

The first route, yielding bulkcatalysts, starts with theprecipitation of a catalystprecursor, which is filtered,

dried, and shaped. Calcinationsteps may be included afterdrying or after shaping.

The second route starts with acarrier material, which can beimpregnated with salt solutionsor coated with powders. Dryingand calcination give the finalsupported catalyst.

Bulk catalysts Supported catalysts



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Preparation of bulk catalysts

Bulk catalysts can be prepared from two routes, precipitation is the mostcommon but a small number of heterogeneous catalysts is prepared by thefusion of mixtures of oxidic or metallic precursors.

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Bulk Catalysts and Supports

Fused Catalysts

Among the thermochemical methods, the fusion processes are rarely appliedas they require extensive special equipment which is generally unavailable toresearch groups, and difficult to use in industry.

Metallic Glasses

Amorphous metals can be prepared in a wide variety of stable and metastablecompositions, with all catalytically relevant elements.

The function of the fused amorphous alloysis to serve as a precursor material for theformation of a metastable active phasecharacterized by an intimate mixture ofphases with different functions (synergism).


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Precipitation and Co-precipitation

Precipitation is the process in which a phase-separated solid is formed fromhomogeneous solution, after super-saturation.

The main advantage of precipitation is the possibility of creating very purematerials.

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Precipitation is and probably will remain one of the most important methodsfor the synthesis of solid catalysts.

For the industrial preparation of catalysts by precipitation, the ease and cost ofthe operation must always be balanced against the catalyst performance.

Advanced precipitation methods, such as microemulsion synthesis, in most

cases do not yield catalysts of sufficiently superior quality to compensate forthe higher efforts required in the synthesis. Moreover, the more complex asynthetic process is, the less robust it is usually, and this represents a majorobstacle for industrial implementation.

Hence, conventional precipitation and coprecipitation from aqueous solutionwill most likely continue to be the workhorse of catalyst synthesis formany years.



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Simplified scheme for the formation of a solid productfrom solution.

The final particle size is determined by theinterplay between nucleation and growth.The more particles nucleate, the smallerthe resulting particles will be.

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Usually, precipitates with specificproperties are desired.

These properties includes chemicalcomposition, purity, morphology,

homogeneity, particle size, pore sizesand pore volume, crystallinity,surface area, separability from themother liquor,

To obtain this properties, manyparameters such as solvent,saturation, precipitating agent,mixingsequence, pH, aging, calcinationtemperature, etc, must be optimized.


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Sol gel Process

Solgel processing is one of the routes for the preparation of porous materialsby solidification (without precipitation) from a solution phase.

The characteristics of the gel dependon the balance between both

hydrolysis and condensation rates

The reaction proceed in two steps:

-M-O-R + H2O == -M-OH + ROH (hydrolysis)

-M-OH + XO-M == M-O-M + XOH (condensation)

Ex. Synthesis de Silica gel

Precuror: tetraethylortosilicate (Si(OC2H5)4 (TEOS)

Si-OR + H2O == Si-OH + ROH (hydrolysis)

Si-OH + OH-Si == Si-O-Si + H2O (condensation)

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Sol gel Process

First appear a clear colloidal solution due to primary condensation of dissolvedmolecular precursors. These colloidal particles, during gelation, forms polymericchains by chemical bonding between local reactive groups at their surface. Thisprevents flocculation, that is a result of isotropic micelle aggregation.

The porous solids (xero- or aerogels) are produced in the next step desolvation depending on the drying mode.

Si-OR + H2O == Si-OH + ROH Si-OH + OH-Si == Si-O-Si + H2O


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Sol gel Process

One of the advantages of the sol-gel process is the flexibility of the process, thatpermits to obtain catalysts in different configurations by templating or coating methods.

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Supported Catalysts

Many industrial catalysts consist of metals or metal compounds finely dispersedon an appropriate supports.

The role of the support is not merely that of a carrier; it may actually contributecatalytic activity. Further, the interaction between the active phase and thesupport phase can affect the catalytic activity.

The advantage of spreading the active phase on a support is to disperse itthroughout the pore system, to obtain a large active surface per unit weight used.

Ni supported on SiO2


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stability under reaction and regeneration conditions

appropriate physical form for the given reactor

adequate mechanical properties

high surface area and porosity

inertness

chemical nature

The selection of the support is based on a series of

desirable characteristics:

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Porous solids are used as bulkcatalysts or as catalyst supports.


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The surface area of the solids is located inside the pores

Concepts

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Some typical porous solidsZeolites

Zeolites are microporous crystalline solids with well-defined structures. Generallythey contain silicon, aluminum and oxygen in their framework and cations, waterand/or other molecules within their pores

Framework Structure

A defining feature of zeolites is that theirframeworks are made up of 4-connectednetworks of atoms.One way of thinking about this is in terms oftetrahedra, with a silicon atom in the middle andoxygen atoms at the corners. These tetrahedracan then link together by their corners to from arich variety of beautiful structures.The framework structure may contain linkedcages, cavities or channels, which are of theright size to allow small molecules to enter

In all, over 130 different framework structuresare now known.

LTA: Structure and Framework Figures


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Zeolite A was the first zeolite to besynthesized on a large scale. Zeolite Ais synthesized in the sodium form togive a pore opening of 0.38 nm. Asimple ion exchange with potassium saltafter the filtration and washing stepsreduces the pore size to 0.30 nm.

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Figure. Standard zeolites and their porediameters.


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Some typical porous solids

Carbon materialsActivated carbons are the microporous carbonaceousadsorbents whose history can be traced back to 1600B.C. when wood chars were used for medicinalpurposes in Egypt.

Commercially available activated carbons areprepared from carbon containing source materialssuch as coal (anthracite or brown coal), lignite, wood,nut shell, petroleum and sometimes synthetic highpolymers.

There are many types of carbon materialswith different origins, thermal

resistance, morphology,porosity, etc and in differentconfiguration such as nanonubes,nanoparticles, foams, powders,pellets, etc

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Some typical porous solids

Carbon materials


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Characterization of porous texture

There are different techniques to evaluate the type and pore volume:

ADSORPTION ISOTHERMS

Adsorption isotherms describe the relation between the amount ofmolecules adsorbed on a unit mass of the solid and the adsorbateequilibrium pressure (or relative pressure) at a given temperature (VSTPvs P/P0).

Intuitively, it is straightforward to see that the amount of adsorbedmolecules forming a dense monolayer on the surface of a solid can beused to calculate its specific surface area, A. The monolayer capacity it isrelated to the specific surface area A (m 2/g) by the simple equation:

A = nm am L

Where nm, the monolayer capacity (molsorbate / gsolid), am is the averagearea occupied by a molecule of adsorbate in the completedmonolayer and L is the Avogadro constant

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Types of adsorption isotherms

Isotherm Iis typical of adsorption in microporesIsotherm II represents multilayer physisorption on a flat surface (valid for manynonporous substances).Isotherms III and V are characteristics of weak gas-solid interactionsIsotherm IV is frequently observed in the study of practical heterogeneouscatalysts. It is characteristic of multilayer adsorption accompanied by capillarycondensation in mesopores.Isotherm VI is obtained when the surface of a nonporous adsorbent isenergetically uniform, the isotherm may have a step-like shape associated tomultilayer formation.

Monolayer adsorbed molecules


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Mercury porosimetry is used for the determination of the pore volume andthe size distribution of meso- and macropores.

The technique uses progressive increase of the external pressure to forcemercury into the pores of the sample studied. An inverse relationship existsbetween the pressure applied P and the pore radius rp. For cylindrical poresthis is the Washburn equation:

where is the surface tensionand the contact angle

MERCURY POROSIMETRY

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Why supported Catalysts?

The activity of solid catalysts is usuallyproportional to the active surface areaper unit volume of catalyst, provided thattransport limitations are not present.

Remember the idea that one isolatedatom can be an active site.

A high activity per unit volume calls forsmall particles.

As most active species sinter rapidly atthe temperatures at which the thermalpretreatment and the catalytic reactionproceed, small particles of the activespecies alone generally do not providethermostable, highly active catalysts.


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The degree of dispersion, definedby the ratio of the number ofsurface atoms to the total numberof atoms in the polyhedron underconsideration, NS/NT.

Dispersion needs to be as close aspossible to unity so as to maximizethe utilization of a catalyst.

Supported Catalysts

For a crystallite of cubic shape NS/NT reachesunity when the size approaches 10A(assuming a unit cell spacing of 2.5A).

The concept of dispersion

For very small clusters it is not

crystallographically sensible to talk ofplanes with Miller indices. Small particlesmay not be favorable when the reaction isstructure-sensitive, and larger particlesmay exhibit a higher activity and/orselectivity per unit surface area.

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The procedures used to prepare supported catalysts can be divided into twomain groups:

(i) the selective removal of one or more component of usually non-porousbodies of a compound containing precursors of the support and the activecomponent(s);

(ii) the application of a precursor of the active component(s) onto a separatelyproduced support

Supported Catalysts

Bulk catalysts Supported catalysts


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Supported Catalysts

- Contacting with a solution of an active precursor and drying- Precipitation of an active precursor in the presence of a support

suspended in the solution

- Decomposition of a gaseous active precursor on the support surface

Precipitation onto a Suspended Support

DepositionPrecipitation within Pre-Shaped Support Bodies

Solid-State Ion Exchange in ZeolitesIncipient Impregnation

Grafting and Anchoring of Transition Metal Complexes to

Inorganic Oxides

Chemical Vapor Deposition

There are also different techniques and the results depend alsoof many factors such as porosity, interactions between bothphases, nature of precursors, thermal treatments, etc

The procedures used to load separately produced supports are:

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Grafting and Anchoring of Transition Metal Complexes to

Inorganic Oxides


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Supported Catalysts from Chemical Vapor Deposition

and Related Techniques

According to IUPAC, deposition which occurs byadsorption or reaction from the gas phase is referred toas chemical vapor deposition (CVD).

In CVD, the correct choice of reactants (the precursors) is veryimportant. These precursors fall into several general majorgroups, which include the halides, carbonyls, alkoxides,acetates, and organic complexes.

The choice of a precursor is governed by certain generalcharacteristics, including:

(i) stability at room temperature;(ii) Sufficient volatility at low temperature so that it can be easily

transported to the reactor;(iii) a capability of being produced at very high degrees of purity;(iv) an ability to react cleanly on or with the support(v) an ability to react without producing side reactions or parasitic

reactions

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From the Precursor to the Final Catalyst

In general, the impregnated support are not jet actives. The catalyst precursordeposited on the support must be transformed to a suitable compound (metal,oxides, sulfurs) in order to acquire the desired activity.

These last stages, calcination, reduction and sulfidation, are usually calledActivation, and their effect strongly depends of experimental conditions andinteractions active phase and supports. (ex. Sintering is favored with longcalcinations and treatment temperature, stabilization of crystallographicphases, different degrees or reduction / non stoichoimetry, or the formation ofsolid solutions by diffusion of the active phase on the supports can occurs )


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The catalytic converter

Steel Catalyst housing

Wire mesh packing forprotection

CeramicMonolithicSupport

Steel Catalyst housing

CeramicMonolithicSupport

Exhaust gas

from motor

PurifiedExhaust gas

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Cordierite monolithic support

Alumina and ceria coating: provide surfacearea and porosity

Active metals dispersion: Pt, Pd, Rh

The ceramic monolith provide low pressuredrop, short diffusion distance and the lack ofattrition by vibrations and thermal shockresistance

The washcoat provide a high internal surface(macro mesopores) to adequately disperse theprecious metal and to act as an adsorbent for thecatalyst poisining elements. The task of the CeO2component is oxygen storage, because:

2CeO2 == Ce2O3 + O2

Depending of the O2 concentration in the exhaustgas


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Three-way catalysts: NO reduction and CO andhydrocarbons (VOCs) oxidations

Three-way catalystscontain Pt : Rh : Pd inproportions that varieswith the properties offuels, the catalystsoperating conditions or theemissions targets to bereached.

Both Pt and Pd arecommon elements inoxidations catalysts, withan increasing proportion

of Pd vs Pt because thismetal is cheaper. The Rhenhance the selectivereduction of NO to N2.

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Catalyst deactivationCatalyst deactivation

The activity and selectivity of heterogeneous catalysts may change during thecourse of reaction. Usually activity decreases due to either chemical or physicalreasons, or a combination both factors.

However, sometimes the selectivity increases, and therefore, is not to be viewedas an entirely negative effect.

Example: In zeolitic and other acid catalysts such as Al2O3, thereis a loss in their activity for the cracking of hydrocarbons, but animprovement in their isomerisation facility, upon exposure tonitrogeneous bases such as pyridine or quinoline.


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Catalyst deactivation can be caused by:

(i) a decrease in the number of active sites(ii) A decrease in the quality of the active sites(iii) degradation of accessibility of the pore space


The main causes of deactivation can be classified as:

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Poisoning: The active sites of a catalyst may become inactive by theadsorption of impurities in the feed stream


It is useful to distinguish between temporary and permanent poisoning.

In the case of temporary poisoning, adsorption of the poison is relatively weak andreversible and removal of the poison from the fluid phase results in restoration ofthe original catalytic activity.

If adsorption of the poison is strong and not readily reversed, the poisoning iscalled permanent.


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Formation of Deposits


The formation of deposits can be of a physical or a chemicalnature.

So-called catalyst fouling is a physical process, which covers all phenomena inwhich material is deposited on the surface of the catalyst to block active sitesand/or pores.

In most cases, the catalyst causes the formation ofundesired by- products that lead to deposits. The most

commonly encountered is so-called coke, which isformed by chemical reactions of hydrocarbons atrelatively high temperature.

When large amounts of coke are deposited, thediffusion characteristics change as a result of narrowingor even complete blocking of the catalyst pores.

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Thermal Degradation


Thermal degradation is a physical process leading tocatalyst deactivation because of high-temperature-inducedsintering, chemical transformations, evaporation, etc.

Sintering is the loss of catalyst active surface due to crystallite growth ofeither the support material or the active phase.

Initially, the metal is supposed to be present as small clusters of atoms (or smallmetal particles), termed monomer dispersion. Surface diffusion of the atomswill lead to two-dimensional clusters and, upon further diffusion, three-dimensional particles will be formed


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Sintering resembles crystallization: largerparticles grow at the expense of smallerones. Particles might move and coalesceor atoms move from one particle toanother, either by volatilization or bysurface migration.

The position of the particle contributes to the ease of sintering. A valley positionis stable, whereas an on-top position is highly unstable.

Sintering is a function of time, temperature, atmosphere,interaction metal support, porosity and morphology of thesupport, etc

Thermal Degradation

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Thermal Degradation

Chemical transformations, reactions in which the support is directlyinvolved can also take place. An example is alumina-supported cobalt oxide.At high temperatures (>800 K), solid-state diffusion becomes noticeable andCo ions diffuse into the support, generating a spinel. It is not surprising thatin this process catalytic activity is lost

Evaporation, leads to a progressive loss of the active phase and ofcourse, is most important in high temperature processes, the majorexamples being steam reforming and catalytic combustion. By a high-temperature treatment (up to 920 K) in an oxygen-containing gas, platinum isslightly vaporized because of the formation of Pt oxides.


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Mechanical Deactivation

Mechanical strength is important in giving the catalystresistance against crushing. In a fluidized-bed reactor,attrition will always occur and the fines formed will be carriedaway with the product flow.

The shape and porosity of the catalyst particles influence mechanical strength: aspherical shape is most favorable, while the higher the porosity, the lower is themechanical strength. In particular, macropores will lead to reduced strength.

In washcoat monoliths, the adherence of the coating to the monolith is veryimportant. The washcoat may be fractured and/or separated from the monolithbecause of thermal stresses.

This is particularly relevant in automotive applications, in which the catalyst issubject to a wide range of temperatures that change very often and very rapidly(engine on off).

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Corrosion / Leaching

Particularly in liquid-phase catalysis, leaching of the active metal bydissolution into the surrounding liquid reaction mixture is often aconcern.

In most cases, leaching is the result of solvolysis of metaloxygenbonds through which the catalyst is attached to the support. This notonly causes loss of catalyst activity, but also results in contamination ofthe product.

The support material itself may be also subjected to leaching. Thereaction medium can be corrosive. Consider alumina at high or lowpH. Above pH 12 it will dissolve.


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Summary

Deactivation phenomena, their causes and effects

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What can be done to eliminate or in any case

reduce the catalyst deactivation?

If the cause of deactivation is understood, logicalmeasures can be contrived:

- At the catalyst level- In the reactor selection and process design- In the optimization of the operating conditions.


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When poisoning is the problem, a more robust active phase or supportmight be selected. Example: Alumina forms a surface sulfate with sulfuroxides, whereas titania is inert.

When fouling is the problem, the option is to optimize the catalyst texture(wide / narrow pores). The coke deposition can be prevented by tuning thereaction conditions or by alloying the metal particles.

Sintering is more easily prevented than cured, as it is often irreversible.Sintering can be reduced by structural promoters or stabilizers. Example,the addition of Re to the Pt/alumina catalyst leads to strongly improvedstability.

The crushing strengths and attrition resistances of catalysts can besignificantly improved, for example by using different preparation orshaping methods

What To Do?

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It will be clear that the time-scale of deactivation influences the choiceof reactor type, the mode of operation and regeneration.

What To Do?

With increasing the deactivation rate, the choice of the reactor typechanges from fixed bed to the riser with continuous regeneration.

The correct choice of the operating conditions strongly influences thecatalyst life, which vary from years to days.


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PGM = Platinum group metals (noble metals)

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In spite of the increasing price and demand of noble metals (platinum group,PGM) the real recycling degree only reach about 30% of the potential


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Some final remarks:

- Catalysis is essential to maintain our quality of life.

- The catalyst only increases the reaction rate

- Selectivity is most important than activity

- Catalysis is a surface phenomenon. The catalyst surface isheterogeneous and active sites of variable strength are related togeometric, electronic, acid-basic aspects, etc

- Different kinds of catalysts: bulk and supported, can be prepared

- The catalytic behavior can be tailored by fitting the catalystcharacteristics (porosity, electronic parameters, etc)

- The catalyst is finally deactivated and the recycling the spent catalystis necessary from an economic and environmental point of view.

Documents

Maldonado 1