3_catalysts and catalysis

Catalysis & Catalysts

Hikmat S. Al Salim

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Facts and Figures about Catalysts Life cycle on the earth Catalysts (enzyme) participates most part of life cycle e.g. forming, growing, decaying Catalysis contributes great part in the processes of converting sun energy to various

other forms of energies e.g. photosynthesis by plant 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2. Catalysis plays a key role in maintaining our environment Chemical Industry ca. $2 bn annual sale of catalysts ca. $200 bn annual sale of the chemicals that are related products 90% of chemical industry has catalysis-related processes Catalysts contributes ca. 2% of total investment in a chemical process

Catalysis & Catalysts

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Catalysis Catalysis is an action by catalyst which takes part in a chemical reaction process

and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions

(This is one of many definitions) Three key aspects of catalyst action taking part in the reaction

• it will change itself during the process by interacting with other reactant/product molecules

altering the rates of reactions • in most cases the rates of reactions are increased by the action of catalysts; however, in

some situations the rates of undesired reactions are selectively suppressed

Returning to its original form • After reaction cycles a catalyst with exactly the same nature is ‘reborn’ • In practice a catalyst has its lifespan - it deactivates gradually during use

What is Catalysis

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Catalysis action - Reaction kinetics and mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction)

Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that

The reaction activation energy is altered

The intermediates formed are different from those formed in non-catalytic reaction

The rates of reactions are altered (both desired and undesired ones)

Reactions proceed under less demanding conditions

Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials

Action of Catalysts

reactant

reaction process

uncatalytic

product

ener

gy catalytic

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It is important to remember that the use of catalyst DOES NOT vary ∆G & Keq values of the reaction concerned, it merely change the PACE of the process

Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of ∆G & Keq, NOT by the presence of catalysts.

In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process.

e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) ∆G°373=151 kJ/mol (100 °C)

∆G°973 =-16 kJ/mol (700 °C)

At 100°C, ∆G°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst

At 700°C, ∆G°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4 and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).

Action of Catalysts

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The types of catalysts Classification based on the its physical state, a catalyst can be

gas liquid solid

Classification based on the substances from which a catalyst is made Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) Organic (organic acids, enzymes etc.)

Classification based on the ways catalysts work Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq) Heterogeneous - reaction system involves multi-phase (catalysts + reactants/products)

Classification based on the catalysts’ action Acid-base catalysts Enzymatic Photocatalysis Electrocatalysis, etc.

Types of Catalysts & Catalytic Reactions

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Industrial applications Almost all chemical industries have one or more steps employing catalysts Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals …

Advantages of catalytic processes Achieving better process economics and productivity

Increase reaction rates - fast Simplify the reaction steps - low investment cost Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption

Reducing wastes Improving selectivity toward desired products - less raw materials required, less unwanted wastes Replacing harmful/toxic materials with readily available ones

Producing certain products that may not be possible without catalysts Having better control of process (safety, flexible etc.) Encouraging application and advancement of new technologies and materials And many more …

Applications of Catalysis

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Environmental applications Pollution controls in combination with industrial processes

Pre-treatment - reduce the amount waste/change the composition of emissions Post-treatments - once formed, reduce and convert emissions Using alternative materials …

Pollution reduction gas - converting harmful gases to non-harmful ones liquid - de-pollution, de-odder, de-colour etc solid - landfill, factory wastes …

And many more …

Other applications Catalysis and catalysts play one of the key roles in new technology development.

Applications of Catalysis

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Research in catalysis involve a multi-discipline approach Reaction kinetics and mechanism

Reaction paths, intermediate formation & action, interpretation of results obtained under various conditions, generalising reaction types & schemes, predict catalyst performance…

Catalyst development Material synthesis, structure properties, catalyst stability, compatibility…

Analysis techniques Detection limits in terms of dimension of time & size and under extreme conditions (T, P)

and accuracy of measurements, microscopic techniques, sample preparation techniques…

Reaction modelling Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry …

Reactor modelling Mathematical interpretation and representation, the numerical method, micro-kinetics,

structure and efficiency of heat and mass transfer in relation to reactor design …

Catalytic process Heat and mass transfers, energy balance and efficiency of process …

Research in Catalysis

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Understanding catalytic reaction processes A catalytic reaction can be operated in a batch manner

Reactants and catalysts are loaded together in reactor and catalytic reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion

Type of reactor is usually simple, basic requirements Withstand required temperature & pressure Some stirring to encourage mass and heat transfers Provide sufficient heating or cooling

Catalytic reactions are commonly operated in a continuous manner Reactants, which are usually in gas or liquid phase, are fed to reactor in

steady rate (e.g. mol/h, kg/h, m3/h) Usually a target conversion is set for the reaction, based on this target

required quantities of catalyst is added required heating or cooling is provided required reactor dimension and characteristics are designed accordingly.

Catalytic Reaction Processes

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Catalytic reactions in a continuous operation (cont’d) Reactants in continuous operation are mostly in gas phase or liquid phase

easy transportation The heat & mass transfer rates in gas phase is much faster than those in liquid

Catalysts are pre-loaded, when using a solid catalyst, or fed together with reactants when catalyst & reactants are in the same phase and pre-mixed It is common to use solid catalyst because of its easiness to separate catalyst

from unreacted reactants and products Note: In a chemical process separation usually accounts for ~80% of cost. That

is why engineers always try to put a liquid catalyst on to a solid carrier. With pre-loaded solid catalyst, there is no need to transport catalyst which is

then more economic and less attrition of solid catalyst (Catalysts do not change before and after a reaction and can be used for number cycles, months or years),

In some cases catalysts has to be transported because of need of regeneration

In most cases, catalytic reactions are carried out with catalyst in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids


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General requirements for a good catalyst Activity - being able to promote the rate of desired reactions Selective - being to promote only the rate of desired reaction and also

retard the undesired reactions Note: The selectivity is sometime considered to be more important

than the activity and sometime it is more difficult to achieve (e.g. selective oxidation of NO to NO2 in the presence of SO2) Stability - a good catalyst should resist to deactivation, caused by

the presence of impurities in feed (e.g. lead in petrol poison TWC. thermal deterioration, volatility and hydrolysis of active components attrition due to mechanical movement or pressure shock

A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.


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Example Heterogeneous Catalytic Reaction Process The long journey for reactant molecules to . travel within gas phase . cross gas-liquid phase boundary . travel within liquid phase/stagnant layer . cross liquid-solid phase boundary . reach outer surface of solid . diffuse within pore . arrive at reaction site . be adsorbed on the site and activated . react with other reactant molecules, either

being adsorbed on the same/neighbour sites or approaching from surface above

Product molecules must follow the same track in the reverse direction to return to gas phase

Heat transfer follows similar track

gas phase

pore porous solid

liquid phase / stagnant layer

gas phase reactant molecule

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Catalyst composition Active phase

Where the reaction occurs (mostly metal/metal oxide)

Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters

Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active

Solid Catalysts

Catalyst

Support

What makes a catalyst?

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Some common solid support / carrier materials

Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic

Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic

Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic

Solid Catalysts

Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g)

pore porous solid

Active site

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Preparation of catalysts Precipitation To form non-soluble precipitate by desired

reactions at certain pH and temperature

Adsorption & ion-exchange Cationic: S-OH+ + C+ → SOC+ + H+

Anionic: S-OH- + A- → SA- + OH-

I-exch. S-Na+ + Ni 2+ S-Ni 2+ + Na+

Impregnation Fill the pores of support with a metal salt

solution of sufficient concentration to give the correct loading.

Dry mixing Physically mixed, grind, and fired

Solid Catalysts

precipitate or deposit

precipitation

filter & wash the resulting precipitate

Drying & firing

precursor solution

Support

add acid/base with pH control

Support

Drying & firing

Pore saturated pellets

Soln. of metal precursor

Amou

nt

adso

rbed

Concentration

Support

Drying & firing

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Preparation of catalysts Catalysts need to be calcined (fired) in order to decompose the precursor and to

received desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right.

Commonly used Pre-treatments Reduction

if elemental metal is the active phase

Sulphidation if a metal sulphide is the active phase

Activation Some catalysts require certain activation steps in order to receive the best performance. Even when the oxide itself is the active phase it may be necessary to pre-treat the

catalyst prior to the reaction

Typical catalyst life span

Can be many years or a few mins.

Solid Catalysts

0 25 50 75

100

500 600 700 800 900 Temperature °C

BET

S.A

. m2 /g

0

40

0 10 Time / hours

BET

S.A

.

Activ

ity

Time

Normal use

Induction period

dead

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Langmuir-Hinshelwood mechanism This mechanism deals with the surface-catalysed reaction in which that 2 or more reactants adsorb on surface without dissociation

A(g) + B(g) A(ads) + B(ads) P (the desorption of P is not rate-determining step r.d.s.)

The rate of reaction ri=k[A][B]=kθAθB

From Langmuir adsorption isotherm (the case III) we know

We then have When both A & B are weakly adsorbed (B0,APA<<1, B0,BPB<<1),

2nd order reaction When A is strongly adsorbed (B0,APA>>1) & B weakly adsorbed (B0,BPB<<1 <<B0,APA)

1st order w.r.t. B

Mechanism of Surface Catalysed Reaction

++=

++=

BB,AA,

BB,B

BB,AA,

AA,A

PBPBPB

PBPBPB

00

0

00

0

1

1

θ

θ

BB,AA,

BAB,A,

BB,AA,

BB,

BB,AA,

AA,i PBPB

PPBkBPBPB

PBPBPB

PBkr

00

00

00

0

00

0

111 ++=

++

++=

BABAB,A,i PP'kPPBkBr == 00

BBB,AA,

BAB,A,i P''kPkB

PBPPBkB

r === 00

00

A B + P

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Eley-Rideal mechanism This mechanism deals with the surface-catalysed reaction in which that one reactant, A, adsorb on surface without dissociation and other reactant, B, approaching from gas to react with A

A(g) A(ads) P (the desorption of P is not rate-determining step r.d.s.)

The rate of reaction ri=k[A][B]=kθAPB

From Langmuir adsorption isotherm (the case I) we know

We then have When both A is weakly adsorbed or the partial pressure of A is very low (B0,APA<<1),

2nd order reaction When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B


AA,

AA,A PB

PB

0

0

1+=θ

AA,

BAA,B

AA,

AA,i PB

PPkBP

PBPB

kr0

0

0

0

11 +=

+=

BABAA,i PP'kPPkBr == 0

BAA,

BAA,i kP

PBPPkB

r ==0

0

A P

B

+ B(g)

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Mechanism of surface-catalysed reaction with dissociative adsorption The mechanism of the surface-catalysed reaction in which one reactant, AD, dissociatively adsorbed on one surface site

AD(g) A(ads) + D(ads) P

(the des. of P is not r.d.s.)

The rate of reaction ri=k[A][B]=kθADPB

From Langmuir adsorption isotherm (the case I) we know

We then have

When both AD is weakly adsorbed or the partial pressure of AD is very low (B0,ADPAD<<1),

The reaction orders, 0.5 w.r.t. AD and 1 w.r.t. B When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B


( )( ) 21

0

210

1 /ADAD,

/ADAD,

AD PBPB

+=θ

( )( )

( )( ) 21

0

210

210

210

11 /ADAD,

B/

ADAD,B/

ADAD,

/ADAD,

i PBPPBk

PPB

PBkr

+=

+=

( ) B/

ADB/

ADAD,i PP'kPPBkr 21210 ==

( )( ) B/

ADAD,

B/

ADAD,i kP

PBPPBk

r == 210

210

+ B(g) P

B

A B

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Mechanisms of surface-catalysed reaction involving dissociative adsorption In a similar way one can derive mechanisms of other surface-catalysed reactions, in

which dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another

associatively adsorbed reactant B on a separate surface site dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another

dissociatively adsorbed reactant BC on a separate site …

The use of these mechanism equations

Determining which mechanism applies by fitting experimental data to each.

Helping in analysing complex reaction network

Providing a guideline for catalyst development (formulation, structure,…).

Designing / running experiments under extreme conditions for a better control

…


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Bulk and surface The composition & structure of a solid in bulk and on surface can differ due to

Surface contamination Bombardment by foreign molecules when exposed to an environment

Surface enrichment Some elements or compounds tend to be enriched (driving by thermodynamic

properties of the bulk and surface component) on surface than in bulk

Deliberately made different in order for solid to have specific properties Coating (conductivity, hardness, corrosion-resistant etc) Doping the surface of solid with specific active components in order perform certain

function such as catalysis …

To processes that occur on surfaces, such as corrosion, solid sensors and catalysts, the composition and structure of (usually number of layers of) surface are of critical importance

Solids and Solid Surface

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Morphology of a solid and its surface A solid, so as its surface, can be well-structured crystalline (e.g. diamond C,

carbon nano-tubes, NaCl, sugar etc) or amorphous (non-crystallised, e.g. glass)

Mixture of different crystalline of the same substance can co-exist on surface (e.g. monoclinic, tetragonal, cubic ZrO2)

Well-structured crystalline and amorphous can co-exist on surface Both well-structured crystalline and amorphous are capable of being used

adsorbent and/or catalyst …


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Defects and dislocation on surface crystalline structure A ‘perfect crystal’ can be made in a controlled way Surface defects

terrace step kink adatom / vacancy

Dislocation screw dislocation

Defects and dislocation can be desirable for certain catalytic reactions

as these may provide the required surface geometry for molecules to be adsorbed, beside the fact that these sites are generally highly energised.


Terrace Step

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Pore sizes micro pores dp <20-50 nm meso-pores 20nm <dp<200nm macro pores dp >200 nm Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides)

Pore size distribution Typical curves to characterise pore size:

Cumulative curve Frequency curve

Uniform size distribution (a) & non-uniform size distribution (b)

Pores of Porous Solids

b

d

a

dw dd

∆d

wt

b a ∆wt

d

Cumulative curve Frequency curve

3_catalysts and catalysis

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