catalyst & catalysis

CH4003 Lecture Notes 5 (Erzeng Xue)

CATALYST &CATALYSIS

Nofal Umair 2k11-Che-148

Catalysis & Catalysts2 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 CO2 + H2O=HC + O2

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

Hetrogeneous Catalysis-Milestones in Evolution-1

3

1814- Kirchhoff-starch to sugar by acid. 1817-Davy-coal gas(Pt,Pd selective but not

Cu,Ag,Au,Fe) 1820s –Faraday H2 + O2 H2O(Pt);C2H4 and S 1836- Berzelius coins”Catalysis”; 1860-Deacon’s Process ;2HCl+0.5O2 H2O + Cl2; 1875-Messel.SO2 SO3 (Pt); 1880-Mond.CH4+H2O CO+3H2(Ni); 1902-Ostwald-2NH3+2.5O2 2NO+3H2O(Pt); 1902-Sabatier.C2H4+H2 C2H6(Ni). 1905-Ipatieff.Clays for acid catalysed reactions;

isomerisation, alkylation, polymerisation.

Milestones in Evolution-24

1910-20: NH3 synthesis (Haber,Mittasch) ; Langmuir 1920-30-Methanol syn(ZnO-Cr2O3); Taylor;BET 1930-Lang-Hinsh &Eley -Rideal models ;FTsyn;EO; 1930-50:Process Engg; FCC / alkylates;acid-base

catalysis;Reforming and Platforming. 1950-70: Role of diffusion; Zeolites, Shape Selectivity;

Bifunctional cata;oxdn cat-HDS; Syngas and H2 generation.

1970- Surface Science approach to catalysis(Ertl) 1990 - Assisted catalyst design using : -surface chem of metals/oxides, coordination chemistry - kinetics,catalytic reaction engg - novel materials(micro/mesoporous materials)

What is Catalysis5 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


By providing an alternative pathway (or mechanism) with lower/ higher activation energy.

Catalyst Characteristic

1. Activity. The ability of a catalyst to increase the rate of a chemical reaction is called activity. A catalyst may accelerate a reaction to as high as 10^10 times.

2. Selectivity. The ability of the catalyst to direct a reaction to give a particular product.3. Small quantity. Only small quantity is need for a reaction.

4. Specific. One catalyst is need for specific reaction only

5. Physical properties may change during a reaction but no it does not take part in the reaction.

6. Catalyst doesn’t influence on the general stoichiometric coefficients.

7. Catalysts decrease activation energy thus increase the chemical rate.

8. Catalysts don’t influence on the equilibrium constant. They only reduce time of reaching the equilibrium and increase the rate of forward and back reaction.

Action of Catalysts8 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


reactant

reaction process

uncatalytic

product

ener

gy

catalytic

PHYSICAL ADSORPTION

10

Steps in a catalytic Reaction: - Diffusion of reactant (bulk, Film, surface) - Adsorption( physical chemical) -Surface reaction - Desorption and diffusion of products Physical Adsorption: - Van der Waals forces;BET surface area Pore Size distribution ( Wheeler, de Boer,

BJH) Influence of pore size on reaction order,

temperature coefficient, selectivity, Influence of poisons …

Types of Catalysts & Catalytic Reactions11

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.


Applications of Catalysis12

Industrial applicationsAlmost 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 Catalysis13

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.


Research in Catalysis14

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 …


Catalytic Reaction Processes15 Understanding catalytic reaction processes

A catalytic reaction can be operated in a batch mannerReactants 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 Processes16

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.


Example Heterogeneous Catalytic Reaction Process

17

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

poreporous solid

liquid phase /stagnant layer

gas phasereactant molecule


Solid Catalysts18

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


CatalystAct

ive

phas

e

Support

Prom

oter

Solid Catalysts19

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


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)

poreporous solid

Active site

Solid Catalysts20

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


precipitate or deposit

precipitation

filter & wash the resultingprecipitate

Drying& firing

precursorsolution

Support

add acid/basewith pH control

Support

Drying & firing

Pore saturated pellets

Soln. of metal precursor

Am

ou

nt

ad

sorb

ed

Concentration

Support

Drying & firing

Solid Catalysts21

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.


0

25

50

75

100

500 600 700 800 900

Temperature °C

BE

T S

.A.

m2/g

0

40

0 10Time / hours

BE

T S

.A.

Act

ivit

y

Time

Normal use

Induction period

dead

catalyst & catalysis

Engineering

catalysis

classification based

catalytic reaction

small quantity

reactant molecules

porous structure

original form

phase