CHM 311

CATALYSIS

Learning Objective

• To understand the definition of a catalyst.

• To know the characteristics of a catalyst

• To understand the types of catalyst.

• To understand catalytic poisoning

• To know what a catalytic converter is.

• To understand autocatalysis

• To understand acid and base catalysis

Definition of a catalyst

• A catalyst is a substance that alters the rate of a chemical reaction but undergoes no net chemical change at the end of the reaction.

• The catalyst lowers the activation energy of the reaction by providing an alternative pathway that avoids the slow, rate-determining step of the uncatalysed reaction

Source: Chemicool Dictionary

Characteristics of a catalyst

• 1. A catalyst remains unchanged in mass and composition at the end of a reaction.

• 2. A small quantity of catalyst is needed to produce the reaction

• 3. A catalyst is more effective when it is finely divided

• 4. A catalyst is specific in action.

• 5.A catalyst cannot initiate a reaction.

• 6. A catalyst does not alter the final position of equilibrium although it shortens the time required to establish the equilibrium.

Types of catalyst

• There are two types of catalyst:

• Homogeneous catalyst and heterogeneous catalyst

• Homogeneous catalyst: This is a catalyst in the same phase as the reactant and evenly distributed throughout.

• This type of catalyst can occur in gas phase or liquid (solution) phase.

Examples of homogeneous catalyst

• 1. Oxidation of sulphur dioxide (SO2) to sulphur trioxide (SO3) in the presence of nitric oxide (NO2) catalyst.

• 2SO2 + O2 + [NO] 2SO3 + [NO]

• Gas gas gas

• 2. Decomposition of ethanal (CH3CHO) with iodine (I2) as catalyst

• CH3CHO + [I2] CH4 + CO

• Vapour vapour gas gas

Examples of homogeneous catalyst

• 3. Decomposition of hydrogen peroxide in the presence of iodide ion as catalyst.

2H2O2 2H2O + O2

• 4. Hydrolysis of cane sugar in aqueous solution in the presence of mineral acid as catalyst.

C12H22O11 + H2O C6H12O6 + C6H12O6 + H2SO4

cane sugar glucose fructose

Examples of homogeneous catalysis

Other examples include the hydrogenation of ethene to ethane, a gas-phase reaction, is accelerated in the presence of a solid catalyst

• such as palladium, platinum, or nickel.

• The metal provides a surface upon which the

• reactants bind.

• This binding facilitates encounters between reactants and increases the rate of the reaction.

Heterogeneous catalysis

• This refers to catalyst that is in a different phase from the reactants.

• The most important example is that in which the reactants are in the gas phase while the catalyst is a solid

• This catalysis occur by contact of the reactant with the catalyst surface.

• This catalyst has industrial significance

Examples of heterogeneous catalysis

• 1. Heterogeneous catalysis with gaseous reactants

• a. Reaction of sulphur dioxide with oxygen in the presence of finely divided platinum or vanadium peroxide, V2O5(Contact process for sulphuric acid)

• 2SO2 + O2 + [Pt] 2SO3 + [Pt]

Examples of heterogeneous catalysis

• b. Oxidation of ammonia to nitric oxide in the presence of platinum gauze ( a stage in the manufacture of nitric acid)

4NH3 + 5O2 + [Pt] 4NO + 6H2O + [Pt]

gas gas solid

• c. Hydrogenation reaction of unsaturated organic compounds are catalyzed by finely divided nickel

CH2=CH2 + H2 + [Ni] CH3-CH3 + [Ni]

Ethene gas solid

Examples of heterogeneous catalysis

• 2. Heterogeneous catalysis with liquid reactants

• The catalysis of the decomposition of aqueous solution of hydrogen peroxide by using manganese dioxide or platinum in colloidal form as catalyst.

• 2H2O2 + [Pt] 2H2O + O2 + [Pt]

Examples of heterogeneous catalysis

• 3. Heterogeneous catalysis with solid reactants.

• Potassium chlorate decomposition in the presence of manganese dioxide

• 2KClO3 + [MnO2] KCl + 3O2 + [MnO2]

• Solid solid

Catalytic converter

• Catalytic converters are made of expensive metals like platinum, palladium and rhodium as the heterogeneous catalyst.

• The catalytic converter changes poisonous molecules like carbon monoxide and various nitrogen oxides in car exhausts into more harmless molecules like carbon dioxide and nitrogen.

• The metals are deposited as thin layers onto a ceramic honeycomb. This maximises the surface area and keeps the amount of metal used to a minimum.

Catalytic poison

• The presence of small amounts of impurities in the reactants can render heterogeneous catalyst ineffective

• A poison destroys the ability of a catalyst to speed up a chemical reaction. The process of destroying the activity of catalyst to speed up a chemical reaction is called catalytic poisoning.

Examples of catalytic poisoning

• 1. Arsenic oxide (As2O3) can poison the platinum catalyst used in the oxidation of sulphur dioxide.

2SO2 + O2 2SO3

• 2. hydrogen sulphide (H2S) poisons the iron catalyst used in the synthesis of ammonia (Haber Process)

• N2 + 3H2 2NH3

Catalytic poisoning

• Catalytic converters can be affected by catalyst poisoning. Lead is a catalyst poison for catalytic converters. It coats the honeycomb of expensive metals and stops it working.

• In the past, lead compounds were added to petrol (gasoline) to make it burn more smoothly in the engine.

• A catalytic converter cannot be used if one is using leaded fuel.

• So catalytic converters have not only helped remove poisonous gases like carbon monoxide and nitrogen oxides, but have also engaged in the removal of poisonous lead compounds from petrol.

Explanation of catalytic poisoning

• .The poison is adsorbed on the catalytic surface in preference to the reactant. The poison forms a layer or layers on the surface of the catalyst thus preventing adsorption of the reactant.

• The catalyst can also chemically combine with the impurity. Example is the poisoning of iron catalyst by hydrogen sulphide

• Fe + H2S FeS + H2

Autocatalysis

• Autocatalysis is the process when one of the products of a reaction acts as a catalyst for that reaction.

• In autocatalysis, the initial rate of the reaction increases as the catalytic product is formed, instead of decreasing.

Autocatalysis

Examples of autocatalysis

• 1. Hydrolysis of an ester. The hydrolysis of ethyl acetate forms ethanoic acid and ethanol. Ethanoic acid acts as a catalyst. For the reaction

• CH3COOC2H5 + H2O CH3COOH + C2H5OH Catalyst

Acid catalysis

• In acid catalysis the crucial step is the transfer of a proton to the substrate:

• Y + HA HY+ + A- HY+ Product

• Acid catalysis is the main process in the solvolysis of esters and keto–enol tautomerism.

• Hydrolysis of ester

• CH3COOC2H5 + H2O CH3COOH + C2H5OH

Base catalysis

• In base catalysis, a proton is transferred from the substrate to a base:

• YH + B Y- + BH Y- Product

• Base catalysis is the main step in the isomerization and halogenation of organic

• compounds, and of the Claisen and aldol condensation reactions.

References

• 1. Atkins P and Paula 2008. Physical Chemistry

• 2. Bahl B. S, Bahl A, Tulin G.D. 2005. Essentials of Physical Chemistry. P. 706-722.

• 3. chemguide

• https://www.chemguide.co.uk/physical/catalysis/introduction.html