heterogenous catalysts

what are heterogeneous catalysts

• In a different state/phase to the reactants

• Heterogeneous catalysts are usually solids whereas the reactants are gaseous or in solution

What type of catalyst do transition metals form

• Transition metals and their compounds form both heterogeneous and homogeneous catalyst

• TM catalysts are usually solid-they provide a surface for the reaction to take place in

Surface adsorption theory

• Adsorption: the reactants adsorb/attach on to the surface of the catalyst-the active site.. Higher concentration of reactants at the solid surface so elading to a higher collision frequency

• Reaction: The bonds within the reactants are weakened so less energy is required for the reaction to occur. activation energy of catalyst is lower than activation energy.

• Desorption: the products leave the surface of the catalyst

what does strength of adsorption determine?

• The strength of adsorption helps to determine the effectiveness of the catalytic activity

• Some metals e.g. W have too strong adsorption and so the products cannot be released

• Some metals e.g. Ag have too weak adsorption, and the reactants do not adsorb in high enough concentration

• Ni and Pt have about the right strength and are most useful as catalysts 

how changing surface area of catalyst affects them?

• Increasing the surface area of a solid catalyst will improve its effectiveness.

• A support medium is often used to maximise the surface area and minimise the cost (e.g. Rh on a ceramic support in catalytic converters)

advantages of heterogeneous catalysts?

• Heterogeneous catalysts can be filtered off and are easy to separate from any liquid or gaseous products

• They are also suited to continuous processes rather than batch processes

how impurities in gas canr educe effectiveness of heterogenous catalysts

• The impurities can: 

  • Adsorb onto catalyst surface and occupy active sites

  • Prevent bond weakening in the reactants

  • 'Take up' the surface area of catalyst by forming strong bonds to surface of catalyst so are less likely to desorb from surface of catalyst

what is the contact process?

• The process used to make sulfuric acid, H2SO4

• The catalyst used is V2O5

• SO2 + ½ O2—>< SO3 all gases

• Oxidation state of vanadium is +5

• The hot gases (440 degrees celcius) are passed over the finely divided V2O5 (s)

Contact process reaction:

• The first step of the process is roasting sulfur in air to produce sulfur dioxide

S (s) + O₂ (g) → SO₂ (g)

1) Adsorption: The SO2 adsorbs onto the surface of the V2O5. A redox reaction occurs. 2SO₂ (g) + O₂ (g) ⇌ 2SO₃(g)

2) Reaction: V2O5 + SO2—> V2O4 + SO3

V has been reduced from +5 to +4

S has been oxidised from +4 to +6.

The vanadium(V) oxide catalyst converts sulfur dioxide into sulfur trioxide and is reduced to vanadium(IV) oxide

3) Desorption:The SO3 leaves the catalyst. The catalyst is regenerated. The oxygen reacts with the V2O4

Redox: O₂ (g) + 2V₂O₄ (s) →  2V₂O₅ (s)

uses of H2SO4

• fertilisers

• detergents

• adhesives

• explosives

• car batteries

use of catalytic converters

These are used to convert carbon monoxide, nitrogen oxides and unburnt hydrocarbons (the fuel) produced by internal combustion engines into CO2, H2O and N2

how CO and NO are produced and their dangers

• CO- incomplete combustion: toxic (binds irreversible with Hb)

• NO- formed from the reaction btw N2 and O2 due to the high temperatures in an engine. Forms acid rain and is a respiratory irritant

what are catalytic converters made of?

• made of allows of platinum, rhodium and palladium

• The transition metal catalysts facilitate the conversion of these pollutants into less harmful products

• They have a large surface area for the reaction to take place on due to the internal honeycomb structure

reaction that occurs in catalytic converter

• Adsorption: CO and NO absorb onto the metal surface. The C triple bond O and N triple bond O are weakened and so they are able to react

• Reaction: 2CO(g) + 2NO (g) —> 2CO2(g) + N2(g)

• Desorption: the products leave the surface and exit the exhaust