13.1.4 Catalytic & Magnetic Properties

• Transition metals are often used as catalysts in the elemental form or as compounds
• The ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily
• This enables them to catalyse certain redox reactions. They can be readily oxidised and reduced again, or reduced and then oxidised again, as a consequence of having a number of different oxidation states of similar stability
• There are two types of catalyst:
  • A heterogeneous catalyst is in a different physical state (phase) from the reactants
    • The reaction occurs at active sites on the surface of the catalyst
    • An example is the use of iron, Fe, in the Haber process for making ammonia

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• • A homogeneous catalyst is in the same physical state (phase) as the reactants

• The hydrogenation or reduction of alkenes makes use of a nickel catalyst

CH₂=CH₂ (g) + H₂ (g) → CH₃CH₃ (g)

• The same reaction is used in the hydrogenation of vegetable oils to form polyunsaturated fats
• The decomposition of hydrogen peroxide is a common reaction in the study of chemical kinetics and uses manganese(IV) oxide as the catalyst

2H₂O₂ (g) →  2H₂O (aq) + O₂ (g)

Catalytic converters

• Catalytic converters are used in car exhaust boxes to reduce air pollution. They usually consist of a mixture of finely divided platinum and rhodium supported on a ceramic base

Diagram of a catalyst on an inert support medium in a vehicle catalytic converter

• Carbon monoxide, nitrogen dioxide and unburnt hydrocarbons are sources of pollution in car exhaust
• The transition metal catalysts facilitate the conversion of these pollutants into less harmful products

2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g)

CH₃CH₂CH₃ (g) + 5 O₂ (g) → 3CO₂ (g)  + 4H₂O (g)  

• Some of the transition metals are precious metals so they can be very expensive
• In order to minimise the cost and maximise the efficiency of the catalyst the following measures can be taken:
  • Increasing the surface area of the catalyst
  • Coating an inert surface medium with the catalyst to avoid using large amounts of the catalyst

• This is achieved by spreading the catalyst over a hollow matrix such as a honeycomb-like structure

Biological catalysts

• Many of the enzyme catalysed reactions in the body make use of homogeneous transition metal catalysts
• An example of this is haemoglobin, abbreviated to Hb, which transports oxygen around the blood

The structure of haemoglobin

The structure of haem

• The iron(II) ion is in the centre of a large heterocyclic ring called a porphyrin
• The iron has a coordination number of four, is square planar and can bind to one oxygen molecule
• The Hb molecule contains four porphyrin rings so each Hb can transport four oxygen molecules

• Materials are classified as diamagnetic, paramagnetic or ferromagnetic according to their behaviour when placed in an external magnetic field
• Diamagnetism is a property of all materials and produces a very weak opposition to an applied magnetic field
  • It arises from the repulsion of electrons to the applied magnetic field

• Paramagnetism only occurs in substances which have unpaired electrons
  • It produces magnetisation proportional to the applied field and in the same direction
  • Transition metal complexes show paramagnetism

• Ferromagnetism has the largest effect and produces magnetisation greater than the applied field

Diamagnetism

• The atoms of diamagnetic materials have paired electrons
• Spinning electrons create a tiny magnetic dipole
• The paired electrons orientate themselves so that the magnetic field they create opposes the external field
• This result is a very weak repulsion force

Argon is diamagnetic with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶

• Many molecules are diamagnetic since all the electrons are paired up in bonds
• It is very hard to demonstrate diamagnetism but it is possible by suspending a sample of the material from a sensitive force meter and lowering it into a strong horseshoe magnet - a slight change in the force should be seen

Paramagnetism

• Paramagnetic materials are attracted to an external magnetic field
• The unpaired electrons can be temporarily aligned to the magnetic field causing attraction into the field

Orbital diagrams for the first row d-block showing the number of unpaired electrons increasing up to chromium

Ferromagnetism

• The metals iron, cobalt and nickel show the unusual property of ferromagnetism
• The alignment of the unpaired electrons in an external field in ferromagnetic materials can be retained so the material becomes permanently magnetised
• If ferromagnetic materials are heated and cooled in a magnetic field, the magnetic field of the electrons remains
• Magnetic regions within the metal that are aligned magnetically are know as domains
• Banging or heating a permanent magnet will weaken the magnetism

Ferromagnetic materials are used to make permanent magnets which produce characteristic magnetic field lines