Heterogeneous catalysis

Hydrogenation of ethene on a solid surface

Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase (ie. solid, liquid and gas, but also oil and water) to the reactants. Heterogeneous catalysts provide a surface on which the reaction may take place.

In order for the reaction to occur, one or more of the reactants must diffuse to the catalyst surface and adsorb onto it. After reaction, the products must desorb from the surface and diffuse away from the solid surface. Frequently, this transport of reactants and products from one phase to another plays a dominant role in limiting the reaction rate. Understanding these transport phenomena and surface chemistry such as dispersion is an important area of heterogeneous catalyst research. Catalyst surface area may also be considered. Mesoporous silicates, for example, have found utility as catalysts because their surface areas may be in excess of 1000 m²/g, which increases the probability that a reactant molecule in solution will come in contact with the catalyst surface and adsorb. If diffusion rates are not taken into account, the reaction rates for various reactions on surfaces depend solely on the rate constants and reactant concentrations. Asymmetric heterogeneous catalysis can be used to synthesize enantiomerically pure compounds using chiral heterogeneous catalysts.^[1] The field is of great industrial and environmental importance. It has attracted two Nobel prizes for Irving Langmuir in 1932 and Gerhard Ertl in 2007.

Examples

• The synthesis of Ammonia is an example of heterogeneous catalysis:

3H₂(g) + N₂(g) ↔ 2NH₃(g) - catalysed by Fe(s).

• The use of Nickel in the hydrogenation of vegetable oils to produce margarine. The unsaturated fat present in the vegetable oils are converted to saturated fat by the addition of hydrogen. This in turn breaks the Carbon-carbon double bonds. In order for this reaction to be catalysed effectively the nickel must present a large surface area so therefore must be finely divided.

-CH=CH- + H₂ → -CH₂-CH₂-

Another example is the reduction of nitriles for instance in a synthesis of phenethylamine with Raney nickel and ammonia:^[2]

• The cracking, isomerisation and re-forming of hydrocarbons to form appropriate and useful blends of Petrol.

Catalytic Converters

These are often used in automobiles. Three main reactions are catalysed by Catalytic converters.

The oxidation of carbon monoxide to carbon dioxide.

2CO(g) + O₂(g) → 2CO₂(g)

The reduction of nitrogen monoxide back to nitrogen.

2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g)

The oxidation of hydrocarbons to water and carbon dioxide. This can occur on any of the hydrocarbons however primarily Petrol or Diesel.

2C₆H₆(g) + 15O₂ → 12CO₂(g) +6H₂O(l)

See also

• Homogeneous catalysis
• Temperature-programmed reduction
• Thermal desorption spectroscopy

References

1. ^ Heitbaum, Glorius, Escher, Asymmetric heterogeneous catalysis, Angew. Chem. Int. Ed. 2006, 45, 4732.
2. ^ Organic Syntheses, Coll. Vol. 3, p.720 (1955); Vol. 23, p.71 (1943). http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf