Our previous discussion has concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). on catalysts which are in the same phase as the reaction being catalyzed. This kind of catalysisThe increase in rate of a reaction due to the presence of a substance that undergoes no net change during the reaction. is called homogeneous catalysis. Many important industrial processes rely on heterogeneous catalysis, in which the catalyst is in a different phase. Usually the catalyst is a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. and the reactants are gases, and so the rate-limiting stepThe step in a reaction mechanism that by its relatively slow rate limits the overall rate of a reaction; also called rate-determining step. occurs at the solid surface. Thus heterogeneous catalysis is also referred to as surface catalysis.
The detailed mechanisms of most heterogeneous reactions are not yet understood, but certain sites on the catalyst surface appear to be able to weaken or break bonds in reactant molecules. These are called active sites. One example of heterogeneous catalysis is hydrogenationAny addition reaction of hydrogen with an organic compound; usually the organic compound has a carbon-carbon double bond or triple bond. of an unsaturatedDescribes 1) a solution that contains less than the equilibrium concentration of a solute, or 2) an organic compound containing double or triple bonds. organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. compoundA substance made up of two or more elements and having those elements present in definite proportions; a compound can be decomposed into two or more different substances. such as ethane (C2H4) by metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. catalysts such as Pt or Ni:
The currently accepted mechanism for this reaction involves weak bonding of both H2 and C2H4 to atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. on the metal surface. This is called adsorptionFormation of a layer of gas, liquid, or solid on the surface of a solid; the adsorbed substance is held by either covalent bonds or by non-covalent (intermolecular) forces. Adsorption differ from absorption in that the adsorbed substance is on the surface only.. The H2 molecules dissociate to individual H atoms, each of which is weakly bonded to a Pt atom:
- H2 + 2Pt(surface) → 2H- - -Pt(surface)
These adsorbed H atoms can move across the metal surface, and eventually they combine with a C2H4 molecule, completing the reaction. Because adsorption and dissociationThe breaking apart of one species into two or more smaller species; often applied to ions in a crystal lattice, which dissociate when the ionic solid dissolves in water. Dissociation refers to separation of particles that already exist; ionization refers to the formation of ions from neutral species, as in the ionization of a weak acid in aqueous solutoin. of H2 on a Pt surface is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference. (ΔHm° >= –160 kJ mol–1), it can provide H atoms for further reaction without a large activation energyThe energy barrier over which a reaction must progress in order for reactants to form products; the minimum energy that reactants must have if they are to be converted to products.. By contrast, dissociation of gaseous H2 molecules without a metal surface would require the full bond enthalpyThe change in enthalpy when a mole of chemical bonds of a particular type are broken; the molecules whose bonds are broken must be in the gas phase. Closely related to bond energy. (ΔHm° = +436 kJ mol–1). Clearly the metal surface makes a major contribution in lowering the activation energy.
Heterogeneous catalysts are used extensively in the petroleum industry. One example is the combination of SiO2 and Al2O3 used to speed up cracking of long-chain hydrocarbons into the smaller molecules needed for gasoline. Another is the Pt catalyst used to reform hydrocarbon chains into aromaticReferring to a hydrocarbon containing benzene-like rings and exhibiting characteristic stability and patterns of reactivity. ring structures. This improves the octane rating of gasoline, making it more suitable for use in automobile engines. Other industries also make effective use of catalysts. SO2, obtained by burning sulfur (or even from burning coal), can be oxidized to SO3 over vanadium pentoxide, V2O5. This is an important step in manufacturing H2SO4.
Another important heterogeneous catalyst is used in the Haber process for synthesisFormation of substances with more complicated sturctures than do their precursors. of NH3 from N2 and H2. As with most industrial catalysts, its exact composition is a trade secret, but it is mainly Fe with small amounts of Al2O3 and K2O added.
The surface catalyst you are most likely to be familiar with is found in the exhaust systems of many automobiles constructed since 1976. Such a catalytic converter contains from 1 to 3 g Pt in a fine layer on the surface of a honeycomb-like structure or small beads made of Al2O3. The catalyst speeds up oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. of unburned hydrocarbons and CO which would otherwise be emitted from the exhaust as air pollutants. It apparently does this by adsorbing and weakening the bond in the O2 molecule. Individual O atoms are then more readily transferred to CO or hydrocarbon molecules, producing CO2 and H2O. This action of the catalytic surface can be inhibited or poisoned if lead atoms (from tetraethyllead in leaded gasoline) react with the surface. Hence the prohibition of use of leaded fuel in cars equipped with catalytic converters.