Heterogeneous Catalysis

In heterogeneous catalysis, the reactant(s) and the catalyst are in different phases -- typically a solid catalyst with gaseous or liquid reactants. The solid catalyst is added to the reaction mixture without dissolving in it, and the reaction itself takes place on the surface of the solid catalyst (which is why the adsorption-based mechanism of Section 11.8 applies specifically to this type of catalysis). Three examples are given. (i) In the Haber process, dinitrogen and dihydrogen combine to form ammonia in the presence of finely divided iron together with K2O and Al2O3: N2(g) + 3 H2(g) --[Fe(s), K2O/Al2O3]--> 2 NH3(g); here Al2O3 and K2O act as promoters of the iron catalyst -- Al2O3 is added to stop the iron particles fusing together, and K2O promotes chemisorption of nitrogen atoms onto the iron surface (molybdenum is also sometimes used as a promoter). (ii) Hydrogenation of vegetable oils to solid fat (vanaspati ghee), used in the food industry, is catalysed by finely divided metals such as Ni, Pd or Pt: the vegetable oil's carbon-carbon double bonds (C=C) are converted to single bonds (C-C) by adding H2 across them, with the reaction occurring on the metal's surface. (iii) Automobile catalytic converters use a heterogeneous catalyst to transform exhaust air pollutants -- …