14.4: Heterogeneous Catalysts

Heterogeneous Catalysis

In heterogeneous catalysis, the catalyst is in a different phase from the reactants. At least one of the reactants interacts with the solid surface in a physical process called adsorption in such a way that a chemical bond in the reactant becomes weak and then breaks. Poisons are substances that bind irreversibly to catalysts, preventing reactants from adsorbing and thus reducing or destroying the catalyst’s efficiency.

An example of heterogeneous catalysis is the interaction of hydrogen gas with the surface of a metal, such as Ni, Pd, or Pt. As shown in part (a) in Figure \(\PageIndex{2}\), the hydrogen–hydrogen bonds break and produce individual adsorbed hydrogen atoms on the surface of the metal. Because the adsorbed atoms can move around on the surface, two hydrogen atoms can collide and form a molecule of hydrogen gas that can then leave the surface in the reverse process, called desorption. Adsorbed H atoms on a metal surface are substantially more reactive than a hydrogen molecule. Because the relatively strong H–H bond (dissociation energy = 432 kJ/mol) has already been broken, the energy barrier for most reactions of H₂ is substantially lower on the catalyst surface.

Figure \(\PageIndex{1}\): Hydrogenation of Ethylene on a Heterogeneous Catalyst. When a molecule of hydrogen adsorbs to the catalyst surface, the H–H bond breaks, and new M–H bonds are formed. The individual H atoms are more reactive than gaseous H₂. When a molecule of ethylene interacts with the catalyst surface, it reacts with the H atoms in a stepwise process to eventually produce ethane, which is released. (CC BY-NC-SA; anonymous)

Figure \(\PageIndex{1}\) shows a process called hydrogenation, in which hydrogen atoms are added to the double bond of an alkene, such as ethylene, to give a product that contains C–C single bonds, in this case ethane. Hydrogenation is used in the food industry to convert vegetable oils, which consist of long chains of alkenes, to more commercially valuable solid derivatives that contain alkyl chains. Hydrogenation of some of the double bonds in polyunsaturated vegetable oils, for example, produces margarine, a product with a melting point, texture, and other physical properties similar to those of butter.

Several important examples of industrial heterogeneous catalytic reactions are in Table \(\PageIndex{1}\). Although the mechanisms of these reactions are considerably more complex than the simple hydrogenation reaction described here, they all involve adsorption of the reactants onto a solid catalytic surface, chemical reaction of the adsorbed species (sometimes via a number of intermediate species), and finally desorption of the products from the surface.

Table \(\PageIndex{1}\): Some Commercially Important Reactions that Employ Heterogeneous Catalysts

Commercial Process	Catalyst	Initial Reaction	Final Commercial Product
contact process	V2O5 or Pt	2SO2 + O2 → 2SO3	H2SO4
Haber process	Fe, K2O, Al2O3	N2 + 3H2 → 2NH3	NH3
Ostwald process	Pt and Rh	4NH3 + 5O2 → 4NO + 6H2O	HNO3
water–gas shift reaction	Fe, Cr2O3, or Cu	CO + H2O → CO2 + H2	H2 for NH3, CH3OH, and other fuels
steam reforming	Ni	CH4 + H2O → CO + 3H2	H2
methanol synthesis	ZnO and Cr2O3	CO + 2H2 → CH3OH	CH3OH
Sohio process	bismuth phosphomolybdate	\(\mathrm{CH}_2\textrm{=CHCH}_3+\mathrm{NH_3}+\mathrm{\frac{3}{2}O_2}\rightarrow\mathrm{CH_2}\textrm{=CHCN}+\mathrm{3H_2O}\)	\(\underset{\textrm{acrylonitrile}}{\mathrm{CH_2}\textrm{=CHCN}}\)
catalytic hydrogenation	Ni, Pd, or Pt	RCH=CHR′ + H2 → RCH2—CH2R′	partially hydrogenated oils for margarine, and so forth