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14.4: Heterogeneous Catalysts

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    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.

    imageedit_3_4962652511.png
    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

    This page titled 14.4: Heterogeneous Catalysts is shared under a not declared license and was authored, remixed, and/or curated by Kathryn Haas.

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