Day 25: Homogeneous and Heterogeneous Catalysis

D25.1 Homogeneous Catalysis

Reactions that are facilitated by catalysts can be divided into two major classes: homogeneous catalysis and heterogeneous catalysis. A homogeneous catalyst is present in the same phase as the reactants. Gas-phase reactions and reactions in solution are homogeneous reactions and we have already discussed several examples where catalyst and reactants are all in the same phase. Here is one more.

In Section D13.5 we described condensation reactions and hydrolysis reactions that occur in aqueous solutions. (Hydrolysis is the reverse of a condensation that produces water as the small molecule.) We also mentioned that condensation and hydrolysis can be catalyzed by strong acids (that is, by H+ ions). This is a homogeneous catalytic reaction because reactants, products, and catalyst are all in aqueous solution.

An example of ester hydrolysis is the acid-catalyzed decomposition of methyl acetate to form acetic acid and methanol. The reaction mechanism is shown in Figure 1.

Figure 1. Mechanism of acid-catalyzed hydrolysis of methyl acetate to form acetic acid and methanol.

Activity 1: Analyzing a Reaction Mechanism

D25.2 Heterogeneous Catalysts

A heterogeneous catalyst is present in a different phase from the reactants. Such catalysts are usually solids, and often function by furnishing an active surface upon which one or more steps in the reaction can occur.

A heterogeneous catalytic reaction has at least four steps in its reaction mechanism:

  1. Adsorption of the reactant(s) onto the surface of the catalyst
  2. Activation of the adsorbed reactant(s)
  3. Reaction of the adsorbed reactant(s)
  4. Diffusion of the product(s) from the surface into the gas or liquid phase (desorption)

Any one of these steps may be slow and thus may serve as the rate determining step. But the overall rate of the reaction is still faster than it would be without the catalyst. Figure 1 illustrates the reaction of alkenes with hydrogen on a nickel catalyst.

In this figure, four diagrams labeled a through d are shown. In each, a square array of green spheres forming a nickel surface is shown in perspective to provide a three-dimensional appearance. In a, the label “N i surface” is placed above with a line segment extending to the green spheres. At the lower left and upper right, pairs of white spheres bonded tougher together appear as well as white spheres on the green surface. Black arrows are drawn from each of the white spheres above the surface to the white sphere on the green surface. In b, the white spheres are still present on the green surface. Above the surface is a grayed-out structure labeled ethene with two C atoms and four H atoms. The label “Ethene” at the top of the diagram is connected to the greyed out structure with a line segment.  Arrows indicating motion point down from this structure to a an ethene molecule with two central black spheres with a single bond indicated by a horizontal black rod between them. Above and below to the left and right, a total of four white spheres are connected to the black spheres with white rods. A line segment extends from this structure to the label, “Ethene adsorbed on surface; pi bond broken.”  In c, the diagram is very similar to b except that the greyed out structure and labels are gone and one of the white spheres near the black and white structure in each pair on the green surface is greyed out. Arrows point from each greyed-out white sphere to each of the two black spheres. There is a label below that says, "H atoms migrate to C H 2". In d, only a single white sphere remains from each pair in the green surface. A curved arrow points from the middle of the green surface to a model above with two central black carbon-atom spheres with a single black rod indicating a single bond between them. Each of the black spheres has three small white spheres bonded as indicated by white rods between the black spheres and the small white spheres. The four bonds around each black sphere are evenly distributed about the black spheres.
Figure 2. There are four steps in the catalytic hydrogenation of ethene on a nickel surface, C2H4(g) + H2(g) ⟶ C2H6(g). (a) Hydrogen is adsorbed on the surface, breaking the H–H bonds and forming Ni–H bonds. (b) Ethene is adsorbed on the surface, breaking the π bond and forming Ni–C bonds. (c) H atoms diffuse across the surface and form new C–H bonds when they reach ethene molecules. (d) The saturated carbon atoms in C2H6 molecules can no longer bond to the surface so the ethane molecules escape from the surface.

The uncatalyzed C2H4(g) + H2(g) ⟶ C2H6(g) reaction would necessitate a transition state where the C=C π bond and the H-H σ bond are breaking while the C-H σ bonds form. Such a transition state is so high in energy that without a catalyst, H2 is considered as being unreactive towards alkenes under most conditions.

Nickel is a catalyst often used in the hydrogenation of polyunsaturated fats and oils to produce saturated fats and oils. Other significant industrial processes that involve the use of heterogeneous catalysts include the preparation of sulfuric acid, the preparation of ammonia, the oxidation of ammonia to nitric acid, and the synthesis of methanol. Heterogeneous catalysts are also used in the catalytic converters found on most gasoline-powered automobiles.

Exercise 1: Catalysis and Reaction Energy Diagram

Exercise 2: Catalytic Mechanisms

Podia Question

Platinum metal is a heterogeneous catalyst for this reaction:

2 NO(g)  →  N2(g)  +  O2(g)

The reaction rate varies with concentration of NO as shown in the graph.

Explain each of these observations. Include in your explanation an atomic-level description of NO molecules, the platinum surface, and how the two interact.

  • The graph is linear with positive slope at low concentrations of NO.
  • The graph is horizontal at high concentrations of NO.

Suggest an experiment that could be done to support or contradict your explanation of the horizontal graph. Describe the hypothesis you propose for what will happen in the experiment if your explanation is correct; also describe what experimental results would contradict your hypothesis.

Two days before the next whole-class session, this Podia question will become live on Podia, where you can submit your answer.

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.