Heterogeneous & Homogeneous Catalysts

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  • 0:04 Types of Catalysts
  • 0:37 Heterogeneous Catalysts
  • 2:55 Homogeneous Catalysts
  • 4:16 Poisoned Catalysts
  • 5:23 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

There are two main groups of catalysts, heterogeneous and homogeneous. In this lesson, we'll learn how they are different, how each type reacts, and look at examples of each.

Types of Catalysts

In order to help change toxic gases that cars produce into nontoxic gases, a catalytic converter is used. This catalytic converter gets its name because it uses a catalyst to help in the reaction to convert the toxic gas into a nontoxic form.

A catalyst is a compound used to help a reaction occur faster by lowering the activation energy. There are two types of catalysts, homogeneous and heterogeneous. A homogeneous catalyst is a catalyst that is in the same phase as the reactants, while a heterogeneous catalyst is in a different phase than the reactants.

Heterogeneous Catalysts

As we just covered, a heterogeneous catalyst is in a different phase than the reactants. Because of this, it can't mix in with the reactants. The most common way these catalysts work is that the reactants adsorb onto the surface of the active site of the catalyst. With the reactants connected to the catalyst, the reaction occurs faster. There are several different mechanisms by which this occurs. Sometimes the reactants are brought into close proximity with each other. Sometimes the connection between one of the reactants and the catalyst helps make the reactant more reactive. After the reaction occurs, the catalyst releases the product. A good catalyst needs to bind tightly enough to the reactant to allow the reaction to occur, but weak enough to release the product. Let's take a closer look at some examples of heterogeneous cataysts.

Examples

There are several processes which use heterogeneous catalysts to help speed up the reaction. The contact process is one of the most widely used methods for sulfuric acid, H2 SO4, production. By adding excess oxygen to sulfur dioxide, sulfuric acid can be formed. But this is a very slow reaction. Vanadium oxide (V2 O5) is a brown/yellow solid on which the oxygen and sulfur dioxide can adsorb in order to react with each other to form sulfuric acid. This reaction is interesting because the vanadium oxide is reduced during the process of this reaction. Simply adding extra oxygen reforms the catalyst so that it can still be used.

Methanol, CH3 OH, can be produced by many different methods. But, by using carbon monoxide and hydrogen, a lot of energy (in the form of heat) is also produced, which can be used as power. The carbon monoxide and hydrogen method isn't as fast as other methods, such as keeping a stream of carbon dioxide, water, carbon monoxide, and hydrogen gases mixed together to form methanol. So, a catalyst such as Cr2 O3 can be used to make this an energy efficient method.

Ammonia, NH3, is an important product used in commercial farming. One way it can be made is through the Haber process, which uses nitrogen and hydrogen to form ammonia. In this process, nitrogen from the air can be used, a plentiful reactant. Iron is used as the catalyst, but it isn't pure iron. Instead, iron and potassium hydroxide work together to help the nitrogen and hydrogen come together to combine into ammonia.

Homogeneous Catalysts

As we mentioned earlier, with a homogeneous catalyst, the catalyst and the reactants are in the same phase. The allows the catalyst to directly interact with the reactants. Sometimes the catalyst can be positively charged and bring two negative charges together.

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