Unit 10. Heterogeneous Catalysis

This website provides learning and teaching tools for a first course on kinetics and reaction engineering. The course is divided into four parts (I through IV). Here, in Part II of the course, the focus is on chemical reaction kinetics, and more specifically, on rate expressions, which are mathematical models of reaction rates. As you progress through Part II, you will learn how rate expressions are generated from experimental kinetics data.

This first section of Part II of the course focuses upon the selection of an equation to be tested as a rate expression. The equation to be tested can be chosen simply for its mathematical convenience. Alternatively, theory can be used to select the mathematical form of the equation to be tested. For some reactions, theory can be applied directly. In other cases the reaction must be described in terms of a group of reactions that comprise what is known as a reaction mechanism. In the latter case theory can be applied to the reactions in the mechanism which are then combined to get the mathematical form of the equation to be tested.

The most common type of heterogeneous catalysis involves gas phase reactants and products and a solid catalyst. The mechanistic steps in such a reaction typically take place on the surface of the catalyst. This unit shows how the rates of these reactions can be modeled using the fractional coverage of the surface by the various species in place of surface concentrations. Similarly, the conservation of catalyst can be expressed in terms of fractional coverages instead of concentrations. With these modifications, the methods considered in previous units for generating a rate expression from a mechanism can be applied to heterogeneous catalytic reaction mechanisms. The unit also shows that if the most of the catalyst surface is covered by one species, then the mechanistic rate expression can be simplified considerably. The unit also offers a comparison of Michaelis-Menten rate expressions for enzymatic reactions to Langmuir-Hinshelwood rate expressions for heterogeneous catalytic reactions.

Learning Resources

• Archive (.zip) - Contains all learning resources listed below for this unit
• Documents to Read:
  • Unit 10 Informational Reading (.pdf)
  • Example 10.1 (.pdf)
  • Example 10.2 (.pdf)
• Videos to Watch (please right-click and save, then play back locally on your computer):
  • Unit 10 Informational Video (.mov)
  • Example 10.1 Video (.mov)
  • Example 10.2 Video (.mov)
• Reference Files:
  • Unit 10 Definitions, Nomenclature and Equations (.pdf)

Teaching Resources

• Archive (.zip) - Contains all teaching resources listed below for this unit
• Sample Class
  • Online Pre-class Quiz (.pdf file)
  • Redacted slides to make available to students before class
    • Keynote 09
    • PowerPoint
  • Class Lesson Plan (.pdf)
  • Complete slides to make available to students after class
    • Keynote 09
    • PowerPoint
  • Solutions to the in-class activities to make available to students after class
    • Activity 10.1 Solution (.pdf)
    • Activity 10.2 Solution (.pdf)
• Alternative Questions (.pdf) that could be used in a pre-class quiz

Practice Problems

1. Suppose that the gas phase conversion of A to B is heterogeneously catalyzed and the mechanism is given by reactions (1a) through (1c). In addition, suppose that gas phase species Z adsorbs reversibly on the catalyst as given in equation (1d). Derive an acceptable expression for the rate of generation of B taking place in the presence of Z. Your rate expression should not include any surface coverages.

(Problem Statement as .pdf file)

2.^* Assume that methane partial oxidation, equation (2), takes place on a metal catalyst according to the simplified mechanistic scheme given in equations (2a) through (2d). Step (2c) may be assumed to be the rate-limiting step. Develop a rate expression for the overall reaction in terms of only the concentrations of the gas phase species (methane, oxygen, and methanol) and the equilibrium and rate constants. If it is assumed that the surface of the catalyst is almost completely covered with adsorbed O (which is the most abundant surface intermediate), how does this simplify the rate expression? Comment upon the result.

(Problem Statement as .pdf file)