10. Catalysis and Catalytic Reactions*


  1. Octane Rating
  2. Steps in Catalytical Reaction
  3. Rate Limiting Step
  4. Regulation for Automotive Exhaust Emissions
  5. Chemical Vapor Deposition
  6. Types of Catalyst Deactivation
  7. Temperature-Time Trajectories
  8. Moving Bed Reactors & Straight Through Transport Reactors

Octane Rating top

A Typical Engine Piston

  1. a uniform burning front

  2. spontaneous combustion producing
    detonation waves and knock

Determine the compression ratio, CR,
to achieve the standard knock intensity.

The more compact molecules are (for a given
number of carbon atoms), the greater the
octane number they will have.



Espanol- Svenska-

Molecular Adsorption

At equilibrium:

Langmuir Isotherms

Dissociative Adsorption

At equilibrium:

Steps in a Catalytic Reaction top

Adsorption on Surface

Surface Reaction

Single Site

Dual Site

Desorption from Surface


Adsorption on Surface

Surface Reaction

Dual Site


Desorption from Surface

Example: Catalytic Reaction to Improve the Octane Number of Gasoline


n-pentane: Octane No. = 62
i-pentane: Octane No. = 90

The difference in octane ratings provides
an economic incentive for carrying out this

Steps in this reaction:

Focusing on the second reaction:

Rate Limiting Steps top


Surface Reaction



Assume surface reaction is rate limiting

If the surface reaction is limiting then:

see also stirctly speaking link


see also strictly speaking link

Site balance:

Substituting for CN-S, CI-S, and CV into CT = CV (1 + KN PN + KI PI) :

where KP is the thermodynamic equilibrium constant for the reactor.

Linearizing the Initial Rate:

on setting rad/ka=0

Single site
Dual Site

Finding the rate law and mechanism for
  Finding the rate law and mechanism for A+B<=>C+D

Regulations for Automotive Exhaust Emissions top
Principle Reactions:
Surface reaction limiting:

CO + NO; -r'CO = f(PCO , PNO , PN2 , PCO2)


Let's see what fraction of sites are covered by CO at the optimum:
Multiplying by CV:
Calculating Fractional Coverage
Dimethyl Ether Examples
Example Exam Questions

Chemical Vapor Deposition, CVD (Chapter 10) top

Manufacturing of a Silicon Layer

We see that a number of the key steps in the microelectronic fabrication involve CVD, we shall consider the CVD of silicon.

I Mechanism
II Rate Limiting Step (Reaction 3)
III Expressing fSiH2 in Terms of Partial Pressures
IV Site / Surface Area Balance:

For the homogeneous reaction:  

Types of Catalyst Deactivation top
Separable Kinetics:

Types of Decay

1.) Sintering
2.) Coking
3.) Poisoning
4.) Slow Decay Temperature-Time Trajectories
5.) Moderate Decay Moving Bed
6.) Rapid Decay STTR

Temperature-Time Trajectories top

The catalyst decay rate is a function of temperature, so you can vary the temperature with time to keep the rate of decay as constant as possible.

or solving for

Decaying Catalyst in a Batch Reactor

Moving Bed Reactors & Straight Through Transport Reactors top

Catalyst Decay Example

The gas-phase, irreversible reaction is elementary with first order decay. The reaction is carried out at constant temperature and pressure.

Batch Reactor Moving Bed Reactor Straight Through
Transport Reactor
Mole Balance:
Rate Law:
Decay Law:
Stoichiometry: gas phase, but , T = T0, and P = P0

Another Catalyst Decay Example

The second-order, irreversible reaction is carried out in a moving bed reactor. The catalyst loading rate is 1 kg/s to a reactor containg 10 kg of catalyst. The rate of decay is second order in activity and first order in concentration for the product, B, which poisons the catalyst. Plot the conversion and activity as a function of catalyst weight down the reactor.

Additional information:



Mole Balance:
Rate Law:
Decay Law:

Conversion vs. Catalyst Weight

Catalyst Activity vs. Catalyst Weight


Moving Bed Reactor

Example 10-7: Strictly Speaking

When there is a change in the velocity due to a change in the number of moles up through the STTR, one cannot directly substitute t = z/U in the coking activity equation:
Instead, one must add another equation to the Polymath program. We know that at any location, the gas velocity up the column is:
where t = 0 at z = 0.

You can use either Polymath or MatLab to solve this equation and substitute it for t in the activity equation:

Along with:

(same as the program in Table E10-7.1)



Catalyst Decay in a Packed Bed
Object Assessment of Chapter 10

  * All chapter references are for the 4th Edition of the text Elements of Chemical Reaction Engineering .