10. Catalysis and Catalytic Reactions^*

Topics

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

A Typical Engine Piston

	a uniform burning front 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.

to further explanation on octane rating

Molecular Adsorption

At equilibrium:

Langmuir Isotherms

Dissociative Adsorption

At equilibrium:

Adsorption on Surface
Surface Reaction	Single Site
	Dual Site
Desorption from Surface

Adsorption on Surface
Surface Reaction	Dual Site
	Eley-Rideal
Desorption from Surface

Rationale:	n-pentane: Octane No. = 62 i-pentane: Octane No. = 90
	The difference in octane ratings provides an economic incentive for carrying out this reaction.
Steps in this reaction:
Focusing on the second reaction:

Adsorption
Surface Reaction

Desorption
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 r_ad/k_a=0

Single site
A)
Dual Site
B)
C)
Eley-Rideal
D)

Finding the rate law and mechanism for

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

Principle Reactions:

Surface reaction limiting:

CO + NO; -r^'_CO = f(P_CO , P_NO , P_N2 , P_CO2)

Example

Let's see what fraction of sites are covered by CO at the optimum:
Multiplying by CV:
(A)
(B)

Calculating Fractional Coverage

Dimethyl Ether Examples

Example Exam Questions

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
(1)
(2)
(3)
II Rate Limiting Step (Reaction 3)
rdep=rs=ksfSiH2
III Expressing fSiH2 in Terms of Partial Pressures
IV Site / Surface Area Balance:

For the homogeneous reaction:
	then
	where:

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

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.

Then:
or solving for

Decaying Catalyst in a Batch Reactor

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
Combine:

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:

Solution:

Polymath
Mole Balance:
Rate Law:
Decay Law:
Stoichiometry:
Combine:

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:
	(1)
Instead, one must add another equation to the Polymath program. We know that at any location, the gas velocity up the column is:
	(2)
Then:
	(3)
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:
etc. (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 .

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