# 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

 Octane Rating top

#### A Typical Engine Piston

 a uniform burning front spontaneous combustion producingdetonation waves and knock Determine the compression ratio, CR, to achieve the standard knock intensity. The more compact molecules are (for a givennumber of carbon atoms), the greater theoctane number they will have.

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 At equilibrium:

#### Langmuir Isotherms

 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 Eley-Rideal Desorption from Surface

Example: Catalytic Reaction to Improve the Octane Number of Gasoline
 Rationale: n-pentane: Octane No. = 62i-pentane: Octane No. = 90 The difference in octane ratings providesan economic incentive for carrying out thisreaction. Steps in this reaction: Focusing on the second reaction:

 Rate Limiting Steps top
 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:

 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

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

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

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

 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 (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:

 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.

 Then: 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
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.

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 .