Elements of
Chemical Reaction Engineering
6th Edition



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Essentials of
Chemical Reaction Engineering
Second Edition

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Chapter 10: Catalysis and Catalytic Reactors

Finding the Mechanism

      

The following data were obtained for the oxidation of CO over a catalyst. All rates are initial rates

.020

0.01

1

.035

0.01

3

.049

.01

6

.060

.01

9

.196

.1

1

.384

.2

1

.902

.5

1

1.653

1

1

4.44

5

1

5.00

10

1

4.44

20

1

2.77

50

1

The initial rate was found to be independent of CO2.  

a)   Suggest a rate law consistent with the data

Hint 1  Sketch versus CO2.

Hint 2  Sketch versus CCO at low concentration.

Hint 3  Sketch versus CCO at high concentration.

Full Solution What is the rate law?






 

b)   Suggest mechanisms consistent with the rate law

Hint 1 What do the trends suggest?

Hint 2 What is the mechanism?

Full Solution

 

Hint 1

Use only the data points for which the concentration of CO is the same.

 

 

 

Back to Hints

Back to Problem

 

Hint 2: Low concentration

For constant CO2

 

We see at low concentration the rate is linear in CO and

Back to Hints

Back to Problem

Hint 3: High concentration

   

At high concentrations the rate decreases with increasing concentration

Back to Hints

Back to Problem

Full Solution: Part A

Combining

For fixed concentration of O2 and low concentration of CO and

                      

 

High concentration

             

Consequently the rate law

Satisfies the data

Back to Part b

 

Hint 1

From the denominator of the rate law, we see that CO is adsorbed and that O2 is either weakly adsorbed (KO2PO21/2 << 1) or not adsorbed at all.  

The 1/2 order with respect to oxygen suggests dissociative adsorption.  

Because the initial rate is independent of CO2, it is either not adsorbed on the surface or weakly adsorbed.  

Back to Hints

Back to Problem

 

Hint 2

The following mechanism is consistent with the rate law

Molecular Adsorption                               

Dissociative Adsorption                                

Surface Reaction                                                    

Back to Hints

Back to Problem

 

Full Solution: Part B

The following mechanism is consistent with the rate law

Molecular Adsorption                               

Dissociative Adsorption                                

Surface Reaction                                                    

Assume surface reaction limits

                                                        

            1)                                     

                                                            

            2)                                       

                                                           

            3)                    

            4)                                          

                                                  

            5)                           

We see that oxygen is weakly adsorbed (very small) such that

                                                         

Back to Chapter 10