Surface Catalyzed Reactions

 Surface Catalyzed Reactions

 Surface Catalyzed Reactions

These are heterogeneous reactions. These reactions consist of solid phase catalyst and liquid/gas phase reactants. They take place at the surface of solid catalyst and thus called surface catalyzed reactions. Reactants react in adsorbed form. In below equations “ g” = gaseous phase and “ad” = adsorbed phase. And the catalyst in the very first equation if platinum.

CO_(g) + O_2(g)          →             CO_2(g)

CO_(g)              →                CO_(ad)

O_2(g)              →               O_2(ad)

CO_(ad)        +     O_2(ad)        →          CO_2(g)

Steps involved in surface catalyzed reactions

Surface catalyzed reactions takes place in 5 steps. (It is a complex mechanism)

1) Diffusion of reactants to the surface of catalyst.

2) Adsorption of reactants at the surface of catalyst.

3) Reaction between adsorbed reactants. This step is chemical reaction step.

4) Desorption of product from the surface.

5) Diffusion of product away from the surface of catalyst.

Out of these five steps, one is rate determining step.

If step (3) is slow (rate determining) then the surface catalyzed reaction is called kinetic controlled reactions or true reactions.

If steps other than (3) are slow (i.e. rate determining) then the reaction is called diffusion controlled reaction.

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All steps other than (3) are physical processes. Rate of reaction depends upon the concentration of reactants in adsorbed form. Adsorbed concentration of reactant can be expressed in terms of Fraction of catalyst surface covered “θ”.

            A           →        P        eq.23.1

A_(g)       →       A_(ad)

A_(ad)      →       P_(ad)

P_(ad)       →      P_(g)

θ = fraction of catalyst surface covered by reactant

θ_A fraction of catalyst surface covered by reactant A

For θ, 

adsorption isotherms are used.

Rate  ∝ θ_A

            Rate = k θ_{A        eq.23.2}

Most common adsorption isotherms are:

1) Langmuir adsorption isotherm

2) Frendlich adsorption isotherm

3) Tempkin adsorption isotherm.

(4) BET (Brunauer-Emmett-Teller) theory

Here we will discuss Langmuir Adsorption isotherm.

From eq.23.1 we have reactant = A and product = P. The value of θ here is

here b = adsorption equation constant.. Reactants will be adsorbate and Catalyst will be adsorbent. For θ_{A the equation will be}                            

 eq.23.3

At low pressure for A, 1 + bAPA ~ 1. So eq.23.3 will become

 θ_{A =} bAPA            eq.23.4

Now we can put this value of θ_A in eq.23.2

                                                                            Rate = k θ_A 

                                                                            Rate = k bAPA 

            

eq.23.5

Here, k and bA are constants so, KbA = apparent rate constant and can be written as "Kap". Hence eq.23.5 becomes                                        

Integrating the above equation we will get,

Following  the integration rule 

, the above equation will becomes,

-ln PA = Kap t + z        eq.25.6

To find the value of z, consider that if t = 0 then PA= Pi 

-ln Pi = Kap (0) = z                             or z = -ln Pi

Putting the value of z in eq.23.6 we got

-ln PA = Kap t - ln Pi

ln Pi - ln PA = Kap t

ln Pi/PA =  Kap t                 eq.23.7

Above is a rate equation for surface catalyzed reaction. The graphical representation will be

The above discussed method was for systems having one reactants. For systems having more than one reactants we have three cases:

Case I : All reactants in gaseous form

A_(g)      +        B_{(g)     →}   P

Case II: One reactant in gaseous form and one in liquid form.

A_(g)      +        B_(l)       →  P

Case III: All reactants in liquid form

A_(l)      +        B_(l)     →    P

For these types of reaction there will be two types of mechanism to occur.

1. Langmuir - Hinshelwood Mechanism

Both reactants will react in adsorbed form.

                                                                    A_(g)       →       A_(ad)

                                                                      _{B(g)       →       B}(ad)

A_(ad)      + B_(ad)          →        P

2. Eley Rideal Mechanism

One of the reactant is in adsorbed form and the other in fluid form. The fluid can be gaseous of liquid.

A    +    B       →    P

A_(g)        →          A_(ad)

A_(ad)    + B_(g)        →        P

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Chain Reactions Or Consecutive Reactions

First Order Reactions

1st Order Type Parallel Reactions