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CRE II Heterogeneous Catalysis L 10

Prof. K.K.Pant Department of Chemical Engineering IIT Delhi. kkpant@chemical.iitd.ac.in

Mechanism of Heterogeneous Catalysis: 1. Bulk Diffusion of reacting molecules to the surface of the catalyst 2. Pore Diffusion of reacting molecules into the interior pores of the catalyst 3. Adsorption of reactants (chemisorption) on the surface of the catalyst 4. Reaction on the surface of the catalyst between adsorbed molecules 5. Desorption of products 6. Pore Diffusion of product molecules to the surface of the catalyst 7. Bulk Diffusion of product molecules

Mechanism of Heterogeneous Catalysis:

Mechanism of Heterogeneous Catalysis:

Pore and film resistances in a catalyst particle

Rate-Determining Step (rds) In a kinetics scheme involving more than one step, it may be that one change occurs much faster or much slower than the others (as determined by relative magnitudes of rate constants). In such a case, the overall rate, may be determined almost entirely by the slowest step, called the ratedetermining step (rds). The rate of the rds is infinitesimal when compared to the rates of other steps. Alternately the rates of other steps are infinite compared to the rate of rds.

Chemisorption: Chemisorption controlled reactions are usually fast Rate increases rapidly with increase in temp. Permits the use of wide reactors Surface reaction: 70% of the reactions which are not controlled by diffusion falls under this case Rate increases rapidly with increase in temp. Permits the use of wide reactors

Desorption: Desorption of a product could also be rate controlling in a few cases Complexities: Theoretically more than one step can be rate controlling Too many possible mechanisms Experimental data is normally fitted to any single rate controlling step, which is then called the most plausible mechanism

Bonding strength between H2 & metal surface increases with increase in vacant d orbital.

Maximum catalytic activity will not be realized

if the bonding is too strong and the products

are not easily desorbed from the surface.


Adsorption at gas/solid interface.

Adsorption: Term used to describe the process Whereby a molecule(the adsorbate) forms a Bond to a solid surface(an adsorbent). Fractional surface coverage Ns number of sites occupied by adsorbate = N total number of adsorption sites When =1,NS=N and an adsorbed monolayer is formed.The fractional coverage depends on pressure of adsorbing gas phase species. This =(P) relationship is called an adsorption isotherm.

Langmuir Adsorption Isotherm

Simple approach to quantitatively describe an adsorption process at the gas/solid interface

N = N +N number of vacant sites V S


Solids surface is homogeneous and contain a number of equivalent sites,each of which is occupied by a single adsorbate molecule.

A dynamic equilibrium exist between gas phase reactant and adsorbed species. No interactions between adsorbed species. Adsorbed species localised,Hads is independent of coverage ..