cre ii l 10
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CRE II Heterogeneous Catalysis
L 10
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
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Mechanism of Heterogeneous Catalysis:
1. Bulk Diffusion of reacting molecules to the
surface of the catalyst2. Pore Diffusion of reacting molecules into
the interior pores of the catalyst
3. Adsorption of reactants (chemisorption) onthe 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 thesurface of the catalyst
7. Bulk Diffusion of product molecules
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Mechanism of Heterogeneous Catalysis:
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Mechanism of Heterogeneous Catalysis:
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Pore and film resistances in a catalyst particle
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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 byrelative magnitudes of rate constants).
In such a case, the overall rate, may be determined
almost entirely by the slowest step, called the rate-determining step (rds).
The rate of the rds is infinitesimal when comparedto the rates of other steps.
Alternately the rates of other steps are infinite
compared to the rate of rds.
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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
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Desorption:
•Desorption of a product could also be rate
controlling in a few cases
Complexities:
•
Theoretically more than one step can berate controlling
•Too many possible mechanisms
•
Experimental data is normally fitted to anysingle rate controlling step, which is thencalled the most plausible mechanism
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•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 productsare not easily desorbed from the surface.
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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 θ
N number of sites occupied by adsorbatesθ=N total number of adsorption sites
å
When θ=1,NS=N∑ and an adsorbed monolayer
is formed.The fractional coverage θ dependson pressure of adsorbing gas phase species.
This θ=θ(P) relationship is called an adsorption
isotherm.
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Langmuir Adsorption Isotherm
Simple approach toquantitatively describe
an adsorption process
at the gas/solid interface
∑
N = N + N number of vacant sitesVS
Assumptions:
• Solids surface is homogeneous and contain a
number of equivalent sites,each of which is
occupied by a single adsorbate molecule.
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• A dynamic equilibrium exist between gas
phase reactant and adsorbed species.
• No interactions between adsorbed species.
• Adsorbed species localised, ΔHads isindependent of coverage θ..