Heterogeneous catalysis (C9981)
Common Pitfalls of Catalysis Manuscripts Submitted to Chemistry of Materials


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Active site/catalyst evaluation
•Turn-over frequency (TOF; [s–1])
= number of catalytic cycles performed by 1 active site per time unit •Precise numbers for
homogeneous and enzymatic catalysis
• Numbers for heterogeneous catalysis???
TOF [s–1]
Hetero: ~1–100 s–1
Homo: ~10–1000 s–1
Enzymes: ~10000–1000000 s–1

Active site/catalyst evaluation
•Turn-over number (TON; [-])
= number of catalytic cycles performed by 1 active site before deactivation (~lifetime) •Precise
numbers for homogeneous and enzymatic catalysis
• Numbers for heterogeneous catalysis???

Common pitfalls - 1
•Improper calculation of turnover frequencies (TOFs)
–Low conversion, early stage of the rxn
–Valid only for specific reactant concetration
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•Improper calculation of turnover numbers (TONs)
–A measure of catalyst’s stability
–Accurate determination requires measurements until the catalyst’s activity is completely lost
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Common pitfalls - 2
•Deactivation studies at full/equilibrium conversion
–Batch vs. continuous flow
–The available amount of reagents limit the conversion = the catalyst could be in fact more active
–Deactivation should be studied at intermediate conversions
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Common pitfalls - 2
•Deactivation studies at full/equilibrium conversion
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Active site/catalyst evaluation
•Selectivity is ability of catalyst to form one product from a pool of products (possibly many)
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•Selectivity (S; [%])
= number of D molecules produced / R molecules converted
reactant R

Common pitfalls - 3
•Comparison of selectivities at different conversion levels
–Selectivity does depend on conversion
–A→B→C
–Always compare at isoconversion
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Diffusional limitation
•Gradient of reactant concentration in
–Fluid film of particle (External diffusion)
–Inside the pore (Internal diffusion)
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Common pitfalls - 4
•Neglect of mass transfer (diffusional) limitations
–External (film) + Internal (pore) diffusion
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Rate determining step
•High temperature
–The chemical reaction is fast
–There is no time for internal diffusion to take place, only external surface employed in catalysis
–Diffusional steps
are limiting
–Eapp = Ea of the diffusion
in the fluid film
(external diffusion)

Common pitfalls - 4
•Neglect of mass transfer (diffusional) limitations
–Batch, both diffusions: stirring rates
–Continuous flow, external diffusion:
–Continuous flow, internal diffusion: particle size, pore volume, pore size
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Diffusional limitations
•Internal diffusional limitations always present to some extent
–We can diminish them at the time of catalyst preparation (pore volume, pore diameter, size of
catalysts grains)
–Good practice is to compare a series of catalysts with similar pore volume, pore diameter, and
size of catalysts grains
•External can be avoided
at the time of catalytic
reaction
–Linear velocity of vector gas,…

Common pitfalls - 5
•Failure to study the catalysts after reaction
–Catalysts can change dramatically during catalytic reactions
–Sintering, coking, pore collapse, poisoning,…
–It is not correct to justify the differences between the catalysts only based on the
characterization of the starting material
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Common pitfalls - 5
•Failure to study the catalysts after reaction
–Catalysts can change dramatically during catalytic reactions
–Coking
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Catalysts | Free Full-Text | Catalyst Stability—Bottleneck of Efficient Catalytic Pyrolysis | HTML

Common pitfalls - 5
•Failure to study the catalysts after reaction
–Catalysts can change dramatically during catalytic reactions
–Sintering
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Before cata
After cata

Common pitfalls - 6
•Failure to consider differences in surface area of catalysts
–Catalytic activity scales with number of active sites
–Common sense: Number of active sites scales with the surface area
–Compare the activity of catalysts per m2
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