Catalysis Lab: Surface Reaction Simulator

Catalyst

Temperature 550 K

Reactant concentration 20

Surface coverage

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Reaction rate

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Products formed

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Rate constant k

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Energy diagram

Eₐ = 38 kJ/mol (Pt) vs 120 kJ/mol uncatalyzed

How it works

This is heterogeneous catalysis: reactant molecules (blue) collide with the solid catalyst surface and, at an open active site, adsorb onto it. Two adsorbed atoms sitting on neighbouring sites can then react together directly on the surface — the classic Langmuir–Hinshelwood mechanism — before the product (green) desorbs back into the gas phase, freeing the site for another cycle.

The catalyst doesn't change the reaction's thermodynamics (ΔG, and the equilibrium it settles at, stay the same) — it opens an alternative reaction pathway with a much lower activation energy Eₐ, so a far larger fraction of collisions have enough energy to react. The rate constant follows the Arrhenius equation:

k = A · exp(−Eₐ / R·T)
rate ∝ k · [siteA_occupied] · [siteB_occupied]

Catalyst — None leaves the surface inert (Eₐ ≈ 120 kJ/mol, nothing sticks, no reaction); Pt gives the lowest barrier (Eₐ ≈ 38 kJ/mol) and Pd a higher one (Eₐ ≈ 50 kJ/mol) — different metals bind and activate reactants differently.
Temperature — raises both the collision/adsorption rate and, through the exponential in the Arrhenius equation, the surface reaction rate k.
Concentration — more gas-phase molecules means more collisions with open sites and faster site turnover.
Surface coverage — the fraction of active sites currently holding an adsorbed reactant; too high and no sites remain open for new molecules, too low and neighbouring pairs rarely form.

Real-world relevance: this exact site-blocking, adsorb–react–desorb cycle is what happens inside a catalytic converter (Pt/Pd/Rh), an ammonia synthesis reactor (Haber–Bosch, Fe catalyst) and industrial hydrogenation — the catalyst is recovered unchanged at the end of every cycle, which is why a small mass of it can process an enormous throughput of reactant.

Reactant (gas) Product (gas) Adsorbed on site Open site

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