In the Langmuir-Hinshelwood mechanism, a heterogeneous catalytic reaction does not happen in the gas phase — both reactants must first adsorb onto neighbouring active sites on a solid catalyst surface before they can react. Here CO molecules (blue) stick to a single empty site, while O₂ molecules (red) dissociate on adsorption and need two adjacent empty sites to land. Only when a CO and an O atom occupy neighbouring sites can they react to form CO₂ (green), which promptly desorbs back into the gas phase.
This exact mechanism runs inside every car's catalytic converter: platinum and palladium sites adsorb CO and O₂ from exhaust gas, letting them combine into harmless CO₂ far faster than they ever could in free gas — a reaction Irving Langmuir and Cyril Hinshelwood both won Nobel Prizes for helping to explain.
A catalyst lattice where CO and O₂ molecules drift down from the gas phase, stick to empty active sites, and react only when a CO and an oxygen atom end up on neighbouring sites — releasing CO₂ back into the gas.
Both reactants must adsorb before they can react — O₂ dissociates and needs two empty neighbouring sites, so an excess of one gas can flood the surface and starve the other, capping the reaction rate.
Set CO and O₂ pressure, raise or lower temperature, and choose how densely active sites are packed. Watch coverage percentages and reaction rate respond, and see the surface tip into saturation.
This is exactly how a catalytic converter cleans car exhaust: platinum and palladium sites adsorb CO and O₂ so they can combine into CO₂ far faster than they ever would in free gas.