Diffusion: Reactant molecules move from the bulk gas or liquid phase toward the solid catalyst surface.
Adsorption: Reactants form temporary chemical bonds with the catalyst's surface atoms at specific locations called active sites.
Reaction: The adsorption process weakens the internal bonds of the reactant molecules, allowing them to react more easily on the surface to form products.
Desorption: The product molecules break their bonds with the catalyst surface and move back into the bulk phase, freeing the active site for new reactants.
The production of ammonia () utilizes a solid iron catalyst to make the process economically viable.
Because the forward reaction is exothermic, low temperatures would favor a high yield but result in an unacceptably slow reaction rate.
A compromise temperature (typically around 450°C) is used alongside the catalyst to ensure a sufficient rate of production while maintaining a reasonable yield.
High pressures (around 20 MPa) are also used to shift the equilibrium toward the product side, as there are fewer gas molecules in the products than in the reactants.
| Feature | Homogeneous Catalysis | Heterogeneous Catalysis |
|---|---|---|
| Phase | Same as reactants | Different from reactants |
| Separation | Difficult (requires distillation/extraction) | Easy (filtration or fixed bed) |
| Mechanism | Forms intermediate compounds | Surface adsorption/desorption |
| Examples | Acid catalysis in esters | Iron in Haber process |
Adsorption vs. Absorption: Adsorption involves molecules sticking to the surface of a solid, whereas absorption involves molecules entering the bulk of the material.
Catalyst Activity vs. Selectivity: Activity refers to how much the catalyst speeds up the reaction, while selectivity refers to the catalyst's ability to favor one specific product over others.
When explaining industrial conditions, always use the term compromise. Explain that while high temperatures increase rate, they may decrease yield in exothermic reactions, necessitating a middle ground.
Always mention that catalysts provide an alternative pathway; avoid saying they 'lower the activation energy of the original pathway,' as the pathway itself changes.
In questions about Maxwell-Boltzmann distributions, remember that a catalyst does not shift the curve; it only shifts the marker to the left, encompassing more area under the curve.
Be prepared to discuss catalyst poisoning, where impurities bind irreversibly to active sites, rendering the catalyst ineffective and increasing industrial costs.
