Topic 11 · page 17 of 21

Solid catalysts in industry

In gas-phase industrial reactions a solid catalyst provides a surface. The gases adsorb, react on the surface and desorb, by a route with a lower activation energy. A bigger surface area gives more places to react.

11 min read11.11

Read first: Activation energy and catalysts

Why a solid for a gas-phase reaction 11.11

Many industrial reactions between gases use a solid catalyst. The catalyst is in a different phase from the reactants, so it is a heterogeneous catalyst (page 4). This suits industry. The gases flow continuously through a bed of the solid, the catalyst stays where it is, and the products leave in the gas stream. No separation step is needed.

Process	Reaction	Solid catalyst
Haber	$\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$	iron
Contact	$\ce{2SO2(g) + O2(g) <=> 2SO3(g)}$	vanadium(V) oxide, V₂O₅
Catalytic converter	$\ce{2CO(g) + 2NO(g) -> 2CO2(g) + N2(g)}$	platinum and rhodium

Providing a surface 11.11

The reaction does not happen throughout the solid. It happens on its surface, which provides a place where gas molecules can meet and react. The mechanism has three stages.

Four panels of a gas reaction on a solid metal surface, drawn as a row of grey atoms at the bottom. Panel 1: nitrogen and hydrogen molecules approach the surface. Panel 2, adsorption: the molecules are held on the surface and their bonds are weakened or broken, leaving separate N and H atoms on the surface. Panel 3, reaction: the atoms bond to each other on the surface to make NH3. Panel 4, desorption: the ammonia molecules leave the surface, freeing the sites. — Haber process on iron, drawn schematically. Adsorption, reaction on the surface, then desorption.

Adsorption. Reactant molecules bond to active sites on the surface. The bonds within the molecules are weakened, and may break. For the Haber process this loosens the very strong N≡N bond.
Reaction. The atoms and fragments are held close to each other on the surface, in the right positions, and bond together to form the product.
Desorption. The product molecules leave the surface. This frees the sites for more reactant.

Adsorption is not the same as absorption. Adsorption is bonding to a surface, and absorption is soaking into a material.

Surface area 11.11

Only the surface does the work, so the surface area matters. More surface means more active sites, so more reactant molecules can adsorb at once and the rate is higher. Industry therefore uses a catalyst with a large surface area:

finely divided or porous solids, such as iron pellets in the Haber process
thin coatings of an expensive metal on a ceramic honeycomb, as in a catalytic converter. The honeycomb has a huge area for its volume, so a small mass of platinum and rhodium is enough.

Check yourself

State what is meant by a heterogeneous catalyst, and name the catalyst used in the Haber process.
Describe how an iron catalyst speeds up the reaction of nitrogen with hydrogen.
Explain why a solid catalyst is often used as a powder, as pellets with pores, or on a honeycomb.
Why is it important that the product molecules desorb from the surface?
Give one advantage of using a solid catalyst for a gas-phase reaction rather than a catalyst dissolved in a liquid.
A student writes that the iron catalyst increases the equilibrium yield of ammonia because it brings the gases together. Explain what is wrong.

Answers cover these until you have tried the questions

A catalyst in a different phase from the reactants. The Haber process uses solid iron.
Nitrogen and hydrogen molecules adsorb onto active sites on the iron surface, and the bonds in the molecules are weakened. The atoms react on the surface to form ammonia, and the ammonia desorbs. This provides an alternative route with a lower activation energy.
The reaction takes place on the surface. A powder, pores or a honeycomb give a large surface area with more active sites, so more molecules can react at the same time and the rate increases.
If the products stay on the surface they block the active sites, so no more reactants can adsorb and the reaction stops.
The catalyst is in a different phase, so it does not need to be separated from the products. The gases flow through the solid and the catalyst stays in place, so the process can run continuously.
A catalyst speeds up the forward and the reverse reactions equally, so it does not change the position of equilibrium or the equilibrium yield. It only helps the system reach equilibrium faster.