Why is intermediate adsorption strength best for a catalyst?

If reactants bind too weakly they are not activated; if they bind too strongly the products cannot leave and block the site. The most active catalysts bind reactants at an intermediate strength, a balance captured by the volcano-shaped relationship between binding energy and activity.

What makes zeolites useful catalysts?

Zeolites are crystalline aluminosilicates with regular molecular-sized pores and acidic sites. Their pore dimensions admit and release only molecules of certain sizes and shapes, giving shape-selective catalysis that favours specific products, which is valuable in petroleum refining and chemical synthesis.

Heterogeneous Catalytic Materials

Heterogeneous catalytic materials are solids whose surfaces accelerate chemical reactions of gases or liquids without being consumed, providing active sites where reactants adsorb, react, and desorb.

Definition

A heterogeneous catalyst is a solid that increases the rate of a reaction occurring in a different phase by providing surface active sites, lowering the activation barrier through adsorption and bond reorganisation while itself remaining unconsumed.

Scope

This topic covers the materials chemistry of solid catalysts: supported metal nanoparticles, metal oxides, and microporous zeolites; the active sites and surface structure that determine activity and selectivity; the adsorption, surface reaction, and desorption steps of the catalytic cycle; and the roles of support, promoters, and porosity. It connects surface and defect chemistry to the performance and durability of industrial catalysts.

Core questions

What are the active sites of a solid catalyst?
How do adsorption, surface reaction, and desorption make up the catalytic cycle?
How do support, promoters, and porosity affect activity and selectivity?
What causes catalyst deactivation?

Key concepts

Active sites
Adsorption and desorption
Supported metal catalysts
Zeolites and shape selectivity
Promoters and supports
Deactivation and poisoning

Key theories

The surface catalytic cycle: Catalysis proceeds by adsorption of reactants onto surface sites, reaction of the adsorbed species, and desorption of products; the strength of adsorption must be intermediate — strong enough to activate bonds but weak enough to release products — giving the volcano relationship between binding energy and activity.
Active sites, supports, and selectivity: Activity resides at specific surface sites whose structure and electronic state set the reaction; dispersing the active phase on a support maximises sites, while promoters and the shape-selective pores of zeolites tune which products form.

Mechanisms

Reactant molecules chemisorb on surface sites, where bonds weaken and rearrange; surface intermediates react and the products desorb to free the site for another cycle. Activity depends on adsorption strength and site structure, and deactivation occurs by poisoning, fouling, sintering, or loss of active phase.

Clinical relevance

Heterogeneous catalysts underpin much of the chemical and energy industries: they make fertilisers and fuels, refine petroleum, control automobile and industrial emissions, and enable cleaner chemical synthesis, so improving their activity, selectivity, and stability has wide economic and environmental impact.

History

Heterogeneous catalysis grew from early-twentieth-century industrial processes such as ammonia synthesis, with Sabatier and Langmuir laying foundations for surface reactivity and adsorption. Ertl's later atomic-scale studies of reactions on well-defined surfaces, recognised by the 2007 Nobel Prize, established the molecular understanding of how solid catalysts work.

Key figures

Gerhard Ertl
Irving Langmuir
Paul Sabatier

Seminal works

chorkendorff2017
thomas1997

Frequently asked questions

Why is intermediate adsorption strength best for a catalyst?: If reactants bind too weakly they are not activated; if they bind too strongly the products cannot leave and block the site. The most active catalysts bind reactants at an intermediate strength, a balance captured by the volcano-shaped relationship between binding energy and activity.
What makes zeolites useful catalysts?: Zeolites are crystalline aluminosilicates with regular molecular-sized pores and acidic sites. Their pore dimensions admit and release only molecules of certain sizes and shapes, giving shape-selective catalysis that favours specific products, which is valuable in petroleum refining and chemical synthesis.