Catalysts are extremely diverse and can be classified according to multiple dimensions such as composition, phase, mechanism of action, and function.

Classification by the phase relationship between the catalyst and the reaction system

This is the most intuitive and engineering-critical classification method, which determines the design and operation mode of the reactor.

Homogeneous catalyst

Definition: A catalyst that is in the same phase as the reactants (usually the gas or liquid phase).

Advantages: High activity and good selectivity (because each catalytic center is uniformly dispersed and in full contact with the reactants).

Disadvantages: Difficult to separate and recover, usually requires complex distillation, extraction and other steps, high cost, and may contaminate the product.

Typical types: acid/base catalysts , such as sulfuric acid (for esterification and alkylation), sodium hydroxide. Rhodium carbonyl (for hydroformylation of olefins to aldehydes). Palladium/platinum complexes (for cross-coupling reactions, such as the Suzuki reaction).

Heterogeneous catalysts

Definition: A catalyst is a substance in a different phase from the reactants. Typically, the catalyst is a solid, and the reactants are a gas or liquid.

Advantages: Easy to separate (through simple filtration and sedimentation), can be produced continuously, has a long lifespan, and is reusable. It is the most important form of catalyst in industry.

Disadvantages: The active centers are located on the surface, resulting in relatively low utilization; mass and heat transfer may become the limiting steps.

Main components: The active component is the catalytic substance (such as Pt, Pd, Ni, V₂O₅, Cu/ZnO) , while the support is generally a porous material that supports the active component, providing high specific surface area and mechanical strength (such as Al₂O₃, SiO₂, TiO₂, activated carbon, molecular sieve).

Typical types : Automotive three-way catalysts , with Pt-Pd-Rh supported on honeycomb ceramics. Ammonia-iron synthesis catalysts , typically supported on Fe-K₂O-Al₂O₃. Desulfurization catalysts , typically supported on Co-Mo/Al₂O₃. Petroleum cracking catalysts , typically in the form of zeolite molecular sieves .

Biocatalysts - Enzymes

Definition: Proteins or RNA with catalytic functions. Essentially, they are special catalysts that exist between homogeneous and heterogeneous phases.

Advantages: Ultra-high efficiency (10⁷-10¹³ times higher than ordinary catalysts) and specificity ("one key opens one lock"), mild reaction conditions (room temperature and pressure).

Disadvantages: Sensitive to the environment (pH, temperature), easily deactivated, high cost, and difficult to recycle.

Typical industrial applications: Immobilized enzymes (which can be considered a type of heterogeneous catalyst) are used to produce high fructose syrup, laundry detergent, and pharmaceuticals (such as penicillin acylase for the production of semi-synthetic penicillin).

Classified by chemical composition and material 

Group VIII transition metals (Fe, Co, Ni, Ru, Rh, Pd, Pt, Ir, etc.) and Cu, Ag, Au.

Features: Primarily used in reactions involving H₂ or O₂, such as hydrogenation, dehydrogenation, oxidation, and reforming.

Examples: Pt/Rh network (ammonia oxidation to nitric acid), Pd/C (hydrogenation reduction), RaneyNi (inexpensive hydrogenation catalyst).

Metal oxide and sulfide catalysts

Features: Heat-resistant, oxygen-resistant, and toxic-resistant; commonly used in oxidation, dehydrogenation, selective reduction, and acid-base catalysis.

Examples: Oxidized forms: V₂O₅ (sulfuric acid and phthalic anhydride production), CuO/ZnO/Al₂O₃ (methanol synthesis), Bi-Mo-O (selective propylene oxidation), TiO₂ (photocatalysis) .

Solid acid/base catalysts

Features: It can replace highly corrosive and difficult-to-recycle liquid acids/alkalis, and is environmentally friendly.

Examples: Solid acids , such as zeolite molecular sieves, heteropoly acids, zirconium sulfate, acidic resins, and Nafion membranes. Solid bases , such as hydrotalcite, supported alkaline earth metal oxides, and basic resins.

Complex catalysts

Definition: A complex ion or molecule formed by a central metal atom and an organic ligand through coordination bonds. It can be used in homogeneous phases or immobilized on heterogeneous supports.

Features: Catalytic performance can be "tailored" by designing ligands to achieve high enantioselectivity (asymmetric catalysis).

Examples: Wilkinson catalysts (RhCl(PPh₃)₃, hydrogenation), Ziegler-Natta catalysts (TiCl₄/AlR₃, olefin polymerization), and various chiral catalysts used in drug synthesis.

 Classified by function or reaction 

Oxidation catalysts: catalysts for the complete/selective oxidation of organic matter or CO (such as Ag catalysts for automobile exhaust purification and the oxidation of ethylene to ethylene oxide).

Hydrogenation/dehydrogenation catalysts: catalysts for hydrogenation of unsaturated bonds or dehydrogenation of saturated hydrocarbons (such as Ni catalysts for hydrogenation of oils and fats, and Fe-K catalysts for dehydrogenation of ethylbenzene to styrene).

Acid catalysts: catalytic cracking, isomerization, alkylation, esterification and other reactions (such as zeolites in petroleum cracking).

Polymerization catalysts: catalysts for the polymerization of olefins and dienes (such as Zn catalysts and metallocene catalysts).

Environmentally friendly catalysts: specifically designed to purify pollutants.

Exhaust gas purification catalysts: automotive three-way catalysts, power plant SCR denitrification catalysts (V₂O₅-WO₃/TiO₂).

Photocatalysts: Catalysts that produce active species after absorbing light energy to catalyze reactions (such as TiO₂ photocatalytic water splitting and photodegradation of pollutants).

Electrocatalysts: catalysts that accelerate electrochemical reactions on the electrode surface (such as Pt/C catalysts for fuel cells and IrO₂/RuO₂ catalysts for hydrogen production by water electrolysis).