Classification by Phase of the Reaction System
This is the primary, classic classification method, comprising three main categories:
Homogeneous catalysts
The catalyst and reactants exist in the same phase (e.g., acid-base solutions or soluble metal compounds); there is no phase interface, and the reaction system is uniform.
Heterogeneous catalysts
The catalyst and reactants exist in different phases; the vast majority are solids (e.g., metals or metal oxides). They are the most widely used in industry due to the ease of separation and recovery.
Biocatalysts
Primarily enzymes; they can efficiently catalyze various biochemical reactions-either inside or outside living organisms-under specific conditions and exhibit extremely high specificity.
Classification by Physical State
Solid catalysts
These account for the largest share of industrial applications (e.g., iron catalysts for ammonia synthesis or molecular sieve catalysts for petroleum cracking); they offer good stability and are easy to recover.
Liquid catalysts
Typically exist in solution form (e.g., sulfuric acid or sodium hydroxide solutions) and are widely used in homogeneous reactions.
Gaseous catalysts
Relatively rare; used only in specific high-temperature gas-phase reaction scenarios.
