Catalysis: Homogeneous and Heterogeneous

• Catalysts accelerate chemical reactions without being consumed
• They participate in the reaction mechanism and are regenerated in their original form
• Catalysts lower the activation energy, providing an alternative reaction pathway
• Reaction rate increases as more reactant molecules overcome the reduced energy barrier
• Catalysts do not alter reaction thermodynamics
• The equilibrium constant and Gibbs free energy change (ΔG) remain unchanged
• Catalysts affect reaction kinetics, accelerating the rate at which equilibrium is reached
• Catalysis is classified into homogeneous and heterogeneous

Homogeneous Catalysis

• Occurs when catalysts and reactants are in the same phase
• Typically, catalysts and reactants are in solution (usually liquid)
• Acid catalysis of ester hydrolysis in aqueous solution exemplifies this, with the acid catalyst (e.g., H+) in the aqueous phase

Heterogeneous Catalysis

• Occurs when catalysts and reactants are in different phases
• Commonly, the catalyst is solid, and reactants are gases or liquids
• Solid catalysts in automobile catalytic converters reduce harmful emissions, a common example

How Catalysts Work

• Catalysts lower the activation energy compared to uncatalyzed reactions
• Activation energy is the energy barrier for a reaction to occur
• Lowering this barrier increases the proportion of molecules with enough energy to react
• Catalysts interact with reactants to form intermediates
• Intermediates proceed to form products, regenerating the catalyst

Steps in Heterogeneous Catalysis

• Reactants diffuse to the catalyst surface from the bulk phase
• Reactants adsorb onto the catalyst surface through chemisorption or physisorption
• Chemical reactions occur on the catalyst surface, forming intermediate species
• Product molecules desorb from the catalyst surface
• Products diffuse away from the catalyst surface into the bulk phase

Catalyst Selectivity

• Selectivity is the ability of a catalyst to direct a reaction towards a specific product
• Some catalysts are highly selective
• Selectivity depends on the catalyst's chemical composition, physical structure, and reaction conditions
• Maximizing desired product yield and minimizing byproducts relies on selectivity in many industrial processes

Examples of Catalysts

• Enzymes are biological catalysts that accelerate biochemical reactions
• Acids and bases catalyze hydrolysis, esterification, and cracking, among other reactions
• Transition metals are used in hydrogenation, oxidation, and polymerization in industrial processes
• Zeolites are used in petroleum refining and petrochemical industries due to their pore structures and acidic properties

Industrial Applications of Catalysis

• Catalysis is integral to many industrial processes
• Petroleum refining uses catalytic cracking, reforming, and isomerization
• Petrochemical production uses catalysts to produce plastics and synthetic fibers
• Chemical synthesis utilizes catalysts for pharmaceuticals and agrochemicals
• Environmental catalysis uses catalytic converters to reduce vehicle emissions, and catalysts to remove pollutants from waste streams

Catalyst Poisoning and Inhibition

• Catalyst poisoning occurs when a substance binds to the catalyst surface, blocking active sites
• Common poisons include sulfur compounds, carbon monoxide, and heavy metals
• Inhibition is the reduction in catalytic activity caused by substances competing for adsorption sites
• Preventing poisoning and inhibition maintains the efficiency and longevity of catalysts