Heterogeneous Catalysis Steps

Heterogeneous Catalysis Steps

• The process of heterogeneous catalysis occurs in five steps:
  • Reactants diffuse to the surface of the catalyst
  • Molecules adhere to suitable sites available for adsorption
  • Reactants bound to the surface form an intermediate compound
  • The intermediate compound gets desorbed from the surface, making it available for adsorption of other molecules
  • The intermediate compound disintegrates to form the final products, which then diffuse out of the internal pores and the external surface of the catalyst

Kinetic Treatment of Heterogeneous Catalytic Reaction

• The rate of catalytic reaction is proportional to the surface coverage (θ)
• The Langmuir isotherm equation is used to describe the adsorption process: θ = b * p / (1 + b * p)
• There are three cases:
  • Partially covered catalyst surface: R = k * b * p (first-order reaction)
  • Fully covered catalyst surface: R = k (zero-order reaction)
  • Adsorption with dissociation: R = k * b^1/2 * p^1/2

Poisoning of Catalysts

• A poison reduces the effectiveness of a catalyst by preferentially adsorbing onto the catalyst surface
• Characteristics of poisoning:
  • Very minute quantities can inhibit catalytic reactions
  • Poisoning can be reversible or irreversible
  • Examples of poisons include volatile compounds of sulfur, HCN, lead, and arsenic compounds

Types of Catalysts

• Non-porous, bulk-solid catalyst particles (e.g. Pt metal sheet)
• Supported catalysts (e.g. Pt on ceramic, TiCl3 on MgCl2, Pt crystallites on Al2O3)
• Porous, bulk-solid catalyst particles (e.g. zeolites, silica, Raney nickel, activated charcoals)

Zeolites

• Microporous crystalline solids with diameters smaller than 2 nm
• Consist of silicon, aluminum, and oxygen in a three-dimensional polymeric framework
• Can have cations such as Na+, K+, Ca2+, Mg2+, water, and/or other molecules within their pores
• Three common types of zeolites: A, X, and Y
• Starting materials include sodium aluminate and sodium silicate
• Synthesis occurs at temperatures between 300-450 K
• Sodium ions can be exchanged for ammonium ions, and acidic forms of zeolites can be produced by heating the ammonium-zeolite to 700 K

Charcoal

• Dark grey residue consisting of carbon and any remaining ash
• Produced by slow pyrolysis of wood or other substances in the absence of oxygen
• Activated charcoal has pores ranging from 1-100 nm
• Physical adsorption is the primary means of removing contaminants from wastewater
• Types of charcoal include granular activated carbon, powdered activated carbon, and impregnated carbon

Catalytic Processes and Their Catalysts

• Ammonia synthesis: iron
• Sulfuric acid manufacture: nitrogen(II) oxide, platinum
• Cracking of petroleum: zeolites
• Hydrogenation of unsaturated hydrocarbons: nickel, platinum, or palladium
• Oxidation of hydrocarbons in automobile exhausts: copper(II) oxide, vanadium(V) oxide, platinum, palladium
• Isomerization of n-butane to isobutane: aluminum chloride, hydrogen chloride

Enzyme Catalysis

• Enzyme catalysis is the increase in the rate of a chemical reaction by the active site within the enzyme
• Enzymes form a transition state with reactants, lowering the free energy of the reaction
• Once the reaction completes, the product leaves the active site, allowing the enzyme to catalyze more reactions
• Criteria for enzyme catalysis:
  • Enzymes are highly selective and active
  • There are over 2000 different enzymes, each catalyzing different reactions
  • Enzymes are large molecules with cross-sectional dimensions up to 100 nm
  • The active site is a small part of the external surface
  • Enzymes can be deactivated over time at moderate temperatures
  • The rate of catalytic enzyme reaction has a maximum value at pH = 7.4
  • The lock-key theory explains the mechanism of enzyme catalysis