8.2 Homogeneous and heterogeneous catalysis

Catalysis is a game-changer in chemical reactions. It speeds things up and makes processes more efficient. This topic dives into two main types: homogeneous and heterogeneous catalysis, exploring how they work and what sets them apart.

From industrial applications to environmental protection, catalysts are everywhere. We'll look at real-world examples, like making ammonia or cleaning car exhaust. Understanding catalysis is key to grasping how many important chemical processes happen.

Catalysis Types and Mechanisms

Mechanisms of homogeneous catalysis

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• Homogeneous catalysis occurs when the catalyst and reactants are in the same phase, typically liquid
  • Catalyst and reactants are molecularly dispersed allowing for efficient interaction
• Coordination of reactants to the catalyst forms an activated complex
• Lowers the activation energy by providing an alternative reaction pathway
• Stabilizes transition states or intermediates through ligand interactions
• Regenerates the catalyst after product formation allowing for multiple catalytic cycles
• Examples of homogeneous catalysts:
  • Transition metal complexes with organic ligands (Wilkinson's catalyst for hydrogenation)
  • Enzymes in biological systems (catalase for hydrogen peroxide decomposition)

Process of heterogeneous catalysis

• Heterogeneous catalysis occurs when the catalyst and reactants are in different phases
  • Typically a solid catalyst with reactants in the gas or liquid phase
• Steps in heterogeneous catalysis:
  1. Adsorption of reactants onto the catalyst surface
  2. Surface reaction between adsorbed species forming products
  3. Desorption of products from the catalyst surface
  4. Regeneration of active sites on the catalyst surface for subsequent catalytic cycles
• Heterogeneous catalysts provide a surface for the reaction to occur
  • Active sites on the surface are responsible for catalytic activity
  • Surface area and porosity of the catalyst are crucial factors in determining catalytic efficiency
• Examples of heterogeneous catalysts:
  • Supported metal nanoparticles (Pt, Pd, or Ni on $Al_2O_3$ or $SiO_2$)
  • Zeolites with acid sites for cracking and isomerization reactions

Homogeneous vs heterogeneous catalysis

• Similarities:
  • Both types of catalysis aim to increase reaction rates and selectivity
  • Catalysts lower the activation energy by providing alternative reaction pathways
  • Catalysts are regenerated after each catalytic cycle
• Differences:
  • Phase: Homogeneous catalysts are in the same phase as reactants while heterogeneous catalysts are in a different phase
  • Separation: Homogeneous catalysts are difficult to separate from products while heterogeneous catalysts are easily separated
  • Reaction conditions: Homogeneous catalysis often occurs under milder conditions while heterogeneous catalysis may require higher temperatures and pressures
  • Selectivity: Homogeneous catalysts generally offer higher selectivity due to well-defined active sites while heterogeneous catalysts may have various active sites with different selectivities
  • Recyclability: Heterogeneous catalysts are more easily recycled and reused compared to homogeneous catalysts

Industrial applications of catalysts

• Homogeneous catalysis:
  • Hydroformylation (oxo process): Converts alkenes to aldehydes using Co or Rh complexes
  • Acetic acid production (Monsanto process): Carbonylation of methanol using Rh or Ir complexes
  • Asymmetric hydrogenation: Synthesizes chiral compounds using Ru or Rh complexes with chiral ligands
• Heterogeneous catalysis:
  • Ammonia synthesis (Haber-Bosch process): $N_2$ and $H_2$ react over an Fe catalyst to produce $NH_3$
  • Catalytic cracking: Breaks down large hydrocarbons into smaller molecules using zeolite catalysts
  • Hydrodesulfurization: Removes sulfur from fuels using Mo or W sulfide catalysts supported on $Al_2O_3$
  • Catalytic converters in automobiles: Reduces $NO_x$, CO, and unburned hydrocarbons using Pt, Pd, or Rh catalysts