Heterogeneous catalysis is a game-changer in chemistry. It's all about solid catalysts speeding up reactions with gases or liquids. These catalysts are super efficient and can be reused, making them key players in industries like chemical manufacturing and pollution control.

To really get how heterogeneous catalysis works, we need to look at the nitty-gritty of what's happening on the catalyst's surface. That's where surface science techniques come in handy, helping us peek at the atomic-level action and figure out how to make even better catalysts.

Heterogeneous Catalysis in Surface Science

Importance and Fundamentals

• Heterogeneous catalysis involves the use of a solid catalyst that is in a different phase than the reactants, typically gas or liquid
  • The catalytic reaction occurs at the surface or interface of the catalyst
• Heterogeneous catalysts are widely used in industrial processes for chemical synthesis, pollution control, and energy production due to their:
  • High efficiency
  • Selectivity
  • Recyclability compared to homogeneous catalysts
• Understanding the fundamental principles of heterogeneous catalysis is crucial for:
  • Designing and optimizing catalytic systems
  • Developing new catalytic materials with improved performance

Surface Science Techniques for Investigating Heterogeneous Catalysts

• Surface science techniques are essential tools for investigating the properties and behavior of heterogeneous catalysts at the atomic and molecular level, including:
  • X-ray photoelectron spectroscopy (XPS)
  • Scanning tunneling microscopy (STM)
  • Temperature-programmed desorption (TPD)

Principles of Heterogeneous Catalysis

Adsorption, Surface Reactions, and Desorption

• Adsorption is the first step in heterogeneous catalysis, where reactant molecules bind to the catalyst surface through:
  • Physisorption (weak van der Waals interactions)
  • Chemisorption (strong chemical bonds)
  • The adsorption process can be described by isotherms (Langmuir isotherm)
• Surface reactions involve the breaking and forming of chemical bonds between adsorbed species on the catalyst surface
  • The rate of surface reactions depends on factors such as:
    • Surface coverage of reactants
    • Activation energy barrier
    • Temperature
• Desorption is the final step in heterogeneous catalysis, where the product molecules leave the catalyst surface
  • The desorption process can be activated by heat or other stimuli
  • It regenerates the active sites on the catalyst for the next catalytic cycle

Mass Transfer, Diffusion, and Catalyst Deactivation

• Mass transfer and diffusion of reactants and products to and from the catalyst surface can be rate-limiting steps in heterogeneous catalysis
  • Especially in porous catalysts with high surface areas
• Catalyst deactivation can occur due to various mechanisms:
  • Poisoning (strong adsorption of impurities)
  • Sintering (loss of surface area)
  • Coking (deposition of carbonaceous species)
• Understanding and mitigating catalyst deactivation is crucial for maintaining long-term catalytic performance

Catalyst Structure and Activity

Surface Structure and Composition

• The surface structure of a heterogeneous catalyst can significantly influence its catalytic properties, including:
  • Arrangement of atoms
  • Presence of defects (steps, kinks)
  • Exposed crystal facets (Pt (111) facet is more active for CO oxidation than the (100) facet)
• The composition of a heterogeneous catalyst can affect its electronic and geometric properties, leading to enhanced catalytic activity and selectivity
  • Type and ratio of metal components in bimetallic catalysts
  • Addition of a second metal (Ru, Sn) to Pt catalysts can improve their selectivity for the hydrogenation of unsaturated aldehydes

Support Materials and Nanoparticle Size/Shape

• The support material can interact with the active metal particles and modulate their catalytic properties through:
  • Electronic and structural effects
  • Providing high surface area and stability
  • Examples of support materials: metal oxides (Al2O3, SiO2), carbon-based materials (graphene, carbon nanotubes)
• The size and shape of metal nanoparticles on the catalyst surface can influence their catalytic activity and selectivity due to the different proportions of surface atoms with varying coordination numbers
  • Small Au nanoparticles (<5 nm) are highly active for CO oxidation, while larger particles are less active
• Surface characterization techniques are essential for understanding the structure-activity relationships in heterogeneous catalysts and guiding the rational design of improved catalytic materials
  • X-ray diffraction (XRD)
  • Transmission electron microscopy (TEM)
  • X-ray absorption spectroscopy (XAS)

Reaction Conditions and Catalysis Performance

Temperature, Pressure, and Reactant Composition

• Temperature plays a crucial role in heterogeneous catalysis, as it affects the rates of adsorption, surface reactions, and desorption
  • Higher temperatures generally increase the reaction rates
  • But they can also lead to catalyst deactivation through sintering or thermal decomposition
• Pressure can influence the surface coverage of reactants and the selectivity of catalytic reactions
  • Higher pressures can increase the concentration of reactants near the catalyst surface, leading to higher reaction rates
  • High pressures may also promote side reactions or catalyst deactivation
• The composition and flow rate of the reactant mixture can affect the catalytic performance by altering:
  • Surface coverage of reactants
  • Residence time in the catalyst bed
  • The optimal reactant ratio and flow rate depend on the specific catalytic reaction and the desired product selectivity

Impurities, Reactor Design, and Optimization

• The presence of impurities or poisons in the reactant stream can significantly impact the catalytic activity and selectivity by:
  • Blocking active sites
  • Modifying the electronic properties of the catalyst
  • Strategies for mitigating the effects of impurities include:
    • Using guard beds
    • Selective adsorption
    • Catalyst regeneration
• The choice of reactor design can influence the mass and heat transfer, as well as the contact time between the reactants and the catalyst
  • Reactor types: fixed-bed, fluidized-bed, membrane reactors
  • The optimal reactor design depends on:
    • The specific catalytic reaction
    • The scale of operation
    • The desired product yield and selectivity