Surface-adsorbate interactions are key to understanding adsorption and desorption. From weak van der Waals forces to strong covalent bonds, these interactions determine how molecules stick to surfaces and affect their behavior.

Active sites on surfaces, like step edges and defects, play a crucial role in adsorption and catalysis. Understanding these sites and how they interact with adsorbates is essential for optimizing surface processes and reactions.

Surface-Adsorbate Interactions

Van der Waals Forces and Hydrogen Bonding

• Van der Waals forces are weak, short-range interactions arising from instantaneous dipoles induced by fluctuations in electron density
  • Include London dispersion forces, Debye forces, and Keesom forces
• Hydrogen bonding is a stronger, directional interaction between a hydrogen atom covalently bonded to an electronegative atom (N, O, F) and another electronegative atom with a lone pair of electrons
  • Plays a crucial role in the adsorption of polar molecules on surfaces (water, alcohols)

Covalent and Electrostatic Bonding

• Covalent bonding involves the sharing of electrons between the adsorbate and surface atoms, resulting in the formation of chemical bonds
  • Characterized by high binding energies and specific geometric arrangements
  • Occurs in chemisorption processes (CO on metal surfaces)
• Electrostatic interactions, such as ionic bonding or dipole-dipole interactions, can also contribute to surface-adsorbate bonding
  • Depends on the nature of the surface and adsorbate (charged species, polar molecules)
• The strength and nature of surface-adsorbate interactions determine the adsorption energy
  • Can be measured using techniques such as temperature-programmed desorption (TPD) or calorimetry

Surface Active Sites

Concept and Characteristics

• Surface active sites are specific locations on a surface where adsorption or catalytic reactions preferentially occur
  • Due to their unique geometric, electronic, or chemical properties
• Examples of surface active sites include step edges, kinks, vacancies, and low-coordination sites
  • Often exhibit higher reactivity compared to terrace sites
• The presence of surface defects, such as atomic vacancies or adatoms, can create additional active sites
  • Alter the local electronic structure, influencing adsorption and catalytic behavior

Role in Catalysis

• In heterogeneous catalysis, the interaction between reactants and surface active sites is crucial
  • Lowers activation barriers and facilitates chemical transformations
• The concentration and distribution of surface active sites can be controlled through surface preparation techniques
  • Ion bombardment, annealing, or chemical treatments
  • Optimize adsorption or catalytic performance

Influence of Surface Properties

Surface Structure and Composition

• The atomic structure of the surface affects the arrangement and binding of adsorbates
  • Crystallographic orientation, lattice spacing, and symmetry
• Surface reconstructions, where the topmost atomic layer rearranges to minimize surface energy, can create unique adsorption sites
  • Alter the surface-adsorbate interaction
• The chemical composition of the surface determines the type and strength of bonding with adsorbates
  • Metal surfaces often favor chemisorption through electron transfer
  • Oxide surfaces may exhibit stronger electrostatic or hydrogen-bonding interactions

Defects and Co-adsorbates

• Surface defects, such as step edges, kinks, or vacancies, can act as preferential adsorption sites
  • Due to their lower coordination number and altered electronic structure
  • Adsorbates may bind more strongly to defect sites compared to flat terraces
  • Leads to enhanced reactivity or selectivity in catalytic processes
• The presence of co-adsorbates or surface modifiers can influence the adsorption behavior of other molecules
  • Through lateral interactions, site blocking, or electronic effects

Spectroscopic Characterization of Adsorption

Infrared and X-ray Photoelectron Spectroscopy

• Infrared spectroscopy, particularly reflection-absorption infrared spectroscopy (RAIRS), provides information on the vibrational modes of adsorbed molecules
  • Reveals the nature of surface-adsorbate bonding
  • Shifts in vibrational frequencies compared to gas-phase molecules indicate the strength and type of interaction with the surface
  • Selection rules in RAIRS help determine the orientation of adsorbed molecules relative to the surface normal
• X-ray photoelectron spectroscopy (XPS) probes the electronic structure of surface atoms and adsorbates
  • Measures the binding energies of core-level electrons
  • Shifts in binding energies relative to reference compounds provide information on oxidation state, chemical environment, and bonding
  • Attenuation of substrate peaks upon adsorption estimates the thickness or coverage of the adsorbate layer

Complementary Techniques

• Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is sensitive to the unoccupied electronic states of adsorbates
  • Probes the orientation of molecular orbitals relative to the surface
• Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) provide real-space imaging of adsorbed molecules
  • Reveal their spatial arrangement on surfaces with atomic resolution
• The combination of multiple surface-sensitive techniques offers complementary information
  • Elucidates the complex nature of surface-adsorbate interactions and bonding