🧂Physical Chemistry II Unit 6 – Surface Chemistry and Catalysis

Key Concepts and Definitions

• Surface chemistry studies chemical reactions and physical processes occurring at the interface between two phases (solid-liquid, solid-gas, liquid-gas)
• Adsorption refers to the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface
  • Physisorption involves weak van der Waals forces (dipole-dipole interactions, London dispersion forces)
  • Chemisorption involves the formation of chemical bonds between the adsorbate and the surface
• Desorption is the release of adsorbed species from a surface back into the gas or liquid phase
• Catalysis is the process of increasing the rate of a chemical reaction by adding a substance (catalyst) that is not consumed in the reaction
• Heterogeneous catalysis occurs when the catalyst and reactants are in different phases (solid catalyst, gas/liquid reactants)
• Homogeneous catalysis occurs when the catalyst and reactants are in the same phase (typically liquid)
• Turnover frequency (TOF) measures the efficiency of a catalyst, defined as the number of reactant molecules converted per active site per unit time

Surface Properties and Characterization

• Surface area and porosity play crucial roles in determining the reactivity and adsorption capacity of materials
• Brunauer-Emmett-Teller (BET) theory is used to calculate the specific surface area of a material based on gas adsorption isotherms
• Pore size distribution can be determined using methods such as mercury porosimetry or gas adsorption-desorption isotherms
• Surface morphology and roughness affect the accessibility of active sites and the overall catalytic performance
  • Techniques like atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide high-resolution images of surface features
• Surface composition and chemical states are analyzed using spectroscopic methods (X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES))
• Surface acidity and basicity influence the adsorption and reaction of molecules on catalytic surfaces
  • Temperature-programmed desorption (TPD) of probe molecules (ammonia, carbon dioxide) quantifies the number and strength of acid/base sites

Adsorption Phenomena

• Adsorption isotherms describe the relationship between the amount of adsorbate on a surface and its pressure or concentration at constant temperature
  • Langmuir isotherm assumes monolayer adsorption on a homogeneous surface with no interaction between adsorbed molecules
  • Freundlich isotherm accounts for multilayer adsorption and heterogeneous surfaces
• Adsorption kinetics govern the rate at which molecules adsorb onto a surface
  • The rate of adsorption depends on factors such as temperature, pressure, and surface coverage
• Heat of adsorption quantifies the strength of the interaction between the adsorbate and the surface
  • Calorimetric methods (differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC)) measure the heat released or absorbed during adsorption
• Competitive adsorption occurs when multiple species compete for the same adsorption sites on a surface
  • Selective adsorption can be achieved by tuning the surface properties or operating conditions to favor the desired species
• Surface diffusion enables adsorbed molecules to move along the surface, facilitating their interaction with active sites or other adsorbed species

Surface Reactions and Kinetics

• Surface reactions involve the interaction and transformation of adsorbed species on a catalytic surface
• Langmuir-Hinshelwood mechanism assumes that the reaction occurs between two adsorbed species on the surface
  • The rate-determining step is often the surface reaction between adsorbed reactants
• Eley-Rideal mechanism proposes that a gas-phase molecule directly reacts with an adsorbed species on the surface
• Mars-van Krevelen mechanism involves the incorporation of lattice oxygen from the catalyst into the product, followed by the replenishment of oxygen from the gas phase
• Microkinetic modeling combines elementary reaction steps to predict the overall reaction rate and selectivity
  • Kinetic parameters (activation energies, pre-exponential factors) are obtained from experimental data or theoretical calculations
• Surface restructuring and reconstruction can occur during reactions, altering the catalytic properties of the surface
  • In situ characterization techniques (X-ray absorption spectroscopy (XAS), infrared spectroscopy (IR)) monitor surface changes under reaction conditions

Types of Catalysis

• Heterogeneous catalysis is widely used in industrial processes (ammonia synthesis, petroleum refining, automotive exhaust treatment)
  • Solid catalysts provide a high surface area for reactions and enable easy separation from products
• Homogeneous catalysis offers high selectivity and milder reaction conditions compared to heterogeneous catalysis
  • Organometallic complexes and enzymes are examples of homogeneous catalysts
• Biocatalysis employs enzymes or whole cells to catalyze reactions with high specificity and efficiency
  • Biocatalysts are biodegradable and operate under mild conditions (aqueous media, ambient temperature and pressure)
• Photocatalysis utilizes light to activate catalysts and drive chemical reactions
  • Semiconductor materials (titanium dioxide (TiO2), cadmium sulfide (CdS)) are common photocatalysts for environmental remediation and solar fuel production
• Electrocatalysis involves the use of electrodes to catalyze redox reactions, often in fuel cells or electrolyzers
  • Platinum group metals (platinum (Pt), palladium (Pd), ruthenium (Ru)) are effective electrocatalysts for hydrogen evolution and oxygen reduction reactions

Catalytic Materials and Design

• Transition metals (iron (Fe), cobalt (Co), nickel (Ni), copper (Cu)) are widely used as catalysts due to their variable oxidation states and ability to form complexes
• Noble metals (platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au)) exhibit high catalytic activity and stability but are scarce and expensive
• Metal oxides (cerium oxide (CeO2), titanium dioxide (TiO2), zirconium dioxide (ZrO2)) possess unique redox properties and acid-base characteristics
• Zeolites are microporous aluminosilicate materials with well-defined pore structures and strong acidity
  • The shape selectivity of zeolites enables selective catalysis based on the size and geometry of reactants and products
• Supported catalysts consist of active components dispersed on high-surface-area supports (alumina (Al2O3), silica (SiO2), carbon)
  • Supports enhance the stability and dispersion of active sites while facilitating heat and mass transfer
• Catalyst design strategies aim to optimize activity, selectivity, and stability
  • Nanostructuring increases the surface-to-volume ratio and exposes more active sites
  • Alloying and doping modify the electronic and geometric properties of catalysts
  • Encapsulation and core-shell structures protect active sites from deactivation and improve selectivity

Industrial Applications

• Ammonia synthesis (Haber-Bosch process) uses an iron catalyst to produce ammonia from nitrogen and hydrogen gases
  • The catalyst enables the reaction to occur at lower temperatures and pressures than the uncatalyzed process
• Catalytic cracking in petroleum refining converts heavy hydrocarbons into lighter, more valuable products (gasoline, diesel)
  • Zeolite catalysts selectively crack large molecules based on their pore structure and acidity
• Automotive catalytic converters reduce harmful emissions (carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx)) from engine exhaust
  • Three-way catalysts (TWCs) simultaneously oxidize CO and HC while reducing NOx using platinum group metals
• Polymerization catalysts enable the production of plastics and synthetic fibers with controlled properties
  • Ziegler-Natta catalysts (titanium chloride (TiCl4) and triethylaluminum (Al(C2H5)3)) produce high-density polyethylene (HDPE) and isotactic polypropylene (PP)
• Hydrogenation reactions are used to saturate unsaturated compounds, reduce nitro groups, and remove sulfur from fuels
  • Raney nickel, a porous nickel catalyst, is commonly used for hydrogenation in the food and pharmaceutical industries

Emerging Trends and Research

• Single-atom catalysts (SACs) maximize the utilization of precious metals by dispersing individual atoms on supports
  • SACs exhibit unique catalytic properties and high selectivity due to their well-defined active sites
• Catalytic nanomaterials (nanoparticles, nanowires, nanosheets) offer high surface areas and tunable properties
  • Colloidal synthesis methods enable precise control over the size, shape, and composition of catalytic nanostructures
• Hierarchical catalysts combine micro-, meso-, and macroporosity to facilitate mass transfer and accommodate bulky molecules
  • Zeolite-templated carbons (ZTCs) and metal-organic frameworks (MOFs) are examples of hierarchical catalytic materials
• Machine learning and computational catalysis accelerate the discovery and optimization of catalysts
  • High-throughput screening and data-driven approaches identify promising catalyst compositions and structures
• Operando characterization techniques monitor catalysts under working conditions to elucidate reaction mechanisms and deactivation processes
  • Synchrotron-based X-ray techniques (XAS, XRD) and environmental transmission electron microscopy (ETEM) provide real-time insights into catalytic processes
• Sustainable and green catalysis focuses on developing environmentally friendly and energy-efficient catalytic processes
  • Biomass-derived catalysts, CO2 utilization, and waste valorization are active areas of research in sustainable catalysis