CHEM 310: Page-by-Page Surface Chemistry & Heterogeneous Catalysis Notes

• Definition of Surface Chemistry: The study of chemical reactions at surfaces, particularly at interfaces between different phases such as solid-gas, solid-liquid, and solid-solid.

• Catalysts: workhorses of the chemical industry; ~85–95% of chemical industry products produced via catalytic processes.

• Roles of catalysts:

  • Production of transportation fuels.

  • Bulk chemical production across industry sectors.

  • Pollution prevention by avoiding waste formation.

  • End-of-pipe pollution abatement (e.g., automotive/industrial exhaust systems).

• A catalyst provides energetically favorable pathways, enabling industrially feasible conditions (P, T, time).These techniques help minimize the environmental impact of chemical processes and improve overall efficiency in reaction systems.

How Catalysts Work (Qualitative)

• A catalyst accelerates a reaction without being consumed.

• Mechanism: forms bonds with reacting molecules, facilitates reaction to product, product detaches, leaving the catalyst available for further cycles.

• Catalysis is cyclic: catalyst participates and is recovered in its original form at cycle end.

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How Catalysts Speed Up Reactions (Energy Perspective)

• Potential energy diagram comparison: non-catalyzed vs catalyzed pathways with a solid catalyst and gas-phase reactants.

• Features:

  • Catalysis provides a lower activation energy Ea.

  • ΔG of reaction is unchanged by catalysis: Ea (catalyzed) < Ea (uncatalyzed); ΔG stays the same.

  • Catalysts affect kinetics, not thermodynamics.

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Implications for Catalyst Design

• Not all catalyst–reactant combinations succeed. Criteria for effective catalysts:

  • Bonding too weak → low conversion.

  • Bonding too strong → active sites occupied by one reactant, blocking other reactants; intermediates/products may bind too strongly and poison the cycle.

• The best catalyst has intermediate interaction strength with reactants and products.

• Catalyst selection requires consideration of many parameters and extensive experimental testing.

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Types of Catalysis I: Biocatalysis (Enzymatic Catalysis)

• Enzymes: highly efficient catalysts with shape-specific active sites.

• Example: Catalase catalyzes decomposition of hydrogen peroxide to water and oxygen.

• Mechanism: Enzymes lower activation energy, allowing reactions to proceed faster and under milder conditions compared to traditional catalysts.

• Advantages: Selectivity, milder reaction conditions, and the ability to catalyze complex reactions.

• Remark on turnover: Catalase can convert millions (~10^7) molecules of H2O2 per second.

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Type of Catalysis I example: Catecholase

• Catecholase example polyphenol, features a dinuclear copper center, embedded in a central four-helix bundle motif.

• Each active copper site is coordinated by three histidine residues and a solvent molecule (H2O).This enzyme catalyzes the oxidation of phenolic compounds, which plays a crucial role in the browning of fruits and vegetables, impacting their flavor and shelf life.

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Michaelis–Menten Enzyme Kinetics (Intro)

• Model: E + S ⇄ ES → P

  • E = free enzyme, S = substrate, ES = enzyme–substrate complex, P = product.

• At equilibrium, [ES] is treated as constant, leading to standard MM derivation with rate equations.This model explains how the rate of product formation (P) is influenced by the concentration of the substrate (S) and the enzyme (E), which is crucial for understanding enzyme efficiency and reaction kinetics.

Summary of MM Parameters

• Km indicates affinity; lower Km means higher affinity.

• kcat is turnover number (s^-1).

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Types of Catalysis II: Homogeneous Catalysis

• Definition: Both catalyst and reactants are in the same phase.

Page 17

Ozone Hole and Montreal Protocol

• Montréal Protocol results: Ozone levels stabilized in the 1990s; recovery projected to pre-1980 levels by ~2075.

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Types of Catalysis III: Heterogeneous Catalysis

• Distinction: catalyst and reactants in different phases (solid catalyst, usually gas-phase reactants).

• Real-world example: Three-way catalytic converter in cars reduces emissions: CO, NOx, VOCs.

• main focus

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Three-way Catalytic Converters

• Objective: minimize CO, VOCs, and NOx emissions.

• Components: reduction catalyst and oxidation catalyst, both on high-surface-area ceramic supports coated with Pt, Rh, and/or Pd nanoparticles (typical particle size < 10 nm).

• Reduction catalyst: NOx reduction: 2NO → N2 + O2 or 2NO2 → N2 + 2O2.

• Oxidation catalyst: oxidizes VOCs and CO: VOCs + O2 → CO2 + H2O; 2CO + O2 → 2CO2.

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What is a Surface?

• A surface: region of a phase modified by proximity to an interface.

• Surfaces underpin heterogeneous catalysis, adhesion, corrosion, flocculation, detergency, etc.

• Heterogeneous catalysis, adhesion, corrosion, flocculation and detergency all involve chemical phenomena occurring at either a gas-solid or liquid-solid interface

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Surface Reactivity Origins

• Solid surfaces are highly reactive due to steps, kinks, adatoms, dislocations, coordinative unsaturation, dangling bonds, and electron density protrusion.

• General rule: surface reactivity increases as the coordination number (CN) of surface atoms decreases.

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Surface Structure of Metals and Planes

• Metals used in catalysis are often active in finely divided or polycrystalline form.

• The exposed surface planes determine reaction pathways; understanding surface plane composition is crucial.

• Common metals are fcc, hcp, or bcc structures with low-index surfaces typically present.

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Importance of Exposed Surface Planes

• To understand surface properties, we need to know the fraction of each surface plane exposed and their properties.

• Macroscopic single crystals can be cut to expose specific planes; most metals adopt low-index planes.

• In principle, therefore, we can understand the surface properties of any material if we

  • know the amount of each type of surface exposed, and

  • have detailed knowledge of the properties of each and every type of surface plane

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Crystallography: Structures and Planes

• Common crystal structures: fcc (face centred cubuic), hcp (hexagonol close packed), bcc (body-centred cubic)

• Each structure exhibits unique packing arrangements, affecting their catalytic behavior and reactivity.

• Infinite possible planes, but practically limited to low-index surfaces.

• For catalytic metals (Pt, Rh, Pd, Ag, Au), fcc is common.

Miller Indices: Surface Notation

• For cubic structures (fcc), common planes: (111), (100), (110) with CN surface values.

• Adsorption sites depend on geometry: on-top, bridging, hollow.

• Reactivity order on fcc surfaces: fcc(111) < fcc(100) < fcc(110).

  

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Surface Notation and Adsorption Sites

• On fcc surfaces, adsorption sites include on-top, bridge, and hollow (three types).

• Adsorbate interaction with these sites influences adsorption energy and reactivity.

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Energetics of Solid Surfaces

• All surfaces have a positive surface free energy (surface tension) because breaking bonds to create a new surface costs energy.

• All surfaces are energetically unfavourable in that they have a positive free energy of formation

• Surface energy minimization strategies:

  • Reduce exposed surface area.

  • Expose low-energy surface planes.

  • Reorganize atoms to lower surface energy.

• Metastable surfaces can exist due to kinetic barriers (e.g., highly dispersed supported metal catalysts).

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Relaxation and Reconstruction of Surfaces

• Relaxation: small, subtle rearrangements of surface layers to reduce surface energy; no change in surface periodicity.

• Unrelaxed vs. relaxed: layer spacing perpendicular to surface changes; in-relaxation, dbulk > d1-2, etc.

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Reconstruction of Surfaces

• Reconstruction: larger, atomic-scale rearrangements; can change surface periodicity and symmetry.

• Common on less stable metal surfaces (e.g., fcc(110)) and abundant on semiconductors.

• Detectable via surface diffraction techniques like LEED/RHEED due to periodicity changes.

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Adsorbate-Induced Reconstruction

• Adsorbates can induce reconstruction by bonding strongly enough to alter surface periodicity.

• In the case of many semiconductors, the simple reconstructions can often be explained in terms of a "surface healing" process in which the coordinative unsaturation of the surface atoms is reduced by bond formation between adjacent atoms

• This process leads to a change in the surface periodicity: the period of the surface structure is doubled in one direction giving rise to the so-called (2x1) reconstruction

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Adsorbate-Induced Reconstructions and Examples

• adsorabte-induced reconstruction: The reconstruction of a surface is frequently induced by the adsorption of molecular or

  atomic species onto the surface

• Adsorbate-induced reconstruction is favoured when adsorbate–substrate bonding competes with substrate–substrate bonding.

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Supported Metal Catalysts – Particulates

• Definition: Metals dispersed on oxide supports; often sub-micron to ~1 nm particles.

• Particle shape is driven by surface free energy and the desire to minimize surface area to volume.

• Ideal equilibrium shape for fixed mass tends toward a sphere; for crystals, shapes like cubo-octahedra arise to balance low-energy surfaces (8×(111) and 6×(100)) with area considerations.

• Surface atom coordination on exposed faces: CNs differ by face type (e.g., CN=9 on 111, CN=8 on 100, CN=6 at corners).

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Nanoparticle Facets and CN

• Center atoms on {111} facets have CN = 9; center of {100} facets CN = 8; corner/intersections CN = 6.

• The concept of achieving a balance between low-energy surface facets and minimized surface area shapes nanoparticle morphology.

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Metal–Support Interaction (MSI)

• MSI is not always weak; strong MSI can lead to greater wetting of the support, increasing metal–support contact area and altering particle morphology.

• In strong MSI cases, one should consider interfacial free energy with the support rather than treating the metal particle in isolation.

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Understanding MSI: Wetting and Contact Angles

• Spreading of liquids (metal nanoparticles on oxide supports) can be analyzed via contact angle and interfacial tensions.

• Surface tensions: γ (N m^-1) for liquids; solids also characterized by surface energy γ.

• Spreading occurs if the solid–liquid interaction reduces the overall Gibbs free energy.

Page 49

Young’s Equation and Wetting

• At the three-phase line (solid–liquid–vapour), interfacial tensions satisfy Young’s equation:

• 

• This relation explains how contact angle θ depends on interfacial tensions and substrate polarity.

• 

Page 50

Wetting and Metal Nanoparticles on Oxide Supports

• Contact angle and wetting behavior depend on substrate polarity and surface energy.

• For metals on oxides, similar wetting concepts apply to nanoparticle dispersion and surface coverage.

• Liquids with low surface tensions wet solids with high surface energies.

  For a given liquid, the contact angle increases with decreasing substrate

  polarity

• Teflon is used for nonstick frying pan because it is a non-polar surface so any liquid will bead to minimize its interactions with the Teflon

• 

Page 51

Metal Nanoparticle Catalysts – Size Effects

• Nanoparticles exhibit unusual electronic/physical properties due to a high fraction of surface atoms.

• Melting point depression: dcluster ≈ 0.0342n (n = number of atoms; dcluster in nm) and bulk Au mp ≈ 1064 °C.

  • melting point decreases at smaller particle diameters

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Au/TiO2 Catalysts and Plasmonic Properties

• Au nanoparticles display surface plasmon resonance, leading to vibrant colours (purple/blue/red) depending on particle size.

• Example: 1.5 wt% Au on TiO2 calcined at various temperatures (400, 600, 800 °C).

• Colours come from localized surface plasmon resonance

  • as particle size gets bigger, the resonance moves to longer wavelengths resulting in more green colours coming into absorption

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Au Nanoparticle Shapes and Optical Properties

• Transmission electron microscopy (TEM) shows shapes: spheres, decahedra, rods, etc.

• Absorption spectra for different geometries: transverse plasmon resonance (TSPR) and longitudinal plasmon resonance (LSPR).

• Example: Au nanorod absorption features across visible to near-IR.

• nanoparticle shape is important

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Catalysis by Supported Nanoparticles: CO Oxidation (Example)

• CO(g) + O2(g) → CO2(g) as a fundamental elementary reaction on supported metal nanoparticles.

• Why use supported nanoparticles? High surface area, tunable facets, and enhanced activity/selectivity.

• reaction rate per metal atom is very dependent on the size of (au) nanoparticles

• more small nanoparticles = more periphery sites = more active sites for carbon monoxide oxidation to CO2

• 

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Gas Adsorption on Solid Surfaces – Process Overview

• Heterogeneous catalysis involves: diffusion of reactants toward surface, adsorption, surface diffusion to active sites, surface reactions, desorption of products, and diffusion of products away.

• 

Page 58

Ethylene Hydrogenation on Pt(111) – A Case Study

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Useful Definitions: Adsorption Concepts

• Adsorption: binding of gas-phase (or solution) molecules to a surface.

• Adsorbate binds to the surface vs. Adsorbent (substrate) which is the surface.

• Desorption: release of adsorbates.

• Physisorption vs. Chemisorption:

  • Physisorption: weak van der Waals interactions; intact adsorbate; non-activated.

  • Chemisorption: true chemical bond formation; may involve dissociation; strong interactions.

  • Differences in energy:

    • Physisorption: low energy changes, reversible process.

    • Chemisorption: significant energy changes, often irreversible.

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fractional Coverage and Sticking Probability

• 

• 

• Rate of adsorption depends on the ability of the surface to dissipate kinetic energy; energy dissipation is crucial for successful adsorption.

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• sticking probability can be defined as the likelihood that a gas molecule will adhere to a surface upon collision, which is influenced by factors such as surface roughness, temperature, and the chemical nature of both the adsorbate and the substrate.

• Observations: sticking probability decreases with coverage; plane orientation affects adsorption statistics (e.g., (110) often shows different behavior).

• 

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Physisorption vs. Chemisorption – Dissociative Adsorption

• Non-activated chemisorption vs. activated chemisorption (via activation barrier) is visualized in energy diagrams:

  

• Dissociative adsorption generally more complex than nondissociative due to multiple bond-forming events.

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Physisorption Characteristics

• Enthalpies: ΔHphys ≈ -20 to -40 kJ/mol; similar to condensation enthalpies.

• Physisorption is non-activated and non-dissociative (does not break bonds).

• Examples: H2 dissociation energies (Hdiss) for H2 ≈ 436 kJ/mol; N2 ≈ 945 kJ/mol; O2 ≈ 497 kJ/mol.

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Chemisorption Characteristics

• Enthalpies: ΔHchem typically > -100 kJ/mol; strong enough to break some bonds and allow dissociation on surfaces.

• Chemisorption often involves bond formation with substrate and potential fragmentation of adsorbates.

• This energy is usually sufficient to break chemical bonds, and thus a chemisorbed molecule may dissociate into (reactive) molecular fragments. In this way surfaces catalyse reactions

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Enthalpies of Chemisorption and Catalytic Activity – C2H4 Hydrogenation (Volcano Plot Concept)

• Example: Ethylene hydrogenation over various transition metals, with Rh showing high activity.

• Concept: Volcano curve arises because optimal catalysts must adsorb reactants strongly enough to activate them but not so strongly that they are immobilized.

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Adsorption Isotherms – Background (θ vs p)

• For physisorption: θ = Vads / Vmon, depends on pressure p.

  • Isotherm describes how θ varies with p at fixed T.

  • 

• Kinetic information can be extracted from adsorption/desorption as a function of pressure.

• The surface coverage (θ) depends strongly on the pressure (p) of the adsorbing gas. Increasing p will increase θ and vice versa

• The variation in θ with pressure p at a fixed temperature is called an

  adsorption isotherm.

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Measurement Methods for θ

• Methods include:

  1. Flow methods (change in uptake rate dθ/dt from gas flow differences).

  2. Flash desorption (heat sample and monitor pressure rise).

  3. Gravimetric (weigh before/after exposure).

  4. Radioactive tracers (isotopically labelled gases).

• Isotherms: Langmuir (Non-dissociative physisorption and dissociative

  chemisorption) and Brunauer, Emmett & Teller (BET) (Multilayer Physisorption) isotherm

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Langmuir Adsorption Isotherm – Non-dissociative Physisorption

• 

• R is reactant

  M is an unoccupied adsorption site

  RM is an occupied adsorption site

  ka is the rate constant for adsorption

  kd is the rate constant for desorption

• Assumptions:

  • Monolayer coverage only.

  • All adsorption sites equivalent.

  • Adsorption at one site independent of neighboring sites.

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Langmuir Isotherm (Mathematical Details)

• With these approximations, the rate of change of θ with respect to time (i.e. dθ/dt) will depend on the relative rates of adsorption and desorption

• 

• 

Page 72

Langmuir Isotherm – Linear Plot for Parameter Determination

• increase pressure should mean increase adsorption until eventually we reach monolayer coverage

• 

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Dissociative Langmuir Isotherm – Derivation Outline

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Comparison: Non-Dissociative vs. Dissociative Adsorption

• Key takeaway: Dissociative adsorption shows weaker pressure dependence than nondissociative adsorption, reflected in the different mathematical forms of the isotherms.

• 

Page 79

Surface Areas from Gas Adsorption Isotherms

• Specific surface area As can be calculated from BET/adsorption data.

• General formula:

• 

  • n = number of moles of adsorbate in a monolayer

  • N_A = Avogadro's number

  • σ = cross-sectional area of the adsorbate molecule (e.g., N2 ≈ 0.162 nm^2)

  • m = sample mass (g)

• This provides a practical route to determine surface area from gas adsorption experiments.

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BET Adsorption Isotherm – Multilayer Physisorption

• If the first adsorbed layer acts as a substrate for additional adsorption, multilayer adsorption occurs.

• BET theory extends Langmuir to multilayer adsorption with the following assumptions:

  • The same adsorption assumptions as Langmuir but allows multilayer adsorption.

  • The second layer adsorbs only on the first layer, the third on the second, etc.; when p = p*, infinite layers form.

  • The heat of adsorption for additional layers equals the latent heat of condensation.

  • At equilibrium, condensation and evaporation rates are equal for each layer.

• p* represents the vapor pressure above the multilayer and is analogous to bulk liquid vapor pressure.

• water activity = %relative humidity/100

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BET Equation Form and Linearization

• In this expression, p* is the vapour pressure above the multilayer of adsorbate and which resembles a pure bulk liquid, and c is a constant.

• 

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BET Data Analysis (Continued)

• Relationships:

  • slope = (c − 1)/ (c V_mon)

  • intercept = 1 / V_mon

• Calculation yields c ≈ 302.5 and V_mon ≈ 815 cm^3 (at STP) for the TiO2 sample.

• Resulting surface area estimation from V_mon and molecular area (0.162 nm^2 for N2) leads to a specific surface area around 356.4 m^2 g^-1.

• z is partial pressure, c is constant

• inert gas as adsorbing gas

  

Practical Use of BET in Industry

• The BET method is widely used by surface-area analyzers.

• Procedure: Room-temperature gas is not used; typical analysis uses cryogenic adsorption (77 K) with nitrogen as adsorbate.

• Steps: measure monolayer volume, convert to surface area per gram of sample.

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Adsorption Isotherm Classification (I–VI)

• Type I: Characteristic of chemisorption isotherms or physisorption in materials with micropores (e.g., charcoal).

• Type II: Macroporous materials with strong adsorbate–adsorbent interactions.

• Type III: Macroporous materials with weak interactions.

• Type IV: Hysteresis due to capillary condensation; multilayer adsorption with mesopores. Staged adsorption (first monolayer then build-up of additional layers) with capillary condensation

• Type V: Multilayer adsorption with capillary condensation (different pattern than IV).

• Type VI: Isotherm with more than one step; multiple adsorption events.

• Porosity classifications:

  • Microporous: pore diameter < 2 nm

  • Mesoporous: 2–50 nm

  • Macroporous: > 50 nm

• hysteresis is when adsorption and deabsorption rates aren’t the same due to the bottleneck/carpark effect - easy to fill a pore, harder to empty, need higher pressure

• 

Page 89

Adsorption Isotherms and Pore Size

• In pores, adsorption and condensation can occur at pressures below bulk condensation due to capillary effects.

Page 90

MOFs – Metal–Organic Frameworks

• MOFs are crystalline porous materials with metal nodes linked by organic connectors.

• They feature well-defined microporosity or mesoporosity depending on linker size.

• High surface areas; used for gas storage and selective separations.

• Example MOFs: Mg-MOF-74 (surface area ≈ 1500 m^2/g; 1D hex channels; pore width 1.1–1.2 nm).

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Isosteric Enthalpy of Adsorption (ΔH_ads)

• Relationship: ΔGads = −RT ln K; ΔGads = ΔHads − T ΔSads.

• Isosteric enthalpy can be determined via temperature dependence of adsorption: use van’t Hoff-like relations.

• For Krypton on a solid (Langmuir-type example): ΔH_ads can be extracted by plotting ln K vs 1/T at fixed θ.

• adsorption is usually exothermic

• 

• 

Page 95

Temperature Dependence of K (Van’t Hoff-like Analysis)

• From Langmuir form Kp/(Kp + 1) = θ/(1−θ), taking ln on both sides and differentiating with respect to T yields a route to ΔH_ads.

• The slope of ln K vs 1/T gives ΔH_ads/R values at fixed θ.

• 

Page 96

Practical Isosteric Enthalpy Analysis

• the amount of gas adsorbed at a particular pressure decreases with temperature

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Adsorption: Applications and Importance

• Adsorption as a purification method: selective adsorption to separate components from mixtures using porous solids (activated carbon, silica gel, activated alumina, zeolites, molecular sieves).

• Regeneration strategies: temperature swing, pressure swing, inert/purge stripping.

• Adsorption as a purification method. A gaseous component can be

  separated from a mixture if it selectively adsorbs onto a porous solid surface, such as:

  • activated carbon (adsorbs organics)

  • silica gel (adsorbs moisture)

  • activated alumina (adsorbs moisture)

  • zeolites and molecular sieves

  • synthetic resins

  When a bed nears saturation, the flow is stopped and the bed is regenerated to cause desorption. The adsorbate can thus be recovered and the adsorbent reused. Regeneration can be accomplished in several ways,

  • temperature swing

  • pressure swing

  • inert/purge stripping

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Adsorption: Applications and Importance (Chemisorption)

• Chemisorption can be used to measure surface area and metal dispersion: a quantitative chemisorption measurement gives available metal surface area and dispersion if the stoichiometry is known (e.g., 2 Pt + H2 → 2 Pt–H).

• 

• only the metal will react which means we can tell if the pores have low or high dispersion

Page 99

Adsorption: Applications – Reaction Microkinetics

• Visualization of catalytic steps: adsorption/desorption are physical steps; chemical steps include adsorption/activation and surface reactions that constitute microkinetics.

• The macro-kinetics of a reaction depend on transport, adsorption, surface reactions, and diffusion steps.

• 

Page 100

Adsorption and Catalysis – Mechanistic Views

• Heterogeneous catalysis commonly involves at least one chemisorbed reactant undergoing modification to a reactive form, followed by diffusion and surface reaction steps.

• Three major mechanisms:

  • Langmuir–Hinshelwood (LH)

  • Eley–Rideal (ER)

  • Mars–van Krevelen (MvK)

Page 101

Transition State Model of Catalyst Activity

• Concept: energy profile along reaction coordinate with distinct contributions:

  • ΔE_hom: energy of the homogeneous (?) transition state

  • ΔE_ads: adsorption energy of reactants on surface

  • ΔE_des: desorption energy of products

  • ΔE_het: energy barrier for surface reaction (RDS)

• In LH kinetics, adsorption/desorption are fast; surface reaction is rate-determining (RDS) with ν = k θ_A or a combination depending on steps.

• 

Page 102

Langmuir–Hinshelwood Mechanism – Unimolecular (A(g) → B)

• Adsorption of A on the surface precedes conversion to product B (A(ads) → B).

• Reaction occurs uniformly across the surface; products desorb weakly.

• Rate expression under Langmuir adsorption: ν = k θA with pA as the gas-phase pressure and θ_A as fraction of sites occupied by A.

• General LH rate: ν = (k pA K)/(1 + K pA) for a simple case (illustrative form).

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LH in Limiting Cases (Unimolecular Reactions)

• Low pressure / weak binding (Kp ≪ 1):

  • Rate ≈ k Kp pA; surface coverage is low; first-order in p_A.

• High pressure / strong binding (Kp ≫ 1):

  • Rate ≈ k; surface coverage ~ 1; rate becomes independent of pA (zero-order in pA).

• 

Page 104

LH Mechanism – Bimolecular (A(ads) + B(ads) → Product)

• Reaction occurs between two adsorbed species on adjacent sites; LH bimolecular step.

• When expressed in terms of partial pressures with Langmuir isotherms for adsorption, the rate can be written as a function of pA and pB using adsorption constants.

• 

Page 105

LH vs ER – Rate Expressions and Site Coverages

• LH bimolecular case: ν ∝ θA θB, often leading to a second-order dependence on surface coverages.

• If A and B compete for the same sites, expressions include Kp terms and θ dependences; the rate can be rearranged into a function of partial pressures using Langmuir isotherms.

• If adsorption is non-dissociative, the Langmuir isotherm for each species can be used to derive rate laws.

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LH Mechanism – Examples (CO + O2 on Metals)

• Example: CO(g) + 1/2 O2(g) → CO2(g) on metal surfaces; a common LH example where CO adsorbs molecularly and O2 adsorbs dissociatively; rate depends on θCO and θO via isotherms.

• Adsorption equilibria: CO and O2 compete for sites; expressions combine K_p terms and θs to yield rate laws.

• 

Page 107

Eley–Rideal Mechanism

• Gas-phase molecule A(g) reacts with adsorbed species A(ads) or B(ads) on surface.

• Rate: ν = k θA pB (for A adsorbing and reacting with B in surface).

• In the ER model, adsorption of A may be small, but the reaction occurs via direct collision with adsorbed species.

• If adsorption of A is significant and follows Langmuir adsorption, rate expressions can reduce to first-order in p_B under certain limits.

• molecule a is adsorbed and molecule b is a gas

• collisional reaction

  

• 

Page 108

ER – Quantitative Forms and Limiting Cases

• When the partial pressure of A is high (KApA >> 1), there is almost complete surface coverage, and thus ν = kpB (overall kinetics first order).

  When the partial pressure of A is very low (KApA << 1), now the extent of surface coverage of A is important in determining the reaction rate. ν = kK ApApB

Page 109

Diagnosis of Mechanisms via Coverage Dependence

• Diagnostic behavior of rates vs. surface coverage:

  • Eley–Rideal: rate increases with A coverage, until saturation.

  • Langmuir–Hinshelwood: rate may show a maximum and then decrease as A occupancy blocks sites; rate can go to zero if the surface becomes fully covered by A.

• Practical implication: experimental plotting of rate against θ_A helps distinguish LH from ER.

• 

Page 110

Mars–van Krevelen Mechanism (MvK)

• Common in oxidations on metal oxides (e.g., CO oxidation).

• Mechanism: CO reacts with lattice oxygen to form CO2; oxide lattice becomes reduced (oxygen vacancies) and must be replenished by O2 activation in a subsequent step, often at defect sites.

• Schematic: CO + O_(lattice) → CO2 + vacancy; O2 activation regenerates lattice oxygen.

• Emphasizes lattice oxygen mobility and oxide redox chemistry in catalysis.

• 

Page 112

Catalytic Reaction Processes and Reactor Types

• Catalytic reactions can be run in different modes:

  • Batch reactors: reactants and catalysts combined; conditions controlled to reach desired conversion.

  • Continuous reactors: steady-state operation; feed rates determine conversion and throughput.

• Considerations include temperature, pressure, mass/heat transfer, catalyst loading, and reactor design.

Page 113

Selecting the Best Catalyst – Key Metrics

• Activity: molar convert rate per unit time; good dispersion and high surface area are desirable.

• Conversion: fraction of reactant transformed per reactor pass.

  • 

• Selectivity: moles of desired product per mole of reactant converted.

  • 

• Yield: conversion × selectivity.

  • 

• Lifetime: time before deactivation; resistance to poisons, thermal stability, hydrolysis, attrition and impurities.

  • 

• Cost: production and deployment costs; catalyst cost is a fraction of total production cost.

Page 115

Catalyst Preparation – Key Considerations

• Desired catalyst properties:

  • High and stable activity and selectivity.

  • Controlled surface area and porosity.

  • Resistance to poisons and temperature variations; mechanical strength; safe handling.

• Manufacturing decisions:

  • Supported vs. unsupported catalysts.

  • Shape of pellets (cylinders, rings, spheres, monoliths) affects void fraction, diffusion, flow, and mechanical strength.

  • Pellet size influences diffusion; smaller pellets improve mass transfer.

  • Oxide-based catalysts are often activated by reduction in H2 within the reactor.

Page 116

Catalyst Composition and Roles

• Active phase: where reaction occurs (often metal or metal oxide).

• Promoter: textural (e.g., Al for NH3 production) or electronic/structural modifiers; prevents poisoning.

• Support/carrier: adds mechanical strength and increases surface area; may or may not be catalytically active.

Page 119

Studying Adsorption and Reaction on Solid Surfaces – Surface Sensitivity

• Surfaces are studied with surface-sensitive techniques to focus on near-surface atoms.

• Bulk techniques average over entire sample; surface techniques emphasize surface layers.

• A key criterion for surface studies: IA/IB ≫ 10^-5 (surface signal is dominant).

• a surface sensitive technique is more sensitive to those atoms

  which are located near the surface than it is to atoms in the bulk

Page 120

Clean Surfaces and Vacuum Conditions

• Surface studies require clean surfaces, typically under ultra-high vacuum (UHV) to avoid contamination and interference.

• Surface cleanliness time depends on gas flux, target monolayer coverage, and sticking probability;

• Practical: at high flux and ambient conditions, surfaces contaminate quickly; UHV is essential for long-duration measurements.

• 

Page 122

Surface Analytical Techniques Overview

• Techniques listed: Temperature Programmed Desorption (TPD), HREELS, RAIRS, STM, and XPS.

• These methods provide complementary information about adsorption, vibrational modes, surface structures, and composition.

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Temperature Programmed Desorption (TPD)

• Experimental setup:

  • Adsorbates are placed on the surface at low temperature (often ~300 K).

  • Surface is heated with a controlled (often linear) ramp while monitoring gas phase species via mass spectrometer.

• Data interpretation: peak area ∝ initial surface coverage; peak temperature relates to adsorption enthalpy.

• Example mechanism for formic acid on Cu surface shown with multiple desorption peaks indicating intermediates.

• 

Page 124

Activated Dissociation of O2 on Pt(111) – TPD Interpretation

• The area under a peak is proportional to the amount originally adsorbed, i.e. proportional to the surface coverage.

• The position of the peak (the peak temperature) is related to the enthalpy of adsorption, i.e. to the strength of binding to the surface

Page 126

RAIRS – Reflection Absorption Infrared Spectroscopy

• RAIRS uses specular reflection from a metal surface to study adsorbates; grazing incidence enhances sensitivity.

• Surface dipole selection rule: only vibrational modes producing a dipole perpendicular to the surface are IR-active.

• 

Page 128

CO Adsorption on Metals – RAIRS Observations

• CO on metal surfaces displays strong C–O stretching vibrations; frequencies depend on adsorption site:

  • Terminal CO: ~2090 cm^-1

  • Bridging CO: ~1870 cm^-1 (or lower depending on site)

• Backbonding from metal d-orbitals into CO π* orbitals weakens C≡O bond, lowering stretch frequency relative to gas-phase CO (2143 cm^-1).

Page 129

CO Adsorption on Surfaces – Mode Assignments (Illustrative Fig)

• Six eigenmodes of CO on a bridging site (C2 symmetry) shown; only CO stretch is a genuine gas-phase mode.

• Other modes are surface hindered translations/rotations; frequencies are guide values.

• three vibrational modes and three rotational modes. for vibrational modes of non-vibrational, its 3N-6. for linear its 3N-5. can’t do this on a surface, because of the hindered vibrational modes so it’s only 3N

• 

Page 134

High Resolution Electron Energy Loss Spectroscopy (HREELS)

• HREELS uses inelastic scattering of low-energy electrons to measure vibrational spectra on surfaces under UHV.

• The energy loss ΔE corresponds to vibrational quanta of adsorbates.

• 

• On striking the surface, electrons may be elastically scattered (E = Eo), or undergo discrete energy losses ΔE = (Eo - E), equal to the vibrational quantum (i.e. the energy) of the vibrational modes of the adsorbate excited in the inelastic scattering process

• in off-specular geometry, all vibrational modes are excited

• ONLY MODES WHICH HAVE A DIPOLE MOMENT CHANGE PERPENDICULAR TO THE SURFACE WITHH BE EXCITED

Page 135

HREELS Selection Rules

• Surface selection rules: For metallic substrates and a specular geometry, only those vibrations giving rise to a dipole change normal to the surface can be observed.

• By contrast, in an off-specular geometry, loss features are relatively weak but all vibrations are allowed and may be observed

Page 139

Scanning Tunneling Microscopy (STM) – Basics

• STM allows imaging of surfaces at atomic scale, visually resolving individual atoms.

• Principle: tunneling current between a sharp tip and surface, modulated by tip–surface distance.

• Tip bias controls electron flow direction; positive tip bias leads to electrons tunneling from tip to sample in the described setup.

• image conductive surface and see what happens when we adsorb things into a surface

• STM images not only display the geometric structure of the surface, but also depend on the electronic density of states of the sample

• hydrogen doesnt show up in tunneling so shows up as a black ‘hole’

  

Page 140

STM Imaging Principles

• To image, the tip scans, maintaining a constant tunneling current by adjusting height (z) as a function of lateral position, plotting height vs. lateral position to create an image.

• The STM tip is mounted on a piezoelectric tube; minute movements are produced by voltage changes.

• STM images reflect both geometric arrangement and electronic density of states.

Page 142

Dissociative O2 Adsorption on Al(111) – STM View

• On Al(111), O atoms appear as dark spots due to lack of tunneling current.

• Observations show O atoms migrate across the surface after dissociation to reduce energy via diffusion and clustering

Page 145

STM Visualization of Molecules on Surfaces

• STM can image adsorbed molecules like benzene and pentacene on graphene; even visualize vacancies in monolayers.

• Demonstrates the precision of STM in identifying surface intermediates in reactions.

Page 146

Naphthalocyanine on Graphite – STM Image & Analysis

• Constant-current STM images reveal monolayer order, vacancies, and molecular orientation.

• Inset shows molecular structure; second panel demonstrates azimuthal orientation analysis.

Page 147

X-ray Photoelectron Spectroscopy (XPS) – Basics

• XPS uses photoelectric effect with X-ray photons (e.g., Mg Kα, Al Kα) to eject core/valence electrons.

• Binding energies (BE) provide chemical state information and elemental composition.

• Kinetic energy relation: KE = h
  u - BE - ext{work function}

Page 148

XPS – Instrumentation and Surface Sensitivity

• XPS requires UHV to avoid contamination, arcing, and to maximize electron mean free path.

• The surface-sensitive measurement probes only near-surface region (surface depth typically ~3 IMFPs).

Page 149

XPS Spectrometer - Photoemission Core Concepts

• Core-level electrons vs. valence electrons; photoemission kinetic energies reflect BE and chemical state.

• Auger emission can accompany photoemission; X-ray fluorescence is another relaxation pathway.

• electrons with low kinetic energies will hit the left side because they were pulled in by the inner positive sphere. if they have high kinetic energy, they hit the right sound and follow the path closer to the outer negative sphere

Page 150

Fate of the Core: Auger vs. X-ray Emission

• Core-hole relaxation can occur via Auger electron emission or characteristic X-ray emission (Kα, Kβ lines).

• electrons from higher levels will drop down to fill in the removed electron. an auger electron

Page 151

Bulk Techniques: XRF (X-ray Fluorescence)

• XRF detects characteristic X-rays emitted by elements; bulk-sensitive technique complementary to surface-sensitive XPS.

Page 152

BE Shifts and Relaxation Processes

• Core-level BE shifts depend on chemical environment and oxidation state; reference standards are needed for interpretation.

Page 153

XPS and Surface Sensitivity (Depth Profiling Concept)

• Inelastic mean free path (IMFP) λ defines information depth; typical XPS depth ≈ 3λ.

• The universal curve provides IMFP vs. kinetic energy for various materials.

• Distance electron can travel in solid depends on (i) material and (ii) electron KE Loss processes (inelastic scattering) reduce KE and can prevent the photoelectron (and Auger electrons) escaping from surface:

Page 154

IMFP Calculations

• Example: At KE = 100 eV, IMFP ≈ 0.554 nm; information depth ≈ 1.66 nm.

• At KE = 1000 eV, IMFP ≈ 1.708 nm; information depth ≈ 5.12 nm.

• 

Page 156

Spin–Orbit Splitting (SOC)

• SOC in core-levels leads to doublets: e.g., 2p, 3d, 4f core-levels split into multiple BE peaks.

• These features help identify elemental oxidation states and chemical environments.

• degeneracy: 2j+1

• 

Page 158

XPS – Quantitative Analysis

• Relative concentrations can be determined via peak intensities and sensitivity factors S:
  C = rac{I / S}{\u00a0 extstyle \, ext{sum over all elements}}

• Output is often given as atomic percent (at%).

Page 159

Relative Sensitivity and Quantification (Continued)

• A chart lists relative sensitivities of elements for XPS, guiding quantitative interpretation.

Page 161

XPS – Case Study: Ni–Cu Alloy (Quantification Example)

• Demonstrates how to read peak areas and apply sensitivity factors to deduce composition.

Page 163

Origins of Binding Energy Shifts – Conceptual View

• Binding energy depends on electron–nucleus attraction, electron–electron repulsion, screening, and Fermi level alignment.

• Shifts can indicate both elemental identity and oxidation state changes.\

Page 170

Hydrogen Economy – Ethanol Steam Reforming (Catalyst Systems)

• Ethanol steam reforming (ESR) to H2 and CO2; catalysts include Rh–Pd on CeO2 (bimetallic, interface sites).

• General reaction: CH3CH2OH + 3 H2O → 6 H2 + 2 CO2; ΔH° ≈ +173 kJ/mol.

• Insights: Rh–Pd/CeO2 catalysts show high activity with low metal loading; active sites often at the Rh/Pd–Ce interface; catalyst activation occurs during reaction.

Page 171

Temperature-Dependent Ethanol Reforming (Experimental Data)

• Temperature–volume percent data for H2, CH4, CO2, CO, etc., under different flow/regimes.

• Regions: I (dehydrogenation to acetaldehyde and H2), II (methanation path), III (full reforming to CO2 and H2).

• Observed selectivity and conversion dependent on temperature and catalyst loading.

Page 172

XPS in Ethanol Reforming Studies

• XPS data for Rh/Pd on CeO2 after reforming_cycle shows oxidation state changes and possible alloy formation during activation.

• Near-surface chemical states monitored during reaction.

Page 173

Angle-Resolved XPS (ARXPS)

• Method: Tilt sample relative to analyzer to enhance surface sensitivity (θ = 15° increases surface sensitivity by approximately 4×).

• Trade-off: higher surface sensitivity reduces signal, needing longer acquisition times.

• 

Page 178

XPS – Summary: Pros and Cons

• Advantages: non-destructive, quantitative elemental composition, chemical state information, shallow information depth (~20–100 Å), sensitivity ~0.1% ML.

• Disadvantages: expensive instrumentation (> ~$1,000,000), requires UHV, surface charging issues for insulators, cannot detect H and He.

Page 179

Case Study – Fischer–Tropsch Syntheses (FTs)

• FT reactions: syngas (CO + H2) → hydrocarbons (CnH2n+2) and H2O using metal catalysts at 150–300 °C and 1–20 atm.

• General reaction: (2n+1) H2 + n CO ightarrow Cn H{2n+2} + n H2O

Page 180

ASF Distribution in Fischer–Tropsch Chemistry

• Product distribution follows Anderson–Schulz–Flory (ASF):
  Wn = n(1- au)^2 au^{n-1} ext{ or } \, ext{log}(Wn/n) = n \, ext{log} au + ext{log}[(1- au)^2 au]

• τ is the chain-growth probability; aiming for τ ≈ 0.7–0.8 to favor liquid (C10–C20) fuels.

Page 181

Photothermal Catalysis – Concept Diagram

• Driving forces include photochemical processes (LSPR) and thermal activation (photothermal effects) with energy transfer pathways: direct energy transfer (DET), resonant energy transfer (RET), chemical interface damping (CID).

• Shows integration of photonics and catalysis (PTC) including metal–oxide interfaces (Rh/Al2O3, etc.).

Page 182

Photothermal Catalysis: Case Studies

• Photothermal CO hydrogenation to C5+ liquids with Ru1Co single-atom alloy (SAA) on Al2O3 via UV illumination under ambient pressures.

• Demonstrates light-assisted catalysis enabling high selectivity to C5+ fuels under mild thermal conditions.

Page 184

Catalyst Synthesis and Characterization – Spectroscopic Data

• UV–Vis diffused reflectance spectra show strong light absorption in UV/visible, correlating with photocatalytic activity.

Page 185

Photothermal Catalysis – Heating Profiles (Experimental)

• Representative heating profiles under UV–Vis irradiation; demonstrates how light exposure translates to catalyst heating and catalytic activity.

Page 186

Photothermal Catalysis – Selectivity and Temperature Profiles

• Ru1Co-SAA demonstrates outstanding selectivity to C5+ liquids under solar irradiation at ~200 °C and low pressures.

• Reaction: (2n + 1) H2 + n CO ightarrow Cn H{2n+2} + n H2O

Page 187

Product Analysis – Gas/ Liquid Phase Products

• Gas-phase GC shows distribution of hydrocarbons; liquid products extracted in cyclohexane show C8–C16 hydrocarbons, consistent with jet-fuel-range products.

Page 188

Module Summary

• Reiterates core topics covered in the module:

  • Crystal and electronic surface structures.

  • Adsorption/desorption and Langmuir/BET isotherms.

  • Surface reaction models (LH, ER, MvK).

  • Structure–reactivity relationships and catalyst fabrication/testing.

  • Modern surface science techniques (TPD, RAIRS, HREELS, STM, XPS) for catalyst characterization, mechanism elucidation, and working-catalyst assessment.

• Case studies highlighted to connect theory to industrial relevance and environmental challenges.

• Closing thanks and acknowledgement of participation.

Key Formulas (summary):

• Langmuir Isotherm (non-dissociative):
  heta = rac{K p}{1 + K p}, \ V{ads} = V{mon} heta = V{mon} rac{K p}{1 + K p} Linear form: plotting rac{1}{V{ads}} ext{ vs } rac{1}{p} yields slope = rac{1}{K V{mon}} and intercept = rac{1}{V{mon}}.

• Dissociative Langmuir Isotherm (qualitative form): heta ext{ vs } p ext{ follows a weaker } p ext{ dependence than nondissociative case; often involves } heta = rac{ ext{something like } rac{K p}{1 + ext{something}}}{ ext{denominator}} ext{ with sqrt(p) behavior in simple cases.}

• BET Isotherm (multilayer physisorption) – standard forms and linearization to obtain V_mon and c (via slope/intercept in the BET plot):

  • Basic form and rearranged linear form (as shown in slides).

• Michaelis–Menten kinetics:
  V = rac{V{max} [S]}{KM + [S]}, \ KM = ext{substrate concentration at } rac{1}{2} V{max}, \ k{cat} = rac{V{max}}{[E]{tot}}, \ ext{Catalytic efficiency} = rac{k{cat}}{K_M}

• Isosteric enthalpy of adsorption (ΔHads) from van’t Hoff analysis: rac{d \, ext{ln} K}{d(1/T)} = - rac{ ext{Δ}H{ads}}{R}

• Surface area from BET:
  As = rac{n NA \, ar{ ext{σ}}}{m}
  where n = moles adsorbed in monolayer, N_A = Avogadro’s number, σ = cross-sectional area of adsorbate, m = sample mass.

If you’d like, I can export these notes as a PDF or provide page-by-page flashcards for rapid review. You can also tell me if you want me to emphasise specific sections (e.g., LH vs ER mechanisms, XPS interpretation, or BET analysis) for exam-prep.