Surface Chemistry, Kinetics & Catalysis Notes

Surface Chemistry, Kinetics & Catalysis

• Reference books: Atkins' Physical Chemistry & Essentials of Physical Chemistry by Arun Bahl
• UNIT 3: Surface Chemistry, Kinetics & Catalysis
  • Adsorption - Characteristics, Classification, Application
  • Adsorption isotherms - Freundlich, Langmuir & BET
  • Chemical Kinetics - Rate law, Arrhenius equation, Transition state theory, Collision theory; Complex reactions
  • Catalysis - Homogeneous and Heterogeneous Catalysis; Mechanism of Catalysis; Industrial Applications of catalysts

Adsorption

• Definition: Adsorption is the process where a substance (adsorbate) accumulates on the surface of a solid (adsorbent).
  • The adsorbate can be in a gas or liquid phase.
  • Adsorption refers to a higher concentration of a component at the surface of a liquid or solid phase.
  • Example: Charcoal in gas masks removing poisons from the air.

Adsorbent and Adsorbate

• Adsorbent: Solids used to adsorb gases or dissolved substances.
  • Examples: charcoal, alumina, silica gel.
• Adsorbate: The adsorbed molecules.
  • Examples: gases like oxygen, nitrogen, etc.

Adsorption vs. Absorption

• Adsorption: A surface phenomenon.
• Absorption: A bulk phenomenon.
• The term "adsorption" was coined in 1881 by Heinrich Kayser.

Surfaces for Adsorption

• Surfaces have different features like terraces, steps, kinks, and adatoms, which influence adsorption.
• Defects on terraces play a role in surface growth and catalysis.
• Activated charcoal can have varying oil adsorption capacities.

Characteristics of Adsorption

• Adsorption decreases the free energy change of the system, establishing equilibrium.
• High surface area leads to more adsorption.
• Adsorption is exothermic ($\Delta H$ is negative), meaning heat is released.
• Adsorbate molecules' movement is restricted, decreasing entropy.
• Adsorption results from surface energy; surface atoms attract adsorbates because they are not fully surrounded by other atoms.

Factors Affecting Adsorption

• Nature of adsorbate and adsorbent:
  • Greater surface area of the adsorbent leads to greater gas adsorption.
  • Different gases are adsorbed differently by the same adsorbent at the same temperature.
• Activation of adsorbent:
  • Increases surface area by making the surface rough or subdividing the adsorbent into smaller grains.
• Experimental conditions:
  • Temperature: Adsorption generally decreases with increasing temperature.
  • Pressure: At constant temperature, adsorption increases with increasing pressure.

Classification of Adsorption

• Physical Adsorption (Physisorption)
  • Involves Van der Waals forces between adsorbate and adsorbent.
  • Weak attraction allows easy reversal by heating or decreasing pressure.
• Chemical Adsorption (Chemisorption)
  • Involves chemical bond forces (comparable to covalent bonds) between adsorbate and adsorbent.
  • Also known as Langmuir adsorption.
  • Strong attraction makes reversal difficult.

Physisorption vs. Chemisorption

• Physisorption
  • Forces of attraction: Van der Waals forces.
  • Low heat of adsorption: 20-40 kJ/mol.
  • Occurs at low temperatures and decreases with increasing temperature.
  • Reversible.
  • Related to the ease of liquefaction of the gas.
  • Not very specific.
  • Does not require activation energy.
• Chemisorption
  • Forces of attraction: Chemical bond forces (usually covalent bonds).
  • High heat of adsorption: 40-400 kJ/mol.
  • Occurs at high temperatures.
  • Irreversible.
  • The extent of adsorption is generally not related to liquefaction of the gas.
  • Highly specific.
  • Requires activation energy.

Applications of Adsorption

• Silica gel packets:
  • Used to keep moisture out of products by adsorbing moisture vapors.
• Pollution Masks:
  • Consist of fabric layers with activated carbon granules or filter sheets to adsorb dust and smoke particles.
• Curing Diseases:
  • Disease-causing germs get deposited on the surface of drugs and are ejected from the body.
• Charcoal Gas Masks:
  • Used in mining to filter out toxic gases.
• Purification of Water:
  • Alum is used to combine impurities, which can then be removed.
• Removing Hardness from Water:
  • Ion exchange resins remove calcium and magnesium.
• Misty Windows:
  • Water vapor deposits on windows, showcasing adsorption.
• Decoloring of Matter:
  • Fuller’s earth or charcoal solutions remove impurities causing color change.
• Heterogeneous Catalysis:
  • Reactants adsorb onto the catalyst surface, react, and products desorb.
• Other Applications:
  • Metallurgy: Froth floatation process for ore concentration.
  • Chromatography: Separating pigments.
  • Virology: Viruses adsorb onto hosts to colonize and cause disease.
  • Polymer science: Non-stick coatings and biomedical devices.
  • Polyelectrolytes adsorption: Oil recovery, nutrition, concrete.

Adsorption Isotherms

• The relationship between the amount of adsorbate on the adsorbent and its pressure or concentration at constant temperature.
• Free and adsorbed gas are in dynamic equilibrium, and fractional coverage ($\theta$) depends on the pressure of the gas.
• Types of adsorption isotherms:
  • Langmuir adsorption isotherm
  • Freundlich adsorption isotherm
  • BET adsorption isotherm

Langmuir Adsorption Isotherm

• Assumptions:
  1. Adsorption cannot proceed beyond monolayer coverage.
  2. All sites are equivalent, and the surface is uniform.
  3. The ability of a molecule to adsorb at a given site is independent of the occupation of neighboring sites.
• Dynamic Equilibrium Equation: $A(g) + M(surface) \rightleftharpoons AM(surface)$
  • $A(g)$ is gaseous adsorbate.
  • $M(surface)$ is solid adsorbent.
  • $AM(surface)$ is the adsorbed material.
  • $\kappa<em>a$ and $\kappa</em>d$ are rate constants for adsorption and desorption, respectively.
• Rate of change of surface coverage due to adsorption is proportional to the partial pressure $p$ of $A$ and the number of vacant sites $N(1 - \theta)$, where $N$ is the total number of sites.
• Rate of change of $\theta$ due to desorption is proportional to the number of adsorbed species, $N\theta$.
• At equilibrium, the rates of adsorption and desorption are equal; solving for $\theta$ gives the Langmuir isotherm.
• Rate of adsorption: $R<em>1 = K</em>1N(1-\theta)P$
• Rate of desorption: $R<em>2 = K</em>2N\theta$
• At equilibrium: $K<em>1(1-\theta)P = K</em>2\theta$
• Langmuir equation: $\frac{x}{m} = \frac{ap}{1+bp}$, where $\frac{x}{m}$ is the mass of adsorbate adsorbed per unit mass of adsorbent.
  Plotting $\frac{p}{x/m}$ vs $P$ gives a straight line, validating the Langmuir isotherm.

Limitations of Langmuir Adsorption Isotherm

1. Assumes monolayer adsorption, but many layers can adsorb in reality.
2. Assumes no interaction between adsorbed molecules.
3. Works well at low pressure but fails at high pressures.
4. The effect of temperature is not well considered.
5. Relation between heat of adsorption and surface area not explained.

• Langmuir adsorption is applicable for monolayer adsorption onto a homogeneous surface with no interaction between adsorbed species.
  • $\frac{x}{m}$ is the amount of adsorbate adsorbed per unit mass of adsorbent
  • $p$ is the pressure of the adsorbate gas
  • $a$ and $b$ are constants
  • $a = K_3b$
  • $b = \frac{K<em>1}{K</em>2} = \frac{K<em>a}{K</em>d}$

Freundlich Adsorption Isotherm

• Empirical relationship between the amount of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature.
• Equation: $\frac{x}{m} = k \cdot p^{\frac{1}{n}}$ (where n > 1)
  • $x$ is the mass of the gas adsorbed on mass $m$ of the adsorbent at pressure $P$.
  • $k$ and $n$ are constants dependent on the nature of the adsorbent and gas at a particular temperature.
• Plotting mass of gas adsorbed per gram of adsorbent against pressure shows the relationship.
• Taking the log of the equation: $\log \frac{x}{m} = \log k + \frac{1}{n} \log P$
• Validity test: Plotting $\log \frac{x}{m}$ (y-axis) against $\log P$ (x-axis) yields a straight line if the Freundlich isotherm is valid.
  • The slope of the line gives the value of $\frac{1}{n}$, and the intercept on the y-axis gives the value of $\log k$.

Limitations of Freundlich Adsorption Isotherm

• Based on the assumption that every adsorption site is equivalent.
• Applicable to physical adsorption.

BET Adsorption Isotherm

• Deals with multilayer adsorption.
• Derived by Stephen Brunauer, Paul Emmett, and Edward Teller.
• Salient features:
  1. Gas molecules physically adsorb on a solid in infinite layers.
  2. Gas molecules only interact with adjacent layers; Langmuir theory is applied to each layer.
  3. The enthalpy of adsorption for the first layer is constant and greater than the second (and higher).
  4. The enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction.
• BET isotherm plots the amount of gas adsorbed as a function of the relative pressure $\frac{P}{P_o}$.
• BET Equation uses the information from the isotherm to determine the surface area of the sample.
  • $X$ is the weight of nitrogen adsorbed at a given relative pressure $\frac{P}{P_o}$.
  • $X_m$ is monolayer capacity (volume of gas adsorbed at STP).
  • $C$ is constant.
• STP (Standard Temperature and Pressure) is defined as 273 K and 1 atm.

BET Isotherm Types

• Type I:
  • When $\frac{P}{P_o} < 1$ and $C > 1$.
  • Pseudo-Langmuir isotherm depicting monolayer adsorption.
  • Characterizes microporous materials (pore diameters < 2 nm).
  • The extent of adsorption increases with pressure until saturation.
• Type II:
  • When C > 1.
  • The formation of a bilayer occurs only after the monolayer has fully formed.
• Type III:
  • When C < 1.
  • The formation of monolayers, bilayers, and trilayers occurs simultaneously.
• Type IV:
  • Shows the formation of a monolayer, followed by multilayers.
  • Characterizes mesoporous materials (pore diameters between 2 - 50 nm).
• Type V:
  • Obtained when intermolecular attraction effects are large.
  • Adsorption takes place in pores and capillaries.
  • Similar to type IV isotherms.