Surface Chemistry (Adsorption And Catalysis)

Catalysis

Catalysis is the phenomenon of increasing the rate of a chemical reaction by the addition of a substance called a catalyst, which is not consumed in the overall reaction.

Characteristics of Catalysts:

• Specificity: A catalyst is usually specific for a particular reaction or a class of reactions.
• Small Amounts: Only a small amount of catalyst is usually required to affect a large amount of reactants.
• No Effect on Equilibrium: A catalyst increases the rate of both forward and reverse reactions equally, hence it does not alter the equilibrium position. It only helps in attaining the equilibrium faster.
• Activation Energy: A catalyst provides an alternative reaction pathway or mechanism by reducing the activation energy ($E_a$). It lowers the energy barrier between reactants and products.
• No Effect on Gibbs Free Energy: A catalyst does not change the overall change in Gibbs free energy ($\Delta G$) of the reaction.

Types of Catalysis:

• Homogeneous Catalysis: The catalyst is in the same phase as the reactants.
  • Example: $2SO_2(g) + O_2(g) \xrightarrow{NO(g)} 2SO_3(g)$ (NO is a gaseous catalyst, reactants are gases).
  • Example: Hydrolysis of ester in presence of acid ($H^+$).
• Heterogeneous Catalysis: The catalyst is in a different phase from the reactants. The most common case is a solid catalyst with gaseous or liquid reactants.
  • Example: $2SO_2(g) + O_2(g) \xrightarrow{Pt(s)} 2SO_3(g)$ (Pt is a solid catalyst, reactants are gases).
  • Example: Haber's process for ammonia synthesis ($N_2(g) + 3H_2(g) \xrightarrow{Fe(s)} 2NH_3(g)$).

Promoters and Poisons:

• Promoters: Substances that are added to the catalyst to increase its activity.
• Poisons: Substances that decrease or destroy the activity of a catalyst.

Adsorption (Applications)

Applications Of Adsorption:

Adsorption is a surface phenomenon with numerous applications in various fields:

1. Production of High Vacuum:

Activated charcoal adsorbs remaining gases from vessels evacuated by a vacuum pump, creating a very high vacuum.

2. Gas Masks:

Adsorbents like activated charcoal and silica gel are used in gas masks to adsorb poisonous gases (like $CO$, $Cl_2$, phosgene) from the air, making it safe to breathe.

3. Humidity Control:

Adsorbents like silica gel and activated alumina are used to adsorb moisture from air, thus controlling humidity, especially in storage of goods and electronic instruments.

4. Colour Removal from Solutions:

Certain adsorbents like animal charcoal (activated charcoal) are used to decolorize solutions. For example, sugar solutions are decolorize by passing them through a layer of animal charcoal.

5. Heterogeneous Catalysis:

The surfaces of solid catalysts adsorb reactant molecules, which then react to form products. The products then desorb from the surface. This is a major application of adsorption.

Examples: $Pt$ adsorbs $H_2$ and $O_2$ in the synthesis of $SO_3$; $Fe$ adsorbs $N_2$ and $H_2$ in Haber's process.

6. Froth Floatation Process:

In the froth flotation method for concentrating ores, collectors (like xanthates) are adsorbed onto the surface of the desired mineral particles, making them hydrophobic and allowing them to attach to air bubbles and float.

7. Chromatography:

Adsorption is the fundamental principle behind various chromatographic techniques, where separation of components is achieved based on their differential adsorption onto a stationary phase (adsorbent).

8. Medicinal Applications:

• Activated charcoal is used to treat poisoning or drug overdose by adsorbing the toxic substance in the stomach.
• Certain drugs adsorb bacteria or toxins from the gut.

9. Detergents:

Detergents work by adsorption. Their molecules have a hydrophobic tail and a hydrophilic head. They adsorb at the oil-water interface, reducing surface tension and helping to emulsify the dirt and oil.

10. Ion Exchange Resins:

Ion exchange processes rely on the adsorption of ions from a solution onto solid resin materials.

Catalysis

Homogeneous And Heterogeneous Catalysis

Homogeneous Catalysis:

• Definition: The catalyst is in the same phase as the reactants.
• Mechanism: The catalyst reacts with one of the reactants to form an intermediate, which then reacts with the other reactant to form the product and regenerate the catalyst.
• Examples:
  • Synthesis of Sulphur trioxide ($SO_3$) from Sulphur dioxide ($SO_2$) and oxygen ($O_2$) using Nitric oxide (NO) as catalyst: $2SO_2(g) + O_2(g) \xrightarrow{NO(g)} 2SO_3(g)$
  • Hydrolysis of ester by dilute acids.

Heterogeneous Catalysis:

• Definition: The catalyst is in a different phase from the reactants. Usually, a solid catalyst with gaseous or liquid reactants.
• Mechanism: Explained by the adsorption theory.
• Examples:
  • Contact process for $SO_3$ synthesis using $Pt$ or $V_2O_5$ (solid).
  • Haber's process for $NH_3$ synthesis using $Fe$ (solid).
  • Hydrogenation of oils using $Ni$ or $Pt$ or $Pd$ (solid).

Adsorption Theory Of Heterogeneous Catalysis:

This theory explains how solid catalysts work in heterogeneous catalysis:

1. Adsorption of Reactants: Reactant molecules adsorb onto the surface of the catalyst. This adsorption weakens the bonds within the reactant molecules, making them more reactive.
2. Formation of Activated Complex: The adsorbed reactant molecules diffuse on the catalyst surface and react to form an activated complex or intermediate.
3. Formation of Products: The activated complex breaks down to form product molecules.
4. Desorption of Products: The product molecules desorb from the catalyst surface, leaving the surface free to adsorb more reactant molecules.

The efficiency of a heterogeneous catalyst depends on the strength of adsorption of reactants and products. If adsorption is too weak, reactants won't adsorb effectively. If adsorption is too strong, products might not desorb easily, blocking the active sites.

Shape-Selective Catalysis By Zeolites:

Zeolites are microporous aluminosilicate minerals with a framework structure of interconnected channels and cavities of molecular dimensions. They act as excellent catalysts in organic chemistry, particularly in the petroleum industry.

• Shape-Selectivity: The catalytic activity of zeolites is based on their shape-selective nature. The pores and cavities in zeolites are of a size comparable to the dimensions of reactant and product molecules. This allows zeolites to preferentially catalyze reactions in which reactants can fit into the pores and form products that can escape.
• Mechanism: Reactants enter the pores, undergo catalytic transformation at the active sites within the pores, and then the products exit. Reactants or products that are too large to fit in the pores cannot participate in the reaction.
• Example: ZSM-5 (Zeolite Socony Mobil-5) is a zeolite used to convert alcohols (like methanol) into gasoline. It selectively produces gasoline hydrocarbons by facilitating the dehydration of methanol to carbocations, which then react to form hydrocarbons of the correct size for gasoline.

Enzyme Catalysis:

Enzymes are complex protein molecules that act as biological catalysts, speeding up biochemical reactions in living organisms.

• High Specificity: Enzymes exhibit high specificity for their substrates, often due to the unique three-dimensional structure of their active sites.
• Mechanism: Similar to heterogeneous catalysis, enzymes provide an alternative pathway with lower activation energy through the formation of an enzyme-substrate complex (ES complex).
• Active Site: The active site is a specific region on the enzyme where the substrate binds and the reaction occurs.
• Models:
  • Lock and Key Model: Suggests a rigid active site fitting a specific substrate.
  • Induced Fit Model: Suggests the active site changes shape upon substrate binding for a better fit.
• Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration, and presence of activators or inhibitors.

Catalysts In Industry:

Catalysts play a vital role in numerous industrial processes, improving efficiency, reducing energy consumption, and enabling reactions that would otherwise be slow or impossible.

• Contact Process (for $H_2SO_4$): $V_2O_5$ is used as a catalyst for the oxidation of $SO_2$ to $SO_3$.
• Haber's Process (for $NH_3$): $Fe$ with $K_2O$ and $Al_2O_3$ as promoters is used for the synthesis of ammonia from nitrogen and hydrogen.
• Hydrogenation of Oils (to form Vanaspati Ghee): $Ni$ is used as a catalyst.
• Ostwald Process (for $HNO_3$): Platinum-gauze catalyst is used for the oxidation of ammonia.
• Fischer-Tropsch Process (for synthetic fuel): Cobalt or Iron catalysts are used to convert carbon monoxide and hydrogen into liquid hydrocarbons.
• Polymerization: Ziegler-Natta catalysts are used for the polymerization of alkenes.

Effect Of Catalyst (from Chemical Kinetics)

Effect Of Catalyst

A catalyst influences the rate of a chemical reaction without being consumed in the overall process. It achieves this by providing an alternative reaction pathway with a lower activation energy.

• Lowering Activation Energy: This is the primary way a catalyst increases the reaction rate. By lowering the energy barrier ($E_a$), a larger fraction of reactant molecules possess sufficient kinetic energy to overcome this barrier at a given temperature, leading to more effective collisions and a faster reaction.
• Alternative Mechanism: Catalysts participate in the reaction by forming intermediate species with the reactants. These intermediates are typically less stable and have lower energy of formation than the transition state of the uncatalyzed reaction.
• No Change in Equilibrium: A catalyst speeds up both the forward and reverse reactions to the same extent. Therefore, it helps the reaction reach equilibrium faster but does not change the equilibrium constant ($K_{eq}$) or the equilibrium concentrations of reactants and products.
• No Change in Overall $\Delta G$:** A catalyst does not alter the thermodynamics of the reaction; it only affects the kinetics by changing the pathway. The change in Gibbs free energy ($\Delta G$), enthalpy ($\Delta H$), and entropy ($\Delta S$) for the overall reaction remain unchanged.

Graphical Representation:

The effect of a catalyst can be visualized using reaction profile diagrams:

In the diagram:

• The uncatalyzed reaction has a higher activation energy ($E_a$).
• The catalyzed reaction has a lower activation energy ($E'_a$), often involving one or more intermediate steps.
• The initial energy of reactants and the final energy of products are the same for both catalyzed and uncatalyzed reactions.

Rate Law: The presence of a catalyst can sometimes change the order of the reaction with respect to reactants, as the mechanism of the reaction is altered.