Chemical Kinetics

• Importance of surface reactions
• Factors affecting the rate of surface reactions
• Activation energy and its significance
• Collision theory and its application to surface reactions
• Rate-determining step in surface reactions

Importance of surface reactions

• Surface reactions play a crucial role in various important chemical processes
• Examples include heterogeneous catalysis, corrosion, and atmospheric reactions
• Surface reactions occur at the interface between two phases – gas/solid or liquid/solid
• Understanding surface reactions helps in designing efficient catalysts and controlling corrosion processes

Factors affecting the rate of surface reactions

• Nature of the reactants and adsorbents
• Temperature and its effect on reaction rate
• Surface area of the catalyst
• Presence of impurities or other catalysts
• Pressure and concentration of reactants
• pH value in case of aqueous surface reactions
• Electrochemical potential in corrosion reactions

Activation energy and its significance

• Activation energy (Ea) is the minimum energy required for a reaction to occur
• It represents the energy barrier that reactant molecules must overcome to convert into products
• Higher activation energy implies a slower rate of reaction
• Catalysts lower the activation energy, thereby increasing the reaction rate
• Understanding activation energy helps in selecting appropriate reaction conditions and catalysts

Collision theory and its application to surface reactions

• Collision theory explains the mechanism of chemical reactions
• According to this theory, chemical reactions occur when reactant molecules collide with sufficient energy and proper orientation
• In the context of surface reactions, collision theory helps in understanding the interaction between reactant molecules and the catalytic surface
• Surface area plays a key role in facilitating the collision of reactant molecules with the catalyst

Rate-determining step in surface reactions

• Rate-determining step (RDS) is the slowest step in a reaction mechanism
• In surface reactions, the RDS is often the step involving the breaking or formation of chemical bonds on the catalytic surface
• Identifying the RDS helps in understanding the overall rate of the reaction and optimizing reaction conditions
• Catalytic promoters are sometimes used to enhance the rate-determining step in surface reactions

Collision Theory and Surface Reactions

• Collision theory explains the mechanism of chemical reactions
• In the context of surface reactions, collision theory helps us understand the interaction between reactant molecules and the catalytic surface
• According to collision theory, reactions occur when reactant molecules collide with sufficient energy and proper orientation
• The probability of successful collision depends on the collision frequency, the fraction of collisions with enough energy (activation energy), and the fraction of collisions with the correct orientation
• The surface area of a catalyst plays a crucial role in increasing the chance of collisions with the catalyst

Surface Area and Reaction Rate

• Increasing the surface area of a catalyst leads to an increase in the reaction rate
• A higher surface area provides more active sites for reactant molecules to collide with
• The rate of surface reactions depends on the number of active sites available on the catalyst surface
• Dividing a solid catalyst into smaller particles or using a catalyst with a highly porous structure increases the surface area, leading to a higher reaction rate
• Examples: Using finely divided platinum as a catalyst in the hydrogenation of ethene

Temperature and Reaction Rate

• Increasing temperature generally increases the rate of surface reactions

• Higher temperature provides reactant molecules with more kinetic energy, increasing the likelihood of successful collisions

• The Arrhenius equation describes the temperature dependence of reaction rates: k = A * e^(-Ea/RT)

  where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin

• As temperature increases, the exponential factor e^(-Ea/RT) becomes larger, leading to a higher rate constant and faster reaction rate

Catalysis and Reaction Rate

• Catalysts increase the rate of surface reactions without being consumed in the process
• They lower the activation energy required for the reaction to occur
• Catalysts provide an alternative reaction pathway with a lower activation barrier, allowing more reactant molecules to possess the required energy to overcome the barrier
• Examples: Platinum catalyst in the catalytic converter of a car, enzymes in biological reactions

Adsorption and Reaction Rate

• Adsorption is the process of sticking to a surface, and it plays a crucial role in surface reactions
• Adsorption of reactant molecules on the catalyst surface increases the chance of successful collisions
• The strength of adsorption influences the reaction rate
• Strongly adsorbed reactant molecules tend to form intermediate species, leading to slower reactions
• Weakly adsorbed reactant molecules tend to have higher reaction rates

Langmuir Adsorption Model

• The Langmuir adsorption model describes the adsorption process on a solid surface

• It assumes that adsorbent sites on the surface are equivalent and that only one molecule can occupy each site at a time

• The adsorption process can be described by the equation: θ = (kp * P) / (1 + kp * P)

  where θ is the fractional surface coverage, kp is the adsorption constant, and P is the pressure of the adsorbate gas

• The Langmuir adsorption isotherm can be used to determine the surface area and adsorption kinetics of a catalyst

Rate-Determining Step (RDS)

• The rate-determining step (RDS) is the slowest step in a reaction mechanism
• In surface reactions, the RDS often involves the breaking or formation of chemical bonds on the catalytic surface
• Identification of the RDS helps determine the overall rate of the reaction and optimize reaction conditions
• Catalyst promoters can be used to enhance the rate-determining step in surface reactions
• Example: In the nitrogen oxide reduction reaction, the RDS is often the dissociation of nitrogen monoxide on the catalyst surface

Catalyst Poisoning

• Catalyst poisoning refers to the reduction in the activity of a catalyst due to the presence of certain substances
• Poisoning agents can bind to active sites on the catalyst surface, blocking reactant molecules from adsorbing and reacting
• Poisoning can be reversible or irreversible, depending on the strength of the bond between the poison and the catalyst
• Examples of catalyst poisons include sulfur compounds in catalytic converters and lead in leaded gasoline

Catalytic Promoters

• Catalytic promoters are substances added in small amounts to enhance the activity of a catalyst
• Promoters can increase the number of active sites available for reactant molecules to adsorb and react on
• They can also modify the electronic or surface properties of the catalyst to improve its catalytic performance
• Examples of catalytic promoters include alkali metals in catalytic cracking of petroleum and transition metals in catalytic hydrogenation reactions

Langmuir-Hinshelwood Mechanism

• In the Langmuir-Hinshelwood mechanism, surface reactions involve the adsorption of reactant molecules on the catalytic surface followed by the reaction between the adsorbed species.
• This mechanism is applicable to many catalytic reactions, including the oxidation of carbon monoxide on platinum catalysts.
• The Langmuir-Hinshelwood mechanism can be described by the following steps:

1. Adsorption of reactant molecules on the catalyst surface:
   • R(g) + * → R*
   • R(g) represents the reactant gas molecule, * denotes an active site on the catalyst surface, and R* represents the adsorbed reactant species.

2. Adsorption of a second reactant molecule:
   • S(g) + * → S*
   • S(g) represents the second reactant gas molecule, S* represents the adsorbed species on the catalyst surface.

3. Reaction between the adsorbed species:
   • R* + S* → P(g)
   • P(g) represents the product gas molecule formed as a result of the reaction between the adsorbed species.

4. Desorption of the product from the catalyst surface:
   • P* → P(g)
   • P* represents the adsorbed product species, and P(g) represents the product gas molecule.

5. Regeneration of the catalyst surface:
   • • → catalyst
   • The active site on the catalyst surface is regenerated for the next cycle of adsorption and reaction.

Examples of Langmuir-Hinshelwood Reactions

• Carbon monoxide (CO) oxidation on platinum (Pt) catalyst:

  1. CO(g) + * → CO*
     • CO(g) adsorbs on active sites on the Pt surface to form CO*.
  2. O2(g) + * → O2*
     • O2(g) adsorbs on active sites to form O2*.
  3. CO* + O2* → CO2(g)
     • Reactions between CO* and O2* lead to the formation of CO2(g) as the product.
  4. CO2* → CO2(g)
     • CO2* desorbs from the Pt surface to form CO2(g).
  5. • → Pt(s)
     • The Pt surface is regenerated for the next cycle.

• Nitric oxide (NO) reduction on platinum catalyst:

  1. NO(g) + * → NO*
     • NO(g) adsorbs on active sites on the Pt surface to form NO*.
  2. H2(g) + * → H2*
     • H2(g) adsorbs on active sites to form H2*.
  3. NO* + H2* → N2(g) + H2O(g)
     • Reactions between NO* and H2* lead to the formation of N2(g) and H2O(g) as products.
  4. N2* + 3H2O* → N2(g) + 3H2O(g)
     • The adsorbed water species (H2O*) further reacts with N2* to produce more N2(g) and H2O(g).
  5. • → Pt(s)
     • The Pt surface is regenerated for the next cycle.

Effect of Catalyst Structure on Surface Reactions

• The structure of a catalyst plays a crucial role in its activity for surface reactions.

• Several factors of catalyst structure affect the reaction rate and selectivity, including:

  1. Crystal planes and surface defects: Different crystal planes of a catalyst may have different catalytic activities due to their varying surface properties.
  2. Particle size: The size of catalyst particles affects the surface area available for reactant adsorption. Smaller particles generally have higher activity.
  3. Porosity: Catalysts with a higher degree of porosity possess a larger surface area, which enhances the reaction rate.
  4. Catalyst support: Some catalysts are supported on substances like alumina or zeolite, which can influence the catalytic activity and selectivity.
  5. Catalyst morphology: The shape and morphology of the catalyst can impact the reactant accessibility and reaction rate.

• Optimization of catalyst structure is essential for improving catalytic efficiency and enhancing the desired reaction pathways.

Reaction Rate and Concentration of Reactants

• The rate of surface reactions is typically dependent on the concentration or pressure of reactants.
• According to the collision theory, the probability of successful collisions increases with increasing reactant concentrations.
• Higher reactant concentrations result in a higher collision frequency and an increased chance of reactant molecules encountering the catalyst surface.
• Increased reactant concentrations also lead to a higher number of reactant molecules available for adsorption on the catalyst surface, promoting reaction kinetics.
• However, beyond a certain point, increasing reactant concentration may not significantly affect the reaction rate, as the catalytic surface may become saturated with adsorbed reactant species.

Reaction Rate and Temperature

• Temperature has a significant impact on the rate of surface reactions.
• Increasing temperature generally increases the reaction rate by providing reactant molecules with more kinetic energy.
• Higher kinetic energy leads to more frequent and energetically favored collisions between reactant molecules and the catalyst surface.
• The Arrhenius equation describes the temperature dependence of the reaction rate constant (k): k = Ae^(-Ea/RT)
  • A represents the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin.
• As temperature increases, the exponential term e^(-Ea/RT) becomes larger, resulting in a higher rate constant and faster reaction rate.
• Understanding the temperature dependence of reaction rates helps in optimizing reaction conditions and selecting suitable catalysts.

Reaction Rate and Pressure

• Pressure influences the rate of surface reactions, particularly in gas-phase reactions.
• For reactions involving gaseous reactants, increasing the pressure tends to increase the rate of surface reactions.
• Higher pressure increases the concentration of gas molecules, leading to an increased collision frequency with the catalyst surface.
• However, beyond a certain pressure, the reaction rate may plateau due to saturation of active sites on the catalyst surface.
• The effect of pressure on the reaction rate can be explained using the collision theory and the concept of partial pressures of reactant gases.

Reaction Rate and Surface Area

• The surface area of a catalyst plays a critical role in determining the reaction rate of surface reactions.
• Increasing the surface area of a catalyst leads to an increased rate of surface reactions.
• A higher surface area provides a larger number of active sites for reactant molecules to adsorb on.
• Increased active sites result in a higher frequency of successful collisions between reactant molecules and the catalyst surface, enhancing the reaction rate.
• Dividing a solid catalyst into smaller particles or using a catalyst with a highly porous structure increases the surface area, leading to a higher reaction rate.
• Examples include using finely divided platinum as a catalyst in the hydrogenation of ethene.

Summary

• Surface reactions are important in various chemical processes.
• Factors influencing the rate of surface reactions include surface area, temperature, reactant concentration, pressure, and catalyst structure.
• Activation energy represents the energy barrier that reactant molecules must overcome for a reaction to occur.
• Collision theory explains the mechanism of chemical reactions, including surface reactions.
• The Langmuir-Hinshelwood mechanism describes the adsorption and reaction steps in surface reactions.
• Catalyst poisoning and catalytic promoters can affect reaction rates and selectivity.
• Optimization of catalyst structure is essential for improving catalytic efficiency.
• Temperature, pressure, reactant concentration, and surface area play crucial roles in determining the rate of surface reactions.