Quick Recap

The session had discussion on seven representative questions covering topics like surface reactions, catalyst deactivation, and diffusion resistance. MSubbu delivered detailed explanations on surface reaction mechanisms, adsorption types, and rate expressions, including derivations and practical applications in catalytic reactions. The discussion concluded with insights on porous catalyst effectiveness, experimental results from basket-type mixed flow reactors, and guidance for future class content and peer discussions.

Next Steps

  • Students: Review the Langmuir-Hinshelwood, Rideal, and Eley-Rideal mechanisms for heterogeneous catalysis.
  • Students: Prepare for the upcoming review session on Thursday.
  • Students: Begin preparing for the Control Systems topic starting Sunday.
  • Students: Study molecular and dissociative adsorption concepts for catalytic reactions.
  • Students: Understand catalyst deactivation models and their effect on reaction rates.
  • Students: Learn about pore diffusion resistance, Thiele modulus, and effectiveness factor for porous catalysts.
  • Students: Access the online course materials on a regular basis as recommended by Professor MSubbu.

SUMMARY

Representative Questions and LHHW Modeling Topics 
MSubbu outlined the agenda for the day, which included three new representative questions on topics such as Langmuir-Hinshelwood-Hougen-Watson modeling, as well as questions on catalyst deactivation and pore diffusion resistance. 

Surface Reaction Kinetics (LHHW) Overview
MSubbu discussed different types of surface reactions, focusing on first and second-order reactions with surface reaction control, and introduced the Langmuir-Hinshelwood mechanism. He explained the concept of rate-limiting steps and how adsorption, surface reaction, and desorption can be in equilibrium, except for the rate-limiting step. MSubbu derived rate expressions for various scenarios, emphasizing the importance of considering forward and backward rates and equilibrium constants. He concluded by highlighting how the rate of reaction depends on the rate-limiting step, which in this case was the desorption of \(R\).

Surface Reaction Rate Expressions

MSubbu discussed the derivation and interpretation of rate expressions for surface reactions, focusing on the relationships between concentration terms, equilibrium constants, and rate-limiting steps. He explained how to determine the appropriate rate expression based on whether adsorption or desorption is rate-limiting and whether a reaction follows a Langmuir-Hinshelwood and Eley-Rideal mechanism. MSubbu emphasized the importance of checking dimensions and simplifying expressions to include only measurable concentrations, and he provided examples to illustrate these concepts.

Dissociative Adsorption and Reaction Rates

MSubbu explained the concepts of molecular and dissociative adsorption, focusing on how different adsorption mechanisms affect the concentration of adsorbed species and the rate of surface reactions. He derived the rate expression for a reaction involving molecular and dissociative adsorption, highlighting the importance of dissociation in altering the reaction order. MSubbu emphasized the need to consider dissociative adsorption when dealing with reactions involving diatomic molecules and explained how to identify dissociative adsorption by the presence of square root terms in the rate expression.

Catalytic Reaction in Batch Reactor

MSubbu discussed a catalytic reaction occurring in a 10-liter isothermal batch reactor with decaying catalyst activity. He explained how to convert the rate expression from mass-based units to volume-based units and derived a simple rate expression for the reaction. MSubbu also described how catalyst activity decreases over time according to a first-order decay process, and he provided a method to calculate the reactant conversion after 24 hours of operation, which resulted in approximately 19% conversion.

Porous Catalyst - Effectiveness Analysis

MSubbu discussed the effectiveness of porous catalysts in chemical reactions, focusing on the Thiele modulus and its impact on reaction rates. He explained that when the Thiele modulus is less than 0.4, pore diffusion resistance is negligible, and the entire catalyst is effective. Conversely, when the Thiele modulus is greater than 4, effectiveness is inversely proportional to the Thiele modulus, indicating strong pore diffusion resistance. 

Catalyst Performance - Reactor Experiments

MSubbu explained the results of three experiments involving catalysts of different sizes and speeds in a basket mixer flow reactor. He concluded that there is no external mass transfer limitation with larger diameter catalysts, and since smaller diameter catalysts also show no limitation, mass transfer is not controlling the reaction. MSubbu also discussed the relationship between effectiveness, diameter, and pore diffusion resistance, and mentioned that he would cover biochemical reactors in future classes. He encouraged the students to review the content, participate in peer discussion sessions, and access the course materials regularly.

Last modified: Friday, 1 August 2025, 10:57 AM