Download presentation

2
**Langmuir Adsorption Isotherm**

3
“Adsorption”! “Isotherm”?

4
**In this case, molecules do not adsorb or “stick” to the surface**

Consider gas molecules striking a surface In this case, molecules do not adsorb or “stick” to the surface Surface

5
**In this case, molecules do adsorb to the surface**

Now, consider the following scenario… In this case, molecules do adsorb to the surface Surface

6
**That is, adsorption at a fixed temperature**

What about… Isotherm - it means…Constant Temperature That is, adsorption at a fixed temperature

7
**molecules in the gas phase molecules adsorbed at the surface**

The objective of Langmuir Equation… …To describe the equilibrium between the molecules in the gas phase and the molecules bound to the surface molecules in the gas phase molecules adsorbed at the surface

8
**The Experiment… We will: Introduce molecules in this chamber**

2. Wait for an equilibrium to reach 3. Plot number of molecules adsorbed versus the equilibrium pressure

9
**The Experiment… No Gas Molecule Nadsorbed Amount Adsorbed Pressure, P**

1 2 3 4 5 6 7 8 9 10 11 12 13 14

10
**Now, Let’s introduce 4 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14

11
**Now, Let’s introduce 4 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 in gas phase 3 adsorbed

12
**Now, Let’s introduce 8 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 in gas phase 6 adsorbed

13
**Now, Let’s introduce 12 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 4 in gas phase 8 adsorbed

14
**Now, Let’s introduce 15 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 7 in gas phase 9 adsorbed

15
**Now, Let’s introduce 21 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 11 in gas phase 10 adsorbed

16
**Finally, Let’s introduce 24 Molecules**

The Experiment… Pressure, P Amount Adsorbed Nadsorbed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 14 in gas phase 10 adsorbed

17
**We can plot this data as a fraction of molecules adsorbed**

Fraction Adsorbed 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Number Adsorbed Fraction Adsorbed Maximum Available sites 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Pressure, P

18
**So, how to describe this behavior?**

In 1916, Langmuir proposed a simple model… Basic Assumptions… Adsorption sites are homogeneous and equivalent 2. The adsorbed molecules are immobile and they do not interact 3. Only a monolayer coverage is possible Irving Langmuir

19
**So, how to describe this behavior?**

In 1916, Langmuir proposed a simple model… Based on these assumptions, Langmuir derived the following equation… Equilibrium constant Fraction Adsorbed Equilibrium Pressure Irving Langmuir

20
**This is the Langmuir Equation describing (fitting) the Data**

Fraction Adsorbed Recall, the data we collected Based on these assumptions, Langmuir derived the following equation… 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Equilibrium constant This is the Langmuir Equation describing (fitting) the Data Fraction Adsorbed Equilibrium Pressure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Pressure, P

21
**This is the Langmuir Equation describing (fitting) the Data**

Fraction Adsorbed Recall, the data we collected 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Notice… This is the Langmuir Equation describing (fitting) the Data And at low concentration, adsorption is linearly proportional to P 1 2 3 4 5 6 7 8 9 10 11 12 13 14 i.e. reaches a monolayer Pressure, P

22
**The Langmuir Equation Now, that’s an equation for another video!**

Stronger binding affinity What about the Equilibrium Constant, K… Larger the value of K, stronger the binding Equilibrium constant can be used to calculate Gibb’s free energy of adsorption using the following equation… Now, that’s an equation for another video!

23
**Langmuir Equation - Application**

Kinetics of Catalytic Reactions A B C Product Surface Reaction Adsorption Pollution Remediation adsorption of contaminants to colloids Material Science e.g. adsorption on DSSCs

24
**Tidbits about Langmuir**

Historical Tidbits about Langmuir Was nearly blind for the first eleven years of his life Graduated from Columbia’s School of Mines, NYC in 1903 Post graduate work at Univ. of Gottingen in Germany in Physical Chemistry under Walther Nernst Irving Langmuir 3rd Law of Thermodynamics Nernst Equation

25
**Tidbits about Langmuir**

Historical Tidbits about Langmuir Was nearly blind for the first eleven years of his life Graduated from Columbia’s School of Mines, NYC in 1903 Post graduate work at Univ. of Gottingen in Germany in Physical Chemistry under Walther Nernst Irving Langmuir Worked at the General Electric Labs 3rd Law of Thermodynamics Nernst Equation

26
**Tidbits about Langmuir**

Historical Tidbits about Langmuir Was nearly blind for the first eleven years of his life Graduated from Columbia’s School of Mines, NYC in 1903 Post graduate work at Univ. of Gottingen in Germany in Physical Chemistry under Walther Nernst Irving Langmuir Worked at the General Electric Labs 1932 – First Industrial Chemist to receive Nobel Prize in Chemistry “for his discoveries and investigations in surface chemistry “ This is the same year Schrodinger won the prize in Physics

27
**Resources The End http://www.britannica.com/biography/Irving-Langmuir**

Music: “Mouton Swing” by Robert R. Putnam – Resources

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google