Session Objectives Adsorption versus absorption Types of adsorption: physisorption and chemisorption Desorption Adsorption isotherms: Freundlich and Langmuir Adsorption isobar Catalysis
Adsorption The phenomenon of higher concentration of any molecular species at the surface than in the bulk Adsorbent The substance on the surface of which adsorption takes place is called adsorbent Adsorbate The substance which is being adsorbed on the surface of another substance. Desorption The process of removal of an adsorbed substance from the surface on which it is absorbed
Activated Carbon Activated Alumina Silica Gel Molecular Sieves (Zeolites) Polar and Non-polar adsorbents Adsorbent Materials
Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate and carbonize (remove volatile components), "Activation" is the process that produces the porous structure essential for effective adsorption by oxidation of carbon with water vapor or CO 2. Activated carbon attracts non-polar molecules such as hydrocarbons. Typical surface areas are 300 to 1500 m 2 /g. Activated carbon
Types of Adsorption Positive adsorption occurs when the concentration of adsorbate is higher on the surface of adsorbent than in the bulk. Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk.
Types of adsorption 1.Physical adsorption 2.Chemical adsorption
Factors affecting adsorption Effect of adsorbate: The easily liquifiable gases like NH 3, HCl, CO 2 etc. are adsorbed to a greater extent than the permanent gases such as H 2, O 2, N 2, etc. Effect of specific area of the absorbent: The greater the specific area of the solid, the greater would be its adsorbing capacity. Effect of temperature:adsorption decreases with increase in temperature. Effect of pressure: An increase in pressure causes an increase in the magnitude of adsorption of an adsorbent.
Over a narrow range of p Freundlich Isotherm A graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at constant temperature is called adsorption isotherm At low pressure the graph is nearly straight line At high pressure x/m becomes independent of p
Rate of adsorption Rate of desorption At equilibrium, r a = r d ; Mono-layer coverage m: mass of adsorbate adsorbed per unit mass of adsorbent f: fraction of surface area covered f 1-f p: partial pressure of the adsorbate Langmuir isotherm
Langmuir adsorption isotherm: The values of constants ‘a’ and ‘b’ depend upon the nature of adsorbate, nature of solid adsorbent and temperature. a = k a x k a ’/k d b = k a /k d Combining equations (1) and (2):
Summary of adsorption isotherms Easy to fit adsorption data Chemisorptions and physisorption Freundlich Useful in analysis of reaction mechanism Chemisorption and physisorption Langmuir NoteApplicationIsotherm equation Name
Adsorption isobar Graph between the amount adsorbed(x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar Chemisorption isobar shows an initial increase with temperature and then expected decrease.The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption.
Application of Adsorption In clarification of sugar In gas masks In catalysis In adsorption indicators In chromatographic analysis In softening of hard water In preserving vacuum In paint industry In removing moisture from air in the storage of delicate instruments
Need to make chemicals faster Most Reactions are too slow to be useful...
Ways to Make Chemicals Faster Pressure Disadvantage--Cause Explosions Catalysts!!!! Disadvantage--Costly Disadvantage--Too hot! Temperature Add other Chemicals Disadvantage--Separate chemicals
The Story of Catalysis Why Catalysis ? What is a Catalyst ? How Catalysts Work ?
Role of a Catalyst Catalysts speed up a chemical reaction without being used up... Catalyst + Reactants Catalyst-Reactants Catalyst + Products
Important properties of catalyst Activity: A reasonable rate of reaction is needed. Selectivity: Byproducts should be minimized. Cost: The acceptable cost depends upon the catalyst lifetime and product value.
Catalysis Positive catalysis Negative catalysis Auto catalysis
Catalysis Types of catalysis Homogeneous catalysis Process Products Catalyst 1. Hydrolysis of an ester. Acid and AlcoholH + 2. Oxidation of SO 2 to SO 3 SO 3 NO in lead chamber process Heterogeneous catalysis ProcessProductsCatalyst 1. Ammonia synthesisNH 3 Fe with Al 2 O 3 2. Methanol synthesisMeOHZnO/Cr 2 O 3
Catalysis Enzyme catalysis ReactionCatalyst 1. Invertase 2. Zymase A coenzyme is an organic non-protein molecule that is a functional part of an enzyme. Coenzymes are not used up in the reactions in which they assist
Do Catalysts Live Forever? NO!!! They can die from: -- poisons which contaminate the catalyst -- large molecules which cover the catalyst -- over heating, over pressurizing -- crumbling/crushing
Automotive Emission Control Automobile catalytic converters need to catalyze several reactions – CH x + O 2 CO 2 + H 2 O – CO + O 2 CO 2 – NO x +CH x N 2 + H 2 O + CO 2 – This is achieved by the use of a supported precious metal catalyst like platinum, palladium etc. – Catalyst needs O 2 to operate, CeO 2 acts as a temporary regenerable source of O 2. CeO 2 Ce 2 O 3 + 1/2O 2