Catalysis

Sunny Suranab.Tech 1st yearu101112fbt145

What is catalysis???.....

• A Catalyst is a substance that initiates or accelerates a chemical reaction without itself being affected. Catalysts provide an alternative reaction mechanism with a lower activation energy.

Effects of catalysis

• Catalyst provide an alternative path for a reaction to proceed it does not lower the activation energy of the reaction.

• It just alters the rate of reaction without itself getting involved in the reaction.

Types of catalysis

• Homogeneous catalyst

---Catalyst is in the same phase as reactants

---Increase rate of reaction caused by catalyst allowing for reaction to occur with alternative mechanism with lower activation energy

---Catalyst is consumed at one stage in the mechanism and reformed at a later stage

---Liquid Phase

• Heterogonous catalyst

---Catalyst is in different phase to reactants

---Catalyst provides reactive surface where reaction can take place; reactants are absorbed onto the surface

---This process weakens the bonding in molecules and brings them into close contact; reaction occurs on the surface and the products are desorbed; this frees the surface for further reaction

---Solid/Gas Phase

Homogenous catalystAdvantages---all the

catalyst is exposed to the reactant, easily modified, reaction mechanisms are well understood by scientists, easy process of diffusion and heat transfer, well defined active site

Disadvantages--- can be difficult to remove from the products for reuse

Heterogeneous catalysis

Advantages---easily removed from products by filtration, easily recycled

Disadvantages---only effective on the surface, reaction mechanisms are poorly understood in general, poorly defined active site

Heterogeneous catalysis

• Heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid,  gas, but also immiscible liquids, e.g. oil and water.

Mechanism of heterogeneous catalysis

Mechanism of heterogeneous catalysis

• Heterogeneous catalysis takes place mainly in 2 steps1. Adsorption.2. Surface reactions.

Adsorption……

• 1st step of heterogeneous catalysis.• Adsorption is when a molecule in the gas phase or in

solution binds to atoms on the solid or liquid surface.• The molecule that is binding is called the adsorbate, and the

surface to which it binds is the adsorbent. • The process of the adsorbate binding to the adsorbent is

called adsorption. The reverse of this process (the adsorbate splitting from adsorbent) is called desorption.

• the catalyst is the adsorbate and the support is the adsorbent.

Surface reactions

• After the adsorption the reaction takes place on the surface of the catalyst and these reactions are known as surface reactions

• The surface reaction are of 3 types1. Langmuir-Hinshelwood mechanism2. Rideal-Eley mechanism. 3. Precursor mechanism.

Langmuir-Hinshelwood mechanism

• The two molecules A and B both adsorb to the surface. While adsorbed to the surface, the A and B "meet," bond, and then the new molecule A-B desorbs.

Rideal-Eley mechanism

• One of the two molecules, A, adsorbs to the surface. The second molecule, B, meets A on the surface, having never adsorbed to the surface, and they react and bind. Then the newly formed A-B desorbs.

Precursor mechanism.

• One of the two molecules, A, is adsorbed on the surface. The second molecule, B, collides with the surface, forming a mobile precursor state. The molecule B then collides with A on the surface, they react, bind and the new molecule desorbs.

Personalities of India working on

heterogeneous catalysis

Dr. Ataullah Khan

• Dr. Khan has a doctorate degree in Heterogeneous Catalysis from Indian Institute of Chemical Technology (IICT), Hyderabad. 

•  At IICT, he designed novel nano-composites based on Ceria for catalytic applications and has a US Patent based on his Ph.D. work. His Ph.D. thesis was honoured with the ‘BEST Ph.D. THESIS AWARD, 2005’ in the discipline of Physical Chemistry & Heterogeneous Catalysis by the Catalysis Society of India.

•  His current research interests include: Catalytic Reformation Technologies, waste-to-value added processes, Biomass Gasification/Pyrolysis , Biofuels, Gas-to-Liquid Technology (GTL), and Solid Sorbents for CO2 Capture

Dr. Subhash chandra laha

• Materials/solid catalysts: zeolites, mesoporous materials, AlPO , redox molecular sieves, metal oxides

• Heterogeneous catalysis: aromatization, dehydrogenation, alkylation, acylation, epoxidation, hydroxylation, oxidation with molecular oxygen, CO2 utilization

• Specialties : Heterogeneous Catalysis, Zeolites, metal oxides, Solid Catalysts

Dr. Sreedevi Upadhyaya

• Kinetics of Heterogeneous reactions• Green Catalysis using Ionic Liquids, heterogenization of IL

catalyzed reactions• Solid Acid catalysts, Zeotype, Mesoporous materials• Tailored heterogeneous catalysts  for•    Utilization of energy and resources•    Petrochemical & Refinery Chemical processes

Applications of heterogeneous catalysis

Process Reactants, product(s) Catalyst

Sulfuric acid synthesis (Contact process) SO2 + O2, SO3 vanadium oxides

Ammonia synthesis (Haber-Bosch process) N2 + H2, NH3 iron oxides on alumina

Nitric acid synthesis (Ostwald process) NH3 + O2, HNO3 unsupported Pt-Rh gauze

Hydrogen production by Steam reforming CH4 + H2O, H2 + CO2 Nickel or K2O

Ethylene oxide synthesis C2H4 + O2, C2H4O silver on alumina, with many promotors

Hydrogen cyanide synthesis (Andrussov oxidation) NH3 + O2 + CH4, HCN Pt-Rh

Olefin polymerization Ziegler-Natta polymerization

propylene, polypropylene TiCl3 on MgCl2

Desulfurization of petroleum (hydrodesulfurization)

H2 + R2S (idealized organosulfur impurity), RH + H2S

Mo-Co on alumina