Chem 201 Lectures 37:

Catalysis Textbook: Ch. 16 WWW: search for “Catalysis ppt site:.edu” Wiki: search for “catalysis” Presentations used to prepare this lecture: 1) by Silberberg (text book) 2) S.A. Green (St. Charles Community College)

Transition State Theory • It shows the energy of the reactants and products (and, therefore, E). • The high point on the diagram is the transition state. • The species present at the transition state is called the activated complex. • The energy gap between the reactants and the activated complex is the activation energy barrier.

What is a “Catalyst” •

A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction. (Wikipedia) k(T) = k0e-Ea/RT Ea′ < Ea k0′ > k0 k′ > k

Ea

Ea′

∆G = ∆G A+B

A+B+ catalyst

∆G C uncatalyzed

∆G C + catalyst catalyzed

Reaction energy diagram of a catalyzed and an uncatalyzed process

intermediate state (reaction intermediates)

Reaction is split into several steps each having lower Ea is compared with single step

CATALYSTS • Each catalyst has its own specific way of functioning. • Ideal catalalyst is not spent during the reaction • In general a catalyst lowers the energy of activation. • Lowering the Ea increases the rate constant, k, and thereby increases the rate of the reaction.

• A catalyst increases the rate of the forward AND the reverse reactions.

• A catalyzed reaction yields the products more quickly, but does not yield more product than the uncatalyzed reaction.

• A catalyst lowers Ea by providing a different mechanism, for the reaction through a new, lower energy pathway.

Types of Catalysts - Enzymes

Triosephosphateisomerase

“TIM” Cytochrome C Oxidase Highly tailored “active sites” Often contain metal atoms

• The “Gold Standard” of catalysts • Highly specific • Highly selective • Highly efficient • Catalyze very difficult reactions – N2  NH3 – CO2 + H2O  C6H12O6 • Works better in a cell than in a 100000 l reactor

Collision theory A+B→C Number of collisions:

Reaction rate:

Steric factor

Probability for molecule to have energy greater than Ea

Catalysis affects steric factor and activation energy

http://en.wikipedia.org/wiki/Collision_theory

Types of Catalysts – Organometallic Complexes • Perhaps closest man has come to mimicking nature’s success • 2005 Noble Prize in Chemistry (Grubbs) • Well-defined, metal-based active sites • Selective, efficient manipulation of organic functional groups • Various forms, especially for polymerization catalysis • Difficult to generalize beyond organic transformations

Types of Catalysts – Homogeneous vs. Heterogeneous

Zeolite catalyst

Catalyst powders

Homogeneous catalysis

Heterogeneous catalysis

Single phase (Typically liquid) Low temperature Separations are tricky

Multiphase (Mostly solid-liquid and solid-gas) High temperature Design and optimization tricky

Types of Catalysts: Crystalline Microporous Catalysts • • •

Regular crystalline structure Porous on the scale of molecular dimensions – 10 – 100 Å – Up to 1000’s m2/g surface area Catalysis through – shape selection – acidity/basicity – incorporation of metal particles

10 Å 100 Å

Zeolite (silica-aluminate)

Silico-titanate

MCM-41 (mesoporous silica)

Types of Catalysts: Amorphous Heterogeneous Catalysts • • • • •

Amorphous, high surface area supports – Alumina, silica, activated carbon, … – Up to 100’s of m2/g of surface area Impregnated with catalytic transition metals – Pt, Pd, Ni, Fe, Ru, Cu, Ru, … Typically pelletized or on monoliths Cheap, high stability, catalyze many types of reactions Most used, least well understood of all classes

SEM micrographs of alumina and Pt/alumina

Important Heterogeneous Catalytic Processes •

Haber-Bosch process – N2 + 3 H2 → 2 NH3 – Fe/Ru catalysts, high pressure and temperature – Critical for fertilizer and nitric acid production

•

Fischer-Tropsch chemistry – n CO + 2n H2 → (CH2)n + n H2O , syn gas to liquid fuels – Fe/Co catalysts – Source of fuel for Axis in WWII

•

Fluidized catalytic cracking – High MW petroleum → low MW fuels, like gasoline – Zeolite catalysts, high temperature combustor – In your fuel tank!

•

Automotive three-way catalysis – NOx/CO/HC → H2O/CO2/H2O – Pt/Rh/Pd supported on ceria/alumina – Makes exhaust 99% cleaner

Automotive Emissions Control System “Three-way” Catalyst CO  CO2 HC  CO2 + H2O NOx  N2

Monolith reactor

Most widely deployed heterogeneous catalyst in the world – you probably own one!

Pt, Rh, Pd Alumina, ceria, lanthana, …

Sample problems When a catalyst is added to a reaction mixture, it A. increases the rate of collisions between reactant molecules. B. provides reactant molecules with more energy. C. slows down the rate of the back reaction. D. provides a new pathway (mechanism) for the reaction. E. None of these choices is correct. A boiled egg can be cooked at 100.0C in exactly 5 minutes. At an altitude of around 2000 m where the boiling point of water is 93.0C, it takes exactly 7.5 minutes to cook the egg to the same amount. What is the activation energy for the reaction involved when an egg k 1t1  k 2 t 2 is boiled?  k1t1  k 2t 2 E E e e   A. 0.5 kJ/mol RT RT t1 k 0 e  t2k 0e B. 4.5 kJ/mol E  1 1     t1 R  T T  C. 7.9 kJ/mol  e t D. 66 kJ/mol 2 t R ln E. >100 kJ/mol t 1

2

1

1

E 

2

1 1  T1 T 2

2