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Reactor Design

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: : catalysis : : ! : / ! : : : : ... : : ...

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Catalysis

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WHAT IS A CATALYST?

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Catalyst is a substance that changes the rate of the reaction without being consumed in the process. Catalyst affects only the rate of the reaction,i.e. Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.

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HOW?

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Do you remember this man?

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A catalyst provides an alternative route for the reaction with a lower activation energy. It does not lower the activation energy of the reaction.

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Since a catalyst makes it possible to obtain an end product by a different mechanism, it can affect both the yield and selectivity. A catalyst can either accelerate or slow the formation of a particular product species.

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CATEGORIES OF CATALYSIS

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Concerns processes in which a catalyst is in a solution with at least one of the reactants. Homogeneous Catalysis Involves more than one phase; usually the catalyst is a solid and the reactants and products are in liquid or gaseous form. Heterogeneous Catalysis

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CATALYST TYPES

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Catalyst Types Porous Molecular Sieves Monolithic

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CATALYST DEACTIVATION

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Aging. Poisoning or fouling. Coking.

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STEPS IN A HETEROGENEOUS CATALYTIC REACTION

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ADSORPTION, SURFACE REACTION & DESORPTION Importaaaant!!!

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Most examples of heterogeneous catalysis go through the same stages: One or more of the reactants are adsorbed on to the surface of the catalyst at active sites. Adsorption is where something sticks to a surface. It isn't the same as absorption where one substance is taken up within the structure of another. Be careful! An active site is a part of the surface which is particularly good at adsorbing things and helping them to react. There is some sort of interaction between the surface of the catalyst and the reactant molecules which makes them more reactive. This might involve an actual reaction with the surface, or some weakening of the bonds in the attached molecules.

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The reaction happens. At this stage, both of the reactant molecules might be attached to the surface, or one might be attached and hit by the other one moving freely in the gas or liquid. The product molecules are desorbed. Desorption simply means that the product molecules break away. This leaves the active site available for a new set of molecules to attach to and react.

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A good catalyst needs to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecules stick more or less permanently to the surface. Silver, for example, isn't a good catalyst because it doesn't form strong enough attachments with reactant molecules. Tungsten, on the other hand, isn't a good catalyst because it adsorbs too strongly. Metals like platinum and nickel make good catalysts because they adsorb strongly enough to hold and activate the reactants, but not so strongly that the products can't break away.

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EXAMPLE

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The hydrogenation of a carbon-carbon double bond

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Hydrogen molecules are also adsorbed on to the surface of the nickel. When this happens, the hydrogen molecules are broken into atoms. These can move around on the surface of the nickel.

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If a hydrogen atom diffuses close to one of the bonded carbons, the bond between the carbon and the nickel is replaced by one between the carbon and hydrogen.

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That end of the original ethene now breaks free of the surface, and eventually the same thing will happen at the other end.

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As before, one of the hydrogen atoms forms a bond with the carbon, and that end also breaks free. There is now space on the surface of the nickel for new reactant molecules to go through the whole process again.

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Presence of Inhibitor

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The animation to the right shows CO reacting with O 2 with the help of a metal catalyst. Oxygen (white) dissociates on the catalyst surface. Carbon (red), which is already attracted to the catalytic surface, picks up an unbonded oxygen atom to form CO 2.

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REACTIONS

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Adsorption Concentration of vacant active sites A Occupied Site Vacant Site

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Surface Reaction Langmuir- Hinshelwood Kinetics

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Desorption

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EXERCISE

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SHEET 8

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1.Derive a Hoogen & Watson type model for the overall reaction 2A --------- P Assuming that the surface reaction is the rate determining step, if: A + S = = = = AS (1) 2AS -------- PS + S (2) PS = = = = P + S (3)

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3.The catalytic hydrogenation of butyraldehyde (B) to butanol (C): H 2 + C 3 H 7 CHO = = = = C 3 H 7 CH 2 OH Has a reported rate equation: R = [k o e E/RT (p A p B p c /K eq )] / [1+K 1 p A +K 2 p B +K 3 p C ] 2 Develop a surface reaction model for the observed kinetics