HETEROGENOUS CATALYST Akamigas Balongan

Catalyst A substance which changes the speed of a reaction without being used itself is called a catalyst. The phenomenon of increasing the rate of reaction by the use of catalyst is called catalysis. If а catalyst increases (accelerates) the speed of а reaction, it is called а positive catalyst and the phenomenon is called positive catalysis. On the other hand, if а catalyst decreases (retards) the speed of а reaction, it is called а negative catalyst and the phenomenon is called negative catalysis.

Catalyst lowers the activation energy for both forward and reverse reactions. Type of catalyst : Homogenous catalyst : catalyst and reactant in one phase Heterogenous catalyst : catalyst and reactant in different phase

Catalysist Catalysts increase reaction rate without themselves being changed Can accelerate a reaction in both directions Do not affect the state of equilibrium of reaction simply allow equilibrium to be reached faster

Properties heterogeneous catalyst Simple construction Easy separated from mixed reaction Stable and resistence for high temperature Easy to construct in pellet form

Heterogenous catalyst Adsorption (STEP 1) Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier Reaction (STEPS 2 and 3) Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur. This increases the chances of favourable collisions taking place.

Heterogenous catalyst Desorption (STEP 4) There is a re-arrangement of electrons and the products are then released from the active sites

Adsorption reactan Desorption product Reaction of adsorbed intermediates

CATALITYC ACTION

REACTION HYDROGENATION OXIDATION OTHER REACTION

Oxide catalyst

Metal Catalyst

Heterogenous catalyst Metals Ni, Pt hydrogenation reactions Fe Haber Process Rh, Pd catalytic converters Cobalt Copper Oxides Al2O3 dehydration reactions V2O5 Contact Process Zeolit Clay

Many cases, the choice of catalyst could affect product Ethanol undergoes different reactions depending on the metal used as the catalyst. The distance between active sites and their similarity with the length of bonds determines the method of adsorption and affects which bonds are weakened.

CATALYTIC REACTION

Mechanism of Catalytic Reaction Example : heterogeneous catalytic hydrogenation of ethylene Σ ΔFi < ΔF*

Surface area catalyst attract atom surrounding gas and solid. Catalyst surface is an irregular crystal which has different form side, making its “attracting force” in each atoms were different. The surface catalyst always has “active center”, where the increasing physic and chemist adsorption phenomena take place.

Irregular crystal form

Physic Adsorption Not specified The solid molecule force in surface to fluid molecule of fluid is weak Can’t explain the catalytic reaction The increasing temperature, the decreasing physic adsorption The physic adsorption was concern for determining the property of solid catalyst ( surface are, size distribution)

Chemist Adsorption The force of chemist adsorption > physic adsorption According to Langmuir, the adsorbed molecule are held to surface by valence energy are the same type as those occurring between atoms in molecules. High heat adsorption

Chemist Adsorption Non- Activated : activation energy near zero, occur rapidly Activated : rate varies with temperature, process slow

Transfer massa liquid - solid

Transfer massa liquid - solid

Transfer massa gas - solid

PHYSICAL PROPERTIES OF CATALYST High porosity Surface area active center Void volume Solid material density Void volume distribution

VOID VOLUME Micropore : void spaces within the particels Macropore : void regions between particels

CATALYST CLASIFICATION Conductor : metals ( Silver, Platinum, Vanadium, Iron) which has chemisorption properties Semiconductor : Oxide (NiO, Cu2O, ZnO) have capabilities to exchange electron Insulator : Silica, Alumina and their combined.

Metal chemisorb oxygen and hydrogen usually effective for catalyst for oxidation-reduction, hydrogenation-dehydrogenation. Example SO2 and Ni effective for hydrogenation. The metal oxide, as semiconductor, catalyze the same kind of reactions, but need high temperatures. Due to relative strength of chemisorption bonds with O2 and CO attached metals, theses gases are poisons when metals are employed as hydrogenation catalyst

CATALYST PREPARATION Promotor/ carier / support : a substance added during the preparation catalyst which improve activity, selectivity, or stabilized catalytic agent so as to prolong its life. (Ex : Al2O3, CaO, K2O) Inhibitor : when added I small amounts during catalyst manufacture, it lessens activity, stability, or selectivity. (Ex: halogen group)

Catalsyt support Example : Alumina, Silica, Carbon ( Carbon black, charcoal, graphite)

Effect heating on catalyst support

Effect particle size of metal support

Model catalyst support in support particels

Egg shell : type of catalyst having the active component on or near the surface is more efficient Uniform : is preferred for reactions in which mass-transfer limitations have been removed. Egg Yolk : action medium contains a catalyst poison or inhibitor, then the use of an “egg white” or “egg yolk” catalyst is desirable so the exterior of the support can adsorb the poison before it can reach the active metal