Heterogeneous catalyis

Heterogeneous catalysis

By Lyn :)

Good Catalyst

Using a transition metal as a catalyst

Needs to absorb reactant molecules strongly enough for them to react but not so strong so that they can be desorbed (not permanently stuck to surface).

e.g. platinum & nickel

Bad Catalyst

Desorbtion

Silver doesn’t form strong enough attachments with reactant molecules

Tungsten absorbs too strongly.

At this stage, both reactant molecules may be attached to the surface or one might be knocked off by another freely moving molecule in the gas/liquid.

Absorption

Product molecules are desorbed – they break away leaving active site available for next set of reactant molecules.

Catalytic Converters

One or more of the reactants are absorbed (stick) on to the surface of the catalyst at the active sites (area which is good at absorbing things – helping them to react)

There is an interaction between the catalyst surface and the reactant molecule which makes them more reactive. This may involve a reaction with the surface or the weakening of the bonds of the attached molecule.

HETROGENEOUS

CATALYSIS

What do they do?

Catalyst Poisoning

Change poisonous molecules e.g. Carbon monoxide and Nitrogen oxides found in car exhausts into less harmful molecules such as Carbon dioxide and Nitrogen.

A molecule not part of the reaction gets strongly absorbed onto the surface inhibiting normal reactant from reaching it.

Use expensive metals e.g. Platinum, Palladium and Rhodium as the catalyst.

E.g. Lead – coats the honeycomb inhibiting their function.

Metals deposited as thin layers onto a ceramic honeycomb – maximize surface area keeping amount of metal used to a minimum

Lead compound used to be added to petrol to make it burn more smoothly however leaded fuel doesn’t go well with the catalytic converter.

The use of a catalyst in a different phase to the reactants