What is a catalyst? How does it work?

A catalyst is a chemical substance which can speed up the
rate of a chemical reaction without being consumed itself. This happens due to the
activation energy required for the reaction to take place getting
reduced.

Catalysts can be substances which are in the same
state as that of the reacts or they can have a different state. State here is used with
reference to solid, liquid or gas. An example of catalysts which have the same state are
enzymes, which speed up, and in many cases allow, reactions within our body to occur.
Both the reactants as well the catalysts are in the liquid state. An example of a
catalyst in a different state as the reactants would be the platinum-rhodium catalyst
used in catalytic converters of cars to break down nitrogen oxides into nitrogen and
oxygen. The catalyst here is solid and the reactants are
gases

The working of catalysts usually follows this
path:

Two substances X and Y have to react to yield Z. The
direct reaction of X and Y requires a lot of activation energy. The catalyst works by
forming an intermediary with X or CX; Y reacts with CX forming CXY; this is converted to
CZ and finally Z is liberated freeing the catalyst C.

The
catalyst is able to facilitate the reaction of X and Y as it requires lesser energy to
form CX and for Y to react with CX to finally result in Z than the direct
reaction.