Most substances are weakly repelled by a strong magnetic field while others are weakly attracted to it. When they are repelled, it is known as diamagnetism and when attracted it is known as paramagnetism.
Paramagnetism arises because electrons can be regarded as spinning on their axes and, just as an electric current flowing through a wire generates a magnetic moment, so does the spinning electron.
Electrons that occupy the same orbital (i.e. have opposite spins), have, of course, zero magnetic moment since the two contributions cancel each other.
A catalyst enables a reaction to take a different reaction pathway or mechanism of a lower overall activation energy.
Catalysts are regenerated at the end of a reaction therefore a small amount of catalyst will catalyse an infinite amount of reactants.
Catalysts do take part in a reaction but are not 'consumed'.
Transition metals are considered to be good catalysts, due to their ability to form a variety of different oxidation states and 'd' electrons allow adsorption onto their surface. This weakens bonds in reactant molecules.
Transition metals tend to be heterogenous catalysts meaning, they are in a different state (usually solid) than the products.
For example: Nickel powder is used in the hydrogenation of C=C, as in ethene. Ni atoms provide sites for holding C=C and H2 in correct orientation for bonding.