Assuming interstellar dust particles being composed at least partly of materials with catalytic activity or of refractory metals and being exposed to the radiation field of the Sun or other stars they can favour the formation of molecules as combinations of the most abundant gases H, C, N and O mainly for three reasons:

(1) By the adsorption at the surface of dust particles the collision probability for two or more atoms is greatly enhanced with respect to the gas phase.

(2) Because of the adsorption bond the excitation and dissociation energies are shifted to lower values.

(3) For highly exothermic reactions dust particles can absorb the excess energy and thus lead to the formation of molecules which in the gas phase can be formed only by three body collisions.