  Surface chemistry on cosmic dust grains plays an important role in the
formation of molecules at low temperatures in the interstellar and
circumstellar environments. For the first time, we experimentally put in
evidence the catalytic role of dust surfaces using the thermal reaction CO2 +
2NH3 = NH4+NH2COO-, which is also a proxy of radical-radical reactions.
Nanometre-sized amorphous silicate and carbon grains produced in our laboratory
were used as grain analogues. Surface catalysis on grains accelerates the
kinetics of the reaction studied at a temperature of 80 K by a factor of up to
3 compared to the reaction occurring in the molecular solid. The evidence of
the catalytic effect of grain surfaces opens a door for experiments and
calculations on the surface formation of interstellar and circumstellar
molecules on dust. Ammonium carbamate on the surface of grains or released
intact into protostellar or protoplanetary disk phases can give start to a
network of prebiotic reactions. Therefore, there should be a great interest to
search for ammonium carbamate and its daughter molecule, carbamic acid, in
interstellar clouds, protostellar envelopes, and protoplanetary disks.
