  Context. It is generally agreed that hydrogenation reactions dominate
chemistry on grain surfaces in cold, dense molecular cores, saturating the
molecules present in ice mantles. Aims. We present a study of the low
temperature reactivity of solid phase isocyanic acid (HNCO) with hydrogen
atoms, with the aim of elucidating its reaction network. Methods. Fourier
transform infrared spectroscopy and mass spectrometry were employed to follow
the evolution of pure HNCO ice during bombardment with H atoms. Both multilayer
and monolayer regimes were investigated. Results. The hydrogenation of HNCO
does not produce detectable amounts of formamide (NH2CHO) as the major product.
Experiments using deuterium reveal that deuteration of solid HNCO occurs
rapidly, probably via cyclic reaction paths regenerating HNCO. Chemical
desorption during these reaction cycles leads to loss of HNCO from the surface.
Conclusions. It is unlikely that significant quantities of NH2CHO form from
HNCO. In dense regions, however, deuteration of HNCO will occur. HNCO and DNCO
will be introduced into the gas phase, even at low temperatures, as a result of
chemical desorption.
