Thermal formation of ammonium carbamate on the surface of laboratory
  analogues of carbonaceous grains in protostellar envelopes and planet-forming
  disks
  The catalytic role of dust grain surfaces in the thermal reaction CO2 + 2NH3
$\rightarrow$ NH4+NH2COO was recently demonstrated by our group. The rate
coefficients for the reaction at 80 K on the surface of nanometre-sized carbon
and silicate grains were measured to be up to three times higher compared to
the reaction rate coefficients measured on KBr. In this study, the reaction was
performed on carbon grains and on KBr in the extended temperature range of 50 -
80 K and with the addition of water ice. The reaction activation energy was
found to be about 3 times lower on grains compared to the corresponding ice
layer on KBr. Thus, the catalytic role of the dust grain surface in the studied
reaction can be related to a reduction of the reaction barrier. Addition of
water to NH3:CO2 ice on grains slowed the reaction down. At the H2O:CO2 ratio
of 5:1, the reaction was not detected on the experimental timescale. This
result calls into question the thermal formation of ammonium carbamate in dense
molecular clouds and outer regions of protostellar and protoplanetary
environments with dominating water ice mantle chemistry. However, it can still
happen in inner regions of protostellar and protoplanetary environments in
crystalline ices.
