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The catalytic role of dust grain surfaces in the thermal reaction CO2 + 2NH3$\rightarrow$ NH4+NH2COO was recently demonstrated by our group. The ratecoefficients for the reaction at 80 K on the surface of nanometre-sized carbonand silicate grains were measured to be up to three times higher compared tothe reaction rate coefficients measured on KBr. In this study, the reaction wasperformed 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 wasfound to be about 3 times lower on grains compared to the corresponding icelayer on KBr. Thus, the catalytic role of the dust grain surface in the studiedreaction can be related to a reduction of the reaction barrier. Addition ofwater to NH3:CO2 ice on grains slowed the reaction down. At the H2O:CO2 ratioof 5:1, the reaction was not detected on the experimental timescale. Thisresult calls into question the thermal formation of ammonium carbamate in densemolecular clouds and outer regions of protostellar and protoplanetaryenvironments with dominating water ice mantle chemistry. However, it can stillhappen in inner regions of protostellar and protoplanetary environments incrystalline ices.

astrophysical journal/the astrophysical journal

Thermal formation of ammonium carbamate on the surface of laboratory  analogues of carbonaceous grains in protostellar envelopes and planet-forming  disks