Thermal Formation of Ammonium Carbamate on the Surface of Laboratory Analogs of Carbonaceous Grains in Protostellar Envelopes and Planet-forming Disks

The catalytic role of dust grain surfaces in the thermal reaction CO₂ + 2NH₃ → NH₄⁺NH₂COO^- was recently demonstrated by our group. The rate coefficients for the reaction at 80 K on the surface of nanometer-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 three 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.