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


IR spectra taken after the deposition of a H2O:CO2:NH3 (2:1:4) mixture on carbon grains at 15 K and after 4 hours at 75 K. Two bands related to NH4 +NH2COO- are marked by asterisks. The 75 K spectrum is vertically shifted for clarity.

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 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.

Alexey Potapov, Cornelia Jäger, Thomas Henning
(Submitted on 4 Apr 2020)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2004.01982 [astro-ph.GA] (or arXiv:2004.01982v1 [astro-ph.GA] for this version)
Submission history
From: Alexey Potapov
[v1] Sat, 4 Apr 2020 17:35:07 UTC (353 KB)
https://arxiv.org/abs/2004.01982
Astrobiology, Astrochemistry

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