Probing surface reactivity of amorphous and crystalline Mg2SiO4 by CO, CO2, CD3CN and HCN adsorption

Forsterite is a nesosilicate with Mg₂SiO₄ composition, representing the iron-free endmember of the family of olivines. Interestingly, aside its finding as mineral component of the Earth’s crust and mantle, it has been observed in extraterrestrial environments where have been found as principal inorganic constituents of meteorites, comets and in the core of dust grains found in the interstellar medium and in protoplanetary disks [1]. It has often been indicated that these grains could be possible sites for the occurrence of astrochemical reactions involving simple prebiotic molecules (SPM, e.g. CO, CO₂, H₂O, HCN, etc.). In order to assess the role of Mg₂SiO₄ in this prebiotic scenario, the understanding of the surface properties of grain-core phases is of primary importance. We thoroughly characterized two Mg₂SiO₄ model samples, namely an amorphous magnesium silicate (AMS) and a crystalline forsterite (Forst), and we compared their surface acid-base properties as a function of their structure (amorphous vs crystalline) [2]. The AMS sample was synthesized via thermal plasma route, as proposed by Koike et al. [3], whereas Forst was obtained by thermal annealing of the former at 1073 K. The acid-base properties of the two systems were investigated by monitoring the adsorption of selected probe molecules (CO, CO₂ and CD₃CN) by IR spectroscopy and then by studying the perturbation of their vibrational fingerprints upon interaction with specific surface functionalities. As an example, the IR spectra of CD₃CN adsorbed on AMS and Forst in the υ(CN) frequency range are shown in Figure 1. The spectra for both materials are characterized by three main bands at 2300, 2266 and 2215 cm^-1. The 2300 cm^-1 component is assigned to CD₃CN interacting with Lewis acid sites, i.e. surface Mg²⁺ cations, whereas the 2266 cm^-1 band is assigned to the features of CD₃CN interacting with weak Bronsted acid sites (surface silanols) and of not interacting CD₃CN forming multilayers. These components are closely similar among both samples. Instead, they remarkably differ with respect to the 2215 cm^-1 band, which is much more intense in AMS than in Forst. We assigned this signal to anionic species formed by deprotonation of acetonitrile in presence of strong surface basic sites. These results, confirmed also by the adsorption of other probe molecules, showed as AMS and Forst have a similar population of surface acid sites, whereas only the former shows a significant surface basicity.

Finally, we investigated HCN adsorption on both samples. HCN is a molecule of particular interest since its adsorption and reactivity on silicates in interstellar media is supposed to play a role in the synthesis of biomolecules contributing to the development of life [4]. The HCN reaction leads to the formation of a variety of oligomers (at least up to 12 terms) and the surface basicity, already highlighted by the other probe molecules, seems to play a key role in the process.

                                                                       

Figure 1. IR spectra of AMS and Forst outgassed for 2 hours at 673 K in vacuum and contacted with 40 mbar of CD₃CN at r.t. (curves a) and outgassed for 30 min at r.t. after CD₃CN contact (curves b). Spectra collected at decreasing CD₃CN pressures are reported as thin lines for both materials. The contribution from the bare activated material has been subtracted. The spectra have been normalized to the specific surface area of the samples.

 

References

[1] T. Henning, Cosmic Silicates. Annu. Rev. Astron. Astrophys. 2010, 48, 21–46.

[2] M. Signorile, L. Zamirri, A. Tsuchiyama, P. Ugliengo, F. Bonino, G. Martra, ACS Earth Sp. Chem. 2020, 4, 345–354.

[3] C. Koike, Y. Imai, H. Chihara, H. Suto, K. Murata, A. Tsuchiyama, S. Tachibana, S. Ohara, Astrophys. J. 2010, 709, 983–992

[4] J. P. Ferris, and W. J. Hagan Jr, Tetrahedron. 1984, 40, 1093-1120.