Atomistic simulation of the formation at NH3 at the pyrite surface under hydrothermal conditions

Stirling1, T. Rozgony1, M. Krack2 and M. Bernasconi3

1.Institute of Organic Chemistry, Research Centre for Natural Sciences of the Hungarian Academy of Sciences, Budapest, Hungary

2.Laboratory for Reactor Physics and Systems Behaviour, Paul Scherrer Institute, Villigen, Switzerland

3.Department of Materials Science, University of Milano-Bicocca, Milano, Italy

Iron sulfides play an important role in various geochemical and environmental processes. Their oxidation is responsible for acid production in mine wastewater which is of great environmental concern. Moreover, iron-sulfides have been proposed to act as catalysts in the synthesis of prebiotic molecules in hydrothermal vents. This feature is the basis for the so called “”iron-sulfur world”” scenario for the origin of life which was stimulated by the discovery of complex ecosystems close to the black smokers in the deep ocean [1]. Seawater penetrating through cracks and fissures hundreds of meters deep into the oceanic crust is heated at supercritical condition. In this environment, iron-sulfide minerals are available abundantly providing conditions suitable for prebiotic redox processes.  An important constituent of the prebiotic pool of reactants is the most reduced nitrogen compound NH3 which is a necessary precursor for nitrogen-containing organic molecules.

In this contribution, we report on the simulation of the prebiotic synthesis of ammonia from 
NO3- and NO2- at FeS2 surfaces under hydrothermal
 conditions [2]. Ab initio metadynamics simulations 
have successfully uncovered a full reaction path consistent with experimental reaction rates 
[3]. We have found that the 
reaction mechanism consists of several stepwise single
 atom transfers. The kinetic bottleneck of the full process is the reduction of NO-  which starts preferably at surface defect sites. The supercritical hydrothermal conditions provide a particularly suitable interface arrangement where surface Fe sites are not covered by water thus providing room for both adsorption and reduction of nitrogen oxides.

[1] G. Waechtershaüser, Microbiol. Rev. 52, 452 (1988); E. Blöchl, M. Keller, G. Waechtershaüser, O.K. Stetter, Proc. Natl. Acad. Sci. U.S.A. 89, 8117 (1992).

[2] A. Stirling, T. Rozgony, M. Krack, and M. Bernasconi, Inorg. Chem. 55, 1984 (2016).

[3] J.A. Brandes et al., Nature 395, 365 (1998).