(Invited) Electrochemical CO2 Reduction By Violarite (FeNi2S4) As a Prebiotic Core of Carbon Monoxide Dehydrogenase
Authors: Nakamura Ryuhei 
Published: February 2020

The discovery of deep-sea hydrothermal vents on the late 1970’s has led to many hypotheses concerning chemical evolution in the prebiotic ocean. Such studies are motivated by the desire to understand microbial ecosystems which are isolated from energy input from photosynthesis and thereby sustained by geochemical energy sources, such as H2S and H2, emitted from hydrothermal vents. Simultaneously, the continuous emission and mineralization of strongly reducing iron sulfides (pirite and greigite etc) in hydrothermal environments leads to the “iron-sulfur world” concept, an idea that the surface of iron sulfides with trace metals such as Ni, Cu, and Mo in the vicinity of hydrothermal vents played an extensive role in prebiotic CO2reduction on the early Earth.
To deepen understanding of prebiotic CO2 reduction, we have examined geochemical processes associated with iron sulfides from the view point of the electrochemical functionalities. In 2010, we obtained a portion of mineral crust from the Mariner hydrothermal field in Black smoker chimney, and found that natural minerals themselves display an excellent metallic electrical conduction over a 10-cm distance and promotes electrocatalytic oxidation of H2S and H2 as well as electroreduction of O2 and ferric ions. [1] This finding promptly led us to propose the new type of CO2 reduction and carbon assimilation in the deep floor. Namely, not only the reductive chemicals such as H2 and H2S emitted from hydrothermal vents, but the high-energy electrons directly delivered from hot reduced hydrothermal trigers the electrocatalytic CO2 reduction with the same operation principal of an electrochemical fuel cell. [1-3]
To confirm the validity of this hypothesis, here, we examined the energetics of electrocatalytic CO2 reduction by iron sulfide (FeS) at slightly acidic pH. Our analyses demonstrated that FeS functions as efficient electrocatalysts for CO2 reduction upon the substitution of Fe with Ni to form FeNi2S4, and that the faraday efficiency for CO2 reduction is further increased by modification of the catalytic surface with amine compounds. [3] These findings are consistent with the concept that Ni/greigite serves as a prebiotic catalyst for CO2 reduction due to its structural similarity with the catalytic centers of carbon monoxide dehydrogenase (CODH) enzyme. [4] Thus, the findings in the present study are expected to provide great insights into the electron conduction and electrocatalytic functions of bisulfide-bearing hydrothermal for triggering and maintaining the electrosynthesis of organic compounds prior to the emergence of the primordial living forms.
References: (1) R. Nakamura,T. Takashima, S. Kato, K. Takai, M. Yamamoto, K. Hashimoto, Angewandte Chemie, 2010, 49, 7692-7694. (2) M. Yamamoto, R. Nakamura, K. Oguri, S. Kawagucci, S. Suzuki, K. Hashimoto, K. Takai, Angewandte Chemie, 2013, 52, 10758-10761. (3) A. Yamaguchi, M. Yamamoto, K. Takai, T. Ishii, K. Hashimoto, R. Nakamura, Electrochemica Acta, 2014, 141, 311-318 (4) M. J. Russell, R. M. Daniel, A. J. Hall, J. A. Sherringham, Journal of Molecular Evolution, 1994, 39, 231.