(Invited) Electrochemical CO2 Reduction By Violarite (FeNi2S4) As a Prebiotic Core of Carbon Monoxide Dehydrogenase

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.