Serpentinizing hydrothermal systems generate H 2 as a reductant and harbor catalysts conducive to geochemical CO 2 conversion into reduced carbon compounds that form the core of microbial autotrophic metabolism. This study characterizes mineral catalysts at hydrothermal vents by investigating the interactions between catalytically active cobalt sites and silica-based support materials on H 2 -dependent CO 2 reduction. Heteroatom incorporated (Mg, Al, Ca, Ti, and Zr), ordered mesoporous silicas are applied as model support systems for the cobalt-based catalysts. It is demonstrated that all catalysts surveyed convert CO 2 to methane, methanol, carbon monoxide, and low-molecular-weight hydrocarbons at 180 °C and 20 bar, but with different activity and selectivity depending on the support modification. The additional analysis of the condensed product phase reveals the formation of oxygenates such as formate and acetate, which are key intermediates in the ancient acetyl-coenzyme A pathway of carbon metabolism. The Ti-incorporated catalyst yielded the highest concentrations of formate (3.6 mM) and acetate (1.2 mM) in the liquid phase. Chemisorption experiments including H 2 temperature-programmed reduction (TPR) and CO 2 temperatureprogrammed desorption (TPD) in agreement with density functional theory (DFT) calculations of the adsorption energy of CO 2 suggest metallic cobalt as the preferential adsorption site for CO 2 compared to hardly reducible cobalt−metal oxide interface species. The ratios of the respective cobalt species vary depending on the interaction strength with the support materials. The findings reveal robust and biologically relevant catalytic activities of silica-based transition metal minerals in H 2 -rich CO 2 fixation, in line with the idea that autotrophic metabolism emerged at hydrothermal vents.

The chemical reactions that formed the building blocks of life at origins required catalysts, whereby the nature of those catalysts influenced the type of products that accumulated. Recent investigations have shown that at 100 °C awaruite, a Ni3Fe alloy that naturally occurs in serpentinizing systems, is an efficient catalyst for CO2 conversion to formate, acetate, and pyruvate. These products are identical with the intermediates and products of the acetyl-CoA pathway, the most ancient CO2 fixation pathway and the backbone of carbon metabolism in H2-dependent autotrophic microbes. Here, we show that Ni3Fe nanoparticles prepared via the hard-templating method catalyze the conversion of H2 and CO2 to formate, acetate and pyruvate at 25 °C under 25 bar. Furthermore, the 13C-labeled pyruvate can be further converted to acetate, parapyruvate, and citramalate over Ni, Fe, and Ni3Fe nanoparticles at room temperature within one hour. These findings strongly suggest that awaruite can catalyze both the formation of citramalate, the C5 product of pyruvate condensation with acetyl-CoA in microbial carbon metabolism, from pyruvate and the formation of pyruvate from CO2 at very moderate reaction conditions without organic catalysts. These results align well with theories for an autotrophic origin of microbial metabolism under hydrothermal vent conditions.

Abiotic synthesis of formate and short hydrocarbons takes place in serpentinizing vents where some members of vent microbial communities live on abiotic formate as their main carbon source. To better understand the catalytic properties of NiÀ Fe minerals that naturally exist in hydrothermal vents, we have investigated the ability of synthetic NiÀ Fe based nanoparticular solids to catalyze the H 2 -dependent reduction of CO 2 , the first step required for the beginning of prebiotic chemistry. Mono and bimetallic NiÀ Fe nanoparticles with varied Ni-to-Fe ratios transform CO 2 and H 2 into intermediates and products of the acetylcoenzyme A pathway-formate, acetate, and pyruvatein mM range under mild hydrothermal conditions. Furthermore, NiÀ Fe catalysts converted CO 2 to similar products without molecular H 2 by using water as a hydrogen source. Both CO 2 chemisorption analysis and post-reaction characterization of materials indicate that Ni and Fe metals play complementary roles for CO 2 fixation.

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Graphene aerogels decorated with nickel/nickel oxide core–shell nanoparticles are synthesized via a one-pot hydrothermal reaction followed by thermal reduction. Nickel loading is varied from 1.5 to 40 wt % in a highly controlled fashion by simply tuning Ni2+ to graphene oxide ratio in the hydrothermal reaction mixture. In fact, graphene aerogel loaded with 40 wt % Ni has a BET surface area of 560 m2 g–1 due to the preservation of the porous structure with the aid of supercritical CO2 drying. The thermal reduction applied for the growth of nanoparticles induces deoxygenation of graphene aerogel structure simultaneously. Valence band spectroscopy results reveal an increase in metallicity with a gradual increase in the nickel loading, which has a direct impact on the final electrocatalytic performance. For the evaluation of the electrocatalytic activity of Ni/NiO loaded aerogels toward oxygen evolution reaction, cyclic voltammetry (coupled with a rotating disk electrode) is utilized. In fact, 40 wt % Ni loaded graphene aerogel requires a low overpotential of 320 mV for the supply of 10 mA cm–2 (with a Tafel slope of 61 mV dec–1 and a TOF of 0.11 s–1), which outperforms most of the Ni-based electrocatalysts reported in the literature.