Synergistic Effects of Silica-Supported Iron-Cobalt Catalysts for CO2 Reduction to Prebiotic Organics

Abstract

To test the ability of geochemical surfaces in serpentinizing hydrothermal systems to catalyze reactions from which metabolism arose, we investigated H₂-dependent CO₂ reduction toward metabolic intermediates over silica-supported Co-Fe catalysts. Supported catalysts converted CO₂ to various products at 180 °C and 2.0 MPa. The liquid product phase included formate, acetate, and ethanol, while the gaseous product phase consisted of CH₄, CO, methanol, and C₂-C₇ linear hydrocarbons. The 1/1 ratio CoFe alloy with the same composition as the natural mineral wairauite yielded the highest concentrations of formate (6.0 mM) and acetate (0.8 mM), which are key intermediates in the acetyl-coenzyme A (acetyl-CoA) pathway of CO₂ fixation. While Co-rich catalysts were proficient at hydrogenation, yielding mostly CH₄, Fe-rich catalysts favored the formation of CO and methanol. Mechanistic studies indicated intermediate hydrogenation and C-C coupling activities of alloyed CoFe, in contrast to physical mixtures of both metals. Co in the active site of Co-Fe catalysts performed a similar reaction as tetrapyrrole-coordinated Co in the corrinoid iron-sulfur (CoFeS) methyl transferase in the acetyl-CoA pathway. In a temperature range characteristic for deeper regions of serpentinizing systems, oxygenate product formation was favored at lower, more biocompatible temperatures.

Keywords: CO2 Fixation; CoFe Alloy; Heterogeneous catalysis; Hydrothermal Vent; Prebiotic Chemistry.

Figure 1

(a) H₂-TPR profiles of calcined Co−Fe catalysts. (b) Comparison of H₂-TPR profiles of selected Co−Fe catalysts after calcination (dark color) and after reduction with H₂ at 450 °C with subsequent surface passivation (light color).

Figure 2

(a) HAADF image, (b) Co and (c) Fe EDX elemental mappings, (d) combined Fe and Co elemental mappings (arrow indicates the region for the EDX line scan), (e) EDX line scan, and (f) HR–TEM micrograph of the silica-supported CoFe catalyst.

Figure 3

Concentrations of oxygenate products from CO₂ hydrogenation with silica-supported Co−Fe catalysts determined by HPLC for the liquid product phase collected after 72 h time-on-stream. Exemplary error bars are shown based on the reproduction of the reaction with different catalyst batches. If not shown, the deviation in the formate and acetate concentrations was below the accuracy of the HPLC method (±0.1 mM).

Figure 4

(a) Catalytic activity and (b) product selectivity for CH₄, methanol, CO, and C₂₊ hydrocarbons determined by online GC after 54 h time-on-stream. Reaction conditions: T = 180 °C, p=2.0 MPa, H₂/CO₂/Ar=6:3:1, 4000 cm³ h⁻¹ g_cat⁻¹. Exemplary error bars are shown based on the reproduction of the reaction with different catalyst batches.

Figure 5

Schematic representation of product formation in CO₂ reduction on silica-supported Fe-rich, Co-rich, and Co−Fe alloy catalysts. For simplicity the materials are presented without the silica support.

Figure 6. H₂-TPD profiles of silica-supported Co−Fe catalysts tracked by QMS at m/z = 2.

Figure 7

Gas phase product selectivities as a function of the reactant gas space velocity (H₂/CO₂/Ar=6:3:1), defined as volume flow per hour and mass of catalyst, for silica-supported (a) Fe, (b) CoFe, and (c) Co. Reaction conditions: T=180 °C, p=2.0 MPa. Exemplary error bars are shown based on the reproduction of the reaction with different catalyst batches.

Figure 8

(a) CO₂ conversions and gas phase product selectivities of (b) CH₄, (c) CO, and (d) methanol as a function of the reaction temperature. Reaction conditions: p=2.0 MPa, H₂/CO₂/Ar=6:3:1, 4000 cm³ h⁻¹ g_cat⁻¹. Exemplary error bars are shown based on the reproduction of the reaction with different catalyst batches.