Effects of Silica Modification (Mg, Al, Ca, Ti, and Zr) on Supported Cobalt Catalysts for H2-Dependent CO2 Reduction to Metabolic Intermediates

Abstract

Serpentinizing hydrothermal systems generate H₂ as a reductant and harbor catalysts conducive to geochemical CO₂ 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 H₂-dependent CO₂ 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 CO₂ 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 H₂ temperature-programmed reduction (TPR) and CO₂ temperature-programmed desorption (TPD) in agreement with density functional theory (DFT) calculations of the adsorption energy of CO₂ suggest metallic cobalt as the preferential adsorption site for CO₂ 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 H₂-rich CO₂ fixation, in line with the idea that autotrophic metabolism emerged at hydrothermal vents.

Figure 1

Dark-field high resolution scanning transmission electron microscopy (HR-STEM) micrographs and energy dispersive X-ray spectroscopy (EDX) elemental mappings of selected catalysts 10 wt % (a) Co/SBA-15, (b) Co/Mg–SBA-15, and (c) Co/Ti–SBA-15.

Figure 2

HPLC results for concentration of oxygenate products from CO₂ hydrogenation with 10 wt % Co/M–SBA-15 catalysts (M = Mg, Al, Ti) collected after 72 h time-on-stream. D: detected, concentration <0.2 mM. 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 and the measurement of the liquid phase sample at different times-on-stream. The deviation for acetate was below the accuracy of the method (±0.1 mM), as well as the deviation for formate in case of Co/SBA-15.

Figure 3

(a) Effect of Co-loading on catalytic performance of Co/SBA-15 catalysts. (b) Catalytic activity and product selectivity for CH₄, methanol, CO, and C₂₊ hydrocarbons of 10 wt % Co/M–SBA-15 catalysts (M = Mg, Al, Ca, Ti, Zr). Reaction conditions: T = 180 °C, p = 2.0 MPa, H₂/CO₂/Ar = 6:3:1, 4000 cm³ h^–1 g_cat^–1, 36 h time-on-stream. Exemplary error bars are shown based on the reproduction of the reaction with different catalyst batches. For the conversion with Co/Zr–SBA-15, the deviation is smaller than the symbol size.

Figure 4

(a) H₂-TPR profiles of Co₃O₄/M–SBA-15 catalysts and (b) CO₂-TPD profiles of Co/M–SBA-15 catalysts. (c) Correlation between the amounts of consumed H₂ in H₂-TPR and desorbed CO₂ in CO₂-TPD of Co/M–SBA-15 materials. Adsorption structures of CO₂ on (d) the Co⁰ and (e) the Co^δ+ species. q(site) represents the sum of the Hirshfeld charges of the two cobalt atoms at the adsorption site before the interaction with CO₂. E_ads is the adsorption energy. The Co₂₀ cluster, CO₂, and SiO₄ units directly bonded to cobalt are highlighted as the ball-and-stick model. The other part of the silica support is shown as the wire-frame model. (f) Correlation between CO₂ conversion and amount of desorbed CO₂ in CO₂-TPD of 5, 10, and 20 wt % Co/SBA-15 catalysts and (g) 10 wt % Co/M–SBA-15 (M = Mg, Al, Ca, Ti, Zr).

Figure 5

(a) FTIR spectra of adsorbed pyridine on M–SBA-15 support materials after evacuation at 150 °C (B: Brønsted acid sites, L: Lewis acid sites). (b) NH₃-TPD profiles of M–SBA-15 support materials (M = Mg, Al, Ca, Ti, Zr).

Figure 6

Simplified reaction pathways with key intermediates of CO₂ hydrogenation to methanol and selected side reactions (dashed arrows). *X indicates adsorbed species.