Direct Synthesis of Formamide from CO2 and H2O with Nickel-Iron Nitride Heterostructures under Mild Hydrothermal Conditions

Abstract

Formamide can serve as a key building block for the synthesis of organic molecules relevant to premetabolic processes. Natural pathways for its synthesis from CO₂ under early earth conditions are lacking. Here, we report the thermocatalytic conversion of CO₂ and H₂O to formate and formamide over Ni-Fe nitride heterostructures in the absence of synthetic H₂ and N₂ under mild hydrothermal conditions. While water molecules act as both a solvent and hydrogen source, metal nitrides serve as nitrogen sources to produce formamide in the temperature range of 25-100 °C under 5-50 bar. Longer reaction times promote the C-C bond coupling and formation of acetate and acetamide as additional products. Besides liquid products, methane and ethane are also produced as gas-phase products. Postreaction characterization of Ni-Fe nitride particles reveals structural alteration and provides insights into the potential reaction mechanism. The findings indicate that gaseous CO₂ can serve as a carbon source for the formation of C-N bonds in formamide and acetamide over the Ni-Fe nitride heterostructure under simulated hydrothermal vent conditions.

Figure 1

TEM (a), HR-TEM (b), STEM (c), corresponding STEM–EDX elemental mapping (d–f), Ni 2p (g), Fe 2p (h), and N 1s (i) spectra of Ni₃FeN/Ni₃Fe prepared at 350 °C for 2 h.

Figure 2

¹H NMR spectra of the products (with their molecular structures) obtained under 25 bar CO₂ at different temperatures (a), under different initial CO₂ pressures at 100 °C (b), and at different initial pH values under 25 bar of CO₂ at 100 °C (c) over the Ni₃FeN/Ni₃Fe-350-2h heterostructure after 16 h in H₂O. Concentrations of obtained products (calculated from related ¹H NMR spectra) at different temperatures (d), diverse initial CO₂ pressures (e), and different reaction pH values (f). Ft: formate, Fd: formamide. Error bars represent the standard deviations of at least two independent reactions.

Figure 3

¹H NMR spectra of the products obtained with different amounts of the catalyst under 25 bar CO₂ for 16 h (a) and products after different reaction times with 0.5 mmol catalyst 25 bar CO₂ at 100 °C (b), concentrations of obtained products with different amounts of catalysts, obtained from ¹H NMR spectra in panel (a) (c), and after different reaction times, obtained from ¹H NMR spectra in panel (b) (d). Ft: formate, fd: formamide, ND: not detected. Error bars represent the standard deviations of at least two independent reactions.

Figure 4

Postreaction XRD patterns of the Ni₃FeN/Ni₃Fe-350-2h catalyst at different reaction temperatures with 25 bar of CO₂ (a), with different CO₂ pressures at a reaction temperature of 100 °C (b), and at different initial pH values at 100 °C after 16 h of reaction time (c). SEM (d), TEM (e), and HR-TEM (f) images of the Ni₃FeN/Ni₃Fe-350-2h after the catalytic reaction under 25 bar of CO₂ at pH 6 at 100 °C for 16 h.

Figure 5

Possible reaction pathway for the formation of amides from CO₂ and H₂O over the Ni₃FeN/Ni₃Fe heterostructure.