New Insight into CO2 Reduction to Formate by In Situ Hydrogen Produced from Hydrothermal Reactions with Iron

Abstract

To reveal the nature of CO2 reduction to formate with high efficiency by in situ hydrogen produced from hydrothermal reactions with iron, DFT calculations were used. A reaction pathway was proposed in which the formate was produced through the key intermediate species, namely iron hydride, produced in situ in the process of hydrogen gas production. In the in situ hydrogenation of CO2, the charge of H in the iron hydride was −0.135, and the Fe–H bond distance was approximately 1.537 Å. A C-H bond was formed as a transition state during the attack of Hδ− on Cδ+. Finally, a HCOO species was formed. The distance of the C-H bond was 1.107 Å. The calculated free energy barrier was 16.43 kcal/mol. This study may provide new insight into CO2 reduction to formate in hydrothermal reactions with metal.

Recent advances in CO2 reduction with renewable reductants under hydrothermal conditions: towards efficient and net carbon benefit CO2 conversion

“…Ultimately, this process leads to the formation of formate accompanied by H 2 O. Furthermore, DFT calculations suggest that the Fe-H δ− species is formed via the dissociation of water 73 (Figure 5c and d). Mulliken charge of H in the iron hydride was −0.135 in HCO 3hydrogenation process, wherein a C-H bond forms in a transition state through the attack of H δ− on C δ+ .…”

Hydrothermal Reduction of CO2 to Value-Added Products by In Situ Generated Metal Hydrides

“…A theoretical study was conducted by using the DFT calculations [ 49 ]. Under hydrothermal reaction conditions, the in situ technique for experimentally studying the reaction mechanism of formate production is very challenging.…”

“… IRC calculation results ( a ) and HOMO ( b ) and LUMO ( c ) orbital shapes of the transition state [ 49 ]. …”

Autocatalysis and CO2: Carbon Capture and Utilization (CCU) Meets Natural Processes

“…Instead, iron(II) carbonate was observed as an intermediate and hence a bicarbonate promoted pathway involving Fe(HCO 3 ), FeCO 3 and FeO was proposed for oxidation of iron(0) to Fe 3 O 4 . [22,29] Nevertheless, intermediate hydroxides were reported with Al, Zn and Mn reducing agents, where Al(OH) 3 , [25] Zn(OH) 2 [30] and Mn(OH) 2 [24] form before their dehydration to the corresponding oxides. Although they are not as abundant as iron on the seafloor, zinc and manganese can still be found in μmol to mmol/kg quantities, [31] sufficient to play a role in catalysis.…”

“…Formation of Fe(OH) 2 , postulated to take place during serpentinization of olivine, was not observed in the laboratory reactions. Instead, iron(II) carbonate was observed as an intermediate and hence a bicarbonate promoted pathway involving Fe(HCO 3 ), FeCO 3 and FeO was proposed for oxidation of iron(0) to Fe 3 O 4 [22,29] . Nevertheless, intermediate hydroxides were reported with Al, Zn and Mn reducing agents, where Al(OH) 3 , [25] Zn(OH) 2 [30] and Mn(OH) 2 [24] form before their dehydration to the corresponding oxides.…”