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

Abstract

An integrated process by coupling hydrothermal reactions, including CO₂ reduction and H₂O dissociation with metals, is proposed. The hydrogen could be rapidly produced under hydrothermal conditions, owing to the special characteristics of high temperature water, generating metal hydrides as intermediates. Hydrogen production from the H₂O dissociation under hydrothermal conditions is one of the most ideal processes due to its environmentally friendly impact. Recent experimental and theoretical studies on the hydrothermal reduction of CO₂ to value-added products by in situ generated metal hydrides are introduced, including the production of formic acid, methanol, methane, and long-chain hydrocarbons. These results indicate that this process holds promise in respect to the conversion of CO₂ to useful chemicals and fuels, and for hydrogen storage, which could help alleviate the problems of climate change and energy shortage.

Keywords: CO2; hydrogen storage; hydrogenation; hydrothermal reduction; metal hydrides.

Scheme 1

Proposed process of CO₂ reduction and hydrogen utilization [15].

Figure 1

Number of hydrogen bonds per water molecule [20].

Figure 2

Maximum theoretical hydrogen yield (a), reaction heat of the metal–water cycle (b) [25].

Figure 3

The effects of temperature and time on the yield of formic acid (a) and the yield of formic acid with longer reaction time (b) [13].

Figure 4

XRD patterns of ZnO (a) and the conversion rate of Zn into ZnO (b) (300 °C, Zn 10 mmol, NaHCO₃ 1 mmol) [14].

Figure 5

IR absorption spectra of samples (NaHCO₃ 1 mmol, 300 °C, Zn 10 mmol) [14].

Figure 6

Effects of Al and NaHCO₃ amounts on the yield of formic acid (300 °C, 2 h) [32].

Figure 7

Effects of reaction time and temperature on the formic acid yield (Al 6 mmol, NaHCO₃ 1 mmol) [32].

Figure 8

Effects of reaction time and reaction temperature on the formic acid yield (NaHCO₃ 2 mmol, Fe 12 mmol) [34].

Figure 9

Effects of temperature and time on the yield of formic acid (NaHCO₃ 1 mmol, Mn 8 mmol) [39].

Figure 10

Zn₅ cluster for the calculation [42].

Figure 11

Gibbs free energy of different patterns [42].

Scheme 2

Proposed mechanism of CO₂ reduction via zinc hydrides [43].

Figure 12

The geometries of TS (left) and activation energy of HCOO^- formation [43].

Figure 13

IRC calculation (a), HOMO (b) and LUMO (c) orbital shapes of the TS [43].

Figure 14

The geometries and charge of initial state (a), TS (b) and final state (c) [48].

Figure 15

IRC calculation (a), HOMO (b) and LUMO (c) orbital shapes of the TS [48].

Figure 16

Optimized geometric parameters of the transition state and the final state [49].

Figure 17

Potential energy diagram for HCOO⁻ formation [49].

Figure 18

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

Figure 19

Effect of Cu amount on the methanol yield (350 °C, 2 h; Zn 60 mmol, NaHCO₃ 40 mmol (or 60 mmol), HCl 2.0 M, water filling 50%) [53].

Figure 20

Proposed mechanism for the methanol formation [53].

Figure 21

In situ FTIR spectra of Cu/ZnO [55].

Figure 22

Effects of time and temperature on the methanol yield (a) 200 °C, (b) 70 h [60].

Figure 23

Proposed mechanism for methane formation [60].

Figure 24

Effects of reaction conditions on the methane yield (Ni 20 mmol, H₂O 10 mL, CO₂ 13 mmol; (a) Fe 0.1 mol; (b) 4 h; (c) Fe 0.1 mol, 300 °C; (d) Fe 0.1 mol, 300 °C, 4 h) [61].

Figure 25

Identification of hydrocarbons synthesized by the hydrothermal reduction of NaHCO₃ [70].

Figure 26

ATR-FTIR spectra (a), XRD patterns of solid products (b), and long-chain product distribution (c) [70].

Figure 27

Proposed pathways for the hydrocarbons formation [70].