Alkali and Alkaline Earth Metal Hydrides Driven Thermochemical Reduction of CO₂ to CH₃OH at Ambient Pressure

Abstract

The reduction of carbon dioxide (CO₂) to methanol (CH₃OH), is presently constrained by energy-intensive processes requiring high temperatures and pressures to overcome kinetic and thermodynamic limitations. This work unveils for the first time a reaction pathway for alkali and alkaline earth metal hydrides (LiH, NaH, KH, MgH₂, CaH₂, and BaH₂), enabling the activation and selective reduction of CO₂ to methanol under ambient pressure. This finding represents a significant departure from the established paradigm, which exclusively produced methane at moderate pressures (0.1–1 MPa). Temperature-programmed reaction studies reveal that these metal hydrides function synergistically as both hydrogen sources and promoters for CO₂ reduction, exhibiting hydrocarbon production within specific temperature windows. Systematic screening identifies lithium hydride (LiH) as the most effective material, with a maximum production rate of 0.182 μmol_MeOH·mol_metal ^–1·h^–1 at 245 °C under ambient pressure, highlighting a structure–activity relationship governed by metal-formate stability. Through mechanistic investigation, we elucidate a formate (HCOO*)-mediated pathway wherein the hydride ion serves as a potent nucleophile, directly reducing CO₂ without the need for high H₂ partial pressures. This discovery provides a transformative framework for the sustainable synthesis of CH₃OH and lays the groundwork for the development of next-generation materials incorporating reactive ionic hydrides for the conversion of CO₂ under mild conditions.

1. Introduction

Global population growth continues to drive an unprecedented energy demand, which has historically been met through the consumption of fossil fuels. − These finite resources, primarily coal, natural gas, and oil, are fundamental to modern society, supporting electricity generation, heating, and the production of chemicals. However, their extensive use comes at a significant cost: the rapid depletion of reserves and the release of anthropogenic carbon dioxide (CO₂) into the atmosphere. − The resulting emissions of CO₂ are a major contributor to global climate change. The consequences are widespread, including global warming, ocean acidification, rising sea levels, and the most frequent occurrences of natural disasters. CO₂ is a nontoxic, abundant, and available one-carbon (C1) feedstock. Consequently, considerable scientific and industrial efforts have been spurred to develop efficient pathways for the conversion of captured CO₂ into value-added products, notably liquid fuels and platform chemicals such as formic acid (HCOOH), methane (CH₄), and methanol (CH₃OH). − Among various potential fuels and chemicals, CH₃OH is an ideal clean fuel and a versatile chemical feedstock for synthesizing numerous value-added products. Furthermore, it serves as an effective H₂ energy carrier and storage material (12.5 wt %) due to its high energy density, limited risk, relatively low toxicity, and easy transportation and storage. , Thus, the production of sustainable methanol via CO₂ conversion represents a cornerstone of the circular carbon economy. Nevertheless, the catalytic conversion of CO₂ to CH₃OH is inherently challenging due to the molecule’s high thermodynamic stability and kinetic inertness. These properties necessitate severe reaction conditions, including elevated temperatures and pressures, to drive the reaction forward at appreciable rates. Moreover, its fundamental molecular structure is a linear, closed-shell molecule in its electronic ground state, which poses a significant challenge, as it impedes effective adsorption and activation during transformation. In general, the reduction of CO₂ to CH₃OH takes place in the presence of H₂ over a catalyst. − It is an exothermic reaction as per eqs and . ,, The selective reduction of CO₂ to methanol proceeds stoichiometrically via eq , consuming four moles of reactants to yield two moles of products. This molar contraction necessitates elevated pressures (3–10 MPa) to favor methanol formation thermodynamically, while moderate temperatures (∼250 °C) balance kinetic activity with suppression of the competing endothermic reverse water–gas shift (RWGS) reaction (eq ). Although RWGS is favored at higher temperatures, the CO produced may subsequently undergo reduction to methanol (eq ), implying a parallel reaction pathway. Optimal process conditions are therefore critical to maximize selectivity toward methanol over CO.

$${\mathrm{C}\mathrm{O}}_2+3{\mathrm{H}}_2{=\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}{\mathrm{\Delta }H}_{298K}=-49.5\mathrm{K}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$ 1

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2=\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}{\mathrm{\Delta }H}_{298K}=+41.1\mathrm{K}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$ 2

$$\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2={\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}{\mathrm{\Delta }H}_{298K}=-90.7\mathrm{K}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$ 3

In contrast to this conventional Thermocatalytic process, which requires harsh conditions, alternative strategies are being developed to mediate CO₂ reduction under milder environments. Catalyst-free CO₂ reduction to value-added chemicals has been widely studied and reviewed. Notably, metal hydrides have recently emerged as promising platforms for the chemical reduction of CO₂, demonstrating efficient pathways to produce methane (CH₄) at significantly reduced pressures. Metal hydrides are a class of superior solid-state materials extensively investigated for chemical hydrogen storage. , Notably, due to their abundant and newly developed bonding sites, and enhanced adsorption–desorption hydrogen reversibility.

Among light metal hydrides, ionic hydrides such as alkali (i.e., LiH, NaH) and alkaline earth metal hydrides (i.e., MgH₂, CaH₂) are particularly attractive due to their high gravimetric hydrogen capacities (4.7–12.5 wt %). , Addressing the significant safety challenges associated with the H₂ molecule, alkali and alkaline earth metal hydrides offer a considerably safer alternative for handling and transportation. Consequently, they are employed as potent direct reductants, leveraging their highly reactive hydride ions (H^–) to activate and convert CO₂. Their strong reducing power, which exceeds that of molecular H₂ gas, enables these reactions to proceed under comparatively milder conditions. Recent years have witnessed growing interest in leveraging metal hydrides for CO₂ methanation, where they function as both a hydrogen source and a reaction promoter. Notably, several studies have demonstrated the feasibility of catalyst-free CO₂ reduction through mechanochemical and thermochemical pathways, utilizing gas–solid or solid–solid reaction systems under 48 h static conditions. ,−

Dong et al. demonstrated that alkali (LiH, NaH) and alkaline earth (MgH₂, CaH₂) metal hydrides can function synergistically as both hydrogen sources and promoters for the catalyst-free thermochemical reduction of CO₂, at 450 °C. Complementary to thermochemical approaches, mechanochemical strategies have also been successfully employed for CO₂ methanation. Independent studies have demonstrated the catalyst-free methanation of CO₂ via ball-milling at ambient temperature and moderate pressures (0.25–1 MPa), with reaction yields being highly dependent on milling rate and operating pressure. , Both thermochemical and mechanochemical reduction reactions over LiH, MgH₂, and CaH₂ were shown to effectively liberate and reduce solid CO₂ from the metal carbonate (MCO₃, where M = Mg, Ca), yielding CH₄ with high selectivity. ,,, Collectively, these studies demonstrate the efficacy of alkali and alkaline earth metal hydrides for activating and reducing CO₂ to CH₄. A catalyst-free approach was advanced by Liu et al. who demonstrated ambient-condition CO₂ reduction to formic acid (HCOOH) in an aqueous system. Their method exploited hydrogen radicals generated in situ by nanobubble–water interfaces, providing a distinct route to this valuable C₁ compounda key intermediate in the CO₂-to-methanol conversion pathway. In contrast, the selective synthesis of CH₃OH via analogous catalyst-free thermochemical routes remains largely unexplored. This gap stems from inherent thermodynamic and kinetic limitations that, at high temperatures, preferentially drive the reaction toward CH₄ and CO formation. To circumvent these limitations, this study employs the intrinsic reducibility and nucleophilicity of hydride ions (H^–) to alter the reaction equilibrium, lower activation energy barriers, and facilitate CO₂ activation and conversion to CH₃OH at ambient pressure and significantly reduced temperatures far below conventional thermochemical processes. Herein, we demonstrate a paradigm shift in the valorisation of CO₂, achieving the novel thermochemical synthesis of CH₃OH at ambient pressure using alkali and alkaline earth metal hydrides. Among the hydrides screened, lithium hydride (LiH) demonstrated the highest activity, achieving the optimal methanol production rate of 0.182 μmol_MeOH·mol_metal ^–1·h^–1 after a 2 h thermochemical reaction.

2. Experimental Section

2.1. Materials and Preparation

2.1.1. Materials

Commercial metal hydrides were used as supplied unless otherwise stated. Lithium hydride (LiH, 98%, Aladdin), magnesium hydride (MgH₂, 99%, Aladdin), and calcium hydride (CaH₂, 98.5%, Aladdin) were used without further purification. Sodium hydride (NaH, 60% dispersion in mineral oil, Aladdin) and potassium hydride (KH, 55% dispersion in paraffin, Aladdin) were washed thoroughly with anhydrous cyclohexane (99.5%, Aladdin) and super dry tetrahydrofuran (99.9%, J & K Chemical), respectively, to remove the protective oil and paraffin coatings. Subsequently, all washed samples were dried overnight under an inert argon atmosphere (>99.999%) within an Ar-filled glovebox (Mikrouna Shanghai Company Ltd., China).

Barium hydride (BaH₂) was synthesized in-house. 400 mg of metallic barium (99.95%, Aladdin) was loaded into a fixed-bed reactor, pressurized with 2 MPa of H₂ gas, and maintained under static conditions for 48 h at room temperature to facilitate initial hydrogen absorption. The sample was then heated to 400 °C at a ramp rate of 20 °C min^–1 and held at this temperature for 5 h under a flowing H₂ atmosphere maintained at 1 MPa.

2.1.2. Ball Milling Procedure

0.32 g of the commercial metal hydride (LiH, NaH, KH, MgH₂, or CaH₂) and synthesized BaH₂ were each loaded into a stainless-steel milling vessel alongside 7 stainless-steel balls (Ø 10 mm and 4.14 mg each) inside an argon-filled glovebox. This resulted in a constant ball-to-powder weight ratio of 90:1. The sealed vessel was then transferred to a planetary ball mill (POWTEQ BM40) and processed for 48 h under a static Ar atmosphere (>99.999% purity) maintained at 0.1 MPa. To prevent overheating, the milling cycle was programmed to pause for 30 min for every 1 h of milling operation. All post ball-milling samples were retrieved and stored under an argon atmosphere within the glovebox with O₂ and H₂O concentrations maintained below 0.01 ppm via a recirculating purification system.

2.2. Temperature-Programmed Reaction Experiments

Thermochemical testing was conducted in a fixed-bed reactor. Precisely weighed quantities of each metal hydride, 50 mg of LiH, or 150 mg of NaH, or 110 mg of KH, or 200 mg of MgH₂, or 200 mg of CaH₂, or 200 mg of BaH_2, was loaded into the reactor. The selection of masses was guided by two parameters: (i) maintaining a comparable mass-based loading to ensure consistent bed geometry and gas–solid contact conditions across experiments, and (ii) representing hydrides with distinct molar masses and intrinsic reactivity. This mass-normalized approach was selected instead of mole-based or hydride-equivalent normalization, as equating these quantities would have generated large variations in bed volume and reactor hydrodynamics, thereby compromising comparability in a fixed-bed configuration. − Prior to reaction, the system was purged with argon (30 mL min^–1 for 5 min) and vented to ensure an uncontaminated environment. The reactions were performed under a total gas flow rate of 12.4 mL min^–1. The feed consisted of CO₂ diluted in argon at a constant Ar/CO₂ molar ratio of 2:1. The weight hourly space velocity (WHSV) was tailored to the mass of each hydride: 14,880 mL g^–1 h^–1 for LiH, 4960 mL g^–1 h^–1 for NaH, 6763 mL g^–1 h^–1 for KH, and a uniform 3720 mL g^–1 h^–1 for MgH₂, CaH₂, and BaH₂. Each experiment was conducted more than two times to minimize experimental errors.

Products analysis was performed by a quadrupole mass spectrometry (HIDEN ANALYTICAL, UK). After establishing a stable mass spectrometry (MS) baseline, the reactor was heated from ambient temperature to 400 °C at a ramp rate of 10 °C min^–1 under ambient pressure (0.1 MPa). Effluent gases were continuously monitored to identify reaction products as a function of temperature.

2.3. Characterization

2.3.1. Methanol Quantification

The liquid reaction products were analyzed by proton nuclear magnetic resonance spectroscopy (Bruker AVANCE NEO 600 MHz, Bruker BioSpin, Germany). Experiments were conducted using the fixed-bed reactor system with identical metal hydride loadings described in Section , and gas flow conditions (Ar/CO₂ = 2:1, WHSV as specified). The temperature was raised to 245 °C and maintained for 2 h to facilitate product formation under reaction conditions (ambient pressure and 245 °C). Furthermore, the products were collected by dissolving them in 1 mL of deuterated dimethyl sulfoxide (DMSO-d ₆, 99.8 at. % D, Innochem). For quantification, 10 μL of chloroform (CHCl₃, 99.8%, admas-beta) was added to each sample as an internal standard. All ¹H NMR spectra were processed using MestReNova software, with proton peak positions calibrated and referenced against the residual DMSO-d ₆ solvent peak.

2.3.2. Materials Characterization

The structural characteristics of the materials, before and after the reaction, were characterized by X-ray diffraction (XRD) on a Bruker D8 ADVANCE diffractometer. Measurements were performed using Cu Kα radiation (λ = 1.5418 Å) with a 2θ range of 10° to 90° and a step size of 0.02°. Crystalline phases were identified by X’Pert High Score software suite. Furthermore, the reaction intermediate species were evaluated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Tests were conducted under a total flow rate of 5 mL min^–1 in an Ar/CO₂ atmosphere (2:1 molar ratio) at 0.1 MPa. The spectra were recorded by a Thermo Fisher Scientific Nicolet iS50 FT-IR spectrometer equipped with a mercury cadmium telleride (MCT) detector in transmission mode. The MCT detector was cooled with liquid N₂, and scans were collected from 740 to 3100 cm^–1 at a resolution of 4 cm^–1 under conditions replicating the temperature-programmed reactions. However, adjustments were made to the maximum temperature (270 °C) and hold time (2 h) as necessary. The background was collected under an inert Argon atmosphere at the corresponding reaction temperatures.

3. Results and Discussions

3.1. Thermochemical Reduction of CO₂ by Alkali and Alkaline Earth Hydrides

To elucidate the heterogeneous interaction between CO₂ and activated lithium hydride (LiH), the thermochemical reactions and products were studied by temperature-programmed reaction coupled with a mass spectrometer (TPR-MS) at ambient pressure. Previous studies have established that the high reactivity of as-milled (activated) alkali and alkaline earth metal hydrides is directly attributable to the mechanical energy transferred and/or increased surface area from grain size reduction generated by ball milling. From the reaction, Figure a presents the product distribution and CO₂ conversion profile for the reaction over lithium hydride (LiH). The onset of CO₂ consumption occurred at 104 °C, reaching a pronounced minimum at 133 °C, which corresponds to the point of maximum conversion. Subsequently, the CO₂ signal intensity increased, indicating a decrease in CO₂ conversion at elevated temperatures. A secondary, lower-intensity CO₂ evolution event was observed at 365 °C. Concurrently, the formation of CH₃OH was detected, emerging at 126 °C and exhibiting maxima at 180 °C, 293 °C, and 364 °C. CH₄ production commenced at 122 °C and rapidly peaked at 135 °C. This finding is consistent with prior reports confirming CH₄ formation below 200 °C over LiH via both thermochemical and mechanochemical pathways. Notably, no CO formation was detected. Water (H₂O) was identified as a major byproduct, with its signal intensity rising significantly to an initial maximum at 151 °C. The H₂O profile displayed further complexity, achieving additional maxima at 365 and 395 °C. Molecular H₂ evolution was observed at 51 °C, with prominent peaks at 147 and 365 °C. This H₂ generation is attributable to the thermal decomposition of LiH. Furthermore, the desorption and/or diffusion of hydride anions at interfaces may contribute to H₂ formation at lower temperatures. ,,

1.

(a) CO₂ conversion and product distributions over LiH (0.1 MPa and 14880 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, (b) XRD patterns taken before and after 400 °C temperature-programmed reaction.

While TPR-MS revealed rapid H₂ evolution at 51 °C during CO₂ reduction, the spontaneous dissociation of thermally stable lithium hydride proceeds at a significantly slower rate, with an onset temperature of 400 °C. This thermal stability demonstrates that LiH does not undergo self-decomposition below 400 °C. Table S1 compares the dissociation temperatures for the current metal hydride reactions with those for spontaneous dissociation. − Notably, under a CO₂ flow, alkali and alkaline earth hydrides dissociate significantly faster at lower temperatures. We attribute this kinetic enhancement to an electrostatic interaction between adsorbed oxygenates (CO _x ) and the hydride ions, which weakens the M–H bond and promotes dissociation at lower temperatures.

To identify the solid-phase products, X-ray diffraction (XRD) analysis was performed on the LiH sample before and after the thermochemical reaction with CO₂ at 400 °C (Figure b). The diffraction pattern of the pristine, activated LiH (PDF no. 01-073-1221) confirms its phase purity prior to reaction. Postreaction, the XRD pattern revealed the presence of three distinct lithium compounds: lithium oxide (Li₂O, PDF no. 01-077-2144), lithium carbonate (Li₂CO₃, PDF no. 01-087-0728), and lithium hydroxide (LiOH, PDF no. 01-076-0911). The sample’s appearance transitioned to black following the reaction, suggesting the potential formation of amorphous carbon, a phenomenon previously documented in analogous studies. , The broad peak between 10° and 25° 2θ is attributed to the polyimide sheet used for handling air-sensitive LiH samples during measurement. By integrating the analysis of solid-phase (Figure b) and liquid–gas products (Figure a), reaction pathways are proposed (eqs –). The standard enthalpy (ΔH _298K°), entropy (ΔS _298K°), and Gibbs free energy (ΔG _298K°) for these reactions were calculated (Tables S2–S26) based on thermodynamic data. − The resulting thermodynamic parameters for these proposed pathways, along with those for selected other alkali and alkaline earth metal hydrides, are compiled in Table .

$$2{\mathrm{L}\mathrm{i}\mathrm{H}}_{(\mathrm{s})}=2{\mathrm{L}\mathrm{i}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 4

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{s})}+2{\mathrm{L}\mathrm{i}\mathrm{H}}_{(\mathrm{s})}={\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_{(\mathrm{s})}+C_{(\mathrm{s})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}$$ 5

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+4{\mathrm{L}\mathrm{i}\mathrm{H}}_{(\mathrm{s})}=2{\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_{(\mathrm{s})}+2{\mathrm{H}}_{2(\mathrm{g})}+{\mathrm{C}}_{(\mathrm{s})}$$ 6

$${\mathrm{C}}_{(\mathrm{s})}+2{\mathrm{H}}_{2(\mathrm{g})}={\mathrm{C}\mathrm{H}}_{4(\mathrm{g})}$$ 7

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+3{\mathrm{L}\mathrm{i}\mathrm{H}}_{(\mathrm{s})}={\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{L}\mathrm{i}}_{(\mathrm{s})}+{\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_{(\mathrm{s})}$$ 8

$${\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{L}\mathrm{i}}_{(\mathrm{s})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}={\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_{(\mathrm{l})}+{\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}}_{(\mathrm{s})}$$ 9

$${\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_{(\mathrm{s})}+{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}={\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_{3(\mathrm{s})}$$ 10

$$2{\mathrm{L}\mathrm{i}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}=2{\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 11

1. Calculated Standard Thermodynamic Parameters for the Proposed Thermochemical CO₂ Reduction Reactions with Alkali and Alkaline Earth Metal Hydrides .

reaction	ΔH° (298)/kJ mol–1	ΔS°(298)/J mol–1 K–1	ΔG° (298)/ kJ mol–1
4	181	148.924	136.598
5	–309.22	–140.49	–267.333
6	–440.29	48.568	–454.771
7	–74.81	–80.848	–50.705
8	a	a	a
9	a	a	a
10	–224.49	–160.94	–176.506
11	–403.34	18.224	–408.774
12	112.6	153.284	66.898
13	–406.46	–176.266	–353.906
14	–224.19	–148.04	–180.051
15	–6.23	10.69	–9.417
16	–322.99	–153.84	–277.123
17	–279.94	17.064	–285.027
18	115.4	160.884	67.432
19	–1068.8	–591.06	–892.595
20	75.3	132.264	35.866
21	a	a	a
22	a	a	a
23	–837.44	–284.5	–752.62
24	–74.81	–80.838	–50.705
25	–100.59	–174.98	–48.42
26	–638.67	21.384	–645.05
27	181.5	130.704	142.531
28	–345.72	–141.39	–303.56
29	a	a	a
30	a	a	a
31	–513.29	46.768	–527.23
32	–74.81	–80.848	–50.705
33	–413.54	32.844	–423.33
34	177	261.168	99.133
35	a	a	a
36	–611.81	–57.422	–594.689
37	a	a	a
38	–373.04	–132.71	–333.472

^aWhere: ΔH° is the standard reaction enthalpy, ΔS° is the standard reaction entropy, and ΔG° is the standard Gibbs free energy. Parameters for reactions marked with “a” could not be fully calculated due to insufficient thermodynamic data.

The thermochemical reduction of CO₂ with NaH was investigated to elucidate its reaction pathways. As depicted in Figure a, CO₂ consumption reached its maximum at 98 °C. A signal at m/z = 16 exhibits a profile closely mirroring that of CO₂, suggesting it may originate from CO₂ fragmentation within the mass spectrometer. At the same time, the signal at m/z = 28 exhibits a similar profile to that of CO₂ at lower temperatures, likely due to interference from CO₂ fragmentation, a substantial and distinct CO evolution event commenced at 275 °C, culminating in a maximum at 375 °C. Meanwhile, CH₃OH formation was initiated at 168 °C, increased to a maximum at 285 °C, and subsequently decreased, becoming undetectable for the remainder of the temperature program. H₂O was observed as a major reaction product, with evolution onset at 74 °C and a maximum intensity reached at 386 °C. Concurrently, molecular H₂ desorption occurred in two distinct stages: a low-temperature event with onset at 38 °C and a maximum at 91 °C, followed by a less intense, and a high-temperature event peaking at 175 °C. This bimodal H₂ release profile is indicative of multiple underlying processes, such as surface hydride decomposition and reaction-intermediate degradation.

2.

(a) CO₂ conversion and product distributions over NaH (0.1 MPa and 4960 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, and (b) XRD patterns after 400 °C temperature programmed reaction.

X-ray diffraction (XRD) analysis of the solid residues following the thermochemical reduction reaction between CO₂ and NaH at 400 °C revealed a multiphase product mixture (Figure b). The diffraction patterns confirm the presence of unreacted NaH (PDF no. 01-076-0172) alongside the reaction products sodium hydroxide (NaOH, PDF no. 00-035-1009), sodium oxide (Na₂O, PDF no. 00-023-0528), and sodium carbonate (Na₂CO₃, PDF no. 00-001-1166). The observed dark gray-black discoloration of the solid material suggests the accompanying formation of elemental carbon. This solid-phase analysis, combined with the detection of liquid gas products −CH₃OH, CO, H₂O, and H₂ by mass spectrometry (Figure a), provides a complete product distribution. Based on these analytical results, we propose the following reaction mechanism to account for the observed products

$$2{\mathrm{N}\mathrm{a}\mathrm{H}}_{(\mathrm{s})}=2{\mathrm{N}\mathrm{a}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 12

$$3{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+6{\mathrm{N}\mathrm{a}\mathrm{H}}_{(\mathrm{s})}={\mathrm{C}\mathrm{O}}_{(\mathrm{g})}+3{\mathrm{N}\mathrm{a}}_2{\mathrm{O}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+2C_{(\mathrm{s})}$$ 13

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+3{\mathrm{N}\mathrm{a}\mathrm{H}}_{(\mathrm{s})}={\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{N}\mathrm{a}}_{(\mathrm{s})}+{\mathrm{N}\mathrm{a}}_2{\mathrm{O}}_{(\mathrm{s})}$$ 14

$${\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{N}\mathrm{a}}_{(\mathrm{s})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}={\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_{(\mathrm{l})}+\mathrm{N}\mathrm{a}\mathrm{O}{\mathrm{H}}_{(\mathrm{s})}$$ 15

$${\mathrm{N}\mathrm{a}}_2{\mathrm{O}}_{(\mathrm{s})}+{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}={\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_{3(\mathrm{s})}$$ 16

$$2{\mathrm{N}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}=2{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 17

The reaction profile for CO₂ over KH exhibits distinct differences from those observed with LiH and NaH. As shown in Figure a, CO₂ consumption commenced at 138 °C, reaching maximum conversion at 159 °C. Critically, no hydrocarbon products (i.e., CH₄, CH₃OH) were detected. Instead, the primary liquid–gas products were H₂ and H₂O. The H₂ evolution onset occurred at a notably low temperature (47 °C), with both H₂ and H₂O signals reaching their maxima concurrently with peak CO₂ consumption (∼159–176 °C). This product distribution suggests a divergent reaction pathway. The absence of carbon-containing products likely implies that CO₂ is not being reduced to hydrocarbons but is instead participating in a direct reaction with the hydride. This mechanism could be initiated by the thermal decomposition of KH, releasing H_2, and is succeeded by a surface-mediated carbonate and formate that decomposes into CO₂ and H₂O.

3.

(a) CO₂ conversion and product distributions over KH (0.1 MPa and 6763 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, and (b) XRD patterns after 400 °C temperature programmed reaction.

The structural evolution of the solid was characterized by XRD over an active 2θ range of 10–60°, both prior to and following the reaction. The analysis of the solid residue confirms that only potassium carbonate (K₂CO₃, PDF no. 00-001-1001) is the sole crystalline product from the reaction (Figure b). The observed darkening of the solid phase suggests the potential coformation of elemental carbon, consistent with the behavior observed in the NaH system. Meanwhile, the diffraction pattern of the pristine KH (PDF no. 01-089-2778) confirms its phase purity prior to reaction. This solid-state product distribution, when combined with the gas–liquid phase analysis, which showed exclusive CO₂ consumption concurrent with H₂ and H₂O evolution and a complete absence of hydrocarbons, constrains the possible reaction pathways. We therefore propose that the following nonsequential reactions account for the observed products

$$2{\mathrm{K}\mathrm{H}}_{(\mathrm{s})}=2{\mathrm{K}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 18

$$4{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+4{\mathrm{K}\mathrm{H}}_{(\mathrm{s})}=2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+2{\mathrm{C}}_{(\mathrm{s})}+2{\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_{3(\mathrm{s})}$$ 19

The reactivity of alkaline earth metal hydrides with CO₂ was subsequently investigated under identical thermochemical conditions (i.e., temperature, pressure, and reaction time). The performance of MgH₂ is presented in Figure a. The thermochemical reduction of CO₂ over MgH₂ commenced at 223 °C, achieving maximum conversion at 373 °C, with a secondary reduction event observed at 391 °C (Figure a). CH₃OH was identified as a primary product, with an onset temperature of 229 °C and a maximum yield at 361 °C. Notably, CO formation was detected at 278 °C, peaking at 371 °C. This confirms the ability of MgH₂ to partially reduce CO₂ to CO via a two-electron transfer pathway (C⁴⁺ to C²⁺). In contrast, CH₄ formation initiated at a lower temperature (263 °C) and reached its maximum at 332 °C. H₂O and H₂ evolution both exhibited maxima around 370 °C; however, their onset temperatures differed significantly, with H₂ detected as early as 40 °C (indicative of MgH₂ decomposition) and H₂O appearing at 223 °C. The concurrent presence of CO and H₂O may be indicative of a reverse water–gas shift (RWGS) reaction pathway contributing to the CO₂ reduction process.

4.

(a) CO₂ conversion and product distributions over MgH₂ (0.1 MPa and 3720 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, and (b) XRD patterns after 400 °C temperature programmed reaction.

Replacing Ar with H₂ in the Ar/CO₂ feed (12.4 mL/min, H₂/CO₂ molar ratio of 3:1) over LiH and MgH₂ as references resulted in a similar overall trend in CO₂ conversion, product distribution, and hydride instability (Figures S1 and S2). A key distinction, however, was a discernible shift to higher onset and peak temperatures in the H₂-containing atmosphere. This indicates that molecular H₂ inhibits the dissociation kinetics of the metal hydride, despite not being consumed as a reactant. Functioning solely as stoichiometric hydrogen sources and reduction reaction promoters, these alkali and alkaline earth metal hydrides do not exhibit catalytic behavior. Their reactivity, however, leads to stable oxide byproducts (i.e., alkali and alkaline earth metal oxides), which are thermodynamically unfavorable to regenerate back into the active hydride phases.

X-ray diffraction (XRD) analysis of the solid residue after reaction at 400 °C revealed a complex mixture of products, including magnesium (Mg, PDF no. 00-004-0770), magnesium oxide (MgO, PDF no. 00-003-0998), magnesium hydroxide (Mg­(OH)₂, PDF no. 01-075-1527), and magnesium carbonate (MgCO₃, PDF no. 01-071-1534), and unreacted magnesium hydride (MgH₂, PDF no. 01-074-0934). The coexistence of the oxidized and carbonated phases clearly demonstrates that the oxidation of the MgH₂ occurs under a CO₂ atmosphere. Based on the comprehensive product analysis from mass spectrometry and XRD, we propose a reaction mechanism to account for the observed pathways.

$${\mathrm{M}\mathrm{g}\mathrm{H}}_{2(\mathrm{s})}={\mathrm{M}\mathrm{g}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 20

$$4{\mathrm{M}\mathrm{g}\mathrm{H}}_{2(\mathrm{s})}+3{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}=({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{M}\mathrm{g}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}+3{\mathrm{M}\mathrm{g}\mathrm{O}}_{(\mathrm{s})}+{\mathrm{C}\mathrm{O}}_{(\mathrm{g})}$$ 21

$$({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{M}\mathrm{g}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}=2{\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_{(\mathrm{l})}+{\mathrm{M}\mathrm{g}(\mathrm{O}\mathrm{H})}_{2(\mathrm{s})}$$ 22

$$2{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+2{\mathrm{M}\mathrm{g}\mathrm{H}}_{2(\mathrm{s})}=2{\mathrm{M}\mathrm{g}\mathrm{O}}_{(\mathrm{s})}+2{\mathrm{C}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}$$ 23

$$2{\mathrm{H}}_{2(\mathrm{g})}+{\mathrm{C}}_{(\mathrm{s})}={\mathrm{C}\mathrm{H}}_{4(\mathrm{g})}$$ 24

$${\mathrm{M}\mathrm{g}\mathrm{O}}_{(\mathrm{s})}+{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}={\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_{3(\mathrm{s})}$$ 25

$${\mathrm{M}\mathrm{g}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}={\mathrm{M}\mathrm{g}(\mathrm{O}\mathrm{H})}_{2(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 26

To further explore the reactivity of alkaline earth metal hydrides, we examined the interaction of CaH₂ with CO₂. Figure presents the CO₂ conversion profile and product distribution for the thermochemical reduction reaction with CaH₂. CO₂ consumption commenced at 70 °C, reaching a pronounced minimum at 290 °C, which corresponds to the point of maximum conversion. The formation of CH₃OH was observed at 226 °C, coinciding with the period of rapid CO₂ consumption and reaching its maximum yield at 280 °C. Notably, the production rates of CH₄, H₂, and H₂O were all found to be maximized at 290 °C. These synchronous peaks suggest that the formation of the products is intrinsically linked to the primary CO₂ activation and conversion over CaH₂.

5.

(a) CO₂ conversion and product distributions over CaH₂ (0.1 MPa and 3720 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, and (b) XRD patterns after 400 °C temperature programmed reaction.

In contrast to the CO₂–MgH₂ system, which yielded a mixture of carbonate and oxide products, X-ray diffraction analysis of the solid residue following the reaction of CaH₂ (PDF no. 00-031-0266) with CO₂ identified only calcium (Ca, PDF no. 01-089-4051), calcium hydroxide (Ca­(OH)₂, PDF no. 00-001-1079) and calcium oxide (CaO, PDF no. 01-075-0264). Consequently, no crystalline carbonate phases were detected. By integrating this solid-phase analysis with the liquid–gas product distribution presented in Figure a, we propose the following reaction pathways to account for the observed products

$${\mathrm{C}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}={\mathrm{C}\mathrm{a}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 27

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+{\mathrm{C}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}={\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+{\mathrm{C}}_{(\mathrm{s})}+{\mathrm{C}\mathrm{a}\mathrm{O}}_{(\mathrm{s})}$$ 28

$$2{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+3{\mathrm{C}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}=({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{C}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{C}\mathrm{a}\mathrm{O}}_{(\mathrm{s})}$$ 29

$$({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{C}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}=2{\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_{(\mathrm{l})}+{\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H})}_{2(\mathrm{s})}$$ 30

$${\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}+2{\mathrm{C}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}=2{\mathrm{C}\mathrm{a}\mathrm{O}}_{(\mathrm{s})}+2{\mathrm{H}}_{2(\mathrm{g})}+{\mathrm{C}}_{(\mathrm{s})}$$ 31

$${\mathrm{C}}_{(\mathrm{s})}+2{\mathrm{H}}_{2(\mathrm{g})}={\mathrm{C}\mathrm{H}}_{4(\mathrm{g})}$$ 32

$${\mathrm{C}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}={\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H})}_{2(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 33

We conclude our investigation into alkaline earth metal hydrides with barium hydride (BaH₂), which also proves to be active for the thermochemical reduction of CO₂. Figure a depicts the CO₂ conversion profile and product distribution for the reduction reaction with BaH₂. The onset of significant CO₂ consumption occurred at 315 °C, reaching a minimum at 352 °C, which corresponds to the point of maximum conversion. The evolution of H₂ and H₂O was detected, with both species reaching their maximum production rates at similar temperatures (363 and 366 °C, respectively). The H₂O signal exhibited a sustained increase with temperature following its initial appearance. CH₃OH formation was observed, with an onset at 275 °C and a maximum yield achieved at 355 °C, closely coinciding with the peak of maximum CO₂ conversion. Notably, aside from CH₃OH, no other hydrocarbon products (i.e., CH₄, CO) were definitively identified. The observed mass fragments at m/z = 16 and m/z = 28 are attributed to ion-source fragmentation of CO₂, rather than genuine reaction products.

6.

(a) CO₂ conversion and product distributions over BaH₂ (0.1 MPa and 3720 mL g_material ^–1 h^–1), heated to 400 °C at a ramping rate of 10 °C min^–1, and (b) XRD patterns after 400 °C temperature programmed reaction.

To confirm the active role of the hydrides, a control experiment was performed. As shown in Figure S3, no notable activity occurred in their absence. This result establishes the dual, essential function of these materials as direct chemical reagents and promoters that facilitate the reduction pathway.

X-ray diffraction analysis of the postreaction solid residue confirmed the presence of unreacted BaH₂ (PDF no. 01-078-1402) alongside the reaction products barium hydroxide (Ba­(OH)₂, (PDF no. 00-001-0630), and barium oxide (BaO, PDF no. 00-030-0143). No barium carbonate (BaCO₃) phases were detected. By integrating the solid-phase composition with the liquid–gas product distribution detailed in Figure a, we propose the following reaction scheme to account for the observed products

$${\mathrm{B}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}={\mathrm{B}\mathrm{a}}_{(\mathrm{s})}+{\mathrm{H}}_{2(\mathrm{g})}$$ 34

$$3{\mathrm{B}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}+2{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}=({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{B}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{B}\mathrm{a}\mathrm{O}}_{(\mathrm{s})}$$ 35

$$3{\mathrm{B}\mathrm{a}\mathrm{H}}_{2(\mathrm{s})}+2{\mathrm{C}\mathrm{O}}_{2(\mathrm{g})}=3{\mathrm{B}\mathrm{a}\mathrm{O}}_{(\mathrm{s})}+2{\mathrm{H}}_{2(\mathrm{g})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+2{\mathrm{C}}_{(\mathrm{s})}$$ 36

$$({\mathrm{C}\mathrm{H}}_3{\mathrm{O})}_2{\mathrm{B}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}=2{\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_{(\mathrm{l})}+{\mathrm{B}\mathrm{a}(\mathrm{O}\mathrm{H})}_{2(\mathrm{s})}$$ 37

$${\mathrm{B}\mathrm{a}}_{(\mathrm{s})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}={\mathrm{B}\mathrm{a}(\mathrm{O}\mathrm{H})}_2(\mathrm{s})+{\mathrm{H}}_2(\mathrm{g})$$ 38

A comprehensive summary of the products identified from the thermochemical reduction of CO₂ with selected alkali and alkaline earth metal hydrides, conducted at atmospheric pressure, is consolidated in Table . This table integrates the liquid–gas products detected by mass spectrometry during temperature-programmed reaction (TPR) experiments with the solid-phase products characterized by X-ray diffraction (XRD) analysis on postreaction samples.

2. Product Distribution for the Thermochemical CO₂ Reduction Reaction Across Selected Alkali and Alkaline Earth Metal Hydrides.

AH	liquid/gas products	solid products
LiH	H2, H2O, CH4, CH3OH	Li2O, C, Li2CO3, LiOH
NaH	H2, H2O, CO, CH3OH	Na2O, C, Na2CO3, NaOH
KH	H2O, H2	K2CO3, C
MgH2	H2O, H2, CO, CH4, CH3OH	Mg, MgO, Mg(OH)2, MgCO3, C
CaH2	H2, H2O, CH4, CH3OH	Ca, CaO, Ca(OH)2, C
BaH2	H2, H2O, CH3OH	BaO, Ba(OH)2, C

3.2. CH₃OH Production Yield Determined by ¹H NMR

Having established the qualitative product distributions, we proceeded to quantify the formation of CH₃OH via a separate analytical approach. Postreaction, the liquid products were collected and dissolved in DMSO-d ₆ solvent, and the resulting solutions were analyzed by ¹H nuclear magnetic resonance (NMR) spectroscopy using chloroform as an internal standard. The ¹H NMR spectrum for the alkali metal hydrides-CO₂ systems (Figures S4 and S5) exhibited two low-intensity signals at 3.17 and 4.2 ppm, corresponding to the methyl (CH₃) and hydroxyl (OH) protons of CH₃OH, respectively. This conclusively confirms the formation of methanol, corroborating the mass spectrometry data. A prominent signal at 3.37 ppm was assigned to water, originating from the reaction and/or adsorbed atmospheric moisture. Additional peaks were attributed to the solvent (DMSO-d ₆, 2.50 ppm) and internal standard (chloroform, 8.31 ppm). This NMR methodology was extended to the alkaline earth metal hydrides (MgH₂, CaH₂, BaH₂; Figures S6–S8). In all cases, characteristic resonances for CH₃OH were identified within the ranges of 3.16–3.17 ppm (CH₃) and 4.1 ppm (OH). The integrated peak areas were used to calculate the concentration and production rate of CH₃OH (see Supporting Information for calculations), normalized by metal loading and presented in Table .

3. Methanol Production Rates from the Thermochemical Reduction of CO₂ Over Alkali and Alkaline Earth Metal Hydrides ^,

AH	mass (mg)	CH3OH concentration (mmol L–1)	Time (h)	CH3OH productivity (μmolmethanol molmetal –1 h–1)
LiH	55	0.007221	2	0.18162
NaH	150	0.00512	2	0.15605
MgH2	200	0.005389	2	0.15431
CaH2	200	0.003609	2	0.16798
BaH2	200	0.001068	2	0.17304

^aRepresents T = 245 °C, P = 0.1 MPa, and t = 2 h.

^bWhere; AH represents alkali or alkaline earth metal hydride.

The methanol (MeOH) production rates, ranked in descending order, are as follows: LiH > BaH₂ > CaH₂ > NaH > MgH₂. LiH demonstrated the highest production rate (0.182 μmol_MeOH·mol_metal ^–1·h^–1), which we attribute to its low onset temperature for methanol formation (126 °C) and its high production intensity (180 °C), as evidenced by the temperature-programmed reaction data. Conversely, the lowest production rate was observed for MgH₂, consistent with its broader distribution of C₁ products (i.e., CO, CH₄). The notably high production rate over BaH₂ aligns with its unique product distribution, where CH₃OH was identified as the sole hydrocarbon product.

3.3. Reaction Mechanisms Revealed by In Situ DRIFTS: A Case Study of LiH

Despite the demonstrated efficacy of alkali and alkaline earth metal hydrides in promoting the thermochemical reduction of CO₂ to value-added C1 productsprincipally CH₃OH, the nature and role of adsorbed intermediate species remain unexplored. In the following part, we selected LiH, leveraging its superior thermochemical activity to probe the intermediate species using in situ DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) during CO₂ reduction.

Figure presents in situ DRIFTS spectra acquired as a function of temperature under a flow (5 mL min^–1) of Ar/CO₂ (2:1 molar ratio) at 0.1 MPa. The spectral regions are displayed in two panels: (a) the 750–1800 cm^–1 region, characteristic of symmetric and asymmetric stretching vibrations of carboxylic groups and the CO vibrations of carbonate species, and (b) the 1900–3100 cm^–1 region, associated with C–H stretching modes. Upon heating from 26 to 270 °C, bands emerge at 1220, 1270, and 1438 cm^–1 (Figure a). These are assigned to the symmetric and asymmetric O–C–O vibrations of surface bicarbonate species (HCO₃*). A weaker band at 1613 cm^–1 is consistent with a tridentate carbonate species (t-CO₃*). , Holding the reaction temperature at 270 °C for 2 h reveals a dynamic evolution of the surface species. The intensities of the HCO₃* and t-CO₃* bands gradually diminish and eventually disappear. This preferential consumption suggests HCO₃* and t-CO₃* act as key reactive intermediates during the initial stages of CO₂ reduction over LiH, behavior consistent with previous catalytic studies of methanol synthesis from CO₂. Concurrently, new spectral features develop, signaling the formation of more reduced species. Bands at 1541 and 1571 cm^–1 are consistent with monodentate carbonate (m-CO₃*) and formate (HCOO*) species, respectively. Notably, the emergence of a band at 1081 cm^–1, indicative of a formyl (CHO*) species, provides direct spectroscopic evidence for C–H bond formation and implies the operation of a CO₂ methanation pathway alongside methanol synthesis. The spectroscopic identification of these intermediates offers a mechanistic basis for our earlier TPR findings, which revealed the simultaneous production of CH₃OH and CH₄ at low reduction temperatures. Specifically, the presence of both HCOO* and CHO* key intermediates as precursors to CH₃OH and CH₄, respectively, provides a mechanistic rationale for this product distribution. Moreover, no spectral bands are observed in the region characteristic of CO adsorption (∼2067 cm^–1), indicating that the RWGS pathway is not a significant reduction reaction pathway over LiH under the present experimental conditions (Figure b). Furthermore, the intense doublet at 2342 and 2361 cm^–1, assigned to the asymmetric stretching vibration of CO₂, exhibits a gradual decrease in intensity with increasing temperature. The observed change in band intensity confirms the progressive activation and conversion of CO₂ over the LiH.

7.

In situ DRIFTS spectra recorded during CO₂ reduction at 0.1 MPa and various temperatures on LiH (CO₂/Ar = 1/2 molar ratio) for 2 h (a) 750–1900 cm^–1, and (b) 1900–3100 cm^–1.

Upon reaching 270 °C, new spectral features emerge at 1801 and 2495 cm^–1. These bands, which remain unassigned, indicate the presence of unique reactive carbon-containing reactive intermediate species that become stable on the surface only at elevated temperatures. Concurrently, bands at 2860 and 2938 cm^–1 emerge at 245 °C and intensify with temperature. These are tentatively assigned to the asymmetric stretching of formate (HCOO*) and methoxy (CH₃O*) species, respectively, − signifying the reduction of carbon intermediates. Collectively, the spectroscopic identification of HCOO* and its subsequent consumption, coupled with the absence of CO, provides compelling evidence that CO₂ reduction over LiH proceeds via the formate pathway for selective methanol synthesis.

4. Discussion

This work presents a systematic study on the noncatalytic thermochemical reduction of CO₂ mediated by ionic metal hydrides, encompassing activated alkali (LiH, NaH, KH) and alkaline earth (MgH₂, CaH₂, BaH₂) families under ambient pressure and low temperature. Extending beyond the well-characterized systems (NaH, LiH, MgH₂, CaH₂), we include the previously underexplored KH and BaH₂ to furnish an extensive ionic metal hydride-CO₂ system overview. The reactivity was found to be highly dependent on the specific metal hydride employed, yielding markedly distinct distributions of C1 products (CH₃OH, CH₄, CO). The chemical alteration of the hydrides postreaction unequivocally confirms their role as chemical reagents and promoters. Based on the experimental evidence, we elucidate distinct mechanistic pathways wherein the metal cation dictates the stability of key intermediates and thus governs the ultimate product distribution.

As summarized in Table , the calculated thermodynamic profiles (ΔH ₂₉₈°, ΔS ₂₉₈°, ΔG ₂₉₈°) show that all proposed CO₂ reduction reactions under standard ambient conditions are exothermic (ΔH ₂₉₈° < 0 kJ mol^–1) and spontaneous (ΔG ₂₉₈° < 0 kJ mol^–1). This confirms their thermodynamic favorability. The notable exception is the initial decomposition of the metal hydrides, which is endergonic (ΔG ₂₉₈° > 0 kJ mol^–1). This analysis confirms that the overall process is thermodynamically driven once the hydride is activated. Therefore, energy input is needed to overcome the ionic M–H bond strength in alkali and alkaline earth metal hydrides. It is important to note that the proposed mechanistic pathways involving metal alkoxide intermediates (i.e., CH₃OLi, (CH₃O)₂Mg, (CH₃O)₂Ca) could not be fully validated thermodynamically. The scarcity of thermochemical data (ΔH _f298° and S ₂₉₈°) for these species in the literature poses a significant limitation for quantitative analysis. The hydrogenation of CO₂ to CH₃OH is an exothermic process, consuming three molecules of H₂ and one of CO₂ to produce one molecule of CH₃OH and one of H₂O. Thus, while elevated pressures are thermodynamically necessary for significant CH₃OH yields, we found that selected alkali and alkaline earth metal hydrides act as thermodynamic promoters, shifting the CO₂ reduction equilibrium toward CH₃OH production at ambient pressure. Crucially, the definitive identification of CH₃OH via ¹H NMR (signature signals for CH₃ and OH groups) proves that alkali and alkaline earth metal hydrides enable the reduction of thermodynamically stable CO₂ to CH₃OH under ambient pressure and low temperaturea transformation previously considered infeasible under such mild conditions. In situ DRIFTS spectroscopy, combined with temperature-programmed reaction (TPR), revealed that intermediate species formed during CO₂ reduction over LiH at ambient pressure exhibit low spectral intensities, likely due to a low steady-state surface coverage of these reactive intermediates. The analysis identified a sequence of carbon-containing intermediates, including t-CO₃*, m-CO₃*, HCO₃*, HCOO*, and CH₃O* species, which appeared and disappeared at distinct temperatures. The absence of detectable CO below 270 °C indicates that CO RWGS is not a common pathway under these conditions. These findings provide direct spectroscopic evidence that CH₃OH synthesis from CO₂ via thermochemical reduction mainly proceeds through the HCOO* pathway, with this intermediate serving as a key precursor on the LiH surface. Moreover, this mechanism is highly efficient, with LiH achieving the highest methanol (MeOH) production rate among the screened ionic metal hydrides, reaching 0.182 μmol_MeOH mol_metal ^–1 h^–1. The quantitative determination of CO and CH₄ was therefore inherently constrained. Their high volatility, coupled with low solubility, under the continuous-flow regime, precluded their effective retention for analysis. In addition, the dynamic nature of the thermal reduction reaction, where C1 product concentrations are nonstationary and change as a function of time, precludes the use of conventional GC for quantitative analysis in this system. Although CO₂ interaction with alkali and alkaline earth metal hydrides is known to favor the production of CH₄, the synthesis of CH₃OH has been elusive. Here, we reported the first successful conversion of CO₂ to CH₃OH via a thermochemical reduction pathway under ambient pressure and low temperature (245 °C), marking a significant departure from previously reported high-pressure (3–10 MPa) catalytic or high-temperature (around 265 °C) metabolic routes.

5. Conclusion

This work demonstrates a paradigm shift in the valorisation of CO₂, achieving the novel thermochemical synthesis of CH₃OH at ambient pressure and enabling the initial onset at low temperature. This was achieved by leveraging the high chemical potential of alkali and alkaline earth metal hydrides to drive CO₂ activation and reduction, circumventing the substantial thermodynamic and kinetic barriers that traditionally necessitate energy-intensive processes. Systematic screening revealed that C1 product distribution is precisely governed by the identity of the metal cation, with LiH emerging as the best candidate for CH₃OH synthesis, underscoring a critical structure–activity relationship based on metal-formate stability and hydride transfer kinetics. Mechanistic evidence robustly indicates that the reaction proceeds primarily through an HCOO* intermediate pathway, wherein the hydride ion (H^–) acts as a potent nucleophile to initiate CO₂ reduction. This insight challenges the existing paradigm that efficient CO₂ reduction must rely on high partial pressures of H₂ and elevated temperatures to activate stable molecular bonds. Instead, our approach utilizes the inherent reactivity of solid-state ionic metal hydrides to provide the necessary driving force, effectively reducing the activation energy from thermal input and opening a new avenue for future thermo-catalytic design. This discovery moves alkali and alkaline earth metal hydrides beyond their traditional role in CO₂ methanation, unveiling them as versatile reagents and promoters for CH₃OH production. By introducing the principle of using chemo-potential to overcome thermodynamic and kinetic barriers, this work opens a new and broadly applicable design space for methanol synthesis via CO₂ reduction. The principal limitation of this systemhydride instability, which stems from the consumption of the reactive reagent-phase metal hydrides. Future work will address this by the development of a robust next-generation ionic hydride-based material, with cofed H₂ at low pressure relative to conventional CH₃OH synthesis pressure for the energy-efficient conversion of CO₂. This approach can achieve two key objectives: (i) in situ regeneration of the active hydride species to ensure reaction activity longevity, and (ii) suppression of surface poisoning during H₂ dissociation and spillover on the material. We anticipate this will unlock sustained CO₂ conversion and higher methanol yields. The implications extend beyond a novel synthetic route, providing a foundational principle for the development of next-generation materials incorporating reactive ionic metal hydrides for the energy-efficient conversion of CO₂.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c11137.

• ¹H NMR figures for the liquid products after the thermochemical reduction of CO₂. Thermodynamic parameters and calculations for the proposed reactions at standard conditions (PDF)

M.A.M.: Investigation, Writingoriginal draft, Visualization, Methodology, Formal analysis. P.L.: Investigation, Methodology, Writingreview and editing. X.O.: Writingreview and editing, Supervision. N.M.M.: Writingreview and editing, Supervision. F.C.: Conceptualization, Methodology, Writingreview and editing, Supervision, Funding acquisition.

This research was supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LR24B030002 and No. LQN26E060004, and the China Postdoctoral Science Foundation under Grant No. 2025M770596. The authors acknowledge the Materials Analysis and Testing Center (YongV) of Yongjiang Laboratory for providing access to the equipment used in the materials characterization experiments conducted in this study.

The authors declare no competing financial interest.