Mechanism study on hydrogen generation from metal hydride-coupled methanol steam reforming

Abstract

Metal hydride (MH)-coupled methanol steam reforming (MSR), in which MSR is coupled with the hydrolysis of MH, is an attractive strategy for onsite hydrogen generation as it offers a high hydrogen density, high hydrogen purity and balanced thermal effect. This study provides a comprehensive mechanism study on the MH-coupled MSR over Cu/CaH₂ using online mass spectroscopy coupled with isotope labelling and in situ Fourier-transformed infrared spectroscopy. Results show that the hydrolysis of MH and MSR over Cu proceeds simultaneously via a formate-mediated pathway. The stepwise dehydrogenation of methanol primarily occurs on the Cu surface, while CaH₂ effectively activates CH₃OH at low temperatures by supplying the hydrolysis heat to the endothermic MSR. Although Ca(OCH₃)₂ and Ca(HCOO)₂ can be formed when CaH₂ is introduced, they are not active intermediates in the MH-coupled MSR due to their high thermal stability. Instead, they are converted back to CH₃OH and HCOOH via hydrolysis.

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Broader context

Hydrogen is a key vector for decarbonizing energy systems, yet efficient and high-purity on-demand hydrogen production remains challenging. Methanol steam reforming (MSR) is attractive due to its high hydrogen density and the handling advantages of a liquid carrier but its intrinsic endothermicity and CO formation limit its practical application. Meanwhile, metal hydrides (MHs) have been extensively studied for their hydrogen production ability, and their potential to participate in hydrogen transfer reactions has recently attracted increasing attention.

In this work, MSR is coupled with metal hydride hydrolysis, where CaH₂ acts simultaneously as a catalyst support and as a reactive component. During hydrolysis, CaH₂ releases heat and hydrogen and captures CO₂, which helps in supplying the heat for MSR and shifts the reaction equilibrium. This coupling not only improves hydrogen production performance but also changes the reaction environment and possible pathways. This work also provides insights into CO₂ capture and may inspire new strategies to further enhance the hydrogen storage density of existing hydrogen storage materials.

1. Introduction

Hydrogen is the ideal energy source to achieve carbon neutrality due to its high abundance, zero emissions, high efficiency, and renewability.^1–3 The development of high-performance hydrogen storage technologies is crucial for its practical utilization.^4–6 Among the chemical hydrogen production methods, methanol steam reforming (MSR) and metal hydride hydrolysis are promising strategies for hydrogen generation for fuel cells though each presents inherent limitations.^7–9 MSR offers a high hydrogen density (12 wt%) and low cost, but its application hindered is by its endothermic nature and the formation of CO as a byproduct. For MSR, most of the researches have predominantly focused on the rational design of efficient and stable catalysts, achieving significant progress in both activity and durability. For instance, Hu et al.¹⁰ developed a highly dispersed Ni-based catalyst supported on spongy mesoporous alumina, which enabled rapid reactant adsorption and efficient methanol dehydrogenation, delivering a high hydrogen yield and excellent long-term stability (12 h). Similarly, Shu et al.¹¹ reported an Ru-based carbon-coated CeO₂ catalyst, where the synergistic interaction between the carbon layer and CeO₂ enhanced metal–support interactions, facilitated electron transfer, and generated abundant oxygen vacancies, leading to improved stability and resistance to carbon deposition. Beyond activity and stability, considerable efforts have been devoted to suppressing CO formation.^12–14 At the catalyst level, extensive research has been devoted to the design of highly selective catalysts.^15–17 For example, Xie et al.¹⁸ demonstrated that the rod-shaped CeO₂ supports enriched oxygen vacancies, promoting both methanol activation and the water–gas shift reaction, thereby reducing CO selectivity. At the system level, new MSR technologies such as absorption-enhanced MSR and electrochemical-assisted MSR^19,20 have been widely developed to enhance hydrogen purity. However, the intrinsic endothermicity remains one of the key challenges in its practical application for onsite hydrogen generation.

Copper-based catalysts are the most widely used MSR catalysts. The reaction mechanism of the MSR over copper-based catalysts has been extensively studied.^21–25 It generally follows two main pathways: the methanol decomposition–water gas shift (MD–WGS) route and the MSR–reverse water gas shift (MSR–rWGS) route.²⁶ The dominant pathway depends on the type of intermediate formed on the metal surface.²⁷ When methanol is adsorbed in a bidentate mode via both carbon and oxygen atoms (η²(C,O)), it tends to decompose into CO and H₂. In contrast, monodentate adsorption via the oxygen atom (η¹(O)) leads to intermediates, such as HCOOCH₃ or HCOOH, which eventually desorb as CO₂ and H₂ without forming CO.²⁸ The metal–support interaction strongly affects the reaction intermediates and plays a crucial role in MSR catalysis.^29–31 Li et al.²² enhanced the performance of a commercial Cu/ZnO/Al₂O₃ catalyst by optimizing the activation process, in which the migration of ZnO_x to the Cu surface increased the Cu–ZnO_x interfacial sites and enhanced catalytic activity and stability. Similarly, Franco et al.³² reported that the partial oxidation of Cu⁰ to Cu^δ+ by CeO₂ in a CuO/CeO₂ catalyst provided active sites for the MSR, while the excellent oxygen storage capacity of CeO₂ facilitated oxygen transfer and enhanced CO₂ selectivity.

In our previous study, we used CaH₂ as the support for Cu catalysts in MSR and demonstrated a new metal hydride (MH)-coupled MSR protocol, in which MSR was coupled with the hydrolysis of MHs.^33,34 The exothermic hydrolysis of metal hydrides supplies heat for the endothermic MSR, and the CO₂ generated from the MSR is captured by hydroxides derived from hydride hydrolysis, thereby shifting the equilibrium and suppressing CO formation. Although similar strategies, such as sorption-enhanced MSR,¹⁹ can also achieve high hydrogen purity (up to 99.6%) by introducing CO₂ absorbents, they are limited by the reduced hydrogen density, challenges in sorbent regeneration, and lack of thermal integration. In contrast, the MH-coupled MSR system intrinsically integrates heat supply and in situ CO₂ capture within a single material platform. As a result, it delivers superior hydrogen production performance, including a higher hydrogen density (6.56 wt%) than standalone hydride hydrolysis (5.1 wt% for CaH₂ hydrolysis),³⁵ near-100% hydrogen purity compared to ∼75% in conventional MSR,⁷ and a more balanced heat profile than MSR, metal hydride hydrolysis, or sorption-enhanced MSR. Beyond the improved reaction performance, from a practical perspective, the coupled system generates a solid product (Cu/CaCO₃), implying that the catalyst is not regenerated in situ. However, Cu can be recovered from the product using appropriate chelating reagents, and the remaining CaCO₃, due to the abundance of calcium, can be disposed of following established procedures or potentially reused as a Ca-based material.

MHs have been increasingly employed as active catalytic components in various hydrogen-involved reactions, such as reversible hydrogen storage in liquid organic hydrogen carriers (LOHCs), CO₂ hydrogenation, and chemical looping ammonia synthesis.^36,37 For example, in LOHC dehydrogenation, hydrogen vacancies in MHs facilitate hydrogen release and accelerate the rate-limiting step.³⁸ In CO₂ hydrogenation, MHs act as supplementary hydrogen sources, enhancing the C₂₊ product selectivity under lower H₂/CO₂ ratios.³⁹ In ammonia synthesis, MHs enable milder reaction conditions by preventing hydrogen over-adsorption on transition metals and promoting N₂ activation through their electron-donating properties.⁴⁰ In these successful examples, the reversible hydrogen absorption and desorption properties of MHs introduce novel hydrogen transfer pathways and active sites, thereby leading to new mechanisms.

Compared to previously reported MH-based catalysis, MH-coupled MSR over Cu/CaH₂ exhibits distinct features. Notably, CaH₂ functions not only as a support for Cu but also as a hydrogen source via hydrolysis, dynamically transforming into hydroxide, accompanied by heat release. In addition, CaH₂ may directly react with methanol and intermediates such as HCOOH, potentially introducing alternative reaction pathways. To uncover the underlying mechanism, this study explores potential intermediates and pathways using online mass spectrometry (MS), isotope tracing, in situ DRIFTS and other comparative experiments, ultimately proposing a detailed reaction mechanism for the coupled system.

2. Experimental

2.1 Preparation and characterization of the samples

The Cu/CaH₂, Cu/Ca(OH)₂ and Cu/Al₂O₃ samples were prepared using the methods described in our previous study.³⁴ Briefly, Cu precursors were mixed with the support via either mechanical milling or impregnation, followed by reduction in Ar/H₂ flow to yield the corresponding catalysts. All the catalysts contain about 9 wt% Cu. Structural characterizations by XRD and XPS suggest that Cu is mainly in the metallic state, and the supports are generally unaffected.

2.2 In situ detection of hydrogen evolution by online MS

The online mass spectrometry in this study was conducted using the setup shown in Fig. S1a in the SI. During the test, CH₃OH and H₂O were pumped into the evaporation chamber, and their molar ratios were controlled by adjusting the pumping rates, which were previously calibrated. The CH₃OH–H₂O mixture was then vaporized in the evaporation chamber and carried by Ar gas into the reactor containing the catalysts to initiate the hydrogen generation reaction. The evolved gas products were continuously monitored online using a residual gas analyzer (Pfeiffer Vacuum OmniStar GSD 320) in the scanning mode, covering an m/z range of 0–59. The major gaseous products and their corresponding m/z values in the MS were 2 (H₂), 3 (HD), 4 (D₂), 18 (H₂O), 28 (CO) and 44 (CO₂). All signals are calibrated relative to the Ar signal (m/z = 40), which flows at a constant rate of 10 standard cubic centimeters per minute (sccm). To avoid air exposure during the reaction, solid sample loading was carried out inside an Ar-filled glove box. The valves and bypass lines were designed to ensure no air intrusion during system installation.

2.3 Isotope tracing experiment

Isotope tracing was also conducted using MS analysis, and the setup is shown in Fig. S1b. All the deuterated samples in the experiment were used as received without further treatment. CH₃OH and H₂O (or their various deuterated counterparts) were first preheated separately to form individual vapors. These vapors were then mixed and introduced into a reactor containing the catalysts to initiate the coupling reaction. This procedure was designed to minimize the H–D exchange prior to the reaction. Bypass lines were designed to eliminate air intrusion during system installation. The molar ratios of H₂, HD and D₂ were obtained using previously calibrated sensitivity factors of 1, 0.73 and 0.73 for H₂, HD and D₂, respectively.

2.4 In situ infrared spectroscopy measurements

The in situ FT-IR test was carried out in the diffuse reflection mode, and the setup is shown in Fig. S2. The sample and KBr were mixed by grinding at a 1 : 100 mass ratio and loaded into the sample holder inside an Ar-filled glove box. During the test, the system was heated to the target temperature in an Ar atmosphere and maintained constant. After baseline correction of the solid sample, a CH₃OH/H₂O mixed vapor was introduced to initiate the reaction. The spectra were collected every 5 minutes.

3. Results and discussions

3.1 Main mechanistic issues in MH-coupled MSR

MH-coupled MSR over Cu/CaH₂ can be written as follows (eqn (1)):

CaH₂ + CH₃OH + 2H₂O → CaCO₃ + 5H₂ (1)

In addition to providing H₂, the hydrolysis of hydride also generates heat to balance the endothermic MSR and captures CO₂ by the hydrolysis product Ca(OH)₂. There are three main mechanistic issues to be solved.

The first is whether the MH-coupled MSR is a synergetic reaction or a simple addition of three stepwise reactions: hydrolysis of CaH₂, MSR over Cu and CO₂ capture by Ca(OH)₂. The MH-coupled MSR involves hydrogen with different bonding natures: protonic hydrogen from –OH groups in water and methanol, covalent hydrogen from the –CH₃ group in methanol, and anionic hydrogen from CaH₂. How these hydrogen species with different bonding natures interact with each other is the second important mechanism issue. Finally, introducing CaH₂ into the MSR system might change the reaction pathway and the key intermediate during the H₂ generation. The third mechanism issue is the possible new reaction pathways and intermediates in MH-coupled MSR over Cu.

3.1.1 Coupled reaction mechanism: synergetic or stepwise. Online MS analysis was carried out to clarify whether the coupled reaction proceeded via a synergetic or stepwise pathway. MSR was carried out over Cu loaded on different supports with real-time monitoring of gaseous products using MS. The experimental conditions and results are summarized in Table S1 and Fig. 1a, respectively.

Fig. 1 Online MS profiles of (a) MSR over Cu supported on CaH₂, Ca(OH)₂ and Al₂O₃. Hydrolysis of CaH₂ is also included for comparison. (b) MSR over Cu/CaH₂ at different methanol-to-water molar ratios. (c) Time-averaged H₂ production rate in the first 40 min, and the corresponding number of hydrogen atoms released per methanol at different methanol-to-water molar ratios.

As shown in Fig. 1a, the hydrogen production of MSR over Cu/CaH₂ initiates within 2 min, which is within the residence time of the measurement system, indicating almost immediate H₂ generation. In contrast, MSR over Cu/Ca(OH)₂ and Cu/Al₂O₃ was notably slower, initiating around 43 and 20 minutes, respectively (Fig. S3). The initiation rate is similar for coupled MSR and direct hydrolysis over Cu/CaH₂ hydrolysis. However, the hydrogen generation rate is much faster for the coupled MSR. After 5 minutes, the peak signal was approximately 1.5 times higher. For comparison, the hydrogen generation rate of MSR over Cu/Al₂O₃ and Cu/Ca(OH)₂ reached only ∼7% and ∼3% of that over Cu/CaH₂, respectively (Fig. S3). In addition, in the first ∼25 min, there is no detectable CO or CO₂ from the coupled MSR over Cu/CaH₂, as the generated Ca(OH)₂ effectively captures CO₂. Our previous study demonstrated that near-complete CO₂ capture was achieved when the generated Ca(OH)₂ was in excess, which agrees well with the online MS analysis.³³

The impurities were observed in only the later stage of the coupled reaction when most of the CaH₂ was converted into CaCO₃. In contrast, a considerable amount of CO and CO₂ was detected by the MSR over Cu supported on Al₂O₃.

These results clearly demonstrate the advantages of coupled MSR over Cu/CaH₂ in terms of the hydrogen generation rate and H₂ purities. Moreover, the Cu/CaH₂ catalyst is significantly more reactive than the Cu/Ca(OH)₂, suggesting a synchronized reaction mechanism. The higher reactivity is attributed to the exothermicity from CaH₂ hydrolysis, which accelerates MSR kinetics.

Next, the effect of the methanol-to-water ratio was investigated to further clarify the contribution of MSR to hydrogen production in the coupled reaction by varying the methanol feed rate while keeping the water flow constant. Experimental conditions and results are presented in Table S2 and Fig. 1b and c.

As shown in Fig. 1b, introducing additional methanol clearly leads to a higher H₂ generation rate. Our previous study showed that the hydrolysis of CaH₂ proceeds nearly completely and much faster compared to MSR.³³ The additional H₂ generation over Cu/CaH₂ compared to that of CaH₂ hydrolysis is attributed to the coupled MSR process. To quantify the contribution of MSR to the H₂ generation, the H₂ signals from the first 40 minutes were integrated and averaged over time to give the time-averaged H₂ generation rate. As shown in Fig. 1c, the H₂ generation rate is 1.5 and 2.0 times higher than that of the hydrolysis reaction at CH₃OH/H₂O molar ratios of 0.3 and 0.6, respectively. The number of hydrogen atoms contributed per methanol molecule was estimated using eqn (2):

n_H(CH₃OH) = [S_H(total) – S_H(H₂O)]/[x(CH₃OH)·S_H(H₂O)], (2)

where S_H(total) and S_H(H₂O) are the integrated H₂ signals for methanol–water and water-only systems, respectively; and x(CH₃OH) is the methanol-to-water molar ratio. As shown in Fig. 1c, n_H(CH₃OH) is calculated to be around 1.84–1.87 when the methanol/water molar ratio is 0.3 and 0.6.

In CH₃OH, the –OH group is more acidic and can directly react with CaH₂ to produce H₂ and (CH₃O)₂Ca (details discussed later), while the –CH₃ group is more inert. Our experiments show that methanol yields ∼50% of the H₂ at the same molar feed rate of water (Fig. 1b), suggesting lower reactivity of the –OH group in methanol. Therefore, the more inert –CH₃ group must also be involved in the H₂ generation, contributing 0.8 (100% contribution of the –OH group, theoretically) ∼1.3 (50% contribution of the –OH group from the experiment data) out of 3 hydrogen atoms. Despite uncertainties in MS quantification, these results highlight that the heat from CaH₂ hydrolysis effectively promotes C–H bond activation in methanol.

3.1.2 Interactions among various hydrogen sources. First, to clarify the role of hydridic H in CaH₂, a comparative study was conducted using CaH₂/KCl/Cu and Ca/KCl/Cu prepared via ball milling. Here, KCl is introduced to assist in the pulverization of large Ca granules into fine powder similar to that of CaH₂. Structural characterization by XRD and SEM (Fig. S4 and S5) suggests that the two samples have a similar structure. The hydrogen production performance of Ca/KCl/Cu and CaH₂/KCl/Cu coupled with MSR at 220 °C and a methanol-to-water molar ratio of 1 : 3 is shown in Fig. 2a (details are shown in Table S3). Both systems showed comparable results, with methanol conversions of ∼66%, hydrogen purity above 99%, and only trace CO detected (0.10% and 0.16%). XRD analysis of the solid products (Fig. 2b) confirmed CaCO₃ formation in both samples, indicating that a coupling reaction occurred on both the Ca and CaH₂ surfaces. This was further supported by IR spectra (Fig. S6), which exhibited characteristic CaCO₃ peaks at 1440 and 876 cm⁻¹.⁴¹

Fig. 2 Comparison of Ca/KCl/Cu and CaH₂/KCl/Cu coupled with MSR: (a) hydrogen yield, methanol conversion, and gas composition. (b) XRD patterns of the hydrolysis (H₂O) and coupling (H₂O + CH₃OH) solid products.

Similar behaviors in coupled MSR on Ca and CaH₂ supported Cu catalysts indicate that the presence of H⁻ in CaH₂ is not essential for the coupling process. If the reaction is exothermic and the hydrolysis products can capture CO₂, effective coupling can be achieved. This aligns with our previous findings on LiH and NaH.³⁴ Nevertheless, CaH₂ offers advantages over Ca, including a higher hydrogen yield density and a more moderate reaction heat.

To further probe the interactions among hydrogen species with different bonding natures, isotope tracing with online MS was employed to analyze isotope distribution in the evolved gas. The gas profiles and the time-averaged deuterium gas percentage D₂% are presented in Fig. 3.

Fig. 3 (a) H₂, HD, and D₂ profiles of CH₃OH and H₂O over Cu/CaH₂ under different deuterium labelling conditions. (b) D₂ percentage in the evolved gas after different reaction times.

It is interesting to note that when CD₃OD/D₂O was fed to the system, notable H₂ formation is still observed (Fig. 3a), which can be tentatively attributed to the solid-state H exchange in the CaH₂–Ca(OD)₂ mixture (eqn (3)), as direct decomposition of CaH₂ requires a higher temperature:⁴²

CaH₂ + Ca(OD)₂ = CaH_2−xD_x + Ca(OD)_2−x(OH)_x. (3)

The H in Ca(OD)_2−x(OH)_x is of a protonic nature and can combine with the hydridic H in CaH₂ directly to give H₂. Similarly, the apparent D₂ formation when feeding is CH₃OD/H₂O and CH₃OH/D₂O can also be explained by such solid-state H exchange.

Such complicated H exchange makes quantitative analysis of the contribution of each type of H very difficult. However, the different reactivity of H with different bonding natures can be observed. The higher D₂% from CH₃OH–D₂O compared to CH₃OD–H₂O clearly indicates that H in H₂O is more reactive compared to that in the –OH group of CH₃OH (Fig. 3b). The peak that appeared at ∼2 min in the H₂ signal after the feeding can be attributed to the rapid hydrolysis reaction (Fig. 3a).

Another interesting observation is that the D₂% in the first 5 min is even lower for CD₃OD/D₂O compared to that of CH₃OD/D₂O (Fig. 3b). This can be explained by the lower reactivity of the –CH₃ group in CH₃OH overlapped with the kinetic isotope effect. However, the D₂% increases much more rapidly with time for CD₃OD/D₂O compared to other D labelling precursors (Fig. 3b), indicating that D in the more stable –CD₃ group is released in the later stage of reaction. Compared to the peak of the H₂ signal at ∼2 min, it can be concluded that the stable –CD₃ group was most likely activated by the heat from the hydrolysis of CaH₂.

3.1.3 Identification of intermediates in the coupled reaction. MSR on transition metals involves intermediates, such as methoxy (–OCH₃), formate (HCOO⁻), formaldehyde (HCHO), and methyl formate (HCOOCH₃).^43–45 Introducing CaH₂ may introduce new pathways and intermediates, potentially including calcium methoxide (Ca(OCH₃)₂) and calcium formate (Ca(HCOO)₂)—formed via reactions of CaH₂ with CH₃OH and HCOO⁻, respectively. The roles of these possible stable intermediates in the coupled reaction are first examined.

Ca(OCH₃)₂ can be formed by the direct reaction of CaH₂ and CH₃OH:

CaH₂ + 2CH₃OH → Ca(OCH₃)₂ + 2H₂. (4)

XRD analysis (Fig. S7) showed that Ca(OCH₃)₂ can be readily formed at 80 °C according to eqn (4) with or without Cu in an autoclave. The formed Ca(OCH₃)₂ remained unchanged within the temperature range of 80–250 °C, indicating its high thermal stability under reaction conditions. TPD–MS analysis (Fig. 4) revealed that Ca(OCH₃)₂ decomposes at ∼530 °C to release H₂, and this temperature is unaffected when combined with CaH₂, CuO, or Cu/CaH₂. A weak H₂ signal at 310 °C in the Cu system was attributed to the desorption of pre-adsorbed hydrogen. These results further indicate that Ca(OCH₃)₂ is too stable to be an active intermediate for H₂ generation below 250 °C.

Fig. 4 TPD–MS H₂ signals for Ca(OCH₃)₂ containing CuO, CaH₂ or Cu/CaH₂.

In the presence of H₂O, Ca(OCH₃)₂ can be converted into CH₃OH and Ca(OH)₂ at 250 °C according to eqn (5).

Ca(OCH₃)₂ + 2H₂O → Ca(OH)₂ + 2CH₃OH (5)

Ca(OH)₂ and CaCO₃ are detected by XRD without and with a Cu catalyst (Fig. S8). Although CH₃OH readily reacts with CaH₂ to form Ca(OCH₃)₂, the high thermal stability of Ca(OCH₃)₂ prevents further dehydrogenation under reaction conditions. The results suggest that despite its high thermal stability, Ca(OCH₃)₂, once formed, undergoes hydrolysis, which is expected to be significantly faster than thermal decomposition to regenerate CH₃OH and return to conventional MSR mechanisms over Cu. In the coupled reaction, the generated CO₂ is absorbed by the Ca(OH)₂ from hydrolysis, yielding CaCO₃. Therefore, even if Ca(OCH₃)₂ is transiently formed, it is rapidly converted back to CH₃OH and funneled into the main formate-mediated MSR pathway.

Then, another possible stable intermediate, Ca(HCOO)₂, was investigated, which may be formed from the following reaction (eqn (6)):

CaH₂ + 2HCOOH = Ca(HCOO)₂ + H₂. (6)

These results suggest that CaH₂ readily reacts with HCOOH to form Ca(HCOO)₂, as confirmed by the XRD results (Fig. S9). Similar to Ca(OCH₃)₂, Ca(HCOO)₂ also exhibits high thermal stability. The decomposition temperature is ∼450 °C. By mixing with Ca(OH)₂ and Ca(OH)₂/CuO, the decomposition temperature is only slightly reduced to ∼410 °C, suggesting that Ca(HCOO)₂ is not a key intermediate for the H₂ generation in the coupled MSR either (Fig. 5). Considering that the decomposition temperature of Ca(HCOO)₂ is far higher than the reaction temperature employed in this study, its thermal decomposition is unlikely to occur under reaction conditions. Instead, any transiently formed Ca(HCOO)₂ would be rapidly converted back to formate-related intermediates in the presence of H₂O, thereby allowing the reaction to proceed through the conventional formate-mediated MSR pathway.

Fig. 5 Thermogravimetric profiles of the Ca(HCOO)₂-containing samples.

After excluding these stable intermediates, we further examined the unstable intermediates using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The main species of interest and the characteristic peaks of their corresponding functional groups^43–45 are listed in Table S4.

Fig. 6 and Fig. S10 summarize the in situ DRIFTS spectra of MSR over Cu/CaH₂ and Cu/Al₂O₃ from 100 to 250 °C. The peaks at 1032 and 1057 cm⁻¹ are related to adsorbed CH₃OH. The key intermediates are formate species, as evidenced by the 1580–1600 and 1367–1376 cm⁻¹ peaks.^46,47 On Cu/CaH₂, peaks corresponding to formate⁴⁸ (1597 and 1371 cm⁻¹) readily appeared even at 100 °C after introducing the CH₃OH–H₂O mixture for only 10 min (Fig. 6a), implying initiation of the coupled MSR at a very mild temperature. These peaks intensified at higher temperatures (Fig. 6a–c). In contrast, only CH₃OH-related peaks from the adsorbed CH₃OH were observed on the Cu/Al₂O₃ surface at 100 °C (Fig. 6a). HCOO⁻-related peaks only began to emerge at 150 °C (Fig. 6b). Peaks corresponding to fully dehydrogenated products CO₂ (2360 cm⁻¹) and CO (2181 cm⁻¹) were only detected above 200 °C and become prominent only at 250 °C (Fig. S10), suggesting that effective MSR over Cu/Al₂O₃ requires high temperature.

Fig. 6 In situ DRIFT spectra obtained after the reaction of Cu/Al₂O₃ and Cu/CaH₂ with the CH₃OH/H₂O mixed vapor (molar ratio 1 : 3) at different temperatures: (a) 100 °C, (b) 150 °C, and (c) 200 °C.

The formate-related peaks are much weaker over Cu/Al₂O₃. On Cu/Al₂O₃ at 250 °C (Fig. S10), there is a clear CO₂-related peak at 2360 cm⁻¹ and CO (2178 cm⁻¹),⁴⁶ which increases over time. In contrast, no clear CO₂ or CO peaks were observed on Cu/CaH₂ (Fig. S10). Instead, carbonate-related peaks at 1425 and 870 cm⁻¹ (ref. 41) are detected, which suggests effective CO₂ capture in the MH-coupled MSR.

To investigate the intermediate evolution during the coupled reaction, temporal changes in methoxy and formate peak intensities over Cu/CaH₂ were monitored after stopping the CH₃OH and H₂O feed (Fig. 7a). As shown in Fig. 7b, the methoxy-related peak at 1030 cm⁻¹ decreased whereas the formate-related peak at 1585 cm⁻¹ increased, indicating a methoxy-to-formate transformation pathway.

Fig. 7 (a) Time-dependent DRIFTS spectra of MSR and (b) peak intensity of methoxy at 1030 cm⁻¹ and formate at 1585 cm⁻¹ over Cu/CaH₂. The CH₃OH/H₂O mixed vapor (molar ratio-1 : 3) was introduced into the chamber for 10 min.

The in situ FT-IR results suggest that the MH-coupled MSR over Cu/CaH₂ also proceeds via the HCOO⁻ intermediate, which is the same as on conventional Cu catalysts. However, the presence of MH facilitates MSR initiation, promotes formate formation, and accelerates its decomposition at much lower temperatures (∼100 °C) due to localized heat and alkaline conditions from CaH₂ hydrolysis. These results are in agreement with the hydrogen generation and online MS data.

3.2 Proposed mechanism for the coupled reaction

Based on the above experimental results, we propose the mechanism of MSR-coupled hydrogen production over the Cu/CaH₂ system, as illustrated in Fig. 8.

Fig. 8 Schematic of the coupled reaction mechanism.

The main evolution pathway of CH₃OH over Cu follows the conventional HCOO⁻ pathway. The MH plays two important roles: (1) supplying heat to initiate MSR, particularly endothermic C–H bond dissociation, at a lower temperature, and accelerating their conversion to CO₂; (2) Ca(OH)₂ captures CO₂, which also facilitates the water–gas shift reaction and suppresses CO formation.

CaH₂ introduces two possible new intermediates: Ca(OCH₃)₂ from the direct reaction of CH₃OH with CaH₂, and Ca(HCOO)₂ from the reaction of HCOOH with CaH₂. However, both species are highly thermally stable and cannot directly release hydrogen. In the presence of water, however, they can be hydrolyzed to regenerate CH₃OH and HCOO⁻ adsorbed on the Cu surface, re-entering the main reaction pathway, as shown in the two branch reactions of Fig. 8.

4. Conclusions

This study provides a systematic study on the mechanism of hydrogen production from MH-coupled methanol steam reforming (MSR) on the Cu/CaH₂ system. The main conclusions are as follows.

(1) The MH-coupled MSR proceeds via a synergetic mechanism instead of the simple addition of separated MSR over Cu and hydrolysis of CaH₂ and CO₂ capture by Ca(OH)₂. Online MS analysis suggests that CH₃OH starts to release H₂ in the initial stage of the reaction from both the hydroxyl and methyl groups in the molecule, which is concomitant with the hydrolysis of CaH₂.

(2) Isotope labelling study suggests a complicated exchange of H with different bonding natures. The hydridic H in CaH₂ is not essential for synergized MSR, while CaH₂ is superior to Ca owing to its higher H density and milder thermal effect.

(3) The evolution of CH₃OH over Cu follows the conventional HCOO⁻ pathway. The MH provides heat to facilitate the MSR at lower temperatures and eliminate both CO₂ and CO. Ca(OCH₃)₂, and Ca(HCOO)₂ might be formed in the synergized MSR process, while they are not active intermediates for hydrogen generation at mild temperatures due to their high thermal stability. These intermediates, once formed, can react with water, regenerate CH₃OH and HCOOH and return to the main reaction mechanism.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this study are available within the article and its supplementary information (SI). Supplementary information is available. This supplementary information document provides comprehensive experimental details, material characterizations, and mechanistic insights supporting this study. It includes detailed schematics of the online MS and in situ IR setups, alongside tables outlining specific reaction conditions. Furthermore, the document presents XRD and SEM-EDS characterizations to confirm the physical and chemical properties of the synthesized catalysts. Finally, it provides quantitative data on hydrogen yield and gas composition, coupled with in situ DRIFTS and IR spectral analyses, to elucidate the reaction intermediates and the underlying hydrogen production pathways. See DOI: https://doi.org/10.1039/d6ey00038j.

Acknowledgements

This work is supported by the MOST of China (No. 2024YFE0207400), NSFC (No. 22579003) and the Key Project of Jiangsu Province Basic Research Program (No. BK20243033).

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