Catalyst, Reactor, and Purification Technology in Methanol Steam Reforming for Hydrogen Production: A Review

Abstract

Methanol steam reforming (MSR) represents a highly promising pathway for sustainable hydrogen production due to its favorable hydrogen-to-carbon ratio and relatively low operating temperatures. The performance of the MSR process is strongly dependent on the selection and rational design of catalysts, which govern methanol conversion, hydrogen selectivity, and the suppression of undesired side reactions such as carbon monoxide formation. Moreover, advancements in reactor configuration and thermal management strategies play a vital role in minimizing heat loss and enhancing heat and mass transfer efficiency. Effective carbon monoxide removal technologies are indispensable for obtaining high-purity hydrogen, particularly for applications sensitive to CO contamination. This review systematically summarizes recent progress in catalyst development, reactor design, and gas purification technologies for MSR. In addition, the key technical challenges and potential future directions of the MSR process are critically discussed. The insights provided herein are expected to contribute to the development of more efficient, stable, and scalable MSR-based hydrogen production systems.

1. Introduction

The growing global energy demand and the urgent need for clean energy solutions have significantly increased interest in hydrogen as an ideal energy carrier [1,2,3,4,5,6]. Hydrogen fuel cells have attracted considerable attention due to their high efficiency and environmental friendliness. These cells can directly convert chemical energy into electricity, thereby circumventing the efficiency limitations imposed by the Carnot cycle and achieving higher energy conversion efficiency [7]. However, the hydrogen compression process consumes substantial energy, resulting in reduced overall system efficiency [8]. Additionally, hydrogen poses serious safety risks due to its explosion potential in collision accidents [9]. In contrast, on-site hydrogen production technologies utilize high-energy-density renewable liquid fuels to generate hydrogen through reforming reactions [10,11], offering advantages such as improved energy efficiency and reduced safety risks.

Methanol, as a liquid hydrogen carrier [12,13], is widely utilized among various renewable low-carbon alcohol-based fuels due to its low reforming temperature, favorable economics, high hydrogen-to-carbon ratio, and ease of storage and transportation [14]. Methanol can be produced from biomass-derived syngas, as biomass is a widely available renewable resource with carbon-neutral characteristics [15]. Biomass-derived syngas can be directly converted into methanol through chemical looping processes [16], including chemical looping gasification [17], chemical looping combustion [18], and chemical looping hydrogen production [19]. Methanol steam reforming (MSR) does not require the cleavage of C–C bonds and proceeds at relatively low temperatures (200–350 °C), whereas the reforming of methane—the primary industrial hydrogen feedstock—requires temperatures exceeding 500 °C [20]. Therefore, methanol is often regarded as one of the most favorable liquid fuels for hydrogen production.

Methanol steam reforming, methanol decomposition, partial oxidation of methanol, and oxidative steam reforming of methanol are the four main methods for hydrogen production from methanol. Each of these approaches has its own advantages and limitations, as summarized in

Table 1. Comparison of methanol-reforming methods [27,28,29,30,31,32,33].

Method	Advantages	Disadvantages
Methanol steam reforming	• Mild reaction conditions • High conversion rate • High hydrogen content • Low CO generation	• Expensive • Requires external heat supply
Methanol decomposition	• H2 can be produced directly from methanol • No extra reactants needed	• Highly selective towards CO • High CO concentrations must be removed for fuel cell use • Requires external energy source
Partial oxidation of methanol	• Exothermic • No extra heating needed	• Low H2 yield • High CO content • Requires pure oxygen supply
Oxidative steam reforming of methanol	• Thermally neutral process • No external heating is required	• High reaction temperature • Requires pure oxygen supply

. Among these, MSR has received the most attention and has been studied most extensively. Compared with the other methods, MSR offers the advantages of mild reaction conditions and the lowest CO concentration in the product gas. MSR represents a viable strategy for the development of proton exchange membrane fuel cells (PEMFCs), which generate electricity using hydrogen fuel and are among the most promising power sources for various mobile applications [21]. PEMFCs have attracted considerable interest due to their high efficiency, high power density, low operating temperature, and environmental compatibility. Given the safety concerns associated with hydrogen storage and transportation, MSR holds great potential as a hydrogen supply strategy for PEMFCs, especially in mobile platforms such as vehicles and aircraft [22,23,24,25,26].

MSR is a chemical process in which hydrocarbons or alcohols react with steam to produce hydrogen, carbon dioxide, and carbon monoxide. Equations (1)–(3) represent the key reactions involved in the methanol steam reforming process. Equation (1) describes the overall steam reforming reaction, while Equation (2) corresponds to methanol decomposition, and Equation (3) refers to the water–gas shift (WGS) reaction [34]. Among these, the reverse water–gas shift reaction is considered the main pathway for CO formation [35,36,37,38].

$$\begin{array}{c}\mathrm{C}{\mathrm{H}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2 \Delta H=49.7 {\mathrm{kJmol}}^{-1}\end{array}$$

(1)

$$\begin{array}{c}C{\mathrm{H}}_3\mathrm{OH}\rightleftharpoons \mathrm{CO}+2{\mathrm{H}}_2 \Delta H=90.2 {\mathrm{kJmol}}^{-1}\end{array}$$

(2)

$$\begin{array}{c}\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2 \Delta H=-41.2 {\mathrm{kJmol}}^{-1}\end{array}$$

(3)

In the MSR process, catalyst selection, reactor design and optimization, and CO removal technologies are critical factors influencing methanol conversion efficiency and hydrogen quality [39]. Catalysts play a central role in facilitating the reaction and are a major focus of current research. Copper-based catalysts are widely used in MSR due to their high activity and selectivity at low temperatures; however, they are prone to deactivation as a result of thermal sintering [40,41,42,43]. Group VIII–X metal catalysts also show promise, offering relatively high activity and stability, though their performance is often influenced by the choice of support materials and is generally associated with lower hydrogen yields. Noble metal–supported oxides, such as those containing platinum, palladium, or gold, typically exhibit excellent activity, stability, and selectivity [44,45,46,47], but their high cost limits large-scale applications. Each type of catalyst has its own advantages and limitations; the incorporation of promoters and support materials can effectively enhance overall catalytic performance [33,48,49].

The reforming reactor serves as the site where MSR takes place, and its heat and mass transfer characteristics significantly influence reforming performance [50]. Based on the catalyst configuration, reforming reactors can be broadly classified into three main types: conventional packed-bed reactors, wall-coated microreactors, and porous catalyst support reactors. Conventional packed-bed reactors typically refer to tubular reactors filled with catalyst pellets [51]. These reactors are often characterized by large volume, high flow resistance, uneven reactant distribution, and the presence of hot or cold spots [52,53]. In contrast, microreactors offer advantages such as rapid mixing at micrometer to sub-millimeter scales, efficient heat and mass transfer, and inherent operational safety [6]. Microreactors are generally categorized into wall-coated microreactors and porous catalyst support reactors. Wall-coated microreactors involve directly coating the catalyst onto the internal surfaces of the reaction channels. Due to their small dimensions, these reactors can achieve high reaction efficiency while maintaining low pressure drop and high flowability. Examples include microchannel reactors [54] and microreactors with post array structures [55]. The structural design of microreactors plays a crucial role in determining their heat and mass transfer performance. Uniform temperature and flow distribution, low pressure drop, high surface-area-to-volume ratio, and scalability are key design goals for microreactors [56]. To further enhance catalyst loading, heat transfer, and gas diffusion, porous catalyst support reactors have been developed. In these systems, catalysts are loaded onto porous scaffolds, increasing the contact time between reactant gases and the active sites. Compared to wall-coated microreactors, porous support reactors generally exhibit superior reforming performance but at the expense of higher pressure drop [32].

During the methanol reforming process for hydrogen production, in addition to the target product hydrogen, the reformate gas typically contains various byproducts and potentially unreacted species, such as CO, CO₂, H₂O, and unreacted CH₃OH. These components not only reduce the purity of hydrogen but may also adversely affect its performance in downstream applications. Carbon monoxide is one of the primary byproducts in MSR and has a significant impact on reaction efficiency and hydrogen purity [57]. The presence of CO affects the active sites on the catalyst, leading to a reduction in catalytic activity. CO competes with reactants for adsorption on the catalyst’s active centers, thereby hindering the adsorption and reaction of the reactants [58]. When integrated with fuel cell systems, the CO in the product poses even greater hazards. CO can block and thus reduce the availability of hydroxyl active sites on the catalyst, impairing fuel cell performance. CO concentrations as high as 100 ppm can cause rapid fuel cell degradation, while levels as low as 10 ppm still adversely affect fuel cell operation [59,60]. Typically, CO concentrations below 0.2 ppm are required to ensure effective long-term fuel cell performance [61,62]. Therefore, efficient CO removal technologies are essential to significantly reduce CO content and improve hydrogen purity in the MSR process. Common CO removal methods include pressure swing adsorption (PSA), separation membranes, CO-selective oxidation, and CO-selective methanation [63]. Each CO removal technology has different application scopes and operating conditions, making the selection of an appropriate CO removal method crucial for MSR technology.

This work reviews recent advances in MSR for hydrogen production, including catalysts, reactors, and purification technologies. A systematic summary is provided on various strategies to improve methanol conversion, hydrogen yield, and hydrogen purity to meet increasing demand. Finally, the application scenarios and challenges of MSR are summarized and discussed.

2. Catalyst

2.1. Copper-Based Catalysts

Copper-based catalysts are the most extensively studied catalysts for MSR. However, these catalysts are prone to thermal sintering, aggregation, and carbon deposition. To address these issues, various promoters, supports, and synthesis methods have been explored. Traditionally, ZnO, ZrO₂, CeO₂, and Al₂O₃ are commonly used as promoters [49,64,65,66,67,68,69]. Other materials such as Ga₂O₃ [70,71], MgO [33], and In₂O₃ [72]; transition metals and their oxides including Fe [73], Ni [74], Ti [75], Sc₂O₃ [76], and Y₂O₃ [72,77]; as well as rare-earth metals and their oxides such as La [78], Sm [79], and Gd [80] have also been employed to enhance the performance of copper-based catalysts. To address these issues, various active metals, promoters, supports, and synthesis methods have been employed to enhance the performance of copper-based catalysts. Transition metals such as Ni [74] and Co are commonly used as active components due to their catalytic activity. Other transition metals and their oxides, including Fe [73], Ti [75], Sc₂O₃ [76], and Y₂O₃ [72,77], are often employed as promoters or structural modifiers to improve catalyst stability and performance. Traditional metal oxides like ZnO, ZrO₂, CeO₂, and Al₂O₃ serve as promoters or supports to enhance catalyst dispersion, thermal stability, and resistance to sintering [49,64,65,66,67,68,69]. Additionally, oxides such as Ga₂O₃ [70,71], MgO [33], and In₂O₃ [72] have been utilized. Rare-earth metals and their oxides, such as La [78], Sm [79], and Gd₂O₃ [80], are also employed owing to their excellent oxygen storage and redox properties.

Table 2. Performance of Cu-based catalysts for steam reforming of methanol.

Catalyst	T (°C)	Methanol Conversion (%)	H2 Selectivity (%)	Activity	CO Selectivity (%)	Refs.
Cu-ZnO	300	40	-	$~45 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	0	[76]
Cu/Se2O3-ZnO	300	90	-	$~130 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	0	[76]
Cu-Ga/ZnO	320	96	-	$720 \mathrm{mmol}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{h}}^{-1}$	-	[70]
Cu/Zr	260	20	-	$25.28 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	-	[81]
Cu/ZrSi	260	73	-	$102.78 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	-	[81]
CuZn3GaZr	275	75	-	312 mlg−1min−1	0.3	[71]
ZrO2-CeO2-Cu/KIT-6	300	96	99.8	-	0.7	[82]
Cu-ZnO-ZrO2/MCM-41	300	97.8	99	-	0.4	[83]
Cu/ZnO/ZrO2	250	88.6	75	806.4 molmol−1h−1	-	[84]
Cu-ZnO-Al2O3-ZrO2	240	100	75	-	-	[85]
Cu/CeO2	250	95.5	-	-	-	[86]
CuO/CeO2	260	100	-	-	2.4	[87]
CuO/ZnO/CeO2/Al2O3	260	98	65	-	-	[88]
Cu-Zn/Y-Al2O2/Al	350	74	-	-	-	[89]
CuFeMg/Al2SO3	250	100	-	$20.28 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	2.3	[90]
Cu/ZnO/Sc2O3	300	87	-	$140 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	0.2	[76]
Cu/SiO2	300	94.67	-	-	0.36	[41]
Cu-In/SiO2	300	96.1	-	-	0.07	[41]
CuZnOx	150	18.8	-	-	-	[91]
CuZnGaOx	150	22.5	-	-	-	[91]
Cu/TiO2	300	90.2	-	-	7.6	[92]
CuNi/TiO2	300	92.6	-	-	9.6	[92]
CuZnAl	200	56	-	-	-	[93]
Cu/Zn/GaOx	195	98	-	$49 \mathrm{\mu }\mathrm{mol}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	0.2	[94]
Ce/Cu/ZnAl	240	92	-	-	0.9	[80]
CuZnAl-Mg	200	68	-	$~51.94 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	-	[93]
CuZnGaZr	275	75	-	$232.14 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	-	[71]
Cu/Zn1.11La1.26Al0.5O4.27	300	96	86	-	-	[95]
Cu/Y1.6Ce0.76Ru0.03O4	300	99.5	98.7	-	-	[96]
Cu0.5Ce0.25Mg0.05/Al	250	100	46.5	-	0.15	[97]
Cu/SBA-15	300	91	-	-	2.8	[98]
Cu/ZnO/SBA-15	300	92.1	-	-	1.7	[98]
Cu/ZnO/CeO2/SBA-15	300	93	-	-	1.1	[98]
Cu/ZnO/ZrO2/SBA-15	300	96.8	-	-	2.1	[98]
Cu/ZnO/CeO2/ZrO2/SBA15	300	95.2	-	-	1.4	[98]
5Cu10Ce	350	99.7	75.1	5.3 molmin−1g−1	0	[99]
5Zn5Cu10Ce	90.9	75	-	4.2 molmin−1g−1	0	[99]
Cu/ZnAl-LDHs/γ-Al2O3	300	100	-	$2.08 \mathrm{\mu }{\mathrm{mol}}_{\mathrm{H}2}{\mathrm{g}}_{\mathrm{cat}}^{-1}{\mathrm{s}}^{-1}$	1	[100]

summarizes the MSR performance of selected copper-based catalysts. It should be noted that some of the references cited in this paper primarily focus on catalyst preparation and reaction performance, with limited reporting on hydrogen selectivity or yield data. Therefore, the hydrogen selectivity data in the table are incomplete, and due to significant differences in experimental conditions, direct comparison of the related data has certain limitations. Future studies will further supplement and improve these aspects in order to provide a more comprehensive performance evaluation.

Extensive studies have confirmed the promoting effect of ZnO. Tajrishi et al. [98] found that ZnO plays a positive role in reducing CO selectivity in their study on Cu/SBA-15 nanocatalysts. Wang et al. [101] investigated nano-Cu₂O/ZnO catalysts using transmission electron microscopy and energy-dispersive spectroscopy and confirmed that the dispersion of copper nanoparticles on ZnO nanorods was improved, enhancing the catalyst’s resistance to sintering. Mrad et al. [99] reported that the addition of ZnO facilitates the reduction of Cu²⁺ to the active species Cu⁺ and Cu⁰ during the MSR reaction. Zhang et al. [102], in their study on CuZnO/Al₂O₃/Al catalysts, observed that the self-activation of copper nanoparticles induces ZnO encapsulation, which not only prevents copper nanoparticle aggregation but also promotes the formation of abundant Cu₂O/ZnO synergistic sites. These synergistic sites significantly reduce the activation energy barrier of the rate-determining step.

ZrO₂ is commonly used as a promoter or support for copper-based catalysts in MSR. ZrO₂ enhances catalytic performance by improving the dispersion of active metal species and increasing catalyst stability [83]. Park et al. [103] found that the presence of ZrO₂ in Cu/ZnO/ZrO₂/Al₂O₃ catalysts facilitated the reduction of CuO and promoted methanol conversion. Chen et al. [104] reported that the addition of ZrO₂ as a promoter in attapulgite-based zeolite-supported copper catalysts led to the formation of Cu⁰/Cu⁺–ZrO_xH_γ interfacial sites. The formation of such interfaces improved the dispersion of active metal species on the catalyst surface, enhanced stability, and suppressed both sintering and carbon deposition. Sanches et al. [84] found that the introduction of ZrO₂ increased the microstrain of CuO and ZnO, reduced their crystallite sizes, and inhibited grain growth. The addition of ZrO₂ nanoclusters or amorphous materials in Cu/ZnO and Cu/ZnO/ZrO₂ catalysts promoted the formation of exposed CuO species. Another study on ZrO₂ revealed that the incorporation of Al₂O₃ or ZrO₂ into CuZnxGa catalysts increased the surface area of the resulting CuZnxGaAl multicomponent catalysts and enhanced their catalytic activity [71]. These findings suggest that ZrO₂, when used as a promoter, may also induce significant structural, textural, and morphological changes in the active metal components [103].

CeO₂ is a key promoter that enhances the dispersion of active catalytic components, improves catalytic activity, suppresses CO formation, and increases catalyst stability [41,42,105,106,107,108,109]. CeO₂ possesses a fluorite cubic structure, which facilitates the migration of oxygen ions to the surface under low-temperature conditions. The oxidation state of Ce in CeO₂ can reversibly shift between Ce⁴⁺ and Ce³⁺, allowing the catalyst surface to form oxygen vacancies and exhibit oxygen storage and release capacity [110]. Owing to this oxygen storage–release property, the addition of CeO₂ can improve thermal stability and resistance to sintering, as ceria can reversibly store and release substantial amounts of oxygen in response to reaction conditions. In the reductive environment of MSR, CeO₂ within the catalyst can be partially reduced, and the resulting active oxygen species can react with carbon deposits, thereby achieving carbon gasification [111]. Furthermore, CeO₂ addition significantly suppresses CO concentration in the reformate, especially when combined with ZrO₂. The resulting CeO₂–ZrO₂ solid solution not only promotes surface methanol dehydrogenation but also exhibits superior oxygen storage capacity compared to pure CeO₂ [107]. However, the loading amounts of CeO₂ and ZrO₂ must be optimized. Bagherzadeh et al. [108] reported that a CuO–ZnO–Al₂O₃ nanocatalyst achieved the highest activity when both ZrO₂ and CeO₂ were loaded at 10 wt%.

In addition to extensively studied promoters and supports such as ZnO, ZrO₂, and CeO₂, other metals including Ga, P, In, Sc, Ti, Fe, Y, La, and Gd have also attracted increasing attention [33,49,112]. These metals enhance the performance of Cu-based catalysts in various ways. Tong et al. [92] found that in Ga-doped CuZnGaOx catalysts, a non-stoichiometric cubic spinel phase containing interstitial Cu⁺ ions was formed. This suggests that Ga doping leads to the formation of defective ZnGa₂O₄ surfaces by combining with Cu and Zn oxides, thereby generating a large number of small copper clusters. The resulting clusters reduced CO formation and increased methanol conversion. Although Ga addition can improve methanol conversion and H₂ production in Cu-based catalysts [70,91,94], it may also lead to an increased deactivation rate [113]. Kim and Kang et al. [114] studied the effect of P on Cu-based catalysts and found that phosphorus plays a critical role in promoting Brønsted acid sites in Cu–Ti–P oxide catalysts, which facilitates strong methanol oxidation and thermal decomposition into acetaldehyde and CO₂. As a result, the presence of P increased H₂ production while suppressing CO formation. F. Bossola et al. [81] reported that adding an appropriate amount of In to MSR Cu/SiO₂ catalysts led to the formation of an InOx buffer phase at the interface between Cu nanoparticles and the silica support. This enhanced catalyst fluidity, increased electron density on Cu nanoparticles, and improved H₂ selectivity and yield. Pu et al. [76] investigated Sc-promoted Cu/ZnO catalysts for MSR in the temperature range of 220–600 °C and found that Sc doping into the ZnO lattice enhanced the metal–support interaction, which improved metal dispersion and enhanced Cu resistance to sintering. Zhao et al. [75] studied Ti-modified catalysts and found that titanium increased the support’s specific surface area, thereby improving Cu dispersion and increasing the number of active Cu sites. Titanium also reduced the number of hydroxyl groups, which suppressed the formation of acidic sites, thus reducing side reactions such as methanol dehydration and favorably enhancing H₂ production. Zhang et al. [90] examined the modification effects of FeOx in CuFeMg/Al₂O₃ catalysts and, through DFT calculations, confirmed that FeOx addition could significantly reduce the reaction energy barrier for MSR over Cu-based catalysts. The catalyst exhibited stable activity during the 100 h durability test, with methanol conversion maintained above 90%. Moreover, FeOx enhanced H₂O adsorption and activation while promoting electronic rearrangement (as shown in Figure 1). Matsumura [72] investigated the promotion effect of Y₂O₃ in Cu/ZnO/ZrO₂/In₂O₃ catalysts. The study found that yttrium oxide improved the dispersion of co-precipitated metal oxides and increased the number of surface Cu species. However, particle aggregation was observed at 500 °C, leading to a decrease in surface Cu species and, consequently, a drop in catalytic activity [77].

In summary, promoters interact with catalysts, influencing their dispersion, reactivity, and stability, thereby improving overall performance. Promoters facilitate better dispersion of the active copper species, enhancing catalytic activity and improving resistance to agglomeration and sintering. The interaction between promoters and catalysts also leads to the formation and exposure of reduced copper species (e.g., Cu⁺ and Cu⁰) on the catalyst surface, which is another positive factor in enhancing MSR performance. However, it is important to note that different catalysts or promoters have specific applicable conditions; under inappropriate operating conditions, catalysts with added promoters may exhibit poorer reforming performance.

2.2. The Influence of Synthesis Methods on the Performance of Copper-Based Catalysts

Different synthesis methods have varying effects on the surface morphology, specific surface area, distribution of active phases, interactions, and dispersion of MSR catalysts [115]. These synthesis methods can also alter reaction pathways, thereby influencing the selectivity of target products and the stability of the catalysts [33,116,117,118]. Currently, commonly used preparation methods for copper-based catalysts include combustion synthesis [88,119,120,121,122,123,124,125,126,127,128,129], hydrothermal synthesis [130], impregnation [131,132,133,134,135], solid-state reaction [136], co-precipitation [137,138], homogeneous precipitation [139,140], stepwise precipitation [137], nanocasting [123], and the microwave-assisted polyol method [141]. Depending on the characteristics of the support, the optimal preparation method for copper-based catalysts may vary. The following section summarizes selected recent examples.

Combustion synthesis has been widely used in the preparation of copper-based catalysts [121,122,127,128]. During the preparation of copper-based catalysts by combustion synthesis, factors such as the gas atmosphere, fuel properties, and precursor types significantly influence the catalyst’s performance and catalytic activity. The pore structure of catalysts prepared by combustion synthesis is notably affected by the calcination temperature; lower calcination temperatures help preserve the pore structure, thereby achieving higher activity and long-term stability in MSR. Ploner et al. [142] found that calcination above 600 °C causes loss of porosity, resulting in more severe sintering and catalyst deactivation.

Hydrothermal synthesis is a method used to prepare copper-based catalysts for MSR. Generally, catalysts prepared by hydrothermal synthesis exhibit higher specific surface areas and superior catalytic activities compared to those prepared by combustion or solid-state methods [119,120,130]. Fasanya et al. [126] demonstrated that one-pot hydrothermal synthesis is suitable for preparing mesoporous CuZn/MCM-41 catalysts, as it enables high dispersion of metal components and better interaction with the support.

Co-precipitation is a commonly used catalyst synthesis method, whose core principle is the simultaneous precipitation of multiple ions in solution to achieve uniform mixing of components at the molecular level, resulting in high-purity and highly homogeneous products [50,143]. Shen [135] prepared Cu/Zn/Al catalysts by co-precipitation, impregnation, and hydrothermal methods, comparing their performance in methanol reforming for hydrogen production. The results showed that catalysts prepared by co-precipitation exhibited higher specific surface areas and could achieve 100% methanol conversion and 71–76% hydrogen yield at low temperatures with low CO concentrations (0.05–0.15%). For catalysts prepared by co-precipitation and hydrothermal methods, additional plasma treatment could further enhance catalytic activity. Plasma surface treatment reduces particle size and prevents agglomeration, thereby improving the physicochemical properties and performance of catalysts [85,124]. Bagherzadeh and Haghighi [124] evaluated the effects of plasma treatment on Cu/ZnO/Al₂O₃ catalysts prepared by hydrothermal and co-precipitation methods. Results indicated that plasma-treated catalysts prepared via co-precipitation showed increased conversion rates and decreased CO selectivity compared to untreated samples under the same test conditions. Applying ultrasound during co-precipitation can further increase catalyst specific surface area and reduce particle size, thereby achieving better catalytic activity [125].

The stepwise precipitation method can achieve higher specific surface area and improved Cu–Zr interfaces compared to conventional co-precipitation, which facilitates the formation of Cu⁺ and surface oxygen species, ultimately enhancing catalyst activity and stability [137]. Talkhoncheh et al. [139] demonstrated that homogeneous precipitation can increase copper dispersion in Cu/ZnO/Al₂O₃/ZrO₂ catalysts, resulting in smaller particle sizes; compared with catalysts prepared by combustion synthesis, these catalysts exhibit higher methanol conversion rates and lower CO selectivity. Shishido et al. [144] reported that Cu/ZnO and Cu/ZnO/Al₂O₃ catalysts prepared by homogeneous precipitation showed excellent catalytic activity and formed highly dispersed copper metal species.

Besides the synthesis methods mentioned above, researchers have explored various other approaches for preparing MSR catalysts. Taghizadeh et al. [115] employed surfactant-assisted impregnation to prepare Ce–Cu/KIT-6 catalysts, showing a 4% increase in the methanol conversion rate and a reduction in CO selectivity to 0.9% compared to conventional impregnation. Bagherzadeh and Haghighi [145] used plasma-assisted oxalate gel precipitation to synthesize CuO/ZnO/Al₂O₃ catalysts, achieving a methanol conversion rate as high as 99% with CO selectivity as low as 0.6%. The microwave-assisted polyol method [141] and active carbon co-nanocasting method [123] also produced catalysts with superior reforming performance compared to co-precipitation. Given the diversity of catalysts and supports, the optimal synthesis method varies accordingly. Novel synthesis techniques merit further exploration, and comprehensive comparative studies are needed to clarify their applicable scenarios.

Given the limited systematic studies in the current literature on the effects of calcination temperature, fuel type, and fuel-to-metal ratio on catalyst performance, and the significant variations in experimental conditions, catalyst compositions, and testing methods across different studies, direct comparison and comprehensive analysis of the relevant data are challenging. Therefore, this review could not conduct an in-depth quantitative analysis or a comprehensive evaluation of these parameters. We will clearly state this limitation in this manuscript and suggest that future research should focus more on systematic and standardized investigations of these critical preparation parameters. By adopting unified experimental designs and evaluation criteria, further insights can be gained into the intrinsic relationships between catalyst preparation methods, their physicochemical properties, and catalytic performance, thereby providing more instructive theoretical and practical guidance for catalyst optimization.

2.3. Noble Metal Catalysts

In MSR reactions, common noble metal catalysts include Pd-based and Au-based catalysts [22,146], which exhibit very high stability. Group VIII metal-based catalysts primarily catalyze methanol decomposition, but the water–gas shift reaction proceeds kinetically slower in the presence of water [147]. Metals such as Pt, Ni, and Ru are active for methanol conversion and have been used as catalysts for SMR; however, they tend to have relatively high CO selectivity, making them unsuitable for CO-sensitive applications like PEMFCs [148,149,150,151,152,153,154,155,156,157]. Nevertheless, Group VIIIB metals used for hydrogen production—particularly Pd-based catalysts—are of special interest due to their higher stability at elevated temperatures (above 300 °C) [147].

Table 3. Performance of precious metal catalysts for steam reforming of methanol.

Catalyst	T (°C)	Methanol Conversion (%)	${\mathbf{H}}_2 \mathbf{Production} \mathbf{Rate} ({\mathbf{cm}}^3{\mathbf{g}}_{\mathbf{cat}}^{-1}{\mathbf{h}}^{-1})$	CO Selectivity (%)	Reference
Pd/ZnO	300	98	-	13.7	[158]
Pd/Al2O3	350	99	-	3	[159]
Pd/La2O3	200	-	248	-	[160]
Pd/Nd2O3	200	-	300	-	[160]
Pd/Nb2O5	200	-	124	-	[160]
Pd/In2O3	220	28.3	-	4.5	[161]
Pd/Ga2O3	220	21.2	-	5.4	[161]
Pd/SiO2	220	15.7	-	100	[161]
Pd/MgO	220	41.0	-	93	[161]
Pd/ZrO2	220	64.3	-	81.6	[161]
Pd/ZnO	220	56.3	-	-	[162]
Pd/In2O3	220	54.2	-	0.8	[148]
Pd/Ga2O3	220	21.2	-	5.4	[148]
Pd/Al2O3	220	58.9	-	69.6	[148]
Pd/MgO	220	41.0	-	93.4	[148]
Pd/CeO2	220	62.4	-	77.3	[148]
Pd/A.C.	220	-	-	100	[148]
Pd/ZnO/Al2O3	250	100	-	-	[163]
Pd–Zr	350	61	≈170	77	[164]
Pd–Zr–Zn	300	92	≈307	≈38	[164]
Pd–Zn	300	87	≈307	≈24	[164]
Pd/Sm–Ce	400	97.4	-	-	[165]
Pd/Zn1Zr1Ox	330	46	100	0	[166]
Pd/ZnO/CeO2	250	100	-	45	[167]
PdZn/ZnO/Al2O3	250	0.998	-	-	[168]
PdZnSc–CNTs	275	21.4	-	10	[44]
Pd–Ru/Al2O3	300	88	-	-	[22]
Ru-Pd/Al2O3	350	100	-	12.5	[159]
Zn-Pd/ZnO	300	98	-	5	[169]
CuPd/ZnO2	220	≈65	≈193	5	[170]
1Nb–Pd–Zr–Zn	300	81.5	≈300	≈7	[164]
Au/CeO2	300	100	-	-	[171]
Pt/In2O3/Al2O3	350	99	-	5	[172]
CuPd/ZrO2	240	87	-	-	[173]
Au/CuO–CeO2	300	100	-	-	[174]
Au–Cu/Ce0.75Zr0.25O2	350	96	-	78	[175]

summarizes commonly used noble metal catalysts for methanol steam reforming.

In the study of noble metal catalysts, we explored changes in the microscopic characteristics and catalytic effects from the perspectives of supports and additives. Iwasa et al. [148,160,161,162,163,176] investigated the influence of different catalyst supports such as SiO₂, La₂O₃, Nb₂O₅, Nd₂O₃, ZnO, Al₂O₃, and ZrO₂ on Pd-based catalysts. The results showed that Pd supported on ZnO exhibited the highest CO₂ selectivity [160]. Unsupported Pd catalysts displayed high selectivity towards the decomposition reaction, while ZnO alone exhibited no catalytic activity [160]. Iwasa et al. [162] also studied the catalytic activity of metals including Pd, Pt, Ni, Co, Ru, and Ir supported on ZnO and found that Pd/ZnO and Pt/ZnO catalysts exhibited exceptional activity; the formation of Pd-Zn alloys was identified as the key reason for the superior catalytic performance of Pd/ZnO [163,176,177]. Danwittayakul and Dutta [178] investigated the mechanism of bimetallic composite catalysts by incorporating ZnO nanorods into Cu, Pd, and Au nanoparticles, discovering that the enhanced catalytic activity of ZnO nanorods in Cu and Pd systems was attributed to synergistic effects of the bimetallic oxides. Au/ZnO nanorods showed extremely high methanol dehydrogenation activity, achieving over 97% methanol conversion at an operating temperature as low as 200 °C. The catalytic performance of Pd/ZnO in SRM depended on the Pd loading amount [179,180,181]. Liu et al. [180] studied how Pd loading influenced the catalytic performance of Pd/ZnO, showing that higher Pd loadings resulted in more active and stable catalysts. Other metals supported on ZnO, such as Ni, Co, Ru, and Ir, showed poorer selectivity for steam reforming and instead promoted methanol decomposition.

Ranganath et al. [182] compared the activity and selectivity of Pd catalysts supported on ZnO and CeO₂ in methanol steam reforming. The Pd/ZnO catalyst exhibited a lower conversion rate but a higher selectivity toward carbon dioxide. This is attributed to the higher density of acidic sites on the ZnO-supported catalyst, which favors reaction pathways converting intermediates into CO₂, whereas the higher density of basic sites on CeO₂ facilitates conversion to carbon monoxide. In Pd catalysts supported on ZnO-CeO₂ nanocomposite carriers [167], it was found that the ZnO phase dispersed better within these carbonate-derived nanocomposites, and bulk and surface PdZn alloys formed in the ternary Pd catalyst supported on nanoceria and zinc oxide, resulting in higher CO₂ selectivity and improved catalytic stability.

Pt/In₂O₃/Al₂O₃ catalyst prepared by adding In₂O₃, where In₂O₃ is uniformly dispersed on the Al₂O₃ surface, effectively enhances the dispersion of Pt nanoparticles [172]. Meanwhile, the addition of In₂O₃ establishes a good correlation between water activation capability and catalytic selectivity. The strong Pt–In₂O₃ interaction and close contact induce a synergistic effect that plays a key role in efficiently activating H₂O, thereby minimizing CO selectivity and improving the stability of the Pt/In₂O₃/Al₂O₃ catalyst. For Pd catalysts supported on Ga₂O₃ thin-film powder [183], hydrogen reduction generates a single Pd–Ga bimetallic species on the oxide support. This bimetallic suppresses methanol dehydrogenation and shows high CO₂ selectivity in MSR. The presence of ZrO₂ in Pd/Zn₁Zr₁O_x catalysts promotes dispersion of ZnO and Pd species [166] and stabilizes Pd species against growth, enhancing catalyst stability. Under reaction conditions, Zn₁Zr₁O_x (with finely dispersed ZnO clusters in the ZrO₂ matrix) exhibits excellent anchoring and stabilization of active sites. Azenha et al. [173] studied the effect of Cu in ZrO₂-supported CuPd catalysts on MSR, finding that Cu addition increases catalytic activity and selectivity. They also investigated the influence of support structure (monoclinic and cubic phases) on product selectivity, revealing that monoclinic ZrO₂ supports better metal dispersion and thus higher catalytic performance. CeO₂–ZrO₂ hybrid oxide solid solution supports prepared by coprecipitation and sol–gel methods [175], loaded with Au–Cu/Ce₀.₇₅Zr₀.₂₅O₂, show excellent activity, achieving complete methanol conversion with very low CO concentration at 350 °C. CNT and Sc₂O₃ dual supports synergistically enhance the catalytic activity and stability of Pd–ZnO catalysts for selective production of H₂ and CO₂ [44].

Noble metal catalysts are modified by adding a third metal component (such as Ru, In, Nb, Cu, Al, Ga, Ce, Mg, Zr, or La) to influence reaction products. Kim et al. [22] found that the addition of Ru enhances the activity of Pd/Al₂O₃ catalysts. TEM analysis showed that Ru addition leads to smaller particle sizes in Pd/Al₂O₃ due to the formation of highly dispersed Pd–Ru alloys on the catalyst surface. The presence of Ru promotes CO conversion and reduces byproduct formation in MSR. Men et al. [184] studied bimetallic Pd–In/Al₂O₃ catalysts used in microstructured reactors for MSR, showing that catalytic performance and selectivity depend on the Pd/In ratio and metal loading. The formation of Pd–In alloys accounts for the high CO₂ selectivity of Pd–In/Al₂O₃ catalysts, whereas Pd catalysts without In on Al₂O₃ exhibit higher CO selectivity. Nb as a promoter in Pd–Zr–Zn catalysts enhances surface oxygen reactions [164], significantly lowering CO selectivity and yield while improving CO₂ production efficiency. The 1Nb/Pd–Zr–Zn catalyst efficiently suppresses CO formation even under high methanol–water feed rates. Increasing N₂ flow shortens the residence time of methanol and steam molecules on active sites, limiting their contact with the catalyst. Azenha et al. [173] reported that adding Cu as a promoter to Pd/ZrO₂ catalysts creates strong interactions between Cu and Pd, enhancing Pd dispersion. Matsumura [185] found that adding a small amount of Al to PdO/ZnO catalysts leads to Pd–Zn alloy particle formation; Al addition improves catalyst dispersion and forms ultra-fine PdO particles, resulting in high surface activity. Mayr et al. [186] observed that Ga addition suppresses methanol dehydrogenation and developed a novel GaPd₂ catalyst with a stacked support film to enrich Pd metal. This catalyst shows high stability in MSR but very low methanol dehydrogenation activity. Kosinski et al. [165] prepared Ce-doped Pd/Sm catalysts by impregnation with Pd(NO₃)₂ solution, which exhibit excellent nanostructure and optimal catalytic activity; Pd is highly dispersed as fine nanoparticles, producing high surface Pd concentration and enhanced catalytic activity.

However, the high cost of noble metal catalysts severely hinders their commercial application. To address this issue, Liu et al. [187] synthesized a PdZnβ alloy catalyst supported on ZnAl₂O₄ with a Pd loading as low as 1000 ppm. Their study showed that the ZnAl₂O₄ spinel facilitates the formation of PdZnβ alloy at low Pd loadings (≤2.5 wt%), whereas at higher Pd loadings (≥7.5 wt%), Pd-rich PdZnα alloys were observed. In the MSR reaction, the 0.1 wt% Pd/ZnAl₂O₄ catalyst containing the PdZnβ alloy phase exhibited a high CO₂ selectivity of 97%, comparable to that of copper, confirming its copper-like catalytic properties [187]. Reducing noble metal loading while improving methanol conversion and H₂ selectivity is an effective approach to advancing the commercialization of noble metal catalysts.

It should be noted that hydrogen production is influenced not only by the properties of the catalysts, but also significantly affected by operational conditions such as reaction temperature, pressure, gas flow rate, and reactant ratios. Due to the considerable variation in reaction conditions across different studies, this review primarily focuses on catalyst preparation methods and performance, without conducting an in-depth systematic comparison or comprehensive analysis of all reaction parameters. Readers are advised to consider the specific experimental conditions when interpreting performance data. Future research in this field could further explore the synergistic optimization of catalysts and reaction process parameters to advance the application and development of methanol steam reforming for hydrogen production.

3. Reactor

3.1. Conventional Packed-Bed Reactor

Conventional packed-bed reactors offer advantages such as low cost and high catalyst availability [188,189,190]. However, they also have some limitations, including high pressure drop and poor heat transfer characteristics within the reactor, as well as significant axial and radial temperature gradients in the catalyst bed [51,52,189]. When compactness of the reformer is required, the reactor’s geometry (size and weight) poses an additional challenge for packed-bed reactors [53].

Karim et al. [189] investigated the non-isothermality in packed-bed reactors of various sizes. The results showed that the temperature gradient decreases as the reactor diameter decreases. However, a drawback of using small-diameter reactors is the high pressure drop in the catalyst bed, and it is difficult to load catalyst into reactors with diameters smaller than 1 mm [52]. This finding is consistent with Vidal et al.’s [191] study of MSR in tubular quartz reactors (TQRs), which indicated the presence of temperature gradients unfavorable for reforming reactions. Nehe et al. [188] focused on internally heated tubular packed-bed reactors. This reactor design addressed the temperature distribution and heat loss issues observed in externally heated systems, resulting in higher methanol conversion and reduced heat loss. Heat was supplied to the steam reforming reaction by introducing a rod-shaped heater in the reactor center (as shown in the Figure 2a). Both experimental and numerical studies demonstrated enhanced methanol conversion under various operating conditions with internal heating. Zhang et al. [192] proposed a novel method combining internal and external heating to improve temperature uniformity in the reactor. Simulations were performed on different MSR reactor configurations, including tubular fixed-bed reactors (TFBRs), TFBRs with internal heating tubes, and TFBRs with spiral fins around the internal heating tubes (as shown in the Figure 2b). The results indicated that, compared to conventional TFBRs, temperature non-uniformity in the spiral-fin TFBR was reduced by 49%. Moreover, at 250 °C, the methanol conversion rate in the tubular fixed-bed reactor with helical fins around an inner heating pipe (TFBRH) increased by 8.5% compared to that in the standard TFBR.

The performance of packed-bed reactors depends on reactor temperature, heating mechanisms, heat transfer, reactor design, and feed flow rate [33]. For example, Ma et al. [193] conducted 2D modeling and numerical analysis of a plant-scale fixed-bed reactor for SRM, showing that the hydrogen production rate increased with rising reactor temperature and methanol flow rate. Peng et al. [194] evaluated the effects of geometric and thermal–fluid parameters on methanol conversion and H₂ production. Their results indicated that a smaller diameter-to-length ratio and a thicker catalyst bed positively influenced methanol conversion and H₂ production. At 300 °C, the optimal conditions to achieve the highest methanol conversion (85.49%) and H₂ concentration (48% in wet basis) were a catalyst bed thickness of 15 mm and a diameter-to-length ratio of 0.08. Additionally, increasing the inlet fuel temperature helped enhance H₂ generation.

3.2. Wall-Coated Microreactor

Microfluidics is a rapidly developing field that involves controlling the behavior of small fluid volumes in channels typically ranging from micrometers to millimeters in size [195,196,197,198,199]. Microfluidic systems have been widely applied across various fields, including biotechnology [200,201,202], chemical reactions [203], electronic cooling [204,205], and small-volume substance transfer [206,207]. A key component of any microfluidic system is the microchannel [208], which typically has a submillimeter cross-sectional dimension [209]. Microchannels are designed to achieve functions required by different applications and serve as critical practical devices in microfluidic systems [210]. In recent years, microreactor technology has been extensively used for various catalytic and non-catalytic H₂ generation reactions involving hydrocarbons. The size of microreactors is generally in the submillimeter range, and compared to conventional reactors, their high surface-to-volume ratio provides multiple advantages [52,211]. The laminar flow regime in microreactors reduces uncontrolled flow occurrences, resulting in more controllable and efficient reactions. Additionally, the lower pressure drop in microreactors allows for reduced pumping power, making them more energy-efficient than packed-bed microreactors [212].

Microchannel reactors have become a major focus of research for MSR processes in onboard fuel cells [54,213,214,215,216]. Compared to conventional packed-bed reactors, they feature a more compact structure, higher specific surface area, and superior heat and mass transfer performance [53,217,218,219]. Lee et al. [218] conducted a comparative study between wall-coated microreactors and packed-bed reformers for SRM performance, finding that the wall-coated microreactor exhibited better heat transfer efficiency due to lower thermal resistance, as well as lower energy consumption than the packed-bed reformer.

The performance of steam reforming reactors depends on flow patterns, heat transfer efficiency, and pressure drop characteristics. When designing microchannel reactors to achieve maximum conversion and selectivity, these key factors must be comprehensively considered [220,221,222,223,224,225]. Pan et al. [221] designed microchannel reactors with two cross-sectional configurations—rectangular and serrated (as shown in the Figure 3)—and compared three microchannel distribution schemes: equally left-right distributed (ELR), densely left and sparsely right distributed (DLSR), and evenly up–down distributed (EUU). Experimental results showed that the ELR configuration exhibited the best reaction performance, indicating that the distribution mode of the channels has a more significant impact on reactor performance than the microchannel cross-sectional shape.

Hao et al. [226] investigated the performance of a microreactor featuring eight non-parallel microchannels, as shown in Figure 4. This structural design directly influences the distribution of species within the microreactor, facilitating optimal conversion. The inlet angles of the microchannels form a narrow channel in the flow direction, while other inlet angles widen along the same direction. The study found that when the inlet and channel tilt angles are 50° and 9°, respectively, the microreactor achieves the highest conversion. The researchers claimed that methanol is completely converted to the desired products at the outlet.

Mei et al. [222] developed a three-dimensional model and conducted numerical analysis of a single-inlet–double-outlet A-type microchannel (as shown in Figure 5), comparing its performance with that of a Z-type microchannel reactor. The results indicated that the flow pattern within the microchannel significantly affects the reactor’s performance. Although the CO mole fraction was slightly higher than that of the Z-type microchannel reactor, the A-type microchannel reactor increased the methanol conversion by 8% and reduced the pressure drop by 0.63 Pa.

Besides straight microchannels, researchers have explored other microchannel shapes. Spiral channels can extend the contact time between reactants and catalysts, thereby enhancing reaction performance. Lu et al. [227] compared the performance of microchannel reactors with sinusoidal and dimpled structures (as shown in the Figure 6). The results indicated that the sinusoidal structure promotes mixing of hot and cold fluids by generating separated and oscillating flows, while the dimpled structure, by creating vortices and secondary flows, increases the surface area and offers a larger heat transfer area.

As shown in Figure 7a, Zeng et al. [228] fabricated a micro cuboid pillar array metal plate with Z-shaped inlet and outlet chambers using micromilling technology. Experiments comparing methanol reforming for hydrogen production over Cu, Zn, Al, and Zr catalysts with different cuboid pillar sizes showed that at 260 °C the Z-shaped channel with smaller micro cuboid pillars exhibited better reaction performance. Figure 7b shows Qian et al.’s [55] improvement on uniform micro cylindrical pillar array metal plates by designing a non-uniform micro-pin-fin arrays metal plate. Experiments demonstrated that this structure effectively enhances mass transfer performance. Mei et al. [229] designed a microreactor with micro-pin-fin arrays (as illustrated in Figure 7c), investigating the effects of fin height and spacing between adjacent fins on the performance of the hydrogen production reactor. Results indicated that increasing the micro-needle fin height and decreasing both the longitudinal and transverse spacing between adjacent fins led to higher methanol conversion.

Zhuang et al. [224,225] designed a novel packed-bed microchannel reactor (as shown in the Figure 8), featuring a bifurcated inlet manifold, a rectangular outlet manifold, and sixteen parallel microchannels for MSR. The authors developed a 3D model and conducted CFD simulations to study the flow distribution uniformity, with experiments verifying a uniformity exceeding 99.3%.

Due to fractal characteristics, some biomimetic channels exhibit improved flow performance. Huang et al. [230] designed tree-like microchannels using fractal methods to enhance methanol conversion (as shown in the Figure 9a). They found that a gradient catalyst layer can reduce CO concentration caused by delays in the methanol reforming reaction. Yao et al. [231] proposed a tree-structured network microchannel (as shown in the Figure 9b), where the reduced size of downstream branch channels provides a larger surface-to-volume ratio for the reaction space and lowers the reactant velocity in the downstream branches.

Introducing secondary configurations within channels is an effective solution to enhance gas mixing. Perng and Wu [232] designed a novel cylindrical cavity microreactor for methanol steam reforming to produce hydrogen (as shown in Figure 10). Under inlet conditions, they studied the effects of cavity depth, cavity diameter, and heated wall temperature on the reactor’s performance and net output power. The results showed that the new cylindrical cavity design significantly improved the microreactor’s methanol conversion and hydrogen production rates. Compared to conventional plate-type microchannel reactors, when the cylinder diameter was 1.4 mm, cavity depth 1.5 mm, and heated wall temperature 250 °C, methanol conversion and hydrogen yield increased by 22.65% and 64.52%, respectively, with a net output power of 22.46%.

Chu et al. [233] designed the plate-type reactor with a ribbed structure as shown in Figure 11 based on the mixing effect. By varying geometric parameters such as the shape, size, and spacing of the ribbed structures, they found that the trapezoidal channel structure can achieve a relatively high heat and mass transfer rate while maintaining a lower pressure drop.

3.3. Application of Porous Catalyst Supports

Porous structures offer significant advantages across various fields, including materials science, chemical engineering, and biomedicine, primarily due to their unique microstructures and physicochemical properties [234,235,236,237,238,239,240,241,242,243,244,245]. Porous catalyst supports typically exhibit features such as short reaction pathways, high surface-area-to-volume ratios, and stable catalyst coating capacity. These characteristics enhance heat and mass transfer, thereby improving the reaction performance of MSR microreactors. A current challenge lies in the effective fabrication of porous catalyst supports with internally interconnected pore networks. From a manufacturing perspective, porous supports can be produced via melt foaming, metal deposition, and fiber sintering techniques. In the melt foaming process, gas is trapped within molten metal to form bubbles, which solidify into metallic foams upon cooling. However, this method suffers from poor structural stability and limited control over the pore architecture [246,247]. In contrast, metal deposition techniques involve physically or chemically coating an organic polymer template with metal, followed by high-temperature sintering to remove the polymer, yielding a porous metallic structure. This approach often produces metal foams with high porosity and well-interconnected, uniform pores [32]. Liu et al. utilized copper foams with various pore array configurations as catalyst supports to optimize the support structure and enhance microreactor performance [248]. Copper foams with small pores (diameter: 0.3–0.47 mm) and large pores (diameter: 0.76–0.8 mm) were fabricated using laser machining, and Cu/Zn/Al/Zr catalysts were loaded via a two-step impregnation method. Three types of copper foam catalyst supports were prepared to investigate the pore array effects, with catalyst loadings of 0.2, 0.3, and 0.4 g per foam layer. As shown in Figure 12, three pore size distribution types were fabricated: pores decreasing radially from the center (Type 1), increasing radially from the center (Type 2), and uniformly distributed (Type 3). Moreover, simulation results indicated that perforated copper foams supported on Type 1 structures increased axial flow velocity and led to the best microreactor performance. Under an inlet flow rate of 10 mL/h, a reaction temperature of 300 °C, a methanol conversion rate of 95%, and an H₂ flow rate of 0.52 mol/h, the microreactor operated stably.

Fiber sintering technology involves the preforming of raw materials such as metal fibers, meshes, or powders, followed by sintering in a protective atmosphere to produce porous sintered felts with high porosity. This method offers advantages such as low cost and controllable porosity, making it particularly suitable for the fabrication of porous supports [249,250,251]. Pan et al. [252] and Zhou et al. [253] prepared oriented linear cutting fiber sintered felts and porous copper fiber sintered felts as catalyst supports using cutting and fiber sintering methods. The results demonstrated that the prepared catalyst supports enhanced hydrogen production performance in MSR microreactors. Ke et al. [254] applied laser micromilling to fabricate rectangular, stepped, and zigzag microchannels on the surface of 80% porous copper fiber sintered felt (PCFSF), as illustrated in Figure 13. Among the different channel geometries, the rectangular microchannel configuration exhibited lower pressure drop, higher permeability, better catalyst loading, and superior reaction performance. When more than 0.5 g of catalyst was loaded onto the surface of 80% porosity PCFSF with rectangular microchannels, improved catalyst distribution and permeability enabled higher methanol conversion rates and increased H₂ production rates.

Wang et al. [255] conducted numerical studies on flow distribution to optimize the porosity distribution of porous copper fiber sintered felts (as illustrated in Figure 14). Semi-optimized PCFSFs with 12 different porosity distributions were fabricated via die compression and solid-state sintering. Two optimal flow distribution configurations were selected: PCFSF with porosity distributed in the left–right (LR) direction, and PCFSF with porosity distributed in the up–down (UU) direction. The results showed that PCFSF-LR exhibited superior reaction performance compared to PCFSF-UU. Regardless of variations in space velocity or reaction temperature, PCFSF-LR with porosity distributions of 0.7–0.9–0.8 and 0.8–0.9–0.7 achieved higher methanol conversion rates and H₂ production rates. Among them, the PCFSF-LR with a 0.7–0.9–0.8 porosity gradient demonstrated the highest H₂ selectivity. These findings indicated that the porosity distribution of PCFSFs has a significant impact on MSR performance and can be effectively optimized based on flow distribution.

Zhou et al. [249] developed a novel porous Cu–Al fiber sintered felt (PCAFSF, as illustrated in Figure 15) as a catalyst support for cylindrical MSR microreactors and investigated the influence of its physical properties—such as fiber material, surface morphology, and composition—on MSR performance. Compared with PCFSF and porous alumina fiber sintered felt (PAFSF), PCAFSF exhibited the highest specific surface area and catalyst loading capacity, along with strong catalyst affinity, good low-temperature activity, and superior reaction performance. Moreover, when compared to smooth PCAFSF with the same porosity (80%), rough-surfaced PCAFSF demonstrated higher methanol conversion and H₂ yield in the MSR reaction. At equal porosities of 70% and 80%, the weight of copper fibers and aluminum was reduced, yet methanol conversion and H₂ yield increased with increasing total fiber mass. These results indicated that surface morphology and fiber mass significantly affect the catalytic efficiency of PCAFSF-based supports.

The aforementioned techniques are primarily used to fabricate metal-based porous structures. However, metals often suffer from poor corrosion resistance and limited catalyst loading capacity, which significantly restrict the service life of microreactors. In contrast, porous ceramics offer higher chemical stability, lower thermal expansion coefficients, and greater catalyst loading capacity, making them highly promising as catalyst supports. Several studies have been conducted to investigate the fabrication of porous ceramic materials [256,257,258]. Guo et al. [259] prepared a porous SiC ceramic support microreactor with a hierarchical “pore-in-pore” structure using a solution combustion synthesis method, as illustrated in the Figure 16. The catalyst loading on the ceramic support reached approximately 20% of the total support weight, with a substantial improvement in loading strength.

The aforementioned processes enable the fabrication of porous catalyst supports with high reaction performance; however, they lack precise control over internal pore structures. Porous catalyst supports with regularly shaped pore architectures can simultaneously enhance both fluid dynamics and reaction performance. Three-dimensional (3D) printing, with its high design flexibility, offers significant advantages for the fabrication of complex and ordered pore structures [50,56,260,261,262]. For metallic materials, techniques such as selective laser sintering (SLS), selective laser melting (SLM), and particle jetting (PJ) can be employed to print metal-based catalyst supports with intricate and well-defined porous architectures [263,264,265].

Lei et al. [263] fabricated an additively manufactured porous stainless-steel felt (AM-PSSF) using selective laser melting (SLM), which was employed as a catalyst support in MSR microreactors. Experimental results showed that the catalyst-coated AM-PSSF exhibited a high porosity (~82.1%), large specific surface area, and high connectivity, leading to superior hydrogen production performance compared to commercial stainless-steel fiber sintered felts. Zheng et al. [265] utilized 3D printing technology to fabricate porous stainless-steel supports with body-centered cubic structures (BCCSs) and face-centered cubic structures (FCCSs), as shown in the Figure 17. They found that the 3D-printed supports with BCCSs and FCCSs demonstrated enhanced catalyst loading strength, making them suitable for large-scale hydrogen production.

Li et al. [266] developed a monolithic structure with a wall thickness of less than one millimeter using ZSM-5 zeolite—a widely used catalytic material—via 3D printing technology. Compared to powder-form catalysts with the same composition, the 3D-printed catalyst exhibited enhanced stability, higher light olefin selectivity, and extended operational lifetime. Tubio et al. [267] fabricated a Cu–Al₂O₃ catalyst with a “wood-pile” structure using 3D printing, as illustrated in Figure 18, and proposed its application in catalytic reactions. Unlike conventional monolithic structures, the wood-pile design allows fluid to be directed in multiple directions (up, down, left, and right). This design improves heat and mass transfer within the catalyst structure during the reaction, thereby enhancing overall efficiency.

Ceramic-based catalyst supports can be fabricated using direct ink writing (DIW), stereolithography (SLA), and fused deposition modeling (FDM) processes [268,269,270,271,272,273]. Chen et al. [274] employed the DIW technique to prepare a 3D-structured SiC ceramic support using a polycarbosilane (PCS)/n-hexane solution. The resulting SiC ceramic maintained its intricate porous architecture after pyrolysis. Gyak et al. [269] fabricated a SiC-based ceramic microreactor for hydrogen production using the SLA process. The microreactor demonstrated excellent chemical and thermal resistance. Wang et al. [257] designed a self-heating microreactor utilizing a silicon carbide (SiC) honeycomb ceramic as the catalyst support (as shown in Figure 19). Due to its high thermal conductivity, the SiC honeycomb efficiently transferred the generated heat to the reforming chamber, enabling the reactor to reach its target operating temperature within 9 min. Experimental results showed that the microreactor achieved a maximum energy efficiency of 67.9%, and under a methanol feed rate of 0.36 mL/min, the hydrogen production rate reached 316.4 mL/min. In addition, a three-dimensional numerical model was established to analyze heat transfer and fluid flow characteristics. The results revealed that the microreactor exhibited significantly lower pressure drop and a more uniform temperature distribution within the SiC ceramic support.

The greatest advantage of 3D printing fabrication lies in the ability to customize catalyst structures for specific reaction systems rather than conforming to standardized forms [275]. Avril et al. [276] utilized 3D printing technology to manufacture an insertable twisted Pt–Ni metal catalyst, which was applied for alcohol and ethane hydrogenation. Compared to catalysts with identical structures produced by conventional commercial methods, the 3D-printed catalyst reduced costs by approximately one-tenth to one-hundredth. Moreover, due to improved fluid mixing, the 3D-printed catalyst exhibited lower pressure drop relative to commercial catalysts. These results indicated that such complex-structured 3D-printed catalysts are well suited for trickle bed reactor (TBR) systems, where reactants or intermediates involved in catalytic reactions possess high heat capacity and viscosity, such as steam or multiphase flows. When applied to reaction systems requiring advanced engineering—such as reactors with complex architectures including microreactors and MSR—3D printing holds promising potential to accelerate the commercialization of on-site hydrogen production technologies via MSR.

4. Purification Technologies

During the methanol reforming process for hydrogen production, in addition to the desired product H₂, several byproducts and unreacted species are typically generated. These mainly include CO, CO₂, H₂O, and unreacted CH₃OH. Such impurities not only reduce the purity of hydrogen but may also negatively impact downstream applications. Therefore, gas purification and separation processes are generally required after the reforming reaction to meet the purity requirements of high-grade hydrogen, especially in applications such as fuel cells that are highly sensitive to gas quality.

Among these impurities, CO is the most critical and challenging to remove. Although typically present at low concentrations, CO exhibits strong poisoning effects on noble metal catalysts—particularly Pt catalysts used in PEMFCs. Even at concentrations below 10 ppm, CO can significantly reduce catalyst activity, severely affecting the efficiency and lifetime of the fuel cell. As a result, one of the core objectives of hydrogen purification is the selective and efficient removal of CO.

In contrast, although CO₂ is also a major byproduct, its chemically inert nature means it does not cause significant catalyst poisoning, and it is usually removed together with other impurities during the purification stage rather than being a primary target. Other species, such as water vapor and unreacted methanol, can be removed through condensation or adsorption methods, and are generally less difficult to handle.

Consequently, current purification technologies mainly focus on CO removal. Common methods include pressure swing adsorption (PSA), separation membranes, selective oxidation, and selective methanation, all of which are employed to ensure that the final hydrogen product meets the required purity standards. Given that the current literature on the effects of purification process parameters (such as temperature, pressure, gas composition, etc.) on purification performance is scattered and experimental conditions vary widely, there is no unified and universally applicable set of optimal process parameters to summarize. Therefore, this paper focuses on introducing the basic principles and current applications of various purification technologies, without delving deeply into specific operating parameters.

4.1. Pressure Swing Adsorption

Adsorption is a widely used method for separation and purification, with broad applications in chemical engineering, agriculture, and biology [277,278,279,280,281,282,283,284,285]. Hydrogen separation and purification are crucial for its extensive utilization, and PSA is an effective technology for hydrogen separation and purification [286]. Due to the differing adsorption selectivities of various adsorbents toward impurity gases, numerous studies have investigated the effects of different adsorbents on hydrogen purification performance. Activated carbon and zeolites are among the most commonly used adsorbents [287,288,289], and layered beds combining multiple adsorbents have also been explored [290,291]. Besides these traditional adsorbents, a novel class of adsorbents known as metal–organic frameworks (MOFs), such as Cu-BTC, have been widely applied in PSA hydrogen production [292,293]. Adsorbents used in PSA systems should possess high adsorption capacities and be capable of removing all impurities. However, the removal of N₂ and CO remains challenging because conventional adsorbents have much stronger affinity for CO₂ and CH₄ [294]. Finding a single adsorbent with high removal efficiency for all impurities is difficult, so multilayer configurations employing different adsorbents are typically used in PSA units [295,296,297,298]. In commercial PSA systems, activated carbon (AC) is commonly used as the first layer to adsorb CO₂ and CH₄, while zeolites serve as the second layer to adsorb CO and N₂ [297]. Abdeljaoued et al. [298] applied a 12-step, four-bed PSA process to a feed containing 69% H₂, 25% CO, and CH₄ and CO totaling 69%, producing hydrogen with a purity of 99.9913% and achieving a recovery rate of 75.5%. Shi et al. [299] designed a 10-bed PSA process using activated carbon (first layer) and 5A zeolite (second layer) for hydrogen production via methanol steam reforming, achieving hydrogen purity of 99.99% and recovery of 85%. The syngas composition in their study was 76% H₂, 20% CO₂, 3.5% CH₄, and 0.5% CO. Therefore, different adsorbents—including various layered combinations—can be selected and optimized in PSA processes depending on the feed gas composition [300].

Multiple operating variables also influence the performance of hydrogen purification via PSA. Parameters such as the feed-pressure-to-flow ratio (P/F), adsorption pressure, and adsorbent bed height significantly affect the production of high-purity hydrogen [301,302]. You et al. [303] enhanced purification performance by combining vacuum and purge steps—known as vacuum pressure swing adsorption (VPSA)—resulting in approximately a 10% increase in hydrogen recovery. The number of adsorption columns and the number of PSA cycle steps are also critical factors influencing hydrogen purification performance, with studies showing that increasing the number of adsorption columns improves hydrogen recovery [304,305]. Thermal effects during PSA cycles cannot be neglected; effective thermal management plays a vital role in hydrogen breakthrough curves and purification efficiency [306,307,308]. Moreover, many operational parameters in PSA design, including adsorption pressure, purge time, adsorption time, and void gas velocity, impact hydrogen purity and recovery [286]. Minimizing operating costs while maximizing hydrogen performance is essential for PSA process design optimization [309].

4.2. Separation Membranes

CO separation via membrane technology in MSR involves the use of selective permeation membranes to remove CO, thereby achieving the separation of CO and hydrogen [310]. This technology is characterized by its simplicity, high efficiency, and energy savings. Since it does not require gas or gas–liquid adsorbents, membrane-based CO separation features lower operational costs and longer service life [311]. The key to membrane CO separation lies in selecting appropriate membrane materials that enable high CO permeation selectivity while maintaining strong hydrogen barrier properties. These materials include polymer membranes, inorganic membranes, and mixed-matrix membranes [312]. The choice of membrane material depends on process conditions, CO permeation performance requirements, and economic considerations. Figure 20 illustrates a Pd-Ag membrane reactor [313].

Polymer membranes are among the most commonly used membrane materials, offering excellent flexibility, processability, and selectivity [314]. Notably, polyamide membranes, polyether ether ketone (PEEK) membranes, and polypropylene (PP) membranes have been extensively studied and applied [315]. By modifying the polymer structure and incorporating fillers, researchers have enhanced the CO selectivity and hydrogen barrier properties of these membranes [316]. Saidi [313] improved CO separation performance by adding functional fillers, such as nanoparticles, to increase the membrane’s CO adsorption capacity. Inorganic membranes exhibit high thermal and chemical stability, making them suitable for methanol steam reforming applications [317]. Common inorganic membranes include alumina, silica, and carbon membranes. Cifuentes et al. [318] tuned the pore structure and surface properties of inorganic membranes to regulate selective transport of CO and hydrogen, thereby improving separation efficiency. Iulianelli et al. [217] fabricated alumina membranes via the sol–gel method; optimal deposition conditions and post-treatment enhanced CO selectivity and hydrogen barrier performance. Mixed-matrix membranes (MMMs), composed of different membrane materials, leverage the synergistic advantages of each constituent to achieve higher CO separation efficiency and stability [319]. Typical MMMs include polymer/inorganic composites and polymer/polymer blends. By adjusting the composition, structure, and interfacial interactions within MMMs, synergistic effects have been realized that improve CO selectivity and hydrogen barrier capabilities [320]. Li et al. [321] enhanced CO adsorption capacity in polymer/inorganic composite membranes by introducing inorganic fillers into the polymer matrix, thereby improving CO separation performance.

The key aspects of membrane material research focus on improving the stability, permeability, and manufacturing processes of membrane materials [322]. Therefore, future studies will continue to emphasize the development of higher-performance membrane materials and the resolution of technical challenges encountered in practical applications, aiming to promote the widespread adoption of CO membrane separation technology in MSR.

4.3. CO-Selective Oxidation

CO-selective oxidation technology in MSR plays a crucial role in enhancing hydrogen purity and reducing CO concentration [323]. This technology oxidizes CO to CO₂, thereby lowering the CO content, increasing hydrogen purity, and improving the overall efficiency of the MSR process [324].

The types, lifetime, regeneration, and support materials of CO-selective oxidation catalysts are key research directions [325]. CO oxidation catalysts are typically based on noble metals such as Pt, Rh, and Pd, which exhibit excellent CO oxidation activity and stability [326]. Different catalyst compositions influence the reactivity and selectivity of CO-selective oxidation. For example, Pt-Rh alloy catalysts demonstrate high activity and selectivity, catalyzing both CO and methane oxidation simultaneously, thereby enhancing CO conversion and hydrogen purity [170]. Catalyst lifetime is an important metric for evaluating the performance of CO-selective oxidation catalysts [327]. In practical applications, catalyst activity and selectivity tend to decline over time due to carbon deposition, poison adsorption, and sintering [328]. Extending catalyst lifespan is thus a critical research goal. The introduction of promoters or modifiers such as Ir, Fe, and Sc can improve catalyst stability and resistance to poisoning [329]. Catalyst regeneration technologies are another vital research focus, aiming to restore catalyst activity and selectivity through methods like thermal oxidation and reduction [330]. Thermal oxidation regeneration, which removes carbon deposits by oxidation at high temperatures, is commonly used to recover catalyst activity [331]. Other methods, including reduction and heat treatment under hydrogen atmosphere, can also be employed to restore catalyst performance [332]. The catalyst support plays an essential role in the activity and stability of CO-selective oxidation catalysts. Common support materials include activated carbon, alumina, and silica [333]. These supports offer high specific surface areas and good stability, providing ample active sites and facilitating catalyst dispersion. In summary, by optimizing catalyst composition, enhancing stability and anti-poisoning properties, developing effective regeneration methods, and tailoring support properties, the activity and selectivity of catalysts can be improved, catalyst lifespan extended, and the efficiency and sustainability of CO-selective oxidation in MSR enhanced.

Another critical area of research focuses on the reaction conditions for CO-selective oxidation, aiming to optimize catalyst activity and selectivity. Reaction temperature is one of the most important parameters, significantly influencing reaction rates and product distribution [334]. While elevated temperatures generally increase the reaction rate, they can also lead to a decline in selectivity [335]. Some studies have adjusted reaction temperature to control the ratio of CO to methane in the products to meet different application requirements [336]. The composition of the reaction gas mixture is a crucial factor affecting both the activity and selectivity of CO-selective oxidation [337]. Research indicates that an appropriate ratio of methanol to oxygen can enhance reaction activity, while the presence of steam helps suppress side reactions [338]. Space velocity, which refers to the gas flow rate through the catalyst bed, influences the residence time of reactants on the catalyst surface [58]. It has been observed that increasing space velocity within a certain range can improve the reaction rate; however, excessively high space velocity may cause diffusion limitations, reducing overall reaction efficiency. Reactor design plays a significant role in the effectiveness of CO-selective oxidation. Researchers optimize reactor performance by adjusting its shape, size, and catalyst loading, as well as focusing on internal fluid dynamics to enhance mass and heat transfer, thereby further improving reaction efficiency [339]. In summary, by fine-tuning reaction temperature, gas composition, space velocity, and reactor structural parameters, it is possible to regulate reaction rates, product selectivity, and catalyst stability.

4.4. CO-Selective Methanation

Selective CO methanation in the MSR process is a common method for CO removal, where CO reacts with H₂ to produce CH₄ and H₂O, thereby reducing the impact of CO on hydrogen quality and catalyst performance [328]. Selective CO methanation is a catalytic reaction carried out at low temperatures, with the reaction equation shown in Equation (4) [340,341]. Through this reaction, carbon monoxide is converted into more stable products, thereby reducing adverse effects on the catalyst and hydrogen quality [342]. The selective CO methanation reaction needs to be conducted under appropriate temperature and pressure conditions and catalyzed by suitable catalysts. During the reaction, CO and hydrogen are first adsorbed onto the active sites on the catalyst surface. Then, carbon monoxide and hydrogen react through a series of intermediates and reaction steps to produce methane and water [233]. The active sites on the catalyst surface play a key role in facilitating the adsorption of CO and hydrogen, as well as regulating the reaction rate and selectivity.

$$\begin{array}{c}\mathrm{CO}+3{\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O} \Delta H=-206 \mathrm{kJ}/\mathrm{mol}\end{array}$$

(4)

Carbon monoxide methanation catalysts require excellent catalytic activity, selectivity, stability, and strong resistance to poisoning [343]. Common catalysts include Ni, Fe, Ru, Pt, and others. The choice of catalyst type and optimization of reaction conditions are key to improving the activity and selectivity of the reaction [344]. Some researchers have improved catalysts by enhancing preparation methods; for example, Akande and Lee [345] used microwave torch technology to improve catalyst preparation, thereby enhancing catalyst performance. Other studies focus on the nanoscale structural design of catalysts; Wang et al. [346] improved the efficiency of selective CO methanation by adjusting the catalyst’s crystal structure and surface active sites. Researchers have also optimized CO-selective methanation conditions by adjusting reaction temperature, pressure, and gas composition [347]. Zhang et al. [348] found that lower reaction temperatures and an appropriate excess of hydrogen can increase methane selectivity and reduce the formation of byproducts. Janošovský et al. [349] reported that moderate amounts of steam can enhance catalyst stability and reaction performance. Regarding reaction mechanism studies, extensive theoretical simulations and experimental characterizations have been conducted. For instance, Zhang et al. [350] used computational chemistry methods and characterization techniques to reveal intermediate states and reaction pathways during the process, providing important insights into catalyst surface adsorption, activation, and reaction mechanisms.

Selective oxidation of CO and CO methanation are two commonly used methods for removing CO from hydrogen-rich gas streams. They have different catalyst requirements, advantages, and disadvantages. The choice between selective oxidation and methanation of CO requires comprehensive consideration of factors such as CO concentration, desired hydrogen purity, and operating conditions. A reasonable selection can achieve more ideal results in a more economical manner.

5. Technical Application Prospects and Challenges

5.1. Application of MSR Technology

MSR technology for hydrogen production has broad application prospects. Besides being stored in high-pressure tanks for subsequent chemical synthesis, hydrogen produced by MSR can also supply fuel cell vehicles at hydrogen refueling stations. MSR hydrogen production systems can be matched with PEMFCs for mobile applications such as portable systems [351,352], drones [353,354], automobiles [355,356,357], submarines [358,359,360], and other fields.

For mobile hydrogen production, it can be used as a power source for portable systems. Zhang et al. [351] developed an integrated reformate methanol fuel cell (RMFC) portable system that can output 30 W of power at a hydrogen production flow rate of 670 mL/min, exhibiting stable performance throughout the operation. Chiu et al. [352] developed an integrated system that produces hydrogen-rich gas via ATMR, providing approximately 1 kW of power, and the purifier significantly reduces CO concentration.

Using fuel cells as the primary power source has become very common in drone electric propulsion systems. Wang et al. [354] compared three types of fuel cells used in drones, including PEMFC, DMFC, and solid oxide fuel cells (SOFCs). The integration of the fuel cell with the drone airframe is shown in Figure 21. The study found that the PEMFC/H₂ system may be the best propulsion candidate for small drones.

In the transportation sector, MSR fuel cell vehicles offer an environmentally friendly option [356]. During onboard hydrogen production, the use of a heat exchanger to capture waste heat from the internal combustion engine exhaust combined with the methanol reformer provides energy support for the endothermic methanol reforming reaction. This technology, known as thermochemical waste heat recovery, is an advanced solution that enables the methanol reformer to operate without an external heat source and achieves the lowest carbon emissions among various types of vehicles. Liu et al. [355] proposed a hybrid system combined with a chemical looping methanol reformer, as shown in Figure 22a. In the vehicle’s operating phase, a high-temperature proton exchange membrane fuel cell (HT-PEMFC) is used as the power source to carry out the SE-POM reaction and regeneration. The regenerator supplies air to the heat storage chamber for exothermic air supply, and under the heat provided by the PEMFC, the methanol reforming reaction (MSR) proceeds while air supply is stopped during operation. Wu et al. [357] evaluated the overall efficiency of MSR and PEMFC systems for automotive applications, as shown in Figure 22b. The results indicated that MSR-PEMFC vehicles are suitable as a new type of environmentally friendly transportation tool.

MSR technology can also be used to power underwater vehicles. Non-nuclear submarines primarily use diesel engines for propulsion, which require air to burn diesel; therefore, submarines need to snorkel to replenish air and maintain propulsion. For submarines, the PEMFC/H₂ system may be a good alternative power supply. Krummrich and Llabrés [358] compared feedstocks for alcohol reforming hydrogen production and found that methanol has the highest H/C ratio and the lowest overall volume requirement. Through high-pressure treatment, waste CO₂ can also be discharged (as shown in the Figure 23a). Ji et al. [360] applied MSR to supply H₂ for fuel cell-equipped underwater vehicles, as illustrated in Figure 23b. The supply of hydrogen peroxide ensures sufficient methanol conversion and H₂ yield at low temperatures, with methanol conversion reaching up to 93.5%.

Generally speaking, MSR technology has broad application prospects in PEMFC–H₂ systems and can achieve good performance. Compared with other energy supply methods, MSR technology features low carbon emissions, low energy consumption, and renewable resources. The combination of methanol reforming technology with PEMFC, along with CO removal processes, can address the challenges currently faced by hydrogen fuel cell applications. Hydrogen fuel cells are expected to be applied in power supply, transportation, agriculture, and even military fields [6,32,361,362,363,364]. Therefore, we believe that the application of MSR technology in PEMFC-H₂ systems holds great potential and performance advantages. The integration of the entire energy system and its stable operation require further research and development.

5.2. Challenges of MSR Technology

Generally speaking, MSR technology is advanced for efficient hydrogen production and holds considerable application prospects across various energy supply sectors. This technology enables hydrogen energy to replace fossil fuels by using methanol as a hydrogen storage carrier, addressing the difficulties of hydrogen storage and transportation. However, many challenges still need to be addressed in future work.

The preparation of more stable and highly active catalysts is crucial for MSR and still requires in-depth research. During the hydrogen production process, the catalyst’s activity and stability directly affect the performance and service life of microreactors. Catalyst deactivation leads to a decline in reaction performance, resulting in a significant reduction in hydrogen production and an increase in byproduct contents (such as CO and CH4), which seriously impacts the system’s output power.

For the most widely used Cu-based catalysts, the MSR reaction pathway is relatively complex, and the synergistic mechanism between Cu⁰ and Cu⁺ has not been fully elucidated. In particular, the deactivation issues under high-temperature conditions require further investigation. Cu-based catalysts are prone to sintering and coke deposition at high temperatures, leading to a reduction in active sites and decreased stability. The introduction of promoters (such as ZnO and CeO₂) and optimization of preparation methods can partially enhance their resistance to sintering and coking, but an in-depth analysis of the mechanisms at the atomic level is still needed.

Enhancing the metal–support interaction (MSI) is crucial for the intrinsic stability and activity of active metal atoms in steam reforming catalysts. To date, various types of MSIs have been applied to improve catalyst reforming performance, but the understanding and full utilization of the MSI mechanism remain insufficient. The formation and presence of MSIs are influenced by the catalyst structure, the type and properties of the metal/support, and the external activation or reaction atmosphere. An excessively strong MSI often leads to overly strong bonding and mass/charge transfer between the metal and support, thereby reducing reforming activity. Therefore, how to resolve this contradiction or better balance these two aspects remains a challenging problem that requires further investigation.

Suppressing CO formation has become an important research goal for optimizing catalytic performance. However, controversies remain regarding the active sites and reaction mechanisms, especially since the reaction pathways are not yet fully elucidated. Copper-based catalysts exhibit low CO selectivity, and further reductions in CO concentration can be achieved by optimizing the support and metal dispersion. It is necessary to enhance the catalyst’s low-temperature reaction performance to better match fuel cells by lowering the reaction temperature, thereby reducing system heat consumption and thermal losses.

For MSR reactors, the significant advantages of microreactors have gradually replaced traditional packed-bed reactors. At the same time, the application of catalyst supports has further enhanced the reforming performance of microreactors. However, the successful application of microreactors still faces many challenges, including insufficient understanding of kinetics, difficulties in microreactor construction, and challenges in increasing power density [365,366]. Overcoming these challenges is crucial to unlocking the full potential of microreactors.

First, studying the reforming reaction kinetics and the entire diffusion mechanism is crucial for advancing microreactor technology. Currently, although steam reforming is widely applied in many numerical studies, comprehensive research on its reaction kinetics remains significantly insufficient. This research gap highlights the necessity of developing more robust models capable of accurately defining molecular diffusion in microscale systems, thereby providing more precise results for microreactors.

In addition, the formation and removal mechanisms of coke during methanol reforming remain insufficiently and systematically studied. Although existing research has identified carbon deposition as one of the main causes of catalyst deactivation and decreased reaction efficiency, there is still no consensus on how different reaction conditions, catalyst compositions, and support types specifically influence coking behavior. For instance, the introduction of promoters (such as alkali metals or rare-earth elements) and support materials has been reported to suppress coke formation or facilitate its oxidation to some extent; however, the underlying mechanisms remain controversial and lack clear theoretical support. A deeper understanding of the coking pathways, coke species, and their correlation with catalyst surface properties is crucial for improving the coke resistance and prolonging the service life of catalysts. Therefore, future studies should focus on mechanistic investigations of coking in methanol reforming, supported by in situ characterization techniques. In particular, integrating surface science, reaction kinetics, and multiscale modeling approaches may help elucidate the specific roles of promoters and supports in coke suppression or removal, thereby providing a theoretical basis for the design of highly stable catalysts.

The construction of microreactors is another challenge that cannot be overlooked. The choice of reactor materials is crucial for long-term performance [367]. Foam-based microreactors may experience metal corrosion during prolonged operation [273,368], and the use of corrosion-resistant ceramic materials can mitigate this issue. Although materials like ceramics offer advantages in certain applications, their brittleness makes conventional processing difficult. Achieving a uniformly interconnected porous structure during manufacturing is another challenge [32]. Additionally, obtaining high porosity remains a significant difficulty. Studies have shown that techniques such as selective laser melting (SLM) struggle to achieve porosities above 90% [369]. It is necessary to develop materials with better performance, higher stability, and improved cost-effectiveness. The heat and mass transfer of fluids inside the reactor significantly affect reaction efficiency, requiring rational design of internal flow channel structures to ensure high heat and mass transfer performance. In self-heating microreactors, the reforming heat is provided by combustion of the same fuel, making heat management particularly complex. Such systems often exhibit significant temperature differences between reaction chambers, making it difficult to maintain precise temperatures for optimal reaction efficiency [370]. Optimizing microreactor structural parameters often involves multi-objective optimization, and AI technology is well suited to address these challenges. AI excels at rapidly analyzing large datasets at minimal cost and providing real-time predictions of outputs [371]. However, only a few studies have currently applied machine learning techniques to optimize microreactor reaction performance [372]. Introducing AI technology into reactor design holds great potential.

At the same time, it is necessary to improve the system’s power density. The main industrial challenge lies in production capacity limitations, especially when compared to traditional reactors. Enhancing the system’s power density is a key factor. Although using multiple microreactors can increase output to some extent, this approach significantly increases the complexity of hydrogen production processes and control systems. This multi-layered strategy not only complicates management but also leads to increased energy consumption [373].

The integrated hydrogen production reaction system is still far from commercial application. Existing research mainly focuses on specific modules of the system, with relatively little discussion on system integration. Therefore, it is necessary to integrate the dispersed modules and build a complete hydrogen production system with commercial value, which should include raw material supply, reaction, purification, and thermal management modules. Especially since reformate gas contains high levels of impurities, which are unfavorable for hydrogen supply and power generation in proton exchange membrane fuel cells. Generally, PEMFC power generation requires hydrogen of extremely high purity. Thus, in-depth research on purification and hydrogen storage processes is needed to achieve widespread, long-term, and high-power applications of MSR technology.

6. Summary

The depletion of fossil fuels and the greenhouse gas emissions caused by their energy production have brought hydrogen energy to the forefront as an alternative energy carrier. Currently, industrial hydrogen is mainly produced via steam reforming of hydrocarbons such as natural gas and naphtha. Methanol, due to its low cost, high hydrogen-to-carbon ratio, low reforming temperature, sulfur-free content, liquid state under normal conditions, and diverse sources, has become an attractive hydrogen carrier. MSR is a promising sustainable and efficient hydrogen production pathway. Valuable insights and advances in related hydrogen production technologies have significantly enhanced the feasibility of MSR as a key component of the hydrogen economy. This review summarizes the following key findings and conclusions: catalyst development remains the research focus, with particular emphasis on improving catalytic activity, selectivity, and stability. Innovative designs such as nanostructured materials and non-precious metal catalysts have shown significant potential to enhance MSR performance. Reactors play a critical role in the MSR process, and optimization of gas flow, heat management, and reactor design can effectively improve heat and mass transfer performance, thereby enhancing overall reactor efficiency. Coupling CO removal technologies further improves the efficiency and sustainability of MSR processes, with CO-selective oxidation and methanation being effective strategies to adjust the composition of reformate gas. In summary, this review systematically analyzes and evaluates these technologies, providing important references for achieving high-purity hydrogen production, enhancing catalyst stability, and addressing key challenges in CO removal during methanol reforming, which is significant for advancing the application and development of methanol steam reforming hydrogen production technology.

Methanol steam reforming catalysts can be divided into two categories: copper-based catalysts and group 8–10 metal-based catalysts. Copper is the most active and extensively studied catalyst. Regarding copper-based catalysts, various methods to enhance catalytic activity have been reported in the literature. Their catalytic performance is significantly influenced by physicochemical properties such as active surface area, particle size, and dispersion, which in turn depend on the choice of promoters and supports, component ratios, and preparation methods. Well-studied promoters for copper catalysts include ZnO, Al₂O₃, ZrO₂, and CeO₂. ZnO improves Cu dispersion and induces strong metal–support interactions. Al₂O₃ increases catalyst surface area and enhances copper dispersion. Zirconia enhances reducibility and dispersion while preventing Cu sintering. CeO₂ enhances catalytic activity and reduces CO formation due to its high oxygen storage capacity. This review focuses particularly on Pd-based catalysts among group 8–10 metal catalysts. Compared to copper-based catalysts, they exhibit lower activity and selectivity in methanol steam reforming. Most studies on noble metal catalysts indicate their catalytic activity is lower than that of copper-based catalysts, but they offer greater stability. An enhanced metal–support interaction (MSI) is crucial for the intrinsic stability and activity of active metal atoms in steam reforming catalysts. For example, the formation of Pd-Zn alloys shows excellent selectivity toward H₂ and CO₂. Current research mainly focuses on the interface between metal particles/clusters and oxide supports, while future studies could explore interaction mechanisms involving metal-carbide and single-atom catalysts (SACs). So far, it is necessary to address issues such as deactivation and stability of copper-based catalysts and the low hydrogen production rates of noble metal alloy catalysts. It is recommended to conduct DFT modeling of proposed catalysts to better understand whether synthetic catalyst development is warranted. Meanwhile, more research should focus on developing novel catalysts to achieve higher conversion, selectivity, and improved stability while suppressing CO formation at lower reaction temperatures. Additionally, intensified studies on the steam reforming reaction mechanism are essential to resolve current academic controversies.

Three typical reactors have been studied for the methanol steam reforming reaction environment: conventional packed-bed reactors, wall-coated microreactors, and porous catalyst carrier reactors. Microscale reactors eliminate transport limitations and provide near-isothermal conditions, offering more efficient heat and mass transfer performance. By utilizing the exothermic methanol oxidation reaction as a heat source, microreactors can achieve autothermal reforming. Moreover, microreactors allow flexible adjustment of internal microchannel structures, distributions, and shapes according to reaction engineering requirements. Well-designed microreactors outperform conventional packed-bed reactors due to their superior heat and mass transfer capabilities and more thorough contact between catalyst surfaces and reactants. The performance of microreactors is jointly influenced by reaction parameters and structural parameters. However, numerical studies mainly focus on structural parameters, which directly affect internal flow and temperature distributions, ultimately determining overall performance. Structural improvements in microreactor design, such as introducing non-parallel microchannels, sinusoidal patterns, pin fins, spiral structures, and tree-like channel architectures, have significantly enhanced hydrogen production performance. Microreactors face challenges such as cold spot formation, catalyst deactivation, and CO generation, which are directly related to fuel choice, reactor materials, catalysts, and microchannel structure. These issues can be mitigated by adjusting relevant parameters. Porous catalyst carriers typically provide larger specific surface areas and improve catalyst coating stability. They also enhance gas mixing and increase contact time between reactants and catalysts, thereby improving the reaction performance of MSR microreactors. Combining porous catalyst carriers with microchannel reactors may represent a future development direction for reforming reactors, integrating the high heat and mass transfer capacity and low pressure drop of microchannel reactors with the large reactive surface area and gas mixing capabilities offered by porous structures to enhance reforming performance. With the development of additive manufacturing technologies, 3D printing has demonstrated tremendous potential in the design and fabrication of porous catalyst supports. Compared to traditional preparation methods, 3D printing enables precise structural control of catalyst supports, including key parameters such as pore size distribution, channel morphology, and connectivity, thereby optimizing reactant flow and mass transfer efficiency. Moreover, 3D printing technology supports multi-material co-fabrication, allowing the integration of different functional components within the support to enhance the overall performance and stability of the catalyst. In the field of methanol steam reforming for hydrogen production, utilizing 3D printing to fabricate high-performance porous supports is expected to significantly improve catalyst activity distribution and coke resistance, promoting innovations in reactor design and process optimization. Although current research in this area is still at an early stage, 3D printing, as a flexible and high-precision manufacturing approach, will undoubtedly play an important role in the future development of catalyst supports and reactor design, deserving further attention and in-depth exploration. Due to the small size of reformers, numerical simulation is essential for a comprehensive understanding of heat transfer and diffusion phenomena occurring in the system. However, current studies rarely consider these factors. The reaction kinetics and flow patterns in the methanol steam reforming process are extremely complex, especially within catalyst carriers possessing micro- and nano-sized pores. It remains uncertain whether existing simulation methods have sufficient accuracy, as these factors are often overlooked in numerical modeling.

Carbon monoxide removal technology plays a critical role in MSR for hydrogen production, directly affecting the hydrogen purity and catalyst stability. Common CO removal methods include PSA, selective oxidation, and methanation. Through appropriate catalyst selection and precise control of reaction conditions, efficient CO conversion and high-purity hydrogen production can be achieved. Membrane separation technology offers a feasible approach for CO removal; by selecting suitable membrane materials and optimizing membrane device designs, efficient CO transport and hydrogen separation can be realized. Future research should focus on optimizing adsorption processes, enhancing catalyst performance, investigating reaction mechanisms, and developing advanced membrane materials, aiming to achieve more efficient, economical, and environmentally friendly CO removal technologies, thereby promoting the advancement and application of MSR.