Recent Advances in Catalyst Design and Process Intensification for Ethanol Steam Reforming

Abstract

Producing hydrogen from ethanol steam reforming (ESR) is a carbon-neutral and environment-friendly method, which has been expected to reduce the excessive emission of environmental pollution and over-exploitation of fossil resources. Currently, great advances have been made on heterogeneous catalysts, but an in-depth and more comprehensive understanding to further promote this reaction process is still required. Herein, the thermodynamic and kinetic analyses of ESR are firstly highlighted. Then, various reaction pathways of ESR are discussed in detail, respectively combined with experimental studies and density functional theory calculations. On this basis, the key factors affecting the catalytic performance over non-noble and noble metal catalysts are summarized, such as alloying, optimization of the preparation methods, promoter addition and support modification. In addition, the process intensification technologies, including catalytic membrane reactors, adsorption-enhanced reforming and microchannel reactors, are analyzed regarding breaking the thermodynamic limitations and improving the heat and mass transfer efficiency. Finally, the challenges and potential strategies of ESR in the research of dynamic reaction mechanisms, regulation of catalyst stability and integration of intensification technologies are summarized.

1. Introduction

Hydrogen has attracted widespread attention as a promising clean secondary energy carrier, owing to its high mass energy density (40 kW·h/kg) and calorific value (1.4 × 10⁸ J/kg). Nevertheless, hydrogen exhibits a low volumetric energy density of only 3.0 Wh/L under ambient temperature and pressure, which poses great challenges to its storage and transportation [1]. The key to the large-scale application of hydrogen lies in the development of safe and efficient production methods [2]. In particular, alcohol steam reforming has been considered as an attractive method because the liquid alcohols have safe and convenient storage and transportation processes under ambient conditions [3]. Among the various alcohols, ethanol has been developed as the best candidate due to its high theoretical hydrogen storage capacity (~6.1 wt.%), non-toxicity, storage and transport in liquid form. Moreover, ethanol is predominantly available as a bio-based product, derivable from starch-rich feedstocks including sugarcane [4] and corn [5], as well as from lignocellulosic biomass [6] (e.g., agricultural and forestry residues) [7]. Therefore, ethanol steam reforming (ESR) emerges as a promising pathway to facilitate renewable energy generation and enable the efficient utilization of biogenic carbon resources.

Generally, the ethanol steam reforming (ESR) process primarily involves the breakage of O-H, C-H, and C-C, showing a high and similar dissociation energy of (463 kJ/mol) (413 kJ/mol), and (347 kJ/mol), respectively. This results in high activation energy barriers and complex parallel and sequential reaction pathways, including dehydration, dehydrogenation, cracking, and water–gas shift reactions [8,9]. Thermodynamically, ESR is a strongly endothermic process accompanied by a net increase in the number of gaseous molecules, making it favorable under high temperatures (>400 °C) and low pressures [10]. These severe reaction conditions often may induce sintering of the active metal phases and carbon deposition on catalysts, resulting in poor catalytic performances [11]. Therefore, the rational design of supported metal catalysts (active components such as Ni [12], Co [13], and Rh [14]; supports such as CeO₂ [15] and Al₂O₃ [16]) is crucial for improving the catalytic performance of ESR.

Recent years have witnessed significant progress in ESR, particularly in the design of efficient heterogeneous catalysts, the application of in situ characterization techniques, and the use of theoretical calculations to probe reaction intermediates and optimize reactor configurations. Furthermore, a number of review articles have documented advances in ESR, though most have centered on facets of the field, such as reaction mechanistic studies [17], catalyst preparation [18,19], or the optimization of reaction conditions [20]. Nevertheless, identifying the key factors that govern catalytic performance remains challenging due to the complex interplay of multiple variables. In order to improve the process of ESR, it is necessary to provide an in-depth and more comprehensive understanding, from the reaction mechanisms to the effective preparation catalysts and their systematic investigations.

Herein, we first summarize the reaction mechanism of ESR through thermodynamic and kinetic analyses. The effects of modification strategies including alloying, synthesis optimization, promoter addition and support design on improving the activity of non-noble and noble metal catalysts are highlighted. Subsequently, recent advances in process intensification technologies such as catalytic membrane reactors, sorption-enhanced reforming, and microchannel reactors are further summarized. Finally, we put forward the current crucial challenges and potential strategies for the future design and characterization of efficient and stable catalytic systems, so as to further encourage future investigation for this promising field.

2. Reaction Characteristics of Ethanol Steam Reforming

2.1. Thermodynamics Analysis

Ethanol steam reforming is typically represented as: ethanol (C₂H₅OH) reacts with steam (H₂O) to form hydrogen (H₂) and carbon dioxide (CO₂). The ESR reaction is a highly endothermic process with an increasing number of molecules, so it is more favorable in a higher temperature (400–700 °C) and lower pressure. The severe reaction conditions are prone to causing a series of side reactions, including ethanol dehydration, decomposition, and dehydrogenation, followed by the formation various by-products such as CO, CH₄, acetaldehyde (CH₃CHO), acetone (CH₃OCH₃), and ethylene (C₂H₄). Additionally, the exothermic side reactions, e.g., the water–gas shift and Boudouard reactions, become dominant at lower temperatures, leading to a reduction in hydrogen selectivity and yield. Thus, ESR is usually regarded as a complex reaction system, involving the numerous parallel and sequential reactions in

Table 1. Main and side reactions and reaction for hydrogen production via ethanol steam reforming.

No.	Reaction Name	Reaction	ΔHr 298 K KJ·mol−1
1	Ethanol Steam Reforming	C2H5OH + 3H2O→6H2 + 2CO2	+174
2	Ethanol Partial Reforming	C2H5OH + H2O→2CO + 4H2	+256
3	Ethanol Dehydration	C2H5OH→C2H4 + H2O	+45
4	Ethylene Steam Reforming	C2H4 + 4H2O→2CO2 + 6H2	+127.7
5	Ethanol Decomposition	C2H5OH→CO + CH4 + H2	+49
6	Ethanol Dehydrogenation	C2H5OH→CH3CHO + H2	+111.5
7	Acetaldehyde Condensation	2CH3CHO→CH3COCH3 + CO + H2	+4.1
8	Methane Steam Reforming	CH4 + H2O→CO + 3H2	+206
9	Water–Gas Shift Reaction	CO + H2O→CO2 + H2	−41
10	CO Methanation	CO + 3H2→CH4 + H2O	−206
11	Boudouard Reaction	2CO→CO2 + C	−172.5
12	Methane Decomposition	CH4→C + 2H2	+74.6
13	Ethylene Polymerization	nC2H4→C2n + 2nH2	/

[21,22].

The ethanol steam reforming (ESR) reaction system involves multiple chemical equilibria, making thermodynamic analysis essential for evaluating reaction limits and optimizing operating conditions. The Gibbs free energy minimization method, implemented via the HSC Chemistry 6 (version 6.00) software [23], is employed to assess the thermodynamic equilibrium behavior. This approach directly calculates the equilibrium composition under specified conditions without presupposing any reaction pathways. The effects of the temperature (300–900 °C), steam-to-ethanol (S/E) molar ratio (1–6), and pressure (1–20 bar) on the hydrogen selectivity and product distribution are systematically investigated.

As illustrated in Figure 1a, the ESR reaction becomes increasingly favorable as the temperature rises. Above 350 °C, the ESR dominates, while the methanation reaction is suppressed, resulting in a product composition primarily consisting of H₂, CO, and CO₂. Moreover, the mole fraction of hydrogen in the outcome increases with the temperature elevating, as in Figure 1b. Meanwhile, the thermodynamic equilibrium content of by-product CH₄ decreases, probably due to suppression by the high temperature. According to Figure 1c, increasing the S/E ratio promotes H₂ selectivity, because it favors the water–gas shift reaction toward the product side. In addition, the higher pressure inhibits the H₂ selectivity, affected by the increasing mole number reaction (Figure 1d). A high temperature, low pressure, and moderate S/E ratio are favorable for the main ESR reaction. Therefore, when comparing the performance of different catalysts, the differences in reaction conditions should be comprehensively considered.

2.2. Reaction Pathway

To provide critical guidance for the rational design of efficient catalysts, various possible reaction paths are summarized and proposed in Figure 2 (pathway A), based on recent theoretical and experimental advances for ESR. Among these ways, the activation of ethanol on the catalyst surface usually serves as the key step, which primarily is divided into parallel dehydrogenation and dehydration paths.

2.2.1. Dehydrogenation Pathway

The dehydrogenation pathway refers to the preferential dehydrogenation of ethanol to ethoxide (CH₃CH₂O*) in Figure 2 (pathway B). Then, ethoxide is successively converted into the acetaldehyde (CH₃CHO*), acetyl (CH₃CO*), ketene (CH₂CO*), ketenyl (CHCO*), acetate (CH₃COO*), methyl (CH₃*), and other intermediate species. Meanwhile, H₂O, as another reactant, dissociates into active OH* species, which is used to react with the C1 intermediates above to form CO₂. CO, via the water–gas shift (WGS) reaction, is further converted into CO₂ and H₂. This pathway is usually subdivided into the stepwise dehydrogenation and acetate pathways [24].

Stepwise Dehydrogenation Pathway

The stepwise dehydrogenation pathway involves the sequential removal of hydrogen atoms from ethanol to form a series of oxygen-containing intermediates CH_xCH_yO*, following by C-C bond cleavage and subsequent oxidation to form CO₂, as shown in Figure 2 (pathway C). Previous studies found CH₃CH₂O* on the catalyst surface by means of operando DRIFTS experiments, attributed to ethanol dissociation in the first step of ESR [25,26]. Grzybek et al. [27] reported that the energy barrier for acetaldehyde desorption (1.27 eV) is higher than that for C-H cleavage (0.79 eV) on KCo/α-Al₂O₃ by density functional theory (DFT) calculations (Figure 3a), and the adsorbed acetaldehyde is dehydrogenated to acetyl (CH₃CO*). Wu et al. [28] showed by DFT that C-C cleavage occurs on CH₂CO* over Rh(111) terraces, but directly on CH₃CO* over Rh(211) steps (Figure 3b). Ma et al. [29] found that continuous dehydrogenation of ethoxy to CH_xCH_yO* intermediates is the rate-determining step.

Acetate Pathway

As for the acetate pathway in Figure 2 (pathway D), ethoxy intermediates are further oxidized by surface oxygen species to form acetate (CH₃COO*). Acetate decomposes into CH₃* and surface carbonate species. Then, CH₃* reacts with OH* from water dissociation to form CO₂ and H₂, and the carbonate species subsequently decomposes into CO₂. This decarboxylation pathway is common on CeO₂-based catalysts (e.g., Ni/CeMgAl and Ni-CeO₂/MgAl₂O₄) [30]. Ogo et al. [31] found that CH₃CH₂O* is oxidized by surface oxygen to acetate using H/D exchange and in situ infrared spectroscopy experiments. Xu et al. [25] identified characteristic bands of CH₃CH₂O* species at ~1050 and 1110 cm⁻¹ and CH₃COO* species at ~1430 and 1560 cm⁻¹ via in situ DRIFTS (Figure 4), confirming the reaction sequence of CH₃CH₂O* converting to CH₃COO*, carbonate, OH* and CH₃* intermediates.

2.2.2. Dehydration Pathway

In the dehydration pathway, CH₃CH₃OH dehydrates to C₂H₄, which, as a key intermediate, may react with water to produce CO₂ and H₂, or directly dehydrogenate to form coke. The ethylene was detected by gas chromatography (GC) studied by Iglesias-Vázquez et al. [32] and Choong et al. [33], indicating the ethanol dehydration pathway. Valecillos et al. [34] conducted ethylene and water co-feeding experiments with online micro-GC analysis at 500 and 600 °C (Figure 5), using a steam/ethylene ratio of three. The products included H₂, CO, and CO₂, with a distribution similar to ethanol steam reforming. In contrast, a pure ethylene feed without water only produced H₂ and carbon. This means that ethylene undergoes steam reforming in the presence of water, and CO is further converted to CO₂ and H₂ via the water–gas shift reaction.

3. Catalysts for Ethanol Steam Reforming

Metals with unpaired outer d-electrons can interact with the C-C, O-H, and C-H bonds and some oxygen-containing intermediates, which may promote ethanol activation. Thus, supported metal catalysts are usually used for hydrogen production by ESR, which are generally divided into non-noble metal (e.g., Ni, Co, and Cu) and noble metal (e.g., Rh, Ru, and Pt)-based catalysts.

3.1. Ni-Based Catalysts

Nickel-based catalysts have attracted extensive attention due to their abundant reserves, low cost, and excellent C-C and C-H bond cleavage performance in ESR. However, Ni-based catalysts are easily sintered at high temperatures and promote the polymerization of intermediates such as C₂H₄, leading to severe carbon deposition and catalyst deactivation. In recent years, researchers have adopted many strategies, including alloying, preparation optimization, support modification, and promoter addition, to improve Ni dispersion, tailor the surface electronic structure, and strengthen the metal–support interaction, thus significantly enhancing the activity, selectivity, and stability of Ni catalysts.

3.1.1. Incorporating Other Metals

Single Ni active sites usually show a high catalytic activity followed by some side reactions such as deep hydrogenation (methanation) and deep dehydrogenation (carbon deposition). To inhibit side reactions, reduce carbon deposition and improve stability, other metals (e.g., Cu, Co, Fe, etc.) are usually introduced to adjust the geometric structure and electronic properties of single Ni by means of forming bimetallic or multi-metallic Ni-based catalysts.

Co, as a common alloying component, is usually used to form NiCo alloy to modulate Ni properties. Aker et al. [35] reported that Ni-Co/Al₂O₃ with a Ni/Co molar ratio of 2:1 achieves complete ethanol conversion and high H₂ selectivity (95.14%) at 600 °C, surpassing the ethanol conversion and H₂ selectivity of the monometallic Ni/Al₂O₃ catalyst. This is because the CoNi alloy showed a higher metal dispersion and smaller metal particle size compared with the single Ni catalyst. In addition, the distinctive metal–oxide interfaces may be formed on a Co and Ni bimetallic catalyst, which could promote the conversion of reaction intermediates. Shi et al. [36] found that Co could be oxidized to CoO and form a Ni-CoO interface, in which the Ni⁰ promoted the dissociation of C-C bonds and the interfacial CoO promoted the activation of H₂O to OH* (surface-adsorbed hydroxyl species). Similarly, Fe shows dual roles of electronic modulation and carbon deposition inhibition in Ni-based catalysts. Wu et al. [37] prepared a series of NiFe/MgAl₂O₄ catalysts, where Ni-Fe alloys of 7–9 nm formed. In situ DRIFTS, CO-TPD, and XPS analyses revealed the dual role of Fe species (Figure 6): Ni-Fe alloys weakened CO adsorption via electron transfer, suppressing methanation; meanwhile, the oxidation capacity of γ-Fe₂O₃ species promoted the transformation of ethoxy to acetate groups and the elimination of carbon deposits. At 400 °C, the H₂ yield increased from 2.7 mol H₂/mol EtOH over the monometallic Ni catalyst to 4.6 mol H₂/mol EtOH on the optimal NiFe bimetallic catalyst. In a 30 h stability test, the NiFe/MgAl₂O₄ catalyst maintained high activity without obvious deactivation, whereas the unmodified Ni/MgAl₂O₄ catalyst deactivated rapidly after 15 h due to severe carbon deposition covering the active sites.

Apart from non-precious metals, precious metals also are used to form alloys and stabilize Ni nanoparticles. Meng et al. [38] prepared a series of RhNi/TiO₂ catalysts via topological transformation of RhNiTi layered double hydroxides. The unique (Rh-Ni^δ−-O_v-Ti³⁺) multifunctional interface may be formed by means of the strong bimetal–support interaction (Figure 7a). This interface could stabilize RhNi nanoparticles through a reversible TiO₂ overlayer, preventing active-site loss and sintering. At 400 °C, ethanol conversion reached 99.7% with stable activity over 300 h (Figure 7b,c), which is much higher than that of Ni/TiO₂ (49.5%) and Rh/Ni (39.3%) catalysts after 40 h.

Recently, Ni-based multi-metal catalysts display the well abilities of cracking, oxidation, trapping, and reconstruction through the multi-metal synergistic effect. Liao et al. [39] prepared a nanorod-shaped quinary metal oxide NiCoMnCeCaO_x via the coprecipitation method. The ethanol conversion of the optimal NiCoMnCeCaO_x catalyst reached up to 99.92% and the hydrogen production rate was 120.5 μmol/s, which is 27.2% higher than that of unmodified NiCo/CeO₂, at 600 °C, an ethanol/water molar ratio of 1:3 and a space velocity of 44.79 h⁻¹. This is because the Mn and Ce may improve the lattice oxygen mobility, enhancing CH_x and CO oxidation and reducing carbon deposition. Meanwhile, the basic sites generated from Ca could promote in situ adsorption of CO₂.

3.1.2. Optimizing Catalyst Preparation Methods

The catalyst preparation methods are usually regarded as the key factors regulating the structure and properties of active sites, including the impregnation, coprecipitation, hydrothermal and sol–gel methods, and so on. Alipour et al. [40] compared the ESR reaction performance of Ni/Al₂O₃ catalysts prepared by the mechanochemical, impregnation, and coprecipitation methods, respectively. Among them, the ethanol conversion achieved 92% and hydrogen yield was up to 86% with the impregnation method due to the higher specific surface area and Ni dispersion. Sosa et al. [41] studied the catalytic performance of Ni/CeO₂ catalysts prepared by magnetron sputtering deposition (SP) and incipient wetness impregnation (IWI) for the ESR reaction. The Ni/CeO₂-SP catalyst exhibited a higher ethanol conversion of 90% and H₂ yield of 8.1 L·h⁻¹·gcat⁻¹ at 500 °C, as seen in Figure 8, relative to the Ni/CeO₂-SP catalyst (60% and 5.8 L·h⁻¹·gcat⁻¹). This is because the former showed the highly dispersed Ni and abundant oxygen vacancies for the formation of weakly encapsulated graphitic carbon, while the latter may tend to form amorphous carbon and cover active sites.

Optimizing preparation methods can regulate the size distribution of Ni particles, and alter the morphology and migration behavior of carbon deposits. Bao et al. [42] prepared a series of Ni/SiC_xO_y catalysts with a “multi-size distribution” feature using inverse microemulsion and precipitation methods. The Ni/SiC_xO_y maintained good ethanol conversion (95%) and H₂ selectivity (69.2%) after 20 h, superior to the H₂ selectivity (56% after 20 h) of Ni/MgO at 500 °C and a steam/ethanol ratio of 8:1. The small Ni particle size may promote the formation of loose carbon nanowires instead of amorphous carbon, and accelerate carbon mass transfer to avoid coverage of active sites. Villagran-Olivares et al. [43] prepared a Ni/MgAl₂O₄-CeO₂ catalyst using an EDTA-assisted impregnation method. The introduction of EDTA reduced the NiO particle size and promoted the strong Ni-CeO₂ interaction, increasing the surface Ce³⁺ concentration and interfacial oxygen mobility. The carbon-containing intermediates may be efficiently oxidized to decrease the coking rate from 30.6 to 1.4 mgC/(gcat·h).

The structure of active phases can be modulated by different preparation methods. Chen et al. [44] reported that the Ni/Al₂O₃ catalyst prepared by the one-pot (OS) method achieved nearly complete ethanol conversion (99%), while the one synthesized by impregnation (IMP) only reached 74% and decreased to 58% after 55 h, at 500 °C and a S/E molar ratio of six. This was mainly attributed to the more active Ni⁰ species based on the highly dispersed NiO in Ni/Al₂O₃-OS. In contrast, the Ni/Al₂O₃-IMP catalyst displayed a large amount of unreducible NiAl₂O₄ spinel and fewer active Ni⁰ sites. Additionally, a suitable preparation method can regulate the electronic properties of Ni, such as the ratio of both Ni⁰ and Ni²⁺ species. Ni⁰ nanoparticles were responsible for C-C bond cleavage, whereas electron-deficient Ni²⁺ sites enriched in the support strengthened C-H activation and suppressed methanation. Tian et al. [45] synthesized a series of Ni/Ni_xMg_1-xO catalysts with unable Ni⁰/Ni²⁺ ratios via a non-equilibrium two-step calcination method at the same total Ni loading (20 wt%). When applying in the ESR at 400 °C with a S/E of six, the pure solid solution without Ni⁰ (15NiMg ss) showed only 20% ethanol conversion and 10% acetaldehyde formation, as in Figure 9a. In contrast, 10ss-5NiO with suitable Ni⁰ (88.6% surface Ni²⁺) achieved nearly complete ethanol conversion, with a H₂ yield of 11.5 L·h⁻¹·gcat⁻¹ and stable operation for 30 h due to the synergistic effect of balanced Ni⁰ and Ni²⁺, as seen in Figure 9b.

3.1.3. Introducing Promoters

The promoters may modify the active site structure and influence the reaction performance, mainly including the rare-earth elements such as La and Pr, as well as alkali and alkaline-earth metals including K and Mg. Rare-earth metal promoters are commonly used to adjust the redox properties and stabilize nickel species in CeO₂-supported Ni-based catalysts. Xiao et al. [46] prepared a La-doped Ni-CeLa₀.₂ catalyst, achieving a high ethanol conversion of 100% and a low coking rate of 0.48 mgC/(g_cat·h) at 600 °C; the unmodified Ni-CeO₂ showed a stability of 70.1% with a coking rate of 2.60 mgC/(gcat·h). DFT calculations indicated that La doping reduced the formation energy of oxygen vacancies on the CeO₂ surface to −0.96 eV (Figure 10a). The formation of oxygen vacancies significantly promoted water molecule dissociation (energy barrier of −0.25 eV) (Figure 10b), which in turn promoted the gasification of surface carbon species.

Pr can also decrease the oxygen vacancy formation energy of CeO₂ supports via co-doping, and facilitate the generation and migration of active oxygen species, so as to improve the anti-coking ability and long-term stability of catalysts. Xiao et al. [47] found that an appropriate amount of Pr doping (10Ni-CePr₀.₂₀), and as revealed by DFT calculations, co-doping of Ni and Pr, lowered the oxygen vacancy formation energy on the CeO₂ (111) surface to −1.39 eV (Figure 11a), far below that of pure CeO₂ (2.14 eV) (Figure 11b). This promoted the generation and migration of active oxygen species, enabling the catalyst to operate stably for over 120 h with an extremely low coking rate of 0.00056 mgC/(gcat·h).

Alkali metal promoters such as K, Ca and Mg neutralize acidic sites on the support surface and may suppress the dehydration reaction pathway, and thereby hinder the formation of carbon deposition precursors. Shi et al. [48] reported the MgAl₂O₄ spinel formed by a Mg promoter could make the acidity of the support decrease from 297 to 119 μmol/g and inhibit ethanol dehydration to ethylene in Process B of Figure 12. Meanwhile, Mg could promote the increase of Ni crystallinity, which suppresses carbon dissolution and diffusion, and prevents Ni₃C formation in Process C of Figure 12. The main carbon species is filamentous carbon (Process A, Figure 12), which barely covers active sites. The coking rate decreases from 15.1 to 3.7 mgC/(gcat·h), indicating good anti-coking stability. Mg also promotes steam reforming of the by-product methane and enhances hydrogen selectivity. Niazi et al. [49] revealed that H₂ selectivity could reach 74% and CH₄ selectivity was as low as 1% at 600 °C on 13Ni-4Mg/CeO₂ (Ni-Mg) catalysts. This is because the surface basicity (CO₂ uptake:35 μmol/g) measured by CO₂-TPD may be a help for the methane steam reforming. The alkali metal K is also used to tune the electronic properties by constructing unique active interfaces. Wang et al. [50] found that the unique Ni-O-K interface formed on a K-modified Ni-K/KL catalyst achieved 97.5% ethanol conversion and 76.3% hydrogen yield. However, the unmodified Ni/KL catalyst deactivated rapidly after 2 h. This might be attributed to the interface-promoted water molecule activation and accelerated reactions such as the water–gas shift.

3.1.4. Modifying Supports

The different supports exhibit various structures and properties, such as the acid–base sites, oxygen vacancy sites and so on, which may promote the catalytic performance of ESR. Seriyala et al. [51,52] found the oxygen vacancy concentration increased from 20% to 35% by means of the doping of La or Zr into CeO₂. This leads to a significantly improved catalytic activity, achieving 100% ethanol conversion and decreasing carbon deposition from 20 wt% to 2.4 wt%. Baig et al. [53] found the coking rate in the ESR reaction decreased from 20 to 2 mgC/(gcat·h), when introducing La₂O₃ into CeO₂-ZrO₂. This was because the La₂O₂CO₃ intermediates, formed by the adsorption and activation of CO₂ on La₂O₃, could react with the deposited carbon species (La₂O₂CO₃ + C=La₂O₃ + 2CO). Liu et al. [54] found that the Ni-Mg/HCa catalyst, with a high specific surface area and strong basic sites, effectively inhibited ethanol dehydration and C-C bond cleavage, so as to obtain a high ethanol conversion of 100% and H₂ yield of 86% at 650 °C.

The spatial confinement effect on the support also plays a key role in inhibiting the sintering of catalytic active metal sites and carbon deposition. Murthy et al. [55] prepared similar Ni-based catalysts for the ESR reaction using the siliceous mesocellular foam (MCF-S) and SiO₂ as support, respectively. The H₂ selectivity of the MCF-S-supported Ni catalyst increased from 47.2% to 68%, compared with conventional Ni/SiO₂. This was because the spatial confinement effect formed by the protective pore channels inhibited sintering and carbon coverage of active sites. Xue et al. [56] synthesized a NiRh@NiO@mSiO₂ catalyst with an alloy-core, oxide-shell, porous-encapsulated structure, and the controlled catalyst NiRh-SiO₂ without a mesoporous shell, shown in Figure 13. At 550 °C, the conversion loss (17 h) of NiRh@NiO@m-SiO₂ was 10.9%, which is much lower than that of 57.8 (1 h) on the NiRh-SiO₂ catalyst. The mesoporous SiO₂ layer suppressed sintering and leaching of the metal sites via the confinement effect, and promoted the effective in-diffusion of water and out-diffusion of carbonaceous products, verified by the results of EXAFS, TEM-EDS mapping, depth profile XPS, N₂ physisorption, H₂-TPR, and XRD. Similarly, An et al. [57] found a core-shell SiO₂@NiCuMgAl catalyst displayed complete ethanol conversion at 500 °C and remained stable for 21 h, shown in

Table 2. ESR reaction performance of some reported Ni-based catalysts at atmospheric pressure.

Catalyst	T °C	GHSV mL·gcat−1·h−1	Ratio S/E	Con. %	Sel. H2 %	Ref.
2Ni-Co/Al2O3	600	57,120	24	100	95.14	[35]
9Ni1Co/CeMgAl	450	10,000	6	~100	~70	[36]
Ni10Fe10/MgAl2O4	400	59,146	8	~100	~70	[37]
0.5RhNi/TiO2	400	16,700	6	99.7	61.7	[38]
NiCoMnCeCaOx	600	30,100	3	99.92		[39]
12.5Ni/Al2O3	600	24,000	3	92	86	[40]
Ni/CeO2-SP	500	30,000	6	90	65	[41]
Ni/SiCxOy	500		8	100	74	[42]
NiEDTA(0.5)/MgAl2O4-CeO2	650	292,000	4.85	~90	~65	[43]
NiEDTA(1)/MgAl2O4-CeO2	650	292,000	4.85	~100	~95	[43]
Ni/Al2O3-OS	400	35,960	8	100		[44]
Ni/Al2O3-IMP	400	35,960	8	74		[44]
5ss-10NiO	400	33,940	6		~55	[45]
Ni-CeLa0.20	600	55,920	4	~95		[46]
Ni-CeTb0.20	600	55,920	4	~100		[46]
Ni-CeZr0.20	600	55,920	4	~95		[46]
10Ni-CePr0.20	600	44,240	4	100		[47]
Ni/Mg-Al2O3	500	60,000	6	~100	~60	[48]
13Ni/CeO2	600	20,000	6	~89	71	[49]
13Ni-4Cu/CeO2	600	20,000	6	100	73	[49]
13Ni-4Mg/CeO2	600	20,000	6	96	74	[49]
13Ni-4Mo/CeO2	600	20,000	6	91	73	[49]
Ni-K/KL	600	19.96 h−1	6	97.5	76.3	[50]
Ni0.56Sn0.57(5)/Ce0.67La0.33O2	400	7697 h−1	12	100	60	[51]
NS5/CZ12	400	12,720	12	100	69	[52]
7.5Ni-7.5La/Ce-Zr	600	2.48h−1	4.5	97	65	[53]
15Ni-HCa	650	32,900	6	100	85	[54]
10 wt% Ni/MCF-S	700	17,160	4	100	68	[55]
NiRh@NiO@m-SiO2	550	21 h−1	4	100		[56]
NiCuMgAl	350	49,100	3	99.43	61	[57]
SiO2@NiCuMgAl	600	49,100	3	100	75	[57]

, superior to the unmodified NiAl catalyst deactivated within 6 h. This was attributed to the highly dispersed active sites and enhanced structural stability. Table 2 summarizes the performance of some Ni-based catalysts in ESR from recent reports mentioned above.

3.2. Co-Based Catalysts

Co-based catalysts are also used for ESR reaction, because they can effectively activate C-C and C-H bonds, promote the water-gas shift reaction. And, with the presence of appropriate carriers or additives, Co catalysts may inhibit carbon deposition and maintain hydrogen selectivity. However, Co-based catalysts show a relatively poor catalytic activity at low temperature limits large-scale industrial application. Optimizing preparation methods, adding alkali or rare-earth promoters, and using redox supports such as CeO₂ could improve their low-temperature activity and balance activity and stability.

3.2.1. Regulating Catalyst Synthesis Methods

Co species tend to agglomerate and migrate when using traditional preparation methods. Optimizing preparation methods can effectively regulate the structure of active phases, enhance the metal–support interaction, and inhibit Co migration. Grzybek et al. [58] compared Co/α-Al₂O₃ catalysts prepared by incipient wetness impregnation (IWI), hydrothermal (HT), and sonochemical (SC) methods. The highly dispersed mixed phases of Co₃O₄ and CoAl₂O₄, formed by SC method, via ultrasonic cavitation, displayed an enhanced metal–support interaction and suppressed Co migration. At 500 °C, the SC-derived catalyst achieved the highest Co mass activity (CTY = 4.1 molEtOH·gCo⁻¹·h⁻¹), 3.7 times that of the IWI catalyst and 2.6 times that of the HT catalyst (Figure 14), with ethylene formation almost completely suppressed. Appropriate preparation methods can simultaneously form porous carbon supports and metal nanoparticles, simplifying synthesis and improving energy efficiency. Efimov et al. [59] synthesized a Co-Ru alloy catalyst supported on carbonized polyacrylonitrile (IR-PAN-Co-Ru) via a one-step infrared radiation heating method. As the carbonized polyacrylonitrile support formed a graphitic carbon shell which protected the metal nanoparticles, under infrared heating, the catalyst operated continuously for 17 h without deactivation, with a hydrogen yield of 5.6 mol/mol EtOH. Its performance was significantly higher than that of the Co/carbon nanotube catalyst (2.5 mol H₂·mol EtOH⁻¹). The impregnation order also influences the catalytic performance via the impregnation method. Escalante et al. [60] found that in the Co-CeO₂/MgAl₂O₄ system, the catalyst prepared by co-impregnation showed higher Co dispersion (68.5%) than that by sequential impregnation (33.8%). At 650 °C, the co-impregnated catalyst exhibited 5.3 mol H₂/mol EtOH H₂ yield, while the sequential impregnated sample gave only 1.6 mol H₂/mol EtOH.

3.2.2. Adding Promoters

The promoters may affect active centers, so as to reduce the energy barrier of key reactions and inhibit carbon deposition through electronic effects, thereby improving the performance of catalysts. Grzybek et al. [61] found that the electron-enriched Co^δ− formed by electron transfer from K to Co could adjust the oxidation state of Co in the K-CoZn|α-Al₂O₃ catalyst. It reduced the energy barrier for C-C bond cleavage, suppressed acetaldehyde desorption and promoted deep reforming to produce H₂ and CO₂. Grzybek et al. [62] found that K-modified K-Co/b-Al₂O₃ achieved 100% ethanol conversion, compared with approximately 80% for unmodified Co/b-Al₂O₃.

The rare-earth Ce may accelerate carbon deposition gasification by promoting oxygen migration on the CeO₂ surface. Greluk et al. [63] found that the Co-Ce interaction and surface oxygen migration could be enhanced by a low Ce loading, promoting carbon gasification in the Co-0.1Ce/CeO₂ catalyst. This catalyst remained stable for 21 h at 500 °C, whereas Co/CeO₂ suffered rapid deactivation at the initial stage due to blocked active sites by carbon deposition. Ranjekar et al. [64] reported that CeO₂ promoted initial ethanol dehydrogenation and eliminated coke precursors, while a Cu-Co alloy facilitated C-C bond cleavage and the WGS reaction (Figure 15). At 673 K, the catalyst exhibited 97.4% ethanol conversion, 92 mol% hydrogen selectivity and stable operation for 120 h.

3.2.3. Optimizing Supports

The Co-based catalysts also need suitable supports to modulate the active phase structure for the enhanced catalytic performance of ESR. Grzybek et al. [65] prepared a series of Co/ZSM-5 catalysts with different Si/Al ratios (32, 750, ∞) via incipient wetness impregnation. As the Al content decreased, the density of Brønsted acid sites significantly reduced, demonstrated by the FT-IR pyridine adsorption, XPS and H₂-TPR characterizations. The ethylene selectivity dropped from 50% to 0%, when the acid density decreased from 445 μmol/g at Si/Al = 32 to 0 μmol/g at Si/Al = ∞. Zeolite supports can regulate the particle size and dispersion of active metals. Da Costa-Serra et al. [66] found a smaller Co particle size and higher dispersion on the Co/ITQ-6 catalyst using layered zeolite ITQ-6 as support, due to strong metal–support interaction, which displayed good ethanol conversion of 90.5% and H₂ selectivity of 41.3% (5 h) at H₂O/EtOH = 13, GHSV = 4700 h⁻¹, 400 °C and atmospheric pressure.

CeO₂-based supports show a good oxygen storage and release capacity, and tunable structural properties (porosity, crystal facets, etc.), which may affect catalyst performance in ethanol steam reforming. Lupa et al. [67] synthesized porous microspherical CeO₂ via hard templating and prepared a Co/CeO₂-XAD catalyst, with commercial nano-CeO₂ as a reference (Co/CeO₂-COM). At 420 °C, Co/CeO₂-XAD showed an initial ethanol conversion near 100%, retained 54% after 21 h, and achieved 72% H₂ selectivity, while Co/CeO₂-COM exhibited only 58%, 27% and 63%, respectively. This stemmed from the smaller size of the metal crystallites, a well-developed internal surface and highly defective particles of support reported by comprehensive studies. Tuning the exposed crystal planes of CeO₂ enables modulation of the electronic valence state of active metals. Li et al. [68] reported the chemical states of Co sites could be tuned by the preferred exposure of CeO₂ facets (Figure 16a) in Co/CeO₂ catalysts revealed by XRD and TEM (Figure 16b,c). The Co/CeO₂-(M) with a (111) facet maintained Co in a low oxidation state, enhancing C-C bond cleavage and suppressing condensation. This results in a high ethanol conversion of 100%, H₂ selectivity of 97%, and stable operation for 42 h. In contrast, the Co/CeO₂-(U) catalyst with a (100) plane induced high Co oxidation states, promoting chain growth and coking, and displayed a low ethanol conversion (87.2%) and H₂ selectivity (65.8%), which declined to 60.5% after 12 h (Figure 16d).

Introducing heteroatoms into the oxide lattice to form solid solutions can induce stronger charge transfer, regulating the valence state and reducibility. Li et al. [69] doped CeO₂ with La³⁺ to form a solid solution on the Co/La_xCe_1−xO_2−y catalyst. The ethanol conversion and H₂ selectivity reached up to nearly 100% and 94.3%, which is higher than that of Co/CeO₂ (73.3% and 65.8%). La³⁺ doping generated surface peroxide anions (O_p) that donated electrons to cobalt, lowering its valence, thus enhancing C-C bond cleavage. Similarity, Fornero et al. [70] found the Ga-incorporated CeO₂ catalyst, Co/Ce₉₀Ga₁₀O_x, was more efficient for ESR, and the production of acetaldehyde, acetone and oxygenated by-products (Figure 17), than Co/CeO₂. This is because the surface area increased from 58 to 108 m²/g and formed highly active Ce-O-Ga interfaces. Li et al. [71] synthesized a Si₄Al₂O₂N₆ solid solution from Si₃N₄ and Al₂O₃ and prepared a Co/Si₄Al₂O₂N₆ catalyst. It exhibited 97% ethanol conversion, much higher than Co/Si₃N₄ (74%) and Co/Al₂O₃ (81%). The electronic effects of this support transferred electrons to cobalt species, lowering its valence state and increasing the Co⁰ content, thereby enhancing its C-C bond cleavage capability.

Table 3. ESR performance of some reported other non-noble catalysts at atmospheric pressure.

Catalyst	T °C	GHSV mL·gcat−1·h−1	Ratio S/E	Con. %	Sel. H2 %	Ref.
Co(IWI)|Al2O3-S3	500		4	~47	50	[58]
Co(HT)|Al2O3-S3	500		4	~56	55	[58]
Co(SC)|Al2O3-S3	500		4	~61	60	[58]
IR-PAN-Co-Ru	550	1341	3	98.7	94.8	[59]
IR-PAN-Co	550	1341	3	82.7	55.61	[59]
IR-PAN-Ru	550	1341	3	54.7	47.50	[59]
4CoCe/S	650		4.9	~87		[60]
8CoCe/S	650		4.9	~100		[60]
K-Co2.6Zn0.4O4	540		4	~100	94	[61]
Co/b-Al2O3-I	500	60,000	4	~90	88	[62]
1%K-Co/b-Al2O3-I	500	60,000	4	~100	92	[62]
Co/b-Al2O3-SC	500	60,000	4	100	88	[62]
Co-0.1Ce/CeO2	540	60,000	4	100	86	[63]
5%Ce-10%Cu-10%Co/MCF	400	2700	6	95	92	[64]
Co|Z(∞)-S	500	52h−1	12	100	95	[65]
Co/ITQ-6	500	4700h−1	13	90.5	41.3	[66]
Co/CeO2-XAD	420	60,000	12	100	73	[67]
Co/CeO2-COM	420	60,000	12	58	68	[67]
Co/CeO2-(M)	500	34,500	6	100	97	[68]
Co/CeO2-(U)	500	34,500	6	87.2	65.8	[68]
Co/La0.25Ce0.75O1.88	500	44,400	6	100	94.3	[69]
1.0 wt% Co/CeO2	500	867	6	100	35	[70]
1.0 wt% Co/Ce90Ga10Ox	500	867	6	100	50	[70]
Co/Al2O3	500	44,400	6	81	83	[71]
Co/Si3N4	500	44,400	6	74	86	[71]
Co/Si4Al2O2N6	500	44,400	6	97	92	[71]

summarizes the performance of other non-noble catalysts in ESR from recent reports mentioned above.

3.3. Noble Metal Catalysts

Noble metal catalysts (Rh, Pt, Pd, Ru, Ir, etc.) show potential in ethanol steam reforming due to their good activity, while the excessive price limits large-scale industrial application. Current research focuses on optimizing metal–support interaction, designing special structures and adding promoters to improve catalytic performance, while reducing noble metal loading and enhancing atomic efficiency to balance performance and cost.

The various noble metals display different structures and properties, and corresponding metal—support interactions, which may influence the dispersion state, electronic properties, and catalytic reaction performance. Silva et al. [72] prepared a series of M/CeO₂ catalysts (M = Au, Pd, Pt, Rh, Ru) via urea-assisted deposition for the ESR reaction. Among them, Rh/CeO₂ achieved 100% ethanol conversion (Figure 18a) and 37.8% H₂ yield (Figure 18b). This is attributed to the strong interaction between Rh and CeO₂, which facilitated the formation and migration of oxygen vacancies, and enhanced the C-C bond cleavage. Bampos et al. [73] found the Ru/Al₂O₃ catalyst exhibited nearly 100% conversion of a butanol–ethanol mixture at 700 °C, H₂ selectivity above 95%, and stable performance without deactivation for 45 h. In contrast, Pt/Al₂O₃ suffered from rapid coke formation due to the generation of C₃H_x and C₂H₄ by-products, resulting in a decline of H₂ selectivity from 96% to 82% within 12 h and poor stability. This can be attributed to the higher C-C bond cleavage activity of Ru than Pt.

Introducing a second component Ce, and controlling its content to construct specific active sites, also plays a key role in improving the catalytic performance. Lin et al. [74] prepared a series of Rh-Ce/Al₂O₃ catalysts with different Ce content from 1 to 10 wt% for the ESR reaction. When introducing 5 wt.% Ce, the Rh-O-Ce sites formed by the CeO₂ nanoparticles trapped single-atom Rh. This structure enhanced water activation, promoted OH species formation, and accelerated acetate conversion and the water–gas shift reaction. This leads to a high H₂ production rate of 22.4 mmol·g⁻¹·min⁻¹, which is higher than that of Ce-free Rh/Al₂O₃ (15.9 mmol·g⁻¹·min⁻¹) at 450 °C. Studies by Martinelli et al. [75] and Rajabi et al. [76] demonstrated that introducing promoters like Na or Cs into Pt/ZrO₂ increased catalyst surface basicity, altering the reaction pathway from ethanol decarbonylation (producing CO) to decarboxylation (producing CO₂), reducing CO selectivity from 34.0% to undetectable levels.

Incorporating Ru into the perovskite lattice and in situ exsolution of highly dispersed Ru nanoparticles may inhibit the sintering of active metals. Tamara S. et al. [77] synthesized Ru solid-solution-doped LaCr_1-xRu_xO₃ via the Pechini method. The unique solid-solution-derived active sites could not only afford abundant active centers but also suppress metal sintering and coke formation significantly via the strong metal–support interaction. The ethanol conversion in the ESR reaction over this catalyst achieved 100% within 85 h. In contrast, the conventional Ru/LaCrO₃ catalyst deactivated rapidly at the initial reaction stage. Lee et al. [78] prepared the Pd₀.₀₁Zn₀.₂₉Mg₀.₇Al₂O₄ catalyst encapsulating Zn and trace Pd into the MgAl₂O₄ spinel framework. At 450 °C with S/E = 3, this catalyst maintained 100% ethanol conversion and 80% H₂ yield after a 30 h reaction, with coke deposition only one-third of that over unmodified MgAl₂O₄. The stable spinel structure inhibits sintering of the embedded Zn and Pd, thereby extending the catalyst durability. Dai et al. [79] found the hollow zeolite-confined Pt@HBZ catalyst can confine active metals and suppress sintering. At 350 °C, this catalyst achieved 100% ethanol conversion, and H₂ selectivity increased from 55% over conventional Pt-beta to 68%.

Introducing a second metal to form bimetallic alloys can generate a functional synergistic effect by constructing bimetallic interfaces. Choong et al. [80] constructed an Ir-Fe_xO_y bifunctional interface by introducing Fe into Ir/Al₂O₃. The Ir sites facilitate C-C bond cleavage, while Fe_xO_y promotes the water–gas shift (WGS) reaction. At 400 °C, this Ir-Fe_xO_y catalyst exhibited a H₂ yield of 4.1 mol H₂/mol EtOH, much higher than 0.6 mol H₂/mol EtOH over the monometallic Ir catalyst, with CO₂ selectivity increased from 0% to 52.7%. Similar to the IrFe alloy, the PtCu alloy can also promote the reaction through the bimetallic synergy effect. Dai et al. [81] found the Pt-Cu@mSiO₂ catalyst displays a good ethanol conversion, H₂ selectivity and stability (150 h), relative to the Pt@mSiO₂ reference catalyst (30 h) at 400 °C. The synergistic promotion originates from Pt facilitating C-C bond cleavage and the WGS reaction, while Cu accelerates ethanol dehydrogenation. In addition, the support morphology may influence Pt dispersion. Kourtelesis et al. [82] prepared Pt/CeO₂ catalysts using CeO₂ with three distinct morphologies: nanocubes (CeO₂-NC), nanorods (CeO₂-NR), and flower-like structures (CeO₂-FL). Among them, the Pt/CeO₂-NR exhibited the highest Pt dispersion and surface Pt⁰ fraction, with no obvious decline in ethanol conversion within 28 h.

Table 4. ESR performance of some reported noble metal catalysts at atmospheric pressure.

Catalyst	T °C	GHSV mL·gcat−1·h−1	Ratio S/E	Con. %	Sel. H2 %	Ref.
Au/CeO2	550	33,120	3	65	3.1	[72]
Pd/CeO2	550	33,120	3	68	4.8	[72]
Pt/CeO2	550	33,120	3	100	34.6	[72]
Rh/CeO2	550	33,120	3	100	37.8	[72]
Ru/CeO2	550	33,120	3	100	31.2	[72]
Ru/Al2O3	700	13,500	12	100	~95	[73]
Ru/MgO-Al2O3	700	13,500	12	100	~97	[73]
Pt/Al2O3	700	13,500	12	100	~82	[73]
Rh/Al2O3	450	450,000	8	98	39.6	[74]
Rh-Ce5/Al2O3	450	45,000	8	100	55.8	[74]
2%Pt/ZrO2	300	381,000	9	63.5	43	[75]
1.8%Na-2%Pt/ZrO2	300	381,000	9	51.3	60	[75]
2.9% Cs-2% Pt/ZrO2	300	190,560	8.77	73	88	[76]
LaCr0.90Ru0.10O3	600	72,000	3	100	~65	[77]
LaCr0.80Ru0.20O3	600	72,000	3	100	~70	[77]
Pd0.01Zn0.29Mg0.7Al2O4	450	6000	3	100	~90	[78]
Pt@HBZ	350	7020	4	100	68	[79]
Ir/Al2O3	400	88,000	9.6	62		[80]
IrFe/Al2O3	400	88,000	9.6	100		[80]
Pt-Cu@mSiO2	400	8900	4	99.99	70.88	[81]
Pt/CeO2-NC	300	39,720	3	80	30	[82]
Pt/CeO2-NR	300	39,720	3	70	28	[82]
Pt/CeO2-FL	300	39,720	3	65	25	[82]

summarizes the performance of some noble metal catalysts in ESR from recent reports mentioned above.

4. Process Intensification Technologies

4.1. Separation Intensification Technologies

Separation intensification technologies drive the reaction equilibrium forward by in situ coupling of the reaction and separation to selectively remove products. The main technologies include catalytic membrane reactors (CMRs) for hydrogen separation via H₂-selective membranes, and sorption-enhanced ethanol steam reforming (SESR), where CO₂ capture by solid sorbents indirectly promotes hydrogen production.

4.1.1. Catalytic Membrane Reactor

The catalytic membrane reactor (CMR), as a low cost and high efficiency hydrogen purification technology, is usually formed by conventional reactors coupled with palladium-based (Pd-based) membranes. Generally, a higher H₂ yield could be achieved compared to traditional reactors. Chen et al. [83] reported a high ethanol conversion of 93% on a Pd-based membrane reactor at a Reynolds number of 500, compared with only 91% conversion in conventional reactors under identical conditions. The difference was mainly attributed to the H₂ removal through the Pd membrane, making the reaction equilibrium system shift towards higher product yields. Lu et al. [84] conducted ESR reactions over metallic nickel hollow fiber membranes (NiHFMs) without additional catalyst loading, as the Ni membrane possesses catalytic activity. At 800 °C and ambient pressure with a S/E molar ratio of three, the H₂ yield was increased from 53.97% in the fixed-bed reactor to 59.95% in the metallic NiHFM reactor.

Membrane materials have an impact on reactor performance. Jia et al. [85] prepared a Pd₇₅Cu₂₅ alloy membrane on a porous ceramic support via electroless plating. At 873 K and 1.3 MPa, the membrane exhibited 98% H₂ purity over 240 h of continuous operation. In contrast, a pure Pd membrane showed that H₂ purity gradually declined from 96.7% to 91.2%. The authors attributed this improvement to Cu incorporation, which mitigated the hydrogen-induced expansion and defect propagation. The performance of the reactor may be affected by the membrane structure. Eremeev et al. [86] used a dense Ni-Cu alloy–neodymium tungstate nanocomposite asymmetric supported membrane combined with a metal honeycomb catalyst for the ESR reaction. Compared with the fixed-bed membrane reactor, the asymmetric supported membrane exhibited a higher hydrogen permeation flux of approximately 3.5 Nml H₂ cm⁻² min⁻¹ at 900 °C, while that of the fixed-bed configuration was only 1.31 Nml H₂ cm⁻² min⁻¹. Meanwhile, the hydrogen yield increased from about 28% to approximately 38%, and the hydrogen recovery rate increased from around 35% to roughly 48%. Chen et al. [87] used a Pd-based membrane in a cross-flow membrane tube reactor for ESR. Under the conditions of an inlet temperature of 600 °C, a S/E molar ratio of three, and a reaction pressure of 6 atm, the hydrogen recovery rate reached 97.78%. Compared with the traditional reactor, the hydrogen yield was significantly increased by 22.26%, while only 0.34 times the amount of catalyst was used (Figure 19).

4.1.2. Sorption-Enhanced Ethanol Steam Reforming

Sorption-enhanced ethanol steam reforming (SE-ESR) employs solid sorbents such as CaO or modified hydrotalcites for in situ CO₂ capture, shifting the reaction equilibrium toward hydrogen production and thus improving the hydrogen yield. Quan et al. [88] found that the hydrogen yield reached a maximum of 64.4 mmol/g using a 10% Fe/SiC catalyst coupled with 0.3 g CaO, 2.88 times that without a catalyst. In situ CO₂ capture by CaO shifted the reaction equilibrium, and CaCO₃ could be regenerated to CaO at lower temperatures under a H₂ atmosphere, with the regenerated CaO maintaining a high CO₂ adsorption capacity (Figure 20).

Traditional CaO sorbents are prone to sintering at high temperatures, leading to micropore collapse and a sharp decrease in surface area. Wang et al. [89] prepared a series of Ni-CaO-CaZrO₃ catalysts using CaZrO₃ as a structural promoter, which effectively inhibited the sintering of CaO. Among them, Ni10-Ca90Zr10 exhibited excellent stability in 10 cyclic tests (Figure 21). Similarly, Oliveira et al. [90] introduced MgO and Al₂O₃ into CaO to prepare Ni-CaO-CaZrO₃, which was uniformly dispersed among CaO particles as a spacer to stabilize Ni species and mitigate CaO sintering. This reduced the deactivation coefficient (kd) by a factor of 3–4 compared to the undoped sample, significantly enhancing cyclic stability.

Modifying hydrotalcite-based sorbents with alkali metals can tune surface basicity and enhance adsorption performance. Avendaño et al. [91] modified Mg/Al hydrotalcite sorbents using K₂CO₃ as a precursor. The hydrogen purity increased to 95% within 5 min, while the outlet CO₂ concentration was nearly zero. In contrast, the unmodified control group achieved only about 80% hydrogen purity. Constructing porous structures using templating methods is an effective physical strategy to mitigate sintering. Xu et al. [92] prepared porous spherical CaO sorbents (CaOS-C5) using a cellulose template. BET results showed a fresh surface area of 23.5 m²/g, much higher than the 11.9 m²/g for template-free CaOS. After 100 cycles, the CO₂ uptake remained 0.166 g/g, far exceeding the 0.085 g/g for natural limestone. When coupled with Ni/Al₂O₃, the SE-ESR system increased H₂ purity from 75% in conventional SRE to over 90%.

4.2. Microchannel Reactor

Microchannel reactors feature microscale channels (typically inner diameter < 1 mm) with a high specific surface area (1000–5000 m²·m⁻³) and fast transport rates, enabling millisecond mixing, precise temperature control to eliminate hot spots, and safe scale-up via numbering-up.

Rahman et al. [93] fabricated a catalytic hollow fiber microreactor (CHFMR) by depositing a 10 wt.% Ni/MgO-CeO₂ catalyst on the inner surface of asymmetric Yttria-stabilized Zirconia (YSZ) hollow fibers. SEM images revealed that 80% of the YSZ fiber cross-section consisted of finger-like voids, forming hundreds of tapered microchannels with diameters of 10–50 μm and lengths of 200 μm, increasing the geometric surface area from 0.57 to 1.40 m²/g. At 510 °C, 18 mg of catalyst produced approximately 8.9 mL/min of hydrogen, comparable to 100 mg of catalyst in a fixed-bed reactor (Figure 22). Cai et al. [94] reported that the microchannel reactor achieved a hydrogen yield of 40 L H₂·gcat⁻¹·h⁻¹, approximately 33% higher than the 30 L H₂·gcat⁻¹·h⁻¹ obtained in packed-bed systems at 400 °C.

The geometric structure of microchannels also affect the reaction performance. Dehkordi et al. [95] compared three fixed-bed microreactors of identical volume loaded with a commercial Pd/γ-Al₂O₃ catalyst, namely conical (A), cylindrical (B) and inverted conical (C) configurations. At 325 °C, the ethanol conversion in reactor A was 2.5 times that in reactor C. This was attributed to the increasing cross-sectional area of A along the flow direction, which raised catalyst loading and gradually prolonged the contact time from inlet to outlet, favoring conversion at low reactant concentrations in the later reaction stage.

5. Summary and Outlook

This review summarizes the latest research progress of hydrogen production via ESR, including the reaction mechanism, catalyst design and process intensification. Firstly, an in-depth analysis of ESR is conducted from thermodynamic and kinetic perspectives. Combined with experimental studies and DFT calculations, various reaction pathways, including dehydration and dehydrogenation routes, are discussed in detail. Based on supported noble and non-noble metal catalyst systems, the key factors affecting catalytic performance are summarized, including metal alloying, optimization of preparation methods, promoter addition, and support modification. These factors significantly influence the adsorption and activation of reactants, C-C bond cleavage, and the selective formation of CO₂ and CO, thereby determining the catalytic performance in ESR. Meanwhile, this review also explores the roles of process intensification technologies such as catalytic membrane reactors, adsorption-enhanced reforming, and microchannel reactors in breaking thermodynamic limitations and improving heat and mass transfer efficiency. However, there are still numerous challenges and opportunities in comprehensively clarifying the evolution process of active sites, catalyst design, and the scale-up of process intensification technologies due to the complexity of heterogeneous catalysis in the furture.

To address these, future research can focus on breakthroughs in the following three aspects: (1) combining in situ/operando characterization with theoretical calculations to deeply reveal the dynamic reaction mechanism and the evolution law of active centers under reaction conditions; (2) achieving precise design of efficient non-noble and noble metal catalysts by means of AI and high-throughput experiments, and breakthroughs in their long-term stability through strategies such as strong metal–support interaction, core-shell structure, and single-atom catalysts; (3) promoting the structural optimization of membrane reactors, microreactors and the material innovation of adsorbents to solve the problems of their compatibility, cyclic stability and scale-up in practical applications.