Tracking the hydrogen spillover of heterogeneous catalysts in hydrogenation: from formation, migration, and regulation to fate

Abstract

Hydrogen spillover is a critical factor in enhancing the hydrogenation of heterogeneous catalysts. However, understanding and orderly controlling spillover are still challenging. The structural design of heterogeneous catalysts significantly promotes hydrogen spillover, which affects the activity, selectivity, and stability of hydrogenation. However, to date, very few systematic reviews have tracked the period of hydrogen spillover in heterogeneous catalysis. Herein, we systematically reviewed the recent research progress on hydrogen spillover from formation and fate to regulation. A strengthening mechanism of hydrogen spillover in hydrogenation for the catalytic reaction process is proposed. In addition, targeted regulatory strategies for promoting hydrogen spillover are summarized via constitutive relationships to guide the development of highly efficient hydrogenation catalysts. Finally, the opportunities and challenges of hydrogen spillover are prospectively discussed, providing theoretical guidance for the catalytic hydrogenation methodology.

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Green foundation

1. This review discusses the application of hydrogen spillover in hydrogenation reactions, which promotes the use of renewable resources.

2. This review emphasizes the importance of hydrogen spillover in refining petroleum and producing fine chemicals, where efficient hydrogenation is essential for product quality.

3. Coupling hydrogen spillover-enhanced processes with renewable energy sources can enable fully sustainable hydrogenation pathways. Solving challenges in catalyst stability and synthesis scalability will be critical for transitioning from lab-scale to industrial applications.

1. Introduction

Hydrogenation reactions are integral to the field of energy conversion.^1–3 As the global demand for renewable energy rises, hydrogenation technology is increasingly employed for the production and storage of hydrogen energy, providing a clean alternative energy solution.^4,5 H₂ can be produced by water electrolysis or hydrogenation using biomass resources.^6,7 Additionally, hydrogenation reactions are vital for converting biomass into biofuels,^8–10 which promote the optimization of the energy structure and contribute to sustainable environmental development. In the chemical industry, hydrogenation is a fundamental step in various chemical productions.^11–13 The production of organic substances, including alcohols,^14–16 ketones,^17–19 and amines,^20–22 involves hydrogenation processes. The applications of hydrogenation range from simple saturation reactions²³ to complex stereochemical control reactions,^24–26 demonstrating their extensive utility in fine chemicals. In addition, hydrogenation reactions play a crucial role in environmental protection.

In recent years, significant progress has been made in catalyst research and the development of hydrogenation reactions. New types of catalysts, such as single-atom catalysts^27,28 and nanotechnology-based²⁹ catalysts, have improved the efficiency and selectivity of reactions and reduced energy consumption. Future research on hydrogenation will continue to focus on enhancing catalytic efficiency, expanding the range of applications, and optimizing industrial processes.³⁰ Additionally, hydrogenation technology will play an increasingly prominent role in energy storage and conversion.³¹ In summary, the importance of hydrogenation lies in its wide range of applications and its pivotal role in advancing energy development, environmental protection, and new material development.³⁵ As catalytic technology develops, the efficiency and potential applications of hydrogenation will further expand and deepen. However, the lack of clear design principles and regulation strategies for catalysts to efficiently utilize hydrogen spillover poses challenges for optimizing hydrogenation reactions.

The spillover effect in catalytic hydrogenation reactions refers to the phenomenon in which activated hydrogen migrates across the catalyst surface or onto other non-catalytic materials^32,33 (Scheme 1), which is crucial for enhancing catalytic efficiency and developing novel catalytic systems. The hydrogen spillover effect begins with H₂ activation, typically on metal catalysts, such as platinum or palladium. Subsequently, H-atom equivalents or ions (H*) can migrate across the catalyst surface and potentially transfer to adjacent supports or other substances,^34–37 which significantly impacts the kinetics and thermodynamic properties of catalytic reactions, thereby influencing the efficiency and selectivity of hydrogenation. This dynamic hydrogen spillover process is referred to as “H* migration” in this paper. Hydrogen spillover creates a more uniform hydrogen distribution on the catalyst surface,³⁸ enhancing the reaction efficiency. More importantly, H* is dispersed in otherwise inaccessible active sites,³⁹ which increases the possibility of a reaction. H* can migrate from a precious metal catalyst onto an oxide support or other auxiliary materials,⁴⁰ allowing typically non-catalytic materials to exhibit catalytic activity, which reduces catalyst costs and improves resource utilization efficiency.

Scheme 1 Schematic of hydrogen spillover, mechanism, and reinforcement.

Hydrogen spillover can enhance the reaction selectivity by altering the reaction pathways.⁴¹ In hydrocracking and hydrodesulfurization, controlling hydrogen migration and distribution can preferentially hydrogenate certain bonds, thereby improving the purity and quality of the final product.⁴² Spillover technology is also useful for developing environmental reactions, such as effective hydrogenation at lower temperatures,³² and reducing energy consumption and undesirable by-products. In-depth research on hydrogen spillover has advanced multi-component catalytic systems, combining the advantages of various materials, such as pairing metals with high hydrogen activation capabilities with stable or specific functional support materials, which creates more efficient catalysts with broader applications.^34,43 As the understanding of spillover deepens, innovative catalytic materials and technologies are anticipated to further advance the fields of chemical engineering, energy, and environmental science. Enhancing the hydrogen spillover can notably improve the activity of the catalysts (Scheme 2). In hydrogenation reactions, H₂ dissociation and activation typically occur on the surfaces of noble metals. These hydrogen atoms can migrate via spillover to adjacent support materials,^41,43 which expands the distribution area of H* and increases the reaction rate. Optimizing support materials, such as employing oxides or carbon materials with high surface activity and suitable electronic structures, can markedly enhance hydrogen spillover.^44–46 Enhancing the hydrogen spillover significantly affects the hydrogenation selectivity. In complex hydrogenation, especially that involving multiple steps, controlling the migration and distribution of H* is crucial for achieving high selectivity,⁴⁷ which is particularly vital in the pharmaceutical and fine chemical industries. In hydrogenation, catalyst deactivation often results from the aggregation or excessive reduction of metal particles. Enhancing hydrogen spillover can avoid excessively high hydrogen concentrations in local areas, preventing the over-reduction and sintering of active metals on the catalyst surface.^48,49 Additionally, well-distributed hydrogen spillover effectively reduces carbon deposition, further extending the service life of the catalyst.

Scheme 2 Development history of heterogeneous catalysts strengthens the hydrogen spillover in hydrogenation reactions. 2020,⁷⁵ 2021,²¹⁶ 2022,³⁷ 2023,²⁰³ 2024.⁴²

By regulating spillover, efficient H₂ activation and the migration of H* can occur at lower temperatures, maintaining high reaction rates and selectivity,⁵⁰ which is crucial for green chemistry and energy conservation. Enhancing hydrogen spillover also opens new avenues for developing multifunctional catalysts. In multi-component catalyst systems, spillover can connect active sites with diverse functionalities, enabling synergistic catalysis.⁵¹ Therefore, H* can be transferred from precious metals to non-metallic catalysts by spillover, which facilitates multiple transformation steps within a single system and enhances catalytic efficiency and selectivity. In conclusion, enhancing hydrogen spillover in hydrogenation has profound implications for improving catalyst activity, selectivity, and longevity. By investigating and regulating spillover, efficient hydrogenation with reduced energy consumption can be achieved. In the future, more efficient and cost-effective hydrogenation catalysts will be investigated, further driving the sustainable development of the chemical industry.

2. Formation of hydrogen spillover

In catalytic hydrogenation research, hydrogen spillover refers to the process in which H₂ dissociates into H* on a metal catalyst surface and then migrates to the surface or interior of the support (oxides or carbon materials). Hydrogen spillover involves several steps, including the adsorption and dissociation of H₂, migration of H*, re-adsorption and diffusion of H*. First, H₂ dissociates on the surface of the metal catalyst to produce H*. Then, the H* migrates across the metal surface and spillover onto the adjacent non-metallic support. Finally, H* diffuses across the surface or into the interior of the support, potentially participating in the subsequent reactions. The generation of spillover hydrogen is a crucial step in catalytic hydrogenation, significantly impacting the performance and reaction efficiency. Understanding this process is essential for advancing hydrogenation reactions.⁵²

2.1. Adsorption and dissociation of H₂ ^53,54

H₂ is first adsorbed on the surface of the catalyst, and the d orbital electrons on the surface of the active metal are partially filled into the anti-bonding orbital of H₂, weakening the H–H bond and dissociating into H*, which typically occurs in precious metals such as platinum and palladium. These precious metals have a low hydrogen adsorption dissociation energy, which makes it possible to effectively generate H*. Adsorption of hydrogen molecules:^61–64 Hydrogen molecules (H₂) are initially attracted to the catalyst surface by the metal components. They adsorbed through van der Waals forces and occupied active sites, thereby setting the stage for dissociation. Ati et al.⁶⁵ investigated the adsorption characteristics of H₂ on nickel penta-nitride flakes utilizing density functional theory corrected by van der Waals interactions. The results showed that positively charged nickel ions are the main adsorption sites for polarized H₂. Liu et al.⁵⁵ found that the layered nature of AlOOH and its surface-rich OH defects contributed to promoting efficient H₂ activation of olefins and alkynes, thereby reducing the dissociation energy of H–H bonds (Fig. 1a). Moreover, Oshida et al.⁵⁶ synthesized Pt/Al(PO₃)₃ catalysts using aluminum metaphosphate as a support, aiming at the hydrogenolysis of various organic compounds. It was shown that hydrogen could induce the formation of surface Brønsted acid sites upon cleavage of Pt nanoparticles (Fig. 1b).

Fig. 1 (a) Schematic process for the fabrication of the AlOOH catalyst.⁵⁵ (b) H₂ cleavage for the formation of the surface Brønsted acid sites on the Pt/Al(PO₃)₃ catalyst.⁵⁶ (c) Schematic of heterolytic H₂ dissociation and CO_x hydrogenation on ZnO under ambient conditions.⁵⁷ (d) Schematic of the chemoselective hydrogenation of nitroarenes on cobalt single-atom catalysts.⁵⁸ (e) Heterolytic and homolytic cleavages of H₂ on the NbOPO₄ catalyst.⁵⁹ (f) Schematic description of the hydrogenation reaction on oxide-supported nanoparticles.⁶⁰

Dissociation of hydrogen molecules: After adsorption, H₂ homolytic cleavage forms two reactive H* species that are essential for spillover formation. The metal surface provides electron density, which facilitates H–H bond breakage, resulting in H* being adsorbed onto the metal catalyst. Ling et al.⁵⁷ explored the dissociation process of H₂ on the ZnO surface at the atomic level utilizing AP-STM, XPS spectroscopy, and DFT techniques (Fig. 1c). It was shown that the heterogeneous dissociation of H₂ on the ZnO surface could be observed by AP-STM, and the presence of CO or CO₂ did not affect the dissociation of H₂. Li et al.⁵⁸ found that primary solvents or alkalis were able to significantly enhance the isomerization decomposition of H₂ over Co single-atom catalysts through the mechanism of hydrogen chelation or deprotonation (Fig. 1d). Moreover, Yang et al.⁶⁶ distinguished the homolytic and heterolytic pathways of H₂ on Ga₂O₃ during hydrogen activation by quantitative and time-resolved analysis. The experimental results showed that the ligand-unsaturated Ga³⁺ sites had a catalytic effect on the dissociation of homolytic hydrogen, which promoted the easy dissociation of H₂ at low temperatures, leading to the formation of highly active and high-coverage hydrides on Ga₂O₃. In addition, Zhou et al.⁵⁹ designed and synthesized a metal-free NbOPO₄ catalyst and investigated its performance in the dissociation and activation of hydrogen molecules as well as the hydrogenolysis of C_sp²–C_sp³ model compounds. It was found that hydrogen could be efficiently adsorbed and dissociated on the NbO_x species in the presence of oxygen vacancies to form stable surface hydrides (Fig. 1e).

2.2. Migration of H* ^67,68

When H* reaches the metal–support interface, it spills over from the metal surface onto the support surface or into the interior of the support, which marks the formation of hydrogen spillover.⁶⁹ Consequently, the composition and structure of the support surface and its interior significantly influence the rate and extent of hydrogen spillover. Smaller metal nanoparticles with larger surface areas offer shorter diffusion paths, leading to faster migration. Dery et al.⁶⁰ explored the local reactivity of oxide-supported Au particles in hydrogenation reactions using nitrofunctionalized ligands as probe molecules. It was found that hydrogen dissociation occurred at the oxide-metal interface and subsequently H* was able to efficiently diffuse from the interface to the entire Au surface (Fig. 1f). Liu et al.⁷⁰ constructed striped MnO (001) monolayers and lattice-like Mn₃O₄ (001) monolayers on Pt (111) substrates and investigated hydrogen spillover. The results showed that the hydrogen species on the Pt substrate exhibited unidirectional diffusion in the striped structure of MnO (001), while they exhibited isotropic diffusion properties on Mn₃O₄ (001). Liu et al.⁷¹ investigated the hydrogen dissociation ability and hydrogen spillover properties of periodic Ni₃S₂ (111) surfaces using DFT calculations and AIMD simulations. The results showed that Ni₃S₂ exhibited excellent performance in hydrogen dissociation and hydrogen spillover. On the Ni₃S₂ (111) surface, there were multiple sites available for hydrogen dissociation, in which a H₂ could be efficiently dissociated between the S3 top site and the neighboring S1-Ni1 site; then, the H* could migrate through the top of S3 to the top of S1 or through S1–Ni1 to the top of S1 (Fig. 2a). In addition, Tan et al.³⁵ investigated the dissociation of H₂ and hydrogen spillover on near-surface alloys embedded with single Pt atoms using DFT calculations and AIMD simulations (Fig. 2b). The results of the DFT showed that sub-surface alloying with the transition metals (V, Mn, and Fe) promoted the initial hydrogen spillover but inhibited the H₂ dissociation.

Fig. 2 (a) Schematic of H₂ dissociation and spillover on the surface of the Ni₃S₂ active phase.⁷¹ (b) Schematic of H₂ dissociation and hydrogen spillover on the Pt_1−x/Cu (111) surface.³⁵ (c) Schematics of hydrogen spillover on the SLG/Pt foil.⁷² (d) Effects of reaction coordination and quantum tunneling on hydrogen diffusion over the graphene surface.⁷³ (e) Schematic of the hydrogen spillover enhanced by oxygenate additives during catalysis.³⁷ (f) Long-distance spillover hydrogenation over Pd₁/Cu (111)/Cu (100).³⁶

The hydrogen atoms chemisorbed on the graphene surface preferred to diffuse toward the 1-NN adsorption sites. He et al.⁷² constructed a closed electrochemical system with abundant SLG/Pt boundaries using single-layer graphene/platinum foil (SLG/Pt) prepared by the chemical vapor deposition (CVD) technique as a substrate. It was found that Pt atoms not covered by monolayer graphene acted as catalytically active sites that can convert H⁺ into H_ad, which were subsequently migrated and chemisorbed onto monolayer graphene via surface diffusion (Fig. 2c).

Tong et al.⁷³ systematically investigated the diffusion of H* on graphene surfaces based on first-principles calculations (Fig. 2d). The results showed that the diffusion of individual hydrogen atoms significantly affected the transition between hydrogen dimer structures. Tan et al.³⁷ successfully prepared bimetallic Pt@-Fe@SiO₂ catalysts via the double template strategy to investigate guaiacol hydrodeoxygenation. The Pt sites were favorable for H₂ adsorption and dissociation, and the addition of oxygen-containing additives promoted the migration of H* from the Pt to the Fe sites (Fig. 2e), which greatly enhanced the performance of guaiacol hydrodeoxygenation. Jiang et al.³⁶ verified the migration of H* from palladium to the surface of copper (100) by dispersing palladium atoms on copper nanomaterials Cu (111) and Cu (100) with different exposure surfaces (Fig. 2f). Gu et al.⁴³ explored hydrogen spillover on the surface of a Pt/Cu (111) single-atom alloy at the atomic scale utilizing DFT and molecular dynamics calculations (Fig. 3a). Gu et al.⁷⁴ designed a water-assisted hydrogen spillover strategy to facilitate H* migration from Pt to MOF-801 (Fig. 3b). Yin et al.⁷⁵ observed hydrogen spillover from Pd to Au (111) and facilitated the hydrogenation of the Au sites by applying the tip-enhanced Raman spectroscopy (TERS) technique. In addition, Wei et al.⁷⁶ investigated hydrogen spillover at the Pt-TiO₂-Au interface and its effect on the activity and selectivity of catalytic hydrogenation using the in situ surface-enhanced Raman spectroscopy (SERS) technique. The H* could be transferred to the Au surface through the surface of TiO₂ after being generated on the platinum surface.

Fig. 3 (a) Scheme of hydrogen spillover in MOF-801 under dry or wet H₂ conditions.⁴³ (b) Atomic-scale perspective of hydrogen dissociation and spillover processes. The white sphere represents H, the orange sphere represents Cu, and the green spheres represent Pt.⁷⁴ (c) Behavior of hydrogen hopping in Ru/MgO (111).⁷⁷ (d) The corresponding schematic of the hydrogen spillover. H_ad and H_ab denote the adsorbed and absorbed hydrogen, respectively.⁷⁹ (e) Schematic of proton hopping from H₂ to H₂O molecules in oxygen vacancy over Pd/MoO_3−x-R.⁸⁰ (f) Correlation between the hydrogen adsorption energy and different crystal planes.⁶²

Hydrogen spillover occurs through several modes and pathways (Scheme 3): surface diffusion represents the predominant transport mechanism, wherein H* generated on the surface of metal catalysts migrates across the interface to the support. This process is governed primarily by the nature and strength of the interfacial interactions between the metal and the support. Notably, such diffusion pathways are typically confined to the interfacial regions bridging metal nanoparticles and the underlying support.

Scheme 3 Modes and pathways of hydrogen spillover, including surface diffusion, bulk diffusion, and hopping diffusion.

Fang et al.⁷⁷ investigated the decomposition behavior of ammonia by Ru atoms on an MgO (111) surface. When NH₃ approached the MgO surface, [Ru²⁺–O²⁻] ion pairs dispersed at the metal-supported interface activated the NH₃ by heterolytic cleavage of the N–H bonds. These generated protons preferentially and rapidly migrated to the surface of oxygen-capped magnesium oxide (111), thus enabling the transfer of H* at the oxygen site (Fig. 3c). Xia et al.⁷⁸ conducted an in-depth study of explicit solvent water in Pd@ZrO₂ catalysts by DFT calculations and AIMD simulations. The results showed that the metal–support interaction of the Pd@ZrO₂ catalyst was enhanced by the dissociation of water at the metal–support interface.

Hydrogen spillover can also occur deeper within porous materials, enhancing the catalyst performance in complex reactants.^81,82 H* can migrate into the interiors of support materials via bulk diffusion, which typically occurs in materials with high porosity or specific lattice structures such as metal oxides (e.g., TiO₂ and CeO₂) or carbon materials. The efficiency of bulk diffusion depends on the lattice structure, defect concentration, and pore size distribution of the support. Although generally slower than surface diffusion, bulk diffusion can become a primary pathway under certain conditions, such as high temperature or pressure.^69,83 Zhao et al.⁷⁹ used lattice hydrogen (LH) as a typical marker to identify dynamic hydrides (LHM) in alkaline hydroxide reduction (HOR). First, platinum (Pt) atoms on the surface promoted the dissociation of H₂ into adsorbed H*, which was subsequently transferred to bulk-phase palladium (Pd_bulk) via hydrogen spillover to form Pd–H*, thus providing the migration of LH to the Pt surface and participation in the HOR (Fig. 3d). In supports with discrete active sites, such as metal atoms or clusters dispersed on the surface, H* can migrate through a “hopping” mechanism, which involves short-range diffusion of H* from one active site to another. Typically manifesting at the nanoscale, this diffusion mode is governed by the spatial distribution and energy landscape of the surface sites on the support.⁸⁴ Zhao et al.⁸⁰ prepared MoO_3−x-supported Pd nanoparticle catalysts containing oxygen vacancies (Pd/MoO_3−x-R) for the hydrogenation of nitrobenzene (NB) (Fig. 3e). The oxygen vacancies in the catalysts could promote water-assisted proton hopping (WAPH), thus accelerating hydrogenation. In addition, the oxygen vacancies contributed to the adsorption of water, which lowered the energy barrier of WAPH and made proton hopping on the metal oxide surface easier.

The support, though often non-catalytic, plays an active role as a medium for H* migration, with its surface structure, chemical properties, and physical characteristics directly determining the accessibility of migration pathways and the stability of active H species. Surface hydroxyl groups on the support serve as critical stepping stones for H* transfer, while structural defects function as high-flux conduits that accelerate migration. Furthermore, the strength of metal–support interactions governs the energy barrier for interfacial hydrogen transfer, establishing a fundamental link between the catalytic metal and its supporting matrix.

2.3. Re-adsorption and diffusion of H* ⁸⁵

H* migrates across the metal surface to the metal–support interface and migrates onto the support. Once on the support, H* can be readsorbed and participate in further reactions through surface or bulk diffusion.⁸⁶ The support surface of the physicochemical properties plays a crucial role in regulating these processes. Mao et al.⁸⁷ found that in a bifunctional nano-TiO_2−xH_y/Fe catalyst, H* from Fe to cascade oxygen vacancies (O_V–O_V) led to hydrogen trapping on the TiO_2−xH_y component, which in turn allowed the iron species to activate N₂ rapidly and further generated NH₃. Xia et al.⁸⁸ found that Au clusters on TiO_2−x were able to induce interstitial Ti defects to diffuse outward and polarize on the surface by combining DFT calculations and AIMD simulations. The interaction between the metal electrons and the support allowed the release of energy and lowered the energy barrier to diffusion.

Stability of surface-adsorbed states: The stability of H* in surface-adsorbed states directly affects its migration. Lower adsorption energy allows H* to move more freely from one site to another, accelerating the spillover process.^61,89 Liu et al.⁶² explored the activity of H* on different Zn crystal surfaces from both thermodynamic and kinetic perspectives (Fig. 3f). The free energy of adsorption (ΔG_H) of H* on Zn (002) and (100) surfaces was higher than that on Zn (101), (102) and (103) surfaces. He et al.⁹⁰ achieved the chemisorption of H* on single-layer graphene (SLG) using a Pt-electrocatalyzed spillover-surface diffusion-chemisorption mechanism (Fig. 4a). Moreover, Zhang et al.⁹¹ prepared Pt/C₆₀ catalysts anchored on C₆₀ with high loading and high dispersion of single atom Pt using a room temperature synthesis strategy (Fig. 4b). DFT showed that the polymeric structure of Pt–C₆₀ facilitated the adsorption of water molecules, while the charge redistribution between Pt and C₆₀ provided favorable adsorption energies for both *H₂O and *H, thus facilitating the rapid kinetics of hydrogen precipitation.

Fig. 4 (a) Schematic of the electrocatalytic spillover-surface diffusion-chemisorption mechanism on the Pt/SLG/PET electrode assuming a chemical adsorption of Had onto the graphene matrix.⁹⁰ (b) Synthetic scheme of Pt/C₆₀ catalysts and the hydrogen spillover on the catalyst surface.⁹¹ (c) Schematic of formaldehyde hydrogenation, including proton transfer steps and the detoxification of OH* on Pt/TiO_2−x in the aqueous phase. The white sphere represents H, the red sphere represents O, and the brown spheres represent C.⁹³ (d) Schematic of H₂ dissociation and spillover at individual isolated Pd atom sites in the Cu surface layer. The inset shows a high-resolution image of individual H* on Cu (111).⁹⁴ (e) Hydrogen spillover in high-temperature proton exchange membrane fuel cells (HT-PEMFCs).¹⁰³ (f) Temperature effects on H* adsorption and schematic showing H₂ adsorption at MSI.³⁴

2.4. Controlling factors of hydrogen spillover

The rate of hydrogen spillover is mainly influenced by the physicochemical properties of the catalyst and reaction conditions. The acidity/basicity and oxygen vacancy concentrations of the support affect the stability and reactivation capability of H*. Additionally, the electronic interactions at the interface determine the distribution and diffusion pathways of H*. The efficient migration and distribution of H* are key to excellent catalytic performance.³⁵ Miao et al.⁹² suggested that optimizing support by introducing specific surface defects, such as oxygen vacancies, enhanced the activity and diffusion capability of H* and improved catalyst efficiency. Cao et al.⁹³ suggested that modulating the electronic properties of the metal and support through doping or adjusting the metal load could significantly influence the efficiency of hydrogen transfer (Fig. 4c). Active sites on the support material, such as oxygen vacancies or defects, are vital in hydrogen spillover, which captures H* from the metal and facilitates their diffusion to the reaction zone. Kyriakou et al.⁹⁴ found that the density and distribution of active sites on the support surface were crucial factors that determined the diffusion rate of H* (Fig. 4d).

The interaction between the active metal and the support plays a crucial role in hydrogen spillover. Supports, such as TiO₂, CeO₂ and Nb₂O₅, can form strong metal–support interactions (SMSI) under reductive environments. SMSI enhances the interface bonding between the metal and the support, provides a direct migration channel for H and reduces the interface migration energy barrier. Wang et al.⁹⁵ synthesized a hexagonal hafnium oxide (Hex-HfO₂) Ru catalyst (Ru/Hex-HfO₂) with a coordination number of 6 and revealed the positive adsorption free energy of H on Hex-HfO₂, indicating that H is more likely to overflow on Hex-HfO₂. In addition, the strong interaction between Ru and Hex-HfO₂ optimizes the desorption of intermediate hydrogen, thereby promoting the spillover of surface H. However, excessive interactions often induce the formation of a reduced oxide overlayer on the metal or support surface, which can physically hinder H* migration from the metal to the support, thereby suppressing hydrogen spillover. Additionally, changes in the electronic properties of the metal, such as electron deficiency, may weaken its ability to dissociate H₂ or reduce the binding energy of adsorbed H atoms, ultimately decreasing the number of hydrogen atoms available for spillover. Macino et al.⁹⁶ found that when Pt/TiO₂ is reduced above 500 °C, the TiO_x layer can cover more than 80% of the Pt surface, resulting in the inaccessibility of H₂ to metal active sites. The hydrogen dissociation rate decreased by more than 90%, and the hydrogen overflow almost stagnated. In contrast, the weak interaction weakens the electron coupling between the metal and the carrier, and the hydrogen atom migration path is blocked, which is not conducive to hydrogen spillover. Karim et al.⁹⁷ found that the spillover of alumina is mediated by three coordinated aluminium canters, which also interact with water and cause hydrogen migration and hydrogen desorption competition. This leads to the overflow of hydrogen 10 orders of magnitude slower than that of titanium oxide and is limited to a very short distance from platinum particles.

The support surface properties,⁹⁸ such as defects and internal pore characteristics, significantly impact the spillover rate. A large specific surface area and the presence of defects provide more adsorption and transport sites, facilitating rapid spillover. Additionally, high porosity allows for greater hydrogen accommodation, ensuring increased throughput along diffusion pathways and enhancing the spillover rate. The size and dispersion of the catalyst particles also significantly influence spillover. Smaller particles expose more active sites, which increases the likelihood of H₂ contacting active components, thus promoting adsorption and dissociation. However, excessive dispersion can interrupt these processes, slowing spillover and potentially hindering hydrogenation. Guo et al.⁹⁹ investigated the competition between SMSI and the hydrogen spillover effect in CO₂ methanation activity by adjusting the size of Ru deposits in Ru/CeO₂ combinations. The results showed that Ru nanoclusters exhibited the best CO₂ methanation activity and selectivity compared with single Ru atoms and larger Ru nanoparticles, which was attributed to the dynamic balance between SMSI and hydrogen spillover on Ru nanoclusters.

The reaction medium also plays a pivotal role in modulating H* migration during catalysis. In acidic media, a high proton concentration facilitates the formation and migration of H atoms. Yan et al.¹⁰⁰ reported that anchoring PtPd alloy clusters on the surface of CeO₂ facilitates short-range hydrogen spillover, effectively lowering the energy barrier for catalytic reactions. The experimental evaluation of PtPd/CeO₂ in the hydrogen evolution reaction (HER) revealed an exceptionally low overpotential of just 5.7 mV at a current density of 10 mA cm⁻² in acidic media, along with outstanding long-term stability. In neutral media, the presence of surface hydroxyl groups on the support is critical for hydrogen spillover. Mahdavi-Shakib et al.³⁴ discovered that in Au/TiO₂ catalysts, the presence of hydroxyl groups on the TiO₂ surface significantly enhances hydrogen spillover under neutral conditions. H atoms adsorb onto these groups, forming loosely coupled proton/electron pairs and enabling spillover. In alkaline media, a high concentration of hydroxide ions introduces competitive adsorption with hydrogen atoms, thereby suppressing hydrogen spillover. Li et al.¹⁰¹ found that in the Pt/CoP system, under alkaline conditions, the hydroxyl groups on the CoP surface are replaced by OH⁻ ions, reducing the number of available adsorption sites for H atoms and consequently restricting the extent of hydrogen spillover.

In addition, some small molecules can actively promote hydrogen spillover via interactions during H* migration. In liquid-phase reactions, carbonyl-containing organic small molecules (e.g., aldehydes and ketones) can function as hydrogen carriers, facilitating hydrogen spillover on inert oxide surfaces via an adsorption-migration-desorption cycle. Tan et al.³⁷ proposed a hierarchical mesoporous SiO₂-supported Pt–Fe bimetallic catalyst (Pt@-Fe@SiO₂) and employed the hydrodeoxygenation of guaiacol, a lignin-derived compound, as a probe reaction. They proved that these carbonyl-containing intermediates facilitate the spillover of H atoms across the SiO₂ surface and their subsequent transport to Fe particles, significantly enhancing the formation rates of aromatic hydrocarbons, such as benzene, toluene, and xylene, at the Fe sites. Gu et al.⁷⁴ discovered that the introduction of water molecules significantly enhances hydrogen spillover in MOF-801. Furthermore, this water-assisted enhancement strategy can be applied to various MOF structures and covalent organic frameworks (COFs). Using ligand transformation model calculations and isotope labelling techniques, the study revealed that water molecules promote the hydrogen spillover process by improving the efficiency of H* migration. In addition, the interactions between substances in the reaction system have an important influence on hydrogen spillover. Fu et al.¹⁰² demonstrated that the abundant Mo(V) species, in synergy with the supported Pt dual-active sites, facilitate hydrogen spillover from Pt to MoO_3−x, thereby optimizing photodriven nitrogen reduction and hydrogen oxidation. Experimental and theoretical investigations under ambient-temperature photocatalytic nitrogen fixation confirmed that the supported Pt nanoparticles act as cocatalysts to initiate hydrogen spillover, promote nitrogen hydrogenation, and further increase the number of active Mo(V) sites.

In addition, the rate of hydrogen spillover is affected by the physical properties of the catalyst and the reaction conditions. Temperature is a crucial factor affecting the rate of hydrogen spillover. Generally, higher temperature increases the kinetic energy of H*, accelerating their migration rate on surfaces or through bulk phases. However, excessively high temperatures may induce catalyst deactivation or structural changes, necessitating a balance for practical applications. Huang et al.¹⁰³ designed a Pd/C anode that exhibited performance comparable to Pt/C in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). The palladium in the anode acted as a hydrogen buffer layer and oxygen absorbing layer, providing additional in situ hydrogen and absorbing infiltrated oxygen, thus maintaining the activity of the hydroxide (Fig. 4e). Additionally, Mahdavi-Shakib et al.³⁴ indicated that certain metal oxide supports provided optimal spillover rates at moderate temperatures, maintaining structural stability while offering sufficient active sites (Fig. 4f).

Reaction pressure is another critical factor influencing hydrogen spillover. Elevated hydrogen pressure increases the surface coverage of H atoms on metal sites, thereby thermodynamically and kinetically facilitating the transfer of H* species to the support. This effect is particularly prominent on reducible oxides, such as TiO₂, WO₃, and CeO₂. Under low H₂ pressure, limited H* coverage results in a substantial interfacial migration barrier, typically ranging from 0.9 to 1.4 eV, which renders spontaneous spillover energetically unfavourable. In contrast, higher hydrogen pressure can partially overcome this barrier, enhancing spillover efficiency.¹⁰⁴

Hydrogen spillover describes the entire hydrogen migration process, while H* refers to specific hydrogen atoms or ions. Optimizing catalytic efficiency by enhancing hydrogen spillover and ensuring that H* is fully utilized in the necessary reaction zones. This involves engineering the metal–support interface, selecting and modifying support materials, and optimizing reaction conditions, such as temperature and pressure. Hydrogen spillover is a crucial phenomenon that describes the migration of H* from the metal catalyst surface to the support surface. By understanding these concepts, researchers can develop more efficient and selective catalytic systems, significantly improving catalytic efficiency and product purity in complex hydrogenation.

3. Fate of hydrogen spillover

Ultimately, the H * transferred to a specific position engages in the hydrogenation reaction.¹⁰⁵ The availability, distribution, and concentration of spillover hydrogen are critical factors influencing reaction rates and selectivity. Hydrogen spillover in catalytic reactions involves various mechanisms and pathways that are influenced by temperature, support material structure, and catalyst particle size. Understanding these mechanisms allows researchers to design and optimize catalysts for efficient hydrogen spillover, enhancing reaction efficiency and selectivity. Hydrogen spillover is widely used in hydrogenation, significantly enhancing catalytic efficiency and selectivity. In the hydrogenation of unsaturated hydrocarbons, spillover accelerates the conversion of olefins and alkynes into saturated alkanes, which are crucial for petroleum refining and organic synthesis. Aromatic compounds also benefit from hydrogen spillover, allowing for the saturation of aromatic rings at lower temperatures and pressures and improving safety and cost-effectiveness, especially in the hydrogenation of benzene to cyclohexane. In petroleum refining, hydrogen spillover optimizes the hydrogenation of heavy oils, improving the quality of challenging feedstocks.¹⁰⁶ It enhances hydrodesulfurization and hydrodenitrogenation, allowing for the efficient removal of sulfur and nitrogen compounds at lower temperatures, reducing energy consumption, and extending catalyst lifespan. Although the traditional Haber–Bosch process is established, applying hydrogen spillover in ammonia synthesis is under investigation, which makes H₂ use more efficient at lower temperatures and pressures. In biomass conversion, hydrogen spillover improves the hydrogenation efficiency of biomass-derived compounds,¹⁰⁷ aiding conversion to high-energy-density liquid fuels. Hydrogen spillover plays a vital role in selective catalytic hydrogenation, enabling chemical transformations under milder conditions. Understanding its mechanism and optimizing catalyst design may increase its importance in the future of the chemical industry.

3.1. Hydrogenation of unsaturated hydrocarbons

Hydrogen spillover plays a key role in alkane hydrogenation. Metal catalysts, such as palladium, platinum, and nickel, adsorb and dissociate H₂ into atoms and then transfer it to alkane molecules, accelerating the catalytic reaction.¹⁰⁸ Mu et al.¹⁰⁹ successfully synthesized various molecular sieve catalysts for the hydrocracking of long-chain n-alkanes. Owing to the preferential cleavage near the end of the carbon chain in the AEL channel, SAPO-11 was able to selectively produce heavy jet fuel fractions during the hydrocracking of the model reactant n-C₁₆. In addition, the weak acidity of SAPO-11 contributed to the desorption of olefins, enhanced hydrogen spillover, and inhibited the secondary cracking of the jet fuel fraction. Metal alloy catalysts also exhibit strong spillover effects. Tan et al.¹¹⁰ prepared a two-component Pd–Cu catalyst (Pd–Cu@CF) for the electrocatalytic semi-hydrogenation of alkynes (EASH) under mild conditions and explored the hydrogen spillover (Fig. 5a). The prepared catalyst achieved efficient H* generation at a low overpotential on the Pd side, followed by rapid hydrogen spillover from Pd–Cu, which led to the simple hydrogenation of alkynes, while olefins were released on the Cu side, thus effectively overcoming the contradiction between productivity and energy consumption in EASH electrocatalysis. Spillover can also improve selectivity in multi-step reactions by controlling H* utilization, suppressing unwanted side reactions, and preventing excessive hydrogenation or cracking.

Fig. 5 (a) Schematic of the proposed tandem EASH mechanism through hydrogen spillover.¹¹⁰ (b) Schematic of hydrogen migration in pristine Pd and the formation of palladium hydrides.⁶⁴ (c) Schematics of the hydrogen spillover on the highly dispersed Bi-modified Cu_xC-Cu catalyst.¹¹⁴ (d) Schematic of the proposed water-assisted hydrogenation mechanism.¹¹⁵ (e) DHPS selectivity and activation energy at 220 °C under 4.0 MPa over a series of Pt catalysts.¹¹⁶

Conventional benzene hydrogenation processes, such as those employing Ni-based catalysts, require high temperatures (200–300 °C) and elevated pressures (3–5 MPa). In contrast, the hydrogen spillover strategy enables efficient conversion under milder conditions through the synergistic design of metal–support interfaces. Bai et al.³⁹ achieved efficient benzene hydrogenation via a water-assisted hydrogen spillover pathway, reaching a conversion rate of 99.7% at 80 °C and 0.5 MPa, with a hydrogen spillover distance exceeding 50 nm. The catalyst features a hydroxyl network within the MOF shell, which facilitates rapid H* migration, resulting in a reaction rate three times higher than that of conventional Pt/Al₂O₃ catalysts, while simultaneously avoiding the need for elevated temperature and pressure. In addition, conventional Pd-based catalysts often suffer from over-hydrogenation in the semi-hydrogenation of alkynes. The hydrogen spillover strategy, through dual active site design, enables highly selective conversion. Huang et al.¹¹¹ developed a Cz-Co-COF-H catalyst that achieved an acetylene semi-hydrogenation rate of 1755.33 μmol g⁻¹ h⁻¹ under visible-light irradiation, with selectivity exceeding 99%. Hydrogen spillover occurs via localized hydrogen transfer within the COF framework, allowing the reaction to proceed under ambient conditions and reducing energy consumption by over 90% compared to conventional thermocatalytic processes. Conventional methods, such as Cu/SiO₂ catalysts, often suffer from poor selectivity in the hydrogenation of α, β-unsaturated aldehydes. In contrast, hydrogen spillover catalysts enable precise conversion through interfacial modulation. Zhang et al.¹¹² designed a Pd-loaded split core–shell catalyst that achieved an TOF of 26 624 h⁻¹ and nearly 100% selectivity in the hydrogenation of crotonaldehyde. Hydrogen spillover is accelerated by the magnetothermal effect at the Fe₃O₄ core-carbon shell interface, enabling efficient conversion at 80 °C and 1 MPa. Compared to traditional Cu–Zn–Al catalysts, which require 200 °C and 5 MPa, this approach reduces energy consumption by approximately 60%.

In addition, hydrogen spillover aids in removing carbon deposits and impurities that can deactivate catalysts, which are crucial in maintaining high activity during hydroprocessing.¹¹³ It facilitates efficient hydrogen transfer, lowering activation energy and enhancing reactant contact. This improves intermediate conversion rates and overall reaction efficiency. The key steps in this reaction include the dissociation of H₂ and the adsorption of H* on metal catalysts, such as platinum and palladium. These metals offer efficient hydrogen dissociation, with atoms diffusing via spillover into the reaction zone. In situ infrared and X-ray absorption spectroscopy validated spillover properties on various surfaces. Liu et al.⁶⁴ investigated hydrogen spillover in different states using a modular signal amplification strategy, which enabled the spectral visualization of reversible hydrogen spillover in metal/MOF composite structures (Fig. 5b). Nanostructured and porous materials enhance diffusion efficiency and improve alkane hydrogenation performance. Overall, hydrogen spillover enhances the reaction rates, catalyst activity, and selectivity in alkane hydrogenation. Designing catalysts with superior spillover capabilities, especially through multi-metal or alloy synergy, can improve the economic and environmental aspects of the process. Zhou et al.¹¹⁴ prepared highly dispersed SiO₂ loaded with copper and/or bismuth carbonate by applying the bulging strategy and used them as precursors for the synthesis of highly dispersed Bi-modified Cu_xC–Cu catalysts (Fig. 5c). The Bi-modified CuBi/SiO₂ (H-TR) catalysts exhibited excellent ethylene selectivity while maintaining good hydrogenation activity. The high ethylene selectivity was due to the addition of Bi to facilitate the transfer of active hydrogen from Cu_xC to Cu. Future research should explore microscopic spillover mechanisms, develop novel catalysts, and optimize conditions to further enhance reaction performance.

3.2. Hydrogenation of aromatic compounds

Hydrogen spillover significantly enhances the hydrogenation rate of aromatic compounds by facilitating efficient H* supply and rapid migration, improving contact between reactants and active sites. Gao et al.¹¹⁵ developed a modular strategy using core–shell micro mesoporous zeolites as a structural model to investigate the hydrogenation mechanism of oxides (Fig. 5d). Oxides were functionally and efficiently separated from noble metals (e.g., Pt) in mesoporous shells and microporous cores (TS-1). TiO₂, CeO₂, and ZrO₂ exhibited excellent hydrogenation properties with the aid of H* species spillover at near-room temperature. Niu et al.¹¹⁶ prepared Pt/HZ-WI, Pt/HZ-SEA, and Pt/HS-SEA catalysts via wet impregnation (WI) and strong electrostatic adsorption (SEA) using hollow multistage ZSM-5 and silica-1 (HS) molecules as support. The SEA catalysts exhibited higher metal dispersion, more defective Pt^δ+ species, and stronger hydrogen spillover capacity, showing superior intrinsic hydrogenation performance (Fig. 5e). By optimizing H* concentrations, cascade hydrogenation reactions are controlled, improving selectivity and catalyst activity. Moreover, hydrogen spillover can enhance catalyst activity and stability, preventing rapid deactivation and reducing the accumulation of unreacted species or poisons.¹¹⁷ The rapid H₂ supply facilitates the quick transformation of intermediates into desired products.¹¹⁸ Multi-metal catalysts, through metal interactions, effectively promote spillover, improving the adsorption, dissociation, and hydrogenation performance of aromatic compounds. Jing et al.¹¹⁹ prepared a series of multifunctional high-entropy alloy nanocatalysts containing PdPtRu synthesized by applying a solvothermal strategy, which achieved complete hydrogenation (100%) of carbon–carbon unsaturated bonds in solid 1,4-bis (phenylalkynyl) benzene (DEB) (Fig. 6a).

Fig. 6 (a) Hydrogenation schematic of solid DEB molecules catalyzed by Pd (left) and PdPtRuCuNi HEA (right) catalysts at 25 °C under ≤1 bar H₂.¹¹⁹ (b) Chain and cyclic olefin conversion for reformate hydrogenation on Pd/Al₂O₃ and Zn₃Ir₁/Al₂O₃.¹²⁰ (c) Schematic of the hydrogenation of nitroarenes over the CePO₄/Pt catalyst.¹²¹ (d) Hydrogen spillover scheme of Mo-S₄-N/C. (e) Hydrogen spillover energy barrier from the Mo center to the graphene support on Mo-S₄-N/C and Mo-S₄/C models.¹³⁰

Owing to hydrogen spillover, the catalyst enhanced the hydrogen uptake of DEB. Moreover, Wang et al.¹²⁰ designed an alloy catalyst for selective hydrogenation to remove olefins from reformed oils by applying DFT calculations and microkinetic modeling. The ideal hydrogen coverage on the surfaces of different alloys under hydrogenation conditions was closely related to the alloy type (Fig. 6b).

The metallic properties of the catalyst influence dissociation energy and hydrogen utilization efficiency. The metal–support interaction is crucial for spillover capability. Optimizing the metal–support interface, using materials with a high specific surface area, such as CePO₄, Al₂O₃, and SiO₂, enhances metal dispersion, hydrogen adsorption, and spillover. Wu et al.¹²¹ synthesized an efficient CePO₄/Pt catalyst specifically for the selective hydrogenation of nitroaromatics. It was shown that the catalyst exhibited remarkable selectivity (>95%) to produce aromatic amines at a high turnover frequency (1476 h⁻¹) (Fig. 6c). Compared with conventional CeO₂/Pt catalysts, the Pt sites on CePO₄ were negatively charged, which facilitated the dissociation of H₂. Overall, hydrogen spillover plays a vital role in the hydrogenation of aromatic compounds. By investigating catalyst surfaces and structures, we can enhance spillover rates, increasing hydrogenation activity and enabling efficient production of target products.

3.3. Hydrogenation of unsaturated fatty acids

Moreover, spillover can promote continuous hydrogenation, ensuring high selectivity and conversion of target products while reducing catalyst deactivation and extending lifespan.^122–124 Hydrogen spillover accelerates intermediate conversion, increasing the hydrogenation rate and reducing intermediate accumulation.¹²⁴ By adjusting the hydrogen supply and reaction conditions (flow rate, temperature, and pressure), hydrogenation is optimized. Catalyst metal characteristics, including type and morphology, determine dissociation energy, affecting hydrogen utilization and reaction kinetics.¹²⁵ The metal–support interface distance influences the diffusion barrier, and modifying this interface enhances hydrogen migration and catalyst performance. Catalyst shape,¹²⁶ such as nanoparticles or nanowires, also impacts spillover capability.

Hydrogen spillover is crucial in the catalytic hydrogenation of unsaturated fatty acids. Optimizing metal–support interactions and controlling catalyst morphology can effectively enhance spillover, improving hydrogenation efficiency.¹²⁷ Overall, hydrogen spillover is crucial in the catalytic hydrogenation of unsaturated fatty acids. Future research should focus on developing novel catalysts that integrate spillover mechanisms to further boost hydrogenation efficiency.

3.4. Heavy oil hydrocracking

In heavy oil hydrocracking, hydrogen spillover is crucial for breaking heavy hydrocarbons into lighter components.¹²⁸ It facilitates active hydrogen atom diffusion into catalyst pores, increasing reactant and active site contact and boosting reaction activity. In NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts, spillover significantly enhances conversion rates.¹²⁹ Optimizing metal distribution and loading within catalysts can further improve conversion efficiency. Spillover enhances hydrogen migration, efficiently reaching and saturating aromatic molecules and preventing catalyst poisoning and deactivation. Sun et al.¹³⁰ found that the reconfiguration of N species in the second shell layer in Mo-S₄-N/C catalysts accelerated the active hydrogen spillover, modulated the electronic structure of Mo-S₄ sites (Fig. 6d), lowered the transfer energy barrier for hydrogen spillover, and facilitated the migration of the active hydrogen species, thereby improving the hydrogenation performance of the catalysts (Fig. 6e). Cao et al.¹³¹ prepared bifunctional catalysts loaded on γ-Al₂O₃ and HY molecular sieves and investigated the effects of hydrogen spillover and active metals on naphthalene hydrocracking performance. It was shown that Ni or Co promoters were able to initiate hydrogen spillover between NiMo and CoMo catalysts, thus improving catalyst stability.

The design of heavy oil hydrogenation catalysts is significantly affected by hydrogen spillover.¹³² The acidity and porous structure of the support influence spillover and hydrogenation activity.⁸² Nanostructured metal particles, such as Pt, Rh, and Mo, effectively dissociate H₂ and facilitate H* migration, enhancing reaction efficiency.¹²⁸ Multi-metal catalysts, such as NiW, NiMo, and CoMo, exhibit stronger spillover effects, improving hydrogenation activity through synergistic metal interactions.¹²⁹ This spillover prevents the condensation of macromolecules in heavy oil hydrogenation, which in turn inhibits coke generation. H* from spillover eliminates unstable intermediates, maintaining long-term stability.¹³³ Spillover also guides H* to surface active sites, mitigating poisoning risk. Designing catalysts with superior spillover properties enhances resistance to deactivation. Overall, hydrogen spillover is indispensable in heavy oil hydrogenation, preventing polycondensation and carbon formation. Optimizing catalyst structures and materials enhances spillover, particularly with novel supports and multi-metal catalysts. These advancements have improved the performance and economics of heavy oil hydrocracking.

Dong et al.¹³⁴ designed and successfully prepared a Pt@SSZ-13@β core–shell catalyst to investigate the effect of hydrogen spillover on naphthalene hydrocracking (Fig. 7a). Pt@SSZ-13@β exhibited higher naphthalene conversion and BTX selectivity, which was mainly attributed to the fact that the hydrogen spillover effectively inhibited the deep hydrogenation of tetrahydronaphthalene in the process of naphthalene hydrocracking, which in turn enhanced the BTX selectivity. In addition, Sun et al.⁴² successfully synthesized a carbon-loaded MoCo dual-atomic site catalyst (MoCo DAC/C), which was able to enhance both HCK and HDS performance during VR hydrocracking (Fig. 7b). The results showed that the oxygen vacancy-induced built-in electric field (BIEF) modulation mechanism facilitated the transfer of active hydrogen from the Mo site to the Co site in the Mo–C–Co bridge bond, which significantly enhanced hydrogenation performance.

Fig. 7 (a) Schematics of the effects of spillover hydrogenation on hydrocracking of naphthalene over the Pt@SSZ-13@β core–shell catalyst.¹³⁴ (b) Catalytic conversion illustration of HCK and HDS over MoCo DAC/C.⁴² (c) Schematic of the reactive adsorption process of thiophene over the Cu/ZnO sorbent.¹³⁵ (d) HDN reaction of quinoline with spillover hydrogenation over the Ni₂P/Beta@SBA-16 core–shell catalyst.¹³⁶ (e) NH₃-synthesis rate as a function of temperature.¹³⁷ (f) Schematic of the refining active sites and hydrogen spillover for boosting visible-light-driven ammonia synthesis on the Pt@Mo-TiO_2−x catalyst.¹³⁸

3.5. Hydrodesulfurization

Hydrodesulfurization¹³⁹ is an important step in the refining process that contributes to reducing sulfur oxide emissions produced during the combustion of petroleum fuels. Additionally, harmful gases can be converted into substances through hydrogenation with a lesser impact on the environment.^140,141 Hydrogen spillover can effectively promote sulfur removal through the transfer of H* in the reaction. Increasing the effective H₂ concentration facilitates sulfur compound hydrogenation, accelerating desulfurization.¹⁴² Wei et al.¹³⁵ demonstrated that the H₂ flow rate and pressure affected the formation of target products through the removal of thiophene by Cu/ZnO (Fig. 7c). Spillover increases surface H* concentration, making active sites more accessible and enhancing catalytic performance. Hydrogen spillover can reduce sulfide accumulation on catalysts, improving stability and durability,¹⁴³ which accelerates the reaction of intermediates, such as hydrogen sulfide and thiols, with hydrogen, promoting their rapid conversion into sulfur-free or low-sulfur products. By regulating hydrogen spillover properties, the microstructure of the catalyst can be optimized. Alloy or core–shell catalysts enhance hydrogen diffusion and utilization efficiency and then optimize performance. Song et al.¹⁴⁴ successfully synthesized Ni₂P/Ti-MCM-41 catalysts using ammonium hypophosphite and nickel chloride, exploring the effect of Ti on the catalytic performance of hydrodesulfurization (HDS). The doping of Ti increased the acidity of MCM-41 and facilitated the generation of the Ni₂P phase, which significantly enhanced the dissociation and migration of H₂. In hydrodesulfurization, metal catalyst active sites dissociate H₂, providing H* for desulfurization. Effective support facilitates spillover, promoting quick hydrogen delivery to desulfurization sites.¹⁴⁵ Multi-metal alloys enhance hydrogen dissociation and migration, accelerating reactions.¹⁴⁶ Spillover significantly impacts hydrodesulfurization by improving the rate, selectivity, and catalyst activity. Designing catalysts to maximize metal–support interactions enhances spillover, optimizing hydrodesulfurization performance.

3.6. Hydrodenitrogenation

By increasing the effective concentration of hydrogen, more H* is available to react with nitrogen compounds. Metal catalysts, such as Ni, Mo, and Pd, exhibit notable rate increases owing to hydrogen spillover.¹⁴⁷ This effect ensures thorough hydrogen distribution, increasing contact rates between nitrogen compounds and H*, and facilitating efficient hydrodenitrogenation. Gong et al.¹³⁶ designed and synthesized a series of Ni₂P/H-beta@SBA-16-x core–shell catalysts for quinoline hydrodenitrogenation (HDN) using H-beta molecular sieve nanoparticles as the core and tunable mesoporous material SBA-16 film as the shell (Fig. 7d). The unique core–shell structure provided a high external surface area, which contributed to the formation of Ni₂P nanoparticles with a more uniform particle size. Meanwhile, the large pore volume of the mesoporous shell layer facilitated the contact between the reactant molecules and the active sites as well as the rapid transfer of H*. Hydrogen spillover also aids in selectively removing nitrogen compounds, reducing by-products, and enhancing catalyst activity.¹⁴⁸

3.7. Ammonia synthesis

Hydrogen spillover positively affects catalyst stability in hydrodenitrogenation, which reduces surface nitrogen compound accumulation and slows catalyst deactivation, thus extending its use cycle. Wang et al.¹³⁷ synthesized ammonia to achieve nitrogen removal under mild conditions by interfering with the transition metal (TM)-mediated catalytic with a second catalytic site (LiH), which exhibited an excellent ammonia synthesis rate and selectivity over a range of temperatures (Fig. 7e). Yao et al.¹⁴⁹ designed and synthesized Cd/In₂O₃(V_O) catalysts on V_O-rich In₂O₃ and catalyzed the generation of NH₃ from NRR under mild conditions to achieve nitrogen removal. These catalysts had a single CdO₅ site, which facilitated the dissociation and reduction of H₂, which in turn reacted with dissolved N₂ molecules to form *N₂H₂ (RDS). Hydrogen spillover accelerates the conversion of intermediates by rapidly supplying H₂,¹⁵⁰ which then speeds up H* migration, allowing faster denitrogenation with high activity and selectivity. Hydrogen spillover begins with H₂ dissociation and adsorption on the catalyst surface in hydrodenitrogenation. The properties of the metal catalyst determine the dissociation energy of H₂, which in turn affects the efficiency of hydrogen utilization and the overall kinetics of the reaction.¹⁵¹ The metal–support interaction is crucial for spillover capability,¹⁵² with high surface area supports, such as γ-Al₂O₃ and SiO₂, improving metal dispersion and H₂ adsorption. Alloy and heterogeneous catalysts enhance H₂ dissociation and migration, optimizing hydrodenitrogenation performance.¹⁵³ By designing and optimizing catalyst structures and conditions, hydrogen spillover can be amplified for more efficient denitrogenation. Future studies should focus on understanding microscopic mechanisms and developing novel catalysts to improve hydrodenitrogenation performance and durability.

Hydrogen spillover can effectively increase the hydrogen concentration in the active site, thus facilitating the ammonia synthesis reaction. Fu et al.¹³⁸ prepared a Mo, Pt-modified TiO_2−x catalyst for efficient ammonia synthesis. It was shown that the doping of Pt promoted the transfer of H* from Pt to V_O, which further promoted the synthesis and desorption of NH₃ and increased the active site of free Mo (Fig. 7f). Effective H₂ supply boosts catalyst activity, especially in the initial stages. High hydrogen concentrations increase the surface coverage of catalysts, enhancing reaction activity. The Cu/H_xWO₃@CC electrocatalyst synthesized by Li et al. achieved an ammonia production rate of 3332.9 mmol g⁻¹ h⁻¹ at +0.1 V vs. RHE with energy consumption as low as 17.6 kWh kg⁻¹, which is significantly lower than the ∼30 kWh kg⁻¹ of the conventional Haber–Bosch process. This high efficiency under ambient pressure is enabled by hydrogen spillover, which is facilitated by the reversible intercalation/deintercalation of hydrogen in H_xWO₃.¹⁵⁴

It also enhances product selectivity by adjusting the reaction network and suppressing side reactions. Zhou et al.¹⁵⁵ synthesized a series of Zr-based Ru catalysts to investigate the effect of Ru anions on ammonia synthesis under mild conditions (Fig. 8a). Compared with Ru/ZrN and Ru/ZrO₂, the Ru/ZrH₂ catalysts exhibited superior performance. The chemical composition of the Zr matrix affected the strength of the interactions between the Ru anions, hydrogen spillover, and the reaction mechanism of ammonia synthesis. In ZrN or ZrH₂-supported Ru catalysts, hydrogen spillover occurred preferentially, significantly accelerating the reaction. Zhang et al.¹⁵⁶ synthesized an efficient cluster-based co-catalyst, C₆₀-TM, for ammonia synthesis by anchoring buckminsterfullerene (C₆₀) on a non-ferrous transition metal (Fig. 8b). The co-catalyst possessed catalytically active sites for hydrogen and nitrogen, which were interconnected by hydrogen spillover and electron buffer, thus achieving the synergistic effect of nitrogen activation on the transition metal and hydrogen activation and migration on the C₆₀ sites, which demonstrates excellent catalytic performance and antitoxic. Under high temperatures and pressure, spillover prevents catalyst degradation. It also facilitates the effective conversion of reaction intermediates by accelerating nitrogen intermediate hydrogenation and boosting synthesis efficiency.⁸⁷ The conventional Haber–Bosch process requires high temperatures (350–500 °C) and high pressures (20–30 MPa). In contrast, hydrogen spillover catalysts, through dual active site design and optimized H* migration, enable efficient ammonia synthesis under milder conditions. Zhou et al.¹⁵⁷ developed a Re/Mo₂CT_x MXene catalyst that achieved an ammonia synthesis rate of 8.2 mmol g⁻¹ h⁻¹ at 400 °C and 1 MPa, outperforming conventional Ru-based catalysts (5.5 mmol g⁻¹ h⁻¹) while reducing energy consumption by 40%.

Fig. 8 (a) Schematic representation of the effects of Ru-anion interactions on hydrogen spillover and N₂ activation pathways induced by different carrier compositions.¹⁵⁵ (b) Schematic of NH₃ synthesis on the C₆₀-promoting transition metal co-catalysts with the activation and migration of hydrogen on C₆₀ sites.¹⁵⁶ (c) Volume of H₂ generated from AB solution at 25 °C catalyzed by CoO_x/Al₂O₃, Al₂O₃/Pt, and CoO_x/Al₂O₃/Pt.¹⁵⁸ (d) Schematic of the plausible formation process of Ru-SA/H_xMoO_3−y hybrids.¹⁵⁹

The effect of hydrogen spillover is modulated by many factors. Alloy catalysts, such as Ru/Co, have low dissociation energy and perform well in H₂ adsorption and dissociation. Support with a high specific surface area can enhance the H₂ adsorption capacity of the catalyst. Zhang et al.¹⁶² synthesized a Ru–Co single-atom alloy catalyst, Ru_xCo₁ SAA, for the synthesis of NH₃. It was demonstrated that the electronic structure of Ru_xCo₁ SAA could be effectively changed by adjusting the molar ratio of Ru to Co, which enhanced the electron transfer and charge redistribution between Ru and Co atoms, a process that facilitated the adsorption and dissociation of H₂, and thus enhanced the reaction rate. In addition, Gao et al.¹⁵⁸ constructed spatially separated NiO/Al₂O₃/Pt two-component catalysts using atomic layer deposition and demonstrated the promotion of hydrogen reverse spillover in the hydrolysis reaction of ammonia borane by XANES. It was found that the catalyst exhibited excellent catalytic activity and achieved efficient H₂ generation (Fig. 8c). In addition, the H species generated at the NiO site would not be consumed or reduced but would reverse spillover to the Pt site, which significantly enhanced the H₂ generation rate. Nanoparticles, nanowires, and metal combinations (alloy catalysts) of catalyst morphology can significantly enhance hydrogen dissociation and migration, optimizing ammonia synthesis performance. Yin et al.¹⁵⁹ synthesized a Ru-SA/H_xMoO_3−y

hybrid material capable of reducing N₂ to NH₃ under visible light irradiation via H spillover (Fig. 8d). In this hybrid material, there was a synergistic interaction between Moⁿ⁺ and Ru SA, with Ru SA contributing to the activation and migration of H₂, and the Moⁿ⁺ species acting as a localized electron trapping site as well as an adsorption and dissociation site for N₂. Enhancing hydrogen spillover through catalyst morphology and support surface optimization is crucial for efficient ammonia synthesis. Continuous optimization of reaction conditions is vital for improving synthesis efficiency and economic viability.

3.8. Biomass conversion

Hydrogen spillover can accelerate the rate of the biomass conversion reaction. Jiang et al.¹⁶⁰ developed a cooperative catalytic system integrating CsPbBr₃ quantum dots and Hansester, which enabled visible-light-driven oxidative depolymerization of lignin with a TOF of 476 h⁻¹, which is 17 times higher than that of reported Ir-complex catalysts (TOF ≈ 28 h⁻¹), while reducing catalyst cost by approximately two orders of magnitude. Peng et al.¹⁶¹ synthesized a PtIr/α-MoC bimetallic catalyst that exhibited outstanding performance in bioethanol reforming for hydrogen production. Under mild conditions (270 °C), the system achieved a TOF of 1882 h⁻¹, which is 2–4 orders of magnitude higher than that of conventional Fe-based catalysts. Moreover, hydrogen production has nearly 90% lower energy consumption than traditional ethanol steam reforming. The PtIr bimetallic sites selectively cleave the C–H and O–H bonds in ethanol while suppressing C–C bond cleavage, thereby avoiding CO₂ formation. Meanwhile, the α-MoC support promotes water activation and accelerates hydrogen spillover, significantly enhancing catalytic activity. Liu et al.¹⁶³ found that increasing H₂ flow accelerated reactant conversion in lignocellulose hydrogenation (Fig. 9a), especially with catalysts such as nickel (Ni), palladium (Pd), and platinum (Pt). H₂ concentration is crucial for selectivity in biomass conversion. Optimizing hydrogen spillover improves product yield and minimizes by-products. Spillover increases surface hydrogen concentration, boosting catalyst activity for hydrogenating reactants, such as sugars and lipids.¹⁶⁴ In addition, hydrogen spillover can improve the selectivity and stability of catalysts in biomass conversion. Guo et al.¹⁶⁵ successfully prepared novel lignin-tannin particles (L_xT_y) by co-assembling tannins with lignin oligomers for the first time via a solvent transfer strategy. Subsequently, Pd was impregnated on these particles, and Pd/L_xT_y particles were synthesized and applied to the hydrogenation reaction of VAN. Under hydrogen spillover, the H* on the L_xT_y supports were released and induced to participate in the reaction, resulting in the efficient conversion of the VAN (Fig. 9b). Spillover accelerates the hydrogenation of intermediates, increasing reaction efficiency,¹⁶⁶ which contributes to catalyst structure optimization, with alloy catalysts showing enhanced hydrogen dissociation and adsorption, thereby improving conversion efficiency.

Fig. 9 (a) Tandem catalytic mechanistic pathway for the conversion of birch lignin into MPs.¹⁶³ (b) k and TOF of 3% Pd/L₂T₂ catalysis for 10 min of hydrogenation of VAN.¹⁶⁵ (c) Apparent activation energy for FAL and FOL hydrogenation and the reaction mechanism of FAL over Ni-ReO_x/TiO₂.¹⁶⁷ (d) Schematic of the distinctive features of spillover hydrogen affecting HDO according to the phase of TiO₂.¹⁶⁹ (e) Proposed reaction mechanism for the direct conversion of furfural into 1,5-PeD over the 5Cu-30Co/CeO₂ catalyst.¹⁷⁰ (f) Schematic of the synthesis of PtAD/C-Al₂O₃.¹⁷¹

Lin et al.¹⁶⁷ prepared a Ni-ReO_x bimetallic catalyst (Ni-ReO_x/TiO₂) by applying a continuous impregnation strategy, which successfully achieved the efficient and selective conversion of furfural (FAL) to tetrahydrofurfuryl alcohol (THFOL) (Fig. 9c). The results showed that the excellent affinity of ReO_x for C=O and H₂ contributed to the spillover of H* and the activation of the C=O bond on nickel, which enhanced the synergistic effect between nickel and ReO_x and improved the reaction rate. Besides, hydrogen spillover can ensure maximum reactant-hydrogen contact, enhancing conversion efficiency. Žula et al.¹⁶⁸ found that catalyst morphology could affect H species utilization and reaction kinetics through lignin variable ether bond (α-O-4) cleavage properties in different solvents. Kim et al.¹⁶⁹ prepared Ru/TiO₂ catalysts by loading Ru onto TiO₂ for the hydrodeoxygenation (HDO) reaction of guaiacol and investigated the influence of pristine rutile and anatase TiO₂ by hydrogen spillover (Fig. 9d). It was found that these two crystal types of TiO₂ exhibited synergistic effects during the reaction. Multimetallic catalysts enhance hydrogen dissociation and migration through synergistic metal interactions, optimizing conversion performance. Wang et al.¹⁷⁰ successfully synthesized a Cu-modified Co-supported CeO₂ catalyst (5Cu-30Co/CeO₂) for the direct hydrogenolysis of biomass-based furfural to produce 1,5-pentanediol (Fig. 9e). The introduction of Cu improved the dispersibility of Co and facilitated the reduction process of both CeO₂ and CoO, which significantly increased the oxygen vacancies on the surface of the catalyst and the generation of active substances.

3.9. Selective hydrogenation

Selective hydrogenation aims to partially hydrogenate compounds without over-hydrogenation or side reactions, including the partial hydrogenation of alkynes to alkenes, benzene rings, and carbonyl compounds.^30,173 Wu et al.¹⁷¹ successfully prepared ultra-highly dispersed platinum atom catalysts (PtAD/C-Al₂O₃) for the selective hydrogenation reaction of quinoline and p-nitrophenol using a carbon modification strategy (Fig. 9f). The catalyst consisted of Pt-O₁C₂ single-atom and Pt nanoclusters, in which the Pt nanocluster sites contributed to the dissociation of H₂ molecules into H atoms, while the Pt single-atom sites were mainly responsible for the adsorption and activation of quinoline molecules. Zhang et al.¹⁷² summarized the active site isolation approach to the challenges of selective catalytic hydrogenation, pointed out the transition and challenges of nanoscale sites to single-atom sites, and discussed the key factors affecting the catalytic activity/selectivity, especially the geometry and electronic structure of the active sites, to further improve the selectivity of hydrogenation.

Hydrogen spillover plays an important role in selective hydrogenation.¹⁷⁴ In acetylene hydrogenation, spillover facilitates hydrogen transfer, accelerating alkene formation and preventing over-hydrogenation.⁹⁴ Zhang et al.¹⁷⁵ found that spillover enhanced unsaturated compound hydrogenation by promoting H* diffusion on catalyst surfaces with Pt-based and Pd-based catalysts (Fig. 10a). Hydrogen spillover stabilizes hydrogen concentration, lowers activation energy, and boosts catalyst activity. Sun et al.¹⁷⁶ synthesized PdHD/WO₃ catalysts loaded with ppm-sized palladium on WO₃ support for the selective hydrogenation reaction of 4-chloronitrobenzene (Fig. 10b). On the PdHD/WO₃ catalyst, the hydrogenation of the reactants was carried out on the WO₃ support by H* species spillover to the palladium. These H* species generated and migrated from Pd to WO₃ can effectively participate in the hydrogenation of the reactants. Hydrogen spillover can regulate H₂ supply, preventing over-hydrogenation and by-products.¹⁷⁷ Moreover, in benzene hydrogenation, spillover enables selective hydrogenation to cyclohexene, preventing further conversion to cyclohexane.^178–180 Hydrogen spillover depends to some extent on metal–support interactions.¹⁸¹ Zhang et al.¹⁸² demonstrated that Pt–Co alloy catalysts boost hydrogen spillover through metal synergy, enhancing hydrogen dissociation and transfer efficiency. Spillover extends the catalyst lifespan by minimizing carbon and impurity deposition. Pd-based catalysts reduce the carbon deposition rate, prolonging the operational life.^41,183 Optimization of catalyst structures, metal–support interactions, and nanoparticle morphology can further enhance spillover, improving overall reaction performance. Yue et al.¹⁸⁴ successfully achieved the selective hydrogenation of nitroaromatics through the synergistic effect of hydrogen spillover and preferential adsorption by utilizing a magnetic Pt@Fe₂O₃ catalyst containing oxygen vacancies. It was found that the catalyst exhibited excellent hydrogenation activity with 99% conversion of 4-nitrostyrene and 99% selectivity of 4-aminostyrene with a TOF value as high as 2351 h⁻¹.

Fig. 10 (a) Proposed hydrogenation of 6-chloroquinoline on Pt/C-H₂O₂ catalysts by hydrogen spillover.¹⁷⁵ (b) Scheme of hydrogenation of 4-chloronitrobenzene on dual-active sites of highly dispersed Pd and the oxygen vacancy on PdHD/WO₃.¹⁷⁶ (c) Schematic of the synthesis process of PPh₃-Rh_AC–SA/N–C.¹⁸⁵ (d) Schematic of the synthesis process for the Rh₁Co SAA/N–C catalyst.¹⁸⁷

3.10. Hydroformylation

Hydroformylation is a catalytic cycle, and the process involves olefin coordination, olefin insertion, carbonyl coordination, carbonyl insertion, and H₂ oxidative addition and the reductive elimination of a series of free radical reactions.^185,186 Hydrogen spillover promotes the diffusion of H* to different active sites, facilitating the introduction of carbonyl groups and increasing reaction activity. Yang et al.¹⁸⁵ designed and constructed a dual active-site coupled catalyst containing triphenylphosphine-coordinated Rh clusters and Rh-N₄ monoatomic site catalyst (PPh₃-Rh_AC–SA/N–C) for the hydroformylation of olefins (Fig. 10c). DFT calculations showed that PPh₃-modified Rh clusters were favorable for the adsorption and activation of olefins and H and that the doped PPh₃ increased the electron density and decreased the energy barrier of the hydrogenation step on the Rh clusters. In addition, Zhang et al.¹⁸⁷ prepared a novel Rh₁Co single-atom alloy (SAA) catalyst and found that there was a hydrogen spillover effect during the reaction, which greatly enhanced the hydroformylation activity of 1-hexene, with close to 100% conversion and selectivity, and good cyclic stability (Fig. 10d). Different crystal facets of the support are suitable for different reactions. Modulating the hydrogen spillover properties, the microstructure and activity of the catalyst are optimized. In addition, SMSI can stabilize individual atoms. Liu et al.¹⁸⁸ prepared a CeO₂ catalyst with cubic, polyhedral, and rod-like morphologies by adjusting the temperature and pH of the solvent. Then, Rh₁/CeO₂ catalysts were obtained by loading the Rh species. The DFT results showed that the unique structure of CeO₂ with (110) facets produced localized stresses and O atoms with dangling bonds upon Rh loading. The formed Rh₁/CeO₂ (110) structure modified the H adsorption properties and enhanced hydrogen spillover.

Hydrogen spillover significantly enhances catalytic performance in hydroformylation reactions. Zhang et al.¹⁸⁹ developed a 0.17RhOD@MEL catalyst that achieved a TOF of exceeding 6500 h⁻¹ and selectivity towards n-butyraldehyde of surpassing 99% in propene hydroformylation. The sinusoidal, 10-membered ring channels of the MEL zeolite suppress the formation of isobutyraldehyde intermediates via spatial confinement while facilitating the migration of H* from the Rh sites to the olefin. Consequently, the reaction rate is almost ten times faster than that of conventional homogeneous Rh catalysts. The Rh/SiO₂ catalyst developed by Liu et al. exhibited an impressive TOF of approximately 50 000 h⁻¹ for styrene hydroformylation, significantly outperforming traditional homogeneous Rh catalysts (TOF ≈ 4700 h⁻¹) and conventional SiO₂-supported Rh catalysts (TOF ≈ 1200 h⁻¹).¹⁹⁰ Moreover, a high conversion rate of 99.6% was achieved under relatively mild conditions (3 MPa syngas, CO/H₂, at 110 °C), markedly reducing energy consumption compared to traditional homogeneous processes, which typically require 10–30 MPa. This enhanced performance is attributed to silanol nests that accelerate H* migration to the Rh active sites via a hydrogen-bonding network while simultaneously enriching the local concentration of olefin substrates, leading to a reaction rate that is almost ten times faster. Zhang et al.¹⁹¹ demonstrated that a cationic Co(II) bisphosphine complex catalyst achieved a TOF of 103.2 min⁻¹ under a relatively low pressure of 50 bar, overcoming the limitation of conventional cobalt catalysts that typically require high-pressure conditions. In contrast, traditional Co-based catalysts generally operate at pressures of around 300 bar.

By rationally designing catalysts, hydrogen spillover promotes optimal mechanistic and practical applications. Future research will integrate computational chemistry and materials science to precisely control spillover, advancing catalytic hydrogenation in industrial and environmental contexts. Thus, structural design and catalyst regulation are crucial for strengthening hydrogen spillover.

4. How to monitor and elucidate hydrogen spillover

Hydrogen spillover involves interfacial migration, reconfiguration of the electronic structure, and the dynamic evolution of the H species at an atomic level. Owing to its complexity and occurrence at the nanoscale or atomic dimensions, accurately and quantitatively characterizing this phenomenon remains a significant challenge. Below, we summarize representative advanced characterization techniques alongside relevant computational modelling approaches.

4.1. In situ Raman spectroscopy

In situ Raman spectroscopy enables the real-time monitoring of metal components, support surface species and defect structures, as well as changes in chemical bonding during reactions. This capability allows for the indirect detection of hydrogen spillover and the tracing of its migration pathways. Yin et al.⁷⁵ employed tip-enhanced Raman spectroscopy (TERS) to investigate the catalytic hydrogenation of chloro-nitrobenzenethiol on a well-defined Pd (sub-monolayer)/Au (111) bimetallic catalyst. By simultaneously mapping surface morphology and chemical properties with nanometer-scale resolution, they obtained direct spectroscopic evidence of hydrogen spillover from Pd to Au. Wei et al.⁷⁶ designed a sandwich structure consisting of Au/TiO₂/Pt and employed surface-enhanced Raman spectroscopy (SERS) to monitor hydrogen spillover with a spatial resolution of 10 nm. By tracking the reduction of p-nitrothiophenol to p-aminothiophenol, they observed hydrogen migration across the TiO₂ surface over distances of up to 50 nm, confirming a water-assisted spillover mechanism.

4.2. In situ X-ray absorption spectroscopy

The migration of H* can alter the oxidation state of metals, their coordination environment or the interfacial structure between the active metal and the support, which can be investigated using X-ray absorption spectroscopy (XAS). Xiong et al.¹⁹² investigated the electronic state changes in CoO_x species using X-ray absorption spectroscopy (XAS) and found that hydrogen spillover could reduce Co species to a lower oxidation state (CoO), thereby enhancing styrene epoxidation selectivity to 94.8%. Kang et al.¹⁹³ employed Quick-XAFS to rapidly capture the dynamic formation of TiO_x domains in a Ru/Ti/Mn catalyst, which reveals the migration pathway of H*, for the first time, achieving the spectroscopic visualization of hydrogen spillover within a metal–organic framework (MOF).

4.3. Ultrafast spectroscopy

Ultrafast spectroscopy enables the investigation of electron redistribution and surface charge transfer processes triggered by H* migration. Gao et al.¹⁹⁴ employed femtosecond spectroscopy to investigate photocatalytic charge support dynamics and found that in B, P co-doped g-C₃N₄, hydrogen spillover is accelerated through a shallow electron trapping process.

4.4. In situ transmission electron microscopy

Although TEM cannot directly visualise hydrogen atoms owing to their low atomic number and weak scattering properties, it can indirectly reveal the hydrogen spillover process by capturing the structural changes that spillover induces, particularly at the metal–support interface. Zhang et al.¹¹² utilized in situ TEM to visualize the hydrogen-driven migration of iron oxide and observed that Pd facilitates the dissociation of hydrogen molecules into activated H*, which subsequently spills over to reduce iron oxide, its gradual outward fragmentation, and its migration through the carbon shell. Sharma et al.¹⁹⁵ directly observed hydrogen spillover on Ni-supported Pr-doped CeO₂ under a H₂ atmosphere using environmental transmission electron microscopy (ETEM).

4.5. In situ diffuse reflectance infrared spectroscopy

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a powerful technique for investigating the mechanisms of surface reactions and the evolution of species in catalysis. In the context of hydrogen spillover, DRIFTS is commonly employed to monitor the formation and migration of H species, as well as their interactions with surface functional groups on the support material. Although H atoms exhibit weak IR responses, the high surface sensitivity of DRIFTS allows for the indirect or supportive identification of hydrogen spillover by capturing characteristic absorption bands associated with reaction intermediates, hydroxyl groups, metal-hydrogen bonds, and changes in surface functionalities following hydrogen interaction. Kang et al.¹⁹³ monitored the dynamic evolution of surface hydroxyl groups during CO₂ electroreduction using DRIFTS and demonstrated that hydrogen spillover across the TiO_x/MnO interface lowers the activation barrier for CO₂ reduction to 0.72 eV, resulting in a CO production rate of 80 mmol g⁻¹ h⁻¹.

4.6. Isotope labelling coupled with mass spectrometry

The combination of isotope labelling and mass spectrometry (MS) offers a powerful approach for directly or indirectly monitoring hydrogen spillover. By substituting conventional H₂ with D₂, or vice versa, and coupling product analysis with isotopic signal tracking, it becomes possible to distinguish clearly the pathways of hydrogen atoms from different sources. This method is particularly effective for determining whether hydrogen migrates from metal active sites to support and subsequently participates in downstream reactions. Xiong et al.⁶⁴ combined D₂ labelling experiments with ab initio molecular dynamics (AIMD) simulations to confirm the reversible hydrogen spillover in Pd/ZIF-8, where the MOF acts as a hydrogen reservoir that dynamically regulates the H concentration on the Pd surface. Through D₂ labeling experiments, Gu et al.⁷⁴ found that with the assistance of water, H atoms activated on Pt nanoparticles originate from H₂ dissociation and subsequently diffuse across the MOF-801 surface. Combined with a ligand transformation model, the hydrogen spillover coverage diameter was calculated to reach up to 100 nm.

4.7. Computational modeling approaches

Density functional theory (DFT) is a quantum mechanical approach that determines the total energy of a system based on its electron density and is widely used to investigate the structure, properties, and reaction mechanisms of materials. In hydrogen spillover, DFT can be employed to calculate the adsorption energies and diffusion barriers of H atoms on catalyst surfaces and supports, thereby providing microscopic theoretical insights into the spillover. Karim et al.⁹⁷ employed density functional theory calculations to reveal rapid hydrogen spillover on titania, where distal iron oxide nanoparticles are reduced via a coupled proton–electron transfer mechanism. In contrast, spillover on alumina is mediated by three coordinated aluminium centres that also interact with water, resulting in competition between hydrogen migration and desorption. Consequently, hydrogen spillover on alumina is approximately ten orders of magnitude slower than on titania and restricted to distances very short from the platinum particles. Liu et al.¹⁹⁶ investigated the mechanism of hydrogen spillover on a Pd-supported covalent organic framework (COF-108) using density functional theory. The results revealed that the dissociation of H₂ on Pd clusters is a barrierless process and that H atoms adsorbed at bridge sites can migrate to the COF surface. Furthermore, the presence of Pd was found to reduce the diffusion barrier of hydrogen atoms by 30%.

The difference in the work function (ΔΦ) between the metal and the support affects the electron distribution at the interface and the charge transfer, thereby affecting hydrogen spillover. A large ΔΦ can induce the formation of a Schottky junction at the interface, leading to charge accumulation, which traps H* and hinders spillover. Conversely, reducing the work function difference minimises interfacial charge transfer and accumulation, thereby lowering the energy barrier for hydrogen spillover. Li et al.¹⁹⁷ found that the Pt₂Ir₁/CoP catalyst exhibits a smaller work function difference (ΔΦ) between Pt and Ir compared to the Pt/CoP catalyst. This reduced ΔΦ alleviates interfacial charge accumulation, leading to charge redistribution and localization within the alloy components. Consequently, H* adsorption at interfacial sites is weakened, which enables the formation of a stepwise thermodynamic pathway for reverse hydrogen spillover, thereby facilitating an efficient reverse spillover process.

The difference in the d-band centers (Δε_d) between two metals affects the variation in hydrogen adsorption free energy on their surfaces, which in turn influences the kinetic barrier of hydrogen spillover. A smaller Δε_d reduces the disparity in hydrogen adsorption strength, resulting in a lower kinetic barrier and consequently enhancing the spillover process. Analysis of hydrogen spillover behavior in various bimetallic alloy nanoparticles during nitrate hydrogenation revealed that catalysts with smaller Δε_d exhibited superior catalytic performance, confirming that a reduced Δε_d lowers the kinetic barrier for hydrogen spillover.¹⁹⁸

The combined approach of molecular dynamics (MD) and machine learning leverages large datasets generated by methods such as density functional theory (DFT) to train machine learning algorithms for constructing potential energy surfaces or force fields. These trained models are then used to perform MD simulations, enabling the investigation of hydrogen spillover in complex kinetic processes. Gu et al.⁴³ developed a high-dimensional potential energy surface for hydrogen spillover on Pt/Cu (111) surfaces using a machine learning approach trained on DFT. Specifically, they employed the embedded atom neural network (EANN) method to model three systems: clean surfaces, surfaces with one adsorbed H atom, and surfaces with two adsorbed H atoms. Through quasi-classical trajectory with electronic friction (QCT-EF) calculations, the authors demonstrated that collisions between H₂ molecules and pre-adsorbed H atoms on Pt sites can drive hydrogen spillover onto the Cu substrate. This mechanism prevents further deactivation of Pt atoms that would otherwise result from the adsorption of additional H atoms during H₂ dissociation. Xiong et al.⁶⁴ found through ab initio molecular dynamics (AIMD) simulations that when the equilibrium of H_ad concentration between the MOF and the bulk Pd is disturbed, H_ad can be released from the MOF, leading to an increase in H_ad concentration on the Pd surface. Through this reversible process, the MOF effectively acts as a reservoir, dynamically regulating and replenishing the surface H atoms on Pd.

Nevertheless, current techniques still suffer from limitations, such as insufficient sensitivity, poor quantifiability, and restricted spatial resolution. Direct observation of hydrogen spillover under complex catalytic reaction conditions remains particularly challenging. Therefore, establishing integrated, multi-technique characterization systems and developing methods with higher spatiotemporal resolution and stronger environmental relevance are essential for advancing the fundamental understanding of hydrogen spillover.

5. Strengthening spillover across catalyst regulation

Hydrogen spillover plays an important role in catalytic hydrogenation, and enhancing this process is of key significance for improving reaction efficiency and selectivity. Recent studies have focused on the enhancement of hydrogen spillover through the design and optimization of catalysts, especially the modulation of microstructure and surface interface properties.

5.1. Regulation of alloy catalysts

Alloy catalysts enhance hydrogen spillover through synergistic metal interactions. The introduction of additional metals results in the formation of alloys with stronger spillover capabilities,¹⁹⁹ such as Pt–Pd and Ni-Cu, which improve H* migration. Adjusting metal ratios in alloys optimizes hydrogen dissociation and migration, significantly enhancing spillover.²⁰⁰ Gu et al.²⁰¹ investigated the spillover kinetics of H* and CH₃* species on a single-atom alloy surface (Rh/Cu (111)) during the dissociative chemisorption of methane (CH₄) using a molecular dynamics approach (Fig. 11a). The results showed that H* and CH₃* were able to spillover on the metal surface and participate in subsequent reactions. In addition, under ambient pressure conditions, collisions of gaseous CH₄ molecules facilitated the spillover of these two species through energy transfer.

Fig. 11 (a) Schematic of adsorbate spillover on Rh/Cu (111) single-atom alloy. The white sphere represents H, the grey sphere represents C, and the blue sphere represents Rh.²⁰¹ (b) Schematic of hydrogen spillover at different supports.²⁰² (c) Schematic of the hydrogenation of aldehydes/ketones to alcohols with hydrogen spillover on h-BN containing abundant nitrogen vacancies.¹⁸¹ (d) Intensification mechanism of hydrogen spillover of the Pt/s-MoO₃ catalyst.²⁰³ (e) Relation between the TOF value and amount of hydrogen spillover and the schematic of the hydrogen spillover on Pt/SBA-15 and 250FAS-Pt/SBA-15.²⁰⁴

5.2. Optimization and regulation of metal–support interfaces

The metal–support interface is critical for hydrogen spillover. Zhou et al.²⁰² found that optimizing this interface enhanced hydrogen adsorption, dissociation, and migration (Fig. 11b). Smaller metal nanoparticles increase the surface area, providing more active sites for hydrogen interaction. Lin et al.²⁰⁵ successfully synthesized Pt atomic dimers on NiOOH support with reverse hydrogen spillover, exhibiting better basic HER activity than Pt single atoms and Pt clusters. The Pt dimers were anchored to NiOOH via Pt–O bonds, which resulted in reverse and enhanced hydrogen spillover. The surface chemical properties of the support can be adjusted by introducing oxygen vacancies, enhancing metal–support interactions. Supports such as TiO₂ and Al₂O₃, which have an abundance of oxygen vacancies, improve hydrogen adsorption and migration.

Zhang et al.¹⁸¹ presented hydrogenation based on hexagonal boron nitride (h-BN)-loaded Pd nanoparticles, aiming to convert aldehydes/ketones into alcohols with hydrogen spillover (Fig. 11c). Nitrogen vacancies in h-BN were the key factor in hydrogen spillover from Pd to h-BN. Both the hydrogenation of aldehydes/ketones and the hydrogen spillover of Pd occurred in the nitrogen vacancies of h-BN. In addition, Liu et al. induced the migration of H_ads from Pt to TiO₂ by optimizing the metal–support interface, which reduced the H_ads coverage on the Pt surface and effectively inhibited the excessive hydrogenation of cyclohexanone.²⁰⁶

5.3. Design and modification of support materials

Supports significantly influence hydrogen spillover through their chemical and physical properties. The high surface area supports appropriate pore structures (mesoporous silica and zeolites), providing pathways for hydrogen diffusion. Karim et al. explored the hydrogen spillover efficiency and spatial range of reducible and nonreducible supports and found that on the titanium oxide support, H* achieved rapid remote spillover through proton–electron coupling transfer and reduced distal iron oxide nanoparticles. On the alumina support, the spillover process is mediated by three-coordinated aluminum sites, which interact with water simultaneously, resulting in competition between hydrogen migration and hydrogen desorption, so that the hydrogen spillover rate is about ten orders of magnitude slower than that of the titanium oxide support and is limited to the extremely short distance near the platinum particles.⁹⁷ Kang et al. constructed TiO_x patches in situ on the surface of MnO support, forming an efficient transport channel for hydrogen spillover¹⁹³ (oxide-oxide interface). Bai et al.²⁰³ employed three-dimensional single-crystal molybdenum oxide (s-MoO₃) as an active platinum (Pt) site support (Fig. 11d). Compared with conventional support materials, s-MoO₃ exhibited interconnected branching morphology, jagged stepped edges, porous surface, and partially reduced properties. Owing to the morphological and structural advantages of s-MoO₃, the subsequently loaded Pt particles were able to induce higher spillover rates and promote the generation of H(O)_xMoO₃ intermediates, thereby enhancing the activity of the hydrogen evolution reaction (HER).

In addition, surface defects or modifications, such as acidic or basic groups and heteroatom doping, enhance H₂ adsorption, promoting spillover. Warczinski et al.²⁰⁷ investigated the specific mechanism of hydrogen spillover using carbon-loaded palladium nanoparticles (Pd/NMC) as a model and analyzed the effect of nitrogen doping on the process. The results showed that nitrogen atoms and water molecules in graphite significantly contribute to hydrogen spillover and provide anchoring sites for the H*. The preferential binding of palladium nanoparticles to pyridine nitrogen atoms in Pd/NMC significantly reduced the energy barrier for hydrogen spillover. In addition, Xing et al.²⁰⁴ employed an organic molecular modification (OMD) strategy obtained via a molecular layer deposition pulsed strategy to promote hydrogen spillover on a non-reducible silica support (Fig. 11e). The results showed that after molecular layer modification with fluoroalkylsilanes (FAS), the catalyst (xFAS-Pt/SBA-15) showed a significant enhancement of hydrogen spillover compared to the pristine Pt/SBA-15 owing to the presence of carbonaceous material.

5.4. Regulation of layered and core–shell structured catalysts

Layered or core–shell structured catalysts can effectively control the adsorption, dissociation, and spillover of hydrogen by spatially separating different functional regions. Zhan et al.²⁰⁸ successfully prepared multi-shell-structured ZIF nanocubes with Matryoshka-type ((ZIFs@)_n−1ZIFs) by stepwise growth of heterogeneous epitaxy in solution and loading Pt nanoparticles on their outer surface to generate H*. The results showed that the enhanced decomposition of ZIF-67 at low temperatures in the core–shell structure of ZIF-67@ZIF-8/Pt can be attributed to the spillover of H* through the ZIF-8 shell layer. In core–shell structures, the core is an active metal or alloy, while the shell regulates hydrogen transfer. Psofogiannakis et al.²⁰⁹ proposed the mechanism of hydrogen spillover: H atoms cannot migrate in the pore network in a chemisorbed state but diffuse in the gas phase. Then, the H atoms were able to attach to all available binding sites in the joints with an almost negligible energy barrier during diffusion. However, diffused H atoms could also extract adsorbed hydrogen atoms via the Eley–Rideal mechanism, resulting in the formation of H₂.

5.5. Design of novel catalyst synthesis methods

Advanced synthesis methods and monitoring technology can precisely control the structure of catalysts and understand the dynamics of active species, significantly enhancing hydrogen spillover. Lykhach et al.²¹⁰ used resonance photoelectron spectroscopy to study hydrogen spillover over explicitly modeled Pt/ceria catalysts. In the Pt/CeO₂(111)/Cu(111) catalyst, hydrogen spillover and reverse spillover caused reversible changes in the oxidation state of the surface cerium ion. The complete loss of hydrogen as well as the reoxidation of the cerium oxide surface may result from the reverse hydrogen spillover and the subsequent desorption of hydrogen from platinum. Li et al. studied the hydrogen spillover of Pt-induced monoclinic tungsten oxide γ-WO₃ by in situ atmospheric pressure X-ray photoelectron spectroscopy, DFT theoretical calculation and microscopic kinetic modeling and explored the dynamic evolution of catalyst surface states at different temperatures.²¹¹ Atomic layer deposition (ALD) technology enables atomic-level precision in controlling the catalyst structure, optimizing metal particle distribution and metal–support interfaces. This enhances spillover by improving metal–support interactions and H₂ adsorption. These methods can deposit uniformly distributed metal nanoparticles on the support, improving the interaction between the metal and the support and promoting hydrogen adsorption and spillover. Continuous optimization of catalyst structure design and regulatory strategies has led to significant progress in enhancing hydrogen spillover in recent years.²¹² These strategies not only help to improve the efficiency and selectivity of hydrogenation but also provide an important theoretical and practical foundation for the development of novel catalysts.²¹³ Future research will focus on interface engineering, multi-metal alloy design, and support material innovations to further advance hydrogen spillover applications in catalytic reactions.

6. Opportunities and challenges

The discovery and rapid development of hydrogen spillover have significantly transformed hydrogenation processes. Advanced characterization techniques have promoted the understanding of hydrogen spillover at the atomic level. Hydrogenation is indispensable in the critical period of energy transformation. As a crucial part of hydrogenation, the investigation of hydrogen spillover should seize the opportunity and face an arduous challenge (Scheme 4).

Scheme 4 Prospect of hydrogen spillover: regulation, challenge and opportunity.

6.1. Opportunities

6.1.1 Development of novel catalyst materials²¹⁴. Utilizing multi-metal alloys or metal–support composites can enhance hydrogen spillover capabilities. Certain metal combinations, such as Pt/Pd and Ru/Ni, significantly boost hydrogen spillover in specific reactions. Developing novel supports with unique structures, such as mesoporous and carbon-based materials, can optimize metal dispersion and improve hydrogen migration efficiency.²¹⁵

6.1.2 Refined catalyst design. Controlling the size and morphology of metal particles,²¹⁶ such as nanoparticles, nanosheets, and nanorods, can significantly increase the surface area of the catalyst, promoting effective hydrogen adsorption and spillover. Layered or core–shell structured catalysts enhance hydrogen diffusion pathways, increasing spillover rates.

6.1.3 Optimization of reaction conditions. Hydrogen spillover improves under optimal temperature, pressure, and pH conditions.²¹⁷ Research suggests that fine-tuning these parameters can enhance catalyst performance. Additionally, selecting the appropriate reaction medium is crucial because it influences hydrogen utilization efficiency.

6.1.4 Advanced catalyst synthesis techniques. In situ synthesis and self-assembly techniques offer precise catalyst control, enhancing performance. Techniques such as atomic layer deposition (ALD) and hydrothermal synthesis provide uniform metal distribution and higher catalytic activity.²¹⁸

6.2. Challenges

6.2.1 Catalyst stability and durability. During the reaction, heterogeneous catalysts may experience aggregation or oxidation, resulting in a reduction in active sites and consequently affecting hydrogen spillover capacity. Researchers need to develop catalysts with higher stability. Strategies such as chemical modification or the introduction of vacancy defects to optimize the surface properties of the support and enhance hydrogen migration efficiency; the design of core–shell structures to protect active sites, reduce noble metal loading, and improve resistance to poisoning; and the regulation of metal–support coordination or the introduction of specific ligands to optimize H* migration pathways and suppress side reactions are important approaches for improving catalyst stability. Under reaction conditions, the structure of the catalysts may change, leading to decreased hydrogen spillover efficiency. Therefore, understanding and predicting the evolution of catalysts during the reaction is a crucial research direction.²¹⁹

6.2.2 Complexity of hydrogen spillover mechanism. Despite significant advancements in both theoretical and experimental studies on the hydrogen spillover mechanism, many mysteries remain unresolved.²²⁰ Understanding the migration mechanisms and kinetic processes of H* at catalyst interfaces is crucial for designing efficient catalysts.²²¹ Existing characterization techniques are still inadequate for real-time monitoring of hydrogen spillover. Therefore, new characterization technologies need to be developed to better understand the properties of catalysts.²²² In the future, advances in techniques such as time-resolved chromatography, isotope labelling, and spatially resolved imaging are expected to gradually provide direct evidence of hydrogen spillover.

6.2.3 Complexity of catalyst design. The performance of catalysts is influenced by the type and morphology of the metal used, the properties of the support and various reaction conditions. Identifying the optimal combination of these variables remains a significant challenge. Additionally, there is a gap between theoretical simulations and experimental results. Bridging this gap to effectively guide catalyst design is a crucial focus for future research.²²³

6.2.4 Obstacles to industrial application²²⁴. Although novel catalysts demonstrate promising performance in laboratory settings, their economic viability for large-scale production remains a concern. High metal costs and complex synthesis may hinder their industrial applications. To ensure widespread use, these catalysts must maintain consistent performance under various reaction conditions, necessitating further research and validation. The structural design and regulation of heterogeneous catalysts in enhancing hydrogen spillover present a wealth of opportunities and challenges. Future research can address these challenges by gaining a deeper understanding of the hydrogen spillover mechanism, developing novel catalyst materials, optimizing reaction conditions, and improving synthesis techniques. By continually innovating and refining these aspects, the development and application of catalytic hydrogenation can be significantly advanced. The potential for the use of heterogeneous catalysts in hydrogen spillover remains promising.

Author contributions

Guangxun Sun, Peng Xue, Lei Wang, Xin Zhang, Zhidong Wang, Qian Zhang, writing – original draft; Yuan Pan, writing – review and editing; and Yuan Pan, funding acquisition. All the authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare that they have no conflict of interest.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22478432, 22522819), Shandong Provincial Natural Science Foundation (ZR2024JQ004), and Taishan Scholars Program of Shandong Province.

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Footnote

† These authors contributed equally.

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