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Review

Probing Heterolytic H2 Dissociation on Heterogeneous Catalysts: A Brief Review of Experimental Strategies

by
Siwen Wang
1,*,
Xuanqing Lou
2 and
Bowei Liu
1
1
Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, NY 13244, USA
2
Center of Innovation for Flow through Porous Media, Department of Energy and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA
*
Author to whom correspondence should be addressed.
Processes 2025, 13(5), 1465; https://doi.org/10.3390/pr13051465
Submission received: 10 April 2025 / Revised: 6 May 2025 / Accepted: 8 May 2025 / Published: 11 May 2025
(This article belongs to the Section Catalysis Enhanced Processes)
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Abstract

:
Hydrogen (H2) has become a more important alternative source in the current energy transition process. Beyond its role in clean energy production, it also serves as a key reactant in a wide range of industrial chemical transformations, such as hydrogenation and hydroprocessing. A fundamental step in many of these processes is the dissociation of hydrogen on catalyst surfaces. This short review provides an overview of the fundamental mechanisms involved in hydrogen dissociation over catalysts, with a specific emphasis on heterolytic pathways. Meanwhile, the influence of surface coordination environments on hydrogen activation is discussed, focusing on key factors—Lewis acid–base pairs, lattice oxygen and oxygen vacancies, and metal–support interfaces. With recognizing the significance of understanding the reaction mechanisms, we provide a critical review of experimental techniques, including spectroscopy, temperature-programmed methods, and kinetic analysis, that have been successfully applied or appear promising for probing active sites, reaction dynamics, chemisorbed intermediates, and elementary steps. Our goal is to highlight how these techniques contribute to a mechanistic understanding and to outline future directions, making this review a valuable resource for both new and experienced researchers.

1. Introduction

Since industrialization, energy demand has expanded rapidly, leading to a larger scale exploitation and consumption of fossil fuels. However, extensive reliance on fossil fuels has raised serious environmental and sustainability concerns, including pollution, greenhouse gas emissions, and resource depletion [1,2,3]. To address these challenges, increasing research effort has been dedicated to exploring alternative energy solutions, such as solar, wind, and biomass. Among these alternatives, hydrogen has emerged as a promising energy strategy, not only as a clean fuel (H2 + 1/2O2 → H2O) but also as a critical reactant in industrial processes such as oil refining, ammonia synthesis, CO2 hydrogenation, hydrocarbon upgrading, and other fine chemical production (Figure 1) [4]. Consequently, with the increasing focus on sustainable hydrogen production, chemical storage, and application, the development of relevant catalytic materials and chemisorption-based approaches has gained significant attention for optimizing hydrogen utilization. Central to these fields are chemisorption and catalytic activation, where the dissociation of H2 on catalyst surfaces dictates reaction kinetics, selectivity, activity, and scalability [5]. To this end, unraveling the atomic-level mechanism of H2 dissociation over catalysts is of great importance to realize the hydrogen economy. Specifically, H2 adsorption and dissociation on solid catalysts’ surfaces potentially follow two distinct routes, namely (i) homolytic dissociation, where the symmetric H-H bond equally breaks to form two adsorbed hydrogen atoms; and (ii) heterolytic dissociation, generating a proton–hydride pair [5,6]. The pathways are governed by the different electronic structures of catalysts and the local coordination environment of active sites leads to unique pathways, leading to completely different reaction mechanisms.
In industrial applications, hydrogenation and hydroprocessing are two essential processes which are predominantly performed over heterogeneous catalysts. The relevant reaction mechanisms are commonly involved with Langmuir–Hinshelwood or Mas–van Krevelen [7]. In addition, hydrogen activation and mitigation are intensively involved in the rate-determining steps and play a crucial role in reactions [8,9], where first-order kinetics in H2 and zero-order dependence on substrates over the catalyst support are commonly observed [10,11,12,13]. To this end, the criteria for the development of heterogeneous catalysts include well-dispersed active substrates, abundant lattice defects, and superior structural and chemical properties [7]. Currently, one of the main challenges is bridging the gap between macroscopic mechanisms and microscopic elementary steps, which include tracking intermediates, quantifying elementary steps, and further optimizing the efficiency and selectivity of the catalysts. While several groups have provided comprehensive reviews of the H2 dissociation mechanism across homogeneous catalysts and heterogeneous catalysts, a systematic review of experimental techniques used to characterize the catalyst and quantify elementary steps during hydrogen activation remains lacking [5,6,7,14]. This review aims to fill this gap by summarizing the fundamental mechanism of H2 activation on heterogeneous catalysts and exploring key experimental techniques that provide quantitative insights into hydrogen-involved reactions. Subsequently, prospective research directions are proposed to advance the understanding of catalytic hydrogen activation and guide future developments in this field.

2. Fundamental Mechanisms of Hydrogen Dissociation on Heterogeneous Catalysts

2.1. Hydrogen Adsorption on Catalyst Surfaces

The interaction of gaseous hydrogen with solid surfaces is governed by surface chemistry. Depending on the surface chemistry, molecular H2 may interact with the solid surface either through weak van der Waals interaction or dissociative chemisorption, as shown in Figure 2. When interacting with inert materials, such as graphite, carbon nanotubes, and zeolites, H2 physisorbs through weak van der Waals forces, leaving the H-H bond intact. Such non-dissociative adsorption is reversible and typically observed under low temperatures, since molecular H2 lacks significant charge, dipole moment, or polarizability [15].
In contrast, H2 undergoes dissociative chemisorption on reactive surfaces such as transitional metals (e.g., Pd, Pt, and Ni) or reducible oxides (e.g., ZnO) [16,17]. Herein, the strong electronegativity of surface atoms polarizes H2 molecules, redistributes electron density, and weakens the H-H bond, leading to the extension of the bond length of H2 molecules. Since it has enough translational energy, overcoming the energy barrier facilitates the dissociation of H2 and the formation of chemical bonds with the solid surface [18,19]. Such an internal bond cleavage goes either through homolytic dissociation or heterolytic dissociation, which is determined by the local electronic and geometric environment of active sites.

2.2. Homolytic Dissociation vs. Heterolytic Dissociation of H2

The schematic of homolytic dissociation and heterolytic dissociation is shown in Figure 3. The type of dissociation is primarily determined by the electronic and geometric properties of active sites. Homolytic dissociation is defined as the H-H bond breaking symmetrically and forming two neutral hydron atoms (H). This process is facilitated by direct orbital overlaps between the H2 antibonding orbital (σ*) and d orbital of metals, leading to the formation of a covalent M-H· bond [5,14,19]. Yang et al. have demonstrated that hydrogen dissociation predominantly goes through homolytic dissociation and leads to the formation of hydrides on single metal atoms or ensembles with several metal atoms [20]. Particularly, it is commonly observed on transitional metals (Pt, Pd, Ni) due to its similar electronegativities to H2 [5,14]. Recently, homolytic dissociation has also been reported over Ga2O3, a metal oxide-supported Au catalyst [20,21].
In contrast, heterolytic dissociation occurs when the local coordination environment of the metal center satisfies the Lewis acid–base pair. Thereby, the effective polarization of H2 molecules will be enabled and create polar reductive ion pairs, namely Hδ+ and Hδ−, which enable regioselective hydrogenation or hydroprocessing reactions [14]. Specifically, this process is driven by the withdrawal of electrons from the H2 bonding orbital (σ) through the interaction with active sites, while electron donation is to the antibonding orbital (σ*) through interactions with electron-rich oxygen or hydroxyl sites. The electron transfer weakens the H-H bond while maintaining an unaltered catalyst surface [14]. Additionally, heterolytic H2 dissociation exhibits several advantages over homolytic dissociation in catalytic reactions and has been extensively utilized in the selective hydrogenation of functional compounds, including aromatics [22,23], aldehydes [24,25,26], CO2 [8,27,28], and in the hydroprocessing of carbonyl-containing compounds [7,29]. Firstly, it favors the hydrogenation of polar bonds, such as C=O, C=N, and N=O, which can serve as effective acceptors for protons and hydrides, leading to higher selectivity in molecule transformations. Next, the energy barrier of heterolytic dissociation is lower than that of homolytic dissociation, making it kinetically more favorable under mild reaction conditions. Lastly, traditional hydrogenation requires direct adsorption of the substrate onto metal active sites, making it difficult to hydrogenate the sterically hindered substrates. On the contrary, heterolytic dissociation enables hydrogen transfer through outer-sphere proton and hydride migration, which allows hydrogenation to proceed without substrates directly binding on the metal sites [5]. These features highlighted the importance of the heterolytic H2 dissociation in designing efficient and selective catalysts for various hydrogenation or hydroprocessing reactions [5,7].
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Figure 3. The schematic of H2 homolytic dissociation and H2 heterolytic dissociation. (a) the simplified surface changes during H2 dissociation; (b) dissociation of H2 in heterogeneous catalysts. Adapted from Refs. [5,30].
Figure 3. The schematic of H2 homolytic dissociation and H2 heterolytic dissociation. (a) the simplified surface changes during H2 dissociation; (b) dissociation of H2 in heterogeneous catalysts. Adapted from Refs. [5,30].
Processes 13 01465 g003

2.3. Classification of Catalyst for H2 Heterolytic Dissociation

Given the distinct mechanistic advantages of heterolytic H2 dissociation, designing catalysts that facilitate heterolytic dissociation is critical for achieving higher selective and efficient hydrogenation reactions. Both homogeneous and heterogeneous catalysts have been applied to hydrogen-involved reactions, and a heterolytic activation of H2 was observed across various catalytic systems.
Different from pure metal surfaces, where metal–metal interactions lead to homolytic H2 dissociation, the homogenous catalyst includes a metal center and a nonmetal atom (ligand) [5]. In homogeneous catalytic systems, such as Ru-PtBuNPy’NPy [31], hydrogen activation is primarily determined by the electronic and geometric environment of the metal center and its coordinated ligands. Since the reactions over homogeneous catalysts usually rely on metal–ligand interactions, intramolecular electron transfer, or frustrated Lewis pairs (FLPs) to achieve heterolytic dissociation, H2 may form different complexes with the catalyst surface, which can be classified according to the types of metal and ligands, with a dihydrogen complex (M-H2), a dihydride complex (Hδ−-M-Hδ−), or a proton–hydride pair(M-Hδ−/B-Hδ+) [5,32]. Thus, heterolytic dissociation is usually observed over three types of homogenous catalysts, which are summarized in Table 1.
Though homogeneous catalysts provide well-defined active sites for heterolytic H2 dissociation, their limited stability and recyclability challenge industrial-scale application [31]. Importantly, the heterolytic dissociation of H2 is not exclusive to homogeneous catalysts; it has also been observed in a wide range of heterogeneous catalysts, such as supported palladium (Pd), gold (Au), nickel (Ni), platinum (Pt), In2O3, etc. [16,31,45]. Notably, heterogeneous catalysts offer superior structural and chemical properties, such as high crystallinity, abundant lattice oxygen, and excellent recyclability [7]. Unlike homogeneous systems, where the active site is dissolved in the solvent [31], heterogeneous catalysis occurs at solid surfaces, where reactants first diffuse and chemisorb onto the surface, undergo surface reactions, and are followed by the desorption of products [46].
Recent studies have extensively investigated the mechanism of heterolytic H2 dissociation over various supported heterogeneous catalysts, including supported metal catalysts and bulk metal oxides, as listed in Table 2. While some catalysts contain metal surfaces that inherently favor homolytic dissociation, heterolytic dissociation can still become the primary mechanism when metal centers interact with Lewis acidic or basic species, such as cationic metal sites, oxygen species, or oxygen vacancies on the support [5,7]. Such interactions subsequently result in polarizing and breaking the H-H bond asymmetrically. Thus, heterogeneous catalysts exhibiting key properties, including strong basicity, abundant lattice oxygen vacancies on the support, and single-atom metal sites, can effectively facilitate the polarization of H2 molecules and electron transfer, further promoting the formation of protons (Hδ+) and hydrides (Hδ−) at metal–support interfaces or lattice oxygen vacancies [7]. Interestingly, certain heterogeneous catalysts, particularly single-atom catalysts, can be metastable when exposed to gaseous H2. For example, Lu et al. demonstrated that hydrogen exposure at 300 K led to the heterolytic dissociation of H2 on a zeolite-supported Ir(C2H4) with direct evidence from IR and XAS [47]. However, this process was accompanied by the bond breaking between Ir and the zeolite support, resulting in the migration and aggregation of Ir species, as confirmed by STEM. Such surface reconstruction causes a short life cycle of catalysts and significant challenges in accurately identifying active sites and attributing observed intermediates to interpret the reaction mechanisms. To mitigate such destabilization, several effective approaches have been reported, including anchoring isolated metal atoms within the pores of nanoporous support or employing organic ligands to stabilize metal sites on nonporous supports. Nevertheless, compared with homogeneous catalysts, a heterogeneous catalyst system exhibits distinct mechanistic advantages, including stability, higher selectivity, and reactivity. Given these advantages, the following sections will focus on the characterization and experimental techniques used to investigate heterolytic H2 dissociation in heterogeneous catalytic systems.

3. Experimental Methods for Studying H2 Adsorption on Metal Oxides

In heterogeneous catalysis, hydrogenation reactions usually occur at accessible surfaces, where the structural and electronic properties of active sites, such as open metal sites and oxygen vacancies, contribute to the H2 dissociation [7]. However, active sites only occupy a small fraction of the catalyst total surface area and are well-dispersed, since selected active components are incorporated with the catalyst support through impregnation, precipitation, coprecipitation, adsorption, or ion exchange [66]. Meanwhile, the reactivity is influenced by the heterogeneity of the catalyst and the interaction between active components and the support. It is of great importance to understand how the coordination environment of surface metals and the nature of support interaction contribute to the heterolytic H2 dissociation on heterogeneous catalysts, which can further optimize reaction rates, fractional conversion, and selectivity. To achieve this, experimental investigations, including spectroscopy and kinetic analysis, can be applied to bridge atomic-scale characterization with macroscale catalytic performance and provide evidence to support theoretical studies. As shown in Figure 4, it illustrated the relative distribution of different methodologies used in heterolytic H2 dissociation research during the past five years. Recent studies kept employing a broad range of methodologies to investigate the surface changes and relevant reactions. As for spectroscopic techniques, they can provide real-time microscopic insights into the intrinsic properties of catalysts, which identify active sites and geometric and electronic surface structures. In addition, the surface reaction and kinetic study offers a quantitative analysis of reaction mechanisms and elementary steps to reveal the intrinsic reactivity of catalysts [67]. The combination of these techniques is essential for uncovering the fundamental principles of governing heterolytic H2 activation in heterogeneous catalysis. In the next few sections, we reviewed the key experimental methodologies that have been utilized to efficiently study hydrogen-involved reactions on heterogeneous catalysts. A detailed review of spectroscopic techniques, surface reaction probing, and kinetic analysis studies are provided in Section 3.1, Section 3.2, and Section 3.3, respectively.

3.1. Spectroscopic Techniques for Catalyst Characterization

The active site is a significant element in heterogeneous catalysis, which represents the active places on the support of a catalyst. To this end, it is obvious that hydrogenation and hydroprocessing reactions occur primarily on active sites, which allow reactants to chemisorb on and further dissociate, mitigate, and react to form products. Since heterogeneous catalysts exhibit diverse structures and compositions, the characterization of active sites, surface changes, and chemisorbed intermediates is essential for understanding the reaction mechanism and optimizing the catalyst activity [68]. A wide range of analytical techniques have been employed to characterize catalysts, most of which are generally classified according to the type of excitation applied, including photons, electrons, ions, electromagnetic fields, heat, or neutral species [67,69]. Given the complexity of heterogeneous catalysts, most techniques cannot directly observe the heterolytic or homolytic dissociation process. However, they can provide evidence of redox shifting, intermediates, and M-H formation and information on composition, crystallography, and oxidation state according to their probes, respectively [67].
To better understand their practical application and select suitable techniques in H2 studies, it is crucial to compare them in terms of spatial and temporal resolution, structure sensitivity, pressure tolerance, and the types of provided information. Techniques based on hard X-rays, such as XRD, XAS, XANES, and EXAFS, are widely applied for bulk structural and electronic characterization. In particular, XAS-related methods exhibit excellent elemental specificity and are applicable under a wide range of environmental conditions, including elevated temperatures and pressures. However, these X-ray techniques are inherently limited in spatial resolution since they primarily probe bulk properties rather than surface phenomena. Additionally, their relatively low time resolution hinders the capture of fast surface dynamics, making them less suitable for real-time, surface-sensitive studies of H2-involved reactions. In contrast, photoelectron-based techniques such as XPS and XPD are highly surface-sensitive and capable of probing the oxidation state and local chemical environment of surface species. However, ultrahigh vacuum conditions are typically required for operation due to the short mean free path of photoelectrons under a gas-phase environment, which limits their application under near-realistic conditions. Although the development of ambient-pressure XPS has partially addressed the pressure limitation, it still faces challenges in terms of temporal resolution and depth sensitivity. Vibrational spectroscopies, such as Raman and IR, are among the most accessible and cost-effective tools for detecting the adsorbed species and intermediates. They have been adapted for operando studies, which are compatible with a wide range of temperatures and moderate pressures in many studies. Meanwhile, operando IR spectroscopy exhibits moderate time resolution, allowing for real-time monitoring of surface reactions. Nevertheless, they suffer from low sensitivity. In particular, the biggest challenge lies in distinguishing between reactive intermediates and spectator species, such as contaminants and pre-existing species. Specifically, hydrides serve as a direct sign of H2 heterolytic dissociation. Thus, examining isolated surface hydrides through advanced spectroscopy has proven to be an effective way to gain direct evidence [69]. However, the detection of hydroxyl groups on a single-atom catalyst exposed to H2 may give rise to either intrinsic hydroxyls on metal oxide support or reaction intermediates, which complicates mechanistic interpretations. Therefore, a complete understanding of the sequence of H2 chemisorption, dissociation, surface reactions, and desorption cannot be achieved by any single spectroscopic technique. A multi-technique approach is often important for accurately identifying surface species, elucidating elementary steps, and revealing reaction pathways. A few reports have utilized a combination of several spectroscopic techniques to observe surface hydrogen species during the hydrogen dissociation process [6,20,64,70,71,72]. To this end, we summarized the widely utilized techniques, roles in H2-involved reactions, and corresponding studies in Table 3. It is worth noting that many other techniques are not listed in the table, which does not imply that they are unimportant or less often used. Generally, multiple spectroscopic techniques contribute together to determine the H2 dissociation pathway. Meanwhile, these results are often analyzed along with surface reactions and kinetic studies to elucidate elementary steps, rate-limiting steps, and the overall reaction mechanisms, which will be discussed in the next sections. In particular, Wu et al. employed a multi-spectroscopic approach, including inelastic neutron scattering (INS), ambient pressure X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and Raman spectroscopy, to examine heterolytic H2 dissociation over CeO2 [70]. According to INS, direct evidence of Ce-H formation was obtained by detecting hydrogen vibrations. XPS revealed that Ce oxidation states shifted, indicating charge transfer due to H2 activation. FT-IR and Raman spectroscopy identified surface chemistry and further distinguished the homolytic and heterolytic pathways. Their study demonstrated that the dissociation of H2 is highly dependent on the oxidation state of Ce and highlighted the role of oxygen vacancies in promoting heterolytic H2 dissociation. Furthermore, they illustrated the effectiveness of multi-spectroscopy approaches in elucidating hydrogen activation mechanisms on ceria-based catalysts.

3.2. Surface Reaction Probing: Temperature-Programmed Techniques

Understanding H2 adsorption, activation, and reaction mechanisms on heterogeneous catalysts requires direct experimental evidence of surface species. While spectroscopic techniques provide atomic-level insights into catalyst composition, crystal structure, surface chemistry, and electronic properties, they still face limitations in monitoring reaction dynamics and precisely quantifying the adsorbed/desorbed species. To overcome these challenges, the temperature-programed experiment (TPX) has been employed as a powerful tool for probing surface reactions, which offers a direct approach to studying catalytic reactivity.
In TPXs, reactants or titrants, such as H2, ammonia, or CO2, are pre-adsorbed onto the catalyst surface at a designed temperature, followed by controlled heating to induce reaction, reduction, or oxidation processes. The desorbed species are monitored in real time through mass spectrometry (MS), flame ionization detectors (FIDs), or thermal conductivity detectors (TCDs). TPXs generally include temperature-programmed oxidation (TPO), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperature-programmed surface reactions (TPSRs) [67]. Meanwhile, TPXs are usually employed along with other spectroscopy, such as FTIR and XPS, to provide deep insight into the surface reactions. The details of each technique and relevant studies are given in Table 4.
Among various TPX techniques, TPD plays a crucial role in quantifying the active sites by integrating the peaks in desorption profiles [82]. In particular, amine-based chemicals are widely used as titrants to quantify Lewis acid sites considered as the site of active H2. According to specific-site characterization, people can further obtain the turnover frequencies (TOFs), which represent the intrinsic catalytic activity normalized per active site. Furthermore, a direct analysis of catalyst efficiency can be obtained [67]. In addition, TPRs have been extensively employed in experimental investigations on heterolytic H2 dissociation, as listed in Table 4. Specifically, several elementary steps are involved within H2-TPRs over reducible oxides, including (i) H2 dissociation, (ii) conversion to form products, and (iii) diffusion of dissociated hydrogen species or oxygen vacancies [30]. Wu et al. investigated H2 heterolytic dissociation on CeO2 [70]. Except for previously discussed spectroscopic techniques utilized in this study, TPRs were also employed to probe the reducibility of CeO2 and the role of oxygen vacancies in facilitating H2 activation. Through analyzing the reduction in temperature and hydrogen consumption, they illustrated that the formation of oxygen vacancies was critical to H2 activation. Their work highlighted the significance of combining the spectroscopic results and surface reactions to provide a comprehensive analysis of H2 activation pathways.
Although TPO and TPSRs have been less frequently used in directly determining the mechanism of heterolytic H2 dissociation, they remain promising techniques that can quantify surface intermediates, track the oxidation–reduction process, and analyze elementary steps. In the future, it is worth applying them in various hydrogenation reactions and providing a more detailed and systematic understanding of H2 activation on heterogeneous catalysts.
Table 4. Summary of temperature-programmed techniques for investigating H2 dissociation on heterogeneous catalysts.
Table 4. Summary of temperature-programmed techniques for investigating H2 dissociation on heterogeneous catalysts.
TechniqueCommon ApplicationsCoupled MethodExample Studies
Temperature-Programmed Desorption (TPD)Titrate active sites with corresponding titrants; qualify the adsorption strengthICP-OES + CO2-TPD[48]
XPS + H2-TPD[48]
NH3-FTIR + NH3-TPD[83]
Temperature-Programmed Reduction (TPR)Quantify the reducible elements or sites in a material; qualify the interaction strengthin situ IR + in situ electron energy loss spectroscopy (EELS) + H2-TPR[54]
[H2S + H2]-TPD + H2-TPR[84]
H2-TPR[85]
in situ electron paramagnetic resonance (EPR) + H2-TPR[86]
Temperature-Programmed Oxidation (TPO)Investigate the oxidation state changesTPR-TPO cycles[87]
Temperature-Programmed Surface Reaction (TPSR)Monitor and quantify the consumption of reactants and the generation of productsmass spectroscopy (MS) + TPSR[88]
[O2 + CO + H2]-TPSR[89]

3.3. Kinetic Studies for Mechanistic Insights

While spectroscopic techniques and surface reaction studies provide significant insights into catalyst composition, surface properties, active site accessibility, and how hydrogen dissociates, kinetics studies are also critical to determine the mechanisms for hydrogen-involved reactions, where the chemisorption and dissociation of H2 (H2 → 2H*) involves rate-determining steps. For instance, Whittaker et al. indicated that H2 was rate-limiting and newly formed H2O was a strong inhibitor for the H2 oxidation reaction over a supported gold catalyst through a low-conversion kinetics study [64]. In this study, conversions, reaction orders, and reaction rates were measured and calculated with controlled gas pressure at the steady state. Meanwhile, conversions were usually required to be kept at a low level to obtain differential reaction conditions. The kinetic data revealed that H2 activation occurs at the metal–support interface through a heterolytic mechanism, which was supported by the strong inhibition of water molecules on active sites. Furthermore, the conventional homolytic model, where the rate was determined by homolytic H2 activation or the step of adsorbed metal–H reacting with adsorbed O species, was inconsistent with the reaction rate.
However, the traditional steady-state kinetic study is not sufficient to provide critical mechanistic details when large reservoirs of intermediates chemisorb on the catalyst surface and slowly transform into products [90]. The elementary steps proceed in sequence at the same rate, leading to less insight into the complex reaction mechanism. To address this limitation, transient kinetic analysis becomes a useful tool to clarify the complex mechanism of different reactions, for instance, NOx reduction [90], CO hydrogenation [91,92], and methanol synthesis [63,93]. Transient flow reactors are commonly employed to introduce the reactant as pulses, which probe unsteady-state dynamics and perform step changes in the concentration or types of reactants. Generally, a small and controlled concentration of reactants is injected as the pulse transient reaction. In the meantime, transition states between two steady states are tracked. It is worth noting that the duration of injections should be limited to 1–100s, and the quantity of reactants should be minor to avoid intensive reactions and thus eliminate temperature fluctuations from exothermic adsorption [94].
Among various types of transient experiments, steady-state transient kinetic analysis (SSITKA) has gained great interest recently in heterogeneous catalysis [95,96]. The schematic of the SSITKA experimental apparatus is shown in Figure 5. It can provide valuable kinetic parameters of intermediates, such as concentration, rate constants, site coverage, surface residence time, and turnover frequency (TOF). Meanwhile, it has been applied in a few hydrogen-involved reactions [53,90,97,98,99]. For example, Nelson and Szanyi reported that the combination of SSITKA and operando IR spectroscopy enabled them to correlate the mean lifetime and surface coverage of CO intermediates in the water–gas shift (WGS) reaction over Pt/CeO2-Al2O3. The reaction was switched between 12CO2 and 13CO2. It was indicated that heterolytic H2 activation at metal–support interfaces was highly related to the formation and reactivity of key intermediates such as carboxyl and formate species [53].
Beyond SSITKA, other advanced transient techniques, such as a temporal analysis of products (TAP) [63,99,100,101], have also shown potential in elucidating complex catalytic mechanisms and surface dynamics over heterogeneous catalysts. In addition, integrating kinetic analysis with spectroscopy studies and microkinetic modeling is a promising approach to combine experimental observations with theoretical predictions, which can generate a more convincing analysis of hydrogen dissociation pathways.

4. Challenges and Future Perspectives

Heterolytic hydrogen dissociation is a highly selective pathway in hydrogen-involved reactions, especially for activating polar functional groups such as C=O and C=N. Over the past decade, the scientific understanding of this pathway has significantly advanced, supported by well-defined catalytic models and the development of various experimental techniques to study the reactions from different perspectives. As discussed in the above sections, spectroscopies and surface reaction probing can study the reactions from a microscopic view and macroscopic view, respectively.
Despite recent advances, several key challenges continue to limit a comprehensive mechanistic understanding of hydrogen dissociation and its broader application in real-world catalytic systems. To achieve promising progress in this field, these fundamental challenges need more attention in the future. First, one of the critical issues is the relatively high energy barrier associated with heterolytic H2 dissociation (typically >0.3 eV), which inhibits the efficient kinetics of reactions. In contrast, homolytic dissociation on metal clusters is often barrierless, making it a more favorable pathway under mild conditions. To address this challenge, the development of catalysts that can stabilize the proton–hydride pairs and facilitate hydrogen polarization is crucial for expanding the scope of selective hydrogenation reactions and enabling the reaction to proceed under milder conditions. Thereby, operando characterization becomes more feasible, as many spectroscopic instruments struggle to operate under extremely harsh conditions. Recent studies have reported that certain single-atom catalysts, such as single-site Pd-C3N4, atomically dispersed Pd-TiO2, and Pt/FeOx, exhibit competitive turnover frequencies (TOFs) in hydrogenation reactions compared with metal clusters [102,103,104]. A more in-depth experimental and mechanistic investigation of heterolytic dissociation on single-atom catalysts could provide valuable insights to enable the development of catalysts with enhanced activity, yield, and selectivity.
In addition, the other major challenges lie in the complex surface, composition, and structure of heterogeneous catalysts, where both homolytic and heterolytic pathways coexist and compete. It is highly possible to observe both homolytic and heterolytic dissociation on single-atom catalysts [45]. Additionally, the catalyst surface itself may not remain static during the reaction. As discussed in the previous section, especially for single-atom catalysts, the nature of the coordination structure and the distribution can be altered when H2 dissociation occurs. Such surface reconstruction introduces further complexity in assigning the true origin of observed intermediates and identifying the heterolytic pathway. Although IR and INS can detect the chemisorbed hydroxyl groups and hydride species appearing during the reaction, it is insufficient to confirm the heterolytic H2 cleavage due to the existence of hydroxyl groups on the metal oxide support. The reason is that hydroxyl groups and hydrides may also arise from alternative mechanisms or the exchange between hydrogen species and chemisorbed -OH on the metal oxide-supported catalyst. To overcome these limitations on identifying intermediates, the most straightforward method is employing complementary techniques, which can quantify the hydrogen species with distinct chemical identities. However, instead of employing a single technique, it is interesting to consider the integration of multiple spectroscopic, X-ray, and imaging tools, which can simultaneously provide a more comprehensive view of the catalytic surface changes and chemisorbed intermediates or products. Recent progress in commercial operando systems has made such studies accessible. For example, with a Thermo Scientific Nicolet is50 FTIR spectrometer combined with an infrared camera, scientists obtained real-time thermos emissions and spectral absorbance [105]. However, technical and cost limitations still hinder widespread adoption. To this end, self-built systems tailored to specific catalytic systems are encouraged in the field. For example, Leif R. Knçpke and co-workers employed a combination of ATR-FTIR, Raman, and UV/Vis-DRS to simultaneously monitor substrate conversion, product formation, and the nature of solid catalysts during the hydrogenation of imines [106]. The combination of multiple techniques enables the characterization of the sample at an identical position during the reaction and avoids discrepancies among different measurements. Therefore, it is promising to provide effort in home-built systems to allow for greater flexibility, such as temperature and pressure, in experimental design.
Beyond the integration of multiple techniques to observe the catalyst and reactions, it is essential to capture the rapid and dynamic transformations of catalytic surfaces under operando conditions. Catalytic reactions often proceed through chemisorption, intermediate formation, surface reactions, and the desorption of products. These sequential steps span a wide range of timescales, from microseconds to hours. Without sufficient resolution, surface intermediates and structural transitions may be missed, resulting in a misinterpretation of mechanisms. Yang et al. presented a notable example by developing a transient kinetic analysis system integrated with in situ FTIR, capable of operating across a broad temperature range (200–770 K) and moderate pressure range (1–2.5 atm). The FTIR- SSITKA have been applied for CO2 isotope-switching experiments at 75 °C and enabling the detection of chemisorbed carbonate species with a surface residence time of approximately 148 s. This system is a role model which demonstrates the potential of combining time-resolved spectroscopy with transient analysis [107].
In summary, techniques applied in the hydrogen-involved reactions must not only endure realistic temperatures and pressures but also be able to monitor rapid surface dynamics. High-temperature and high-pressure reactors have been well-developed to simulate industrial applications and provide the macroscopic perspective of reactions but lack access to the detection of surface chemistry. To this end, it is essential to integrate characterization tools, which can be operated under realistic conditions. Prerequisites for other characterization instruments must be fulfilled [108].
  • The instrument must withstand elevated temperature, pressures, and a hydrogen-rich environment.
  • The instrument or the group of instruments can identify the active intermediates and substrates.
  • The characterization tools have enough time resolution to capture the formation of transient intermediates or surface changes.
  • The combination of these real-time or operando techniques and transient analysis is crucial to building meaningful correlations between surface chemistry and catalytic kinetics. Thus, it can draw a complete mechanism for the reaction.

5. Conclusions

This review provides an integrative summary of current widely applied experimental methodologies for studying hydrogen dissociation, with a particular emphasis on those relevant to heterolytic dissociation. Instead of simply listing the techniques, we aim to clarify how each method contributes to mechanistic implications and emphasize their roles in revealing the reaction pathways. This summary serves not only as a technical reference but also as a practical entry source for new researchers entering this field. Furthermore, it is crucial to apply cross-disciplinary strategies, particularly combining surface science, spectroscopy, kinetics, and theoretical study to gain a deeper understanding of heterolytic hydrogen activation and guide the rational design of next-generation catalytic systems.
Beyond understanding the reaction pathway, bringing experimental insights together with practical catalytic performance remains a significant challenge for industrial applications. Traditional industrial hydrogenation catalysts, such as Cu- or Ni-based systems, often suffer from limited selectivity and deactivation after prolonged operation. As a result of these limitations, extensive effort has been applied to develop novel heterogeneous catalysts with improved activity and selectivity in the lab. Among them, single-atom catalysts exhibit excellent activity and selectivity under controlled conditions. To better understand and further enhance the performance of novel catalysts, experimental findings from operando studies reveal the mechanism from the molecular level and provide insights into the directions of optimizing catalysts. These molecular-level observations not only advance the understanding of catalytic mechanisms but also pave the way for designing large-scale, robust, recyclable catalytic systems suitable for industrial use, such as CO2 capture and utilization, oil refining, and amine synthesis.

Author Contributions

Conceptualization, S.W. and X.L.; methodology, S.W.; validation, S.W., X.L. and B.L.; formal analysis, S.W.; investigation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, X.L.; visualization, S.W. and X.L.; supervision, B.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This work was conducted independently of any ongoing research projects in the authors’ current institution.

Conflicts of Interest

The authors declare there are no known conflicts of interest or personal relationships that could have influenced the work presented in this paper.

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Figure 1. Current popular H2 production methods and applications. Content supported by Ref. [4].
Figure 1. Current popular H2 production methods and applications. Content supported by Ref. [4].
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Figure 2. The schematic of different interactions between H2 and the solid surface.
Figure 2. The schematic of different interactions between H2 and the solid surface.
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Figure 4. Relative abundance of studies with various experimental techniques for heterolytic H2 dissociation (2020–2024).
Figure 4. Relative abundance of studies with various experimental techniques for heterolytic H2 dissociation (2020–2024).
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Figure 5. Schematic diagram of a steady-state transient kinetic analysis reactor. R and *R represent a pair of isotopes. The design and principle are supported by Refs. [93,99].
Figure 5. Schematic diagram of a steady-state transient kinetic analysis reactor. R and *R represent a pair of isotopes. The design and principle are supported by Refs. [93,99].
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Table 1. Classification of homogeneous catalysts for heterolytic dissociation of H2.
Table 1. Classification of homogeneous catalysts for heterolytic dissociation of H2.
CatalystFeaturesApplicationExamples
Ionic hydrogenation catalyst Metal serves as a hydride acceptor with a base as a proton acceptorHydrogenation of chemicals containing polar bonds, such as C=O, C=N, etc.HMo-(CO)3(C2H5) [33]
[Mo(Cp)(CO3)]H] [34,35]
[W(Cp)(CO)3H)] [34,36]
[W(CP)(CO)2(PR3)(O=CEt2)]+ [35]
Bifunctional catalystContains a proton acceptor (N or O) and a hydride acceptor (Ru, Ir, Fe) Hydrogenation of CO2 to form methanol under mild conditionsRu-PtBuNPyNPyNEt [31,37]
Ru-MACHO-BH [31,38]
Ru-bisPN [31,39]
Mn-PtBuNPy’NPy/Mn-PtBuNPyNtBu [31,40]
Mn-PiPrNPiPr [31,41]
Fe-PiPrNPiPr [31,42]
Frustrated Lewis pairs catalyst Contains bulky Lewis acids and bases, which hinder full interaction with each other, enabling the activity and cooperative H2 activation Metal-free hydrogenation reactions;
CO2 activation;
reduction in ketones		tBu3P/B(C6F5)3 FLP [43]
(Me3Si)3P-CO2-B(p-C6F4H)3 [43]
1,1-bis-(C6F5)2BOB(C6F-)2 [43,44]
Table 2. Heterogeneous catalysts involved with heterolytic H2 dissociation.
Table 2. Heterogeneous catalysts involved with heterolytic H2 dissociation.
Catalyst SystemAdditives/SubstratesTarget ReactionsRef.
Ru/MgOAromaticsHydrogenation[48,49]
Ru (III)/γ-Al2O3Unsaturated hydrocarbonsSelective hydrogenation[50]
Pd/SiO2H2 + O2 (with solvents)Direct H2O synthesis[51]
Fe3O4 (001)-supported single Pd atomsH2 + H2OH2 dissociation[52]
Pd-/nitrogen-doped carbonAromatic carbonyl compoundsSelective deoxygenation of carbonyl groups[29]
Pd/Al2O3; Pd/CeO2; Pd/CeO2-Al2O3CO2 + H2Reverse water–gas shift (rWGS)[53]
Pt/CeO2H2H2 activation[54]
Ni (II)@ChabaziteAcetylene + H2Selective hydrogenation[55]
Ni/ZnO (>500 °C)Fatty acid ester + H2Hydrogenation to fatty alcohols[56,57]
Re/In2O3CO2 + H2Methanol synthesis[58]
In2O3/ZrO2CO2 + H2Methanol synthesis[59,60]
Au/ZrO2ButadieneSelective hydrogenation[61,62]
Au/ZnOCO2 + H2Methanol synthesis[63]
Au/TiO2;
Au/Al2O3		H2 + O2, H2OH2 oxidation[64]
Cu/Al2O3Glycerol + H2Glycerol hydrogenolysis to 1,2-propanediol[65]
Table 3. Spectroscopic techniques for investigating H2 dissociation on heterogeneous catalysts.
Table 3. Spectroscopic techniques for investigating H2 dissociation on heterogeneous catalysts.
TechniqueType of ProbeDetected InformationRole in H2 Dissociation StudiesRef.
X-ray Photoelectron Spectroscopy
(XPS)		X-ray photons (photons)Surface elemental composition and oxidation stateInvestigate the interaction between H2 and solid surface; monitor changes in oxidation state[73]
X-ray Absorption Spectroscopy
(XAS/XANES/EXAFS)		X-ray photons (photons)Bulk oxidation states of metal or metal ions; coordination environmentsTracks metal–support interactions and redox/structure changes during H2 activation[74]
Infrared Spectroscopy
(IR)		Infrared light (photons)Vibrational modes of surface-chemisorbed speciesDifferentiates between homolytic/heterolytic pathways based on intermediates according to surface chemistry[54,64,75]
Raman SpectroscopyMonochromatic laser
(photons)		Vibration of metal oxides; organic residualsA great complement to IR and EPR, which can provide insights into formed complexes[76]
Inelastic Neutron Scattering
(INS)		Beams of neutrons (neutrons)Surface M–H species or H-H speciesIdentify chemical nature of hydrogen content and quantify surface hydrogen species and bulk hydrides[77]
1H Nuclear Magnetic Resonance Spectroscopy
(1H NMR)		Strong magnetic field (field)Hydrogen bonding, diffusion, surface hydridesInvestigate formed chemisorbed hydrogen species during reduction[78,79]
Electron Paramagnetic Resonance
(EPR)		Magnetic field (field)Radicals, transition metal states, defectsDetermine whether heterolytically dissociated H+/H− are exhibited[71]
UV-Vis Diffuse ReflectanceUltraviolet or visible light (photons)d-d transitions of metal ionsMonitor changes in metal centers during reduction[74,80]
Inductively Coupled Plasma Optical Emission Spectroscopy/Mass Spectrometry
(ICP-OES/MS)		Ions or charged particles (ions)Analyze composition of catalystQuantify dispersed metal component in catalysts[81]
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Wang, S.; Lou, X.; Liu, B. Probing Heterolytic H2 Dissociation on Heterogeneous Catalysts: A Brief Review of Experimental Strategies. Processes 2025, 13, 1465. https://doi.org/10.3390/pr13051465

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Wang S, Lou X, Liu B. Probing Heterolytic H2 Dissociation on Heterogeneous Catalysts: A Brief Review of Experimental Strategies. Processes. 2025; 13(5):1465. https://doi.org/10.3390/pr13051465

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Wang, Siwen, Xuanqing Lou, and Bowei Liu. 2025. "Probing Heterolytic H2 Dissociation on Heterogeneous Catalysts: A Brief Review of Experimental Strategies" Processes 13, no. 5: 1465. https://doi.org/10.3390/pr13051465

APA Style

Wang, S., Lou, X., & Liu, B. (2025). Probing Heterolytic H2 Dissociation on Heterogeneous Catalysts: A Brief Review of Experimental Strategies. Processes, 13(5), 1465. https://doi.org/10.3390/pr13051465

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