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Review

Advances in In Situ Investigations of Heterogeneous Catalytic Ammonia Synthesis

by
Weiyi Su
1,2,
Xi Cheng
2,
Suokun Shang
2,3,
Runze Pan
2,3,
Miao Qi
2,
Qinqin Sang
4,
Zhen Xie
4,
Honghua Zhang
4,
Ke Wang
2,4,* and
Yanrong Liu
2,3,4,*
1
Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450003, China
2
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Mesoscience and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
3
School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
4
Longzihu New Energy Laboratory, Zhengzhou Institute of Emerging Industrial Technology, Henan Univesity, Zhengzhou 450000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(2), 160; https://doi.org/10.3390/catal15020160
Submission received: 5 January 2025 / Revised: 29 January 2025 / Accepted: 7 February 2025 / Published: 9 February 2025
(This article belongs to the Special Issue Applications of Heterogeneous Catalysts in Green Chemistry, 2nd Edition)
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Abstract

:
Ammonia is a key “platform” raw chemical for fertilizers and nitrogen-containing chemicals, with a global annual production of ~180 million tons. Recently, ammonia has also come to be seen as an excellent hydrogen-containing liquid promising for long-term, large-scale hydrogen storage and transport. Therefore, artificial N2 fixation, an ammonia synthesis reaction, will play a pivotal role influencing food and energy for human society. Till now, industrial ammonia synthesis has relied on high temperature and high pressure (420~500 °C, 10~15 MPa). Researchers are devoted to developing new catalysts as well as optimizing the traditional Fe-based catalysts continuously. However, the relation between the catalysts’ detailed structure and ammonia production efficiency are not yet fully understood, which is crucial to provide guidance on further improving the efficacy of this importance reaction. Recently, in situ characterization techniques have achieved significant improvements and new understandings have been achieved on the central topic of catalysis. In this review, recent advances in in situ investigations of heterogeneous catalytic ammonia synthesis are summarized and the key results are discussed. In the end, a concluding remark and perspective are proposed, with the hope of inspiring future investigations dedicated to unveiling the principles of designing catalysts for ammonia synthesis.

1. Introduction

Ammonia (NH3) plays a crucial role in various sectors of the contemporary chemical industry, being essential for the manufacture of fertilizers, pharmaceuticals, explosives, and various nitrogen-based compounds [1,2,3,4,5,6]. Moreover, NH3 is seen as an attractive carbon-free medium for the extensive utilization and long-term deposition of intermittent renewable energy. Among liquid hydrogen and other liquid hydrogen carriers like methanol, toluene, and cyclohexane, NH3 stands out as a favored option due to its impressive energy density (4.32 kWh L−1), significant hydrogen content by weight (17.6 wt%), and substantial hydrogen storage capacity by volume (121 kg m−3) [7,8,9,10,11,12]. The use of liquid NH3 is poised to mitigate the high expenses associated with hydrogen transportation, thanks to its established and secure storage and conveyance methods. Currently, iron-based catalysts are predominantly employed in the industrial Haber–Bosch (H-B) process, which operates under harsh conditions of high temperature (420–500 °C) and high pressure (10–15 MPa) [13,14,15,16,17]. Since the 1970s, Ru was found to outperform Fe-based catalysts by an order of magnitude and deemed as a new kind of ammonia synthesis catalyst. It was proven that replacing the third catalyst bed of Fe catalysts with Ru/AC would significantly improve the overall net value of NH3 (the NH3 concentration difference between outlet and inlet converter). The worldwide need for ammonia has led to the Haber–Bosch process producing approximately ~180 million tons of NH3 annually, which consumes roughly 1% to 2% of the global energy output each year. The reliance of this reaction process on coal has led to emissions of about 4.2 tons of CO2 for every ton of NH3 produced. Producing ‘green ammonia’ is crucial not only for ensuring sustainable fertilizer manufacturing, but also for realizing ammonia’s potential as a zero-carbon fuel and a storage medium for hydrogen. As water electrolysis technology, powered by renewable electricity, becomes more mature, integrating the green hydrogen with the H–B process could address the carbon emission issue in ammonia synthesis [18,19,20]. However, the high temperatures and pressures required by the H–B process are not aligned with the conditions of the electrolytic hydrogen production system, resulting in high energy consumption for integration and limited adaptability. Consequently, there is an acute need to design and create a new, more efficient process for ammonia synthesis that operates at lower temperatures/pressures, and maintains a higher net value of NH3, which necessitates innovative developments in catalyst technology.
In the classic view of heterogeneous ammonia synthesis, this reaction initializes with N≡N bond breaking of adsorbed N2* at the C7 and B5 sites for Fe and Ru, respectively, which was proven by Gerhard Ertl with surface science experiments, for which he was awarded a Nobel prize in 2007 [21]. Additionally, a significantly successful theoretical model has been established by Nørskov and co-workers known as the “scaling relation-volcano plot for ammonia catalyst”. By plotting the theoretical kinetic rate (derived from transition-state energy EN-N) against EN (N adsorption energy), this single-descriptor (EN) model accurately explained why Ru and Fe are the highest active catalysts for ammonia synthesis. Optimizing the adsorption of N is the key to achieving a high catalytic ammonia synthesis performance [22]. However, due to the limitation of “scaling relation” (EN-N and EN are linearly correlated over different metals/active sites of metals), it seems that the conventional catalyst design of active metal supported by high-surface-area supports has closely approached its limit after a century’s optimization. Therefore, researchers are working on novel designs that would “break the scaling relation” to find an ammonia synthesis catalyst that can efficiently work at “milder conditions”.
Recently, many new catalysts have been proposed that work well at low pressure and low temperature; however, the relation between the structure of these new catalysts and the corresponding performance is much less understood. In industrial ammonia synthesis conditions of high temperature and high pressure, the coverage of N* is significantly different from surface science experiments and the dynamic surface nitridation/reconstruction is more prone to happen. The roles of surface meta-stable phases and the complex promoter electronic/structural effects are far from fully elucidated. The long-debated “pressure gap” and “material gap” are under more active discussion than ever before. During last decade, in situ characterizations have undergone a significant improvement with respect to sensitivity, accuracy, and adaptability to reaction condition. Many cutting-edge techniques have been adopted to unveil the mechanism underpinning the heterogeneous catalytic NH3 synthesis. In the case of mechanism elucidation, new findings, including the associative mechanisms that predominate over the sub-nanometer Ru cluster and MvK mechanisms over nitride/hydride catalysts, have significantly emerged in literature [23,24]. For the catalyst structure and evolution under reduction condition, new insights have been reported for both traditional Fe/Ru catalysts as well as the new catalysts, especially on how the promoters participate in electronic and structural evolution. New catalysts, such as the hydrides, have been proven to outperform Ru in low-temperature and low-pressure conditions [25], although their surface structure evolution and true active sites have not been fully understood. In situ investigation of heterogeneous catalytic ammonia synthesis is one of the most active research fields in the catalysis community.
This article provides an overview of the latest developments in the field of in situ studies related to ammonia synthesis. The important results are highlighted and discussed. It is hoped that this study provides an important reference for future research on the mechanism of synthetic ammonia and the design of new catalysts.

2. Reaction Mechanisms of Ammonia Synthesis

The core challenge of NH3 synthesis is how to effectively activate N2 and achieve efficient conversion. At present, the NH3 synthesis mechanisms proposed in the field of heterogeneous catalysis mainly include associative (Figure 1a,b), dissociative (Figure 1c) and Mars van Krevelen (MvK) mechanisms (Figure 1d) [23,24,25].
The main feature of the associative pathway is the direct hydrogenation of adsorbed N2 molecules without the dissociation of the N≡N bond. The associative pathway can be further subdivided into two forms in terms of the order of adding hydrogen to the N2* [26,27,28]. In the distal mechanism, N2 initially adheres to the catalyst surface with the N≡N triple bond intact. The subsequent hydrogenation first happens on one of the N atoms. Once this N atom has been completely hydrogenated, a NH3 is then released. Then, the other N atom continues to hydrogenate until another NH3 is formed and desorbed. The alternative mechanism is similar except that the hydrogenation is carried out alternatively between two N atoms. In these two associative mechanisms, with the progress of hydrogenation, the strength of the N≡N bond continuously weakens, and its fracture becomes easier. Therefore, the associative mechanism has a lower kinetic energy barrier than the dissociative pathway and is more feasible at lower temperature. The dissociative pathway initiates with dissociative N2 adsorption. The N≡N bond in the N2 molecule breaks and dissociates into two independently adsorbed N atoms. Each N atom undergoes a hydrogenation process, and finally converts into NH3 and desorbs from the surface [29,30,31]. The MvK mechanism is also an important pathway for ammonia synthesis [32]. This reaction pathway is dependent on the formation and replenishment of N vacancies and is therefore common on the surface of nitride catalysts. First, the lattice N of the catalyst binds with the dissociated hydrogen atom at the interface of the supported metal to form NH3 and is desorbed from the surface to form the lattice N vacancy. N2 is then adsorbed at the N vacancy, and single-ended hydrogenation forms NH3 and desorption. Another N atom achieves the replenishment of the lattice N vacancy, completing the catalytic cycle. The preferential adsorption of N2 by nitrogen vacancy in the MvK mechanism breaks the scaling relation limitation of the traditional metal surface catalysis and ensures a wider design space for new catalyst.
In summary, the pathways of ammonia synthesis are relatively complex, and the reaction mechanism is closely related to the fine structure of the catalysts. On the other hand, ammonia synthesis reacts in a hydrogen-rich, high-pressure environment, and the produced NH3 contains lone pair electrons and has high reactivity. As a result, the catalyst’s surface is susceptible to dynamic reconstruction under the reaction conditions, and the actual reaction mechanism is different from that of the fresh catalyst. In situ characterization of spatial and temporal resolution is a necessity to further understand the real microscopic reaction mechanism of ammonia synthesis, clarifying the structural evolution, active site and key reaction intermediates of the catalyst under reaction conditions. Gaining a comprehensive insight into the correlation between catalytic activity and catalyst structure is crucial for the development of novel, high-efficiency catalysts for ammonia production at a milder operation condition.

3. Recent Advances on In Situ Investigations

3.1. In Situ Diffuse Reflectance Fourier Transform Infrared Spectroscopy

In situ diffuse reflectance Fourier transform infrared spectroscopy (In situ DRIFTS) is a kind of in situ characterization technique that plays a pivotal role in heterogeneous catalysis research. The intermediates adsorbed on the catalyst surface could be qualitatively and quantitatively investigated by the vibrational frequencies of molecular motifs. More importantly, the evolution process of adsorbed species on the surface could be studied by time-resolved spectroscopy to deduce the reaction kinetics. Another important analysis method is to use CO as a molecular probe. Analyzing the CO* vibration frequency is powerful enough to unveil the electronic structure as well as the exposed active sites of the catalyst. Important advances and results with respect to the above two applications are summarized below.

3.1.1. Detection of Reaction Intermediates

The dissociative mechanism is the most widely accepted reaction mechanism of ammonia synthesis. The initial step of the reaction is the N≡N bond breaking, and the high activation energy barrier is the main reason for the high reaction temperature. Cai et al. [33] developed a range of Ru/ZSM-5 catalysts varying in pore size distribution. Their study revealed that the Ru/ZSM-5 with microporous structure exhibited an outstanding rate of ammonia synthesis, significantly outperforming the Ru/ZSM-5 with a macroporous or mesoporous structure. In situ DRIFTS was conducted on the Ru/ZSM-5 catalysts with varying pore sizes under a 25% N2-75% H2 atmosphere. The peak shapes for the bands at 1049, 1154 and 1288 cm−1 were associated with the symmetric vibration of ammonia. The in situ DRIFTS analyses confirmed that ammonia synthesis on Ru/ZSM-5 catalysts with different pore sizes all proceeded via a dissociative pathway. The NH2 * was found as the primary intermediate on Ru/ZSM-5 and no N2Hx* species was observed, which indicated that the rate-determining step is the dissociative activation of nitrogen.
The association reaction mechanism reduces the N-N bond breaking energy barrier by N2 hydrogenation, which is one of the potential reaction mechanisms for low-temperature and high-efficiency ammonia synthesis. Li et al. [34] examined the surface spectra of the Ru/LaN/ZrH2 catalyst at temperatures of 300~400 °C under a 25% N2-75% D2 atmosphere, with the objective of identifying the key intermediates during ammonia synthesis. The findings are depicted in Figure 2a–c. This study revealed that the spectral peak at 2389 cm−1 corresponded to the characteristic vibrational frequency of N2D2 *, and this peak’s intensity progressively rose with increasing temperature, reaching a maximum at 350 °C and dropping after further heating. This observation suggested that a relatively low temperature facilitated the formation of the N2D2* intermediate (2362 cm−1), the key evidence supporting an associative pathway. As the temperature continued to rise to 400 °C and above, this peak almost completely disappeared, indicating that the reaction might occur through another reaction mechanism or the kinetic lifetime of N2D2 * became extremely short. These results showed that N2 did not dissociate directly on the Ru/ZrH2 catalyst promoted by LaN under mild conditions but through the associative pathway. It was also proven that a small amount of lanthanum nitride (LaN) interacted synergistically with the ruthenium (Ru) active sites, enhancing the activation and hydrogenation of N2. Nonetheless, an overabundance of LaN led to the obstruction of Ru sites, impeding the formation of N2H2 * and diminishing the catalyst’s efficacy. By means of the associative pathway, LaN-modified Ru/ZrH2 catalysts circumvented the impediment of direct N2 dissociation, thereby enabling ammonia synthesis under moderate conditions.
Zhou et al. described the creation of a ruthenium nanoparticle (NP) catalyst, which included subnanometer Ru clusters (0.8 nm) supported on hollow nitrogen-doped carbon spheres [35]. In situ DRIFTS experiments were carried out in a 25% N2-75% D2 environment, detecting the N-D stretching vibration of the N2Dx* intermediate at 2321 cm−1. The intensity of this absorption peak significantly decreased with increasing temperature, indicating that elevated temperature facilitated the conversion of *N2Dx. It was believed that the NH3 synthesis over subnanometer Ru NPs followed an associative N2 activation route and N2H4* species are the major intermediates, which bypassed the direct dissociation of the N≡N triple bond with a high-energy barrier and thereby enabled facile conversion of N2 to NH3 under mild conditions. Additionally, Wang et al. [36] developed a collection of Ru-based catalysts with controlled particle sizes through a colloidal deposition method, facilitating an extensive exploration of the impact of size on the ammonia synthesis process. To delve into the activation of N2 molecules, in situ DRIFTS was applied to identify crucial intermediates on the Ru surface. The use of 1.4 nm Ru NPs led to the generation of N2Dx* intermediates, suggesting that N2 activation occurs via an associative pathway. The intensity of the N2Dx* species peaks intensified up to 350 °C but waned at 400 °C, which could be due to the decomposition of N2Dx* on the catalyst surface or its conversion into ND3 as the end product at higher temperatures. In comparison to the 5.0 nm Ru NPs, which exhibited much weaker N2Dx* signals, the 1.4 nm Ru NPs, abundant in corner sites, enhanced the activation of the N2 molecule, thereby yielding superior performance in ammonia synthesis.
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Figure 2. In situ DRIFTS results of (a–c) Ru/LaN/ZrH2 at 300 °C, 350 °C and 400 °C after exposure to 25% N2-75% D2. Reproduced with permission from ref. [34], copyright 2020 Elsevier Ltd. In situ DRIFTS results of (d) Ru/ZrH2, (e) Ru/ZrN and (f) Ru/ZrO2 after exposure to 25% N2-75%D2 at different temperatures. Reproduced with permission from ref. [37]. Copyright 2022 American Chemical Society.
Figure 2. In situ DRIFTS results of (a–c) Ru/LaN/ZrH2 at 300 °C, 350 °C and 400 °C after exposure to 25% N2-75% D2. Reproduced with permission from ref. [34], copyright 2020 Elsevier Ltd. In situ DRIFTS results of (d) Ru/ZrH2, (e) Ru/ZrN and (f) Ru/ZrO2 after exposure to 25% N2-75%D2 at different temperatures. Reproduced with permission from ref. [37]. Copyright 2022 American Chemical Society.
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Metal–anion interaction is widely acknowledged in supported metal catalysts, significantly regulating metal sites and reaction performance. Zhou and coworkers [37] reported a strategy to control the intensity of the interaction between ruthenium and the anion by changing supporting anion from hydrogen (ZrH2) to nitrogen (ZrN) and oxygen (ZrO2). In situ DRIFTS studies on Ru/ZrH2, Ru/ZrN, and Ru/ZrO2 were carried out to investigate surface intermediates formed during N2 molecule activation (Figure 2d–f). These results revealed that on the Ru/ZrH2 catalyst, N2 gradually undergoes hydrogenation via an associative pathway to form the N2H* species, rather than through a dissociative pathway. Aslan and colleagues [38] employed in situ DRIFTS technology to monitor the N2 hydrogenation over 1 wt% Ru/Vulcan under ambient conditions. The absorption signals detected at 3017 and 1302 cm−1 were linked to the formation of NHx* intermediates, as well as the asymmetric stretching and bending vibrations of gaseous ammonia at 3381 and 1650 cm−1. Due to the buildup of NHx* species on the catalyst surface, the intensity of the related peaks intensified with the ongoing cycles of hydrogen, argon and nitrogen gas at ambient temperature and pressure. Zhang and colleagues [39] carried out in situ DRIFTS studies on N2 adsorption onto Ru/ACC, Ba/Ru/ACC, Ce/Ru/ACC and Ba/Ce/Ru/ACC catalysts to examine the influence of electronic interactions among Ba/Ce promoters, and Ru clusters on the activation of the N≡N triple bond. The detection of N2Dx* intermediates on Ru indicated that N2 underwent a stepwise hydrogenation process via an associative pathway to produce NH3 over both Ru/ACC and Ce/Ba-promoted Ru/ACC. The stronger observed peaks of N2Dx* indicated that the N2 associative pathway for NH3 synthesis on Ru/ACC was accelerated by the electronic interactions between Ru atomic clusters and Ba/Ce.
For NH3 synthesis over a complicated catalyst surface, both dissociative and associative mechanisms may coexist. Zhang and colleagues [40] explored the impacts of alloying Ru with rare earth metals (La or Y), featuring a novel type of active sites, on the catalytic performance of NH3 synthesis. To comprehensively elucidate the enhancement imparted by the Ru-M alloy on the adsorption and activation of N2 and H2 in the process of ammonia synthesis, in situ DRIFTS technology was performed under 400 °C with a 25% N2-75% D2 feed to distinguish the surface N-containing intermediates after N2 activation. The band at 2377 cm−1 was associated with the N-D torsional vibration of N2D4*. The intensity of the 2377 cm−1 peak increased significantly as the exposure time was prolonged from 1 to 30 min. The formation of Ru-La or Ru-Y alloys enhanced the electron donation from rare earth elements to the Ru center, thus promoting ammonia production via the dissociative mechanism of N2. Meanwhile, the alloyed Ru-M inhibited the sintering of Ru NPs, maintaining their diminutive dimensions and securing an adequate display of terrace active centers. As a result, this facilitated the activation of nitrogen on the Ru-La sites and step sites through an associative mechanism. The activation of N2 on the Ru-M catalysts proceeded through the synergistic effect of both dissociative and associative pathways, resulting in outstanding NH3 synthesis performance.

3.1.2. CO Molecular Probe Adsorption to Determine the Electronic Structures of Catalysts

Utilizing CO as a molecular probe for DRIFTS (CO-DRIFTS) constitutes a reliable in situ method for pinpointing the accessible active sites, determining their quantity and elucidating the electronic configurations of these sites in catalysts for ammonia synthesis. For example, Qiu and colleagues [41] developed a siliceous zeolite-supported Ru single-atom catalyst, denoted as RuSAs/S-1. After conducting H2 reduction of RuSAs/S-1 and subsequent exposure to CO, CO-DRIFT spectra at room temperature showed multiple CO* absorption peaks at around 2174, 2120, 2060 and 2000 cm−1. The bands at 2060 and 2000 cm−1 were attributed to the formation of bi-carbonyl Ruδ+ (CO)2 species, which was adsorbed on the oxidated Ru sites. The bands at 2174 cm−1 and 2120 cm−1 were assigned to the mono-carbonyl species Ruδ+ (CO) and the tricarbonyl species Ruδ+ (CO)3, respectively. After conducting an in situ NH3 synthesis reaction for 3 h at 633 K using an H2/N2 (3:1) gas mixture, RuSAs/S-1 was cooled to room temperature and exposed to CO molecular probes. The obtained absorption peaks and intensities were nearly identical to the catalysts after direct reduction, indicating that single-atom Ru did not aggregate after NH3 synthesis. Cai et al. [33] carried out in situ CO-DRIFTS on Ru/ZSM-5 catalysts having different pore sizes to elucidate the nature of the Ru active sites. The DRIFTS spectra for Ru/ZSM-5 displayed peaks at 2108 and 2180 cm−1, assigned to the stretching oscillations of tricarbonyl entities adsorbed onto Run+ surfaces. In the case of Ru/ZSM-5 with mesopores, an extra peak at 2014 cm−1 was observed, corresponding to the stretching vibration of CO adsorbed on Ru0. This additional peak was presumed to arise from the reduction in oxidized Ru species in the presence of 10% CO/Ar. Zhou and coworkers [42] developed a Ru catalyst for ammonia synthesis using ZrH as the support and titanium carbonitride (TiCN) as the promoter, achieving good low-temperature catalytic activity. To investigate the effects of TiCN on the B5 sites of the Ru, the authors used DRIFTS to compare the vibrational peaks of CO adsorbed on the surfaces of Ru/TiCN/ZrH2, Ru/TiN/ZrH2 and Ru/ZrH2. Ru/TiCN/ZrH2 showed stronger CO* signals than Ru/ZrH2, suggesting that the incorporation of TiCN to Ru/ZrH2 leads to smaller Ru NPs and an increased dispersion.
Drummond and coworkers [43] conducted CO-DRIFTS experiments on Ru/PrOx, Ru-Cs/PrOx and Ru-Ba-Cs/PrOx catalysts, investigating the role of Cs as a promoter on Ru/PrOx (Figure 3a–c). Firstly, the catalyst without promoter only exhibited linear adsorption of CO (2006 cm−1) and multi-dentate adsorption peaks (2058 cm−1). By comparison, the catalyst with promoters showed additional bridged (1962 and 1959 cm−1) and tridentate (1890 and 1897 cm−1) adsorption peaks. It is also worth noting that there was no significant difference between the single-promoted and double-promoted catalysts, as each adsorption peak only showed a slight shift in wavenumber, indicating that Ru active sites as well as the electronic structure were not significantly affected. However, the catalyst with the double promoter showed a new small peak at 2117~2120 cm−1, which was due to the tri-dentate adsorption of CO on Ru. This absorption peak was only observed on Ru NPs, indicating that promoted catalysts contained Ru NPs. Comparatively, catalyst without promoters only contained atom-dispersed Ru or Ru clusters.
Zhang and colleagues [40] conducted a CO-DRIFTS study on Ru-M alloy catalysts (where M represents La or Y) at a temperature of 50 °C. As depicted in Figure 3d,e, alongside the gas-phase CO absorption featured at 2175 cm−1 and 2117 cm−1, a distinct absorption peak at 1990 cm−1, indicative of bridged CO adsorption, was notably detected on the surface of the Ru-La/HZ catalyst. Following a 30 min purge with He on the Ru-La/HZ and La/Ru/HZ catalysts, the gas-phase CO absorption peaks nearly vanished, yet the peak associated with bridged CO adsorption remained steady. The Ru-La/HZ catalyst showed much stronger CO absorption peaks than the La/Ru/HZ catalyst, indicating the presence of a larger number of Ru sites on the Ru-La alloy. Peng and coworkers [36] synthesized a range of Ru catalysts, varying in particle size from 1.4 to 5.0 nanometers, through a colloidal deposition technique. Their research revealed a strong correlation between the accessible active sites on Ru and its sizes. As the size of Ru decreased, the proportion of different surface sites (such as corners, steps and terraces) also increased accordingly, which was an important parameter influencing ammonia synthesis performance. The CO-DRIFTS results for Ru of different sizes are shown in Figure 3f. The absorption peaks at 2018, 2009 and 1980 cm−1 corresponded to CO adsorption at step sites (monodentate), corner sites (multi-dentate) and step sites (bridged). With the reduction in Ru NP size from 5.0 nm to 1.4 nm, the adsorption force of CO on the corner sites intensified, suggesting a progressive increase in the concentration of corner sites.

3.2. In Situ X-Ray Photoelectron Spectroscopy

With the growing demand for the analysis of catalyst electronic structures under reaction conditions, XPS technology has gradually evolved from ultra-high vacuum environments to the ability to perform XPS analysis under near-ambient pressure conditions (near-ambient/ambient pressure X-ray photoelectron spectroscopy, NAP/AP-XPS). Using in situ NAP-XPS technology, researchers can monitor, in real time, the changes in the oxidation states of catalysts and the electronic structure of active sites during catalytic processes, thereby exploring the true reaction sites. For example, Wang and coworkers [44] developed a Co-N-C catalyst that demonstrated excellent ammonia synthesis rates under low temperature and low pressure. In situ XPS studies revealed that pyrrolic nitrogen anchored a single Co atom in the Co1-N3.5 form. This active site facilitated N2 adsorption and the stepwise hydrogenation of N2 to HNNH*, NH-NH3 * and NH2-NH4 *, ultimately leading to the production of NH3 through the N-N bond cleavage of NH2-NH4 *. Additionally, experimental results showed that under reaction conditions, both dynamic and steady-state single-atom active centers coexisted in the Co-N-C catalyst. This Co1-N3.5 active site successfully tackled the challenge of N2 dissociation, enabling the synthesis of ammonia under moderate reaction conditions. Foo and colleagues [45] synthesized a series of Ru-based ammonia synthesis catalysts supported on barium zirconate, where the B-site of the ABO3 perovskite structure was doped with varying proportions of divalent cations, resulting in different proton conductivity behaviors. The researchers studied the hydrogen migration mechanism during ammonia synthesis using in situ NAP-XPS technology. They found that an appropriate balance between oxygen vacancy concentration (determined by B-site doping), proton capture site concentration and the activation energy for proton hopping is essential. The electronic configuration of the Ru site on the catalyst is intrinsically linked to the activation of N2 gas.
Although NAP-XPS is already a huge step forward to the reaction condition of NH3 synthesis, the gap between NAP and a real reaction condition is still large. Goodwin and colleagues developed an innovative in situ XPS system capable of mimicking pressurized conditions to study the surface valence states of Fe and Ru catalysts during ammonia synthesis [46]. Figure 4a demonstrates how the system simultaneously measured reaction rates via mass spectrometry while analyzing the electronic structure evolution of catalysts using XPS with total pressure up to 1 bar. The inset in Figure 4a presents the N 1s spectrum of the Fe (110) surface under the reaction condition of 423 K and 200 mbar (N2:H2 = 3:1). The deconvoluted peaks were assigned to NH3 * (blue), NH2* (purple), NH* (red), surface nitrogen species (green) and N2 * (yellow). To monitor NH3 production, the mass spectrometer monitored the concentration evolutions of m/z = 15 and 16, as illustrated in Figure 1b. Chemical reactions, shown in Figure 1c, were characterized by comparing the NH3 signal against those of other components using mass spectrometry. This approach allows the calculation of NH3 production rates at surface sites, normalized to the maximum activity observed under different temperatures. It was found that reaction rates increased with temperature, and the step-edged Fe (210) surface exhibited greater catalytic activity than the flat Fe (110) surface, consistent with previous findings from high-pressure reactor studies. As expected, Ru displayed higher surface activity than Fe. Interestingly, for Ru, the maximum reaction rate was observed at 623 K, rather than at the highest tested temperature of 723 K.

3.3. In Situ X-Ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) is a powerful analytical method used to explore the valence states and structural characteristics of catalysts by examining the X-ray absorption spectra with oscillatory patterns. These oscillations result from the interference between photoelectron waves emitted from central atoms and elastically scattered waves from neighboring atoms. The XAS spectrum is influenced by factors such as the wavelength of the photoelectrons, the types of coordinating atoms, and the atomic arrangement. X-ray absorption fine structure (XAFS) consists of two key components: (1) the X-ray absorption near-edge structure (XANES), which provides detailed structural information near the absorption edge, and (2) the extended X-ray absorption fine structure (EXAFS), which reveals details about the center atom’s coordination information. By utilizing in situ XAS, the evolution of catalyst structures during ammonia synthesis can be precisely monitored, offering critical insights into the surface reconstruction of catalysts during reactions.
Zhou et al. [47] explored the correlation between the structure and catalytic activity of Re/Mo2CTx through in situ XANES and EXAFS spectroscopy, providing insights into the nature of active sites during ammonia production. As depicted in Figure 5a, the in situ Mo K-edge XANES spectra for Re/Mo2CTx revealed a shift towards lower energy in the absorption edge after H2 reduction, signifying a decrease in the oxidation state of Mo. Subsequent to the reaction in a N2-H2 atmosphere at 400 °C for 15 to 45 min (Figure 5a), the absorption edge shifted upwards, signaling a rise in the oxidation state of Mo, which is attributed to the transfer of electrons from Mo sites to the anti-bonding π* orbital of N2. Furthermore, the in situ EXAFS spectra using Mo as the center atom, as presented in Figure 5b and its inset, distinctly indicated that with the progression of the reaction, the first-shell coordination number of Mo-C (N) increased from 5.6 to 6.3. Concurrently, the second-shell Mo-C-Mo coordination in the Re/Mo2CTx catalyst remained largely unchanged. Gondo et al. [48] investigated the behavior of Co catalysts for low-temperature ammonia synthesis under applied electric field. The reduced Co/Ce0.5Zr0.5O2 catalyst demonstrated superior ammonia synthesis performance at 473 K. In situ XAFS analysis suggested that the activity of the 5 wt% Co/Ce0.5Zr0.5O2 catalyst was predominantly affected by the reduction level and particle size of Co. Kikugawa et al. [49] formulated Ru/CeO2-PrOx catalysts for ammonia production by employing a urea-based homogeneous co-precipitation method and investigated the influence of Pr doping in the CeO2 support on the catalyst’s performance. Within a specific Pr content range (25–75%), NH3 synthesis activity was promoted. In situ XAFS studies indicated that the enhanced activity stemmed from the improved Ru dispersion, as opposed to the frequently cited Ce3+/Ce4+ ratio. However, an excessive amount of Pr doping impeded the reduction of Ru. In the Ru/CeO2-PrOx catalyst system, the catalytic activity was primarily controlled by the number of active sites, which could be fine-tuned by adjusting the Pr content.
Li et al. [23] employed in situ EXAFS to investigate the structural evolution of Ru atomic clusters during ammonia synthesis. The Ru K-edge EXAFS spectra demonstrated negligible variations over time, with the coordination numbers of Ru-N and Ru-Ru shells remaining largely unchanged at 400 °C. The EXAFS results suggested that the active sites within the atomic clusters were resilient under the specified reaction conditions. Miyazaki et al. [50] conducted the in situ Ti K-edge XAFS characterization of Fe/BaTiO3Ny to analyze the evolution of nitrogen vacancies in the catalyst. The XANES spectra for Fe/BaTiO3−xNy were collected under N2/H2 atmosphere, spanning temperatures from room temperature to 600 °C. Figure 5a–c presents the XANES region, the pre-edge region and the peak region, respectively. Significant spectral changes in the pre-edge and peak regions corresponded to alterations in the coordination structure and electronic state of Ti atoms. In Figure 5b, the pre-edge peak intensity near 4966 eV decreased with rising temperature, consistent with an increase in Ti coordination. Meanwhile, Figure 5c highlighted an enhancement in absorption intensity at 4984 eV during high-temperature in situ measurements, suggesting that nitrogen vacancies adjacent to Ti in BaTiO3−xNy were filled by N2 under ammonia synthesis conditions.
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Figure 5. (a–c) In situ XAFS data at the Ti K-edge for Fe/BaTiO3, presented in normalized form. Permission to reproduce granted by reference [49], in accordance with the CC BY 4.0 license. (d) Analysis of in situ Co K-edge XANES and (e) EXAFS spectroscopy for Co2 ACCs, conducted following exposure to a gas mixture of 25% N2 and 75% H2 at 350 °C over varying durations. Permission to reproduce granted by reference [51]. Copyright 2022 American Chemical Society.
Figure 5. (a–c) In situ XAFS data at the Ti K-edge for Fe/BaTiO3, presented in normalized form. Permission to reproduce granted by reference [49], in accordance with the CC BY 4.0 license. (d) Analysis of in situ Co K-edge XANES and (e) EXAFS spectroscopy for Co2 ACCs, conducted following exposure to a gas mixture of 25% N2 and 75% H2 at 350 °C over varying durations. Permission to reproduce granted by reference [51]. Copyright 2022 American Chemical Society.
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Peng et al. [51] synthesized cobalt atomic dimers supported on nitrogen-doped carbon for ammonia synthesis and identified a unique mechanism of N2 activation on Co2 atomic clusters (Co2 ACCs), which differed from the mechanisms seen in cobalt single-atom catalysts (Co SACs) or Co NPs. As depicted in Figure 5d,e, the in situ Co K-edge XANES and EXAFS spectroscopy of Co2 ACCs exposed not only exceptionally reactive sites with robust metal-support interactions but also significant interactions between clusters. This unique structural attribute amplified the electron transfer from the d-orbitals of cobalt to the anti-bonding orbitals of N2 *, thereby promoting effective N2 activation.

3.4. In Situ Neutron Diffraction

Inelastic neutron scattering (INS) is an advanced technique used to investigate the motion and positions of atoms in condensed matter systems. Similar to in situ X-ray diffraction (XRD), the structural evolution of catalysts during catalytic ammonia synthesis can be revealed and analyzed with temporal resolution. For example, Schlögl et al. [52] conducted a comprehensive in situ XRD investigation on the reduction of “ammonia-iron”. Significant anisotropic α-Fe XRD patterns were found for reduced Fe catalysts and the meta-stable structure was believed to be the origin of its high catalytic performance. The in situ experiment provided important understanding as to why a well-controlled reduction was a necessity for the “ammonia-Fe” catalyst, which is now accepted as industry consensus. Compared to XRD, a feature of INS is its high sensitivity to hydrogen dynamics, which makes it particularly valuable for providing additional information about H/H-containing species inside the crystal and on the surface of the catalysts. For example, to understand the enhanced ammonia synthesis activity of manganese after lithium doping, Laassiri et al. [53] utilized in situ neutron diffraction to explore the structural changes in lithium-doped and undoped manganese nitride samples under a 75%H2/Ar mixture at 400 °C. The in situ diffraction data of the undoped θ-Mn6N5+x sample showed no significant structural changes in the θ-Mn6N5+x and η-Mn3N2 phases after reaction, supporting the thermochemical stability of manganese nitrides. For the lithium-doped manganese nitride material (1% wt Li/Mn6N5+x), in situ INS patterns at low temperatures (≤200 °C) displayed no notable changes, with only the θ-Mn6N5+x phase clearly identified. This aligned with the observation that θ-Mn6N5+x exhibited low reactivity in ammonia synthesis below 300 °C. As the temperature increased, nitrogen occupancy began to decrease progressively, suggesting that the reduction of θ-Mn6N5+x occurred via the formation of nitrogen vacancies. When the nitrogen vacancy concentration reached a critical threshold (n = 0.88), the reduction process was accompanied by a structural rearrangement, leading to the emergence of two distinct Mn sites, and the resulting structure was closely related to η-Mn3N2. The chemical looping process using Ni/BaH was reported to produce ammonia at temperatures as low as 373 K and under ambient pressure. Moon et al. [54] investigated the structural change in Ni/BaH2 during the N2-H2 chemical looping process by in situ INS. BaH2 was found converted to barium imide (BaNH) after reacting with N2.
The electrides supporting the metal NP catalyst is a new type of ammonia synthesis catalyst, showing excellent performance at low temperature and pressure. However, the structure of electrides and the strengthening mechanism of ammonia synthesis are less understood. INS is a powerful tool to investigate the dynamics of H within electride support as well adsorption on the surface. For instance, Kammert et al. [55] reported that the hydrides in C12A7 electrides were stable through in situ INS. The results suggested that the hydride was unlikely to participate in ammonia synthesis. In contrast, the H adsorbed on the surface is responsible for the catalytic activity of the reaction on the Ru/C12A7 electride-based catalyst. Yu et al. [56] investigated the structural change in the electride catalyst of Ru/Ca2N:e− during ammonia synthesis through in situ INS, the results of which are shown in Figure 6. It was proposed that Ca2N:e− was converted into a new phase of Ca2NH during the reaction. The obtained Ca2NH has a segregated structure of H and N atoms separated by the Ca atomic layer, which was different from the previously known structure of Ca2NH.

3.5. In Situ Environmental Transmission Electron Microscopy

In situ environmental transmission electron microscopy (in situ ETEM) is an advanced microscope technique used to observe the structure and behavior of the catalyst at atomic scale under a simulated reaction atmosphere and elevated temperatures. ETEM can provide direct information of the structure of the ammonia synthesis catalyst and its responses towards reaction atmosphere.
Hansen et al. [57] studied the structural evolution of the Ru/Ba/BN catalyst under a reaction atmosphere and temperature with the in situ ETEM. It was found that an amorphous coating on the surface of Ru NPs was formed under the reaction atmosphere, but the structure and morphology of Ru NPs did not change. These results showed that the total number of active sites on the surface did not change regardless of the enhancement of catalytic activity. Ba was found highly mobile and existing in the form of oxides via ETEM, with atomic resolution and electron energy loss spectroscopy (EELS) characterization. Ward et al. [58] investigated the dynamic structures of Ru NPs on carbon supports in ammonia synthesis conditions with the in situ ETEM. It was found that the Ru sintering phenomenon was size-dependent. After 1 h reduction in H2 gas at 450 °C, the size of Ru NPs mainly fell into the range of ~1 nm to ~2.5 nm. After treatment at a reaction atmosphere at 300 °C for 2 h and 450 °C for 1 h, the Ru NPs migrated and sintered. Larger, multifaceted Ru NPs were formed while Ru NPs smaller than a certain size tended to remain unchanged. Ding et al. [59] studied the effects of catalyst promoters on the behaviors of Ru catalysts under simulated ammonia synthesis temperature and atmosphere by in situ ETEM. The images of Ru/MS, MgO-Ru/MS and Cs@MgO-Ru/MS at different reaction times are shown in Figure 7. The structure of Ru before treatment in the reaction atmosphere and temperature is shown in Figure 7(a1–c1). The Ru/MS structural evolution in a simulated reaction condition is shown in Figure 7(a2–a5). As presented in Figure 7(a2), the crystal plane d-spacing expanded and the structure of Ru evolved to amorphous, and Ru atoms even showed a tendency to detangle from the particle surface. The surface of Ru NPs alternated between amorphous and crystalline states at a longer time-scale, as shown in Figure 7(a3–a5), indicating that Ru NPs were prone to surface reconstruction under reaction conditions. The dynamic behavior of Ru NPs changed after adding MgO promoters (Figure 7(b2–b5)). The Ru atoms at the edge and step sites were very stable and the lattice d-spacing remained unchanged, which was attributed to metal oxide interaction. The catalyst activity of ammonia synthesis might be enhanced by the strong interaction between Ru and metal oxide, which restricted the surface reconstruction of Ru NPs. As a result, the B5 active sites could be well preserved under reaction conditions.

4. Summary and Prospect

In situ characterization technology has provided invaluable insights on the formation and transformation of surface intermediates and final products with respect to the surface of ammonia synthesis catalysts in real time, as well as the structural evolution of catalysts under reaction conditions. Based on in situ characterization results, researchers have revealed the different reaction pathways, key steps of ammonia synthesis and the true active sites of catalysts. Although significant progress and important results in the field of heterogeneous catalysis of ammonia synthesis were obtained using in situ characterization techniques, there are still key challenges that need to be addressed in the future: (1) the high-temperature and high-pressure reaction conditions of industrial ammonia synthesis are difficult to achieve for in situ characterization. The in situ information is invaluable but still has discrepancy with respect to “operando” information; (2) the intermediates of ammonia synthesis are complicated and feature weak adsorption. The existing technology has limitations in terms of sensitivity, time and spatial resolution. In the future, in situ characterization investigations of heterogeneous catalytic ammonia synthesis can be further developed in the following three aspects: (1) to overcome the limitations of a single technology, future research is recommended to couple multiple in situ characterizations and achieve a more comprehensive analysis; (2) comparing in situ characterization results with advanced theoretical calculations such as AIMD and Monte Carlo to enhance the depth of the mechanism analyses; (3) designing new types of in situ reactors to adapt to high-temperature and high-pressure conditions or circumventing the difficulties of harsh condition requirements by innovating small chips, high-speed air flow, etc., that can simulate conditions closer to industrial working conditions; (4) high-sensitivity, high-spatio-temporal resolution detector/detection and multi-dimensional characterizations are desired to establish real kinetic/thermodynamic relations between ammonia synthesis reactivity and catalyst structure. In situ cells able to hold high pressure and high temperature are under active development, aiming to address the “pressure gap” and “materials gap” for heterogeneous catalysis. For example, diamond anvils can easily increase the pressure inside to the level of GPa so that it is ready for X-ray/Raman spectroscopic measurement. With the fast development of microfabrication, new micro/nano cells have been reported being able to hold pressure up to the MPa level for gases, introducing reasonable flow patterns for excellent mass transfer [60]. The ultrathin layer of gases within the cell ensures a controllably small interaction between incident beam and gases, which makes these micro/nano cells promising for spectroscopy and even microscopy investigations. While there are few studies in the literature right now, new understandings with respect to ammonia synthesis using these new setups are expected in the future, with improved experimental protocols as well as analytic methods.
In summary, in situ characterizations are vital for elucidating the mechanisms behind ammonia production, which provides invaluable scientific guidance on the design and development of advanced catalysts. It is hoped that this review inspires future research contributing to the evolution of ammonia synthesis, one of the most crucial reactions for human civilization, to lower energy consumption and increase NH3 yields.

Author Contributions

Conceptualization, K.W.; software, S.S., Z.X. and X.C.; writing—original draft preparation, W.S.; writing—review and editing, M.Q., Q.S., K.W. and Y.L.; visualization, R.P. and H.Z.; supervision, Y.L.; funding acquisition, K.W. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Key Research and Development Program of China (No. 2022YFA1505300), the Natural Science Foundation of Henan Province (242300421141), the National Natural Science Foundation of China (22378397, 22208348, 22278402), the Key R&D Program of Henan Province (No. 231111241800) and the Frontier Basic Research Projects of the Institute of Process Engineering, CAS (No. QYJC-2023-03).

Data Availability Statement

Data sharing is not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
MvKMars van Krevelen
DRIFTSDiffuse reflectance Fourier transform infrared spectroscopy
NPNanoparticle
NAP-XPSNear-ambient pressure X-ray photoelectron spectroscopy
AP-XPSAmbient pressure X-ray photoelectron spectroscopy
XASX-ray absorption spectroscopy
XAFSX-ray absorption fine structure
XANESX-ray absorption near-edge structure
EXAFSExtended X-ray absorption fine structure
ACCAtomic cluster
SACSingle atom catalyst
XRDX-ray diffraction
INSInelastic neutron scattering
ETEMEnvironmental transmission electron microscopy
EELSElectron energy loss spectroscopy

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Figure 1. General classification of mechanisms for heterogeneous catalytic ammonia synthesis: (a) associative-distal, (b) associative-alternative, (c) dissociative and (d) MvK pathways.
Figure 1. General classification of mechanisms for heterogeneous catalytic ammonia synthesis: (a) associative-distal, (b) associative-alternative, (c) dissociative and (d) MvK pathways.
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Figure 3. CO-DRIFT spectra of (a) Ru/PrOx, (b) Ru-Cs/PrOx and (c) Ru-Ba-Cs/PrOx. Reproduced with permission from ref. [43], under the license of CC BY 4.0. CO-DRIFT spectra of (d) Ru-M/HZ, (e) RuxLay/HZ catalysts of different Ru/La molar ratios and (f) the as-synthesized Ru samples. Reproduced with permission from ref. [40], copyright 2021 Elsevier Ltd.
Figure 3. CO-DRIFT spectra of (a) Ru/PrOx, (b) Ru-Cs/PrOx and (c) Ru-Ba-Cs/PrOx. Reproduced with permission from ref. [43], under the license of CC BY 4.0. CO-DRIFT spectra of (d) Ru-M/HZ, (e) RuxLay/HZ catalysts of different Ru/La molar ratios and (f) the as-synthesized Ru samples. Reproduced with permission from ref. [40], copyright 2021 Elsevier Ltd.
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Figure 4. In situ XPS experimental setup and relative reactivity measurements. (a) The sample is positioned facing an array of apertures that deliver reactant gases, while products are collected via a mass spectrometer and emitted electrons are analyzed by the photoelectron spectrometer. The inset shows the XPS spectrum of N chemical states on the Fe (110) surface under 200 mbar. (b) Mass spectrometer signals of m/z = 15, 16 corresponding to NH3 production rate over Ru at 673 K. The feed gas composition is tuned from 150 mbar N2 to 300 mbar N2:H2 (1:1). (c) The relative chemical reactivities of Ru and Fe with different exposed faces determined using the above-mentioned equipment. Reproduced with permission from ref. [46], under the license of CC BY 4.0.
Figure 4. In situ XPS experimental setup and relative reactivity measurements. (a) The sample is positioned facing an array of apertures that deliver reactant gases, while products are collected via a mass spectrometer and emitted electrons are analyzed by the photoelectron spectrometer. The inset shows the XPS spectrum of N chemical states on the Fe (110) surface under 200 mbar. (b) Mass spectrometer signals of m/z = 15, 16 corresponding to NH3 production rate over Ru at 673 K. The feed gas composition is tuned from 150 mbar N2 to 300 mbar N2:H2 (1:1). (c) The relative chemical reactivities of Ru and Fe with different exposed faces determined using the above-mentioned equipment. Reproduced with permission from ref. [46], under the license of CC BY 4.0.
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Figure 6. INS spectra of Ru/Ca2N in H2 at 350 °C in the first cycle (black) and the comparison with simulated spectra derived from (a) bulk, segregated and (b) molecular dynamics models Reproduced with permission from ref. [56]. Copyright 2023 American Chemical Society.
Figure 6. INS spectra of Ru/Ca2N in H2 at 350 °C in the first cycle (black) and the comparison with simulated spectra derived from (a) bulk, segregated and (b) molecular dynamics models Reproduced with permission from ref. [56]. Copyright 2023 American Chemical Society.
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Figure 7. In situ ETEM images of the structural evolution over time of Ru NPs in (a1–a5) Ru/MS, (b1–b5) Ru/MgO/MS and (c1–c5) Ru/Cs@MgO/MS catalysts. Experimental details: the temperature was maintained at 500 °C; the pressure ranged from 0.12 to 0.13 mbar; and the atmosphere consisted of a H2/N2 mixture (3:1 v/v). The images were captured at the timestamps of 0, 10, 20, 30 and 40 s for each catalyst sample, respectively. (d) Diagram illustrating the functions of MgO and CsOx as promoters. Adapted from reference [59]. Copyright 2020 Wiley-VCH GmbH.
Figure 7. In situ ETEM images of the structural evolution over time of Ru NPs in (a1–a5) Ru/MS, (b1–b5) Ru/MgO/MS and (c1–c5) Ru/Cs@MgO/MS catalysts. Experimental details: the temperature was maintained at 500 °C; the pressure ranged from 0.12 to 0.13 mbar; and the atmosphere consisted of a H2/N2 mixture (3:1 v/v). The images were captured at the timestamps of 0, 10, 20, 30 and 40 s for each catalyst sample, respectively. (d) Diagram illustrating the functions of MgO and CsOx as promoters. Adapted from reference [59]. Copyright 2020 Wiley-VCH GmbH.
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MDPI and ACS Style

Su, W.; Cheng, X.; Shang, S.; Pan, R.; Qi, M.; Sang, Q.; Xie, Z.; Zhang, H.; Wang, K.; Liu, Y. Advances in In Situ Investigations of Heterogeneous Catalytic Ammonia Synthesis. Catalysts 2025, 15, 160. https://doi.org/10.3390/catal15020160

AMA Style

Su W, Cheng X, Shang S, Pan R, Qi M, Sang Q, Xie Z, Zhang H, Wang K, Liu Y. Advances in In Situ Investigations of Heterogeneous Catalytic Ammonia Synthesis. Catalysts. 2025; 15(2):160. https://doi.org/10.3390/catal15020160

Chicago/Turabian Style

Su, Weiyi, Xi Cheng, Suokun Shang, Runze Pan, Miao Qi, Qinqin Sang, Zhen Xie, Honghua Zhang, Ke Wang, and Yanrong Liu. 2025. "Advances in In Situ Investigations of Heterogeneous Catalytic Ammonia Synthesis" Catalysts 15, no. 2: 160. https://doi.org/10.3390/catal15020160

APA Style

Su, W., Cheng, X., Shang, S., Pan, R., Qi, M., Sang, Q., Xie, Z., Zhang, H., Wang, K., & Liu, Y. (2025). Advances in In Situ Investigations of Heterogeneous Catalytic Ammonia Synthesis. Catalysts, 15(2), 160. https://doi.org/10.3390/catal15020160

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