Decoupling Kinetically Coupled Steps via Hierarchical Relay Catalysis on High‐Entropy Alloy for Efficient Ammonia Decomposition

ABSTRACT

Ammonia (NH₃) is a promising hydrogen carrier owing to its high gravimetric hydrogen storage density (17.6 wt%), but its practical utilization is hindered by sluggish decomposition kinetics at moderate temperatures. For commonly used Ru‐based catalysts, the reaction remains constrained by kinetically coupled elementary steps, including both N‐H bond activation and inhibited H desorption. Herein, we propose a hierarchical relay catalysis strategy enabled by a RuNiCoFeMo high‐entropy‐alloy (HEA) catalyst, wherein distinct metallic centers are rationally integrated to regulate individual elementary steps within a single catalytic framework. Specifically, NiCoFe sites preferentially promote sequential N‐H bond scission, while Mo incorporation electronically modulates the alloy to weaken metal‐H interactions and suppress hydrogen poisoning, thereby breaking scaling constraints across multiple elementary steps. Consequently, the optimized catalyst exhibits ca. 56% and 54% higher NH₃ and H₂ reaction orders relative to the monometallic Ru, respectively, indicating enhanced NH₃ activation and alleviated hydrogen poisoning. These kinetic benefits translate into a threefold increase in NH₃ conversion and H₂ production, reaching ca. 80% NH₃ conversion at 450°C, which is among the highest activities reported for Ru‐based catalysts. This work provides a general design principle for catalyst design in complex multistep reactions beyond NH₃ decomposition.

A hierarchical relay catalytic system based on a RuNiCoFeMo high‐entropy alloy spatially partitions elementary steps in NH₃ decomposition. Distinct metallic sites cooperatively regulate N‐H activation, N‐N recombination, and H₂ desorption, thereby breaking kinetic scaling constraints and enabling markedly enhanced NH₃ conversion and H₂ production.

1. Introduction

Hydrogen (H₂) has been widely recognized as an ideal alternative to fossil fuels [1, 2]. However, its intrinsically low volumetric energy density, coupled with the persistent challenges in storage and transportation, has long hindered its large‐scale deployment [3, 4]. In this context, ammonia (NH₃) has emerged as a particularly attractive hydrogen carrier owing to its high gravimetric hydrogen storage density (17.6 wt%), relatively mild liquefaction conditions (25°C, 1.0 MPa), and carbon‐free decomposition into only nitrogen (N₂) and H₂ [5, 6]. Nevertheless, the sluggish kinetics of NH₃ decomposition at low temperatures remain a major obstacle to its practical utilization [7, 8]. Among various catalysts reported, Ru‐based materials are the most effective for NH₃ decomposition, attributed to their moderate Ru‐N bond strength [9]. However, they still suffer from several intrinsic limitations, including high energy barriers for N‐H bond cleavage [10], sluggish H₂ desorption arising from strong Ru‐H interactions [11], and the high cost of Ru [12]. Consequently, developing Ru‐based catalysts with reduced noble‐metal loading and high intrinsic activity capable of meeting industrial performance requirements has become a central research focus in this field [13, 14].

To enhance the NH₃ decomposition activity of Ru‐based catalysts, extensive efforts have been devoted to regulating elementary reaction steps through strategies such as alloying, support modulation, and morphology engineering [12, 15]. Representative studies have demonstrated that introducing a second metal can optimize specific steps, such as balancing M‐H dehydrogenation and N₂ desorption [15], or that tailored support interfaces can promote both NH₃ activation and nitrogen species desorption simultaneously [12]. Despite these advances, synergistic regulation has largely remained confined to isolated segments of the reaction pathway, and extending cooperative regulation across the key kinetically coupled steps in NH₃ decomposition remains a significant challenge [10].

High‐entropy alloys (HEAs) represent a promising catalytic platform due to their broad compositional flexibility [16], tunable electronic structures [17], and inherent multisite cooperation [18]. By integrating multiple metallic elements within a single framework, HEAs offer the possibility of designing hierarchical relay catalysis, in which distinct elements can preferentially facilitate different elementary steps in a coordinated manner [19]. Such features are particularly appealing for thermal NH₃ decomposition, which demands concerted optimization across multiple coupled reaction steps [20].

Herein, guided by computational screening of N‐H scission barriers across different transition‐metal sites (Figure 1a, see the Supporting Information for details) [21] and systematic analysis of d‐band center modulation via additional metal integration (Figure 1b; vide infra), we construct a hierarchical relay‐catalysis platform based on a RuNiCoFeMo HEA catalyst to decouple the kinetically intertwined elementary steps of NH₃ decomposition. As schematically illustrated in Figure 1c, multiple metallic components are deliberately integrated within a single multimetallic framework to enable functional partitioning at the atomic scale. NiCoFe ensembles preferentially drive sequential N‐H dehydrogenation, while Ru‐Mo motifs cooperatively promote N‐N recombination and H* removal by alleviating kinetic site‐blocking effects. Through this hierarchical relay mechanism, the catalyst redistributes rate‐limiting steps across complementary active centers, thereby overcoming conventional scaling constraints inherent to single‐site systems. As a result, RuNiCoFeMo/CG delivers a threefold increase in H₂ production and ca. 80% NH₃ conversion at 450°C, placing it among the top‐performing Ru‐based NH₃ decomposition catalysts reported to date. This work establishes a composition‐programmable strategy for breaking kinetic coupling in multistep heterogeneous catalysis.

FIGURE 1.

(a) Calculated N–H scission barriers on different metal sites. (b) Fifth element‐dependent modulation of the Ru d‐band center. (c) Schematic diagram of the hierarchical relay catalysis of NH₃ decomposition on RuNiCoFeMo/CG.

2. Results and Discussion

2.1. Material Synthesis and Characterization

RuNiCoFeMo HEA nanoparticles (NPs) were synthesized through a facile solvothermal approach on an interconnected three‐dimensional carbon nanotubes (CNTs)/graphene oxide (GO) hybrid framework (denoted as RuNiCoFeMo/CG) (Figure 2a, Figures S1 and S2). Transmission electron microscopy (TEM) images reveal that the RuNiCoFeMo NPs are uniformly dispersed on the hybrid support, with an average particle size of ca. 2.64 nm (Figures 2b and S3). Atomic‐resolution high‐angle annular dark‐field‐scanning transmission electron microscopy (HAADF‐STEM) images reveal slight differences in the lattice fringes between RuNiCoFeMo NPs and monometallic Ru NPs (Figure 2c, Figures S4 and S5, see the Supporting Information for details). The lattice spacing of RuNiCoFeMo/CG is determined to be ca. 0.202 nm (Figure 2c), slightly smaller than that of pure Ru (ca. 0.208 nm; Figure S5a), which is attributed to the incorporation of smaller‐radius transition‐metal atoms [22]. HAADF‐STEM images of RuNiCoFeMo/CG (Figure 2d,e) further exhibit pronounced contrast heterogeneity, with randomly distributed bright and relatively dim atomic columns, indicating the coexistence of elements with different atomic radii within individual NPs [23]. In contrast, Ru NPs display uniform contrast throughout (Figure S5), consistent with a single‐element composition. The energy‐dispersive x‐ray spectroscopy (EDS) elemental mapping (Figure 2f) confirms the homogeneous spatial distribution of Ru, Ni, Co, Fe, and Mo within individual NPs. Moreover, XRD patterns show that RuNiCoFeMo/CG exhibits a diffraction peak at ca. 44.6°, which is slightly shifted to higher angles compared with that of monometallic Ru (Figure 2g) [24], indicating lattice contraction induced by alloying with smaller‐radius atoms. The absence of additional diffraction peaks corresponding to other metallic or oxide phases confirms the formation of a single‐phase, homogeneously alloyed structure [25]. Together with the HAADF‐STEM observations, these results demonstrate the successful formation of quinary RuNiCoFeMo HEAs with well‐mixed constituent elements [26].

FIGURE 2.

(a) SEM image, (b) TEM image, (c, d) HAADF‐STEM images, and (e) HAADF line‐scan profiles of the RuNiCoFeMo/CG. (f) HAADF‐STEM image and EDS elemental mappings of RuNiCoFeMo/CG. (g) XRD patterns of relative catalysts. (h) HR‐XPS spectra of Ru 3p. (i) XANES spectra of Ru K‐edge. (j) FT‐EXAFS spectra of Ru R space. (k) WT‐EXAFS spectra of Ru K‐edge.

The elemental composition of RuNiCoFeMo/CG determined by x‐ray photoelectron spectroscopy (XPS) reveals an equimolar ratio (Figures S6–S10), consistent with the results obtained from inductively coupled plasma‐optical emission spectrometry (ICP‐OES) (Tables S1 and S2), thereby verifying the successful incorporation of multiple metallic components. The high‐resolution Ru 3p spectra (Figure 2h) show that Ru predominantly exists in the metallic state across all Ru‐containing catalysts [27]. Compared with RuNiCoFe/CG (a sample without a fifth element for electronic structure modulation, Figure S11), the Ru 3p binding energy in RuNiCoFeMo/CG has a negative shift by ca. 0.3 eV, suggesting that the incorporated Mo serves as an electron donor to Ru and thereby enriches the Ru surface electron density. X‐ray absorption fine structure (XAFS) measurements were performed to elucidate the electronic state and local coordination of Ru. The Ru K‐edge x‐ray absorption near‐edge structure (XANES) spectrum of RuNiCoFeMo/CG closely resembles that of Ru foil (Figure 2i), confirming that Ru is predominantly metallic [28]. Quantitative analysis of the spectral features, referenced to the metallic‐state standard (inset, Figure 2i), yields an average Ru valence of ca. 0.3 for RuNiCoFeMo/CG, notably lower than ca. 0.8 observed for RuNiCoFe/CG. This trend aligns with the Ru 3p XPS shift and supports the occurrence of electron donation from Mo to Ru upon Mo incorporation [22]. In the Fourier‐transformed (FT) Ru K‐edge EXAFS (Figure 2j) spectra, RuNiCoFeMo/CG displays a prominent peak at ca. 2.20 Å, assigned to Ru‐M coordination [27]. This peak appears at a significantly shorter radial distance than the Ru‐Ru coordination in Ru/CG (ca. 2.36 Å), consistent with local bond‐length contraction induced by alloying with smaller‐radius transition‐metals [27]. To further verify this assignment, we performed quantitative EXAFS fitting in both R‐space and k‐space. The fitting results reproduce the main peak well and support the presence of dominant Ru‐M coordination in RuNiCoFeMo/CG, in agreement with the literature and the observed shift relative to Ru‐Ru scattering (Figures S12 and S13, Table S3) [29]. Wavelet‐transform (WT)‐EXAFS (Figure 2k) analysis further corroborates this assignment, revealing a scattering signature for RuNiCoFeMo/CG that is clearly distinct from that of Ru/CG and characteristic of Ru‐M coordination. Together with XPS results, these observations indicate that Ru is embedded in a multimetallic local environment, further confirming the formation of a well‐alloyed quinary nanoalloy.

2.2. Performance Evaluation

Comparative activity measurements reveal that the optimized RuNiCoFeMo/CG exhibits markedly enhanced activity at moderate temperature. At 450°C and a gas hourly space velocity (GHSV) of 22 000 mL·g_cat ⁻¹·h⁻¹, it achieves an NH₃ conversion of ca. 80%, substantially outperforming RuNiCoFe/CG (52%) and Ru/CG (25%) under identical conditions (Figure 3a). Upon further increasing the temperature, RuNiCoFeMo/CG reaches complete NH₃ conversion (100%) at 500°C, which is 150°C lower than the temperature required for Ru/CG (Figure 3a), highlighting the significantly reduced operating temperature enabled by the multimetallic design. In terms of hydrogen productivity, RuNiCoFeMo/CG delivers an efficient H₂ production rate of 19.6 mmol_H2·g_cat ⁻¹·min⁻¹ at 450°C (Figure 3b), considerably higher than those of RuNiCoFe/CG (12.7 mmol_H2·g_cat ⁻¹·min⁻¹) and Ru/CG (6.0 mmol_H2·g_cat ⁻¹·min⁻¹). Control experiments confirm that the CG support is inactive towards NH₃ decomposition, exhibiting 0% conversion across the temperature range from 350°C to 600°C (Figure S14a). Moreover, other catalysts with the same metals supported on different substrates also show inferior performance compared to RuNiCoFeMo/CG (Figure S14b,c). Combined with the inferior activity observed for FeCoNi/CG (Figures S15 and S16), this result demonstrates that enhanced catalytic performance originates from synergistic interactions among the multimetallic components. Binary catalysts (RuNi/CG, RuCo/CG, and RuFe/CG, Figure S17) and ternary catalysts (RuNiCo/CG, RuNiFe/CG, and RuCoFe/CG, Figure S18) were also evaluated. However, none matched the activity achieved by the NiCoFe‐containing system, suggesting that the simultaneous presence of Ni, Co, and Fe is essential to fully promote NH₃ decomposition. Moreover, the catalytic performance of RuMo/CG under identical conditions was evaluated (Figure S19). Although RuMo/CG exhibits higher NH₃ decomposition activity than Ru/CG, its performance remains inferior to RuNiCoFeMo/CG. The above results highlight that the combination of Mo with Ni, Co, and Fe is essential for achieving superior catalytic activity. Additional performance assessments over a broader range of temperatures and space velocities are provided in Figures S20–S23, and RuNiCoFeMo/CG consistently delivers the highest NH₃ conversion, underscoring its stable performance across a broad range of operating conditions.

FIGURE 3.

(a) NH₃ conversion and (b) H₂ production rate as a function of temperature. (c, d) Reaction orders of NH₃ and H₂. (e) Relationship between electronegativity and NH₃ conversion, ΔE_a , and n_H2. (f) A comparison chart of NH₃ conversion rates, H₂ production rate, ΔE_a , and reaction orders of n_NH3 and n_H2 for RuNiCoFeMo/CG, RuNiCoFe/CG, and Ru/CG catalysts. (g) Long‐term durability test of the RuNiCoFeMo/CG (conditions: 450°C, GHSV = 5500 mL·g_cat ⁻¹·h⁻¹, 150 h). The insets show the XRD patterns and Ru 3d HR‐XPS spectra before and after reaction. (h) Comparison of the NH₃ decomposition activity of the RuNiCoFeMo/CG with reported catalysts.

To correlate the performance differences with intrinsic kinetics, apparent activation energies (ΔE _a) were determined by keeping the NH₃ conversion below 15% at different space velocities (Figure S24) [30]. RuNiCoFeMo/CG exhibits the lowest ΔE _a of 50.62 kJ·mol⁻¹ (65.41 kJ·mol⁻¹ for RuNiCoFe/CG and 76.06 kJ·mol⁻¹ for Ru/CG), indicating a substantially reduced effective kinetic barrier for NH₃ decomposition. Furthermore, the higher NH₃ reaction orders for RuNiCoFe/CG (0.75) and RuNiCoFeMo/CG (0.81) relative to Ru/CG (0.52) and RuMo/CG (0.56) reflect a stronger kinetic dependence on NH₃ partial pressure, consistent with facilitated NH₃ activation on NiCoFe‐containing catalysts. This behavior suggests that the incorporation of Fe, Co, and Ni promotes N‐H bond cleavage (Figures 3c and S25) [31]. In addition, all catalysts exhibit negative reaction orders with respect to H₂, ranging from −0.69 to −0.32, which quantitatively confirms the competitive adsorption between H₂ and NH₃ molecules on the active sites [7, 31]. Both the monometallic Ru and RuNiCoFe/CG show the most pronounced inhibitory effect, with H₂ reaction orders of −0.69 and −0.63, respectively, indicating that the accumulation of hydrogen product blocks the active surface. In contrast, the RuNiCoFeMo/CG demonstrates a significantly alleviated hydrogen poisoning effect, as evidenced by its increased H₂ order value of −0.32, suggesting that Mo incorporation within the multi‐component structure weakens the H adsorption strength effectively (Figures 3d and S26) [32]. Meanwhile, varying the N₂ partial pressure exerts a negligible influence on the reaction rate (Figure S27), suggesting that N₂ desorption is not rate‐limiting under the present conditions [13].

Following the kinetic analysis based on NH₃ and H₂ reaction orders, we further investigated whether the observed hydrogen‐related effects persist upon substitution of Mo with other transition metals while maintaining comparable catalytic performance (Figures 3e and S28–S34, Table S4). A systematic trend emerges across the Mn‐, Cu‐, Mo‐, and W‐containing catalysts, in which a progressive attenuation of hydrogen inhibition coincides with lower ΔE _a and enhanced NH₃ conversion under identical conditions. The Mo‐containing catalyst occupies the most favorable position within this series, simultaneously exhibiting the weakest hydrogen inhibition and the lowest activation barrier, in agreement with its superior catalytic activity (Figures 3e and S32–S34). This compositional dependence is accompanied by variations in the electronic characteristics of the substituted elements, particularly in the electron transfer energetics, which are expected to influence the electronic structure of the alloy and the associated hydrogen‐related surface processes. In this context, Mo provides particularly effective electronic modulation, reinforcing the reaction‐order‐based conclusion that tuning hydrogen‐related surface kinetics plays an important role in governing the overall catalytic performance. The overall performance metrics summarized in Figure 3f consolidate the above kinetic analyses. Collectively, the incorporation of Fe, Co, and Ni is associated with enhanced NH₃ activation, whereas additional Mo incorporation predominantly mitigates metal‐hydrogen interactions.

The long‐term stability of RuNiCoFeMo/CG was further evaluated at 450°C under a GHSV of 5500 mL·g_cat ⁻¹·h⁻¹. The catalyst maintains stable NH₃ conversion over 150 h without noticeable deactivation, demonstrating robust stability (Figure 3g). To assess structural integrity after the test, the spent catalyst was characterized by XRD, XPS (inset of Figure 3g), and TEM (Figure S35). The alloy catalyst maintains its structural and surface chemical integrity after prolonged operation, as evidenced by unchanged diffraction features, XPS spectra (Table S5), particle size distribution, and lattice fringes (Figure S35). The same trend is maintained at a GHSV of 22 000 mL·g_cat ⁻¹·h⁻¹ (Figures S36–S39). Under the tested conditions of 450°C and a GHSV of 22 000 mL·g_cat ⁻¹·h⁻¹), RuNiCoFeMo/CG delivers catalytic performance that compares favorably with those of previously reported Ru‐based catalysts (Figure 3h and Table S6) [31, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45].

2.3. Mechanistic Insight Into the Activity Enhancement

CO‐diffuse reflectance infrared fourier transform spectroscopy (CO‐DRIFTS) was employed to probe the electronic structure of Ru active sites, given the well‐established sensitivity of the C‐O stretching frequency to the electron density of metal centers [12]. For linearly adsorbed CO on Ru, the stretching vibration typically appears in the range of 1990–2070 cm⁻¹, with lower wavenumbers corresponding to increased electron back‐donation from Ru to CO [46, 47]. As shown in Figure 4a–c, the incorporation of Mo induces a systematic redshift of the CO stretching vibration, with RuNiCoFeMo/CG displaying the lowest wavenumber at 2036 cm⁻¹, indicative of the highest electron density at Ru sites [46]. This observation directly evidences the electron enrichment of Ru driven by multimetallic interactions involving Mo. Upon increasing the temperature from 50°C to 250°C, all catalysts exhibit progressive redshifts of the CO adsorption bands, reflecting thermally enhanced metal‐CO back‐donation [32]. Importantly, this effect is most pronounced for RuNiCoFeMo/CG, whose CO stretching frequency shifts to as low as 1998 cm⁻¹ at 250°C, confirming the strongest electron enrichment at Ru sites among the examined catalysts [48]. These spectroscopic results are fully consistent with the electronic structure trends revealed by XANES and XPS analyses, collectively demonstrating that Mo incorporation promotes substantial electron transfer to Ru.

FIGURE 4.

CO‐DRIFTS of (a) RuNiCoFeMo/CG, (b) RuNiCoFe/CG, and (c) Ru/CG measured under He purging, preabsorbed with 10% CO/He at 50°C–250°C. (d, e) In situ DRIFTS spectra of the steady‐state mode under 10% NH₃/He over the RuNiCoFeMo/CG. (f) NH₃‐TPSR‐MS curves. (g) NH₃‐TPD‐MS curves. (h) The initial desorption temperatures of various gas species on different catalysts.

To directly probe surface intermediates involved in NH₃ decomposition, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ NH₃‐DRIFTS) was conducted under steady‐state NH₃/He flow over RuNiCoFeMo/CG as a function of temperature (Figures 4d,e). At low temperature, characteristic bands at 1626 and 3333 cm⁻¹ are assigned to the N‐H bending and stretching vibrations of molecularly adsorbed NH₃, respectively [49, 50]. Upon increasing temperatures, these NH₃‐related bands progressively weaken, accompanied by the emergence of dehydrogenation‐related features at 1595 cm⁻¹ (‐NH₂) and 1438 cm⁻¹ (‐NH) [51, 52], evidencing stepwise N‐H bond cleavage during NH₃ decomposition. Nevertheless, the ‐NH₂ and ‐NH bands on RuNiCoFeMo/CG remain relatively weak over the entire temperature range and do not accumulate significantly, indicating rapid surface turnover of N‐H‐containing intermediates. This behavior is consistent with the higher NH₃ reaction order and facilitated N‐H scission kinetics inferred from kinetic analysis [51]. In contrast, these ‐NH₂ and ‐NH features on NiCoFe‐free catalysts (Ru/CG and RuMo/CG) intensify continuously with increasing temperature, indicating the accumulation of N‐H intermediates and slower surface turnover (Figures S40c and S41a). By comparison, the incorporation of Ni, Co, and Fe accelerates N‐H bond cleavage, and thereby promoting NH₃ decomposition. In addition, a distinct band at ca. 1866 cm⁻¹ assigned to surface metal‐hydrogen species is clearly observed on Ru‐based catalysts without Mo (Figures 4d and S40) [52]. This signal is strongly suppressed or nearly absent on RuNiCoFeMo/CG across the temperature range, indicating that Mo incorporation effectively mitigates H accumulation and facilitates hydrogen removal from the surface.

We subsequently performed temperature‐programmed surface reaction coupled with mass spectrometry (TPSR‐MS; Figure 4f, Figures S42 and S43) and temperature‐programmed desorption coupled with mass spectrometry (TPD‐MS; Figure 4g, Figures S44 and S45) measurements to systematically probe the temperature‐dependent surface reaction and desorption behaviors. The onset temperatures derived from the various temperature‐programmed measurements are collectively summarized in Figure 4h. During NH₃‐TPSR‐MS measurements (Figure 4f, Figures S42 and S43), RuNiCoFeMo/CG exhibits the earliest onset of NH₃ consumption at 214°C, accompanied by the evolution of H₂ at 242°C and N₂ at 245°C. The close proximity of these onset temperatures indicates a tightly coupled reaction sequence from NH₃ activation to subsequent dehydrogenation and N‐N recombination [32]. In contrast, Ru/CG shows a delayed NH₃ consumption onset at 254°C, while RuNiCoFe/CG displays later product evolution (H₂: 305°C; N₂: 261°C). For Ru‐free NiCoFe/CG, N₂ evolution is further postponed to 385°C, indicating inefficient nitrogen removal [50]. Overall, the onset temperatures follow the order RuNiCoFeMo/CG < RuNiCoFe/CG < Ru/CG < NiCoFe/CG, underscoring the superior low‐temperature reactivity of RuNiCoFeMo/CG.

The release kinetics of surface intermediates were further investigated by NH₃ temperature‐programmed desorption coupled with mass spectrometry (NH₃‐TPD‐MS), where the onset and peak desorption temperatures reflect the stability and turnover of adsorbed species (Figure 4g, Figures S44 and S45). RuNiCoFeMo/CG exhibits markedly lower desorption temperatures for both NH₃ (onset/peak at 100/190°C) and H₂ (245/311°C), indicating facilitated desorption of N‐containing intermediates (e.g., NH and NH₂) as well as accelerated removal of hydrogen species [13]. Compared to Ru/CG, RuNiCoFeMo/CG significantly decreases the NH₃ onset and peak desorption temperatures by 53°C and 66°C, respectively, evidencing enhanced turnover of N‐H‐derived intermediates. Further incorporation of Mo induces an additional downshift of the H₂ desorption onset and peak temperatures by 36°C and 61°C compared with RuNiCoFe/CG, highlighting a pronounced promotion of H removal [49]. This trend directly corroborates the attenuated hydrogen inhibition inferred from reaction‐order analysis and the suppressed M‐H features observed in in situ NH₃‐DRIFTS.

To probe nitrogen removal, N₂‐TPD was performed (Figures S46 and S47) [53]. RuNiCoFeMo/CG displays the lowest N₂ desorption onset temperature (273°C), indicating facile N recombination and desorption, whereas Ru‐free NiCoFe/CG shows a much higher onset temperature (338°C), reflecting sluggish nitrogen removal in the absence of Ru. This behavior is fully consistent with the N₂ evolution profiles observed in NH₃‐TPD‐MS and NH₃‐TPSR‐MS measurements (Figure 4g,f), underscoring the essential role of Ru in enabling efficient nitrogen removal. In parallel, H₂‐TPD (Figures S48 and S49) reveals substantially accelerated hydrogen release on RuNiCoFeMo/CG, with an H₂ desorption onset at 251°C, much lower than that of RuNiCoFe/CG (336°C) [54]. Together with the NH₃‐TPD‐MS and NH₃‐TPSR‐MS results, these findings demonstrate that Mo incorporation electronically modulates the alloy to weaken metal–hydrogen interactions, thereby promoting H removal and sustaining rapid surface turnover during NH₃ decomposition. In summary, this analysis provides a coherent kinetic picture of how composition‐dependent electronic regulation governs the turnover of key surface intermediates during NH₃ decomposition.

2.4. DFT Calculations and Mechanism

Subsequently, density functional theory (DFT) calculations were carried out to elucidate the origin of the enhanced NH₃ decomposition activity. From a mechanistic perspective, NH₃ adsorption on metal surfaces proceeds primarily via σ donation from the nitrogen lone‐pair orbital to metal d states, forming a M‐N coordination bond. Subsequent N‐H bond activation is enabled by effective hybridization and electron redistribution between metal d states and the N‐H σ* antibonding orbitals, which weakens the N‐H bond and facilitates stepwise cleavage. Accordingly, both the energetic distribution of metal d states relative to the Fermi level and their degree of electronic localization are key electronic descriptors governing NH₃ adsorption and activation [15].

Guided by this consideration, we first compared the electronic structures of different metal sites (Figures 5a and S50–S55). Element‐resolved projected density of states (PDOS) analysis reveals that, in the RuNiCoFeMo system, Fe, Co, and Ni exhibit higher d‐state contributions near the Fermi level, with d‐band centers located at −1.16, −1.28, and −1.36 eV, respectively, significantly closer to the Fermi level than that of Ru (−1.84 eV). In addition, the d states of Fe, Co, and Ni are more narrowly distributed, indicative of a more localized electronic character. For NH₃/NH_x activation, which is dominated by σ donation and σ* hybridization, such localized d states are more favorable for forming directional and effective orbital overlap, thereby stabilizing NH_x intermediates and promoting stepwise N‐H bond activation [55]. In contrast, although Mo displays a relatively shallow d‐band center (−0.84 eV), its d states are markedly broadened and energetically dispersed over a wide range, reflecting a more delocalized and metallic electronic character. This feature is more consistent with an electronic modulation role within the multimetallic framework rather than serving as a site providing selectively matched local states for NH_x activation.

FIGURE 5.

(a) PDOS for the RuNiCoFeMo, RuNiCoFe, and Ru. (b) ‐ICOHP values of the N‐H bond in NH₃ adsorbed on the metal sites of RuNiCoFeMo and Ru. (c) Schematic illustration of bond formation between the catalyst surface and adsorbates. (d) Charge density difference analysis of RuNiCoFeMo. Comparison of reaction energy changes for elementary steps of NH₃ decomposition, including (e) dehydrogenation, and (f) H* adsorption and desorption.

Notably, the incorporation of Mo shifts the d‐band center of Ru from −1.58 eV in RuNiCoFe to −1.84 eV in RuNiCoFeMo, indicating substantial electronic regulation of Ru induced by Mo through intermetallic interactions, which provides an electronic basis for optimizing subsequent hydrogen‐related processes [30]. We further evaluated the N‐H bond strength of adsorbed NH₃ on the different metal sites of RuNiCoFeMo and Ru by calculating the integrated crystal orbital Hamilton population (ICOHP) of the N‐H bond. Notably, the Fe site shows the smallest ‐ICOHP value (5.90 eV), indicating the most weakened N‐H bond. This reduced ‐ICOHP reflects diminished bonding interactions together with enhanced antibonding contributions near the Fermi level, thereby favoring cleavage of the N‐H bond in adsorbed NH₃ (Figure 5b) [56]. As schematically illustrated in Figure 5c, Mo incorporation induces electron transfer to neighboring Ru sites, resulting in a discernible downshift of the Ru d‐band center from −1.58 to −1.84 eV. Such electronic modulation may reposition the H‐derived antibonding states closer to, or below, the Fermi level, thereby weakening the Ru‐H interaction and altering the dehydrogenation energetics [30], as reflected by the charge‐density analysis in Figure 5d [55]. To further assess the electronic effects of different fifth elements, we analyzed the Ru d‐band centers in the RuNiCoFeX series, yielding values of −2.11 eV (Mn), −1.90 eV (Cu), −1.84 eV (Mo), and −1.46 eV (W) (Figure S56), highlighting the electronic regulation induced by Mo incorporation.

We further calculated the Gibbs free‐energy profiles of the NH₃ decomposition process (Figure 5e,f). On RuNiCoFeMo, the N‐H dehydrogenation occurs without requiring external energy, while the N‐N coupling step requires 0.94 eV. In contrast, on monometallic Ru, N‐H dehydrogenation requires extra energy, and the NH* → N* step demands as much as 2.04 eV. These results indicate that NiCoFe plays a critical role in facilitating N‐H bond cleavage in RuNiCoFeMo (Figure 5e) [57]. Importantly, N‐H bond cleavage on RuNiCoFeMo does not occur at a fixed site. The adsorption configuration evolves along the dehydrogenation sequence, with the first H abstraction most favorable at Fe, followed by Co in the second step and Ni in the third step (inset, Figure 5e). N‐N coupling is another key step governing nitrogen removal (Figure S57). On RuNiCoFeMo/CG, the Ru site exhibits the lowest N‐N coupling barrier (2.31 eV), distinctly lower than those on Ni (2.86 eV), Co (3.16 eV), and Fe (2.79 eV). This result indicates that Ru offers the most favorable energetics for N coupling and subsequent N₂ release, helping to suppress nitrogen accumulation and sustain rapid NH₃ decomposition [12]. This spatially distributed dehydrogenation sequence embodies a stereochemical relay‐catalysis mechanism, wherein the generated hydrogen species are subsequently transferred to Mo‐modified Ru sites, on which H removal is thermodynamically more favorable (Figures 5f and S58) [27]. Collectively, the theoretical calculations reveal that stereochemical relay catalysis in RuNiCoFeMo originates from electronic modulation and site‐specific functional partitioning, wherein Fe/Co/Ni sequentially mediate N‐H dehydrogenation while Mo‐modified Ru sites accommodate and weaken Ru‐H interactions. This cooperative, site‐resolved pathway enables coordinated regulation across the full NH₃ decomposition sequence.

3. Conclusion

In summary, a hierarchical relay catalysis platform based on a RuNiCoFeMo HEA catalyst was constructed to decouple the kinetically intertwined elementary steps of NH₃ decomposition. By deliberately orchestrating distinct active sites, the HEA catalyst enables a synergistic regulatory mechanism, in which NiCoFe components promote stepwise N‐H bond activation, while Mo incorporation electronically modulates the alloy to weaken M‐H binding and facilitate H desorption. This spatial orchestration of distinct functionalities allows the optimized catalyst to transcend conventional scaling‐relation limitations, achieving 3‐fold enhancement in NH₃ conversion and H₂ production at a moderate temperature of 450°C. This work demonstrates that the design of multisite high‐entropy architecture provides a powerful paradigm for precisely steering reaction pathways in complex, multistep catalytic processes beyond NH₃ decomposition.

Author Contributions

Fashuo Du: methodology, investigation, writing – original draft, formal analysis. Guobin Lai: software, investigation, validation. Bowen Liu: software, visualization, investigation. Dechen Wang: software, data curation. Linjie Zhao: writing – review and editing, visualization. Baoguang Mao: resources, writing – review and editing. Husitu Lin: investigation, data curation. Dan Wang: resources, investigation, writing – review and editing. Zichun Wang: funding acquisition, resources, writing – review and editing. Chuangang Hu: conceptualization, supervision, resources, writing – review and editing, project administration, funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting Information is available from the Wiley Online Library or from the author.Supporting File: anie72870‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.