(Ce(NO₃)₃·6H₂O > 99%), ruthenium (III) chloride hydrate (RuCl₃·3H₂O, > 99%), 1,3,5-benzenetricarboxylate (BTC), and ethanol (> 99%), were procured from Sigma-Aldrich and used without further purification; Deionized (DI) water.

Ce-MOF was synthesized following a reported procedure^[28] and subsequently calcined at 500 °C to obtain CeO₂. Ru-MOF was prepared via a solvothermal method using RuCl₃ and BTC. The XRD [Supplementary Figure 1] and SEM of the prepared Ce-MOF are presented in Supplementary Figure 2A, confirming the successful preparation of Ce-MOF with nanorod morphology. Supplementary Figure 2B shows the SEM of the obtained CeO₂. The nRu/CeO₂ catalysts were synthesized by sonication-assisted mixing of Ce-MOF and Ru-MOF precursors, followed by drying and calcination at 500 °C. Ru loading was varied between 0.2-2 wt.%, and catalysts were denoted as nRu/CeO₂, where n represents the Ru content (wt.%). Detailed synthesis procedures are provided in Supplementary Materials.

Trace elemental analysis was performed using a Thermo Scientific™ iCAP™ 7600 ICP-OES. Powder XRD was recorded on a X’Pert PRO (Cu Kα, 1.5406 Å, 45 kV, 40 mA) over 2θ = 5°-80° (0.02° step, 10 s/step) using a zero-background holder. Raman spectra were obtained using Witec Alpha 300 RA (532 nm). Surface area and pore distribution were measured on a 3Flex Micromeritics system at 77 K after 8 h degassing at 120 °C (BET and BJH methods). TEM, high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM), and elemental mapping were performed on an FEI TITAN Cs-corrected ChemiSTEM; post-catalysis HR-TEM was acquired on Titan 80-300 ST (300 kV). H₂-TPR and H₂-TPD were conducted on 70 mg of catalyst in a U-tube microreactor (AutoChem 2920). Ex-situ XPS was performed on ESCALAB Theta Probe (Al Kα, 1,486.6 eV), calibrated to C 1s (284.8 eV). In-situ XRD was carried out on an INEL EQUINOX 3000 with XRK900 reactor under H₂/N₂ (3:1) with stepwise heating to 200-700 °C (1 h per step) and cooling to room temperature (RT). In-situ high-resolution XPS was conducted on ULVAC-PHI Genesis 900 under H₂/N₂ (3:1) at RT, 700 °C (2 h), and 400 °C. Full experimental details and characterization data are provided in the Supplementary Materials.

NH₃ synthesis and decomposition assessment

NH₃ synthesis was evaluated in a fixed-bed quartz tube reactor using catalysts diluted with SiC. Reactions were conducted at 400 °C, pressures of 10-50 bar, and a WHSV of 10,000 mL g^-1 h^-1 with an N₂/H₂ (1:3) feed. Catalysts were pre-reduced under H₂/N₂ at 800 °C. Helium was used as an internal standard, and outlet NH₃ was quantified online using GC-TCD. NH₃ decomposition was performed in the same reactor after reduction at 700 °C under H₂/N₂. Measurements were carried out at a WHSV of 24,000 mL g^-1 h^-1 over a temperature range of 350-550 °C, with NH₃ as the feed gas and He as an internal standard. Each experiment was repeated thrice.

Catalyst stability was assessed under continuous NH₃ synthesis conditions at 400 °C and 50 bar for 45 h following reduction at 800 °C. NH₃ concentration in the outlet stream was continuously monitored to evaluate long-term performance. Each experiment was repeated thrice. Full experimental procedures and operating details are provided in the Supplementary Materials.

The Ru content in the as-synthesized catalysts, prepared via the MOF-derived method described in Section "Materials", was quantified using ICP-OES. The actual Ru compositions are provided in Table 1. Across all samples, the measured Ru content was slightly lower than the nominal loading, which is consistent with known losses due to the volatility of ruthenium oxides during thermal treatment^[29]. Previous studies^[28-30] have reported that CeO₂ can interact strongly with RuO_x species, effectively stabilizing them and minimizing their volatilization compared to other oxide supports. In line with this, the ICP-OES data reveal minimal Ru loss for the 0.2Ru/CeO₂ and 0.5Ru/CeO₂ catalysts, especially at lower loadings. This suggests that stronger metal-support interactions may occur at reduced Ru content, enhancing the retention of Ru during synthesis. SEM analysis [Supplementary Figure 3A-C] revealed that the Ce-MOF-derived CeO₂ support consists of interwoven nanorods (100-200 nm in diameter, 1-2 µm in length) forming a fibrillar network^[31]. After calcination at 500 °C, the rod-like morphology is largely preserved, indicating strong shape inheritance from the Ce-MOF precursor. Ru deposition (0.2-1 wt.%) does not alter the primary ceria morphology; the nanorods remain cross-linked with open interparticle voids. At higher Ru loadings (0.5-1 wt.%), slight bundle thickening and local necking are observed, but no Ru agglomerates are detected, suggesting high Ru dispersion. The preserved open rod-based framework provides short diffusion paths, mechanical stability, and high external surface area, which are favorable for catalysis and consistent with previous reports^[30-32].

Supplementary Figure 4 displays the XRD patterns of the as-prepared CeO₂ supports and their corresponding nRu/CeO₂ catalysts. Figure 1A shows the XRD patterns of the ex-situ reduced (under H₂/Ar at 700 °C) catalysts, revealing the characteristic planes of CeO₂ as observed for the fresh catalysts with enhanced crystallinity across the board. The diffraction peaks observed at 28.7°, 33.3°,47.7°, 56.5°, 59.4°, 69.7°, 76.9°, and 79.3° correspond to the CeO₂ (111), (200), (220), (311), (222), (331), (420), and (422) planes, respectively. However, when compared, it was noted that crystallinity decreases as the Ru loading increases, along with slight peak broadening. The decrease in crystallinity with increased Ru loading is often due to the introduction of structural disorder (defects), formation of amorphous phases, or interference with the host material's crystal lattice^[33,34]. These effects are typically more pronounced at higher loadings, where the influence of Ru on the material's structure becomes more significant, suggesting that the doping of Ru species could suppress the crystallinity of CeO₂, probably due to the substitution of Ce⁴⁺ (0.97 Å) in CeO₂ by Ru ions with a smaller radius (< 0.7 Å)^[35]. The crystallite size of fluorite CeO₂ support and nRu/CeO₂ catalysts were in the range of 21-28 nm. The calculated lattice parameters for CeO₂ support and the Ru-loaded CeO₂ catalysts were 5.316 Å (CeO₂), 5.3363 Å (0.2Ru/CeO₂), 5.334 Å (0.5Ru/CeO₂), 5.347 Å (1Ru/CeO₂), and 5.365 Å (2Ru/CeO₂). Interestingly, this slight expansion of the lattice with Ru incorporation appears counterintuitive, given that Ru⁴⁺ (ionic radius ~0.64 Å, 6-fold coordination) is smaller than Ce⁴⁺ (0.97 Å, 8-fold coordination)^[36], which would typically lead to lattice contraction. Nevertheless, similar trends have been reported in previous studies, where Ru incorporation led to lattice expansion, likely due to structural distortions or defect formation^[37,38]. To further support this observation, Raman spectroscopy was employed, discussed below in detail. CeO₂, with its fluorite-type cubic structure, normally exhibits a prominent Raman band corresponding to the F_2g symmetric oxygen breathing mode. Upon Ru addition, this peak was found to shift slightly toward lower wavenumbers (459.7 cm^-1), providing additional evidence for lattice expansion caused by Ru substitution^[39]. The N₂ adsorption/desorption and pore-size distribution of the as-prepared CeO₂ supports, and the corresponding nRu/CeO₂ catalysts are shown in Figure 1B and C, respectively. The N₂ adsorption/desorption shows that both the CeO₂ supports and the corresponding nRu/CeO₂ catalysts exhibit a typical type IV adsorption isotherm, with a clear H3-type hysteresis loop appearing between p/p₀ = 0.6 and 1.0, suggesting that the CeO₂ supports and nRu/CeO₂ catalysts possess a mesoporous structure. Additionally, there is an increase in the N₂ adsorption at higher pressures, indicating the presence of macropores within the CeO₂ support. The BET surface area, pore diameter, and pore volume of the samples are provided in Table 1. The BET specific surface areas of the catalysts, ranging from 48 to 74 m²/g, are in good agreement with values reported in the literature for similar CeO₂-based materials^[40-43]. The observed increase in surface area following Ru impregnation and subsequent thermal treatment is likely due to surface modifications of CeO₂ that create or expose additional interparticle voids, thereby enhancing N₂ adsorption capacity^[44]. The increase in surface area with increasing Ru loading could also be due to the possible formation of surface defects (O vacancies)^[45]. High defect content is beneficial to generate a high specific surface area, as also observed by Zhou et al.^[46], where they observed increasing surface area with increasing defect density within the Ce MOF structure.

For the nRu/CeO₂ catalysts, XRD analysis was conducted under controlled reduction conditions to monitor structural changes in the 200-800 °C temperature range. The initial XRD measurement was performed at RT. The catalyst was then heated up to 200 °C and held for 1 h under a reducing gas mixture atmosphere (H₂/N₂ = 3:1), followed by the acquisition of the XRD measurement. This process was repeated sequentially at 400, 600, and 700 °C, with each temperature maintained for 1 h before collecting the XRD measurements. After the final step at 700 °C, the catalyst was cooled down to RT under the same reducing atmosphere, and a final XRD analysis was performed to assess any post-reduction structural changes. As observed from Figure 2, the XRD peaks of the catalysts exhibit no changes in the characteristic CeO₂ diffraction peaks under atmospheric conditions or with increasing temperatures during reduction atmosphere, apart from an improvement in crystallinity. Notably, no Ru-related peaks are observed in the in-situ XRD patterns. This absence is attributed to the exceptionally small Ru particle size, determined to be less than 1.5 nm by HR-TEM, which falls below the detection limit of XRD analysis. Additionally, HR-TEM analysis (discussed below) confirmed the uniform dispersion of Ru nanoparticles across the catalyst surface, further explaining the absence of distinct Ru peaks in the in-situ XRD results.

Oxygen vacancies (O_Vs) are widely recognized as crucial descriptor for catalytic activity, as they not only lower the activation energy barrier for N₂ adsorption but also serve as active reaction sites. To investigate these defect sites, Raman spectroscopy was employed on ex-situ reduced (under H₂/Ar at 700 °C) catalysts, as illustrated in Figure 3. The Raman spectra of CeO₂ primarily feature a prominent F_2g band at 465.3 cm^-1, characteristic of the fluorite phase. Additionally, weaker bands are observed at 257, 598, and 1,172 cm^-1, which are attributed to the second-order transverse acoustic (2TA) mode, defect-induced (D) mode, and second-order longitudinal optical (2LO) mode, respectively^[47-49]. A sharp and symmetric F₂g band at ~465.3 cm^-1 indicates high crystallinity and a well-ordered fluorite structure, also observed from XRD results above. In addition, the incorporation of Ru metal causes a noticeable red-shift and broadening of the F₂g vibrational band. The shift to 465.3 cm^-1, accompanied by band broadening, suggests alterations in the Ce-O bonding environment due to Ru interaction, an effect that aligns with observations reported by Giri et al.^[47], indicating that the deposited Ru lowers the symmetry of the Ce-O bond, by introducing O_Vs and Ru-Ce-O bond formation^[47]. It has been reported that for pure RuO₂, bands at 528, 644, and 716 cm^-1, corresponding to the E_g, A_1g, and B₂g modes, are typically observed^[50]. However, these bands have low intensity relative to the CeO₂ bands and are not detectable in the Ru/CeO₂ spectra. This is likely due to the small amount of Ru present or the interaction between the Ru species and ceria, as supported by the XRD results. The I_D-to-I_F2g ratios, specifically I_(598+1172)/I_465.3, which reflect intrinsic defect concentrations such as oxygen vacancies (O_V)^[51], are summarized in Table 1. The CeO₂ support synthesized in this study exhibited a notably higher ratio compared to CeO₂ supports reported in the literature, implying an oxygen vacancies abundance. This enhanced defect density is attributed to the unique synthesis route involving Ce-MOFs, which offers a more porous structure conducive to O_V formation. Notably, a previous study reported I_(1175+600)/I₄₆₂ ratios of only 0.07 and 0.04 for CeO₂-rod and CeO₂-cube, respectively, indicating low vacancy levels, while their corresponding Ru-loaded counterparts (Ru/CeO₂-r and Ru/CeO₂-c) showed slightly elevated ratios of 0.40 and 0.31^[14]. In contrast, the present work exhibits systematically higher I_(598+1172)/I_465.3 ratios, increasing with Ru loading from 1.02 (CeO₂) to 1.35 (0.2Ru/CeO₂), 1.64 (0.5Ru/CeO₂), 1.75 (1Ru/CeO₂), and 1.84 (2Ru/CeO₂). This underscores the enhanced O_V population due to both the preparation strategy and the synergistic effect of Ru incorporation. In our work, the CeO₂ support exhibits a characteristic peak at {111}, which aligns with the theoretical calculations that indicate oxygen vacancy formation energies vary with the exposed crystal plane in the order {110} < {100} < {111}^[52,53]. The presence of this peak at {111} suggests that oxygen vacancies are effectively created on this plane, supporting the higher formation energy of vacancies observed for the {111} plane. This confirms that the {111} plane is more favorable for oxygen vacancy formation, as seen in our experimental results, where the catalytic activity is enhanced due to the presence of these vacancies. Moreover, DFT studies suggest that Ru dopants activate lattice oxygen by elongating Ce-O and Ru-O bond^[54], lengths relative to their parent oxides, thereby facilitating O_v generation by acting as nucleation centers for O_V clusters on CeO₂ surfaces^[55].

To gain deeper insight into the metal-support interactions and redox characteristics of the synthesized catalysts, H₂-TPR was conducted over the 35-800 °C temperature range of [Figure 4A]. H₂-TPR profiles of nRu/CeO₂ catalysts exhibit a prominent low-temperature reduction peak in Region I (~50-150 °C) corresponding to the reduction of RuO₂ to Ru, which shifts to lower temperatures with increasing Ru loading^[56]. In contrast, the surface reduction peak of CeO₂ in Region II (150-500 °C) appears less pronounced in nRu/CeO₂ catalysts compared to pure CeO₂^[57]. Additionally, the bulk CeO₂ reduction Region III (> 500 °C) is significantly suppressed in Ru-containing catalysts^[58]. The downward shift of the RuO₂ reduction peak (observed around 50-150 °C) with increasing Ru loading from 0.2 to 0.5 wt.% indicates improved reducibility of the catalyst. This behavior suggests that Ru nanoparticles are highly dispersed at the atomic scale, as confirmed by HRTEM, where the particle size is below 1.5 nm. The distribution of active components on the catalyst surface plays a crucial role in the reduction of surface oxygen. With higher dispersion of Ru metal, hydrogen activation is facilitated more efficiently, leading to an increase in H₂ consumption^[59]. Such fine dispersion promotes hydrogen spillover^[60,61], enabling H₂ to dissociatively chemisorbed and migrate onto the CeO₂ support more effectively. As the Ru loading increases from 0.2 to 0.5 wt%, a greater number of well-dispersed Ru species become available, enhancing the overall reduction process at lower temperatures. In case of 1Ru/CeO₂ catalyst, two step reduction of RuO₂ happens (observed around 50-150 °C), leading to the formation of RuO and Ru species. The multiple reduction peaks in H₂-TPR profiles of 1RuO₂/CeO₂ catalysts can be attributed to different RuO₂ particle sizes^[62,63] and their varying interactions with the support, as seen in HRTEM where some larger Ru nanoparticles were observed. The weaker CeO₂ surface reduction in nRu/CeO₂ implies that Ru promotes Ov formation at lower temperatures, leading to a more gradual, extended reduction. More surface oxygen is removed below 150 °C, generating abundant Ov compared to bare CeO₂, likely via hydrogen spillover^[64-66]. Among the samples, 0.5Ru/CeO₂ shows the most favorable reduction profile, suggesting easier Ov formation. Enhanced oxygen vacancies facilitate N₂ dissociation^[67-69], improving NH₃ synthesis performance.

The H₂-TPD analysis of the nRu/CeO₂ catalysts shown in Figure 4B revealed two distinct regions. Region I corresponds to the weakly adsorbed H-species on highly dispersed Ru nanoclusters with two different H adsorption sites observed: one on Ru metallic sites of varying sizes. Region II corresponds to the strongly adsorbed dissociative H-species that are taken up by the Ru-CeO₂ metal-support interface, which is closely associated with oxygen vacancies on the CeO₂ support. In the higher temperature range (150-350 °C), the peak associated with strongly adsorbed hydrogen on Ru-CeO₂ interfaces or O vacancies showed a slight increase in intensity with higher Ru loading. This suggests the increase in O vacancies along with enhanced hydrogen spillover from Ru to CeO₂ with increased Ru loading, leading to more hydrogen being stored and subsequently desorbed from these interfacial sites. The crystallite sizes [Supplementary Table 1] obtained from the H₂-TPD analysis are provided in Supplementary Materials.

To gain a clearer understanding of the oxidation states and local environment of Ru species, XPS was performed on the catalysts following ex-situ reduction in H₂/Ar at 700 °C. This analysis primarily aimed to evaluate the extent of Ru reduction and the interaction between Ru and the CeO₂ support. The Ru 3p peaks at 461.9 and 484.1 eV are indicative of metallic Ru, whereas those at 463.7 and 485.9 eV correspond to oxidized Ru in the form of RuO₂, as previously reported by Wang et al.^[70]. Deconvolution of Ru 3p signals was carried out using Shirley background correction, revealing distinct electronic environments for the different loadings, and is shown in Figure 5A-C. The 0.5Ru/CeO₂ catalyst predominantly exhibited metallic Ru, with characteristic peaks at 461.8 and 484.3 eV. In contrast, 0.2Ru/CeO₂ and 1Ru/CeO₂ catalysts showed a noticeable shift toward higher binding energies (463.1/485.3 eV and 462.4/484.7 eV, respectively), suggesting a greater proportion of oxidized Ru species (Ru⁴⁺). This shift likely arises from variations in the chemical surroundings or electron transfer between Ru and the CeO₂ support, thus driving the Ru oxidation state and of the SMSI phenomena between Ru and ceria support.

The O 1s XPS spectra [Figure 5D-F] revealed three distinct oxygen environments in all three catalysts under study. The peak at ~528.8 eV corresponds to lattice oxygen (O_L), while those around ~530 and 532-533 eV are attributed to oxygen vacancies (O_v) and surface species such as hydroxyls or carbonates, respectively^[71]. Deconvolution using Shirley background correction showed peaks at 528.4, 530.1, and 532.0 eV for 0.2Ru/CeO₂, 528.8, 530.9, and 532.8 eV for 0.5Ru/CeO₂, and 528.4, 530.1, and 532.0 eV for 1Ru/CeO₂. Notably, the 0.5Ru/CeO₂ catalyst exhibited a comparatively lower intensity of carbonate-related peaks. The reduced presence of carbonate species is beneficial, as it implies fewer surface-blocking contaminants, thereby exposing more catalytically active sites. This cleaner surface enhances metal-support interactions and facilitates the formation and preservation of oxygen vacancies, both of which are critical for promoting N₂ activation and hydrogen dissociation. Moreover, a lower population of carbonates also contributes to improve surface basicity and support reducibility, ultimately leading to superior catalytic performance of 0.5Ru/CeO₂ in NH₃ synthesis under mild reaction conditions.

The Ce 3d XPS spectra shown in Figure 5G-I display a complex pattern, characteristic of cerium oxide, and have been deconvoluted based on established literature assignments^[72-74]. The spectral features at ~915.4, 906.4, 899.7, 897.1, 887.4, 881.3 eV correspond to the presence of Ce⁴⁺ oxidation states. In contrast, the peaks centered around 902.6 and 884.1 eV are attributed to the presence of Ce³⁺ species, indicating the partial reduction of cerium within the oxide lattice.

An in-situ XPS study was performed to examine the surface composition of the catalysts under different conditions. XPS spectra were collected for fresh samples at RT, after reduction in N₂/H₂ (1:3) at 700 °C for 2 h, and under reaction conditions at 400 °C. Supplementary Figures 5-7 show the corresponding core-level XPS spectra of the nRu/CeO₂ catalysts (n = 0.2, 0.5, and 1), provided in the Supplementary Materials.

The reduction process performed under the N₂/H₂ atmosphere significantly impacts the surface composition trends of the nRu/CeO₂ catalysts as presented in Table 2. Initially, at 25 °C, the catalysts exhibit a lower atomic% of Ru and varying amounts of Ce and O, reflective of the as-synthesized state of catalysts before reduction. As the temperature increases to 700 °C (reduction condition), the Ru atomic percentage rises notably, indicating that the reducing environment promotes Ru migration toward the surface and enhances its dispersion on the ceria support. This trend is further corroborated by XPS analysis, which reveals a temperature-dependent increase in the surface Ru/Ce ratio accompanied by a decrease in the O/Ce ratio, evidencing Ru surface enrichment and partial reduction of CeO₂ to CeO_2-x^[75]. The resulting Ru-rich, oxygen-deficient surface provides an optimal population of Ru-CeO_2-x interfacial sites. The elevated temperature likely enhances the reduction of both Ru and CeO₂, facilitating an interaction between metal and support that stabilizes Ru in a metallic form, which is often more catalytically active. After this step, when the temperature is reduced back to 400 °C (reaction condition), the atomic% of Ru remains almost similar with a slight reduction. This suggests that the stability of Ru on the surface is maintained after the reduction.

Moreover, oxygen vacancies (O_v)^[76-78] are prevalent anionic point defects commonly observed in transition and f-block metal oxides. These vacancies typically arise in materials containing cations that can alternate between multiple oxidation states, such as Ce³⁺/Ce^4+[77,79,80] and Ti³⁺/Ti^4+[81]. The formation of Ov is often induced by high-temperature treatments^[82] or under reducing environments^[83], where lattice or surface O^2- ions are removed alongside the reduction of metal cations, all while preserving the overall crystal framework. Such defect sites are particularly significant in the context of heterogeneous catalysis^[77,79,80]. In the case of CeO₂, which adopts a cubic fluorite structure (space group Fm$$\overline{3}$$m), the introduction of O_v is associated with the partial reduction of Ce⁴⁺ to Ce³⁺. This process modifies the surface electronic environment and introduces positively charged sites^[84], which are known to enhance catalytic activities in reactions like NH₃ synthesis, CO oxidation, the water-gas shift reaction, and CO₂ conversion^[3,77,78].

At ambient conditions (25 °C), the surface environment of the catalyst shows an O/Ce ratio close to 2, indicating a well-oxidized surface (where O_v population is expected to be low)^[85]. However, under reducing conditions, the O/Ce ratio decreases to ~1.7 signifying the removal of O atoms from the lattice, thus leading to the formation of O vacancies (CeO_2-x). In the case of 0.5Ru/CeO₂, most O_v (O/Ce = 1.77) was observed at the reaction condition (400 °C) as compared to the rest of the catalysts. These vacancies play a critical role in enhancing catalytic performance by serving as active sites and improving the mobility of lattice oxygen for redox reactions^[86]. Additionally, the removal of oxygen reduces Ce⁴⁺ to Ce³⁺, altering the electronic environment and influencing the adsorption and activation of reactants^[87]. The formation of O vacancies also induces slight lattice distortions, which can enhance interactions with other species, such as Ru in the nRu/CeO₂ catalysts^[88]. This demonstrates the importance of the reducible nature of CeO₂ in enabling its superior catalytic activity under reaction conditions as discussed below.

HR-TEM was carried out on the reduced catalysts to examine their microstructure and elemental distribution under reaction conditions. The HRTEM images shown in Supplementary Figure 8 reveal that all three catalysts exhibit a rod-like structure with elongated particles. This suggests a distinct morphology of the nRu/CeO₂ catalyst system, likely influenced by the MOF-derived preparation method.

Figure 6A-C present the HRTEM images of the nRu/CeO₂ catalysts. The inset in each image shows the Ru particle size distribution, which indicates an increase in particle size across the three catalysts. Specifically, the particle sizes range from 0.9 to 1.06 to 1.2 nm as the n values increase from 0.2 to 0.5, and then to 1, respectively. This indicates a clear relationship between Ru loading and particle size growth, as anticipated. Additionally, good dispersion of Ru was observed for all three nRu/CeO₂ catalysts. However, at a Ru loading of 1 wt.%, slight agglomeration of Ru was observed, particularly at the edges of the surface. This agglomeration can be attributed to the high surface energy of Ru (~2.5-3.0 J/m²) at the edges, which promotes particle growth as a way to minimize surface energy (J/m²)^[89]. Moreover, HR-TEM images of the reduced 0.5Ru/CeO₂ catalyst [Supplementary Figure 9A-C] show lattice fringes indexed to the CeO₂ (220) and (111) planes, with d-spacings of ~0.19 and ~0.32 nm, respectively. The corresponding FFT patterns (insets) are consistent with a retained CeO₂ fluorite structure after reduction.

Elemental mapping [Figure 7] further confirmed the distribution of elements (Ce, Ru, and O) across the nRu/CeO₂ catalysts. The mapping data showed that Ru is homogeneously dispersed throughout the catalyst, providing evidence of uniform distribution even at varying Ru loadings in the range studied. These findings highlight the interplay between Ru loading, particle size, and dispersion characteristics, which are critical for catalytic interface and performance afterall.

To critically evaluate the MOF-derived catalyst, Figure 8 compares HR-TEM images and Ru particle size distributions of 0.5Ru/CeO₂ catalysts prepared using MOF-derived CeO₂ and commercial CeO₂ supports. The MOF-derived catalyst exhibits well-defined nanorod morphology and uniformly dispersed Ru nanoparticles with a smaller average size (d_avg ≈ 1.06 nm), whereas the commercial CeO₂-based catalyst shows no distinct morphology and significantly larger, less uniform Ru particles (d_avg ≈ 3.7 nm). The nanorod structure formed via MOF templating enhances Ru dispersion due to higher surface area, abundant oxygen vacancies, and exposure of reactive {111} and {100} facets, which promote strong metal-support interactions and suppress Ru sintering. These features provide a higher density of active sites for N₂ activation, favoring NH₃ synthesis, consistent with previous reports highlighting the superior performance of Ru/CeO₂ nanorods over other morphologies^[90,91].

CeO₂ is widely used in catalysis for its Ce⁴⁺/Ce³⁺ redox behavior, oxygen storage capacity, and electronic structure, which promote NH₃ synthesis by creating oxygen vacancies (Ov) and modulating Ru species^[19,92,93]. Beyond reduction behavior, CeO₂ morphology, Ru loading, and dispersion strongly influence catalytic performance. Nanorods and polyhedrons enhance activity by providing high surface area, abundant Ov, and exposed {111}/{100} facets, which stabilize small, low-crystallinity Ru species with higher surface Ru⁴⁺, whereas nanocubes form larger, poorly dispersed Ru particles with weaker metal-support interactions^[19,90,94]. Ru-O-Ce linkages further generate Ov via partial Ce⁴⁺ reduction, facilitating H₂ and N₂ adsorption and N₂ dissociation, a rate-limiting step in NH₃ formation^[95,96]. Similar effects of Ov on N₂ activation have been observed in photocatalytic systems such as BiOBr^[97], TiO₂^[98], and TiO₂/Au nanorods^[99], highlighting the critical role of oxygen-deficient sites. The NH₃ synthesis of nRu/CeO₂ catalysts (n = 0.2, 0.5, 1) was evaluated using a 25% N₂-75% H₂ feed. Figure 9 and Supplementary Table 2 illustrate the NH₃ synthesis rates at 400 °C across a pressure range of 10-50 bar for the CeO₂ support and nRu/CeO₂ catalysts.

Given the high reversibility of NH₃ synthesis, we also evaluated the applicability of the nRu/CeO₂ catalysts in NH₃ cracking at 350-550 °C and 1 bar. NH₃ cracking (2NH₃ → 3H₂+N₂, ∆H = 92.44 kJ/mol) involves the cleavage of N-H bonds followed by the recombination of nitrogen and hydrogen atoms to form N₂ and H₂ gases^[112]. As an endothermic reaction, its catalytic efficiency is highly dependent on temperature. The influence of temperature on NH₃ conversion% over the nRu/CeO₂ catalysts is presented in Figure 9C. Within the range of 300 to 550 °C, NH₃ conversion steadily increases with temperature across all nRu/CeO₂ catalysts. Notably, the 0.5Ru/CeO₂ catalyst exhibits the highest activity, achieving an impressive 93% NH₃ conversion at 550 °C. At this temperature, the conversion follows the trend: 0.2Ru/CeO₂ (80%) < 1Ru/CeO₂ (87%) < 0.5Ru/CeO₂ (93%). A similar trend is observed in the hydrogen production rate at 550 °C, with 0.5Ru/CeO₂ reaching 25,282 µmol g^-1 min^-1, outperforming both 0.2Ru/CeO₂ (21,832 µmol g^-1 min^-1) and 1Ru/CeO₂ (23,784 µmol g^-1 min^-1) as shown in Figure 9D. Remarkably, the 0.5Ru/CeO₂ catalyst demonstrates superior NH₃ conversion and a faster decomposition rate compared to previously reported Ru/CeO₂ catalysts with Ru nanoparticles. The enhanced catalytic performance of the nRu/CeO₂ catalyst for NH₃ cracking can be attributed to several key factors. First, the increased concentration of oxygen vacancies (O_vs) in the catalyst plays a critical role in improving catalytic activity^[10,107,108]. These O_v act as active sites for the adsorption and dissociation of N₂ and H₂, which are essential steps in the NH₃ cracking reaction. Raman and XPS analysis confirmed the higher O_v concentration, as indicated by the Raman spectroscopy, which correlates with improved catalytic efficiency. Additionally, the hydrogen spillover effect, observed through H₂-TPR analysis, further facilitates the NH₃ cracking process. The spillover effect allows for the migration of atomic hydrogen from the Ru nanoparticles to the CeO₂ support, thereby enhancing the cleavage of N-H bonds and improving the overall hydrogen production rate^[109]. Moreover, the fine dispersion of Ru on the MOF-derived CeO₂ support, as confirmed by HR-TEM, ensures that a large surface area of Ru is available for the reaction, preventing agglomeration and maximizing the number of active sites^[110]. These combined effects: higher oxygen vacancy concentration, effective hydrogen spillover, and enhanced Ru dispersion contribute to the superior catalytic performance of the nRu/CeO₂ catalyst, which outperforms previously reported Ru/CeO₂ catalysts in NH₃ cracking.

When benchmarked against other reported Ru/CeO₂ systems with varying Ru weight loadings, our 0.5Ru/CeO₂ catalyst [Supplementary Table 3, Figure 10] demonstrated superior performance, further underscoring the importance of precise control over Ru particle size and metal-support interactions. These findings validate the previously discussed mechanism, where Ru size not only alters site populations and electronic structure (thus affecting work function and N₂ dissociation) but also determines the operative reaction pathway: associative or dissociative, especially near the critical ~1 nm (Ru particle size) threshold.

The post-catalytic surface and structure characterization was performed after conducting the following tests: NH₃ synthesis, NH₃ cracking (300-550 °C), and the NH₃ synthesis stability test. The provided post-catalytic assessment results in the XRD [Figure 11A, Supplementary Table 4] and HRTEM images [Figure 11B-D] provide important insights into the structural changes and metal dispersion on nRu/CeO₂ catalysts after the catalytic reaction for NH₃ synthesis and cracking. The XRD patterns show the characteristic peaks of CeO₂, indicating that the ceria support retains its fluorite structure after the catalytic reaction. The peaks observed at 28.4°, 33°, 47.4°, 56.3°, and 59°, 65.6°, 69.4°, 76.6°, and 79° correspond to the CeO₂ (111), (200), (220), (311), (222), 400, (331), (420), and (422) planes, respectively. After the reaction, the CeO₂ diffraction peaks shift slightly to lower 2θ value, indicating increase in lattice parameters [Supplementary Table 4]. The increase in lattice spacing observed in the XRD studies is also observed in post catalytic HRTEM analyses, particularly the shift in the d-spacing corresponding to the characteristic CeO₂ plane (111) from 3.08 to 3.12 Å after the catalytic reaction, suggests that structural changes occurred in the catalyst during the NH₃ decomposition process. This change in lattice spacing could be attributed to the formation of oxygen vacancies in the CeO₂ support during the reaction. The creation of oxygen vacancies typically leads to local distortions in the crystal lattice^[111], which can cause an expansion of the lattice as the ceria undergoes reduction. Furthermore, the interaction between the Ru nanoparticles and the CeO₂ support might contribute to this lattice expansion^[87,112]. The Ru species, especially when atomically dispersed or present as small clusters, can influence the electronic environment of the CeO₂ support, potentially weakening the Ce-O bonds and facilitating the formation of vacancies^[113]. This expansion of the lattice upon oxygen vacancy formation is consistent with previous studies, where CeO₂ lattice is observed to expand upon reduction due to the transformation of Ce⁴⁺(0.94 Å) to Ce³⁺ (1.04 Å)^[114,115]. However, the post-reaction XRD data confirms that the CeO₂ support remains stable, and there are no significant signs of phase segregation or crystallite growth for Ru, indicating a well-dispersed catalyst.

The post-catalytic HRTEM images [Supplementary Figure 10A-C] reveal that the nRu/CeO₂ catalysts are rod-shaped, with widths ranging from 60 to 70 nm and lengths of approximately 1 μm. The lattice fringe spacings observed are 0.19, 0.27, and 0.32 nm, which correspond to the (220), (200), and (111) facets of CeO₂^[107]. Notably, no identifiable Ru nanoclusters were observed on the surfaces of the nRu/CeO₂ catalysts, suggesting that the Ru species are likely dispersed at the subnanometer or even single-atom level^[116].

The EDX mapping results [Figure 12A-C] further confirm the homogeneous distribution of Ru species across the entire nanostructure. High-magnification HAADF-STEM images reveal no clusters, indicating that Ru does not exist as clusters or particles in the observed regions^[117].