Enhanced activity of Ru-based catalysts for ammonia decomposition through nitrogen doping of hierarchical porous carbon carrier

Wang, R., Chen, X., Hao, H., Wang, B., Yu, H., Wang, M., Xie, Y., Wang, J., and Si, H. (2024). “Enhanced activity of Ru-based catalysts for ammonia decomposition through nitrogen doping of hierarchical porous carbon carriers,” BioResources 19(3), 4313-4334.

Abstract

Activated carbon (AC) materials, renowned for their high specific surface area, excellent conductivity, and customizable functional groups, are widely employed as catalyst carriers. However, enhancing the activity of Ru-based catalysts supported on AC (Ru/AC) for ammonia decomposition remains a challenge. In this study, commercial AC was utilized as a substrate, with glucose and urea employed as modifiers. Specifically, the surface of the AC was modified via a hydrothermal pyrolysis method, resulting in the successful post-treatment in situ co-doping of nitrogen (AC-GN). Experimental results revealed that Ru/AC-GN exhibited a hydrogen production rate 46% higher than that of Ru/AC at 475 °C, indicating improved activity and stability. The characterization of AC-GN demonstrated that nitrogen doping primarily occurred on the external surface and macropores of the AC, increasing the nitrogen content in the carrier, particularly pyrrolic nitrogen content, while preserving the original structural and morphological integrity of the AC. The enhanced dispersion of Ru, combined with the improved electronic transmission capabilities and strengthened interactions between the metal and the modified carrier, were identified as pivotal factors contributing to the enhanced low-temperature efficacy of Ru/AC-GN. This paper presents a novel direction for the large-scale preparation of efficient catalysts for ammonia decomposition.

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Enhanced Activity of Ru-based Catalysts for Ammonia Decomposition through Nitrogen Doping of Hierarchical Porous Carbon Carriers

Rui Wang,^a Xiuxiu Chen,^a Han Hao,^b Bing Wang,^a Hewei Yu,^a,* Meng Wang,^cYongjun Xie,^d Jianmei Wang,^a,* and Hongyu Si ^a,*

DOI: 10.15376/biores.19.3.4313-4334

Keywords: Activated carbon; Nitrogen doping; Glucose; Ammonia decomposition

Contact information: a: Shandong Provincial Key Laboratory of Biomass Gasification Technology, Energy Research Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, China; b: Jinan Xinhang Experimental Foreign Language School, Jinan, 250014, China; c: The Li Xia Squadron of the Ecological Environment Protection Comprehensive Administrative Law Enforcement Detachment in Jinan City, Shandong Province, China; d: 101 Shanjian Road, Economic Development Zone Linyi, Shandong, China; *Corresponding authors: yhw@qlu.edu.cn (H. YU), wangjm@sderi.cn (J. Wang), sihy@sderi.cn (H. Si).

GRAPHICAL ABSTRACT

INTRODUCTION

Hydrogen is widely recognized as an efficient and clean energy source, offering potential solutions to environmental pollution and the energy crisis (Mehrpooya and Habibi 2020; Wan et al. 2021). However, its low volumetric density and challenges associated with its storage and transportation, particularly liquefaction, pose significant barriers to its widespread application (Abe et al. 2019; Luo et al. 2020). In recent years, ammonia has garnered considerable attention as an effective hydrogen carrier owing to its high hydrogen content of 17.7 wt% alongside its liquid state at room temperature (25 °C) and low pressure (6 atm). Additionally, its CO_X-free decomposition process and the relative maturity of ammonia storage and transport technologies make it a highly promising medium for H₂ storage and transportation (Chang et al. 2021; Sun et al. 2022).

The decomposition of ammonia for hydrogen production is considered a promising pathway for transitioning to clean energy and developing a hydrogen economy, despite existing technical challenges, particularly in enhancing decomposition efficiency and reducing costs (Lee et al. 2021). A key strategy involves the development of highly active catalysts that can operate effectively at temperatures below 500 °C (Mukherjee et al. 2018). Commonly used active metals in ammonia decomposition catalysts include Ru, Ni, and Co. Among them, Ru-based catalysts are the most frequently utilized, owing to their exceptional activity at low temperatures (Cao et al. 2022; Nakamura et al. 2022). Notably, catalytic activity is greatly influenced by the dispersibility, morphology, and size of Ru nanoparticles (Yao et al. 2011). The B5-type surface of Ru contains high-activity sites, and nanoparticles approximately 2 nm in size are considered to provide an increased number of B5-type sites (Zheng et al. 2007; Su et al. 2024). Karim et al. (2009) have demonstrated that the catalytic activity of spherical Ru nanoparticles ranging from 1.8 to 3 nm in size was the highest. Furthermore, extensive research has revealed a close relationship between the efficiency of NH₃ decomposition and factors such as alkaline environments and electron transfer capabilities (Tan et al. 2012; Nagaoka et al. 2014), which can be modulated by employing different carriers.

Porous carbon-based materials, known for their expansive specific surface area, adaptable functional groups, and exceptional conductivity, are widely employed as catalyst carriers (Yin et al. 2004a). The primary focus of research has centered on activated carbon (AC) and carbon nanotubes (CNTs) (Chen et al. 2021). For instance, Yin et al. (2004b) synthesized K-Ru/CNTs at 450 °C with an NH₃ flow rate of 60000 mLg_cat^-1 h^-1, achieving a decomposition efficiency of 97.3% and hydrogen formation rate 32.6 mmol g_cat^-1min^-1, which significantly outperformed the 9.6 mmol g_cat^-1min^-1 hydrogen formation rate of Ru/AC. This enhancement is attributed to the high degree of graphitization in CNTs, facilitating electron transfer between the carrier and metal. There is a notable increase in the H₂ production rate by doping nitrogen into CNT carriers, primarily owing to enhanced electron interactions between the carrier and the metal, as well as the increased anchoring sites provided by nitrogen, which improve Ru particle dispersion (Hien et al. 2015; Ren et al. 2017). However, research on modifying AC via nitrogen doping for ammonia decomposition remains relatively limited.

Biomass carbon materials are considered excellent carrier materials from both environmental and cost perspectives. However, current research indicates that heteroatoms such as S, Cl, and N on the surface of AC can inhibit ammonia decomposition owing to low graphitization levels, impeding effective electron transfer between the metal and the carrier (Yin et al. 2004b). While heteroatoms can be removed by introducing H₂ at high temperatures (Raróg-Pilecka et al. 2003), the presence of defects benefits Ru anchoring, thereby enhancing dispersion (Rodríguez-reinoso 1998). Nevertheless, oxygen-containing functional groups on the surface can diminish the activity of Ru owing to their electron-withdrawing effect (Rarogpilecka et al. 2005). Consequently, a suitable nitrogen doping method must be chosen to strike an optimal balance. In this context, a pioneering approach uniformly anchors micrometer-sized carbon spheres onto the surface and within the large pores of commercial AC using glucose via a hydrothermal method, revealing a new direction for post-treatment nitrogen doping (Liu et al. 2010).

To enhance the performance of the AC carrier, a nitrogen-doping method based on hydrothermal pyrolysis was employed. Concurrently, nitrogen-doped hydrothermal carbon was deposited on the surface of the AC, achieving the post-treatment in situ co-doping of nitrogen. This method aimed to preserve the original pore structure and degree of graphitization to the greatest possible extent, while the introduction of nitrogen provides additional sites for Ru loading. Consequently, the performance of AC as an ammonia decomposition catalyst carrier is substantially enhanced, thereby offering a novel approach for preparing high-activity Ru/AC catalysts.

EXPERIMENTAL

Materials

AC, derived from coconut shells, was procured from Shanghai Ron Chemical Technology Co., Ltd., with the item number R019806. Urea (> 99%), glucose (C₆H₁₂O₆ > 99%), nitric acid (65 to 69%), and ruthenium (31.3%) were sourced from commercial suppliers and employed without additional purification.

Fig. 1. Experimental process and detailed schematic diagram

Sample synthesis

The carrier was prepared by grinding a suitable amount of commercial AC to 20–40 mesh, referred to as AC henceforth in this paper. Subsequently, 5 g of AC was mixed with 200 mL of nitric acid for oxidative pretreatment, followed by multiple washes with deionized water and drying at 80 °C for 12 h, resulting in AC-O. Then, 1 g of AC-O and 1 g of urea were added to 20 mL of distilled water and stirred at room temperature for 30 min, followed by transferring the mixture to an autoclave. The autoclave was placed in an oven and maintained at 180 °C for 6 h. After cooling to room temperature, the sample was filtered and dried at 80 °C for 12 h (Figure 1a). The obtained sample was then placed in a tube furnace and calcined under a N₂ flow of 100 mL min^-1. The pyrolysis protocol involved heating the sample to 800 °C at a rate of 10 °C min^-1, holding the sample at this temperature for 1 h, and finally cooling it to room temperature. The resultant sample was denoted as AC-N (Fig. 1b). Furthermore, AC-G and AC-GN were prepared using the same procedure, with the only difference being the substitution of 1 g of urea with 1 g of glucose and a combination of 1 g urea and 1 g glucose, respectively.

The catalysts were prepared using an equal volume impregnation method to achieve a 3 wt% Ru loading. Specifically, AC, AC-N, AC-G, and AC-G-N were ultrasonically mixed for 20 min, allowed to stand for 2 h, and then dried at 80 °C for 12 h to obtain catalysts with 3 wt% Ru loading, designated as Ru/AC, Ru/AC-N, Ru/AC-G, and Ru/AC-GN, respectively (Fig. 1c). The prepared catalysts were stored in sealed conditions.

Methods

Sample characterization

The samples underwent degassing under an N₂ atmosphere at 120 °C for 12 h. Specific surface area and pore volume were determined using an automatic surface area and porosity analyzer (BET, Beijing Jingwei Gaobo Company JW-BK132F, China). Surface composition analyses and semi-quantitative assessments of the samples were conducted using an X-ray photoelectron spectroscope (Thermo Scientific K-Alpha, USA). X-ray diffraction (XRD) scans were performed from 10° to 80° at a rate of 2° min^-1 using a Rigaku D/max-2200pc instrument (Japan). Raman spectra were recorded with a Horiba LabRAM HR Evolution Raman spectrometer (Japan) using an excitation wavelength of 532 nm. Elemental composition was analyzed with an element analyzer (Elementar UNICUBE, Germany). Surface characteristics were explored via scanning electron microscopy (SEM, TESCAN MIRA LMS, Czech Republic), and the morphology and dispersion of catalysts, along with elemental types and contents, were investigated using a transmission electron microscope (JEOL JEM-F200). Crystal structure and elemental analyses were performed using a high-resolution transmission electron microscope combined with an energy-dispersive spectrometer. Thermogravimetric analysis (TG, Netzsch STA449F3, Germany) was performed under an N₂ atmosphere from 30 to 800 °C with a heating rate of 10 °C min^-1. H₂ temperature-programmed reduction (H₂-TPR) and N₂ temperature-programmed desorption (N₂-TPD) analyses were carried out using a temperature-programmed chemical adsorption instrument (Micromeritics Auto Chem II 2920, USA), with detailed procedures provided in the supplementary materials.

Catalyst Activity Tests

The catalytic activity of the various samples for ammonia decomposition was assessed at atmospheric pressure and 400 to 550 °C in a quartz tube reactor with an 8 mm inner diameter. Specifically, a 200 mg catalyst sample was activated under a 50% H₂ and 50% Ar mixed gas flow at 300 °C for 2 h. Following reduction, the system was purged with NH₃ for 30 min before initiating ammonia decomposition tests. Furthermore, the effluent gases (H₂, N₂, and NH₃) were analyzed using a gas chromatograph equipped with a thermal conductivity detector and a Porapak-Q column (Zhejiang Fuli GC-9720P, China). The corresponding methodological and calculation details are provided in Fig. S1.

Experimental and detection data were compiled using Excel 2021, while linear fitting and graph plotting were performed using Origin 2021. XPS peak deconvolution and fitting were conducted using Avantage 6.6. Phase analysis was performed using MDI Jade 6, while particle size statistics and lattice fringe measurements were performed using Nano Measurer 1.2 and Digital Micrograph 3.5.

RESULTS AND DISCUSSION

Structural Characteristics of Carriers and Catalysts

Figure 2 presents the adsorption–desorption isotherms and pore size distributions of the various samples. The specific surface area and pore structure of a carrier play a crucial role in determining the dispersion and size of active metal particles in supported catalysts (Ren et al. 2023; Du et al. 2024). As shown in Fig 2a, AC, AC-N, AC-G, and AC-GN exhibited similar N₂ adsorption–desorption isotherms, namely Type I isotherms with H1-type hysteresis loops (Calzaferri et al. 2023). The pore size distribution, depicted in Fig. 2b, indicates that the various modification techniques did not substantially alter the structure of AC, thereby preserving its hierarchical porous carbon architecture (micropore–mesopore–macropore), which facilitates the dispersion of active metals.

Fig. 2. N₂ sorption isotherms, (a) and (c), and pore size distribution curves, (b) and (d), of various supports and catalysts

Further analysis of specific surface area and pore structure data from Table 1 revealed that AC-N exhibited the highest specific surface area (750 m²g^-1) and pore volume (0.49 cm³g^-1), representing increases of 27% and 8%, respectively, compared to AC, while the average pore diameter decreased by 13%. This decrease can be attributed to the etching effect on the carbon structure caused by radicals generated during urea decomposition, leading to enhanced porosity (Stöhr et al. 1991). By contrast, AC-G showed a slight decrease in average pore diameter, specific surface area, and pore volume relative to AC. This decrease is attributed to the deposition of new carbon structures formed by glucose on the surface and within the pores of AC. The specific surface area and pore volume of AC-GN were greater than those of AC-G, likely owing to urea decomposition. Given that the carbon microspheres formed during the hydrothermal reaction of glucose ranged between 50 to 200 nm in size (Liu et al. 2010), it is inferred that the carbon structures derived from glucose adhered to the external surface and large pores of AC. These findings suggest that the introduction of glucose did not significantly block the pores, thereby minimally impacting the micro and mesopores of AC. Consequently, this modification method preserved the original micro and mesoporous structures of the carrier alongside facilitating the formation of novel nitrogen-doped carbon structures on the outer surface and in the large pores. Figures 2c and 2d illustrate the post-catalyst-loading adsorption–desorption isotherms and pore size distribution curves. The most significant changes in average pore size and pore volume were observed in AC after loading, with reductions of 19% and 22%, respectively. These changes are likely owing to the agglomeration of Ru active metal atoms within the AC pores, which blocked some micropores. Similarly, the extent of changes observed in Ru/AC-G surpassed those observed in AC-N and AC-GN. Thus, consistent with initial observations, the carriers AC-N and AC-GN were more conducive to the dispersion of the Ru active metal atoms.

Table 1. Textural Properties of Various Carriers and Catalysts

Note: S_BET : Brunauer-Emmett-Teller (BET) specific surface area

The SEM and TEM analyses were performed to characterize the morphologies of the carriers and catalysts. Large pores and mesopores were observed, as depicted in Fig. 3 parts a and b. The introduction of urea for nitrogen doping resulted in the generation of radicals at elevated temperatures, thereby etching the carbon carrier, consistent with the BET results. Carbon deposition on the surface and within the large pores of AC-GN is revealed in Fig. 3c. This occurred owing to the dehydration of glucose during the hydrothermal process at 180 °C, gradually separating from the solution to form an aqueous emulsion. Further dehydration of the sugar resulted in the formation of oligomeric nuclei within the sugar micelles, which subsequently grew into nanoscale spheres (Sakaki et al. 1996; Xu et al. 2008). These carbon microspheres, deposited on the carbon surface and further carbonized at 800 °C, were not observed in AC-GN. As depicted in Fig. 3d, the TEM analysis of AC-GN clearly revealed micro and mesoporous structures, confirming that the carbonization of glucose did not lead to the blockage of the micro and mesopores. The energy-dispersive spectroscopy (EDS) elemental distribution map (Fig. 3e) indicated the doping of nitrogen atoms into the aromatic carbon framework, with Ru nanoparticles being uniformly dispersed on the AC-GN carrier. Furthermore, graphitic carbon lattice fringes and Ru nanoparticles with a lattice spacing of 0.206 nm, corresponding to the Ru (101) plane, are evident in Fig. 3f-h (Agarwal and Ganguli 2014). In addition, the electron diffraction patterns revealed two distinct diffraction rings, corresponding to the (002) and (101) planes of graphitic carbon and Ru, respectively (Wang et al. 2023a).

Fig. 3. SEM images of AC (a), AC-N (b), and AC-GN (c). TEM images of AC-GN (d). EDS elemental mapping of Ru/AC-GN (e). High-resolution transmission electron microscopy image of Ru/AC-GN (f-h). SAED pattern of Ru/AC-GN.

The XRD was used to analyze the carbon crystalline structures and the metallic Ru crystalline structures of the four Ru-based catalysts. A prominent diffraction peak near 26° was observed for all the catalysts, as shown in Fig. 4a (Kulkarni et al. 2017). This peak is attributed to the (002) diffraction of graphitic carbon. There was no considerable difference in the diffraction peak at 26° between Ru/AC and Ru -N, indicating that the hydrothermal post-treatment nitrogen doping process had a minimal impact on the crystalline structure of the carbon. The diffraction peaks observed for Ru/AC-G and Ru/AC-GN were less intense and broader than those for AC and AC-N, signifying a slight variation in the crystallinity of the carbon carrier. This suggests that the nitrogen-doped carbon generated from the glucose–urea mixture was successfully deposited on the AC surface and underwent carbonization at 800 °C. Notably, distinct absorbance peaks corresponding to the Ru (101) plane were observed for Ru/AC and Ru/AC-G near 43°, while similar peaks were not observed for Ru/AC-G and Ru/AC-GN, in contrast with the TEM results(Tee et al. 2015). This discrepancy was attributed to the uniform dispersion of Ru nanoparticles on the AC-N and AC-GN carriers (Wen et al. 2018). Raman spectroscopy was employed to further analyze the graphitization degree of the four carbon carriers, as illustrated in Fig. 4b. Peaks were noted near 1345 cm^-1 (D band) and 1575 cm^-1 (G band), with a lower D to G band intensity ratio (I_D: I_G) indicating a higher level of graphitization (Odedairo et al. 2014). AC exhibited the lowest I_D: I_G ratio of 1.05, signifying the highest degree of graphitization. In the case of AC-N, the I_D: I_G ratio was 1.12, suggesting a reduction in graphitization owing to the disruption of carbon chains by nitrogen doping. Furthermore, AC-G exhibited an I_D: I_G value of 1.14, revealing that the hydrothermal carbon was graphitized at 800 °C, consistent with the XRD results. AC-GN displayed the highest I_D: I_G ratio of 1.23 among the carriers, indicating the lowest degree of graphitization and an increased number of defect structures. This is a consequence of the combined effects of the disruption of carbon chains by nitrogen atom doping and the deposition of nitrogen-doped carbon on the AC surface.

Fig. 4. XRD patterns of the catalyst samples (a) and Raman patterns of the support samples (b)

Table 2 presents the elemental analysis results of the different carbon carriers. The results indicate that the respective nitrogen contents in AC and AC-G, which were not doped with urea, were only 0.5 and 0.4 wt%. Upon the addition of urea alone, the nitrogen content increased to 1.8 wt%. This is because the generation of ammonia at high temperatures led to the formation of NH₂, NH, and N radicals, thereby doping N atoms into the aromatic carbon framework. In AC-GN, the nitrogen content rose to 4.7 wt%, indicating that glucose addition not only facilitated the doping of N atoms into the highly aromatic carbon framework but also generated new nitrogen-doped carbon deposits on the AC surface, achieving in situ nitrogen doping. Notably, compared to traditional post-treatment nitrogen doping methods, this method resulted in increased nitrogen doping. The thermal stability of the carriers, a critical factor affecting ammonia decomposition catalyst activity at reaction temperatures between 400 and 550 °C, was determined through TG. The TG curves of the four carbon carriers (Fig. 5a) exhibited similar weight loss profiles, with varying degrees of loss before 200 °C, primarily due to the vaporization of water. For AC-G and AC-GN, slight mass loss was observed between 500 and 600 °C. Ultimately, the mass loss for AC and AC-G was within 1%, while that for AC-G and AC-GN was approximately 2%, presumed to be caused by the decomposition of certain unstable structures generated by glucose carbonization at high temperatures(Liu et al. 2020). Given that the optimal reaction temperature for Ru/AC-GN in ammonia decomposition is around 500 °C, AC-GN possesses adequate thermal stability.

Table 2. Summary of the Elemental Analysis Results of the Various Carriers

Fig. 5. Mass loss of various carrier composites as a function of temperature between 50 and 600 °C, measured with a heating rate of 10 °C/min under a nitrogen atmosphere (a). XPS survey spectra of AC-N, AC-N, AC-G, and AC-GN (b). XPS N1s spectra of AC-N and AC-GN (c)

X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition and chemical bonds of the carriers. The XPS spectra of the Ru/AC-N and Ru/AC-GN samples revealed deconvoluted peaks attributed to C 1s, N 1s, and O 1s (Fig. 5b)(Yu et al. 2014). By contrast, Ru/AC and Ru/AC-G lacked a peak corresponding to N 1s, and the N1S peak was substantially higher in AC-GN, consistent with the elemental analysis results. The high-resolution N1s peak in Fig. 5c revealed the presence of pyridinic-N, pyrrolic-N, graphitic-N, and oxide-N (Han et al. 2019). Nitrogen species play a crucial role in the performance of the carriers, as shown in Table S1, which displays the content of nitrogen species in the catalysts obtained from the two nitrogen doping methods. Notably, the highest pyridinic-N and pyrrolic-N contents were observed in Ru/AC-N and Ru/AC-GN, respectively. Furthermore, Ru/AC-GN exhibited a higher content of pyrrolic-N compared to Ru/AC-N, while Ru/AC-N had a higher pyridinic-N content. Studies have indicated that pyridinic-N and pyrrolic-N can serve as effective sites for active metals, enhancing the dispersion of active metal components and facilitating the binding between metallic Ru and nitrogen-doped carbon carriers (Zhong and Aika 1998). However, the mechanism of action of these two nitrogen species remains unclear (Miao et al. 2017; Wang et al. 2023b). With increasing pyrolysis temperature, the pyridinic-N content decreased gradually, transforming into pyrrolic and graphitic nitrogen. Excessive pyridinic-N appears unsuitable for high-temperature ammonia decomposition systems. Therefore, the AC-GN carrier, with high pyrrolic-N and graphitic-N contents, is deemed more suitable.

Structure–Activity Relationship between Carriers and Active Metals

Currently, there exists a consensus regarding the mechanism of NH₃ decomposition on active metal surfaces, which can be delineated into eight steps (Fig. 6). These steps encompass ammonia adsorption, dissociation, and the formation and desorption of decomposition products (Chen et al. 2021; Su et al. 2024). Initially, gaseous NH₃(g) adsorbs on the active sites of the catalyst(ad), generating adsorbed NH₃(ad) (step 1). Subsequently, NH₃(ad) dehydrogenates to produce adsorbed N(ad) and H(ad) (steps 2 to 4). Eventually, N(ad) and H(ad) combine to form adsorbed N₂(ad) and H₂(ad) (steps 5 to 6), respectively, which subsequently desorb to yield gaseous N₂(g) and H₂(g) (steps 7 and 8). However, the rate-limiting step remains a subject of debate. Enhanced Ru nanoparticle dispersion facilitates the conversion of reaction gases between gaseous and adsorbed states (step 1) or from adsorbed states back to gaseous states (steps 7 and 8). Moreover, the efficiency of gas transport also influences the aforementioned reaction process, necessitating the provision of more anchoring sites and a porous structure in the carrier.

Fig. 6. Schematic of NH₃ decomposition reaction in the presence of Ru catalyst

As previously mentioned, the nitrogen-doped carbon structures deposited in AC-GN predominantly reside on the outer surface and within large pores, preserving a hierarchical porous structure conducive to gas transport. Additionally, the outer surface and large pores of AC-GN offer more defect sites, conducive to anchoring Ru, thereby enhancing its dispersion and stability. Wu et al. (2023) found that active metals loaded on the outer surface of the carrier exhibit superior ammonia decomposition performance, attributed to the ease of gas adsorption–desorption on the outer surface. Consequently, Ru situated on the outer surface and large pores of AC-GN would theoretically enhance the overall catalytic performance. The gradual dissociation of NH₃(ad) (3-5) also plays a pivotal role in influencing the catalytic reaction. Wang et al. (2023b) observed that the electron density on the outer surface of CNTs surpasses that on the inner surface, resulting in heightened activity of the active metals loaded on the outer surface. The introduction of N alters the electron distribution of C, inducing electron-rich states and enhancing the electron transfer capability. Furthermore, AC-GN retains its original degree of graphitization, accompanied by a notable improvement in the electrical conductivity of its outer surface, thereby facilitating the dissociation of NH₃(ad).

To determine the dispersion of Ru nanoparticles on the carrier, the diameters of at least 50 Ru particles were analyzed statistically. Figure S2 presents TEM images of three catalysts, namely Ru/AC, Ru/AC-N, and Ru/AC-GN, with Ru nanoparticle diameters of 3.39, 2.98, and 2.79 nm, respectively. The Ru nanoparticles in Ru/AC-GN exhibited the smallest diameter and the most optimal dispersion. To further elucidate the presence of the active metal Ru on AC-N and AC-GN compared to AC, Ru 3p spectra were deconvoluted and fitted. Figure 7a-c illustrates two main peaks corresponding to Ru3p^3/2 and Ru3p^1/2. The peaks near 462.4 and 484.7 eV correspond to Ru⁰, whereas those at 464.4 and 486.7 eV correspond to Ruⁿ⁺ (Tang et al. 2015; Wang et al. 2018). The relative contents of Ru⁰ and Ruⁿ⁺ were determined based on the peak areas (Fig. 7d). The Ru⁰ contents of Ru/AC, Ru/AC-N, and Ru/AC-GN were found to be 50%, 70%, and 74%, respectively, indicating that the Ru⁰ content increases with nitrogen content. This demonstrates that nitrogen doping can regulate the electron cloud density of the carbon carrier, thereby enhancing electron transfer between the metal and carrier. Consequently, electron transfer occurs between N atoms and metallic Ru in Ru/AC-N and Ru/AC-GN, resulting in higher Ru⁰ content. Notably, among the four carriers, AC-GN exhibited the greatest performance.

Fig. 7. Ru 3p XPS spectra of Ru/AC (a), Ru/AC-N (b), Ru/AC-GN (c), and Ru⁰ and Ruⁿ⁺ contents of various catalysts (d)

The H₂-TPR curves of the unreduced catalysts indicated that 300 °C is a suitable reduction temperature for Ru (Fig. 8a). Excessively high reduction temperatures might result in Ru particle agglomeration, while temperatures below 300 °C might not effectively reduce Ru oxides to Ru. In the H₂-TPR spectra, the peaks observed for Ru/AC-GN showed an increase of approximately 10 °C compared to Ru/AC-N, a consequence of the interactions between the nitrogen-doped carriers and Ru oxides, which intensified with increased nitrogen content (Chen et al. 2017). Such metal–carrier interactions prevent Ru nanoparticle agglomeration. Furthermore, the issue of catalyst poisoning is crucial to the study of catalyst performance. It has been reported that Ru-based catalysts are primarily affected by hydrogen poisoning in ammonia synthesis reactions (Lin et al. 2021), while activated carbon carriers may generate gases such as CO_X (CO, CO₂) at high temperatures. Due to the porous nature of activated carbon, these CO_X and H₂ molecules excessively adsorb on the catalyst surface, occupying active sites and thereby inhibiting or hindering the catalytic reaction (Chen et al. 2024). Since the treatment temperature of activated carbon (800 °C) is much higher than the temperature of ammonia decomposition reactions, unstable oxygen-containing functional groups are decomposed in advance, making the impact of CO on the catalyst negligible. Ren and others have studied the desorption behavior of hydrogen on catalysts through H₂-TPD, finding that hydrogen begins to desorb at approximately 100 °C and is essentially fully desorbed by around 200 °C (Ren et al. 2017). Under the conditions of ammonia decomposition reactions at 400 to 500 °C, hydrogen desorbs more readily. This suggests that nitrogen may have a more significant impact on catalyst performance in ammonia decomposition reactions. Current research considers nitrogen desorption as the rate-controlling step in ammonia decomposition on Ru catalysts (Zhang et al. 2014). The N₂-TPD curves exhibited three peaks (Fig. 8b) near 140, 450, and 720 °C. The desorption peak at 140 °C, attributed to the physical desorption of N₂ from the carbon surface and pores, suggests that the other two peaks at higher temperatures correspond to deep pore desorption and chemical adsorption. Ru/AC-GN desorbed N₂ primarily between 400 and 500 °C, whereas Ru/AC-N displayed a minor peak after 700 °C. This is possibly owing to the larger surface area and smaller average pore size of AC-N, leading to greater N₂ desorption resistance in the pores of Ru/AC-N. It is anticipated that Ru loaded on AC-GN will demonstrate superior NH₃ decomposition performance.

Fig. 8. H₂-TPR (a) and N₂-TPD (b) profiles of the catalysts fabricated in this study

Assessment of NH₃ Decomposition Performance

Figure 9a demonstrates that the NH₃ conversion rates of the four catalysts exhibit a direct correlation with temperature. NH₃ decomposition is an endothermic reaction. Therefore, the NH₃ conversion rate rises with increasing reaction temperature (Wu et al. 2023). Among the four catalysts, Ru/AC-GN displayed the highest activity, followed by Ru/AC-N, whereas Ru/AC and Ru/AC-G showed no significant difference in activity. Notably, within the low-temperature range of 425 to 500 °C, Ru/AC-GN exhibited a markedly superior conversion efficiency compared to the other catalysts. At a gas hourly space velocity (GHSV) of 15000 mL g_cat^-1 h^-1, Ru/AC-GN achieved 69% NH₃ conversion at 475 °C, corresponding to an H₂ generation rate of 11.5 mmol g_cat^-1 min^-1 (Table S2). Moreover, at 525 °C, it achieved 94% NH₃ conversion, equivalent to an H₂ generation rate of 15.8 mmol g_cat^-1 min^-1. Figure 9b presents Arrhenius plots illustrating the logarithm of the H₂ formation rate versus the reciprocal of the absolute temperature. The determined apparent activation energies (Ea) for Ru/AC, Ru/AC-N, and Ru/AC-GN were78.13, 61.80 and 60.28 kJ mol^-1, respectively. As previously discussed, the dispersion of Ru on the outer surface and large pores of AC-GN facilitated higher gas adsorption–desorption rates, resulting in the lowest E_a and high activity at low temperatures in the case of Ru/AC-GN. Compared with previously reported Ru-based carbon-supported NH₃ decomposition catalysts (Table S3), the performance of Ru/AC-GN is significantly superior, and it is comparable to the high-performing MgO, offering a cost-effective alternative through the valorization of biomass resources (Wang et al. 2004; Lucentini et al. 2019).

Fig. 9. NH₃ conversion in the presence of various catalysts at a GHSV of 15000 mL g_cat^-1 h^-1 (a) and the Arrhenius plots obtained between 400 and 475 ℃ (b)

The catalytic stability of Ru/AC-GN for NH₃ decomposition was evaluated at 500 °C with a GHSV of 15000 mL g_cat^-1 h^-1. Over a 24 h reaction period, Ru/AC-N and Ru/AC-GN demonstrated substantially better stability than Ru/AC (Fig. 10a), with Ru/AC-GN exhibiting particularly notable effects owing to the interactions between the nitrogen-doped structures and Ru within the outer surface and large pores of the AC-GN carrier. The XRD patterns in Fig. 10b show no significant change in Ru size after 24 h of durability testing. However, the stability of the Ru/AC-GN catalyst must be further improved. The addition of a second metal or promoters could further enhance the low-temperature activity of the catalyst and reduce the reaction temperature, thereby extending the catalyst lifespan.

Fig. 10. Stability test results of various catalysts for NH₃ decomposition at 500 ℃ with a GHSV of 15000 mL g_cat^-1 h^-1 (a), and XRD patterns of the aged catalyst samples (b)

CONCLUSIONS

Nitrogen-doped composite materials were successfully synthesized via a hydrothermal pyrolysis method. Unlike conventional nitrogen doping techniques, this approach not only introduces N atoms into carbon chains but also deposits nitrogen-containing carbon materials on the surface and within the large pores of AC.
By incorporating glucose, in situ nitrogen doping was achieved, boosting the nitrogen content of the carbon carrier while preserving its hierarchical pore structure and degree of graphitization.
This methodology improved the effective dispersion of Ru nanoparticles on the external surface and within the large pores of the carrier, enhancing electron transfer capabilities. Leveraging of the inherent advantage of easier gas adsorption–desorption on the external surface of the catalyst, the low-temperature activity of the catalyst was increased in this study.
Examination of nitrogen species revealed that nitrogen doping enhanced metal–carrier interactions, effectively improving the thermal stability of the catalyst. The relatively stable pyrrolic and graphitic nitrogen were more suitable for the ammonia decomposition reaction.
Finally, catalytic performance tests demonstrated that the Ru/AC-GN catalyst exhibited outstanding activity and catalytic stability for NH₃ decomposition, notably surpassing other carbon-supported Ru catalysts at low temperatures. This nitrogen doping method in AC, characterized by relatively high nitrogen doping levels, excellent hierarchical pore structures, and superior catalytic performance, provides valuable insights into the design and preparation of catalyst carriers for ammonia decomposition reactions.

ACKNOWLEDGEMENTS

This study was supported by the National Natural Science Foundation of China (No. 32202606); Youth Foundation Natural Science Foundation of Shandong Province(No. ZR2022QC056); The central government guides local Science and Technology Development Fund Projects-Yellow River Basin-Xinjiang (No. YDZX2023007); The central government guides local Science and Technology Development Fund Projects-Yellow River Basin-Shanxi (No. YDZX2023024); Jinan City science and technology plan social livelihood special (No. 202317009); 20 New Items of Universities” funding project of Jinan (No. 202228043); Science, education and industry integration pilot training fund (No. 2023PY045); Shandong Province Science and Technology-based Small and Medium-sized Enterprises Innovation Capacity Enhancement Project (No. 2023TSGC0235); R&D and Demonstration of Key Technologies for Green and Efficient Hydrogen Production (No. 22JBZ02-03); Talent Research Project of Qilu University of Technology (Shandong Academy of Sciences) (2023RCKY170); Shandong Province building and transportation double carbon innovation joint venture research and development plan (STGTT0101202302) and (STGTT0101202303).

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Article submitted: March 7, 2024; Peer review completed: April 24, 2024; Revised version received and accepted: April 27, 2024; Published: May 15, 2024.

DOI: 10.15376/biores.19.3.4313-4334

APPENDIX

Supplementary

Appendix contains:

Section S1: Sample characterization (H₂-TPR and N₂-TPD)

Section S2: Catalyst Activity Tests

Table S1: N1s XPS analysis of various carbon materials.

Figure. S2: TEM images and Ru particle size distribution histograms of Ru/AC (a, d), Ru/AC-N (b, e) and Ru/AC-GN (c, f).

Table S2: H₂ formation rate (mmol min^-1 g^-1_cat) for various catalysts.

Table S3 Catalytic activity of the supported Ru catalysts for NH₃ decomposition.

Section S1 Sample characterization (H₂-TPR and N₂-TPD)

For the H₂-TPR test, 50 mg of unreduced catalyst loaded in a U-shaped quartz tube was initially heated from room temperature to 100 ℃ at a heating rate of 10 ℃ min^-1and held at 100 ℃ for 1 h in an Ar gas flow (50 mL min^-1) to remove impurities such as water. Subsequently, it was heated from 50 to 800 ℃ at a rate of 10 ℃ min^-1 in a 10% H₂/Ar gas flow (50 mL min^-1), and the amount of H₂ consumed during this heating process was recorded using a thermal conductivity detector (TCD).

For the N₂-TPD test, 50 mg of reduced catalyst loaded in a U-shaped quartz tube was first heated from room temperature to 150 ℃ at a rate of 10 ℃ min^-1and maintained at 150 ℃ for 1 h in a He gas flow (50 mL min^-1) to remove impurities such as water. After cooling to 50 °C, it was purged with nitrogen for 1 h to saturation, and finally, under a helium atmosphere, ramped to 800 °C at a rate of 10 °C min^-1 for desorption. The desorbed gases were detected using a TCD.

Section S2 Catalyst Activity Tests

Fig. S1. Schematic of the experimental setup used for the NH₃ decomposition test

A pure ammonia (NH₃) flow rate of 50 mL min^-1, corresponding to a GHSV of 15,000 mL g_cat^-1 h^-1, was maintained. Temperature-dependent NH₃ decomposition tests were conducted in the temperature range of 400 to 550 °C in 25 °C increments (60 min at each temperature). Measurements were taken three times at each temperature point, and the average was calculated. The NH₃ conversion rate () and H₂ formation rate (r) were calculated according to Eqs. (1) and (2):

where V_N2 and V_NH3 are the molar flow rates of N₂ and NH₃ in the outlet streams, respectively.

The apparent activation energy of the catalyst was determined based on the Arrhenius equation (3) in the temperature range of 400 to 475 °C. Subsequently, a linear regression was conducted between ln r and 1000/T, yielding a linear relationship from which the apparent activation energy was derived.

where r represents the rate constant of the reaction, A represents the pre-exponential factor, E_a represents the activation energy, R represents the universal gas constant, and T represents the thermodynamic temperature.

Table S1. N1s XPS Analysis of Various Carbon Materials

Fig. S2. TEM images and Ru particle size distribution histograms of Ru/AC (a,d), Ru/AC-N (b,e) and Ru/AC-GN (c,f)

Table S2. H₂ Formation Rate (mmol min^-1 g^-1_cat) for Various Catalysts

Table S3. Catalytic Activity of the Supported Ru Catalysts for NH₃ Decomposition

Note: [a] represents: wt%; [b]: °C; [c]: mL h^-1 g ^-1_cat; [d]: %; [e]: mmol min^-1 g_cat^-1

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