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Article

Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen

by
Ling-Feng Zhang
1,2,†,
Zhong-Pan Hu
2,†,
Shi-Hang Liang
3,
Feng Xu
4 and
Zhong-Yong Yuan
2,*
1
Changzhou Vocational Institute of Engineering, Changzhou 213164, China
2
School of Materials Science and Engineering, Nankai University, Tianjin 300350, China
3
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
4
Tianjin Workstation, Technology Center of Shanghai Tobacco Group Co., Ltd., Tianjin 300163, China
*
Author to whom correspondence should be addressed.
†
These authors contribute equally to this work.
Symmetry 2022, 14(12), 2627; https://doi.org/10.3390/sym14122627
Submission received: 23 October 2022 / Revised: 22 November 2022 / Accepted: 2 December 2022 / Published: 12 December 2022
(This article belongs to the Special Issue Heterogeneous Catalysis: Topics and Advances)
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Abstract

Ammonia decomposition to hydrogen technique is an effectively way to solve the problems associated with the storage and transportation of hydrogen, but the development of a high-performance catalyst for ammonia decomposition is a great challenge. Ni species supported on activated attapulgite clay (AATP) is prepared by a homogeneous precipitation method for ammonia decomposition to COx-free H2. The structural properties of the Ni/AATP catalysts are characterized by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy, H2 temperature-programmed reduction, and N2 sorption technique. It is revealed that the porous structure and high surface area of rod-like symmetric AATP results in highly dispersed NiO particles because the presence of a strong interaction between AATP and NiO particles. In particular, the Si-OH in AATP can react with Ni species, forming Si-O-Ni species at the interface between Ni and AATP. The Ni/AAPT catalysts are used for ammonia decomposition, the 20%-Ni/ATTP catalyst shows a 95.3% NH3 conversion with 31.9 mmol min−1 gcat−1 H2 formation rate at 650 °C. This study opens a new way to utilize natural minerals as an efficient support of catalysts towards ammonia decomposition reaction.

1. Introduction

Hydrogen (H2), as a potential clean energy, has attracted much attention. However, the main challenge in its applications is H2 storage. Great efforts have been made in the research and development of H2 storage materials over the past few decades. Ammonia is considered as a suitable carrier for hydrogen storage because of its high H2 content (17.6 wt%) and high energy density (3000 Wh/kg) [1,2]. More importantly, ammonia decomposition can generate high-purity COx-free H2, which can meet the requirement of proton-exchange membrane fuel cells (PEMFC) for hydrogen purity [3,4,5]. In this reaction, the exploitation of a high-efficiency catalyst is of great importance because the N-H dissociation is so difficult. Ru-based catalyst shows the best catalytic activity in ammonia decomposition, but its application is hindered by high price and low availability [6,7]. Many non-noble metal (such as Ni, Fe, Co, etc.) based catalysts have then been reported [8,9,10,11,12,13,14]. Among them, Ni-based catalysts are considered as one of the most promising catalysts for NH3 decomposition, due to their superior catalytic activity, significant price advantage and long-term stability.
In order to improve the activity and stability of Ni-based catalysts, a large number of supports (such as metal oxides, carbon nanotube, zeolites and layered double hydroxides) were employed to load active Ni nanoparticles [15,16,17,18]. After loading on different supports, some properties of Ni particles, including particle size and distribution, reducibility, electronic structure and textural properties would be changed, which can significantly affect the catalytic performance. It was reported that Na-ZSM-5 nanocrystal supported only a 5% Ni catalyst and exhibited excellent catalytic activity and stability for ammonia decomposition due to small Ni particles (mean size ~ 2.3 nm) with homogeneous dispersibility and abundant basic sites of the support [19]. Hu et al. prepared Ni/ZSM-5 catalyst by a modified solid state ion exchange method, which exhibited excellent stability due to strong metal-support interaction between Ni and ZSM-5 [20]. Furthermore, rare earth metal oxides were used as supports for the preparation of high catalytic performance Ni-based catalysts. It was reported that Ni loaded on Y2O3 support exhibited the best catalytic activity for ammonia decomposition among the five rare-earth oxide supports (Y2O3, CeO2, La2O3, Sm2O3 and Gd2O3) because Y2O3 could promote the desorption of hydrogen atoms adsorbed on the nickel surface [21]. Deng et al. [22] prepared mesoporous Ce0.8Zr0.2O2 solid solutions loaded Ni catalyst for ammonia decomposition. The mesoporous structure of support promoted the mass transportation and could expose more active centers during a catalytic reaction. However, the preparation of these porous supports is complex and time-consuming. Therefore, the exploitation of a new support with both porous structure and low cost is still a huge challenge.
Attapulgite (ATP) clay, a kind of natural rod-like symmetric nano-structural silicate clay mineral, possesses a zeolite-like pore structure and a high surface area, as well as thermal stability and quite a low cost [23,24,25,26,27]. ATP can be used as support for preparation of a number of supported metal catalysts, such as Ti-ATP for selective catalytic reduction of NOx with NH3 [23], Ni/ATP for dry reforming of methane [24], and CuO/ATP for CO oxidation [28]. However, ammonia decomposition was carried out under relatively harsh conditions, a high reaction temperature (400–700 °C) and a strong reductive atmosphere, which easily caused active species agglomeration and sintering [29]. For solving this problem, Li et al. [30] synthesized Ni/ATP catalyst enwrapped with porous silica, showing strong anti-sintering activity due to its core-shell structure, though the preparation process of core-shell structure is complicated. Thus, it is of great demand to synthesize ATP supported Ni catalyst with a high catalytic performance, especially strong stability by a simple method.
In this work, the ATP was treated by HCl to wipe off a number of framework Al species, resulting in an activated ATP (AATP) with many Si-OH groups and a series of Ni supported on AATP were prepared through a simple one-pot homogeneous precipitation method. Owing to the reaction between Ni species and Si-OH in AATP, the Ni/AATP catalysts possess many highly dispersed Ni species. The Ni/AATP catalysts with a porous structure were used for ammonia decomposition, exhibiting long-term stability and high catalytic activity.

2. Experimental Section

2.1. Catalysts Preparation

Attapulgite clay was obtained from Anhui Tianjiao Co., and denoted as ATP. Ni(NO3)2·6H2O (>99.0, purity), HCl (36–38%, purity), NH3·H2O (30%, purity) and urea (99.0%, purity) were obtained from Tianjin Guangfu Technology Development Co., Ltd. Firstly, 10 g ATP was added into 240 mL of distill water, then 60 mL of 6 M HCl was dropped under stirring. The mixture was stirred at 90 oC by a water-bath heater for 2 h. Afterwards, aqueous ammonia was put dropwise into the above mixture to pH ≈ 8, followed by stirring for another 30 min. Then, the precipitation was collected by filtering, washed with water and ethanol for several times, and dried at 120 oC for 12 h. The activated ATP sample was denoted as AATP.
Ni/AATP catalysts were synthesized by the homogenous precipitation method. Typically, the calculated Ni(NO3)2·6H2O and appropriate excess of urea (the mole ratio of urea to Ni is 4:1) were dissolved in 100 mL of distilled water under room temperature and then mixed with 1 g of AATP under vigorous stirring. The mixture was heated to 90 °C for 5 h. After that, the mixture was filtered, washed with water, dried, milled and calcined at 600 °C for 3 h. The obtained catalyst was denoted as x%-Ni/AATP, where x% represents the content of Ni (x% = 5, 10, 15, 20, 25 and 30 wt%). The formation process is shown in Scheme 1. The influence of the calcination temperature on the catalyst activity was investigated by a series of Ni/AATP catalysts obtained with different calcined temperatures by similar manner. As a comparison, NiO particles were prepared without the utilizing of AATP support.

2.2. Characterization

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus diffractometer using Cu Kα radiation, operated at 40 kV and 40 mA with a scanning rate of 12 °C min−1. The compositions of the Ni/AATP catalysts were characterized on an X-ray fluorescence (XRF) spectrometer (Philips Magix-601). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed on an FEI Tecnai G20 microscope at 200 kV and a Jeol JSF-7500L microscope at 5 kV, respectively. Thermogravimetric analysis (TGA) was carried out on a TA SDT Q600 instrument in high purity air (100 mL min−1) with a heating rate of 10 °C min−1. H2 Temperature-programmed reduction (H2-TPR) experiments were conducted on a Quantachrome ChemBET-3000 analyzer equipped with a U-shaped quartz micro-reactor. Approximately 50 mg of catalyst is reduced by the mixture of 5% H2 in Ar flowing (30 mL min−1) with a heating rate of 10 °C min−1. A thermal conductivity detector was used to detect the hydrogen consumption.

2.3. Catalytic Testing

The catalytic performance of Ni/AATP catalysts for ammonia decomposition was carried out in a quartz fixed-bed reactor (inner diameter, 6 mm) at atmospheric pressure. 0.1 g catalyst was placed into the bed of the reactor first, then reduced by H2 (20 mL min−1) at 600 °C for 2 h. After cooling to room temperature, pure ammonia (50 mL min−1) was inputted into the reactor and purged for 30 min. The catalytic reaction temperature was in the range of 300 to 650 °C with 25 °C interval. The product was measured on-line by a gas chromatograph (SP-6800A6) equipped with a thermal conductivity detector and Poropak Q column. The catalytic activity was described by the conversion of ammonia, and the H2 formation rate was obtained from the H2 content in out gas.
The apparent activation energies (Ea) are obtained from the Arrhenius relationship between the rate constant (k) and the temperature (T), which can be described as ln k = −Ea/RT + constant. The rate constants were measured in the range of 450–550 °C, since the Ea values obtained in such a way were not influenced significantly by mass transfer limitation. The activation energy (Ea) can be obtained from the slope of the resulting linear plot of ln k versus 1/T.

3. Results and Discussion

The thermal analysis of AATP and the uncalcined 20%-Ni/AATP catalyst are shown in Figure 1. The TG curve of AATP shows a slight weight loss of 4% from room temperature to 800 °C, while with the uncalcined 20%-Ni/AATP sample a continuous mass loss of 17.4% from 30 to 600 °C occurs. Typically, weight loss below 120 °C (~6%) can be attributed to the release of adsorbed water in Ni/AATP. The slight weight loss from 120 to 290 °C (~3%) corresponds to the remaining crystallization water of the precursor [31]. In the temperature range of 290~550 °C, the weight loss (~8%) is attributed to the decomposition of a large amount of Ni(OH)2 [32], a small amount of NiCO3 [33,34,35] and some NiCO3·2Ni(OH)2·4H2O to NiO, and a release of H2O and CO2. When the temperature is higher than 550 °C, almost no weight loss is detected, indicating that the Ni/AATP has been transformed into a stable state. Therefore, all the Ni/AATP samples are calcined at 600 °C before the test.
The XRD patterns of the different supports (natural ATP, AATP and 600 °C-calcined AATP) are shown in Figure 2a. In the natural ATP sample a typical peak at 2θ = 8.3° is observed, which is the basal space of the ATP framework [36]. The diffraction peaks at the 2θ of 13.7°, 16.3°, 19.7° and 20.7° represent the Si-O-Si crystalline layers in the clay [36]. In addition, an intense peak situated at 2θ = 26.7° can be attributed to crystalline SiO2 that exist in the clay [30]. The XRD pattern of AATP shows similar diffraction peaks to the ATP, indicating the ATP after the acid digestion and alkali reprecipitation treatment still maintains the ATP structure. After calcinating at 600 °C, the peak at 2θ = 8.3° of AATP-600 °C is disappeared, owing to amorphization and destruction of the crystal structure of attapulgite at high temperature [37]. Figure 2b provides the XRD patterns of x%-Ni/AATP samples calcined at 600 °C. No obvious NiO characteristic peaks can be observed when the loading amount of Ni is below 25%, which may be due to the formation of highly dispersed NiO particles on the surface of AATP. When the Ni content increases to 25%, three weak and broad diffraction peaks at 2θ = 37.1°, 43.1° and 62.6° can be observed, which can be assigned to NiO crystal phase (JCPDS 65-2901). Further increasing the content of Ni, stronger and narrower peaks can be seen, probably owing to the aggregation of NiO nanoparticles and the increasing of NiO particle size in the Ni/AATP system. To investigate the thermal stability of the as-prepared Ni/AATP catalysts, the 20%-Ni/AATP sample is calcined at different temperatures (600, 700 and 800 °C), and their XRD patterns are shown in Figure 2c. Noticeably, a very weak increase of NiO characteristic peaks can be observed, indicating a faint growth of nanoparticle size and the crystallinity of NiO during the thermal treatment.
The N2 adsorption–desorption isotherms and the corresponding pore size distribution curves of the natural ATP clay, AATP support and Ni/AATP catalysts with different Ni content calcined at 600 °C are shown in Figure 3. The textural properties are listed in Table 1. The N2 sorption isotherms of all the samples belong to classical type III isotherms according to the IUPAC classification. At a relative pressure greater than 0.85, a strong increase of N2-adsorbed volume is observed, indicating the presence of an appreciable amount of macropores [38]. It represents unrestricted monolayer–multilayer adsorption, and suggests a macroporous material whose pores and voids communicated with the surface of the particles [39]. All fresh and activated ATP clay and the supported NiO catalysts show an H3 type hysteresis loop, indicating the materials comprised of aggregates (loose assemblages) of plate-like particles forming slit-like pores [39]. The specific surface area of the ATP clay is 139 m2/g, which increases to 262 m2/g after activation. At the same time, the pore volume increased from 0.48 to 0.60 cm3/g. This can be attributed to removal of some soluble components during the process of acid–alkali treatment. After loading of NiO particles, the surface area and pore volume of Ni/ATTP catalysts decrease with the Ni loading amount (Table 1). When the Ni loading increases to 30%, a high surface area of 179 m2/g with the pore volume of 0.55 cm3/g of 30%-Ni/ATTP is maintained, indicating that the Ni loading has weak impact on the structural property of the AATP support. It is also confirmed by pore size distribution analysis (Figure 3b) that all the samples exhibit three different peaks centered at 2.8, 5.0 and 7.5 nm without significant difference.
The SEM images of AATP support are shown in Figure 4a,b. The ATP and AATP are composed of highly dispersed fiber-sharped attapulgite with 20–50 nm in diameter and 0.2–2 μm in length. After loading with Ni species, the surface symmetric morphology of 20%-Ni/AATP catalyst possess many nanoparticles (Figure 4c,d). It is notable that the Ni/ATTP remains high dispersion, leading to high surface area and large pore volume (Table 1). To investigate the Ni distribution on AATP, EDX elemental mappings of Si, Al, Fe and Ni are shown in Figure 4e–i. The homogeneous brightness and distribution indicate the high dispersion of Ni species on Ni/AATP system.
TEM images of the AATP and 20%-Ni/AATP samples are shown in Figure 5. The rod-like clusters of AATP can be observed obviously in Figure 5a,b. Compared with the original ATP [28], AATP shows a smooth surface, indicating that the impurities have been removed from the surface of ATP after the activation process. For the 20%-Ni/AATP catalyst, a number of small nanoparticles can be observed on the surface of the symmetric AATP support (Figure 5c–e), and the particles show a uniform distribution with a size of 4–10 nm. This result is in good consistency with the XRD patterns, suggesting the formation of highly dispersed NiO nanoparticles in 20%-Ni/AATP. The high-resolution TEM image in Figure 5f illustrates that the NiO nanoparticles are fixed onto the AATP matrix. The lattice fringes of the nanocrystals with interplanar spacing of 0.21 nm index to the (2 0 0) plane of the NiO [40].
Figure 6 shows the H2-TPR curves of the Ni/AATP catalysts and AATP support. NiO nanoparticle is also presented for comparison. It is shown that the AATP has a weak H2 consumption peak at a high temperature range of 700–800 °C, which may be caused by the reduction of impurity FeO to Fe [41]. The result suggests that a small amount of Fe species exist in the AATP support (consistent with the element mapping image in Figure 4i). H2-TPR curve of NiO particles shows an H2 consumption peak at temperature range of 300–450 °C, corresponding to the reduction of NiO to Ni [22]. Differently, the reduction temperatures of all the Ni/AATP catalysts shift to a higher temperature of 500–750 °C, probably due to the presence of a strong interaction between NiO and support [10,42]. It is known that Si-OH can react with metal species, forming Si-O-M bonds [10]. Herein, the AATP generates much of the Si-OH groups during the activating of ATP. Thus, Si-O-Ni species can be formed in the catalyst, increasing the reduction temperature of NiO species to Ni0.
Figure 7 presents the catalytic performances of the synthesized Ni/AATP catalysts for ammonia decomposition as a function of reaction temperature. All the samples show a similar tendency that ammonia decomposition activities increase with the increasing reaction temperature, indicating a higher temperature favors faster NH3 decomposition. The detailed data of NH3 conversion and H2 production are summarized in Table 1.
Figure 7a presents the catalytic activities of the prepared Ni/AATP catalysts calcined at 600 °C with different Ni content. For comparison, the catalytic performances of pure NiO particles and 600 °C-calcined AATP support are provided. The pure NiO sample exhibits the lowest catalytic activity. It can be seen that the pure NiO sample exhibits the lowest catalytic activity and the AATP exhibits an ammonia conversion of ~25% at 650 °C. The catalytic activity of the pure AATP support can be attributed to Fe component as an active metal for ammonia decomposition [15,16]. After loading with Ni species, the Ni/AATP samples exhibit high ammonia decomposition performance. The 20%-Ni/AATP sample exhibits a 95.3% NH3 conversion at 650 °C. This indicates the existence of a strong interaction between the AATP support and Ni species, which can promote the ammonia decomposition activity. The strong interaction can generate many highly dispersed Ni species, which are responsible for ammonia decomposition [32].
The ammonia conversions of Ni/AATP catalysts increase with the increase of Ni loading from 5 to 20%, and the highest catalytic activity is obtained in the 20%-Ni/AATP sample with 95.3% NH3 decomposition and 31.9 mmol min−1 gcat−1 H2 production at a reaction temperature of 650 °C. This catalytic performance is comparable to noble metal-based Ru/MgO catalysts [43] and much higher than that of other Ni and Fe-based catalysts (Table 2). At the same time, it can be seen that low content of Ni of 5% induces slightly enhanced catalytic activity. When the Ni content reached 10%, NH3 conversion jumps to approximately 80% at a reaction temperature of 650 °C. However, too much Ni content over 20% results in a negative effect on conversion of ammonia, probably because the excessive Ni species cover the surface of the catalyst and reduces the interaction between NH3 and active metal. Furthermore, Ni species tend to aggregate into larger particles at a high Ni loading, which is inactive for ammonia decomposition [44].
Ammonia decomposition to hydrogen is a typical high temperature catalytic reaction system, and the thermal stability of the catalyst is most important during the process of reaction. In order to improve the stability and reduce the sintering of active metals, many core-shell structured catalysts have been utilized in ammonia decomposition [30,48,49]. Thus, the thermal stability of Ni/ATTP catalyst systems is also investigated through calcinating a 20%-Ni/ATTP sample at a higher temperature (700 and 800 °C). In Figure 7b, a negligible catalytic activity difference between 20%-Ni/ATTP-600 and 20%-Ni/ATTP-700 samples is observed, suggesting the absence of strong structural change of active metal Ni species after 700 °C calcination. When the calcination temperature increases to 800 °C, the catalytic activity shows a slight decrease. It is interesting that almost the same NH3 conversion is observed in both two catalysts at 650 °C reaction temperature. This indicates that the growth and agglomeration of NiO during a higher calcination temperature (agreement with the XRD characterization in Figure 2b). However, due to the fixing of NiO particles in AATP matrix (proofed by TEM), its structure and size of particles are still suitable for catalyzing decomposition of ammonia. Therefore, the calcination temperature of 600 °C is appropriate, and the catalyst can undergo higher reaction temperature than 600 °C and maintain the original structure characteristic.
The decomposition of ammonia has been demonstrated to be a structure-sensitive reaction [50,51]. The performance of catalysts can be affected by the content and dispersion of the active component on the support. The pure NiO particles without support show the lowest catalytic activity for ammonia decomposition, and its activity is even lower than ATTP support (Figure 7a). This can be attributed to aggregation of NiO nanoparticles proved by XRD result. However, highly dispersed NiO on ATTP supports, according to SEM and TEM observation, obtains a high activity due to the cooperation effect between active metal and support. Moreover, Ni species can tightly anchor on the support that efficiently prevents NiO particles from aggregating, and the catalytic activity can be enhanced obviously. The catalytic activity presents a volcano type trend with the increase of Ni content. Excessive Ni species leads to aggregation of NiO into larger particles, resulting in buried Ni sublayers losing their catalytic activity [52]. The same, low content of Ni due to few active sites leads to low ammonia conversion.
The apparent activation energy (Ea) of x%-Ni/ATTP-600 catalysts are obtained from the Arrhenius relationship between the rate constant (k) and the temperature (T), which can be described by the equation [22]: ln(k) = -Ea/RT + constant, and corresponding results are shown in Table 1. Ea value decreases with the increase of Ni content from 5% to 20%, and then increases with the further increase of Ni species to 30%; the 20%-Ni/AATP sample shows the highest catalytic activity and the lowest activation energy (79.1 kJ/mol). This data is much higher than previously reported 4Ni/Ce0.8Zr0.2O2-SA (54 kJ/mol) [22] and K-Ni/MCM-41(IMP) (53.5 kJ/mol) [51] but much lower than Ni/CNTs catalyst (90.3 kJ/mol) [53].
Figure 8 shows the stability of 20%-Ni/AATP catalyst under selected reaction condition in order to investigate if the catalytic activity can be steady at a given temperature or a higher temperature for a particular period of time. Ammonia conversion is ca. 60% at 550 °C; its activity almost retains after reacting at 650 °C for 12 h with 95% ammonia decomposition and lowering to 550 °C. This indicates that Ni/AATP catalyst system possesses high stability under high a temperature reaction condition.

4. Conclusions

Activated attapulgite clay (AATP) loaded Ni catalyst exhibits significant catalytic performance for ammonia decomposition to COx-free H2. The 20%-Ni/ATTP catalyst shows a 95.3% NH3 conversion with a 31.9 mmol min−1 gcat−1 H2 formation rate at 650 °C. The excellent catalytic activity can be attributed to the small size and high dispersion of NiO particles loaded on the symmetric support. Long-term stability is due to strong interaction between Ni species and support, which efficiently inhibited Ni particle agglomeration at a high temperature. It is demonstrated that AATP is a promising support of catalysts for ammonia decomposition.

Author Contributions

Conceptualization, L.-F.Z. and Z.-P.H.; methodology, Z.-P.H.; investigation, L.-F.Z., Z.-P.H., S.-H.L. and F.X.; writing—original draft preparation, L.-F.Z.; writing—review and editing, Z.-Y.Y.; supervision, Z.-Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available on request.

Acknowledgments

This work was supported by State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC), and Changzhou Vocational Institute of Engineering, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Symmetry 14 02627 sch001
Scheme 1. The formation process of Ni/AATP catalyst.
Scheme 1. The formation process of Ni/AATP catalyst.
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Figure 1. TG curves of AATP and uncalcinated 20%-Ni/AATP.
Figure 1. TG curves of AATP and uncalcinated 20%-Ni/AATP.
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Figure 2. XRD patterns of (a) natural ATP, AATP and 600 °C-calcined AATP, (b) x%-Ni/AATP catalysts (calcined at 600 °C) and (c) the 20%-Ni/AATP samples calcined at different temperature.
Figure 2. XRD patterns of (a) natural ATP, AATP and 600 °C-calcined AATP, (b) x%-Ni/AATP catalysts (calcined at 600 °C) and (c) the 20%-Ni/AATP samples calcined at different temperature.
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Figure 3. (a) The N2 adsorption-desorption isotherms of AATP and Ni/AATP catalysts and (b) the corresponding pore size distribution curves on the basis of NLDFT method.
Figure 3. (a) The N2 adsorption-desorption isotherms of AATP and Ni/AATP catalysts and (b) the corresponding pore size distribution curves on the basis of NLDFT method.
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Symmetry 14 02627 g004
Figure 4. SEM of AATP (a,b) and 20%-Ni/AATP (c,d). EDS elemental mapping images of 20%-Ni/AATP (e–i).
Figure 4. SEM of AATP (a,b) and 20%-Ni/AATP (c,d). EDS elemental mapping images of 20%-Ni/AATP (e–i).
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Figure 5. TEM images of AATP (a,b) and the 20%-Ni/AATP (c–f).
Figure 5. TEM images of AATP (a,b) and the 20%-Ni/AATP (c–f).
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Figure 6. H2-TPR curves of NiO and the Ni/AATP catalysts.
Figure 6. H2-TPR curves of NiO and the Ni/AATP catalysts.
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Figure 7. The conversions of the Ni/AATP catalysts with different Ni content and calcination temperature.
Figure 7. The conversions of the Ni/AATP catalysts with different Ni content and calcination temperature.
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Figure 8. The stability testing of 20%-Ni/AATP catalysts calcined at 600 °C.
Figure 8. The stability testing of 20%-Ni/AATP catalysts calcined at 600 °C.
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Table 1. The physical properties and catalytic activities of the investigated catalysts.
Table 1. The physical properties and catalytic activities of the investigated catalysts.
SamplesSBET
(m2/g)		Vtot
(cm3/g)		Ni Content 1 (wt%)Conversion 2
(%)		H2 Production
(mmol min−1 gcat−1)		Ea
(kJ/mol)
ATP1390.48----
AATP2620.60-31.810.6-
NiO---23.67.9-
5%-Ni/AATP2410.494.939.213.193.7
10%-Ni/AATP2240.617.578.626.391.8
15%-Ni/AATP2240.5612.184.528.385.9
20%-Ni/AATP2270.5514.995.331.979.1
25%-Ni/AATP1990.5317.888.930.479.5
30%-Ni/AATP1790.5522.686.729.081.8
1 Calculated from XRF. 2 The data is obtained at 650 °C under pure ammonia flow of 50 mL/min.
Table 2. Catalytic decomposition of ammonia over different catalysts.
Table 2. Catalytic decomposition of ammonia over different catalysts.
CatalystMetal Content (wt%)T (oC)GHSV (mL/h/g)Conv. (%)Ref
5%Ru/MgO-DP3.5% Ru45030,00056.5[43]
Ru/graphene-45060,00062[45]
15%Ni/MRM15%Ni70030,00097.9[32]
Ni-30/ATP@SiO28.7%65030,00073.4[30]
Ni-50/ATP38.6%65030,00089.9[30]
15%-Ni/rGO15%70030,00076.5[46]
25Ni@Al2O325%60024,00093.9[18]
25Ni@Al2O325%65024,00099.1[18]
Fe-CNFs/mica3.5%Fe600650098.9[46]
Fe/mica3.5%Fe600600051.3[47]
20%-Ni/AATP14.9%Ni65030,00095.3This work
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Zhang, L.-F.; Hu, Z.-P.; Liang, S.-H.; Xu, F.; Yuan, Z.-Y. Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen. Symmetry 2022, 14, 2627. https://doi.org/10.3390/sym14122627

AMA Style

Zhang L-F, Hu Z-P, Liang S-H, Xu F, Yuan Z-Y. Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen. Symmetry. 2022; 14(12):2627. https://doi.org/10.3390/sym14122627

Chicago/Turabian Style

Zhang, Ling-Feng, Zhong-Pan Hu, Shi-Hang Liang, Feng Xu, and Zhong-Yong Yuan. 2022. "Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen" Symmetry 14, no. 12: 2627. https://doi.org/10.3390/sym14122627

APA Style

Zhang, L.-F., Hu, Z.-P., Liang, S.-H., Xu, F., & Yuan, Z.-Y. (2022). Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen. Symmetry, 14(12), 2627. https://doi.org/10.3390/sym14122627

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