A Simple Fabrication of Tourmaline-Supported Ni-NiAl₂O₄ Nanocomposites for Enhanced Methane Dry Reforming Activity

Abstract

Ni-based catalysts have been widely used in catalytic reactions by researchers due to their advantages such as abundant resources, high catalytic activity and lower prices than precious metals. However, the problems of easy agglomeration and poor dispersion of Ni-based catalysts have hindered their large-scale application. Therefore, it is necessary to select a suitable preparation method to reduce the agglomeration of the catalyst and improve its dispersion. In this paper, the Ni-NiAl₂O₄/tourmaline composite material was prepared by using the microwave hydrothermal reduction method. The most favorable conditions for preparing NiAl₂O₄/tourmaline are as follows: using TEOA as the additive, the microwave hydrothermal temperature is 220 °C, the calcination temperature is 800 °C, and the addition amount of tourmaline is 7.4 wt.%. NiAl₂O₄ has a good dispersion over the surface of tourmaline support and the optimal NiAl₂O₄/tourmaline catalyst exhibits a specific surface area of 106.5 m²/g. Metallic nickel was reduced at 650 °C to further obtain Ni-NiAl₂O₄/tourmaline composites. Finally, the Ni-NiAl₂O₄/tourmaline composites showed significantly improved catalytic dry reforming of methane (DRM) activity compared to Ni-NiAl₂O₄ sample under low-temperature conditions (500–600 °C), meaning that the tourmaline carrier could effectively optimize the low-temperature catalytic performance of Ni-NiAl₂O₄.

1. Introduction

Over the past few years, the scientific community has focused its research on developing clean energy and reducing greenhouse gas emissions. Due to its low cost and excellent reactivity [1,2], Ni-based catalysts have found extensive applications in the fields of dry reforming of methane DMR, glycerol reforming, hydrogenation, water–gas shift, and ammonia synthesis [3,4,5,6]. Compared with precious metals, the low cost and abundant resources of Ni provide a reasonable choice for industrial applications [7,8]. However, Ni-based catalysts still have some drawbacks, such as easy agglomeration and low dispersion degree of active components [9,10], which limits the large-scale application of Ni-based catalysts. To address this issue, an effective way to reduce the agglomeration of Ni-based catalysts is to improve their dispersion [11,12,13]. One of the feasible methods is to disperse the active components on supports [14,15]. It is reported that the interaction between oxide, mesoporous molecular sieve and graphene carriers [16], and Ni-based catalysts can improve the dispersion of Ni-based catalysts and prevent the passivation of active metals [17,18,19,20]. The high cost and cumbersome production process make it difficult to meet the standards of industrial applications [21].

Using NiAl₂O₄ as the precursor of Ni-based catalysts can suppress the size of Ni [22,23,24], but it cannot achieve the purpose for dispersing Ni [25,26,27]. Due to the abundant reserves and economic prices of natural minerals, Ni-based catalysts supported by them have received extensive research attention [28,29,30]. Tourmaline, as a structurally complex natural cyclic borosilicate mineral, has the most notable feature of its spontaneous polarization effect, which can generate a continuous electrostatic field. At the same time, it has excellent far-infrared radiation capabilities. In addition, its piezoelectric and thermoelectric effects promote the dissociation of water molecules, thereby continuously releasing negative ions [31]. The microelectric field caused by the spontaneous polarization of tourmaline will attract and capture the surrounding ions, causing these ions to lose their free mobility and thereby reducing the agglomeration of particles after calcination. Moreover, the surface of tourmaline has a relatively abundant nucleation site, which is conducive to the dispersion of the catalyst [32]. The microwave hydrothermal reduction method has attracted much attention in the preparation of catalysts because they can cause rapid and uniform heating, resulting in an even distribution of particles [25]. Herein, NiAl₂O₄/tourmaline composites were obtained through the microwave hydrothermal method. The impact of additives, microwave hydrothermal temperature, calcination temperature, and the addition amount of tourmaline on the microstructure of NiAl₂O₄/tourmaline were studied. The ideal synthesis parameters for the composite materials were identified by a series of characterization techniques of XRD, FTIR, TG-DTA, H₂-TPR, XPS, BET, SEM, and TEM. The NiAl₂O₄/tourmaline under the optimal conditions was reduced in a hydrogen atmosphere to obtain the tourmaline-supported Ni-NiAl₂O₄ composite material. The optimal reduction temperature was determined by XRD and other testing methods, and the performance of the catalytic DMR reaction of Ni-NiAl₂O₄/tourmaline composite material was tested. This research provides a low-cost and effective method for synthesizing uniformly dispersed nickel-based catalysts.

2. Results and Discussion

2.1. Catalysts Characterization

The XRD patterns and FTIR spectra of the NiAl₂O₄ prepared with different organic additives (Figure 1). The characteristic peaks of the samples prepared without any additives at 37.25°, 43.29° and 62.85° correspond to the (101), (012), and (110) crystallographic planes of NiO (PDF#44-1159), which indicates that most of the products prepared without any additives are NiO. The samples prepared with EG as the additive showed diffraction peaks at 45° and 65.5°, which belongs to the (400) and (440) crystal planes of NiAl₂O₄ (PDF#10-0339). When the mixed additive of SDBS and TEOA was added, the characteristic diffraction peak of NiO in the product weakened, the diffraction peak at 62.85° almost disappeared and the characteristic peak of NiAl₂O₄ on the (311) crystal plane appeared at 37° [33]. When the additive was TEOA, the diffraction peak of NiO at 66.8° completely disappeared, and the intensity of the peaks of NiAl₂O₄ at 45° and 65.5° significantly increased, indicating that most of the prepared products were NiAl₂O₄ (Figure 1a). The absorption peaks near 3464 and 1633 cm⁻¹ (Figure 1b) correspond to the O–H stretching vibration modes and bending vibration modes of the water molecules upon adsorption [34]. The 2859 and 2928 cm⁻¹ bands are C–H vibrations [35]. The vibration peaks near 600 and 720 cm⁻¹ represent the Al–O bonds in octahedrally coordinated aluminum versus tetrahedrally coordinated aluminum, respectively [36]. The Ni–O bond in the spinel NiAl₂O₄ structure appeared at 493 cm⁻¹ in all three samples with additives, while no peak appeared in the sample without additives at this location, indicating that NiAl₂O₄ was not present in the sample, which was consistent with the XRD analysis results.

The precursors of the composite materials were prepared by using different microwave hydrothermal temperatures. After calcination at 800 °C, the NiAl₂O₄/tourmaline composite materials were obtained, and XRD analysis was conducted on the samples (Figure 2a). Diffraction peaks of NiAl₂O₄ crystal planes of (311), (400), and (440) exist at 2θ = 37°, 45°, and 65.5°, indicating that NiAl₂O₄ can be generated within the microwave hydrothermal temperature range of 180–240 °C. The diffraction peaks at 13.89°, 22.22°, 34.69° and 44.28° belong to the (101), (220), (051) and (152) crystal planes of tourmaline (PDF#43-1464). The peak of NiAl₂O₄ becomes sharper at 45° with the microwave hydrothermal temperature increasing from 180 to 220 °C, indicating that the higher hydrothermal temperature is more conducive to the formation of NiAl₂O₄. However, when the hydrothermal temperature was 240 °C, the strength of the diffraction peaks of tourmaline and NiAl₂O₄ in the sample decreased, indicating that an excessively high hydrothermal temperature would affect the stability of the NiAl₂O₄/tourmaline composite material structure and be unfavorable for subsequent experiments. When the temperature of the hydrothermal process is 220 °C, the diffraction peak intensities of tourmaline and NiAl₂O₄ are relatively balanced. Therefore, 220 °C is selected as the reaction temperature for the subsequent experiments. Figure 2b shows the FTIR images of NiAl₂O₄/tourmaline composite materials with different microwave hydrothermal temperatures. The peak observed at 1256 cm⁻¹ corresponds to the B–O bond stretching vibration in the tourmaline structure [BO₃]. The vibration at 1037 cm⁻¹ represents the Si–O bond [37]. The vibration peaks at these two positions prove the existence of tourmaline. When the hydrothermal temperature was 240 °C, the intensities of the two vibration peaks became weakened significantly. The Al–O bond corresponding to [AlO₆] at 719 cm⁻¹ and the Ni–O bond corresponding to [NiO₄] at 499 cm⁻¹ proves the existence of NiAl₂O₄ in the composite material.

The thermal behavior of NiAl₂O₄/tourmaline composite is illustrated in Figure 3 through its TG-DTA curve. As the temperature increases, the mass of the precursor progressively decreases, and this reduction process can generally be separated into four stages. The small endothermic peak at 66.32 °C on the DTA curve is attributed to the removal of adsorbed water and crystalline water with the weight loss of 4.12 wt.% [38]. The weight loss range in the second stage was 203.6–508.2 °C. During this stage, obvious thermal weight loss occurred, with a weight loss rate of approximately 12.76 wt.%, which was caused by the thermal decomposition of the intermediate products generated during the microwave reaction. The third stage was 508.2–873.4 °C. Throughout this procedure, the weight loss rate of the sample was approximately 3.04 wt.%. At around 800 °C, the TG curve gradually stabilized, indicating that most of the NiAl₂O₄ had been generated at this time. Near 868.9 °C, there is a weakly endothermic peak in the DTA curve, which may be caused by the destruction of the tourmaline structure [39]. The fourth stage is after 873.4 °C. During this stage, the mass is relatively stable, with a weight loss rate of only 0.25 wt.%, indicating that the formation of NiAl₂O₄ is more complete and the structure of the composite material is more stable after this temperature. Based on the above analysis, the calcination temperature for the preparation of the NiAl₂O₄/tourmaline composite material is set at 750–900 °C.

To determine the ideal calcination temperature, the precursor powder was calcined in muffle furnaces at 750 °C, 800 °C, 850 °C and 900 °C for 3 h. The XRD patterns can be observed from Figure 4a that the typical peaks of NiAl₂O₄ appeared in four samples near 37°, 45° and 65.5°, corresponding to the (311), (400) and (440) crystal planes. This indicates that although NiAl₂O₄ can be generated at 750–900 °C, the peak intensities of NiAl₂O₄ exhibit significant variations. When calcined at 750 °C, the diffraction peaks of NiAl₂O₄ are relatively wide and weak in intensity. When the temperature further rises, the peak shape of the diffraction peaks of NiAl₂O₄ becomes more acute, suggesting that elevated temperature inhibits the generation of NiO. However, when the temperature rises excessively, the strength of the diffraction peaks of tourmaline at 13.89°, 22.22°, 34.69° and 44.28° gradually weakens. When the temperature is 900 °C, the peaks of tourmaline become extremely weak, meaning that the composition of tourmaline begins to be broken and the structure of the composite material is unstable. The findings align with the TG-DTA analysis results. As the temperature rises, the intensity of the vibration peaks corresponding to the B–O bond (1256 cm⁻¹) and Si–O bond (1037 cm⁻¹) in tourmaline decreased in Figure 4b. At a calcination temperature of 900 °C, the peaks at two positions disappear, meaning that the structure of tourmaline has been damaged. The peaks near 719 and 499 cm⁻¹ represent the Al–O bond in [AlO₆] and the Ni–O bond in [NiO₄], indicating the presence of NiAl₂O₄ within the range of 750–900 °C. Although high temperatures are conducive to the formation of NiAl₂O₄, excessively high temperatures can cause structural damage to tourmaline, which is not beneficial to the structural stability of composite materials. Therefore, the calcination temperature was determined to be 800 °C. At this temperature, NiAl₂O₄ can be generated, and the destruction of the tourmaline structure can be avoided, which matches the XRD analysis results.

As shown in Figure S2, NiAl₂O₄ exhibits a plate-like structure. Figure 5 shows the SEM of the composite materials obtained by calcination at 750–900 °C, respectively. Similarly, the NiAl₂O₄ with a flaky shape is loaded on the smooth surface of tourmaline at 750–850 °C. At a calcination temperature of 900 °C, platelet-like NiAl₂O₄ becomes attached to the structurally disrupted outer layers of tourmaline. Figure 5a shows the sample prepared at a calcination temperature of 750 °C, at which point the generation of NiAl₂O₄ is relatively small, and only a small amount of NiAl₂O₄ is loaded on the surface of the tourmaline. As the temperature rises, the formation of NiAl₂O₄ becomes more stable. Figure 5b indicates that at a calcination temperature of 800 °C, the surface of tourmaline is loaded with more NiAl₂O₄, and no agglomeration phenomenon occurs. Upon increasing the calcination temperature to 850 °C or even 900 °C, the agglomeration of NiAl₂O₄ in the composite material becomes more severe. Moreover, an excessively high calcination temperature can easily lead to an overly stable structure of NiAl₂O₄, making it difficult for the composite material to reduce metallic Ni in subsequent reactions (Figure 5c,d). Therefore, combined with the previous analysis, the optimal calcination temperature was established as 800 °C.

Figures S3 and S4 respectively present the XRD patterns and FTIR spectra of NiAl₂O₄/tourmaline composites (calcined at 850 °C) prepared with different tourmaline additional content (the tourmaline content was 11.8, 9.1, 7.4, and 6.3 wt.%, respectively). As illustrated in Figure S3, the diffraction peaks of tourmaline and NiAl₂O₄ in the XRD curves of the four samples exist relatively completely. When the addition amount is 11.8 wt.%, the diffraction peak intensity of tourmaline is relatively high, indicating that the addition amount of tourmaline in the composite material is excessive while the content of NiAl₂O₄ is low currently, which does not facilitate the economical production of the composite material and the subsequent reduction. With the decrease of the tourmaline additional amount, it could be clearly observed that the peak at 34.69° weakened, while the diffraction peaks intensities of NiAl₂O₄ at 45° and 65.5° increased. When the additional amount of tourmaline was reduced to 6.3 wt.%, the diffraction peak of tourmaline was weaker in intensity compared to other samples, while the diffraction peak intensity of NiAl₂O₄ was too high, indicating that the content of the composite material was relatively low. When the addition amount of tourmaline was 9.1 and 7.4 wt.%, The sample showed all diffraction peaks of tourmaline and NiAl₂O₄, and the intensity of the diffraction peaks was relatively high, and the composition of the composite material was balanced. Furthermore, as shown in Figure S4, the absorption peak at 1256 cm⁻¹ is related to the vibration of the B–O bond of [BO₃] in tourmaline, and the band at 1037 cm⁻¹ is the vibration peak of the Si–O bond. Their intensities gradually weaken with the decrease of the additional amount of tourmaline. When the addition amount of tourmaline decreased, the vibrational peaks corresponding to the Al–O bond in [AlO₆] at approximately 719 cm⁻¹ and the Ni–O bond in [NiO₄] at around 499 cm⁻¹ progressively sharpened. This suggests an increase in the NiAl₂O₄ proportion within the composite material, aligning with the XRD analysis findings.

The effect of reduction temperature on the synthesis of metallic nickel nanoparticles in the Ni-NiAl₂O₄/tourmaline composite catalysts with three tourmaline additional amounts (9.1, 7.4, and 6.3 wt.%) was investigated by the programmed temperature reduction (H₂-TPR) technology (Figure S5). All the H₂-TPR curves show absorption peaks of different intensities. The two reduction peaks at lower temperatures should be observed with emphasis. When the addition amount of tourmaline was 9.1 wt.%, the reduction peaks at lower temperatures were located near 538 °C and 741 °C. At 6.3 wt.% tourmaline content, two reduction peaks appeared around 532 °C and 742 °C respectively. With a 7.4 wt.% tourmaline addition, the reduction peaks positioned at 521 °C and 738 °C. Compared with the other two NiAl₂O₄/tourmaline samples, the reduction temperature was relatively low for the composite with 7.4 wt.% of tourmaline, and it was more suitable for reduction at a lower temperature. The contents of Ni and Al elements in the NiAl₂O₄/tourmaline sample with an addition amount of 7.4 wt.% of tourmaline were measured (Table S1). It is calculated that the molar ratio of Ni/Al is approximately 0.47.

After determining the optimal parameters of the NiAl₂O₄/tourmaline composite material, the prepared samples were reduced by 3 h in a gaseous environment with V(H₂)/V(N₂) = 5/95, and the temperatures were set at 550 °C, 600 °C, 650 °C and 700 °C to obtain the Ni-NiAl₂O₄/tourmaline composite material. Figure 6a shows the XRD spectra of the Ni-NiAl₂O₄/tourmaline generated at distinct reduction temperature points. It has obvious diffraction peaks corresponding to NiAl₂O₄ and tourmaline, but the diffraction peak of Ni obtained by reduction at 550 °C is not obvious. When the reduction temperature was 600 °C, the sample exhibited diffraction peaks corresponding to metallic Ni at 2θ = 44.5° and 51.8° (PDF#04-0850), indicating that metallic Ni was generated in the reduced sample at this temperature, but the intensity of its diffraction peaks was relatively weak. By raising the temperature, the diffraction peaks of NiAl₂O₄ at 37°, 45° and 65.5° gradually weaken, while the peak of Ni becomes stronger. When it rises to 650 °C, the diffraction peak of Ni on the (220) crystal plane appears at 2θ = 76.4°, and the diffraction peaks at 44.5° and 51.8° are significantly increased. With the temperature steadily climbing to 700 °C, the Ni peak exhibited a greater relative intensity compared to that of NiAl₂O₄, indicating that the sample contained a large amount of metallic Ni at this time. Moreover, the grain size of Ni at 2θ = 44.5° is approximately 13 nm, as determined by the Scherrer formula. To further identify the ideal reduction temperature, the tourmaline-loaded Ni-NiAl₂O₄ composites reduced at 600 °C, 650 °C and 700 °C were subjected to TG tests respectively (Figure 6b). The tourmaline-supported Ni-NiAl₂O₄ composites obtained by reduction at 600 °C, 650 °C and 700 °C have obvious differences in their mass changes. Firstly, when the reduction temperature is 600 °C, the mass of the composite material shows an overall downward trend. This is because the mass of the trace metal Ni increased after oxidation, which is insufficient to offset the reduced mass of the composite material. When the reduction temperature is 650 °C, the reduced metal Ni is oxidized within the range of 250–350 °C, resulting in a greater mass of the sample. When the temperature reduction continues to rise to 700 °C, the mass increase of the sample within the range of 250–300 °C becomes more obvious due to the excessive reduction of metal Ni which is easy to cause the agglomeration. Therefore, in combination with XRD analysis, 650 °C was selected as the optimal reduction temperature for the composite material.

To better observe the morphological changes of the composite materials at varying reduction temperatures, the samples mentioned above were examined by SEM characterization. Figure 7 presents the SEM images of the composite materials after reduction at temperatures between 550 and 700 °C. Among them, the lamellar NiAl₂O₄ is loaded on the smooth surface of tourmaline. The dark fine particles that appear on the sample surface are the reduced metallic Ni particles. The surface morphology of the Ni-NiAl₂O₄/tourmaline composite obtained by reduction at 550 °C shows almost no change (Figure 7a). When reduced at 600 °C, the diffraction peak of Ni has already appeared in the XRD curve, but the dark Ni particles can hardly be observed (Figure 7b). As the temperature continues to rise to 650 °C, metallic Ni particles are sporadically distributed across the sample’s surface (Figure 7c). When the temperature reaches 700 °C, most of the NiAl₂O₄ in the composite material has been reduced to metallic Ni. Therefore, the surface of tourmaline exhibits numerous dark Ni particles, but the presence of NiAl₂O₄ is almost invisible (Figure 7d). The SEM results corroborate the structural information obtained from XRD analysis. Figure S6 shows the SEM image and elemental distribution of the Ni-NiAl₂O₄/tourmaline at a reduction temperature of 650 °C. We can more clearly observe the Ni metal particles.

After determining the optimal reduction temperature, the NiAl₂O₄/tourmaline composites obtained by calcination at three temperatures were reduced at 650 °C for 3 h, and XRD analysis was conducted. Figure S7 shows the XRD patterns of the reduction of NiAl₂O₄/tourmaline obtained by calcination at 750 °C, 800 °C and 850 °C respectively. The diffraction pattern reveals the presence of characteristic peaks corresponding to metallic nickel in the composite materials after reduction at 650 °C. When the calcination temperature is 750 °C, the (311) crystal plane diffraction peak of NiAl₂O₄ in the reduced Ni-NiAl₂O₄/tourmaline is weak, and its intensity increases with rising temperatures, while the diffraction peak intensity of metallic Ni weakens. This indicates that although NiAl₂O₄ will be generated at a lower temperature, its structure is unstable. This leads to the easy reduction of Ni²⁺ on the surface of NiAl₂O₄. During calcination at 850 °C, due to the increasingly stable structure of NiAl₂O₄, it is difficult for Ni to be reduced to the surface of the composite material. Therefore, the intensity of the diffraction peaks of Ni at 2θ = 44.5° and 76.4° weakens after reduction. When the calcination temperature of the composite material prepared was 800 °C, the XRD pattern exhibited relatively balanced diffraction peak intensities for both NiAl₂O₄ and Ni following the reduction process. The NiAl₂O₄/tourmaline composites prepared with tourmaline addition amounts of 11.8, 9.1, 7.4 and 6.3 wt.% were reduced at 650 °C for 3 h to obtain Ni-NiAl₂O₄/tourmaline composites. As shown in Figure S8, the diffraction peaks of metallic Ni at 44.5° and 51.8° all existed after the composite materials were reduced at 650 °C. When the additional amount of tourmaline is 11.8 wt.%, the less amount of NiAl₂O₄ leads to a lower average loading, and less Ni is reduced. Therefore, the peak intensities of NiAl₂O₄ and Ni are relatively weak. With the decrease of the tourmaline addition amount, the NiAl₂O₄ content is relatively high, thus more metallic Ni will be reduced, resulting in a notable enhancement in the peak intensity at 44.5°. When the addition amount was reduced to 6.3 wt.%, the tourmaline content was relatively low, while the excessive content of NiAl₂O₄ was not easy to disperse, resulting in a low reduction rate of metallic Ni and agglomeration, which was not conducive to the application of composite materials in catalytic reactions. When the addition amount of tourmaline is 7.4 wt.%, the strengths of NiAl₂O₄ and Ni in Ni-NiAl₂O₄/tourmaline are in a relatively balanced state, meaning that the reduction effect is the best.

To investigate the specific surface area of the composite materials, nitrogen adsorption and desorption tests were performed on the samples of raw tourmaline, pure-phase NiAl₂O₄, NiAl₂O₄/tourmaline, and Ni-NiAl₂O₄/tourmaline. Figure S9 demonstrates that all four samples belong to type IV adsorption and desorption isotherms, and there is an H3-type hysteresis loop within the relative pressure range of 0.5–1, indicating that the samples have a mesoporous structure [40]. The test results showed that their specific surface area values were 20.6, 71.6, 106.5 and 109.0 m²/g respectively. Obviously, when NiAl₂O₄ is combined with tourmaline, its specific surface area is significantly increased compared to that of tourmaline and pure-phase NiAl₂O₄. This is because a large amount of NiAl₂O₄ is generated on tourmaline’s surface, thereby increasing the specific surface area. The Ni-NiAl₂O₄/tourmaline composite obtained after hydrogen reduction has a further increase in specific surface area due to the well-dispersed metallic nickel formed on the NiAl₂O₄ surface after reduction.

To determine the elemental composition and valence state distribution, the Ni-NiAl₂O₄/tourmaline composite material was characterized by XPS analysis. Figure 8a shows the full spectrum of the composite material, from which peaks of the main elements such as Ni, Al, O, and Si can be observed. The high-resolution Ni 2p spectrum exhibits two distinct spectral features (Figure 8b). The signal peak near 852.0 eV belongs to the elemental metal Ni⁰ [41]. The signal peaks with binding energies near 855.5 and 873.0 eV correspond to the 2p_3/2 and 2p_1/2 peaks of Ni²⁺. The binding energy of 2p_3/2 is close to the reported value of 856.0 eV of NiAl₂O₄ in the literature, It indicates that Ni²⁺ in the composite material exists in the form of NiAl₂O₄ [42]. In the high-resolution spectrum of Al 2p in Figure 8c, the signal peak of Al 2p appears near 73.9 eV, which is near the reported Al³⁺ binding energy of 74.3 eV in NiAl₂O₄. Furthermore, the area ratio of Ni⁰ to Ni²⁺ on the catalyst surface obtained from the present Ni 2p fitting spectrum is approximately 1: 10.

To better observe the microstructure of tourmaline-supported Ni-NiAl₂O₄ composite material reduced at 650 °C, TEM analysis was conducted on the sample (Figure 9). The loading of Ni-NiAl₂O₄ on the surface of tourmaline minerals can be observed from Figure 9a. Figure 9b shows the HRTEM image of part b in Figure 9a. Distinct lattice fringes can be observed in the metals Ni and NiAl₂O₄. The interfacial spacings of these metals are 0.18, 0.24 and 0.28 nm, which correspond to the (200) planes of Ni, the (311) and (220) planes of NiAl₂O₄, respectively.

2.2. Catalytic Performance

To explore the effect of tourmaline support on the performance of the catalyst, the Ni-NiAl₂O₄ was prepared under the controlled optimal conditions. Under the conditions of normal pressure, space velocity GHSV = 112.5 L·g⁻¹·h⁻¹, CH₄: CO₂: N₂ = 1: 1: 10 (total flow rate 30 mL/min), and 500–650 °C, the DRM catalytic performance of Ni-NiAl₂O₄ and Ni-NiAl₂O₄/tourmaline was compared and tested. Figure 10a,b shows that the CH₄/CO₂ conversion rates of both catalysts increase with the rise in temperature, which is in line with the endothermic reaction characteristics. It should be noted that within the range of 500–600 °C, the conversion rate of samples containing tourmaline is greater than unmodified samples. However, when the temperature reaches to 650 °C, the conversion rates of the two tend to be consistent, indicating that the tourmaline carrier can effectively improve the catalytic performance of Ni-NiAl₂O₄ in the low-temperature zone. The spontaneous polarization characteristics of tourmaline surface and its far-infrared radiation capacity can collaboratively promote the homogeneous distribution of active components. Meanwhile, the far-infrared rays can enhance the kinetic energy of reactant molecules, thereby increasing the effective collision frequency between reactants and active sites [43]. In addition, the addition of tourmaline carrier improves the dispersion of the catalyst, making the reduced Ni particles uniformly dispersed and less prone to agglomeration. Figure 10c shows the hydrogen-carbon ratio of Ni-NiAl₂O₄ and Ni-NiAl₂O₄/tourmaline catalysts. Among them, the hydrogen-to-carbon ratio of the Ni-NiAl₂O₄ catalytic product is stable at around 1.1. Compared with Ni-NiAl₂O₄, the Ni-NiAl₂O₄/tourmaline catalyst demonstrated a higher hydrogen-to-carbon ratio in the catalytic tests at 550–650 °C. The Ni-NiAl₂O₄/tourmaline composite material exhibited the highest hydrogen-carbon ratio (1.32) at 550 °C. However, with the increase in temperature, the hydrogen-carbon ratio showed a downward trend. The rise in reaction temperature enhanced the reverse water gas shift reaction, that is, more H₂ and CO₂ in the catalytic products reacted, thereby reducing the hydrogen-carbon ratio. The Ni-NiAl₂O₄/tourmaline catalyst synthesized by microwave hydrothermal reduction shows a higher conversion efficiency of CH₄ and CO₂ in the DRM, and the H/C of the product is significantly better than that of the conventional carrier catalyst. This phenomenon demonstrates the enhancing role of tourmaline carriers in catalytic performance, which can increase the conversion rate of reactants and optimize the product distribution.

3. Materials and Methods

3.1. Materials

The tourmaline was acquired from Xinjiang, China, and its main chemical composition was SiO₂ (36.88 wt.%), Al₂O₃ (28.96 wt.%), Fe₂O₃ (10.46 wt.%), and MgO (3.85 wt.%). The tourmaline sample sourced from Xinjiang exhibits prismatic morphology with an average particle size of approximately 1 μm (Figure S1) and a specific surface area of 71.6 m²/g (Figure S9). NiCl₂·6H₂O and ethylene glycol (EG) were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). NaOH was purchased from Tianjin Zhiyuan Chemical Reagent Factory (Tianjin, China). Al(NO₃)₃·9H₂O, triethanolamine (TEOA) and sodium dodecylbenzene sulfonate (SDBS) were provided by Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). All the reagents are analytical grade and used without further purification.

3.2. Preparation of Ni-NiAl₂O₄/Tourmaline Composite

Firstly, the optimal synthesis conditions of NiAl₂O₄/tourmaline composites were determined by adjusting the microwave hydrothermal temperature, calcination temperature and the addition amount of tourmaline. Then, the effect of reduction temperature on the behavior of Ni species in NiAl₂O₄/tourmaline composites was systematically studied to identify the most suitable reduction temperature. First, an appropriate amount of nickel chloride hexahydrate (NiCl₂·6H₂O) and aluminum chloride hexahydrate (AlCl₃·6H₂O) was dispersed in 40 mL of distilled water to form a precursor solution (Ni²⁺: Al³⁺ molar ratio = 1:2). To determine the optimal tourmaline loading, NiAl₂O₄/tourmaline composite materials were synthesized with tourmaline additions of 11.8 wt.%, 9.1 wt.%, 7.4 wt.%, and 6.3 wt.% relative to the precursor mass. Then, the above-mixed solution was agitated for 30 min followed by 20 min of ultrasonication. Next, 3M NaOH was added dropwise to adjust the pH of the suspension to 13, and stirring was continued for another 30 min after adding the additives (10 vol.% of TEOA/EG or 0.25 wt.% of SDBS). Later, 50 mL aliquot of the suspension was placed in a 100 mL Teflon-lined autoclave and maintained at 220 °C for 3 h. After that, the suspension was rinsed with DI water until the solution was adjusted to near-neutral pH, dried at 60 °C, and ground to acquire the precursor powder. Then, the powder was calcined at the designed temperatures (an interval of 50 °C from 750–900 °C) for 3 h to obtain NiAl₂O₄/tourmaline composites. NiAl₂O₄/tourmaline was subjected to reduction treatment in a mixed atmosphere of H₂/N₂ (with different reduction temperatures set at 550–700 °C and each 50 °C interval as a variable), and finally Ni-NiAl₂O₄/tourmaline catalyst was obtained.

3.3. Catalytic Performance Test

The catalytic performance of Ni-NiAl₂O₄/tourmaline composite catalyst (16 mg is uniformly diluted with 500 mg of quartz sand) for CO₂ DRM was evaluated in a fixed-bed reactor. It takes CH₄ and CO₂ as reactants, H₂ as the catalyst pretreatment reducing agent, and N₂ as the carrier gas to dilute the reaction gas. The catalyst bed is placed in the central section of the reaction tube, with quartz wool fixed at both ends to ensure uniform gas flow distribution and prevent the loss of catalyst particles. Experimental conditions: Under normal pressure, the ratio of raw gas is CH₄:CO₂:N₂ = 1:1:10, and the space velocity is 112.5 L·g⁻¹·h⁻¹. Catalyst pretreatment: Reduce in a 10% H₂/N₂ mixed gas at 30 mL/min for 2 h at 650 °C, purge with N₂ for 0.5 h, and then cool to 450 °C. The temperature range for the DRM reaction test is 500–650 °C, and the performance is evaluated with a gradient of 50 °C. The calculation formulas for the conversion rates of CH₄ and CO₂ and the product of H₂/CO in the methane dry reforming reaction are as follows:

$${\mathrm{C}}_{{\mathrm{CH}}_4}=1-{\displaystyle \frac{{\mathrm{F}}_{{\mathrm{CH}}_{4,\mathrm{out}}}}{{\mathrm{F}}_{{\mathrm{CH}}_{4,\mathrm{in}}}}}\times 100\%$$

(1)

$${\mathrm{C}}_{{\mathrm{CO}}_2}=1-{\displaystyle \frac{{\mathrm{F}}_{{\mathrm{CO}}_{2,\mathrm{out}}}}{{\mathrm{F}}_{{\mathrm{CO}}_{2,\mathrm{in}}}}}\times 100\%$$

(2)

$${\displaystyle \frac{{\mathrm{H}}_2}{\mathrm{CO}}}={\displaystyle \frac{{\mathrm{F}}_{{\mathrm{H}}_{2,\mathrm{out}}}}{{\mathrm{F}}_{{\mathrm{CO},}_{\mathrm{out}}}}}$$

(3)

Among them, F_{i, in} and F_{i, out} respectively represent the inlet and outlet flow rates of gas i (CO₂, CH₄), and C_i represents the conversion rate of gas i.

4. Conclusions

In summary, the optimal preparation conditions for preparing the Ni-NiAl₂O₄/tourmaline catalyst by microwave hydrothermal reduction method were determined as follows: using 10 vol.% TEOA as the additive, the microwave hydrothermal temperature was 220 °C, the calcination temperature was 800 °C, the addition amount of tourmaline was 7.4 wt.% and the reduction at 650 °C for 3 h. During the material formation process, NiAl₂O₄ promotes the nucleation and dispersion of metallic Ni nanoparticles and provides sufficient metal precursors for the subsequent reduction to Ni. Finally, the DRM reaction results indicated that, compared with Ni-NiAl₂O₄, the low-temperature (500–600 °C) activity of the Ni-NiAl₂O₄/tourmaline composite material was significantly enhanced, and the conversion rates of CH₄ and CO₂ at 600 °C were 70% and 67%, respectively. Meanwhile, the hydrogen-to-carbon ratio of the catalytic product also increased accordingly, reaching the maximum value of 1.32 at 550 °C. This indicates that the tourmaline carrier can effectively optimize the low-temperature catalytic performance of Ni-NiAl₂O₄.