A Facile Fabrication of Supported Ni/SiO₂ Catalysts for Dry Reforming of Methane with Remarkably Enhanced Catalytic Performance

Abstract

Ni catalysts supported on SiO₂ are prepared via a facile combustion method. Both glycine fuel and ammonium nitrate combustion improver facilitate the formation of much smaller Ni nanoparticles, which give excellent activity and stability, as well as a syngas with a molar ratio of H₂/CO of about 1:1 due to the minimal side reaction toward revserse water gas shift (RWGS) in CH₄ dry reforming.

1. Introduction

The availability of natural gas (or shale gas) in large reserves makes CH₄ serve as a suitable feedstock used in C1 chemistry to produce desired fuels and chemicals [1]. Unfortunately, the chemical inertness of CH₄ results in direct conversion, which constitutes a great challenge for highly efficient utilization [2]. Ideally, the best use of CH₄ occurs when it is converted into syngas, which can facilitate further downstream conversion [3] by means of the methanol route [4] and Fischer–Tropsch synthesis (FTS) [5,6,7,8,9,10,11,12] due to good reactivity, unlike the CH₄ which has a high dissociation energy C–H bond [1]. Among the most widely investigated technologies, there are comparable advantages associated with the dry reforming of CH₄ (DRM) with CO₂ for producing syngas [13]. On the one hand, compared to the other reforming processes, there is a 20% lower operating cost for DRM [14]; on the other hand, the reforming of CH₄ using CO₂ not only produces high purity syngas [15,16] but also reduces the emissions of two abundantly available greenhouse gases to alleviate global climate change [17,18,19,20].

In spite of the above-mentioned merits, DRM suffers from serious carbon deposits on the surface of Ni nanoparticles, which leads to a remarkable loss of active sites [21,22,23,24,25]. Recently, DRM research efforts have resulted in strategies to improve the stability of the catalyst [26]. Based on the fact that smaller Ni nanoparticles efficiently improve catalytic performance by avoiding carbon accumulation [27,28,29,30,31,32], the general concept is to develop the catalyst preparation protocol to obtain small Ni nanoparticles encapsulated in the support or confined by the stable porous oxide layer to prevent sintering [33,34]. For example, Tomishige et al. reported that the solid solution catalyst of nickel–magnesia, which was prepared by the co-precipitation method, showed high and stable activity without carbon deposits for 100 days [35,36]. Kawi et al. synthesized a Ni-yolk@Ni@SiO₂ nanocomposite with a yolk-satellite shell structure to efficiently inhibit the sintering of Ni, which resulted in negligible carbon deposition, and the CH₄ conversion was 10% after the first 2 hours of reaction under the conditions of 800 °C, a gas hourly space velocity (GHSV) of 1440 L·g⁻¹cat·h⁻¹, a Wcat of 0.01 g, and a CO₂:CH₄:N₂ ratio of 1:1:1 [37]. Similarly, Wang et al. pointed out that the Ni nanoparticle cores encapsulated by the mesoporous Al₂O₃ shells show superior coke resistance because of the confinement effects which prevent the Ni nanoparticles from agglomeration at high temperatures, and the CH₄ and CO₂ conversions under the reaction conditions of 800 °C, CO₂/CH₄ of 1/1, and a weight hourly space velocity (WHSV) of 36 L·h⁻¹·gcat⁻¹ were about 88% and 92%, respectively [38].

Herein, different from the above-mentioned encapsulated Ni catalysts with relatively complicated preparation procedures, we propose a facile one-step strategy to prepare the SiO₂ supported Ni catalysts toward the controlled formation of nanoparticle size and Ni-support interaction, which could lead to high activity and stability. Following the conventional impregnation method, glycine (C₂H₅NO₂) and ammonium nitrate (NH₄NO₃) were introduced into the impregnated solution of nickel precursor (Ni(NO₃)₂·6H₂O), as shown in Scheme 1. It was expected that the mixed materials with C₂H₅NO₂ as fuel and NH₄NO₃ as combustion improver reacted exothermically after ignition which finished within a short time-frame with a very high temperature and release of a large quantity of gases, such as CO₂, water, and N₂. We thought this process might facilitate the formation of smaller crystalline materials and regulate the metal-support interaction, resulting in improved catalytic performance in the DRM reaction. To demonstrate the effects of the above combustion process on the catalytic performance, several characterizations, such as Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and thermogravimetric (TG), were employed to characterize the catalyst.

2. Results and Discussion

2.1. Characterization of the Catalyst Sample

As shown in Figure S1, all the fresh Ni/SiO₂ catalysts exhibit apparent diffraction peaks at 2θ values of 37.3°, 43.2°, 63.0°, 75.4°, and 79.4° assigned to the NiO (JCPDS 22-1189). For the reduced catalysts (Figure 1a), Ni/SiO₂-0/0 prepared by the conventional wetness impregnation method displayed the most intensive diffraction peaks at 2θ values of 44.5°, 52.2°, and 77.0°, which are the characteristic peaks of metallic Ni (JCPDS 1-1206). According to Figure S1, the peak at 37.3° should be assigned to NiO. As the NH₄NO₃ was introduced into the impregnated solution with nickel nitrate, the resulting catalyst (Ni/SiO₂-0/1) exhibited almost the same diffraction peak intensity at 44.5°. However, for the case of C₂H₅NO₂, Ni/SiO₂-2/0 displays a much weaker diffraction peak. Interestingly, the addition of both C₂H₅NO₂ and NH₄NO₃ results in almost no detectable diffraction peaks for Ni nanoparticles (Ni/SiO₂-2/1), suggesting that smaller Ni nanoparticles can be obtained by synergistic effects of fuel and combustion improver in the combustion process, as presented in Scheme 1.

TEM images of the reduced catalysts are depicted in Figure 1b,c. The Ni/SiO₂-2/1 displays an average Ni nanoparticle size of only 6.1 ± 2.7 nm which is significantly smaller than that for Ni/SiO₂-0/0 (31.3 ± 13.5 nm). The significant difference in the Ni nanoparticle size further confirms the synergistic effects of C₂H₅NO₂ and NH₄NO₃ in reducing the Ni nanoparticle size. The combustion process between N₂O and NH₃ is highly exothermic. The decomposition of nickel nitrate produces N₂O gas at 250 °C, while the decomposition of C₂H₅NO₂ gives NH₃ along with CO₂ and H₂O. The combustion process is triggered by the reaction between N₂O and NH₃ to form N₂ and H₂O [39]. When NH₄NO₃ is further added, NH₃ and N₂O can be formed via its decomposition at a low temperature of about 200 °C, thereby promoting combustion. The high-temperature stage in a short-duration favors the formation of ultra-small nanoparticles in a short time which may be in the order of seconds [40].

Figure 1d exhibits the reduction behavior of fresh Ni/SiO₂ catalysts with different molar ratios of C₂H₅NO₂ to NH₄NO₃. As expected, NH₄NO₃ does not obviously change the H₂-TPR profile compared to the case of Ni/SiO₂-0/0, as both catalysts show a strong reduction peak at 300–450 °C with a small right shoulder peak at 450–510 °C. However, C₂H₅NO₂ only (Ni/SiO₂-2/0) notably weakens the peak at lower temperatures, accompanied by a shift in the right shoulder peak to the higher reduction temperature with enhanced intensity. For Ni/SiO₂-2/1, the high temperature reduction peak is further intensified and shifts to a higher reduction temperature range. This result suggests that the smaller Ni nanoparticle size results in a more difficult reduction owing to a stronger metal-support interaction [41]. The reduction profiles correspond to the XRD and TEM results.

2.2. Activity Evaluation

Figure 2 shows the catalytic performance in the CH₄ dry reforming reaction with CO₂ over the as-prepared catalysts. In the case of catalytic activity, the CH₄ conversion over Ni/SiO₂-0/0 exhibits a rapid drop from 78.3% to 53.0% in the early ten hours and then gradually becomes stable. In contrast, Ni/SiO₂-0/1 gives a milder and continuous decrease in CH₄ conversion until the end of the reaction. Surprisingly, Ni/SiO₂-2/0 exhibits a stable and higher CH₄ conversion over the whole reaction period of 50 hours. Furthermore, Ni/SiO₂-2/1 displays a more stable and even higher CH₄ conversion. The CO₂ and CH₄ conversions are similar for all Ni/SiO₂ catalysts. However, in the corresponding reaction period, the CO₂ conversion is always slightly higher compared to the CH₄ conversion. When the DRM reaction over Ni/SiO₂-2/1 is stable, the conversion rates of CH₄ and CO₂ are 83.6% and 90.6%, respectively, which are slightly lower than their equilibrium conversion rates at 91% and 95% calculated by HSC chemistry 6.0 (Table S1). Also, in the case of the H₂/CO molar ratio, it follows the same trend as that for CH₄ conversion over all the Ni/SiO₂ catalysts. Specifically, for the Ni/SiO₂-2/1 catalyst, the desired H₂/CO molar ratio at the value of 1/1 is obtained, which results from the efficiently suppressed reverse water gas shift (RWGS) reaction. How the Ni/SiO₂ morphology affects the catalytic performance is discussed briefly in the following part.

The DRM reaction is extremely endothermic. Equation (1) shows that the DRM process can produce a syngas with an H₂/CO ratio of 1:1. During the DRM process, several reactions simultaneously occur, like CH₄ dissociation (Equation (2)), reduction of CO₂ to CO (Equation (3)), and the RWGS reaction (Equation (4)).

CH₄ + CO₂ = 2CO + 2H₂ (ΔH_298K = +247 kJ mol⁻¹)

(1)

CH₄ = C(s) + 2H₂ (ΔH_298K = +75 kJ mol⁻¹)

(2)

C(s) + CO₂ = 2CO (ΔH_298K = +171 kJ mol⁻¹)

(3)

CO₂ + H₂ = CO + H₂O (ΔH_298K = +41.2 kJ mol⁻¹)

(4)

The driving force for Equations (2)–(4) strongly depends on the temperature, reactant partial pressure and catalyst structures. In the investigated Ni/SiO₂ catalysts, both activation of CH₄ and CO₂ can occur on the active Ni surface since SiO₂ support is inert material. It is believed that CH₄ activation tends to form an intermediate, like CH_x or a formyl group, but dissociates directly to C species and H₂ at high temperature. Essentially, the DRM reaction of Ni catalysts might follow a dynamic redox type mechanism as the CO₂ oxidizes Ni⁰ to Ni^+δ to give CO, and the oxidative state Ni^+δ is reduced to Ni⁰ by C species as a result of CH₄ dissociation. As seen from the above reaction cycle, it is clear that the presence of O from CO₂ helps the dissociation of CH₄. To avoid the catalyst deactivation resulting from carbon accumulation, the C species from CH₄ dissociation must react timely with CO₂ to give CO. The reaction rate of this step is closely related to the Ni nanoparticle size, as the larger Ni surface favors the formation of multicarbon C_n species, which are potential precursors of carbon deposits such as coke. The smaller Ni nanoparticles allow a smaller amount of carbon species on the Ni nanoparticle surface. Thus, it is easier to keep the monoatomic C species isolated, and in time, they are oxidized by CO₂ to CO. By minimizing the rate of C species combination, the carbon accumulation could be effectively suppressed. Indeed, as shown in Figure 3, Ni/SiO₂-0/0, with an average nanoparticle size of 31.3 ± 13.5 nm, gives the highest amount of carbon deposits with 2.7 mg carbon deposits gCH₄⁻¹ as the BET surface area is decreased to the largest extent (Table S2). In contrast, the Ni/SiO₂-2/1 with a smaller nanoparticle size of 6.1 ± 2.7 nm is significantly coke-resistant, as the amount of carbon deposits decreases to 0.9 mg carbon deposits gCH₄⁻¹. The above experimental results reflect that the smaller Ni nanoparticle size is favorable to lower carbon deposits and thereby improve the catalyst stability, as shown in Figure S2. It should be noted that, in spite of the significant decrease in carbon deposits over Ni/SiO₂-2/1 catalyst, a considerable amount of coke is still formed during the DRM reaction of 50 hours. It can be deduced that most of the carbon deposits might not locate on the Ni nanoparticle surface but are located on the SiO₂ support since the catalytic activity is quite stable. It is reasonable for us to imagine that the Ni nanoparticles are lying on the SiO₂ support and not confined by porous layer material, which provides a chance for the carbon species to grow continuously along the SiO₂ support surface initiated by the Ni nanoparticle and finally form strips of nanofiber.

As seen from Figure 2, the H₂/CO molar ratio is highly dependent on the CO₂ conversion. A lower CO₂ conversion can cause a decrease in the molar ratio of H₂/CO to a large extent as a result of the RWGS reaction, as the higher concentration of CO₂ drives the reaction to the right side (Equation 4). At 800 °C, the standard free energy for the RWGS reaction (ΔG⁰ = –8545 + 7.84T) and the reduction of CO₂ to CO (ΔG⁰ = 39810 − 40.87T) [13] is −132.68 kJ mol⁻¹ and −4043.51 kJ mol⁻¹, respectively. It can be speculated that the reduction of CO₂ to CO, C(s) + CO₂ = 2CO, occurs more easily as a result of the lower ΔG. Comparing the value of ΔG in the RWGS reaction, the CO₂ that oxidizes the C species to CO is more thermodynamically favored than its RWGS reaction. As the lower CO₂ conversion corresponds to lower CH₄ conversion, the C(s) species dissociated from CH₄ is not sufficient for its reaction with CO₂. Therefore, the CO₂ reacting with H₂ toward the RWGS reaction is promoted. In order to minimize the side reaction toward the RWGS, it is necessary to operate the DRM reaction with a high CO₂ conversion rate.

3. Materials and Methods

3.1. Catalyst Preparation

The supported Ni catalysts were prepared with SiO₂ support (Tosoh Kabushiki-gaisha, Tokyo, Japan) by the combustion method, and the combustible materials contained hydrate glycine (C₂H₅NO₂), ammonium nitrate (NH₄NO₃) and nickel nitrate (Ni(NO₃)₂·6H₂O) with different C₂H₅NO₂/Ni(NO₃)₂·6H₂O and NH₄NO₃/Ni(NO₃)₂·6H₂O molar ratios. Briefly, the aqueous solution of the desired amounts of C₂H₅NO₂, NH₄NO₃ and Ni(NO₃)₂·6H₂O was added into the SiO₂ support at room temperature by incipient wetness impregnation, followed by drying with a rotary evaporator for 2 hours at 80 °C, and then overnight at 120 °C. Afterwards, the dried solid materials were calcined in air for 1 hour at 300 °C with a heating rate of 1 °C/min and another 3 hours at 550 °C with a heating rate of 2 °C/min. The calcined samples were denoted as Ni/SiO₂-x/y, where x and y indicate the molar ratio of C₂H₅NO₂/Ni(NO₃)₂·6H₂O and NH₄NO₃/Ni(NO₃)₂·6H₂O, respectively. The metallic Ni loading was 10 wt%. The samples were then crushed and sieved into a 40–60 mesh size for subsequent catalytic tests.

3.2. Catalyst Characterization

Fresh, reduced and spent samples were characterized by several techniques to identify and infer the effects of combustible materials such as C₂H₅NO₂ and NH₄NO₃ on the catalyst morphology and the resulting catalytic performance. N₂ adsorption-desorption isotherms for each sample were collected on a Micromeritics ASAP 2020 system (Micromeritics, Norcross, GA, USA). The surface area, pore size and pore volume were calculated with the N₂ adsorption-desorption isotherms via the conventional Barrett–Joyner–Halenda (BJH) and Brunauer–Emmett–Teller (BET) methods. Prior to the measurements, the samples were outgassed under vacuum for 5 hours at 200 °C. The X-ray diffraction (XRD) patterns of each reduced sample were obtained with a Bruker AXS D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation (λ=1.5406 Å) at a scanning rate of 6°/min with the 2θ range of 10–90°. The reducibility of the catalyst was studied by the H₂ temperature-programmed reduction (H₂-TPR) in an auto-controlled flow reactor system of TP-5076, which is equipped with a thermal conductivity detector (TCD, Tianjin Xianquan Co., China). The sample of 50 mg was pretreated in N₂ stream at 200 °C for 1 hour. Additionally, when the temperature cooled down to 30 °C, the sample was heated to 950 °C at a heating rate of 10 °C/min in the H₂/N₂ flow (5 vol.% H₂ in N₂) of 30 mL/min. The H₂-TPR spectra were obtained at the temperature range of 50–950 °C. The carbon accumulation in spent samples after reaction for 50 hours was determined by thermogravimetric (TG) analysis on a Mettler–Toledo TGA-1100SF thermogravimetric analyzer (Mettler-Toledo, Greifensee, Switzerland).

3.3. Catalytic Test

The dry reforming of CH₄ with CO₂ was performed at atmospheric pressure in a continuous-flow fixed bed quartz tube reactor with an inner diameter of 9 mm. For the typical experiment, 200 mg of shaped catalyst was filled into the center of the reactor. Before starting the reforming reaction, the catalyst was pre-reduced to 750 °C and atmospheric pressure for 2 hours in an H₂ flow of 60 mL/min with a heating rate of 10 °C/min. After that, the reactor temperature was elevated to 800 °C, and then a flow of gas mixture with a molar ratio of CH₄/CO₂/N₂ = 9/9/2 was fed with a flow rate of 160 mL/min. The products were analyzed by online gas chromatography (Agilent GC 7820A, Agilent, USA). CH₄, CO₂, H₂, N₂ and CO were measured by a TCD detector with a 5A molecular sieve column and a Porapak Q column. Additionally, 10% of N₂ was employed as an internal standard. The conversions of CH₄ and CO₂ were calculated with the following formulas:

X_CH4 = (F_CH4-in − F_CH4-out)/F_CH4-in × 100%

(5)

X_CO2 = (F_CO2-in − F_CO2-out)/F_CO2-in × 100%

(6)

where X and F indicate the conversion and flow rate of i gas in the feed or the effluent, respectively.

4. Conclusions

In summary, the combustion method was applied to prepare SiO₂ supported Ni catalysts which showed remarkably smaller Ni nanoparticle sizes due to the synergistic effects of C₂H₅NO₂ and NH₄NO₃ in the combustion process. This kind of Ni/SiO₂ catalyst exhibits excellent coke-resistance performance and effectively suppresses the side reaction toward RWGS compared to that prepared with the conventional wetness impregnation method. As a result, there is almost no loss of activity with the H₂/CO molar ratio close to the theoretical value at 1/1 after a 50-hour stability test over the Ni/SiO₂-2/1 catalyst.