Aqueous Miscible Organic LDH Derived Ni-Based Catalysts for Efficient CO₂ Methanation

Abstract

Converting CO₂ to methane via catalytic routes is an effective way to control the CO₂ content released in the atmosphere while producing value-added fuels and chemicals. In this study, the CO₂ methanation performance of highly dispersed Ni-based catalysts derived from aqueous miscible organic layered double hydroxides (AMO-LDHs) was investigated. The activity of the catalyst was found to be largely influenced by the chemical composition of Ni metal precursor and loading. A Ni-based catalyst derived from AMO-Ni₃Al₁-CO₃ LDH exhibited a maximum CO₂ conversion of 87.9% and 100% CH₄ selectivity ascribed to both the lamellar catalyst structure and the high Ni metal dispersion achieved. Moreover, due to the strong Ni metal–support interactions and abundant oxygen vacancy concentration developed, this catalyst also showed excellent resistance to carbon deposition and metal sintering. In particular, high stability was observed after 19 h in CO₂/H₂ reaction at 360 °C.

1. Introduction

The concentration of CO₂ in the atmosphere has risen from 270 to 385 ppm in the past 200 years which has been causing serious problems such as the greenhouse effect, global climate change, and glaciers melting [1,2,3]. Regarding these environmental issues, three ways to reduce CO₂ content in the atmosphere have been suggested [4,5,6,7], namely CO₂ emission reduction, CO₂ capture and storage (CCS), and CO₂ conversion and utilization.

CO₂ is chemically stable and it needs to be activated in the presence of hydrogen in order to convert it to CH₄. Although CO₂ activation is an endothermic process, thus requiring energy input, it can usually be integrated with fuel production from renewable energy, making CO₂ conversion in the Power-to-Gas (PtG) process a very promising green technology [4,8,9]. Due to a series of technological issues of storage and transportation, renewable energy can hardly be a primary source of energy for the society [10].

CO₂ methanation appears to be one of the largely investigated scientific topics [11,12]. Herein, CO₂ is hydrogenated to methane at relatively high temperatures, ca. 300–400 °C, known as the Sabatier reaction, which is a very promising approach for the storage of renewable energy [6]. Based on the PtG concept, electricity generated by renewable energy is then used to produce hydrogen by electrolysis. The thus formed hydrogen is then used to produce selectively CH₄ by the reaction with CO₂ [13,14,15].

CO_2(g)+ 4 H_2(g) → CH_4(g) + 2 H₂O_(g) (ΔH° = −165 kJ/mol)

(1)

Presently, CO₂ activation catalysts are under extensive investigation. Commonly used catalysts are group VIII noble metal-based catalysts, such as Rh, Ru, etc [16,17]. Moreover, non-precious metal-based in the group VIII, like Ni, Co, and Fe can also be used [18,19]. Noble metal-based catalysts show the best methanation catalytic performance, but their high cost limits large-scale utilization [20,21]. Among all the non-precious metals, although Fe-based catalysts are preferable because of their very low cost, they suffer from problems of low catalytic activity, coking, and deactivation after long time-on-stream [22]. On the other hand, Ni-based catalysts are currently used in Sabatier reaction due to their high activity and low cost [23,24,25]. For Ni-based catalysts, γ-Al₂O₃ has been widely used as support for its large specific surface area (SSA), high-temperature thermal resistance, and good chemical stability [26,27]. Benefiting from these features, Ni/γ-Al₂O₃ composite is the most broadly used catalytic system for CO₂ methanation.

LDHs is a family of ionic lamellar solid compounds consisting of positively charged brucite-like layers and exchangeable interlayer anions [28,29]. Group VIII metal ions can be inserted into the LDH precursor and then be reduced to form catalytically active metal nanoparticles [30,31,32]. Thus, the LDHs derived materials can be tuned for the carbon dioxide methanation to fulfill the following requirements: high basicity that is crucial for CO₂ activation (adsorption strength and capacity), and appropriate nickel dispersion and reducibility, necessary for this redox process [33,34,35]. Liu et al. [36] demonstrated that the interaction between metal nanoparticles and support can be strengthened due to chemical bonding, thus improving the anti-coking and anti-sintering properties of the supported metal catalysts. In addition, a novel method called aqueous miscible organic (AMO) treatment was described in our previous work to increase the SSA and total pore volume of the LDHs materials [30]. In this method, the LDHs were synthesized by the conventional co-precipitation strategy, but the final wet slurry was washed with an AMO solvent. If organic solvents are completely miscible with water, they will then replace surface bound water, which can often lead to dispersion into thin nanosheets or exfoliation to even single layers [37,38]. AMO treatment gives a much-diminished driving force for aggregation to dense agglomerates of LDHs, while improving the SSA and surface basic site density of catalysts, and ultimately enhancing the catalytic activity.

Herein, we report the use of AMO-LDH as precursors for the preparation of solids suitable for the CO₂ methanation reaction. Ni-based CO₂ methanation catalysts derived from NiAl LDHs precursors have been prepared by co-precipitation and further treatment with the AMO method [30,39,40]. Precursors and catalysts were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), and surface texture. The impact of Ni loading (wt%) and the factors influencing the CO₂ conversion and CH₄ selectivity over these AMO-NiAl layered double oxides (LDOs) catalysts, as well as their stability during a long-term CO₂ methanation reaction were examined.

2. Results and Discussion

2.1. Influence of Interlayer Anions of AMO-Ni₃Al₁ LDHs on CO₂ Methanation

Figure 1 shows the powder XRD patterns of LDH precursors with different interlayer anions (NO₃⁻ and CO₃²⁻) before and after reduction. LDH precursors present typical characteristic peaks at 11.7°, 22.8° and 35.1°, corresponding to the reflections of (003), (006) and (012) facets of the LDH crystal phase, respectively, indicating the successful synthesis of LDHs [41]. The peak intensity of Ni₃Al₁-CO₃ LDH is higher than that of Ni₃Al₁-NO₃ LDH, suggesting that CO₃²⁻ intercalated LDH is better crystallized than the NO₃⁻ intercalated one. The XRD patterns of reduced samples show two intensive peaks at 2θ = 44.4 and 51.8°, corresponding to the reflections of (111) and (200) facets of metallic Ni, and no diffraction peaks due to the NiO phase could be observed [42]. This result shows that NiO is rather completely reduced into metallic Ni. However, if strong Ni support interactions prevail, NiO might not be completely reduced into metallic Ni since small NiO particles (<4 nm) cannot be detected by XRD. Moreover, the γ-Al₂O₃ phase was not detected, result that might be related to its very low crystallinity. According to literature reports [30], the Al₂O₃ phase derived from LDH precursors generally exists in an amorphous phase. In order to better understand the dispersion of Ni on the catalyst surface, the TEM image and EDX element mapping images of the reduced AMO-Ni₃Al-CO₃ LDH solid are shown in Figure 2 and Figure 3, respectively. Black dots in the TEM image represent the Ni particles, where a mean diameter of ~10 nm was estimated. From Figure 3b, it can be observed that Ni is evenly dispersed in the sample. Overall, it can be concluded that highly dispersed Ni-based catalysts were successfully synthesized from Ni-Al LDH precursors.

The CO₂ conversion and CH₄ selectivity over the AMO-Ni₃Al₁-CO₃-LDO and AMO-Ni₃Al₁-NO₃-LDO solids were compared and results are presented in Figure 4. These two catalysts show extremely high CH₄ selectivity, ca. 100% at T < 400 °C, although the conversion level is not remarkable in the low-temperature window, i.e., temperatures lower than 260 °C. Above 450 °C, the CH₄ selectivity starts to decrease, due to the competition with the reverse water-gas shift reaction (RWGS) which converts CO₂ to CO through hydrogenation (CO₂ + H₂ → CO + H₂O) [43]. For both catalysts, the CO₂ conversion reaches maximum value at 360 °C, namely 87.9 and 84.2% over AMO-Ni₃Al₁-CO₃-LDO and AMO-Ni₃Al₁-NO₃-LDO, respectively. According to our previous study [44], interlayer anions influence the morphology and the interlayer distance of LDH, and the LDH with CO₃²⁻ interlayer has a larger SSA than the LDH with NO₃⁻ interlayer. The values of SSA of the pre-reduced catalysts, namely 158 m²/g (AMO-Ni₃Al₁-CO₃-LDO) and 134 m²/g (AMO-Ni₃Al₁-NO₃-LDO) are in good agreement with the previous study. Owing to the larger SSA, Ni produced after reduction will be likely better exposed, which might be one of the intrinsic reasons for the better activity performance of AMO-Ni₃Al₁-CO₃-LDO catalyst.

2.2. The Influence of Catalysts Preparation Procedure on the CO₂ Methanation

The N₂ adsorption-desorption isotherms were measured for the as-synthesized carbonate intercalated LDHs derived catalysts, AMO-Ni₃Al₁-CO₃-LDO and C-Ni₃Al₁-CO₃-LDO, by coprecipitation and AMO treatment. These pre-reduced catalysts show different SSA, namely 117 m²/g (C-Ni₃Al₁-CO₃-LDO) and 158 m²/g (AMO-Ni₃Al₁-CO₃-LDO). AMO-Ni₃Al₁-CO₃-LDO, indicating that AMO treatment has a positive effect on increasing the SSA of LDH-derived solids. As previously reported [30,37], AMO treatment greatly reduces the driving force for aggregation to dense agglomerates, resulting in the increase of SSA.

The CO₂ methanation catalytic performance (CO₂-conversion and CH₄-selectivity) of AMO-Ni₃Al₁-CO₃-LDO and C-Ni₃Al₁-CO₃-LDO solids is illustrated in Figure 5. There is a significant difference in the CO₂-conversion (%) over the two catalysts between 250 and 350 °C. The optimal CO₂ conversion of AMO-Ni₃Al₁-CO₃-LDO is 88%, while that of C-Ni₃Al₁-CO₃-LDO is 82.5%. As discussed above, the AMO treatment is beneficial to increase the SSA of the solid, and this could lead to better Ni dispersion and thus to an increased CO₂ methanation activity per gram of solid basis.

2.3. Influence of Ni/Al Ratio on Catalyst CO₂ Methanation Activity

A series of NiAl-LDHs with different Ni/Al atom ratios were synthesized and their XRD patterns are shown in Figure 6. The X-ray diffraction peaks at 11.76, 22.84 and 35.2° prove that LDH precursors were successfully synthesized, and there are no obvious differences in the crystal structure of LDHs with different Ni/Al ratios. The precursors were then calcined and reduced (see Section 2.1) before CO₂ methanation activity tests. The CO₂ conversion (%) and CH₄ selectivity (%) vs temperature profiles obtained over these NiAl-LDHs catalysts are presented in Figure 7. AMO-Ni₂Al₁-CO₃-LDO presented the lowest CO₂ conversion among all the samples, especially at low temperatures. Upon increasing of the Ni/Al ratio to 3:1, the CO₂ conversion increase. As the Ni/Al ratio continues to increase to 4:1, the CO₂ conversion levels off, and becomes almost constant. This is likely because overloaded Ni can cause aggregation of Ni particles [45]. Hence, we identify a threshold for the positive impact of increasing Ni content on the catalytic CO₂ methanation activity being the Ni/Al ratio 4:1 the experimental limit to boost conversion. As for the selectivity to CH₄, by increasing the Ni/Al ratio no change is observed except a slight decrease at high temperatures (ca. 500 °C, Figure 7) due to the RWGS reaction.

2.4. Influence of Reduction Temperature in Hydrogen on Catalyst CO₂ Methanation Activity

Since the active component of the as-synthesized CO₂ methanation catalysts is the Ni metal, NiO (formed after calcination) should be reduced to metallic Ni (H₂ is used) before catalytic activity testing. The SEM images of AMO-Ni₃Al₁-CO₃ LDH, AMO-Ni₃Al₁-CO₃ LDO, and reduced AMO-Ni₃Al₁-CO₃ LDO solids are presented in Figure 8. Stacked and interlayered platelet-like crystals of LDH particles of a crystal particle size of ~60 nm are observed in Figure 8a, which is consistent with previous reports [46,47]. This typical platelet-like morphology remains after calcination. However, the morphology greatly changed after reduction in hydrogen.

As shown in Figure 8c, spherical particles with a diameter of 100 ~ 200 nm are formed. The hydrogen treatment conditions had a great influence on the particle dispersion, morphology and structure, and these features would eventually affect the activity performance of the catalyst [30,48]. The powder XRD patterns of samples reduced at 500 and 600 °C are presented in Figure 9. After hydrogen treatment at 600 °C for 1 h, the sample shows relatively strong diffraction peaks at 44.48 and 51.67° corresponding to the Ni phase. The main diffraction peaks for the NiO phase (ca. 37.25 and 44.48°) were not detected, indicating that NiO has been fully reduced to Ni [49]. However, the sample exhibited relatively weak peaks of Ni phase and clear peaks of NiO phase after H₂ treatment at 500 °C.

The H₂-TPR profile of the AMO-Ni₃Al₁-CO₃-LDO solid (Figure 10) shows its reducibility characteristics. It can be inferred that complete reduction of NiO cannot be accomplished at 500 °C, and it is envisaged that the full reduction temperature is at least 550 °C [50]. This is consistent with the powder XRD results that the sample reduced at 500 °C is not as complete as that reduced at 600 °C. It can be concluded that there exist strong metal–support interactions between NiO and Al₂O₃ surface [36,51,52,53].

The effect of reduction temperature on the CO₂ conversion and CH₄ selectivity over the AMO-Ni₃Al₁-CO₃-LDO solid was investigated and results are presented in Figure 11. In all case, practically 100% CH₄-selectivity (no CO formation) was detected below 400 °C. The CO₂ conversion reaches its maximum value at 360 °C, namely 87.9%, 86.3%, 85.5%, and 77.5% for the catalysts reduced at 600, 550, 500, and 400 °C, respectively. A significant improvement of CO₂ conversion can be seen after increasing the hydrogen reduction temperature from 400 to 600 °C, especially for the CO₂ methanation reaction occurred at 260 °C. As discussed above, with the increase of reduction temperature, more NiO was converted into Ni, which leads to a high activity. Based on the results of powder XRD, H₂-TPR and catalytic tests, it can be stated that the degree of supported Ni reduction is influenced by the hydrogen reduction temperature and it has an important impact on the CO₂ conversion. It appears that 600 °C is the best reduction temperature for the present LDHs derived Ni-based CO₂ methanation catalysts. However, according to the H₂-TPR profile, the catalyst should be reduced completely at 550 °C. Since the H₂-TPR experiment is conducted dynamically, the reduction peak maximum may not appear at the exact temperature recorded. To ensure the thorough conversion of NiO into Ni, the hydrogen reduction temperature of the sample was set at 600 °C.

2.5. Stability Performance of AMO-Ni₃Al₁-CO₃-LDO

A stability test was performed on AMO-Ni₃Al₁-CO₃-LDO at 360 °C for 19 h and results are shown in Figure 12. Before any measurements, the catalyst was reduced at 600 °C for 1 h. The results obtained indicate that AMO-Ni₃Al₁-CO₃-LDO derived catalyst showed enhanced stability for long time-on-stream (TOS) since no obvious deactivation was observed (Figure 12). The CO₂ conversion remained above 87% and no CO was detected in the effluent gas stream. After the stability test, the spent catalyst was characterized by SEM. Figure 13 indicates the surface morphology of the spent catalyst. Compared with Figure 8c, the SEM images of spent catalysts show no obvious change in the particle configuration, which means that there were no structural changes in the solid catalyst. It was reported [22] that deactivation of CO₂ methanation catalysts is mainly caused by possible structural changes, such as sintering of the metal, and the generation of carbon deposits. By using AMO-Ni₃Al₁-CO₃ LDH as precursor, the obtained supported Ni nanoparticles exhibit a high degree of dispersion and strong metal–support interactions. Furthermore, the LDO support may provide oxygen vacancies [54], which are beneficial to the improvement of anti-sintering and anti-coking properties by providing an alternative route for CO₂ dissociation into CO and O (lattice oxygen), the latter being able to oxidize carbon (formed on Ni) formed by the reverse Boudouard reaction [55], so as to ensure its high activity during long-term operation.

3. Experimental

3.1. Preparation of Catalysts

LDHs were first synthesized by the co-precipitation method. An aqueous solution containing nitrates of the metallic salts, ca. Ni(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were added dropwise into a vigorously stirred Na₂CO₃ (NaNO₃ for NiAl-NO₃ LDH) solution. The pH of the resulting solution was kept constant at 10 by the addition of NaOH solution (4 M). The resulting slurry was stirred continuously for ~12 h at 30 °C, then filtered and washed several times with deionized water until pH reached 7, followed by drying at 60 °C in an oven. For AMO treatment, after the above-mentioned washing step, the resulting slurry was re-dispersed in ethanol for 2 h. The final solids were collected by filtration and then dried at 60 °C for further characterization. To investigate the influence of Ni content, NiAl LDHs with different Ni/Al atom ratios (x = Ni/Al) were synthesized, which were denoted as Ni_xAl₁-LDH (x = 2, 3, and 4). The obtained LDHs were calcined at 400 °C for 5 h and reduced in 10 vol% H₂/Ar gas atmosphere (1 bar) before being used in any catalytic experiment.

3.2. Catalysts Characterization

Powder X-ray diffraction (PXRD) patterns of the solid AMO-NiAl LDOs samples were recorded using a Shimadzu XRD-7000 instrument in reflection mode and a Cu Kα radiation. The X-ray tube was operated at 40 kV and 40 mA while the accelerating voltage was set at 40 kV with 30 mA current (λ = 1.542 Å). Diffraction patterns were recorded within the range of 2θ = 5–75° with a scanning rate of 5°/min and a step size of 0.02°. Textural properties of the solids were analyzed using nitrogen adsorption-desorption isotherms obtained from a physisorption analyzer (SSA–7000, Builder, Beijing, China). SSA was estimated using the Brunauer−Emmett−Teller (BET) method. Before each measurement, ~0.1 g catalyst sample was degassed in a N₂/He gas mixture at 220 °C for 4 h.

Hydrogen temperature-programmed reduction (H₂-TPR) experiments were carried out in a multifunction chemisorption analyzer (PCA-1200, Builder, Beijing, China) equipped with a quartz U-tube reactor and a thermal conductivity detector (TCD) for gas analysis. For each test, a ~0.05 g sample was utilized. Before switching to the H₂/Ar gas stream, the sample was pretreated in Ar (40 mL/min) at 200 °C for 30 min and then cooled in Ar gas flow to 50 °C. The sample was then heated from 50 to 800 °C with a ramping rate of 10 °C/min in 5 vol% H₂/Ar gas mixture flow (30 mL/min). A field emission scanning electron microscope (SU-8010, Hitachi, Tokyo, Japan) was used to characterize the morphology of Ni33Al LDO (secondary agglomerated particles). A carbon tape was used to stick the powder to the SEM stage. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai G2 F30 (Hillsboro, USA) which was operated at 300 kV. Energy-dispersive X-ray (EDX) mapping characterizations of the composite oxides-based catalysts was made to investigate the distribution of the elements in the samples using the same TEM instrument.

3.3. Catalytic Activity Tests

The CO₂ methanation catalytic activity tests of the synthesized AMO-NiAl LDOs were carried out at 1 atm pressure in a fixed-bed stainless-steel micro-reactor. The stainless-steel micro-reactor was installed in a vertical split-tube furnace equipped with a proportional-integral-derivative (PID) temperature controller and several mass flow controllers. Before the catalytic performance test, a catalyst amount of W_cat = 0.1 g was first pretreated in 20 vol% H₂/N₂ gas mixture (50 mL/min) at 600 °C for 1 h, and then cooled to 200 °C in N₂ gas flow. Subsequently, a gas mixture of H₂, CO₂ and N₂ with a molar ratio of H₂/CO₂/Ar = 4:1:5 (ca. 40 vol% H₂/10 vol% CO₂/50 vol% Ar) was introduced into the reactor with a total flow rate of 80 mL/min corresponding to a space velocity of 48,000 mL g_cat⁻¹ h⁻¹. The methanation reaction was tested in the 200–500 °C range. After the reaction rate was stabilized at a given temperature after about 30 min, the composition of the effluent gas stream was analyzed online using a gas chromatography (SP-7890, Shandong Lunan, Zaozhuang, China) equipped with a TCD and FID detectors. The CO₂ conversion and CH₄ selectivity were estimated from the following Equations (2) and (3), respectively:

$$C{O}_2 conversion(\%)=[\frac{[C{O}_{2,in}]-[C{O}_{2,out}]}{[C{O}_{2,in}]}]\times 100\%$$

(2)

$$C{H}_4 selectivity(\%)=\frac{[C{H}_4]}{[C{H}_4]+\left[CO\right]}\times 100\%$$

(3)

In Equation (3), the [CO] concentration at the outlet of reactor refers to the non-selective CO₂ hydrogenation reaction towards CO and H₂O products (reverse water-gas shift reaction). Also, given the CO₂ feed composition used (10 vol%), the change in the outlet total molar flow rate of the methanation reaction was small with respect to the inlet total molar flow rate, and this was considered in the estimation of CH₄ selectivity (Equation (3)).

4. Conclusions

Selective CO₂ methanation reaction was conducted over NiAl LDH derived catalysts which were prepared by coprecipitation and AMO treatment. AMO treated LDH exhibits higher SSA, which benefits Ni dispersion, than the conventional one. Highly dispersed Ni particles on the catalyst surface led to higher catalytic activity for CO₂ methanation. The Ni loading was found to play a key role in determining the catalytic activity by influencing the concentration of active sites along with the SSA. However, excessive Ni loading may result in Ni particle agglomeration, thus reducing the catalytic activity. As shown by the performed experiments, an optimal Ni/Al atom ratio of 3:1 was found. The catalyst reduction temperature by hydrogen was found to also play a role in its activity, which should be higher than 550 °C to ensure complete reduction of NiO to Ni. After hydrogen reduction at 600 °C, AMO-Ni₃Al₁-CO₃ LDH derived catalyst showed the highest CO₂ conversion (87.9%) and 100% CH₄ selectivity at 360 °C, due to the enhanced dispersion of Ni nanoparticles and SSA. Moreover, this AMO-Ni₃Al₁-CO₃ LDH derived catalyst shows remarkable stability in a 19-h activity test (above 87%) as the result of maintaining a highly dispersed Ni state, strong metal–support interactions, and abundant oxygen vacancy concentration.