CO 2 Methanation Process over Highly Active and Nanostructured NiO-Al 2 O 3 Catalyst Synthesized by Various Methods

doi:10.21203/rs.3.rs-5035417/v1

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CO 2 Methanation Process over Highly Active and Nanostructured NiO-Al 2 O 3 Catalyst Synthesized by Various Methods

https://doi.org/10.21203/rs.3.rs-5035417/v1

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This study is centered on the synthesis of NiO-Al2O3 catalysts using multiple preparation methods, which encompass mechanochemical, impregnation, sol-gel, co-precipitation, and combustion techniques. These various methods were employed to create catalyst samples, subsequently utilized in the carbon dioxide methanation process. Comprehensive characterization of the prepared samples encompassed H2-TPR, XRD, BET, and FESEM analyses. The outcomes of the BET and XRD analyses unveiled that the 20wt.% NiO-Al2O3 catalyst, synthesized via the mechanochemical preparation approach, exhibited exceptional efficiency in relation to CO2 conversion and selectivity of methane. This was especially pronounced at lower temperatures. Notably, this catalyst showcased a specific surface area measuring 240.7 m2/g, coupled with a reduced crystal size of 29.4 nm. The 20wt. % NiO-Al2O3 catalyst demonstrated a carbon dioxide conversion of 68%, coupled with a methane selectivity of 96% under the operational condition of 400 ℃. Notably, this catalyst demonstrated the highest degree of stability when compared to the other catalysts studied. To comprehensively assess the impact of varying nickel loadings, spanning from 5 to 25 wt. %, on both textural attributes and the catalytic efficacy of mechanochemically synthesized NiO-Al2O3, an in-depth investigation was undertaken. The experimental findings from this investigation unveiled that the augmentation of nickel loading up to 20 wt% led to a discernible enhancement in CO2 conversion efficiency. However, beyond this threshold, a decline in CO2 conversion was detected. This can be linked to the phenomenon of particle sintering, which subsequently leads to a decrease in the dispersion of the active catalytic phase. Furthermore, the study delved into the exploration of processing conditions and the temperature of calcination, assessing their influence on the catalytic efficiency of the chosen catalyst.

methanation of CO2

Nickel-based catalyst

Synthesis method

Mechanochemical

Over the past few decades, greenhouse gas emissions increased with the industrialization of cities and the higher usage of fossil fuels, which leads to the higher production of carbon dioxide as a greenhouse gas. The increase in CO₂ emission has caused environmental degradation and global warming [1–3]. The conversion of carbon dioxide into valuable chemicals is widely regarded as the most effective approach for mitigating its concentration in the environment [4, 5].

Hydrogen is an energy carrier and can be produced in many ways. Catalytic hydrogenation of carbon dioxide, involving the reaction between carbon dioxide and hydrogen, presents a method of transforming a greenhouse gas into valuable substances such as hydrocarbons, alcohols, and aromatics. Additionally, it serves as a process for purifying hydrogen-rich streams by eliminating CO₂ impurities. Among these diverse components, methane (CH₄) emerges as a captivating fuel due to its dual role as both a hydrogen carrier and a potential fuel source [6, 7].

The process of hydrogenating carbon dioxide to produce methane has attained notable significance. This is not only because it aids in reducing CO₂ levels but also because it yields methane as a valuable end product [8–10]. The procedure of CO₂ methanation has garnered substantial attention within industries, primarily owing to its capacity to counter CO presence in hydrogen streams. Through the application of CO₂ methanation, the effective removal of CO becomes achievable, thereby preventing catalyst poisoning in critical applications like fuel cells and ammonia synthesis within the industrial context [11–13].

For the first time, Sabatier [14, 15] performed the hydrogenation reaction of conversion of carbon dioxide into methane (Eq. 1), also known as the carbon dioxide methanation reaction.

CO₂ + 4H₂ → CH₄ + 2H₂O ∆H = -165 kJ/mol, ∆G=-113 kJ/mol (1)

According to the thermodynamic aspects of this reaction, carbon dioxide methanation is spontaneous due to the negative amount of ΔG, but synthetically this reaction occurs slowly. The strong bond between carbon dioxide and oxygen and the high activation energy to break this bond are the significant reasons for the slow rate of carbon dioxide methanation. Hence, a highly active and stable catalyst with desirable features is required to perform this reaction with high efficiency [16–19].

Various metals such as nickel, iron, rhodium, ruthenium, copper, and palladium oxides supported on MgO, TiO₂, SiO₂, Al₂O₃, MgAl₂O₄, and CeO₂ with high specific surface area and porosity have been used in the carbon dioxide methanation [20–23]. Among them, nickel has more advantages such as lower price, high availability, acceptable activity, and strong metal interaction with some supports such as alumina. However, the main problem with this catalyst is its deactivation at high temperatures. For this reason, recent studies have concentrated on enhancing the stability of this catalyst by doping sufficient promoters or using various preparation methods. The catalyst synthesis method directly affects the catalyst structure and activity [24–26].

Sol-gel, impregnation, co-precipitation, and combustion methods are the general methods that have been used to synthesize various nanomaterials. In the past few years, the mechanochemical method has been the subject of extensive investigation as a new and efficient method for synthesizing various components [27–29].

Burger et al.[30] Employed the co-precipitation method to synthesize the Ni-Al catalysts with varying concentrations of Mn or Fe as the promoter and investigated the catalytic performance of the promoted catalysts in terms of methane efficiency and carbon dioxide conversion. Tada et al.[31] employed the wet impregnation method to synthesize nickel (Ni) catalysts supported on various materials, including CeO₂, MgO, TiO₂, and Al₂O₃. The aim was to investigate the influence of various supports on the catalytic efficiency of the carbon dioxide methanation process. Wu et al. [32] utilized the sol-gel procedure to synthesis the Ni-M/Al₂O₃-ZrO₂ (M = Co, Cr, Fe, or Mn) catalysts. The experimental findings indicated that the manganese-promoted catalyst demonstrated enhanced effectiveness concerning CO₂ conversion and selectivity of CH₄.

The authors revealed that the catalyst preparation methods could affect the stability and activity in carbon dioxide methanation [33–35]. Italiano et al. [36] synthesize nickel-based catalysts supported on various materials with the solution combustion method. Subsequently, the synthesized catalysts were utilized for the methanation of CO₂. The 15wt%Ni-Al₂O₃ showed 29% CO₂ conversion and 15wt.% Ni/Y₂O₃ showed 52% CO₂ conversion at 350°C with GHSV = 10000 h^− 1.

Lin et al.[37] Synthesized Ni/Al₂O₃ catalyst with 10 to 25 wt. % of nickel by partial hydrolysis. In this study, the carbon dioxide conversion and methane selectivity were evaluated under specific conditions, including a GHSV of 6000 h-1 and an H₂/CO₂ molar ratio of 4. Also, they reported that the best performance was obtained for the 25 wt. % of Ni/Al₂O₃ catalyst at 360°C with 77.2% CO₂ conversion and 99.9% methane selectivity.

The catalytic activity of the Ni/Al₂O₃ catalyst were compared with other Ni- based catalysts as reported in the literature and the results are summarized in Table 1.

Table 1

the catalytic activity comparison between the present study and other literatures.
Catalyst	Feed	GHSV	CO₂ Conversion	Ref
20% Ni/Al₂O₃	H₂/CO₂/N₂ = 60: 15: 20 ml/min	56.7 h^− 1	49%	[38]
15% Ni/Al₂O₃	H₂/CO₂/N₂ = 12: 3: 5 ml/min	60000 ml/h.g_cat	67%	[39]
15% Ni/Al₂O₃	H₂/CO₂ = 4	-	57%	[40]
15% Ni/Al₂O₃	H₂/CO₂/N₂ = 72: 18: 10 ml/min	150 ml/min	66%	[41]
10% Ni/Al₂O₃	H₂/CO₂ = 4	6000 ml/h.g_cat	68%	[42]
20% Ni/Al₂O₃	H₂/CO₂ = 4	20000 ml/h.g_cat	72%	[43]
20% Ni/Al₂O₃	H₂/CO₂ = 4	15000 ml/h.g_cat	67.75%	-

The main aim of this study was to delve into the catalytic activity exhibited by the Ni/Al₂O₃ catalyst across a range of nickel loadings during the carbon dioxide methanation process. In addition, a comprehensive evaluation was undertaken to scrutinize the impact of various preparation methods on the textural properties and catalytic efficiency of the selected catalyst.

2.1. Catalyst preparation

Five various preparation methods were used in this research.

Ammonium carbonate ((NH₄)₂CO₃) served as a deposition agent for Ni(NO₃)₂.6H₂O and Al(NO₃)₃.9H₂O in the solid-state mechanochemical preparation method. At first, a calculated amount of (NH₄)₂CO₃ was added to the equimolar quantities of Ni(NO₃)₂.6H₂O and Al(NO₃)₃.9H₂O. Subsequently, the components were thoroughly mixed and subjected to milling in an agate mortar for a duration of 20 minutes.

In the impregnation method, the mechanochemically synthesized Al₂O₃ was used as catalyst support. In this method, a desired concentration of nickel nitrate was added to the prepared support material in the form of an aqueous solution. The addition procedure occurred at ambient temperature, and the blend was stirred consistently for a duration of 4 hours.

In the method of combustion, the certain amounts of Al(NO₃)₃.9H₂O and Ni(NO₃)₂.6H₂O along with glycine (molar ratio of glycine/nitrate = 2) were solubilized in 50 ml of distilled water. The mixture was subsequently stirred at 150°C until complete evaporation of all water content. After that, the sample was subjected to a treatment at 600 ℃ for a duration of 20 minutes, employing a heating rate of 3°C/min.

In the co-precipitation method, predetermined amounts of Al(NO₃)₃.₉H₂O and Ni(NO₃)₂.6H₂O were solubilized in 50 ml of distilled water. Sodium hydroxide solution with a concentration of 1M was subsequently introduced into the prepared solution. The pH level of the solution was adjusted to 11. The prepared suspension was aged at 60°C for 4 hours, and then was filtered and washed several times with distilled water.

In the catalyst preparation using the sol-gel method, predetermined amounts of aluminum nitrate and citric acid (C₆H₈O₇/Al(NO₃)₃.9H₂O = 1) were solubilized in 30 ml of distilled water. Subsequently, the solution was heated and held at a consistent temperature of 60°C until a gelatinous substance was generated. In the next step, the mixture of nickel nitrate, citric acid, and water was added to the prepared gel and stirred to complete the gel formation.

In all the preparation methods mentioned, the resultant samples underwent a drying procedure at 100°C overnight. Following the drying phase, the samples were subsequently subjected to calcination, which involved heating them to a temperature of 500°C. The calcination process was conducted for a duration of 3 hours, employing a controlled heating rate of 3°C per minute.

2.2. Characterization

The N₂ physisorption method was employed to assess the BET surface area, pore volume, and pore size distributions for the catalysts, utilizing a BET Belsorp mini II instrument. To elucidate the crystal structures present within the catalysts, X-ray diffraction (XRD) analysis was employed. The XRD measurements were conducted using a PANalytical X'Pert Pro diffractometer, covering a 2θ range extending from 10 to 80 degrees.

To assess the reducibility properties inherent to the distinct Ni-Al₂O₃ catalysts, an analysis involving temperature-programmed reduction (H₂-TPR) was conducted. Additionally, the analysis of morphological characteristics displayed by both the initial and exhausted catalysts was conducted using Field Emission Scanning Electron Microscopy (FESEM). This investigation was facilitated through the utilization of a MIRA3 TESCAN instrument. The comprehensive specifics of the catalyst characterization analyses have been expounded upon in our earlier publication.

2.3. Catalytic test

A quartz fixed-bed micro-reactor was utilized for evaluating the performance of the catalysts in the carbon dioxide methanation process. This micro-reactor possessed an internal diameter of 7 mm, facilitating a controlled environment for the execution of catalytic reactions. The carbon dioxide methanation reaction was carried out under atmospheric pressure, spanning a temperature range from 200 to 500°C, with increments of 50°C in temperature intervals.

In preparation for the experimentation, a meticulously measured quantity of 200 mg of the catalyst, previously sieved to ensure a uniform particle size, was precisely loaded into the reactor's center. Subsequently, a reduction step was implemented to activate the catalyst. This reduction phase involved exposing the catalyst to a pure hydrogen flow at 500°C for a duration of 2 hours. Following the reduction process, the temperature within the reactor was gradually decreased to 200°C, in readiness for the ensuing reaction.

At this point, a reactant stream was introduced into the reactor, comprising a molar ratio of 4:1 hydrogen (H₂) to carbon dioxide (CO₂). The composition and flow rate of this reactant stream, encompassing both H₂ and CO₂, were meticulously regulated in accordance with the predetermined temperatures stipulated for the experiment. To eliminate moisture from the resulting products, a cold trap was positioned between the reactor outlet and the gas chromatograph.

An online gas chromatograph (Younglin M600D) was used to specify the feed flow and product concentrations. The CO₂ conversion and CH₄ selectivity were determined using the following mathematical expressions, Eqs. (2) and (3):

The concentrations of CO₂, CH₄, and CO in the product stream were utilized in this formula to calculate the conversion (XCO₂) and selectivity (SCH₄) of the carbon dioxide methanation process.

3.1. Influence of various preparation methods

Figure 1 depicts the X-ray diffraction (XRD) patterns of the 20NiO-Al₂O₃ samples synthesized employing diverse methods. The discerned diffraction peaks located at 2θ = 37.6°, 45.2°, 63.7°, and 66.3° were linked to the existence of nickel aluminate (JCPDS 10–0339)[44]. The X-ray diffraction (XRD) patterns demonstrated distinct peaks at 2θ = 43.4° and 76.4°, which can be ascribed to the existence of the NiO phase (JCPD 04-0850)[45]. At 2θ = 37.6°, 63.7°, and 66.3°, the diffraction peaks of the NiAl₂O₄ have overlapped with Al₂O₃ diffraction peaks (code N. 73-1519) [45].

The outcomes suggested that the utilization of the sol-gel metod resulted in the formation of the 20NiO-Al₂O₃ catalyst exhibiting the most significant crystallite size when compared to the other catalysts synthesized. Regarding the sample synthesis employing the sol-gel technique, the X-ray diffraction analysis unveiled distinct peaks aligning with the NiO phase at 43.4° and 76.4°. However, notably absent were any discernible diffraction peaks corresponding to nickel aluminate, as observed at 45.2° and 63.7°. Due to the low intensity of the peaks in the sample synthesized by mechanochemical and impregnation methods, the NiO dispersion on the catalyst surface was found to be higher in these samples compared to the others.

Figure 2a depicts the adsorption/desorption isotherms, while Fig. 2b exhibits the pore size distributions of the 20NiO-Al₂O₃ catalysts synthesized employing varied preparation methods. The isotherm shapes observed for the 20NiO-Al₂O₃ catalysts, excluding the specimen synthesized through the combustion technique, can be categorized as type IV isotherms with an H2-type hysteresis loop. This indicates the presence of a mesoporous structure with cylindrical-shaped pores in these materials.

The results unveiled that the sample synthesized through the combustion method displayed an isotherm of type II, accompanied by an H3-type hysteresis loop. This indicates the existence of a mesoporous and macroporous structure characterized by non-uniform sizes or shapes within the catalyst. The pore size distributions of all catalysts, except for the combustion catalyst, were observed to fall within the 2–20 nm range, as illustrated in Fig. 2b. The 20NiO-Al₂O₃ catalyst prepared by the combustion method has a much larger pore size than the sample synthesized with the mechanochemical method [46–48].

Table 2 provides the textural characteristics of the NiO-Al₂O₃ catalysts synthesized utilizing various methods. The results indicated that the sample synthesized via the co-precipitation technique displayed the highest specific surface area (241.5 m².g^− 1), whereas the sample prepared via the combustion method demonstrated the lowest specific surface area (49.4 m².g^− 1). Nevertheless, comparable trends were noted in relation to the pore volume of the samples prepared through different methods.

Table 2

Textural properties of the 20NiO-Al₂O₃ catalysts prepared by different preparation methods
Catalyst	Specific surface area (m²/g)	Pore volume (cm³/g)	Pore size (nm)	Average crystallite size (nm)
Mechanochemical	240.7	0.32	5.28	29.4
Sol-gel	138.5	0.17	4.95	53.8
Combustion	49.4	0.10	12.37	38.2
Impregnation	205.7	0.28	2.97	33.5
Co-precipitation	241.5	0.32	2.53	36.8

The Scherrer equation was utilized for the computation of the crystallite size of nickel oxide. The calculations showed that the sample synthesized through the sol-gel method had the largest crystallite size (53.8 nm) among all the investigated catalysts. This result is consistent with the XRD findings previously discussed. Based on the obtained results, it was observed that the sample synthesized using the mechanochemical method exhibited the smallest crystallite size. This implies that the mechanochemical method resulted in better dispersion of NiO on the catalyst surface compared to the other preparation methods.

The reducibility characteristics of the calcined 20NiO-Al₂O₃ catalysts, prepared using diverse methods, were assessed using temperature-programmed reduction (TPR) analysis. The TPR curves acquired for the catalysts are illustrated in Fig. 3. The obtained results demonstrated that the various preparation methods have various reduction behaviors. The TPR analysis disclosed that the 20NiO-Al₂O₃ catalysts displayed a reduction peak occurring in the temperature range of 600–1000°C. This peak corresponds to the reduction of NiO, signifying a notable interaction between NiO and the alumina support. Additionally, the reduction of the formed NiAl₂O₄ phase in the catalyst was also observed during the TPR analysis.

The results showed that the samples synthesized with combustion and co-precipitation methods had less reduction intensity among the prepared samples. Furthermore, it was noted that the specimen synthesized through the sol-gel method displayed the most pronounced reduction peak. The highest temperature of this reduction peak was also elevated in comparison to the other samples. This behavior can be ascribed to the higher content of NiAl₂O₄ species, which requires higher temperatures for reduction. Additionally, it was noticed that the reduction peak of the sample synthesized via the impregnation method shifted towards lower temperatures. This shift suggests a weaker interaction between the metal and the support.

The catalytic performance of the 20NiO-Al₂O₃ catalysts synthesized through different methods was evaluated in the carbon dioxide methanation process. Figure 4a and 4b illustrate the conversion of CO₂ and selectivity of CH₄ for the diverse catalysts under investigation. The results indicated that the sol-gel and mechanochemical synthesis methods resulted in the 20NiO-Al₂O₃ catalysts exhibiting higher carbon dioxide conversion rates at lower temperatures.The higher CO₂ conversion can be linked to improved reducibility and higher concentration of active species exhibited by the catalysts synthesized using these methods. Also, the catalyst synthesized by the impregnation method showed the lowest CO₂ conversion during the reaction. The results of methane selectivity also showed the high methane selectivity for the catalysts synthesized by combustion, mechanochemical, and sol-gel methods.

At a temperature of 400°C, the 20NiO-Al₂O₃ catalyst prepared through the mechanochemical method, possessing a specific surface area of 240.7 m2/g, exhibited carbon dioxide conversion rate of 68.17% with a methane selectivity of 96.06%. This was while the highest carbon dioxide conversion (79.19%) at 450°C belonged to the 20NiO-Al₂O₃ catalyst synthesized using the co-precipitation method.

Overall, the results highlight that the catalyst synthesized using the mechanochemical method showcased the most elevated CO₂ conversion rates and CH₄ selectivity, particularly at lower temperatures. Consequently, the mechanochemically NiO(20)-Al₂O₃ was selected in this section, and the influence of the various nickel loading was studied over this catalyst.

3.2. NiO(X)-Al₂O₃ catalysts

Figure 5 presents the X-ray diffraction (XRD) patterns of the NiO-Al₂O₃ catalysts featuring diverse nickel loadings spanning from 5 to 25 wt.%. The X-ray diffraction (XRD) profiles of the NiO- Al₂O₃ catalysts revealed the emergence of the NiO phase peak at 37.4°, and the diffraction peaks of Al₂O₃ and NiAl₂O₄ appeared at 37.4°, 46.2° and 66.8° (JCPDS 10–0339), respectively [44].

The XRD pattern depicted in Fig. 5 unveils the distinct diffraction peaks, signifying the coexistence of multiple overlapping species. The results exhibited an augmentation in the intensity of the diffraction peaks situated at 2θ = 37.4°, 46.2°, and 66.8° as the nickel content increased. The diffraction peaks associated with the Al₂O₃ and NiAl₂O₄ phases were appeared at the angle of 19.8° by increasing the nickel contents. The acquired results indicated that an increase in the intensity of the diffraction peaks linked to nickel oxide cuased by the elevation in nickel content. Furthermore, the larger particles formed in the catalyst possessing a higher nickel content.

Due to the similar lattice parameters between gamma-alumina and nickel aluminate, it is difficult to distinguish between the diffraction peaks of Al₂O₃ and the NiAl₂O₄ phase. This overlap in the diffraction patterns could be attributed to the pseudo spinel structure of gamma-alumina, further complicating the identification of these phases. When the percentage of nickel was increased increased, resulting in an excess amount of alumina in the Ni-Al₂O₃ catalyst, this led to a heightened presence of particles in contact with alumina. This increased contact between nickel and alumina promoted the formation of the NiAl₂O₄ spinel phase [49, 50].

Figures 6a and 6b showcase the nitrogen adsorption/desorption isotherms and pore size distributions of the NiO-Al₂O₃ catalysts, each characterized by distinct nickel loadings. Based on the analysis of the isotherms shown in Fig. 6a, the isotherm classifications can be attributed to type IV, a characteristic indicative of materials possessing a mesoporous structure with cylindrical-shaped pores.

The presence of an H2-type hysteresis loop within the isotherms signifies the presence of mesopores within the structure of the synthesized samples. This specific arrangement of the H2 hysteresis loop is often associated with spherical solids composed of interconnected particles featuring varying sizes and shapes, resulting in the formation of edge pores. The presence of the hysteresis loop at low P/P0 values in all the catalysts indicates the formation of small pores.

The detection of an H2-shaped hysteresis loop at elevated partial pressures within the sample possessing higher nickel loadings implies an augmentation in the pore size of the catalyst. However, largest pore size was obtained for the catalyst containing 15 wt. % of Ni. According to the data presented in Fig. 6b, the pore size distribution profiles ranged from 2 to 9 nm and shifted towards higher values with increasing Ni loadings.

Table 3 outlines the textural characteristics of the NiO-Al₂O₃ catalysts with differing nickel loadings. As the nickel loading increased, there was a reduction in pore volume from 0.38 to 0.31 cm3/g and a decline in specific surface area from 268.8 to 235.6 m2/g. This can be attributed to the combination of NiO powder, which possesses a low specific surface area, with alumina, which exhibits a high specific BET area. The interaction between these constituents results in a notable decrease in the specific surface area. The pore size of the samples exhibited a rise with the increment in nickel loading, reaching its maximum at 15 wt%. However, further increases in nickel loading led to a reduction in pore size. This observation is consistent with the findings presented in Fig. 6b.

Table 3

Textural properties of NiO-Al₂O₃ catalysts with various nickel loadings
Catalyst	Specific surface area (m²/g)	Pore volume (cm³/g)	Pore size (nm)	Average crystallite size (nm)
5NiO-Al₂O₃	268.8	0.38	5.49	11.0
10NiO-Al₂O₃	257.3	0.36	5.63	13.9
15NiO-Al₂O₃	249.2	0.34	5.78	20.9
20NiO-Al₂O₃	240.7	0.32	5.28	29.4
25NiO-Al₂O₃	235.6	0.31	5.21	30.8

The application of the Scherrer equation allowed the calculation of nickel oxide crystallite size. The findings indicated that augmenting the nickel content from 5 to 25 wt. % within the catalyst structure resulted in an escalation of the average NiO crystallite size, ranging from 11.0 to 30.8 nm. This suggests a reduction in the dispersion of nickel oxide within the catalyst.

The results obtained from the temperature-programmed reduction (TPR) analysis carried out on the NiO-Al₂O₃ catalysts featuring different nickel loadings are presented in Fig. 7. The observed reduction peak in the TPR analysis corresponds to the reduction of highly interactive NiO species with alumina, as well as the reduction of the NiAl₂O₄ spinel phase. The limited reduction characteristics have resulted in the absence of reduction peaks for the catalysts containing 5 and 10 wt. % of NiO. With the increase of nickel loadings from 15 to 25 wt. %, the reduction peak was detected within the temperature range of 660–1000°C. This indicates the reduction of NiO species that exhibit strong interactions with alumina and the reduction of the NiAl₂O₄ spinel phase [16].

The creation of NiAl₂O₄ spinel during the calcination process is primarily attributed to the potent interaction between nickel species and alumina at high temperatures. This strong interaction promotes the creation of the spinel phase in the catalyst. By elevating the nickel loading in the catalyst, it was noted that the highest temperature (Tmax) of the main reduction peak was detected at lower temperatures. This shift indicates a weaker interaction between Al₂O₃ and Ni in the catalyst. The enhanced nickel loading led to an increase in the size of the nickel crystallites. The reduction peak area and intensity decreased as the percentage of nickel decreased due to the lower concentration of nickel and reduced H₂ consumption. Samples featuring a low nickel percentage displayed diminished reducibility at high temperatures, primarily attributed to the formation of NiAl₂O₄ [50, 51].

The comparison of the catalytic performance among the NiO-Al₂O₃ catalysts with NiO contents spanning from 5 to 25 wt. % is presented in Fig. 8. The equilibrium conversion has been obtained through simulating the equilibrium reaction of CO₂ methanayion in HYSYS software.

As depicted in Fig. 8a, the catalysts performance exhibited enhancement with the augmentation of nickel loading, reaching its peak efficiency at 20 wt.%. This can be attributed to enhanced reducibility and the increased concentration of active species within the catalysts as the nickel content rises. Nevertheless, a reduction in the conversion rate was discerned upon elevating the NiO loading to 25 wt. %. This can be explained by the negative influence of the high concentration of active metal on the dispersion of nickel and the lower BET surface area observed in this particular catalyst. A decrease in nickel loading is recognized to diminish the count of active sites, leading to a reduction in catalyst activity. It is important to highlight that the number of active sites is closely linked to the dispersion of nickel particles on the surface of the catalyst. With a rise in nickel content, the active site count decreases, thereby leading to a reduction in the dispersion of nickel throughout the catalyst. At a specific nickel loading, the catalyst manifests its peak activity, stability, and selectivity.

According to Fig. 8b, the NiO (20)-Al₂O₃ catalyst showed the best performance for methane selectivity. For catalysts with 20 and 25 wt. % Ni loading, the carbon dioxide conversion showed improvement up to 450°C. However, at higher reaction temperatures, both the conversion of CO₂ and selectivity of CH₄ exhibited a decrease. This can be ascribed to the concurrent operation of the reverse water-gas shift reaction alongside the primary reaction [38, 52, 53]. The 20NiO-Al₂O₃ catalyst showed 71.47% conversion of carbon dioxide along with 94.82% methane selectivity at 450 ℃. The 20wt%Ni-Al₂O₃ catalysts results showed 48% CO₂ conversion and 5wt.% Ni/Al₂O₃ showed 1.8% CO₂ conversion at 350°C with GHSV = 16000 h^− 1 and H₂/CO₂ = 4. The 20wt%Ni-Al₂O₃ catalysts showed 48% CO₂ conversion at 350°C with GHSV = 56.7 h-1 and H₂/CO₂/N₂ = 60:15:20 ml/min.

3.3. Temperature of calcination:

Figure 9 displays the XRD patterns of the 20NiO-Al₂O₃ catalysts synthesized through mechanochemical method and calcined at various temperatures. Elevating the calcination temperature promoted the generation of NiAl₂O₄ and led to heightened crystallinity within the prepared samples. As the calcination temperature was raised, the intensity of the peaks related to the NiO, Al₂O₃, and NiAl₂O₄ phases was intensified. This can be linked to the higher crystallite size of the catalysts and a reduction in the dispersion of nickel particles on the catalyst surface.

As the calcination temperature is elevated, the data in Table 4 demonstrates an augmentation in the crystallite size alongside a reduction in the BET surface area. These alterations can be ascribed to the sintering process experienced by particles at higher calcination temperatures. The collapse of the pore structure increased by raising the calcination temperature. This can be associated with dominant impact of the calcination temperature, which alters the catalyst's structure. Furthermore, raising the calcination temperature led to an increase in pore size. This can be attributed to the destruction and collapse of small mesopores at higher calcination temperatures.

Table 4

Textural properties of the mechanochemically 20NiO-Al₂O₃ catalyst calcined at different temperatures
Catalyst	Specific surface area (m²/g)	Pore volume (cm³/g)	Pore size (nm)	Average crystallite size (nm)
Calcined at 400 ̊C	276.3	0.36	5.28	26.9
Calcined at 500 ̊C	240.7	0.32	5.25	29.4
Calcined at 600 ̊C	205.1	0.38	7.33	32.4

Figure 10a portrays the N₂ adsorption/desorption isotherms of the 20NiO-Al₂O₃ catalysts subjected to various calcination temperatures. Figure 10b presents the corresponding pore size distribution profiles. The physisorption isotherms depicted in Fig. 10a can be categorized as type IV, along with an H2-type hysteresis loop. The results demonstrated that as the calcination temperature increased, the hysteresis loop emerged at higher partial pressures, implying a rise in pore size. The profiles of pore size distribution for the 20NiO-Al₂O₃ catalysts after calcination at varying temperatures, as shown in Fig. 10b, exhibited a range of pore sizes from 2 to 20 nm. The results revealed that with the increase of the calcination temperature, the distribution of pore sizes shifted towards larger magnitudes, signifying an increase in the proportion of larger pores within the catalysts.

Figure 11 illustrates the TPR profiles of the 20NiO-Al₂O₃ catalysts that have undergone different calcination temperatures. All samples exhibited one broad reduction peak between 600 to 1000°C, indicating the reduction of nickel oxide with a strong interaction with alumina and the generation of nickel aluminate. Nevertheless, the results demonstrated that with the increase of the calcination temperature, the reduction peak shifted towards higher temperatures. This shift can be ascribed to the increased formation of nickel aluminate spinel and a stronger interaction between nickel and alumina. For the catalyst calcined at a temperature of 400°C, the Tmax value was approximately 800°C, suggesting the reduction of NiO accompanied by an interaction with the alumina support. The peak intensity became more pronounced as the calcination temperature increased. The catalyst with a calcination temperature of 600°C exhibited a peak at approximately 900°C, attributable to the reduction of NiAl₂O₄, an inactive phase in the methanation reaction.

Figure 12 presents the results of FESEM analysis, which was carried out to investigate the impact of the calcination temperature on the morphology of the prepared catalysts. According to the obtained results, it can be deduced that raising the calcination temperature led to particle aggregation, with the catalyst calcined at 600°C displaying the largest particles and the lowest specific surface are

Figures 13a and 13b illustrate the outcomes of CO₂ conversion and CH₄ selectivity, respectively, for the 20NiO-Al₂O₃ catalysts subjected to different calcination temperatures. The results exhibited a reduction in the efficiency of the calcined 20NiO-Al₂O₃ catalyst in the CO₂ methanation process as the calcination temperature was incresed. This suggested that the interaction between the different phases of the 20NiO-Al₂O₃ catalyst was altered with the elevation of calcination temperature. Raising the calcination temperature resulted in a decrease in the quantity of accessible active sites and the specific surface area of the catalyst. Based on the findings, the 20NiO-Al₂O₃ catalyst subjected to calcination at 400°C showcased the most superior performance. The 20NiO-Al₂O₃ catalyst calcined at 400°C showed 80.68% carbon dioxide conversion along with 98.3% methane selectivity at 400°C.

3.4. Effect of operating conditions:

Figure 14 depicts the influence of Gas Hourly Space Velocity (GHSV) on the performance of the 20NiO-Al₂O₃ catalyst during the CO₂ methanation process at a temperature of 350°C. The results demonstrated a decline in CO₂ conversion and CH₄ selectivity as the GHSV value increased.This can be ascribed to the increased flow rate of the inlet feed to the reactor, resulting in reduced adsorption time of carbon dioxide and shortened reaction time between carbon dioxide and hydrogen on the catalyst surface [13, 18]. The highest performance was achieved at a GHSV value of 9000 ml/h.gcat, resulting in a carbon dioxide conversion of 54.24% and methane selectivity of 97.87%.

Figure 15 depicts the impact of the H₂/CO₂ molar ratio on the carbon dioxide conversion and the methane selectivity at an operational temperature of 350°C. The carbon dioxide conversion increased from 37.74–52.35% and the methane selectivity increased from 89.89–94.42% as the H₂/CO₂ ratio was increased from 3 to 4.5. The enhancement observed in the performance of the 20NiO-Al₂O₃ catalyst, achieved through an elevation in the H₂/CO₂ molar ratio, can be attributed to the heightened presence of hydrogen available for the reaction with carbon dioxide. This increase in availability facilitates an augmented interaction between CO₂ and H₂ on the catalyst surface, a phenomenon that has been elucidated in previous research [52].

The exploration of the impact of temperature at which reduction occurs on the effectiveness of the 20NiO-Al₂O₃ catalyst encompassed the alteration of temperatures within the range of 400 to 600°C. The corresponding outcomes are presented in Fig. 16a and 16b. The efficiency of the 20NiO-Al₂O₃ catalyst within the CO₂ methanation process demonstrated an upward trend in efficiency corresponding to increasing reduction temperature. Under the operational temperature of 400°C, a substantial rise in carbon dioxide conversion was observed, elevating from 7.6–82.55%, accompanied by a remarkable augmentation in methane selectivity, ranging from 18.37–97.87%. This improvement in performance stemmed from the heightened availability of active sites capable of adsorbing carbon dioxide. At 400 ℃, the incomplete reduction of the catalyst led to a reduced concentration of active sites located on the surface of the catalyst, thereby contributing to suboptimal catalytic performance.

Figure 17 depicts the thermal stability of the 20NiO-Al₂O₃ catalyst in the process of carbon dioxide methanation at 350°C for 12h. The 20NiO-Al₂O₃ catalyst exhibited remarkable stability, as the CO₂ conversion and CH₄ selectivity remained nearly constant throughout the reaction period. The alteration in morphology between the fresh and spent 20NiO-Al₂O₃ catalysts was evaluated through FESEM analysis, and the corresponding results are depicted in Fig. 17b and 17c. No indications of carbon deposition on the catalyst surface were observed following the stability test. Furthermore, it is noteworthy to mention that an augmentation in particle size was noted after the stability test, which can be attributed to thermal sintering and enhanced particle agglomeration at elevated temperatures.

The CO Methanation reaction was carried out using the 20NiO-Al₂O₃ catalyst, and a comparison between CO and CO₂ methanation is presented in Fig. 18. The activity of the surface active sites on the 20NiO-Al₂O₃ catalyst varied between the CO and CO₂ methanation processes. The 20NiO-Al₂O₃ catalyst demonstrated higher performance in the CO Methanation process compared to the CO₂ methanation process. The selectivity of methane in the CO Methanation process remained nearly constant and reached 100%. The RWGS reaction was not performed under this condition in CO Methanation. At a temperature of 450°C, the conversion of CO to methane reached 98.16%, while the conversion of CO₂ to methane at the same temperature reached 71.24%. The higher carbon monoxide conversion (CO) compared to carbon dioxide (CO₂) can be attributed to the stronger interactions between CO and the catalyst surface. The findings suggested that the existence of nickel on the catalyst surface might obstruct specific adsorption sites for carbon dioxide (CO₂), leading to a lowered catalytic activity for the CO₂ methanation process [54–56].

In conclusion, the impact of various preparation methods on the textural attributes and catalytic efficacy of the NiO-Al₂O₃ catalyst for the purification of hydrogen using the CO₂ methanation process was investigated. The outcomes revealed that all the prepared samples displayed a nanocrystalline structure characterized by a substantial BET surface area. The NiO-Al₂O₃ catalyst prepared using the mechanochemical method exhibited the highest efficiency in the CO₂ methanation reaction compared to the other catalysts. However, various NiO-Al₂O₃ catalysts with varying nickel oxide contents (ranging from 5 to 25 wt. %) were synthesized through the solid-state mechanochemical method. Subsequently, these samples were evaluated for their performance in CO₂ methanation. The findings demonstrated that increasing the nickel loading up to 20 wt. % enhanced the CO₂ conversion in the methanation process. The maximum CO₂ conversion was 68% at 400 ℃ over this catalyst. The impact of calcination temperature on the performance of the NiO(20)-Al₂O₃ catalyst was explored, and the results demonstrated a decrease in catalyst performance with increasing calcination temperature. This decrease in performance can be ascribed to the sintering of particles that takes place at higher calcination temperatures. Furthermore, the influence of reduction temperature, GHSV, and feed ratio on the catalytic activity and selectivity of the NiO(20)-Al₂O₃ catalyst was evaluated. The findings illustrated that the catalytic performance of the chosen catalyst was markedly affected by the processing conditions.

Author Contribution

Kianoush Tamimi: Writing – original draft, Methodology, Formal analysis, Data curation. Seyed Mehdi Alavi: Writing – review & editing, Project administration, Investigation, Conceptualization. Mehran Rezaei: Writing – review & editing, Project administration, Funding acquisition, Data curation, Conceptualization. Ehsan Akbari: Writing –review & editing, Supervision, Conceptualization.

Acknowledgment

Giving access to the facilities from the Iran National Science Foundation (INSF) under the grant number of 97017638 to perform this project is kindly appreciated and acknowledged.

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No competing interests reported.

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CO 2 Methanation Process over Highly Active and Nanostructured NiO-Al 2 O 3 Catalyst Synthesized by Various Methods

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Catalyst preparation

2.2. Characterization

2.3. Catalytic test

3. Results and Disscussion

3.1. Influence of various preparation methods

3.2. NiO(X)-Al₂O₃ catalysts

3.3. Temperature of calcination:

3.4. Effect of operating conditions:

Conclusion

Declarations

Author Contribution

Acknowledgment

References

Additional Declarations

Supplementary Files

Status:

Version 1

CO 2 Methanation Process over Highly Active and Nanostructured NiO-Al 2 O 3 Catalyst Synthesized by Various Methods

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Catalyst preparation

2.2. Characterization

2.3. Catalytic test

3. Results and Disscussion

3.1. Influence of various preparation methods

3.2. NiO(X)-Al2O3 catalysts

3.3. Temperature of calcination:

3.4. Effect of operating conditions:

Conclusion

Declarations

Author Contribution

Acknowledgment

References

Additional Declarations

Supplementary Files

Status:

Version 1

3.2. NiO(X)-Al₂O₃ catalysts