Dry Reforming of Methane (DRM) over Hydrotalcite-Based Ni-Ga/(Mg, Al)O_x Catalysts: Tailoring Ga Content for Improved Stability

Abstract

Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into syngas, which can be further utilized to synthesize value-added chemicals. One of the main challenges for the DRM process is finding catalysts that are highly active and stable. This study explores the potential use of Ni-based catalysts modified by Ga. Different Ni-Ga/(Mg, Al)O_x catalysts, with various Ga/Ni molar ratios (0, 0.1, 0.3, 0.5, and 1), were synthesized by the co-precipitation method. The catalysts were tested for the DRM reaction to evaluate their activity and stability. The Ni/(Mg, Al)O_x and its Ga-modified Ni-Ga/(Mg, Al)O_x were characterized by N₂ adsorption–desorption, Fourier Transform Infrared Spectroscopy (FTIR), H₂-temperature-programmed reduction (TPR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Raman techniques. The test of catalytic activity, at 700 °C, 1 atm, GHSV of 42,000 mL/h/g, and a CH₄: CO₂ ratio of 1, revealed that Ga incorporation effectively enhanced the catalyst stability. Particularly, the Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni ratio of 0.3 exhibited the best catalytic performance, with CH₄ and CO₂ conversions of 66% and 74%, respectively, and an H₂/CO ratio of 0.92. Furthermore, the CH₄ and CO₂ conversions increased from 34% and 46%, respectively, when testing at 600 °C, to 94% and 96% when the catalytic activity was operated at 850 °C. The best catalyst’s 20 h stream performance demonstrated its great stability. DFT analysis revealed an alteration in the electronic properties of nickel upon Ga incorporation, the d-band center of the Ga modified catalyst (Ga/Ni ratio of 0.3) shifted closer to the Fermi level, and a charge transfer from Ga to Ni atoms was observed. This research provides valuable insights into the development of Ga-modified catalysts and emphasizes their potential for efficient conversion of greenhouse gases into syngas.

1. Introduction

In recent years, the quest for sustainable energy solutions has gained unprecedented momentum, prompting researchers and scientists to explore innovative approaches for mitigating the environmental impact of industrial processes. Among the various challenges faced, the transformation of greenhouse gases, such as methane (CH₄) and carbon dioxide (CO₂), into valuable products has emerged as a crucial area of investigation. In this context, the dry reforming of methane (DRM) reaction has garnered significant attention for its potential to simultaneously convert these two greenhouse gases, methane and carbon dioxide, into valuable synthesis gas (syngas), a mixture of hydrogen (H₂) and carbon monoxide (CO), which is a vital precursor for the production of liquid fuels, chemicals, and other value-added products [1,2,3]. DRM can be represented by the following endothermic reaction (Equation (1)). However, the DRM reaction is challenging due to the required high operating temperatures, as well as issues with catalyst deactivation from coke formation and sintering [3].

CH₄ + CO₂ ⇌ 2CO + 2H₂ (△H°₂₉₈ = 247 kJ/mol)

(1)

Nickel-based catalysts are attractive for DRM due to their high catalytic activity, economic viability, and natural abundance. Nonetheless, they are susceptible to issues related to catalyst deactivation, through coking and sintering, which severely limit their practical applications [4,5,6]. One approach to improving the performance of DRM catalysts is the use of bimetallic systems, where the addition of a second metal can enhance catalytic activity, stability, and selectivity.

Recently, a computational screening study, based on density functional theory (DFT) predictions, has shown that a nickel–gallium (Ni-Ga) bimetallic system could be a promising candidate for catalyzing the DRM reaction and could help mitigate these issues [7,8].

This has been confirmed through numerous experimental investigations, which showed that the incorporation of gallium (Ga) as a secondary metal into a Ni-based catalyst has a positive impact on the catalytic performance during the DRM reaction and promotes the catalysts with a strong carbon-deposition resistance [9,10,11,12,13,14].

The support material has a pivotal role in enhancing the performance of nickel catalysts, as it affects the dispersion of active sites, thermal stability, and interaction with reactants. The good textural qualities, high surface area, and mechanical and thermal stability of Al₂O₃ make it a popular choice for supporting nickel [15,16]. Generally, as an acidic support, Al₂O₃ catalyzes the unwanted side reaction of methane cracking (Equation (2)) that can produce coke [17,18]. As such, unless the deposition mechanism is effectively inhibited or the deposited carbon is successfully removed by the reversed Boudouard reaction (Equation (3)), catalyst deactivation takes place [19,20,21]. In this sense, a suitable basic oxide, such as MgO, is needed to neutralize the acid sites of Al₂O₃.

CH₄ → C + 2H₂ (△H°₂₉₈ = 75.0 kJ/mol)

(2)

C + CO₂ → 2CO (△H°₂₉₈ = 172.0 kJ/mol)

(3)

It is well documented that catalysts supported on MgO–Al₂O₃ mixed oxides show higher catalytic activity and higher carbon resistance in the CO₂ reforming of CH₄. The role of MgO is thought to provide moderate basicity that can enhance the dissociative adsorption of CO₂, and thereby the carbon deposition can be suppressed by promoting a carbon gasification reaction [22,23,24]. Furthermore, it has been reported that the application of MgO as a promoter for alumina could not only prevent the amount of deposited carbon but also alter the nature of carbon toward less graphitic and more destructive forms [25].

Hydrotalcites, also known as layered double hydroxides (LDHs), are natural or synthetic laminar materials, with a similar structure to brucite (Mg(OH)₂) and natural hydrotalcite (Mg₆Al₂(OH)₁₆CO₃.4H₂O) [26]. They emerge as a promising class of catalyst precursors. Their chemical composition may be presented by the general formula [M²⁺_1−x M³⁺_x (OH)₂]^x+ (Aⁿ⁻)_x/n.yH₂O where M²⁺ and M³⁺ are divalent and trivalent metal cations, respectively; x is the mole fraction of the trivalent cation; Aⁿ⁻ is the anion of compensation; and y is the degree of hydration. Generally, M²⁺ may be Ni²⁺, Co²⁺, Cu²⁺, or Zn²⁺; M³⁺ may be Al³⁺, Ga³⁺, Ce⁺³, or Fe³⁺; and Aⁿ⁻ may be CO₃⁻² or NO³⁻ [27]. The layered structure of hydrotalcite facilitates the incorporation of metal ions and the formation of well-dispersed mixed oxides upon thermal decomposition. These mixed oxides exhibit high surface areas, tunable acid-base properties, and structural stability, which can enhance the overall catalytic performance in DRM [28,29].

To our knowledge, no one has reported on the modification of NiMgAl hydrotalcite (HT), which uses a fixed M²⁺/M³⁺ molar ratio, with Ga as the DRM catalyst. In this work, Ni/(Mg, Al)O_x and gallium-modified Ni-Ga/(Mg, Al)O_x catalysts from their corresponding hydrotalcite precursors are prepared. HTlcs with different Ga/Ni molar ratios (i.e., Ga/Ni = 0, 0.1, 0.3, 0.5, and 1.0) were synthesized by the co-precipitation method. The prepared HTlcs, their calcined forms, and the spent catalysts were subjected to characterization using N₂ adsorption, FTIR, XRD, H₂-TPR, TPO, TGA, and Raman spectroscopy. The catalytic performance of these materials in terms of activity, selectivity, and stability for the dry reforming of methane (DRM) reaction was evaluated at 700 °C within a vertical fixed-bed reactor.

2. Results

2.1. Characterization of the Catalysts

FTIR is a powerful technique used to analyze the functional groups in materials and to gain knowledge of their structure and composition. The FTIR spectroscopy of the as-prepared materials, measured in the range from 4000 to 400 cm⁻¹, is shown in Figure 1. As can be seen from the figure, all the prepared samples showed similar FTIR spectra, which is typical for hydrotalcite-like materials. The broad and strong absorption band centered at around 3500 cm⁻¹ is ascribed to the stretching vibration of (OH⁻) groups, indicating that they result from hydrogen-bonded, interlayered H₂O molecules or are due to the M-OH (M = Mg, Al) vibrations [30]. The band centered at around 1650 cm⁻¹ is due to the H–O–H bending vibration modes, confirming the presence of water molecules in the interlayer region [31]. The bands centered at 1520 cm⁻¹ and 1380 cm⁻¹ are associated with the carbonate species (CO₃⁻²) in the interlayer spaces; the former is attributed to the ν₃ antisymmetric stretching modes, while the latter is associated with the O–C–O asymmetric vibrations [32,33].

These results demonstrate the successful preparation of layered double hydroxides (LDHs) as precursor catalysts.

The XRD patterns of the as-prepared hydrotalcites are demonstrated in Figure 2. As can be seen from the diffractograms, all the patterns are very similar and show reflection peaks typical of the 3R rhombohedral layered double hydroxide structure. The diffraction peaks match well with those reported in the reference data (JCDPS: 96-900-9273).

The reflection peaks at 11.6°, 23.2°, 34.5°, 39.1°, 46.5°, 59.9°, and 60.9° are associated with the plane families (003), (006), (009), (015), (018), (110), and (0015), respectively. No peaks of other phases are observed, irrespective of the Ga content, indicating the high purity of the as-prepared LDHs and pointing to the successful incorporation of both Ni²⁺ and Ga³⁺ cations into the brucite layers [31,32].

The BET-specific surface area, pore volume, and pore size distribution for the calcined samples were determined by nitrogen physisorption. The BET adsorption–desorption isotherms and pore size distribution for the calcined samples are presented in Figure 3A,B, and the corresponding results are summarized in

Table 1. Physicochemical properties of dried and calcined catalysts: BET surface area, total pore volume, and pore size.

Sample	Surface Area (m2/g)		Pore Volume (cm3)		Pore Size (nm)
	As-Prepared	Calcined	As-Prepared	Calcined	As-Prepared	Calcined
Ni/(Mg, Al)Ox	131.2	215.4	0.51	0.68	14.2	12.4
Ni-Ga/(Mg, Al)Ox (Ga/Ni = 0.1)	124.2	205.7	0.55	0.73	16.8	15.4
Ni-Ga/(Mg, Al)Ox (Ga/Ni = 0.3)	150.4	189.9	0.66	0.70	16.6	15.4
Ni-Ga/(Mg, Al)Ox (Ga/Ni = 0.5)	122.2	197.7	0.59	0.80	17.8	17.0
Ni-Ga/(Mg, Al)Ox (Ga/Ni = 1.0)	150.4	253.1	0.64	0.96	15.9	15.6

. While the BET adsorption–desorption isotherms for the as-prepared hydrotalcites are provided in Figure S2. As can be seen from Figure 3A, all the calcined catalysts show a type IV adsorption–desorption isotherm with an H1-type hysteresis loop [34], indicative of the presence of mesoporous structures in these catalysts, except for Ni-Ga/(Mg, Al)O_x (Ga/Ni = 1.0), which displays a hysteresis loop of type H3, often associated with materials with platelet-like porous structures [35].

Table 1 also compares the textural characteristics of the as-prepared and calcined catalysts in terms of specific surface areas and pore volumes. All calcined catalyst samples exhibit a larger specific area and pore volume than the as-prepared samples, which is attributed to the dehydroxylation and decarbonization process that takes place during the thermal decomposition upon calcination [36]. The specific surface areas for the as-synthesized samples are in the range of 124–150 m²/g, while those for calcined samples are in the range of 190–253 m²/g. Furthermore, the pore volumes for the as-synthesized samples are in the range of 0.51–0.66 cm³/g and increased to a range of 0.68–0.97 cm³/g for the calcined samples. The pore size distribution, shown in Figure 3B, illustrated that all samples exhibited a unimodal pore size in the range of 12.4–17 nm, and the incorporation of gallium broadened the pore size distribution. The relatively high specific surface areas and pore volumes for these catalysts would facilitate the mass transport process of the reactants and products during the DRM reaction.

The H₂-temperature-programmed reduction profile for the hydrotalcite samples is shown in Figure 4A, while that for the calcined ones is depicted in Figure 4B. In the TPR of HT samples, Figure 4A, two significant negative peaks, at temperature ranges of ~165 and ~340 °C, are observed. These two peaks might be attributed to the dehydroxylation (removal of interlayer water) and decarbonization (removal of CO₂) processes during the H₂-TPR analysis [37]. The presence of interlayer water and carbonate anions in the HT structures leads to the evolution of H₂O and CO₂ upon thermal decomposition during the H₂-TPR analysis, which contributes to the appearance of negative responses in the TPR profile, unlike with the case for the calcined samples, shown in Figure 4B, where H₂ is consumed, due to the reduction process, which appears as a positive response. Generally, the strongly interacted NiO species are harder to reduce than the moderately interacted NiO species. For the unmodified catalyst sample, a significant reduction peak appears at a temperature of ~850 °C. This peak corresponds to the hydrogen uptake upon reduction of nickel oxide to metallic nickel (NiO to Ni⁰), which is in strong interaction with the mixed oxide support [38]. Furthermore, a small reduction peak at ~550 °C is detected, ascribed to the reduction of surface NiO with weak interaction with the support [39]. The absence of a reduction peak below 400 °C indicates that no free NiO species exist over the catalyst surface. Upon Ga modification, the high-temperature peak, the peak at ~850 °C, is shifted toward higher reduction temperatures, and this shifting gradually increases with increased Ga loading. It has been reported that the reduction process of Ga₂O₃ requires higher temperatures [40]. Therefore, the shifting in reduction temperatures for the Ga-modified samples toward higher temperatures can be attributed to the simultaneous reduction of Ni²⁺ and Ga³⁺ to metallic Ni and Ga in the form of a Ni-Ga alloy. This also is in good accordance with the XRD of reduced catalysts, as will be discussed later. Similarly, as shown in

Table 2. Quantitative summary of H₂-TPR analysis for the as-prepared hydrotalcite samples.

HT Sample	Peak Temperature (°C)	Total H2 Consumed (mmol/gcat) a
Ni/(Mg, Al)	845	0.92
Ni-Ga/(Mg, Al) (Ga/Ni = 0.1)	851	0.95
Ni-Ga/(Mg, Al) (Ga/Ni = 0.3)	855	1.35
Ni-Ga/(Mg, Al) (Ga/Ni = 0.5)	858	1.10
Ni-Ga/(Mg, Al) (Ga/Ni = 1.0)	860	1.21

^a Calculated from the H₂-TPR profiles over the whole temperature range.

, hydrogen consumption also increases upon Ga modification. Notably, the small reduction peak at the low-temperature range (at ~550 °C) diminishes for the Ga-modified catalyst. These results indicate the enhancement of the metal-support interaction and a growing number of active sites upon Ga incorporation into the catalyst. All the observed reduction peaks are assigned to Ni and Ga oxide species reduction into metallic Ni and Ga since the Mg-Al oxides are not reducible in the H₂-TPR conditions. Similar TPR profiles are also observed for the calcined samples, considering the high-temperature reduction peak at ~850 °C, as shown in Figure 4B, except for the disappearance of the two negative peaks at low temperatures, related to dehydroxylation and decarbonization. The quantitative summary of H₂-TPR analysis for the calcined samples is also provided in Table S2 of the Supplementary Materials.

The XRD diffractograms of the HT-like precursors after calcination at 550 °C for 5 h are demonstrated in Figure 5A. All the samples showed similar XRD patterns, except that the peak intensity decreases when Ga content increases. The diffraction peaks at 2θ angles of 34.7, 43.3, 62.6, and 79.4° are ascribed to the (111), (020), (022), and (222) planes of MgO periclase (JCDPS: 96-900-7060) [31,32]. These peaks can also indicate the formation of periclase-like mixed oxides, in the form of a (Mg(Ni,Ga,Al)-O)-type structure. The diffraction peaks at 2θ angles of ca. 11.4 and 23.1° are associated to the (003) and (006) plane families of hydrotalcites, which can either be attributed to the incomplete decomposition of the HT-precursors or to the partial reconstruction of the calcined mixed oxides into HTs, upon exposure to the air atmosphere, due to the so-called memory effect [41].

To reveal the crystalline structure of reduced Ni/(Mg, Al)O_x and Ni-Ga/(Mg, Al)O_x (with different Ni:Ga molar ratios) catalysts, firstly a reductive treatment was performed for the as-prepared LDHs at 700 °C for 2 h in 30 mL of H₂ flow. The XRD patterns of the resulting catalysts are shown in Figure 5B. The distinct diffraction peaks for Ni/(Mg, Al)O_x catalysts observed at 2θ angles of 43.5° and 76.5° correspond to the (111) and (220) atomic planes of the metallic Ni with fcc structure (JCDPS: 00-04-0850), respectively [42]. Particularly, these two peaks are slightly shifted toward lower 2θ angles for the Ni-Ga/(Mg, Al)O_x (with different Ni: Ga molar ratios) catalyst samples, suggesting the formation of Ni-Ga intermetallic compounds due to the larger atomic radius of Ga (0.135 nm) than that of Ni (0.125 nm) [9,43,44]. The sharp reflection peaks at 2θ angles of 36.9, 43.5, 62.9, and 76.5° could also be ascribed to the periclase-like phase of mixed oxides of the (Mg(Ni,Ga,Al)-O)-type structure [45]. The reflection peak located at ca. 63.3° could also be attributed to the (220) facets of the face-centered cubic NiO phase (JCDPS: 01-089-5881), due to the incomplete reduction process [46].

2.2. Catalytic Activity Tests

The activity of Ni/(Mg, Al)O_x and Ni-Ga/(Mg, Al)O_x (Ga/Ni = 0.1, 0.3, 0.5, and 1.0) catalysts in terms of CH₄ conversion and CO₂ conversion are displayed in Figure 6. For all studied catalysts, the CH₄ conversion is always lower than the CO₂ conversion, which is due to the reverse water gas shift (RWGS) (CO₂ + H₂ → CO + H₂O) reaction accompanying the DRM. The unpromoted catalyst, Ni/(Mg, Al)O_x, showed initial CH₄ and CO₂ conversions of ∼65% and 74%, respectively; however, these conversions tend to gradually decrease over time on stream.

For Ga-promoted catalysts, Ni-Ga/(Mg, Al)O_x (Ga/Ni = 0.1, 0.3, 0.5, and 1), initial conversions were comparable to the unpromoted catalyst. However, after 5 h on stream, significant differences in catalytic stability emerged. Of all the catalysts tested, the Ni-Ga/(Mg, Al)O_x catalysts with (Ga/Ni = 0.3) exhibited the best performance in terms of catalytic stability without noticeable deactivation.

The improved catalytic stability on Ga incorporation might be attributed to the enhanced process of CO₂ activation. As pointed out by many researchers, the hydrotalcite-derived mixed oxides substituted with Ga have good CO₂ sorption (capture) characteristics at elevated temperatures [47,48]. This in turn will facilitate carbon removal through the gasification reaction (C + CO₂ ⇋ 2CO). A proposed mechanism for the enhanced stability of this system involves the formation of Ni₃GaC_0.25 through the interaction of carbon from methane decomposition with the Ni₃Ga alloy. This carbide is subsequently oxidized to CO by CO₂ in a cyclic process, preventing carbon accumulation and polymerization [8].

The catalytic performance of the Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni = 0.3 was further tested at 700 °C and GHSV of 42,000 mL/h/g for a 20 h-duration TOS. The results provided in Figure S4 of the Supplementary Materials show stable CH₄ and CO₂ conversion profiles over the entire reaction period.

The influence of reaction temperature on the catalytic performance of Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni = 0.3 is displayed in Figure 7. As can be seen from the figure, on increasing the reaction temperature from 600 to 850 °C, CH₄ conversion rises from 35% to 94%, whereas CO₂ conversion increases from 46% to 96%. This is due to the endothermic nature of the DRM reaction. Comparing CH₄ and CO₂ conversions displays that CH₄ conversion is always lower than CO₂ conversion, for the studied temperature range, which indicates the coexistence of the reverse water gas shift (RWGS) reaction under these reaction conditions. Furthermore, the ratio H₂/CO increases with reaction temperature from 0.77 at 600 °C to 1.00 at 850 °C. A comparison of the catalytic activity of the Ni-Ga/(Mg, Al)O_x (Ga/Ni = 0.3) catalyst with other Ga-modified catalysts, which was reported in previous studies, is provided in Table S3 of the Supplementary Materials. As shown in the table, the catalytic performance of the Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni = 0.3 outperforms the analogs presented in the literature, which is most likely due to the promotional effect and improved structural properties upon Ga modification.

2.3. Characterization of Spent Catalysts

Ni-based catalysts typically suffer from severe coking during the DRM reaction, due to the accumulation of carbonaceous species on the catalyst’s surface, which lead to the deactivation of the catalytic active sites [3]. Therefore, for a specific catalyst, the catalytic activity and stability are closely related to the amount of carbon species formed on its surface during the DRM reaction.

Thermogravimetric analysis (TGA) could provide useful information on quantifying the deposited carbon on spent catalysts. The TGA and the differential thermal analysis (DTA) of spent catalysts are shown in Figure 8A,B. Figure 8A shows that the addition of Ga enhances the catalytic stability compared to the unmodified catalyst. The unmodified catalyst showed a relatively high weight loss of ~24 wt.%, indicating its poor resistance to coke deposition. The incorporation of Ga resulted in good coke resistance, indicated by the reduced amount of deposited carbon. The Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni ratio = 0.3 showed the best catalytic stability, with only ~13 wt.% weight loss. From the DTA profiles, Figure 8B, a peak at a temperature of ~300 °C, with comparable intensity, appears for all the catalyst samples, which is associated with combustion of the less-ordered (amorphous) carbon. The high-temperature peak is related to the highly ordered form of carbon (>650 °C) (graphitic carbon) that requires higher temperatures to decompose [49]. The intensity of the later peak is the highest for the unmodified Ni/(Mg, Al)O_x catalyst and is significantly reduced upon Ga incorporation. Interestingly, a graphitic carbon peak is not detected for the Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni ratio = 0.3, indicating that the carbon deposited on such a catalyst is mainly amorphous and can easily be gasified.

TPO analysis could provide additional information on the type and amount of deposited carbon on the spent catalysts. The TPO profiles of spent catalysts are depicted in Figure 9. The TPO profiles for all the catalysts show two significant weight loss regions, the low-temperature region between 100–400 °C, corresponding to the combustion of the amorphous carbon or metal carbide species, and the high-temperature above 500 °C, related to the combustion of the graphitic carbon.

The latter type of carbon (graphitic carbon) is less reactive and responsible for catalyst deactivation [49]. From the TPO profiles, it can be seen that the type and amount of each carbon species are significantly altered upon gallium addition. For the unmodified catalyst, Ni/(Mg, Al)O_x, the main coke species deposited on its surface is graphitic. Also, the Ni-Ga/(Mg, Al)O_x catalyst with Ga/Ni ratio = 0.3 had both the least graphic carbon type and the highest amount of amorphous carbon. This result is in good consistency with the TGA analysis and indicates that catalyst deactivation is significantly reduced upon Ga incorporation, due to the formation of reactive-type carbon instead of a graphitic one, which in turn secures in situ gasification of deposited carbon and hence better catalytic stability. These results are in good accordance with the catalytic activity tests discussed previously.

Raman spectroscopy is a useful tool that provides information about the structural disorders and defects in carbon-based materials. Raman spectra for the spent catalysts are shown in Figure 10. The peak at the Raman shift of ~1380 (D-band) is due to disordered carbon species (e.g., amorphous or defective filamentous carbon), whereas the peak at the Raman shift of ~1589 (G-band) arises from the first-order scattering E2g mode of ordered graphite [49,50]. Generally, the relative intensity between the D and G bands is an indicator of the graphitic degree of the deposited carbon; the lower the value, the more graphitic the structure [51]. As shown in the Raman spectra, the unmodified catalyst showed a more ordered graphite structure with a high graphitization nature, indicated by the lowest value of I_D/I_G ratio of 0.93. On Ga modification, this ratio increased to greater values (>1). The Ni-Ga/(Mg, Al)O_x (with Ga/Ni = 0.3), among all the studied catalysts, has the highest value of I_D/I_G of 1.80. This indicates that for this specific catalyst the degree of crystallinity for the deposited carbon is the least during the DRM reaction. This result is also in good agreement with the trends observed in the DTA and TPO analyses discussed previously, shown in Figure 8 and Figure 9, respectively.

2.4. DFT Mechanistic Insights on the Effect of Ga Modification

DFT calculations were performed to obtain mechanistic insights into the enhanced CO₂ activation due to Ga incorporation into the Ni₃Ga catalyst. The density of states (DOS), d-band center, charge density difference, and Bader charge were calculated, and the results are presented in Figure 11. As can be seen from the projected density of states (PDOS) profiles shown in Figure 11A, there is a clear overlapping between the d-band of Ni atoms and the p-bands of Ga atoms in the Ni₃Ga(111) system; this orbital overlapping indicates the hybridization of occupied Ga 4p orbitals with the unoccupied Ni 3d. This orbital hybridization results in electron transfer from Ga atoms to Ni ones. Furthermore, the d-band center of Ni on the Ni₃Ga(111) surface shifts closer to the Fermi level with a value of ~−1.11 eV compared with ~−1.37 eV for that of the Ni(111) surface. From the Bader charge analysis, Figure S3, the estimated specific charge amount of the Ni atoms increased from ~10.04 e on the Ni (111) surface to ~10.12 e on the Ni₃Ga surface.

The electron transfer from Ga to Ni atoms is further confirmed through the analysis of the charge density, shown in Figure 11. The charge density difference shows a clear charge localization, indicated by the electronic clouds around Ga atoms in the figure, which is due to electron redistribution between Ni and Ga atoms, where a partial electronic charge is transferred from Ga atoms to Ni ones.

Overall, it is believed that this charge localization will render Ni atoms to act as Lewis basic centers “pushing” electrons to the electrophilic carbon atom of CO₂, while the Lewis acidic Ga centers facilitate stabilization of the bound CO₂ by “pulling” electron density from the electron-rich CO₂ molecule. Therefore, The Lewis acid and Lewis base synergistically interact with CO₂ boosting its chemisorption and activation. This will eventually result in an in-situ carbon removal process, preventing carbon deposits from self-polymerizing, and consequently leading to enhanced catalytic stability. This result is in good agreement with prior experimental X-ray absorption near-edge structure (XANES) investigations [52,53] and DFT theoretical calculations [54,55].

3. Materials and Methods

3.1. Catalyst Preparation

The monometallic Ni and bimetallic Ni-Ga catalysts were synthesized from their respective layered double hydroxide (LDH) precursors via the following procedure. The LDH samples, with a targeted M²⁺/M³⁺ molar ratio of 3 and a total active metal loading of 10 wt.%, were prepared through the co-precipitation method at the high supersaturation method. In this method, five LDH precursors were synthesized based on the targeted molar ratio of gallium to nickel on the final catalyst (i.e., Ga/Ni = 0, 0.10, 0.20, 0.50, and 1.0). According to the calculations of stoichiometric ratios (Table S1 in the Supplementary Materials), the required weight of metal nitrates, Ni(NO₃)₂·6H₂O (Sigma Aldrich Chemical Company Inc., Milwaukee, WI, USA), Ga(NO₃)₃·xH₂O (Sigma Aldrich Chemical Company Inc., Milwaukee, WI, USA), Al(NO₃)₃·9H₂O (LOBA Chemie PVT. Ltd., Mumbai, India), and Mg(NO₃)₂·6H₂O (Sigma Aldrich Chemical Company Inc., Milwaukee, WI, USA), were mixed in 100 mL of deionized water. An anionic precursor solution of Na₂CO₃ (VWR Chemicals BDH, Radnor, PA, USA) and NaOH (LOBA Chemie PVT. Ltd., Mumbai, India) was also prepared, in which Na₂CO₃ was used in excess, twice that of the calculated stoichiometric ratio, to ensure that the charge compensation anion was carbonate CO₃⁻² instead of nitrate NO₃⁻ Both solutions were kept on a magnetic stirrer for 15 min before final mixing. A nitrate solution containing the divalent and trivalent cations was added drop-wisely to the anionic precursor solution under vigorous stirring using a graduated burette. During the addition, the solution medium was kept at a temperature of 65 °C and a pH of 9.5–10.5 by adding a 0.2 M NaOH solution when required. The final mixture was then aged for approximately 18 h at room temperature with continuous stirring, which resulted in a suspension of the aforementioned hydrotalcite. The resulting suspension was then centrifuged, and the solid mixture was washed several times with distilled water, to ensure the entire removal of nitrate and sodium ions. After this, the samples were dried overnight at 110 °C. After drying, the samples were divided into two parts: one was calcined at 550 °C for a duration of 5 h for further characterizations, while for the other part, the dried samples were crushed, sieved to the desired fraction (~0.15 µm size), and stored in a desiccator for the subsequent activity tests. The unmodified catalyst samples are denoted as Ni/(Mg, Al)O_x, and the gallium-modified ones are denoted as Ni-Ga/(Mg, Al)O_x (Ga/Ni = y), where y refers to the above-stated Ga/Ni molar ratio (Ga/Ni ratio = 0.1, 0.3, 0.5 and 1). More details on the prepared catalysts are provided in Table S1 in the Supplementary Materials.

3.2. Catalyst Characterization

The BET surface area, pore volume, and pore diameter of the catalyst samples were measured from N₂ adsorption–desorption data obtained using a Micromeritics Tristar II 3020 instrument (Micrometrics, Norcross, GA, USA). For each analysis, ~70 mg of catalyst was used. The analysis was performed at −196 °C, and before the measurement, the samples were degassed at 250 °C for 180 min to remove the moisture and other gases adsorbed on the surface of the catalysts.

The FTIR measurements were performed in a Shimadzu IRPrestige-21 (Shimadzu Corporation, Kyoto, Japan) infrared spectrophotometer. The pellets were prepared by mixing the sample with spectroscopic grade KBr. The collected spectra of all the studied samples were measured under ambient conditions in a wavelength range between 400 and 4000 cm⁻¹ with a scan resolution of 4 cm⁻¹ and a total of 32 scans.

Hydrogen-temperature-programmed reduction (H₂-TPR) was performed in an Autochem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA). Typically, 70 mg of the calcined catalyst was purged with a pure argon stream at 150 °C for 60 min, followed by cooling to ambient temperature. Then, a mixture of 10 vol.% H₂/Ar flowing at 40 mL/min was introduced to the sample while the temperature was increased from room temperature to 1000 °C, with the heating rate being 10 °C/min. The signal of H₂ consumption was monitored by a thermal conductivity detector (TCD). A similar approach was adopted for performing the oxygen-temperature-programmed-oxidation (O₂-TPO) analysis, except that degassing was performed with He, and a mixture of 10% O₂/He was used.

The crystalline phases of the as-prepared HT, calcined, and reduced samples were determined using a Miniflex Rigaku X-ray powder diffraction (XRD) instrument (Rigaku, Tokyo, Japan) using a Cu Kα radiation source operated at 40 kV and 40 mA. The scans were collected in the 2θ range 5–90° (step size: 0.05° and time per step 0.8 s).

Thermogravimetric analysis (TGA) was performed for the used catalyst samples on a Shimadzu TGA-51 (Shimadzu Corporation, Japan). Typically, 10–15 mg of the spent catalyst sample was placed in an alumina macro crucible and heated from room temperature up to 1000 °C (at 20 °C/min temperature ramp) in a flowing air atmosphere. The amount of coke deposited was calculated based on the mass change of the spent catalyst sample during the experiment.

The graphitization degree and the type of carbon deposited over the surface of spent catalysts were characterized using a Laser Raman (NMR-4500) spectrometer (JASCO, Tokyo, Japan). The analysis was carried out using a diode laser beam with a wavelength of 532 nm. The spectra were collected under ambient conditions using a Raman shift in the range of 500–3500 cm⁻¹. The collected spectra were an accumulation of 20 with 5 s acquiring time per each.

3.3. Catalytic Activity Performance

The DRM catalytic tests were performed in a fixed-bed reactor, with an inner diameter of 0.92 cm, at a temperature of 700 °C and under atmospheric pressure. Initially, 100 mg of catalyst sample was reduced in situ for 2 h under H2 flow (20 mL/min) at 700 °C. Subsequently, nitrogen gas (50 mL/min) was introduced for 15 min to purge the residual H₂. Then, a mixture of CH₄ (30 mL/min), CO₂ (30 mL/min), and N₂ (10 mL/min) was introduced simultaneously into the reactor with a total GHSV of 42,000 mL/h/g. Nitrogen gas was used as the internal standard for volume-change estimation in the reaction. All volumetric flow rates given in this study are related to 25 °C and atmospheric pressure. The best-performing catalysts were also tested as a function of temperature in a stepwise mode from 600 to 850 °C. The outlet gases were analyzed by an online gas chromatograph (Shimadzu Technologies, Santa Clara, CA, USA). The reactant conversions (X) and ratio of hydrogen to carbon monoxide were calculated based on the formulas given below:

$${\mathrm{X}}_{{\mathrm{CH}}_4}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}-{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{out}}}{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}}}\times 100$$

(4)

$${\mathrm{X}}_{{\mathrm{CO}}_2}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{F}}_{{\mathrm{CO}}_2,\mathrm{in}}-{\mathrm{F}}_{{\mathrm{CO}}_2,\mathrm{out}}}{{\mathrm{F}}_{{\mathrm{CO}}_2,\mathrm{in}}}}\times 100$$

(5)

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

(6)

where ${\mathrm{F}}_{\mathrm{i},\mathrm{in}}$ and ${\mathrm{F}}_{\mathrm{i},\mathrm{out}}$ are the inlet and outlet gas flow rates of i gas (CH₄, CO₂, H₂, and CO).

3.4. DFT Calculations

DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP.6.1.0) [56,57,58]. The generalized gradient approximation with the Perdew–Burke–Ernzerhof functional (GGA-PBE) was used to account for the electron exchange and correlation [59]. The core electrons were described using the projector augmented wave (PAW) method [60,61], with a plane-wave basis set that was expanded up to a energy cutoff of 400 eV. The reciprocal first Brillouin zones were approximated by a sum over special k points chosen using the Monkhorst–Pack scheme [62]. The bulk Ni and Ni₃Ga systems were sampled using 6 × 6 × 6 k-point meshes, while for Ni (111) and Ni₃Ga(111) surfaces, a mesh of 5 × 5 × 1 k-points was used. More details on the DFT calculation method and the modeled slab surfaces are provided in the Supplementary Materials, paragraph S1 and Figure S1.

4. Conclusions

This study investigated the impact of Ga modification on the catalytic performance of Ni/(Mg, Al) O_x catalysts for dry reforming of methane. Catalysts were prepared from hydrotalcite precursors with Ga/Ni ratios, i.e., 0, 0.1, 0.3, 0.5, and 1.0, through the co-precipitation method. The prepared catalysts were extensively characterized both before and after the DRM reaction. The characterization results of the as-prepared catalysts indicated the successful preparation of hydrotalcite precursors. The addition of Ga did not alter the mesoporous structure of the catalysts, as evidenced by the BET results. Moreover, the H₂-TPR analysis revealed an improvement in the metal-support interactions due to Ga incorporation. Among the various Ga-modified catalysts, the catalyst with a Ga/Ni ratio of 0.3 showed the best catalytic performance with superior carbon-deposition resistance compared to the unmodified one, which resulted in the highest CH₄ and CO₂ conversions at the reaction temperature of 700 °C. Characterizations on spent catalysts from TGA/DTA, TPO, and Raman spectroscopy showed enhanced carbon-deposition resistance and reduced degree graphitization of the deposited carbon on Ga-modified catalysts. The firm stability of 20 h time on stream showed stable CH₄ and CO₂ conversion profiles. DFT calculations revealed a charge transfer from Ga to Ni atoms, which is thought to enhance CO₂ chemisorption and activation, ultimately resulting in better catalytic stability due to the in situ process of carbon removal. These findings provide valuable insights into the role of Ga promotion in Ni-based catalysts and highlight the potential of Ga-modified catalysts for enhancing DRM efficiency. Future research should focus on exploring different supports, understanding deactivation mechanisms, and conducting longer-term stability tests. In situ characterization and advanced computational modeling can further refine our understanding and guide catalyst development.