Dry-Oxidative Reforming of Biogas for Hydrogen Generation over Ca and Mg-Promoted Titania-Supported Nickel Catalyst

Abstract

Hydrogen is gaining significant interest from researchers because of its renewable and clean nature. In this study, we explored the effects of promoters and oxygen addition on biogas reforming. The promotion of catalysts with alkaline earth metals (Ca and Mg) improved the basicity of the catalyst, leading to enhanced catalytic activity and stability. The promotion of the Ni/TiO₂ catalyst with Ca showed higher CH₄ conversion and H₂ yield compared to the bare and Mg-Ni/TiO₂ catalysts. The enhanced activity of Ca-Ni/TiO₂ could be attributed to its high dispersion, small particulate size, and strong metal–support interaction. Adding oxygen to the reactor feed improved the activity and stability of the catalyst due to the simultaneous occurrence of dry and partial oxidative reforming. The maximum CH₄ conversion and H₂ yield of 81.13 and 37.5% were obtained at 800 °C under dry reforming conditions, which increased to 96 and 57.6% under dry-oxidative reforming (O₂/CH₄ = 0.5). The CHNS analysis of the spent Ca-Ni/TiO₂ catalyst also showed carbon deposition of only 0.58% after 24 h of continuous dry-oxidative reforming compared to 25.16% under continuous dry reforming reaction. XRD analysis of the spent catalyst also confirmed the formation of carbon deposits under dry reforming. Adding oxygen to the feed resulted in the simultaneous removal of carbon species formed over the catalytic surface through gasification. These findings demonstrate that Ca promotion combined with oxygen addition significantly improves the catalyst efficiency and durability, offering a promising pathway for stable, long-term hydrogen generation. The results highlight the potential of Ca–Ni/TiO₂ catalysts for integration into biogas reforming units at an industrial scale, supporting renewable hydrogen production and carbon mitigation in future energy systems.

1. Introduction

The over-reliance on fossil fuels and fluctuating oil prices, coupled with the energy security crisis, has thrust the quest for renewable and clean energy sources. Hydrogen has been foreseen as a future fuel due to its renewable and clean nature [1,2,3]. Moreover, it can be produced from various sources, such as natural gas, water, biogas, and coal. The easy and domestic availability of biogas makes it an attractive candidate for hydrogen generation applications. The two significant constituents of biogas, i.e., methane (CH₄) and carbon dioxide (CO₂), produce hydrogen through the dry reforming route [4].

Dry reforming (DR; Equation (1)) has gained significant interest in recent years due to its environmentally friendly nature, owing to its ability to consume two major greenhouse gases (CH₄ and CO₂) for the production of valuable synthetic gas [5,6,7]. It produces synthetic gas with an H₂/CO ratio of approximately one, which can be utilized for various downstream applications such as hydrogen, methanol, ammonia, and synthetic fuels [8,9,10,11]. DR is highly endothermic and requires a minimum temperature of at least 550 °C. Various sources, such as landfill gas, biogas, and natural gas, can be employed in this reforming [12,13,14,15].

$${\mathrm{CH}}_4+{\mathrm{CO}}_2\to 2{\mathrm{H}}_2+2\mathrm{CO}\hspace{1em}\Delta H_{298K}^{°}=247 \mathrm{kJ}/\mathrm{mol}$$

(1)

The main dry reforming is usually accompanied by reverse water gas shift reaction (RWGS; Equation (2)), which consumes CO₂ and H₂ to produce H₂O and CO. It also reduces the overall H₂/CO ratio in the reforming reaction.

$${\mathrm{CO}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\Delta H_{298K}^{°}=41 \mathrm{kJ}/\mathrm{mol}$$

(2)

The other important side reaction is methane cracking (Equation (3)) and the Boudouard reaction (Equation (4)), which also co-occur with the primary dry reforming reaction and cause the deposition of carbon and catalyst deactivation.

$${\mathrm{CH}}_4\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2\hspace{1em}\Delta H_{298K}^{°}=75 \mathrm{kJ}/\mathrm{mol}$$

(3)

$$2\mathrm{CO}\leftrightarrow \mathrm{C}+{\mathrm{CO}}_2\hspace{1em}\Delta H_{298K}^{°}=-172 \mathrm{kJ}/\mathrm{mol}$$

(4)

Various noble (Pt, Pd, and Ru), as well as non-noble metals (Ni and Co), supported over metal oxides, have been reported extensively for dry reforming reactions [16,17,18,19]. However, nickel-based catalysts are primarily used owing to their comparative activity, easy availability, and lower cost when compared to noble metals [20,21,22,23]. Nonetheless, these catalysts are vulnerable to deactivation owing to the rapid carbon formation and reoxidation of the active phases. Therefore, efforts have been made to enhance their stability and activity by altering the nature of support, adding promoters, and making bimetallic catalysts [24,25,26,27,28].

The addition of promoters influences the acidity and basicity, dispersion of active sites, and metal–support interaction of the catalysts, significantly controlling their activity and stability [29]. It has been shown that an increase in the adsorption of CO₂ promotes the gasification of carbon deposits (CO₂ + C → 2CO), and thus retard the deactivation of catalysts [30,31]. The slightly acidic nature of CO₂ promotes the addition of basic compounds to the catalyst for increasing its alkalinity. Various alkali, alkaline, and rare-earth metals have been extensively used for this purpose. Liu et al. (2018) [32] presented the promotional effects of Mg on the natural clay-based Ni-catalysts for dry reforming reactions. They demonstrated that enhanced basicity of the catalyst and higher dispersion of active nickel metal contributed to improved catalytic performance. However, carbon deposition was also found to be higher due to enhanced activation of the C-H bond [32]. Al-Fatesh et al. (2019) [33] inferred the effects of CeO₂ and Mg promotion on the titania-doped Ni/γ-Al₂O₃ catalyst and showed improved catalytic activity when compared to the un-promoted catalyst due to improved metal–support association, textural properties, and reduction behavior of the promoted catalysts. However, the stability of the catalyst was not improved due to the formation of graphitic carbon with the addition of promoters [33]. Wei et al. (2018) [34] showed that the addition of Mg to a Ni-Al catalyst improved the surface area, metal dispersion, and metal–support interaction. The promoted catalysts also possessed higher basicity than the unpromoted catalysts. The promoted catalyst demonstrated high CH₄ conversion, H₂/CO ratio, and H₂ selectivity and suppressed the coke formation in combined partial oxidative-dry reforming [34]. Ma et al. (2019) [35] presented the effects of MgO and CeO₂ on the Ni/Al₂O₃ catalyst for partial oxidative reforming of methane. They demonstrated that the promoters’ synergetic product enhances the catalyst’s reducibility and greatly improves the catalyst’s performance [35]. Sun et al. (2020) [36] showed that promoting Ni-HMS catalyst with CaO enhanced the metal–support interaction and inhibited the formation of carbon deposits. They postulated that CaO promotion enhanced the adsorption of CO₂, which accelerated carbon removal [36]. Hambali et al. [37] also studied the effects of various promoters (Ca, Ga, Mg, and Ta) on the Ni/ZSM-5 catalyst for methane dry reforming. They showed that promoters significantly altered the acid-basic site distribution and the metal–support interaction, resulting in improved catalytic activity and stability [37]. Singha et al. [38] also presented that redox properties of CeO₂ and basic nature of MgO effectively controlled the coke deposition and showed good stability for time of stream (TOS) of 24 h at 800 °C, owing to robust metal–support interaction, smaller particle sizes, and improved dispersion [38]. Fakeeha et al. [39] also explored the promotional effects of CuO, ZnO, Ga₂O₃, and Gd₂O₃ on the activity and stability of Ni/γ-Al₂O₃ catalyst and reported that moderate and strong metal–support interaction provided high stability to catalysts [39].

The promotional effects of Ca and Mg on various supported catalysts have been extensively reported. However, a comparative study on their promotional effects on TiO₂-supported Ni catalysts has not been reported previously. Therefore, the present study focused on the promotional effects of Ca and Mg on the catalytic activity of Ni/TiO₂ catalysts.

Biogas dry reforming is environmentally friendly; however, its high endothermic nature and susceptibility to coke deposition on the catalyst surface hinder its industrial application. Moreover, adding oxygen to the reaction feed enhanced the performance of the reforming reactor. The O₂ introduction to the feed retards the carbon accumulation on the catalytic surface through the gasification reaction, which enhances the catalyst’s stability [40]. It also lowers the maximum temperature required for the reforming response due to the synergic effects of partial oxidation and higher affinity of CH₄ towards O₂ [41].

2. Materials and Methods

2.1. Catalyst Synthesis

The promoted catalysts were synthesized by the two-step wet-impregnation method for investigating the effects of Ca and Mg. A two-step impregnation method was adopted instead of conventional sequential and co-impregnation methods to understand the effects of the promoters [2,28]. The concentrations of nickel metal and promoters were kept at 11 wt.% and 5 wt.%, respectively, based on previous studies [28]. Prior to impregnation, to remove the impurities, 5.0 g of TiO₂ support was calcined at 650 °C for 2 h in air. A 0.5 M Ni(NO₃)₂·6H₂O (supplied by Loba Chemie, Mumbai, India) solution was prepared in 50 mL of DI water–ethanol mixture and added dropwise under vigorous stirring. After impregnation, the mixture was aged for 12 h at room temperature, dried overnight at 110 °C, and calcined in air at 600 °C for 4 h. For promoter addition, 0.25 M Ca(NO₃)₂·4H₂O and Mg(NO₃)₂·6H₂O (both procured from Loba Chemie, Mumbai, India) solutions were similarly impregnated, followed by drying overnight at 110 °C and calcining at 600 °C for 4 h in a muffle furnace. The catalysts were reduced at 800 °C in a 10% H₂/N₂ flow for 4 h prior to catalytic testing. The synthesized promoted catalysts were termed NT, NTC, and NTM, representing Ni/TiO₂, Ca-promoted Ni/TiO₂, and Mg-promoted Ni/TiO₂, respectively.

2.2. Catalyst Characterization

XRD analysis for obtaining the diffraction patterns was performed using Cu Kα radiation (λ = 1.54060 Å) with a Rigaku Ultima IV Advance diffractometer housed at SAI labs, TIET Patiala. The scanning step for both fresh and spent catalysts was maintained at 0.02° in the range from 20° to 80°. The average nickel metal crystallite sizes corresponding to the (111) diffraction peak were calculated using the Scherrer equation ($L=0.9\lambda /\beta \mathit{cos}\theta$).

The surface morphology of the prepared catalysts was assessed using FESEM-EDS analysis performed on a Hitachi SU 8010 instrument working at an operating voltage of 5.0 kV. Hydrogen temperature-programmed reduction (H₂-TPR) experiments were performed using a mass spectrometer (Pfeiffer Vacuum, model Thermo star Omni GSD350). Prior to analysis, the samples were pre-treated under a N₂ atmosphere at 150 °C for one hour and then cooled to room temperature to remove surface impurities and moisture. For H₂-TPR analysis, all catalysts were heated in a quartz tube reactor under H₂ (10%)/N₂ flow rate of 20 cm³/min from RT to 800 °C with a constant heating rate of 10 °C/min.

The characterizations of spent catalysts were also performed using a CHNS analyzer (Make-Thermo Scientific Flash 2000, Dreieich, Germany) and XRD. The carbon deposition (wt. %) using a CHNS analyzer was analyzed through the flash combustion (at 960 °C) of samples for their complete and instantaneous oxidation. The combustion products were identified using a thermal conductivity detector (TCD).

2.3. Catalytic Reforming

The lab-synthesized catalysts were employed for dry and dry oxidative reforming in a down-flow, fixed-bed reactor under ambient pressure conditions. The setup details of the reforming reactor have been previously provided [28]. The catalytic activity was tested in the temperature range of 650–800 °C. The catalytic dry reforming reaction was conducted with a 1.5:1.0 ratio of CH₄:CO₂ since biogas is comprised of a 1.5:1.0 ratio of CH₄:CO₂, and the O₂/CH₄ ratio was varied from 0 to 0.5 for dry-oxidative reforming. The sandwiched catalyst (150 mg) between two layers of quartz wool was placed in the middle of the tubular reactor. Prior to the reforming reaction, the catalysts were reduced by keeping them in a hydrogen atmosphere at 800 °C for 4 h. The reaction temperature was attained with the help of carrier inert gas (N₂) before introducing the reactant gases. The outlet gases from the reactor were then dried through a condenser and a liquid–gas separator, and then monitored using a continuous biogas analyser. The catalytic activity was assessed after every 2 h of reaction at a fixed temperature and O₂/CH₄ ratio. The better-promoted catalyst and the Ni/TiO₂ catalyst were then compared for stability and coke deposition tests. The reactant conversion and product yield were calculated according to previously reported formulas [28].

3. Results and Discussions

3.1. Characterization of Catalysts

The lab-synthesized catalysts were characterized through XRD, FESEM-EDS, BET, and H₂-TPR techniques. The amount of carbon deposition over spent catalysts was calculated through the CHNS analyzer.

Figure 1 shows the XRD pattern of the lab-prepared fresh catalysts. The diffraction peaks of different phases of the TiO₂ support were obtained in the XRD spectra. The characteristic peak at 2θ = 27.5°, 36.2°, 39.3°, 41.3°, 54.5°, 56.8°, 62.9°, 64.3°, and 70.0° corresponding to (110), (101), (200), (111), (211), (220), (002), (310), and (112) indicate the rutile phase of TiO₂ support (JCPDS No: 01-078-1510). Whereas the diffraction peaks at 25.4°, 37.9°, 42.8°, and 55.2° correspond to (101), (004), (200), and (211), respectively, indicating the presence of anatase phase TiO₂ support (JCPDS No: 01-073-1764). The diffraction peaks of nickel oxides [JCPDS No: 01-070-0989] were also observed at 44.6°, 51.9°, 76.5°, corresponding to (111), (200), (220) phases, respectively. The XRD spectra of the magnesium-promoted catalyst demonstrated the interaction of Mg and Ni/TiO₂ catalyst. The formation of magnesium complexes with titanium oxide support can be confirmed through the XRD spectra. The diffraction peaks obtained at 32.9°, 35.5°, 40.6°, 49.2°, 56.9°, 62.1°, and 75.1° correspond to (104), (110), (11-3), (024), (018), (12-4), and (220) phases magnesium titanium oxide [JCPDS No: 01-079-0831]. In Mg-promoted Ni/TiO₂ catalyst, the diffraction peaks at 24.1°, 32.9°, 35.5°, 40.7°, 49.2°, 53.7°, 62.2°, and 63.8° corresponding to (012), (104), (110), (11-3), (024), (11-6), (12-4), and (300) phases represent the nickel-titanium oxide [JCPDS No. 01-083-0199]. The XRD spectra of Ca-promoted Ni/TiO₂ also confirmed the formation of the calcium complex with titanium oxide. The peaks at 23.2°, 33.3°, 47.6°, 54.7°, and 59.4°, corresponding to (110), (200), (004), (131), and (204) phases, respectively, demonstrated the presence of calcium titanium oxide [JCPDS No: 01-082-0228]. The presence of the spinal phase in promoted catalysts shows superior metal–support interaction as compared to pristine catalysts [42]. The XRD spectrum of all lab-synthesized catalysts (un-promoted and promoted) showed that the alkaline earth metals have a good interaction with the support material of the catalyst.

The NiO crystallite sizes were obtained using the Scherrer formula on the XRD peak at 2θ = 44.6° and recorded as 23.97, 23.95, and 18.92 nm for NT, NTM, and NTC, respectively. The literature also shows that crystallite size reduces with the incorporation of promoters, owing to the improved dispersion of metal with promoter assistance [29,43]. Similarly, Mosavati et al. [44] showed a reduction in the Ni crystallite size with the incorporation of Ce promoters. Furthermore, they showed that metal particle agglomeration was also countered by the improved metal–support interaction due to promoters’ “spacer” effect [44]. Moreover, the larger ionic radius of Ca²⁺ allows it to be incorporated into the TiO₂ lattice, as evidenced by the spinal peaks in the XRD pattern, which leads to a stronger interaction with NiO. This led to the suppression of Ni agglomeration and a smaller Ni crystallite size, as shown by the weakened NiO peak in the XRD pattern. This can lead to greater CO₂ adsorption and stronger metal–support interaction for the CaO-promoted catalyst as compared to the MgO-promoted catalyst [45].

The FEMSEM images and corresponding EDS results of the reduced lab-synthesized catalyst are shown in Figure 2 and Figure 3, respectively. The FESEM images show the morphology of the lab-synthesized catalysts. The images show that the surface morphology of the catalysts was significantly altered with the promotion of alkaline earth metals. The Ca-promoted catalyst (Ca-Ni/TiO₂) had a finer degree of dispersion of metal crystallites on the surface of the catalyst compared to the other two catalysts. The Mg-promoted catalyst (Mg-Ni/TiO₂) also exhibited better dispersion over the catalytic surface than the unpromoted catalyst. EDS analysis provides the surface concentration of metals. EDS analysis of all the lab-synthesized catalysts demonstrated the presence of metals on their surfaces. For the promoted catalysts, the EDS analysis confirmed the presence of Ca and Mg metals on the surface of the corresponding promoted catalysts. The higher amount of Ca over the catalytic surface compared to Mg also confirmed the higher dispersion of Ca metal over the catalytic surface. A higher amount of promoters on the catalytic surface provides more active sites on the catalytic surface. Although only one representative image and dataset are presented here for clarity, SEM and EDS measurements were performed at multiple randomly selected regions of each catalyst to ensure representativeness and reliability.

Figure 4 shows the H₂-TPR analysis results of the lab-synthesized promoted catalysts. H₂-TPR analysis was performed only up to 800 °C, in accordance with the maximum reaction temperature. The peaks observed in the analysis indicated the amount of H₂-uptake during the analysis. The intensity of the peak center also represents the degree of interaction between the metal and support. For both promoted catalysts, a higher peak was observed in the high-temperature region of approximately 650 °C. The calcium-promoted Ni/TiO₂ catalyst showed two peaks during the analysis, one smaller peak around 400 °C represented the reduction in loosely interacted metal oxide to metal, and the low intensity of the peak also shows the low concentration [46]. The maximum intensity peaks for the Ca-Ni/TiO₂ and Mg-Ni/TiO₂ catalysts were observed at 695.3 and 637.9 °C, respectively. The higher reduction temperature of the calcium-promoted catalysts resulted in spinal formation and strong metal–support interactions for the catalyst. The strong metal–support interaction (SMSI) is desired for a stable and active catalyst [47,48]. The deactivation phenomena, such as sintering through the accumulation of metal crystallites and carbon formation, could be countered with SMSI [49,50,51]. The reducibility of the Ca-promoted catalyst was also higher than that of the Mg-promoted catalyst, as shown by the 10% higher H₂ consumption during reduction and the large peak area obtained from the TCD signal. This improvement could be attributed to the smaller crystallite size of NiO in the case of Ca-Ni/TiO₂, which helped in the revelation of more metal surfaces to the reduction under a reducing atmosphere of H₂ [52,53]. These results are consistent with the XRD results.

3.2. Catalytic Activity

3.2.1. Effects of Temperature on Catalytic Activity

The activity of the lab-synthesized promoted catalysts for dry reforming was evaluated in the temperature range of 650–800 °C under ambient pressure. The results of the dry reforming are shown in Figure 5 and Figure 6. The activity of both catalysts was evaluated by comparing them with that of the unpromoted Ni/TiO₂ catalyst.

The high endothermic nature of dry reforming and the rapid movement of gaseous particles contributed to increasing catalytic activity with increasing temperature [48]. The CO₂ conversion was always higher than the CH₄ conversion for both the promoted and un-promoted catalysts owing to the occurrence of the reverse water-gas shift reaction. The higher activation energy of the C-H bond compared to the C-O bond could also be attributed to the lower CH₄ conversion compared to CO₂ [54]. The experimental analysis showed that the catalytic activity of the Ca-promoted Ni/TiO₂ catalyst was higher than that of the other two catalysts. The Ca-Ni/TiO₂ catalyst exhibited 81.13% CH₄ conversion and 94.65% CO₂ conversion at 800 °C, with H₂ and CO yields of 37.5% and 41.52%, respectively. In contrast, the Mg-Ni/TiO₂ catalyst showed 76.6% CH₄ conversion and 86.9% CO₂ conversion at 800 °C with H₂ and CO yields of 34.5 and 39.3%, respectively. The higher activity of the Ca-Ni/TiO₂ catalyst could be attributed to the higher basicity of CaO, strong metal–support interaction (supported by H₂-TPR analysis), and higher dispersion of active metal sites (supported by XRD analysis). The higher basicity of CaO promoted the preferential adsorption of CO₂ due to its slightly acidic nature, which resulted in higher catalytic activity. Haung et al. and Naeem et al. also reported that the presence of basic components in catalysts improves the adsorption of acidic gases, such as CO₂. They also reported that a higher amount of surface electrons and alkaline oxides improves Ni-catalysts’ activity through the adsorption of acidic gases [55,56].

The occurrence of various side reactions, such as methane decomposition (CH₄ → C+ 2H₂), reverse water–gas shift reaction (CO₂ +H₂ → CO + H₂O), and Boudouard reaction (2CO → C+ CO₂), significantly alters the product yield and product ratio [57]. In the present work, side reactions also play an important role, as shown by the lower product ratio than the stoichiometric value of 1. The lower H₂ yield when compared to the CO yield also showed the existence of RWGS. Wei et al. also showed that the catalytic activity of the Ni-alumina catalyst was improved after promotion with basic metal oxide due to enhanced CO₂ adsorption and Ni metal dispersion [34].

3.2.2. Effects of O₂/CH₄ Ratio on Catalytic Activity

Figure 7 represents the effects of oxygen addition on the performance of the Ca-Ni/TiO₂ catalyst. The oxygen effects were evaluated by varying the O₂/CH₄ ratio from 0 to 0.5 in the temperature range of 650–800 °C. The oxygen introduction has shown the positive effects for CH₄ conversion, H₂ yield, and H₂/CO ratio. The CH₄ conversion shown in Figure 6a was improved significantly with O₂ addition due to the higher affinity of CH₄ with O₂ and the presence of two oxidants in the inlet feed [58]. The co-occurrence of dry and partial-oxidative reforming improved the conversion of CH₄. Moreover, the combined effects of increasing temperature and O₂/CH₄ ratio had a greater impact on CH₄ conversion. The maximum conversion of 96% was observed at an O₂/CH₄ ratio of 0.5 at 800 °C. The increased CH₄ conversion also improved the H₂ yield and H₂ selectivity, which in turn improved the H₂/CO ratio. The higher H₂ yield at a lower temperature range could also be attributed to the occurrence of water-gas shift (WGS) reaction, which was also supported by reduced CO₂ conversion [10,28]. The WGS reaction is favorable at lower temperatures and consumes carbon monoxide to produce H₂ and CO₂. The H₂ yield and H₂/CO ratio attained the maximum values of 57.6% and 1.25, respectively, with an O₂/CH₄ ratio of 0.5 at 800 °C. Figure 7b shows that an increase in the O₂/CH₄ ratio negatively affects CO₂ conversion. The reduction in CO₂ conversion with increasing O₂/CH₄ ratio could be attributed to the higher affinity of CH₄ for O₂ compared to CO₂ and the occurrence of WGS. However, an increase in temperature improved the CO₂ conversion owing to the inhibition of the WGS reaction and promotion of dry reforming. The maximum CO₂ conversion was observed under dry reforming reaction conditions (O₂/CH₄ ratio = 0) at 800 °C. Wei et al. also showed similar effects of O₂ addition for combined reforming in their study for Ni-Mg-Al, Ni-Al, and Ni/Al₂O₃ catalysts [34].

3.3. Stability Analysis of Promoted Catalyst

The stability of the calcium-promoted Ni/TiO₂ catalyst was studied for both dry and oxidative reforming reactions. The results of the stability study are shown in Figure 8. The stability analysis was performed at 800 °C with O₂/CH₄ ratios of 0 and 0.5 for dry reforming and dry-oxidative reforming, respectively.

The results of the stability and carbon deposition studies are presented in Figure 9. The methane conversion was higher in the case of combined reforming owing to the presence of CO₂ and O₂. The catalyst exhibited excellent stability under both reforming conditions for up to 20 h of continuous reaction. After 20 h of continuous reforming, the catalytic activity started to decrease under dry reforming conditions owing to coke formation on the catalytic surface. The XRD patterns of the used catalyst (Figure 10) also confirmed the presence of carbon on the catalytic surface. However, under dry-oxidative reforming conditions, carbon deposition was not observed in the XRD pattern. The simultaneous gasification of carbon species formed on the catalytic surface inhibited their accumulation on the surface. The un-promoted Ni/TiO₂ catalyst presented maximum CH₄ conversions of 74.07 and 91.10% for dry and dry-oxidative reforming, respectively, and dropped significantly to 50.48 and 85.74%, respectively, after 24 h of continuous reforming. CaO promotion significantly improved the catalytic activity and stability. Ranjbar et al. also showed that CaO addition to Ni/Al₂O₃ catalyst successfully countered the carbon formation due to smaller particle size and high basicity [59]. Matos et al. also reported that the presence of CaO decreases the tendency of carbon formation through the stabilization of NiO particles due to improved metal–support interaction [60].

4. Conclusions

The present study elaborates on the effects of Ca and Mg promotion on the activity of the Ni/TiO₂ catalyst for both dry and dry-oxidative reforming. This study involved the synthesis, characterization, and utilization of promoted Ni/TiO₂ catalysts. Characterization showed strong metal–support interactions between the promoters and the catalyst. The Ca-promoted Ni/TiO₂ catalysts markedly improved the catalytic activity, H₂ yield, and stability for both dry and dry-oxidative reforming of biogas, outperforming Mg-promotion and the unmodified catalyst. The higher activity of Ca-Ni/TiO₂ could be attributed to its higher surface area, smaller particle size, better dispersion, and strong metal–support interaction compared to the other catalysts. The oxygenation of the reforming reaction also improved the CH₄ conversion and H₂ yield owing to the simultaneous occurrence of both dry and partial oxidative reforming. The maximum CH₄ conversion and H₂ yield of 95.42 and 57.6%, respectively, were achieved at 800 °C and an O₂/CH₄ ratio of 0.5 using a Ca-Ni/TiO₂ catalyst. The stability of the catalyst was also observed to be higher under dry oxidative reforming than under dry reforming because of the simultaneous gasification of carbon species formed on the catalyst surface by the oxygen present in the feed. Carbon deposition of only 0.6% was observed during dry oxidative reforming using a Ca-promoted Ni/TiO₂ catalyst. The Ca-promoted catalyst is a strong candidate for integration into industrial biogas upgrading units, where combined dry and oxidative reforming can deliver high-purity syngas or hydrogen, while simultaneously reducing greenhouse gas emissions. The study showed that the promotion of the Ni/TiO₂ catalyst with alkaline earth metals and the addition of oxygen to the feed had positive impacts on both catalytic activity and stability.

Future work should include the exploration of long-term stability (100 h) for the promoted catalyst. Additionally, further advanced characterizations, such as EDS mapping and CO₂/CH₄ chemisorption, are required to support and provide a quantitative analysis of metal dispersion.