Partial Oxidation of Methane over CaO Decorated TiO₂ Nanocatalyst for Syngas Production in a Fixed Bed Reactor

Abstract

Syngas is a valuable entity for downstream liquid fuel production and chemical industries. The efficient production of syngas via catalytic partial oxidation of methane (CPOM) is an important process. In this study, partial oxidation of methane (POM) was carried out using CaO decorated TiO₂ catalysts. The catalysts were synthesized employing the sol-gel method, while the decoration of TiO₂ with CaO was achieved in an aqueous solution by wetness impregnation method. The prepared catalysts were characterized by employing XRD, Raman, TG-DTG, and SEM-EDX for structural and morphological analysis. On testing for POM, at 750 °C the catalysts demonstrate excellent CH₄ conversion of 83.6 and 79.5% for 2% and 3% CaO loaded TiO₂, respectively. While the average H₂/CO ratio for both 2% and 3% CaO loaded TiO₂, 2.25 and 2.28, respectively, remained slightly above the theoretical value (H₂/CO = 2.0) of POM. The improved POM performance is attributed to the optimally loaded CaO on the TiO₂ surface that promotes the reaction where TiO₂ support ensure less agglomerated particles, resulting into a fine distribution of the active catalytic sites.

1. Introduction

Natural gas utilization is an increasing trend due its consumption in the energy sector and its conversion into high value chemicals such as syngas, used in various industrial processes [1,2,3]. Primarily, syngas was used as an important precursor for downstream liquid fuel production via different methods, including Fischer-Tropsch (FT) method, methanol synthesis, etc. [4,5]. Different routes for CH₄ utilization have been studied influentially in chemical society [6] and a major challenge in the use of methane for obtaining useful products is due to its stable four sigma bonds (ΔH_d = 440 kJ/mol) [7]. The use of potential catalysts is an attractive approach to convert methane into syngas and different reforming techniques have been employed such as; steam methane reforming (SMR) [8,9]; dry methane reforming (DRM) [10,11,12]; and partial oxidation of methane (POM) [13,14,15]. Although SMR is a commercialized method for H₂ rich syngas production, its endothermic and energy utilization makes it more challenging [16]. In comparison, DRM, an endothermic and less energy intensive than SMR, is considered an important method for converting CH₄ and CO₂ (greenhouse gases) into a valuable chemical. However, this method encounters low H₂/CO ratio, catalyst deactivation readily due to extreme coke deposition and sintering [17].

In recent years, POM has been extensively studied and investigated for CH₄ conversion as it is a fast and cost-effective method with high CH₄ conversion efficiency, selectivity, and short residence time. It produces a H₂/CO ratio of 2.0 (theoretically) that is more suitable for use in FT synthesis [16,18]. POM proceeds either by a direct or indirect mechanism. In the indirect pathway, CH₄ is converted into CO₂ and steam, followed by its reforming to syngas, while in the direct method, syngas can be produced by CH₄ in a single-step, as suggested by Hickman and Schmidt [19]. An assortment of catalysts have been studied for POM including transition metals, noble metals, and perovskites [20], however, the key challenges of metal sintering, hotspot formation and coke deposition that deactivates the catalyst still remained unresolved [21]. While the noble metals are costly in commercial prospect. Therefore, mixed metal oxides with different support materials have been tested to improve conversion and deter catalyst deactivation [22].

The investigation of metal oxides, zeolites, hydrotalcite and alkaline earth metal oxides have shown high product yield with small catalyst loadings [23,24]. For instance, loading of Ni on La₂O₃ exhibited excellent stability, and enhanced activity owing to the favored metal support interaction. Ni nanoparticles were found highly decorated over La₂O₃ that reduced coke formation [25]. While the loading of Co on Yb₂O₃ also proved an efficient catalysts for POM reaction and showed increased conversion rate and stability [26]. Likewise, perovskite based catalysts have been widely used as metal-supported catalysts and received much attention over the past decades as a result of their high activity and thermal stability in POM reactions [27]. Moreover, carbon nanotubes [28], nanoparticles [29], nanocrystals [30], nanocomposites [31], ceria-zirconia containing catalysts [32], nanoclusters [33], ceramic materials [34], hydrotalcite (HT) catalysts [35] and various other structural materials have been used as strong and powerful precursors for the POM. Conventional catalysts broadly utilized in CH₄ partial oxidation are relatively more expensive as compared to waste derived catalysts. Recently, Co loaded waste derived biomass fly ash (BFA) [36] and Co/CeO₂-BFA [37] were employed for methane decomposition and due to the presence of various oxides (SiO₂, Al₂O₃, Fe₂O_3, etc.), is one of the most competitive products to use as a direct support material to improve physicochemical properties and catalytic activity. CaO is another competitive candidate among waste derived catalyst that can be easily obtained from eggshells. CaO has been broadly studied previously due to its low cost and high catalytic activity [38] and due to its strong basic nature. It has been agreed that the active sites of alkaline earth metal oxides are the surface basic sites, produced by the surface metal ion presence that acts like a Lewis acid (electrophile) and an oxygen ion that behaves like a Bronsted basic site. However, these catalysts ca be deactivated by the poisoning of basic sites due to H₂O adsorption at surface sites [39]. Furthermore, alkaline earth metal oxides have a low surface area [40] that promotes onto the surface metal a support interaction, which is favorable for POM [41]. These issues restricted the utilization of simple and environmentally benign catalysts [42]. To cater for these challenges, CaO was impregnated on different supports to increase active sites, surface area and reduce the poisoning of CaO in the reaction medium. Among various supports, such as alumina, zirconia, zeolites, zinc oxide [41], etc., TiO₂ has been extensively studied as TiO₂ is a reducible metal oxide with several crystal structures, and as such, it also possesses multiple oxidation states with moderate chemical and thermal stability [43]. It is encouraging to use TiO₂ as support due to its competitive cost, occurrence, and nontoxic nature in catalytic reactions [44]. Previously, Ni/TiO₂ has been studied for hydrogen production, however, it suffered from coke deposition [45]. Ru/TiO₂ has also shown a promising conversion of methane to syngas [46,47]. Metal support interaction is more crucial in heterogeneous catalysis and a well-known interaction can be more influential for catalyst stability and activity [48] as TiO₂ with VIII B metals suppresses coke deposition due to well-decorated surfaces for metal deposition and the presence of TiO₂ prevents metal agglomeration by increasing metal active sites [49]. Previous reports on Pt/TiO₂ strongly support this hypothesis [49,50]. Furthermore, different preparatory methods have been developed for TiO₂ synthesis, including continuous reaction [51], precipitation [52], sol-gel [53,54] etc. Among these, sol-gel has gained more attention as the highly crystalline powder can be synthesized with enhanced purity even at low temperatures. It is also possible to control the stoichiometry and structure of the catalyst at the same time, using a prepared via sol-gel method [55].

This work reports the study of waste-derived CaO loaded TiO₂ catalysts for H₂ rich syngas production via POM reaction. The sol-gel method was followed to obtain well defined TiO₂ nanostructures, and eggshells-derived CaO loading over TiO₂ nanostructures was carried out via wetness impregnation route. The synthesized catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric-differential thermal gravimetric (TGA-DTG), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) for structural, stability, morphological and elemental analysis. The influence of different CaO loadings on TiO₂ was progressively studied to obtain an optimum loading. CaO, being basic in nature, can provide more oxygen vacancies to improve the conversion efficiency of TiO₂ and can prevent cooking to avoid the catalyst deactivation. As we know that more oxygen vacancies are responsible for weakening and destabilizing the metal oxygen bonds in TiO₂, which can help to reduce the TiO₂ into metallic phase, so, this study is conducted to find an optimum loading of CaO over TiO₂ showing the most conversion efficiency, as well as stability of the catalyst. Finally, the spent of the catalyst was collected and characterized by Raman and SEM/EDS to study carbon deposition on the catalyst surface.

2. Results and Discussion

2.1. Physicochemical Properties of the Catalyst

XRD patterns of different catalysts are given in Figure 1. The diffraction peaks of 1–3% CaO-TiO₂ and pure TiO₂ powder demonstrated the tetragonal arrangement of TiO₂ anatase phase with (I41/amd(141)) space group and are in good agreement with JCPDS 21-1272 data card peaks [56]. Its major peaks are detected at 2θ = 25.3°, 38.6°, 48°, 53.9°, 55°, and 62.7° with indices (hkl) values of (101), (112), (200), (105), (211), and (204) having a dominant peak at (101) (one of the more stable facets having high coordinate Ti) indicating the growth of TiO₂ along c-axis. No additional characteristic peak is observed, showing that prepared samples are free from impurities and pure anatase phase of TiO₂ is obtained without rutile or brookite phase [57]. Moreover, the peak sharpening is observed due to impregnation of CaO. Change in intensity is also noted in CaO impregnated TiO₂, likely due to the metal oxide and support interaction. Moreover, a small peak at 20.39° is observed, indicating the formation of calcium titanium oxide (CaTi₄O₉). No distinct peak for CaO is observed due to the low concentration of CaO in doped TiO₂ [58]. Moreover, no shift in diffraction lines of TiO₂ with the increase in CaO content can be possibly due to metal oxide metal interaction instead of substitution of Ti sites with Ca. Crystallite size of TiO₂, 1% CaO-TiO_2, 2% CaO-TiO_2, and 3% CaO-TiO₂ is 12 nm, 21 nm, 19 nm, and 17 nm, calculated using the Scherrer equation [59]. The change in crystallite size is attributed to CaO loading, which may help to improve the crystallinity of titania.

The Raman spectrum is further used for the phase and structural identification of pure and impregnated TiO₂ catalysts. As is shown in Figure 2, three characteristic vibrational modes of TiO₂ are observed at 395, 512, and 640 cm⁻¹ with symmetries of B_1g, A_1g/B_1g, and E_g, respectively, clearly indicating the existence of anatase phase and no additional peak for rutile or brookite phase is detected [60,61]. Thus, the results are in good agreement with XRD results. Moreover, CaO impregnated TiO₂ (1–3%) does not present any peak shifts while intensity and broadness were affected by the concentration and impregnation of metal. The peak shifting strongly depends on the number of impurity defects while in (1–3%) CaO impregnated TiO₂ concentration of impurities is low, thus no shifting was observed [62]. Only minor changes were observed in the symmetry of TiO₂ due to lattice distortion generated due to defects with CaO substitution [63], where change in intensities supports CaO loading. All the peaks of CaO loaded TiO₂ samples show a significant increase in intensities with an increase in the concentration of CaO loading that support the results obtained in XRD patterns.

FTIR spectra of undoped and CaO doped titania is presented in Figure 3a–d. The spectrum is in the range of 600 and 4000 cm⁻¹. In Figure 3a, the peaks at 3200–3600 cm⁻¹ are due to OH bonds. In Figure 3c, the intensity of peak around 1200 cm⁻¹ and around 2200 cm⁻¹ is increased due to the addition of CaO content into the titania lattice. From Figure 3b–d, the intensity of these situated peaks reduces by increasing the amount of the CaO. In Figure 3c, bending vibration is up to a maximum, which is then again reduced when doped with 3% CaO loaded TiO₂, as shown in Figure 3d. This supports the assumption that 2% loading is an optimized loading. There are some broad peaks at ~ 3600 cm⁻¹ then 3100 cm⁻¹ indicating the presence of a hydroxyl group, while peaks around 1630 cm⁻¹ are found to be corresponding to the bending vibration of H-O-H [64]. The peak near 1140 cm⁻¹ corresponds to the possible interaction of the Ti-O-Ca bond. In addition, varying intensity of vibration of the hydroxyl group with increasing doping of up to 2% CaO loading, and then suppression with 3% CaO loading, indicate the presence of various polarities on the effective doping of CaO into the titania lattice, resulting in an increased number of oxygen vacancies with increasing loading up to 2% CaO loading and decreased for 3% CaO loaded TiO₂ [65]. Overall, all the grown catalysts follow the same pattern and indicate 2% CaO loading is an optimized loading and confirm the successful synthesis of desired catalysts as observed in XRD analysis.

The thermal stability of synthesized materials was confirmed by TG analysis. It is shown in Figure 4 that pristine and CaO impregnated TiO₂ illustrated major weight loss in the same zone. However, the amount of weight loss was insignificant for both catalysts. The weight loss is further divided into three different zones for more clarity. First, weight loss in the range of 0–125 °C was related to the removal of moisture [66]. Second, weight loss (125–400 °C) was attributed to the loss of hydroxyl groups and the decomposition of different layers of amorphous carbon [67], while the third zone in the range of 400–790 °C was related to the decomposition of any organic impurity and condensation of TiO₂ anatase phases [68]. A total of 4.5% weight loss is observed for pristine titania powder, while 2.5% is observed in the case of doped titania. The thermal stability of CaO loaded catalysts is likely due to the better metal support interaction as proposed in R7 in the reaction mechanism section. Further high coordinate TiO₂ facet 101 on interaction with support CaO can provide more oxygen vacancies that possibly limit the thermal degradation [69]. From the curves of the DTA, a high magnitude of the sharp exothermic response on both Figure 4a,b can be observed. The exothermic temperature was 399 °C in case of undoped Titania, which was reduced to 310 °C for doped Titania. While an earlier phase transformation, from the amorphous to crystalline, in the case of CaO doped titania as compared to pure titania is observed [70]. These results correspond highly with XRD results as samples are highly crystalline which are thermally stable at high temperatures. The substitution of CaO into the Titania lattice lowers its temperature of crystallization [70,71], and this likely attributes to the thermal stability of doped TiO₂ due to metal impregnation.

The surface morphology of pure and CaO impregnated TiO₂ was characterized by SEM analysis. Figure 5a depicts a very closely packed spherical and well-dispersed bubble-shaped nanostructure of pure TiO₂ [72]. Less agglomeration and well dispersion were due to the aggregation of TiO₂ nanoparticles at high calcination temperature that was more influential for crystalline growth [73]. Surface structure was changed after CaO impregnation, and more uniform and homogenous distribution with irregular spherical morphological agglomeration was attained, as shown in Figure 5b. Thus, SEM analysis demonstrated the difference in morphology between TiO₂ and CaO-TiO₂ [74].

In addition to morphological study, the elemental mapping of TiO₂ and CaO-TiO₂ was performed by EDX analysis, and its results are represented in Figure 6. The two intense peaks in Figure 6a represent O (65.90%) and Ti (31.10%) elements, and, 0.39% by weight Ca loading was also detected during analysis represented in Figure 6b that confirmed the presence of CaO in synthesized material, for which no extra characteristic peak and peak shifting was observed in XRD and Raman spectroscopy due to low metal loading that could only be attributed to a change in intensities.

2.2. Catalyst Performance Analysis

The synthesized catalysts were tested for POM and depicted in Figure 7a–d. The catalysts were allowed to be activated for two hours before sampling. The average of each three readings is taken and used for plots to minimize the error. TiO₂ and CaO loaded TiO₂ showed a smooth and stable trend of conversion on time of the stream. Apart from the trend, an increase in conversion was observed with the CaO loading. However, 2 and 3% CaO loaded TiO₂ have shown almost the same conversion, and a very small change in conversion is observed after 2% loading for the same amount of loading, assuming 2% is an optimized loading. CaO loading on the surface of TiO₂ support resulted in an active phase exposed for the catalytic reaction [75] due to the combination of earth metal oxide (CaO) and a metal oxide with higher basicity (TiO₂) [76]. An active exposed phase increase with an increase in loading prevents the catalyst from coking and activating the oxidation [77,78], consequently enhancing the conversion efficiency. Furthermore, the activity of CaO on TiO₂ support is related to the increased porosity and oxygen vacancies, resulting in more active sites due to the reduction in metal oxide into a metallic state with an optimal loading [79] at higher temperatures in a hydrogen environment. All the trends for CH₄ conversion (X_CH4), selectivity of H₂ (S_H2), selectivity of CO (S_co), and H₂/CO ratio under experimental conditions reaction temperature =750 °C and flow rate CH₄ = 20 mL min⁻¹ O₂ = 10 mL min⁻¹ and N₂ = 10 mL min⁻¹ are shown in Figure 7. Where CH₄ conversion (X_CH4) for 5 h is shown in Figure 7a and an average for 2 and 3% CaO loaded TiO₂was approximately 83.56 and 79.54%, respectively. Figure 7b–d illustrate the selectivity of H₂ production, selectivity of CO, and H₂/CO ratio. Selectivity of CO for pristine and 1–3% CaO loaded TiO₂ is almost the same as shown in Figure 7c, however, among all the catalysts, 1% CaO loaded TiO₂ have shown the lowest CO selectivity, likely due to the low concentration of CaO as observed in FTIR analysis. After activation, all the catalysts have shown a stable trend, and not much variation has been observed.

The H₂/CO ratio is also an important parameter in POM, and the H₂/CO ratio for all the catalysts has shown slightly higher values than the stochiometric value, which is 2.0 [80], except TiO₂, which has shown a slightly lower value than the stoichiometric value. The variation in H₂/CO ratio could be evidence of incomplete conversion of CO₂ or a side reaction using CO [81] that may oxidize into CO₂. However, CO₂ can be dissociated into CO due to the presence of anatase phase of TiO₂, as observed in XRD that have a lower energy barrier than rutile phase [82]. However, during CO₂ formation, adsorbed oxygen atoms are consumed resulting in more active sites due to oxygen vacancies. More exposed active sites attributed to CH₄ adsorption, consequently increasing H^* ions that are responsible for an enhanced H₂ production [82,83], which is likely the reason for the higher H₂/CO ratio. The average value of H₂/CO for 2 and 3% CaO loaded TiO₂ was 2.25 and 2.28, respectively, which is slightly above the theoretical value for POM. CaO, which is used as promoter, is highly basic in nature when loaded over TiO₂ 101 high coordinate facet, providing more oxygen vacancies that results in an enhanced catalytic activity and hinders coke formation. In addition, the basic character of CaO is further increased the adsorption and prevent the coking consequently activating the oxidizing agent for gasification of carbon. More oxygen vacancies in a result of CaO loading on TiO₂ high coordinate 101 facet are responsible to weaken and destabilize the metal oxygen bond in TiO₂ resulting in the formation of metallic phase during POM reaction at high temperatures such as 750 °C [69]. Highly reduced Ti during POM in a hydrogen environment at high temperature possibly created a synergetic effect resulting in a more stable catalyst and enhanced conversion efficiency.

2.3. Characterization of Spent Catalyst

Spent confirmation is carried out by using Raman spectroscopy as presented in Figure 8, which demonstrates two additional peaks around 1330 (cm⁻¹) and 1580 (cm⁻¹) along with the peaks of pristine CaO loaded TiO₂ confirming the presence of carbon. Moreover, the peak broadening and shifting also support the argument that carbon has been deposited during time on stream. The peak being around 1330 (cm⁻¹) (D Band) validated the existence of disordered graphitic carbon [84,85], though the peak being around 1580 (cm⁻¹) (G Band) is an illustration associated to the ordered graphitic carbon.

Furthermore, SEM/EDX was used for the characterization of the spent catalyst to assess the morphological changes and elemental composition. An SEM micrograph of CaO-TiO₂ spent catalyst is presented in Figure 9a and morphological changes can be seen in comparison to the fresh catalyst, likely due to the carbon deposition on the surface of catalysts and exposure to high temperature. In addition, morphological changes can also be associated to metal oxide, and uncover atoms which may be present in a different form of adsorbed oxygen along with Ca²⁺ and Ti²⁺ ions, where uncovered oxygen atoms are responsible for H⁺ abstraction and Ca²⁺ and Ti²⁺ ions for C⁻ from the molecule of high acidity (CH₄) [86]. Further EDX spectrum presented in Figure 9b also confirmed the presence of carbon, an intense carbon peak with 3.25% by weight was observed in the EDX spectrum along with Ti and O₂ peaks.

2.4. Reaction Mechanism

Catalytic partial oxidation is a complex reaction and its mechanism for H₂ and CO formation has not been clearly elucidated yet, but there is a possibility of two different mechanisms known as direct partial oxidation (DPO) and indirect combustion and reforming reaction [87]. Catalyst component, reaction conditions, metal-support interaction and various other factors affect the reaction mechanisms [88].

In direct partial oxidation, CH₄ dissociation followed by the formation of methyl radicals, H* and C* species (CH₃*, CH₂*, CH*, H*, C*) and oxidation of adsorbed molecular oxygen is subsequently converted into syngas. Thus, syngas is the primary product in direct partial oxidation Equation (1) [89]. While in indirect combustion and reforming reaction mechanism, CO₂ and H₂O formed as primary products, which may undergo steam or dry reforming and finally be converted into syngas Equation (2) [90].

$$C{H}_4+1/2O_2\to CO+2H_2$$

(1)

$$C{H}_4+1/2O_2\to C{O}_2+H_{2}O\underset{C{H}_{4_{(unreacted)}}}{\to }CO+H_2$$

(2)

In POM heterogeneous catalysis, a solid base catalyst is generally characterized by the exposure of basic groups on the surface of the catalyst. Generally, basicity is illustrated by different metal oxides, such as CaO, TiO₂, MgO, ZrO₂, etc. [86]. Therefore, both oxides act as bases in CaO-TiO₂. Catalysts were activated for two hours before sampling in the presence of H₂ where CaO and TiO₂ donate their electron pairs through an interplay in a reduced atmosphere. Both metal oxides uncover O atoms, which may exist in different forms mainly O²⁻ surface adsorbed (SA) and O²⁻ surface lattice adsorbed (SLA) oxygen atoms [91] along with the formation of Ca²⁺ and Ti²⁺ ions Equations (3) and (4)). These positive ions or active sites are responsible for CH₄ adsorption and H₂ formation via different routes expressed in Figure 10. Uncovered O atoms are responsible for H⁺ abstraction from the molecule of high acidity (CH₄) [86] and as a result, OH⁻ ions are formed which are further converted into water and lattice oxygen (O²⁻) via OH⁻ coupling Equation (5). It is also noticeably significant that the conversion of CaO into its ions Equation (1), in which Ca²⁺ acts as Lewis acid (electrophile) and O²⁻ acts as Bronsted base. According to the definition of Lewis acid, it can accept the pair of non-bonding electrons, thus CaO reacts with a lone pair of H₂O (formed as a result of OH⁻ coupling) Equation (5) and converts into H₂ and Ca(OH)₂ Equation (6) that is quickly changed back into oxide again as a result of its thermal instability at high temperatures [39]. Lizuka et al. [92] explained the electrophilicity of Ca²⁺ cation, which is a weak acid due to its electronegativity and based on that, its conjugated base (O²⁻) shows strong basicity. Thus, it can be concluded on the basis of the above discussion that more active sites along with added lattice oxygen will be available on support material that increases the basic nature of heterogeneous catalyst for enhancing catalytic performance. Furthermore, Equation (7) represents the interaction of Ti and Ca, resulting in the formation of H₂ molecules. Syngas can also be synthesized directly by the combination of adsorbed C and H₂O Equation (8). As a result of the increased oxygen consumption, more active sites are exposed to CH₄ adsorption and increased H₂ generation via different routes. [82,83] (Figure 10).

$$CaO \to C{a}^{2+} + O^{2-} \underset{H_2}{\to } C{a}_{(metallic)}$$

(3)

$$Ti{O}_2 \to T{i}^{2+} + O^{2-} \underset{H_2}{\to } T{i}_{(metallic)}$$

(4)

$$O{H}^{-}+O{H}^{-}\to H_{2}O+O^{2-}$$

(5)

$$C{a}^{2+} + 2H_{2}O \to Ca{(OH)}_2 + H_2$$

(6)

$$Ti{O}_2+2Ca{H}_2\to T{i}^{2+}+2CaO+2H_2$$

(7)

$$C+H_{2}O\to CO+H_2$$

(8)

In combination with the literature and the above-mentioned analysis, it was observed that more active sites and uncover O atoms were generated with the involvement of both CaO and TiO₂ and these results propose direct POM over CaO-TiO₂ [82]. CO may oxidize further into CO₂ according to the following paths: Equations (9) and (10) [93]. However, it can be easily dissociated again into CO Equation (11) due to the involvement of anatase phase of TiO₂ as analyzed in XRD. It has also been explained in the literature that a low energy barrier is required for CO₂ dissociation into CO on anatase phase rather than that on a rutile phase of TiO₂ [82].

$$CO+OH\to C{O}_2+H^{+}$$

(9)

$$CO+O^{2-}\to C{O}_2$$

(10)

$$C+C{O}_2\to 2CO$$

(11)

3. Materials and Methods

3.1. Synthesis of CaO Doped TiO₂

A schematic illustration of waste derived CaO loaded TiO₂ nanostructures is depicted in (Figure 11). Titanium (IV) Tetra isopropoxide (TTIP) was used as a precursor to prepare TiO₂ sol by the sol-gel method. Analytical grade hydrochloric acid (HCl) was used for peptization and deionized (DI) water was utilized as dispersing media in TiO₂ synthesis [94]. All the chemicals were bought from Sigma Aldrich, St. Louis, Missouri, USA. Initially, the water-acid mixture was prepared and stabilized at 60 °C based on optimized methodology as reported in the literature [95]. This temperature was kept constant throughout the experiment with continuous stirring and 5 mL TTIP was added into the reaction mixture dropwise for one minute and the resulting solution was continuously stirred at 1500 rpm for 1 h. As a result, a white thick precipitate was prepared that was gradually peptized after 2 h and a clear sol was obtained [96].

At neutral pH, TiO₂ sol is chemically unstable and can undergo agglomeration when it converts into gel form. Therefore, a solution of 30–50% concentrated HCl or NH₄OH was added dropwise into the reaction mixture to keep the pH acidic (3–7) and control the instability of the catalyst. Sol was converted into gel after vigorous stirring and a white suspension was prepared with high viscosity. The resulting mixture was allowed to dry at 110 °C in the oven for 24 h and a pale-yellow powder was obtained that was further washed with an organic solvent (ethanol) and water subsequently to remove impurities, and was again dried at the same temperature to get the final product [97].

CaO was obtained from eggshells by simply grinding followed by sieving with 1 μm sieve. Finally, waste derived 1%, 2% and 3% by weight CaO loading on TiO₂ was carried out by employing a wetness impregnation method. Briefly, 1 g TiO₂ was added in water and kept on stirring for 30 min, a specified percentage of CaO was then added under continuous stirring at 1500 rpm and again continuously stirred for 5 h. A final homogenous solution was placed overnight in the oven and a final product was calcined at 850 °C for 5 h to remove the traces of impurities.

3.2. Material Characterisation

D8 Advance (Bruker Advanced, Germany) equipped with CuKα radiations of wavelength 1.5406 Å at 40 kV was used for the structural analysis of synthesized catalysts [98]. Ni filter was used for filtering background radiations. XRD scans were obtained in the range of 2θ = 10–90° with a scan rate of 2°/min. Furthermore, the crystallographic properties were ascertained by utilizing MDI JADE 6.5 software. Further crystallite size of samples was obtained with the help of the Scherrer equation (Equation (12)).

$$D_p=\frac{0.94\lambda }{\beta Cos\theta }$$

(12)

where D_p is the crystallite size, β is line broadening in radian, θ is Bragg angle and λ is X-ray wavelength.

Raman spectroscopy was performed by using BWS415-532S, USA for structural identification and carbon deposited on the surface of the catalyst during the time on stream. FTIR was carried out using a model Cary 630 (Agilent Technologies, Santa Clara, CA, USA) to investigate the chemical analysis and functional groups. Spectra were recorded in the range of 4000 to 650 cm⁻¹. TGA-DTG was used for the evaluation of thermal stability and weight loss in both fresh catalysts and spent. Experiments were carried out in TGA 500 (TA Instruments, New Castle, DE, USA) where the catalyst was purged by the N₂ flow rate of 35 mL/min for 30 min. The morphological properties and elemental analysis of fresh and spent catalyst were observed by SEM-EDX using the JEOL JSM- 6490A (JEOL Ltd., Tokyo, Japan) having 200 nm resolution.

3.3. Partial Oxidation of Methane Setup

A fixed bed thermal reactor (Parr instruments 5401, St, Moline, IL, USA) was used for methane cracking as shown in Figure 12. The experimental setup consists of three main parts including a gas feed system (GFS), vertical stainless steel fixed bed reactor (SS 316) having a length of 300 mm with an inner diameter of 12 nm, and an analysis system for product gases. Initially, the catalyst was placed in the middle of the thermal reactor and reactant gases (CH₄ (99.99%) and O₂) were allowed to pass through and mass flow controllers (Brooke instruments, Hatfield, PA, USA) were used to control the flow of the reactant gasses. Process controller (4871, Parr instruments) integrated with thermocouples and pressure gauge (2.5 bar) was used to control the temperature and pressure of the reactor, respectively. An online SCADA system was employed to monitor the flow rate and temperature as POM reaction was carried out at 750 °C with a total flow rate of 30 mL/min. Finally, CH₄ and the product gases were analyzed by gas chromatography (GC-TCD) (GC-2010 Plus, Shimadzu, Kyoto, Japan) equipped with a thermal conductivity detector (TCD) (RT-MS5A, 30 m × 0.32 mm, ID 30 μm) for the detection of different gases [37].

3.4. Calculations

The performance of catalyst for POM was analyzed based on CH₄ conversion (Equation (13)), H₂ selectivity (Equation (14)) and CO selectivity (Equation (15)) and H₂/CO ratio (Equation (16)).

$${\mathrm{CH}}_{4 }\mathrm{conversion} \left({\mathrm{X}}_{{\mathrm{CH}}_4}\right)\%=\left[\frac{{{(\mathrm{n} \mathrm{CH}}_4)}_{\mathrm{converted} }}{{{(\mathrm{n} \mathrm{CH}}_4 )}_{\mathrm{feed}}}\times 100\right]$$

(13)

$${\mathrm{H}}_2 \mathrm{selectivity} \left({\mathrm{S}}_{{\mathrm{H}}_2}\right)\%=\left[\frac{{(\mathrm{nH}}_{2 }{) }_{\mathrm{produced}} }{(2\times \mathrm{n}{\mathrm{CH}}_4{) }_{\mathrm{converted}}}\times 100\right]$$

(14)

$$\mathrm{CO} \mathrm{selectivity} \left({\mathrm{S}}_{\mathrm{CO}}\right)\%=\left[\frac{{\left(\mathrm{nCO}\right) }_{\mathrm{produced}}}{{(\mathrm{nCH}}_4{) }_{\mathrm{converted}} }\times 100 \right]$$

(15)

$${\mathrm{H}}_2/\mathrm{CO} \mathrm{ratio} =\left[\frac{\mathrm{selectivity}({\mathrm{S}}_{{\mathrm{H}}_2})}{{\mathrm{selectivity}(\mathrm{S}}_{\mathrm{CO}}) }\right]$$

(16)

4. Conclusions

TiO₂ anatase phase free from impurities and other phases such as rutile were successfully synthesized by employing the sol-gel method. Pristine TiO₂ and CaO decorated on the surface of TiO₂ by wetness impregnation method were confirmed by utilizing structural, elemental, and morphological techniques. The findings revealed that the TiO₂ nanostructures influenced the surface metal oxide deposition resulting in the fully exposed active phases. Among the catalyst, both 2% and 3% CaO loaded TiO₂ have shown higher conversion efficiency in comparison to pure and 1% CaO loaded TiO₂. No distinct change in conversion efficiencies for 2% and 3% were observed confirming that an optimal loading is achieved. Finally, 2% and 3% CaO loaded TiO₂ have shown the average conversion efficiency of 83.56 and 79.54 and H₂/CO ratio 2.25 and 2.28, respectively. These catalysts prepared during this study have the advantage of a low cost and simple preparation method of TiO₂, even at low temperatures along with high catalytic activity for syngas production via the POM route.