Performance Analysis of TiO 2 -Modiﬁed Co/MgAl 2 O 4 Catalyst for Dry Reforming of Methane in a Fixed Bed Reactor for Syngas (H 2 , CO) Production

: Co/TiO 2 –MgAl 2 O 4 was investigated in a ﬁxed bed reactor for the dry reforming of methane (DRM) process. Co/TiO 2 –MgAl 2 O 4 was prepared by modiﬁed co-precipitation, followed by the hydrothermal method. The active metal Co was loaded via the wetness impregnation method. The prepared catalyst was characterized by XRD, SEM, TGA, and FTIR. The performance of Co/TiO 2 – MgAl 2 O 4 for the DRM process was investigated in a reactor with a temperature of 750 ◦ C, a feed ratio (CO 2 /CH 4 ) of 1, a catalyst loading of 0.5 g, and a feed ﬂow rate of 20 mL min − 1 . The effect of support interaction with metal and the composite were studied for catalytic activity, the composite showing signiﬁcantly improved results. Moreover, among the tested Co loadings, 5 wt% Co over the TiO 2 –MgAl 2 O 4 composite shows the best catalytic performance. The 5%Co/TiO 2 –MgAl 2 O 4 improved the CH 4 and CO 2 conversion by up to 70% and 80%, respectively, while the selectivity of H 2 and CO improved to 43% and 46.5%, respectively. The achieved H 2 /CO ratio of 0.9 was due to the excess amount of CO produced because of the higher conversion rate of CO 2 and the surface carbon reaction with oxygen species. Furthermore, in a time on stream (TOS) test, the catalyst exhibited 75 h of stability with signiﬁcant catalytic activity. Catalyst potential lies in catalyst stability and performance results, thus encouraging the further investigation and use of the catalyst for the long-run DRM process. selectivity and yield. It is associated to good metal–support interactions that contributed to activity and provided good stability [39]. Due to its basic nature, the presence of the MgAl 2 O 4 support in the composite resulted in CO 2 adsorption and dissociation that produced CO, while the cobalt in the composite aided CH 4 activation to produce more H 2 and surface carbon. The TiO 2 support played role in the reaction of CoTiO 3 species with surface carbon to avoid coke deposition.


Introduction
The rising concerns over greenhouse gas (GHG) emissions on global warming and climate change has motivated many industrial production facilities to reconsider plans on how to effectively control and recycle GHGs to produce synthetic fuels [1,2]. Synthesis gas (mainly made of H 2 and CO) is a vital fuel gas mixture currently being employed as an alternative to petroleum-based fuels. Syngas is considered the primary feedstock in the production of some important liquid fuels, such as ammonia and methanol, and it can also The DRM has some notable disadvantages, as it promotes reverse water-gas shift reaction (RWGS) and carbon formation via methane decomposition [14,15]. RWGS mainly occurs when the H 2 to CO ratio becomes less than unity. The carbon formation is expressed by the famous Boudouard reaction, which can result in the deactivation of catalysts [16,17] Equations (2) and (3).
2 CO → C + CO 2 ∆H It is understood that carbon formation has adverse effects on the reaction mechanism and results in carbon deposition, thus leading to the deactivation of catalysts that affect the reaction lifetime and altering the H 2 /CO ratio [18,19]. To have a better understanding of coke formation, understanding the thermodynamics of the DRM is important for commercial DRM process implementation. Thus, the development of a catalyst with a high catalytic performance and resistance to coking is an important objective [20]. In this regard, catalyst performance depends on active metal and support properties, their interaction, and their catalytic behaviour.
In the catalytic DRM, noble metals like rhodium (Rh), palladium (Pd), and ruthenium (Ru) are recognised as stable catalysts that can inhibit carbon formation but are commercially unviable [21][22][23]. Nickel (Ni) is the most widely used commercially active metal due to its availability and good catalytic activity. The carbon deposition rate and sintering of the Ni catalyst still need to be addressed in the DRM [24][25][26]. Cobalt (Co) exhibits a better stability to carbon deposition, but its performance in the DRM must be improved [27][28][29][30]. For instance, El et al. [31] investigated the effects of confinement on the catalytic performance of two silica supports (SiO 2 and SBA-15) on Co loading. The results indicated that Co/SBA-15 showed stability against the sintering of mesopores during the DRM and that the addition of rhodium to Co/SBA-15 led to increased stability and activity with less carbon coking.
The effect of the support on the catalyst depends on the nature of the support, which can play an essential role in the activity and stability of the catalyst [32]. MgAl 2 O 4 is an excellent support material with a high sintering resistance, good hydrothermal stability, and excellent mechanical strength [3,14,33]. During the DRM, CO 2 dissociation and adsorption proceed at a faster rate with the use of basic support, and, in this regard, MgAl 2 O 4 can be used as a support material due to its basic nature and high surface area [34][35][36]. DRMs done using different Ni loadings on nanocrystalline MgAl 2 O 4 (with its high surface area) have shown high catalytic activity and stability, with an increase in carbon deposition following the increase in Ni loading [14,37].
Bi-support catalysts can be very effective in improving mechanical properties and catalytic performance. For example, TiO 2 provided some favourable properties for the DRM [38]. The strong metal-support interaction between the TiO 2 support and metal exhibited a high resistance towards coke deposition while increasing catalyst activity and stability. The activity and stability of TiO 2 can be improved using a structured TiO 2 -based catalyst [38,39]. In catalysis, a higher catalytic activity has been observed with the use of metal supported on TiO 2 nanowires. The use of a low-temperature SRM using Ni/TiO 2 as a catalyst revealed that stronger metal-support interactions assisted low-temperature methane activation while Ni species aided in hydrogen production via WGSR. It was reported that Ni/TiO 2 showed an enhanced coking resistance [40]. Though TiO 2 nanowires have been investigated for other photocatalytic and fuel cell applications, their effect on the DRM with a combination of the Co transition metal and added bi-support has yet to be investigated [41]. The combination of TiO 2 and MgAl 2 O 4 as co-supports and Co as an active metal could enhance the CH 4 and CO 2 activation, resist carbon formation, and aid stability.
In this study, the catalytic performance of a TiO 2 nanowire-modified Co/MgAl 2 O 4 catalyst was investigated for the DRM in a fixed bed reactor. Co/TiO 2 -MgAl 2 O 4 was synthesised by the co-precipitation process, the hydrothermal method, and the wetness impregnation method. The catalyst was characterised using various techniques such as XRD, SEM, EDS, TGA, and FTIR. The catalyst was tested for the DRM activity and the effect of metal loading, and stability tests were conducted for longer times on stream (TOS). Furthermore, the spent catalyst was characterised by XRD, SEM-EDS, and TGA to analyse the carbon formation during the TOS and to present a possible reaction mechanism.

Synthesis of MgAl 2 O 4 and TiO 2 Nanoparticles
To prepare the MgAl 2 O 4 spinel, a modified co-precipitation method followed by the hydrothermal process was used, as shown in Figure 1a. A stoichiometric amount of Mg (NO 3 ) 2 ·6H 2 O (99.99%, Sigma Aldrich) and Al (NO 3 ) 3 ·9H 2 O (98%, Honeywell Fluka) was added in deionised water and set to mild stirring conditions until a homogeneous solution formed. An ammonia solution (32%) was used as a precipitating agent, which was added dropwise into the solution with continuous stirring to maintain a pH of at 10.5 because higher PH would have resulted in a lower crystallite size and a higher specific surface area [42]. The stirring and heating continued until the stabilisation of the pH at 10.5 and the appearance of a milky white solution. The solution was then transferred to a hydrothermal autoclave maintained at 160 • C for 24 h and left to cool at room temperature overnight. The precipitates were separated using a centrifuge and washed with warm DI water and absolute ethanol until the pH of the solution reached 7.0 [14]. The slurry was dried at 110 • C overnight. The prepared sample was a ground and calcined in a furnace maintained at 800 • C for 5 h in static air. The collected sample was ground until fine MgAl 2 O 4 particles were formed.
In the typical synthesis of TiO 2 nanowires (NWs), 0.70 g of titanium (IV) oxide (99%, Sigma-Aldrich) nano-powder were added in 70 mL of a 10 M solution of NaOH and continuously stirred until the formation of a homogeneous solution. The solution was then shifted to a hydrothermal autoclave, heated to 160 • C for 5 h, and allowed to cool at room temperature. The solution was centrifuged to collect the white precipitates, and then it was washed with a 0.1 M HCl solution and warm deionised water several times to bring the pH to 7.0 [43]. The sample was oven-dried at 110 • C, followed by calcination at 500 • C to obtain a fine white powder.

Preparation of Co/TiO 2 -MgAl 2 O 4 Nanocomposite
A schematic representation of Co/TiO 2 -MgAl 2 O 4 nanocomposite preparation is shown in Figure 1a. For the preparation of the Co/TiO 2 -MgAl 2 O 4 nanocomposite, the wetness impregnation method was used with the addition of Co(NO 3   In the typical synthesis of TiO2 nanowires (NWs), 0.70 g of titanium (IV) oxide (99%, Sigma-Aldrich) nano-powder were added in 70 mL of a 10 M solution of NaOH and continuously stirred until the formation of a homogeneous solution. The solution was then shifted to a hydrothermal autoclave, heated to 160 °C for 5 hrs, and allowed to cool at room temperature. The solution was centrifuged to collect the white precipitates, and then it was washed with a 0.1 M HCl solution and warm deionised water several times to bring the pH to 7.0 [43]. The sample was oven-dried at 110 °C, followed by calcination at 500 °C to obtain a fine white powder.

Preparation of Co/TiO2-MgAl2O4 Nanocomposite
A schematic representation of Co/TiO2-MgAl2O4 nanocomposite preparation is shown in Figure 1a. For the preparation of the Co/TiO2-MgAl2O4 nanocomposite, the wetness impregnation method was used with the addition of Co(NO3)2.6H2O (98%, Merck) on MgAl2O4 and TiO2 NW supports. In catalyst preparation, 1.0 g of MgAl2O4 and 0.1 g of TiO2 NWs were dispersed in a 0.1 M Co(NO3)2.6H2O solution and allowed to stir for 5 hrs at 110 °C. The resulting slurry was oven-dried at 110 °C overnight and calcined at 750 °C for 5 hrs. The prepared sample was named 5%Co/TiO2-MgAl2O4. Similarly, composite samples of 2.5 and 7.5% Co loading were prepared.

Materials Characterisation
The crystalline structure and phase transition of the prepared catalysts were analysed by the X-ray diffraction method. The identification of the peaks of the calcined and ground samples was carried out via a Bruker D8 advanced X-ray diffractometer with an irradiation wavelength of 1.5418 Å at 40 kV and 40 mA operating conditions. XRD patterns were obtained using diffraction angles over the range of 5-90° with a step size of 0.05°. The Scherrer equation was used to estimate the average crystallite size [44].
The surface morphology of the prepared fresh samples and the spent samples were examined by SEM. To obtain the micro-level images of the samples, a JSM-6490A JEOL

Materials Characterisation
The crystalline structure and phase transition of the prepared catalysts were analysed by the X-ray diffraction method. The identification of the peaks of the calcined and ground samples was carried out via a Bruker D8 advanced X-ray diffractometer with an irradiation wavelength of 1.5418 Å at 40 kV and 40 mA operating conditions. XRD patterns were obtained using diffraction angles over the range of 5-90 • with a step size of 0.05 • . The Scherrer equation was used to estimate the average crystallite size [44].
The surface morphology of the prepared fresh samples and the spent samples were examined by SEM. To obtain the micro-level images of the samples, a JSM-6490A JEOL SEM (Japan) was used. The resolution of the microscope was set to 3 nm, and it was operated at 30 kV. EDX was used to analyse the elemental composition of catalyst.
TGA was performed using a TGA 5500 from TA Instruments (USA) to observe the thermal stability of the fresh catalyst and the amount of carbon deposited on spent samples by maintaining a nitrogen flow of 25 mL min −1 and a heating rate of 10 • C min −1 to a maximum temperature at 900 • C. The weight of the sample used for the analysis was 10 mg. The percentage of weight loss was observed at different temperatures based on the reactive carbon.
FTIR was used to identify the functional groups in the sample and the interactions between them. The FTIR analysis was carried out on a Cary 630 FTIR (Agilent Technologies, USA), with the wavenumber ranging from 4000 to 650 cm −1 .

Experimental Setup
The experimental setup scheme for the DRM is illustrated in Figure 2. The fixed bed reactor of PARR (Model # LSP-2.38-0-32-1C-2335EEE, Moline USA) was used for the DRM process. The reactor consisted of a 2.5 ft SS-316 material tube with a 1 2 inch diameter. Th tube openings were fixed with SS RED union at both ends and connected to a gas mixer. The supply of the feed gas was controlled with digital MF4600 series mass flow controllers. The reactor temperature was controlled by the 4871 Series Process Controllers. The K-type thermocouple was used to control the temperature of the catalyst bed. The catalyst was sandwiched in between the glass wool and kept in the centre of the reactor using suitable support. Before each experiment, 60 mL min −1 of N 2 were introduced to purge the reactor system, and 20 mL min −1 of H 2 in the presence of 60 mL min −1 of N 2 gas were used to reduce the catalyst at 750 • C for 1 h by activating the active sites due to the requirement of higher conversions and longer stability runs for the DRM. During experimental runs, the mass flow rates of the feed gas CH 4 (99.99%) and CO 2 (99.99%) were controlled at 10 mL min −1 each and then introduced into the reactor that was set at 750 • C and 0.5 g of catalyst loading. The reaction products were allowed to pass through the condenser before gas analysis. The gases were analysed with a gas chromatograph (GC 2010 plus Shimadzu) equipped with a thermal conductivity detector (TCD) [45]. The temperature of the GC column was set at 200 • C, while the peak retention time was set for 10 min.

Experimental Setup
The experimental setup scheme for the DRM is illustrated in Figure 2. The fixed bed reactor of PARR (Model # LSP-2.38-0-32-1C-2335EEE, Moline USA) was used for the DRM process. The reactor consisted of a 2.5 ft SS-316 material tube with a ½ inch diameter. Th tube openings were fixed with SS RED union at both ends and connected to a gas mixer. The supply of the feed gas was controlled with digital MF4600 series mass flow controllers. The reactor temperature was controlled by the 4871 Series Process Controllers. The K-type thermocouple was used to control the temperature of the catalyst bed. The catalyst was sandwiched in between the glass wool and kept in the centre of the reactor using suitable support. Before each experiment, 60 mL min −1 of N2 were introduced to purge the reactor system, and 20 mL min −1 of H2 in the presence of 60 mL min −1 of N2 gas were used to reduce the catalyst at 750 °C for 1 hr by activating the active sites due to the requirement of higher conversions and longer stability runs for the DRM. During experimental runs, the mass flow rates of the feed gas CH4 (99.99%) and CO2 (99.99%) were controlled at 10 mL min −1 each and then introduced into the reactor that was set at 750 °C and 0.5 g of catalyst loading. The reaction products were allowed to pass through the condenser before gas analysis. The gases were analysed with a gas chromatograph (GC 2010 plus Shimadzu) equipped with a thermal conductivity detector (TCD) [45]. The temperature of the GC column was set at 200 °C, while the peak retention time was set for 10 min.

Catalytic Activity Calculations
Catalytic activity tests were conducted to analyse the performance and stability of the DRM catalyst in the fixed bed reactor. The DRM activity test included reactant conversion, selectivity, and yield, as presented in Equations (4)-(10). The term 'n' represents the number of moles in the following equations.

Characterization of Catalyst
The XRD of the prepared samples is shown in Figure 3.  [46,47]. A rutile phase with a tetragonal symmetry was detected for TiO 2 (PDF:21-1276) diffraction peaks, with the major phase (211) observed at 54.32 • and a d-spacing of 0.16 nm [48]. Upon adding cobalt with MgAl 2 O 4 , some peaks observed in MgAl 2 O 4 XRD went missing due to the merging of peaks with the neighbour peaks. Similarly, cubic phase CoAl 2 O 4 (PDF:44-0160) showed a major peak at 36.7 • (hkl; 311) with a d-spacing 0.244 nm and a space group of 227:Fd3m, while the other peaks observed at 31.74 • and 44.63 • with the corresponding planes of (220) and (400), respectively [49]. Diffraction peaks were observed at 31.2 • and 36.8 • with planes (220) and (311), respectively, with a space group of 227:Fd3m of the cubic phase, thus indicating the presence of Co 3 O 4 (PDF:43-1003) [50]. CoTiO 3 (PDF:15-0866), with a hexagonal phase with a major peak observed at 32.8 • (hkl; 104) and a d-spacing of 0.27 nm, showed the space group of R-3(148) [51]. An SEM micrograph of MgAl2O4 particles is shown in Figure 4a,b, which indicates the formation of porous-structure, block-shaped crystallites depicting the sintering of particles because of the higher temperature calcination. Figure 4c,d demonstrates the formation of mixed, very fine TiO2 particles and TiO2 nanowires due to the hydrothermal method [52]. Figure 4e,f indicates the modification of MgAl2O4 particles, while Co and TiO2 dispersed over the surface and extended to the pores, ultimately showing a mixed An SEM micrograph of MgAl 2 O 4 particles is shown in Figure 4a,b, which indicates the formation of porous-structure, block-shaped crystallites depicting the sintering of particles because of the higher temperature calcination. Figure 4c,d demonstrates the formation of mixed, very fine TiO 2 particles and TiO 2 nanowires due to the hydrothermal method [52].  Figure 5a-c. The possible compounds are present in the spectrum, and the extra peaks are associated with carbon tape and gold, that were added during coating before the EDX analysis. The Co loading was confirmed by EDX, as it was presented in the composite.   Figure 6 shows the thermal stability of the prepared samples. A total of 5% weight loss was observed in the MgAl2O4 sample TGA analysis, as shown in Figure 6a. A weight loss of 3.5% was observed up to 250 °C because the material was subjected to the removal  loss of 3.5% was observed up to 250 • C because the material was subjected to the removal of the adsorbed moisture and the combustion of nitrates that went unreacted during the reaction, while the rest of the weight loss was due to the dihydroxylation of the mixed oxide to oxides [14]. Furthermore, the TGA analysis of the TiO 2 sample showed a minimal weight loss of up to 2%, thus exhibiting the stability of the sample, and the weight loss was due to adsorbed moisture removal and the decomposition of organic compounds (as presented in Figure 6b), while the slight weight gain after 600 • C was attributed to the rearrangement of the particles after impurity removal and reaction completion. The TGA of the fresh Co/TiO 2 -MgAl 2 O 4 nanocomposite showed a gradual weight loss up to 5% with the increase in temperature, most of which was rooted in moisture removal, while the sudden weight loss at a high temperature was due to the decomposition of bonds and the formation of the intermediate complexes of a composite, as presented in Figure 6c [14].
Energies 2021, 14, x FOR PEER REVIEW 10 of 2 of the adsorbed moisture and the combustion of nitrates that went unreacted during the reaction, while the rest of the weight loss was due to the dihydroxylation of the mixed oxide to oxides [14]. Furthermore, the TGA analysis of the TiO2 sample showed a minima weight loss of up to 2%, thus exhibiting the stability of the sample, and the weight los was due to adsorbed moisture removal and the decomposition of organic compounds (a presented in Figure 6b), while the slight weight gain after 600 °C was attributed to the rearrangement of the particles after impurity removal and reaction completion. The TGA of the fresh Co/TiO2-MgAl2O4 nanocomposite showed a gradual weight loss up to 5% with the increase in temperature, most of which was rooted in moisture removal, while the sudden weight loss at a high temperature was due to the decomposition of bonds and the formation of the intermediate complexes of a composite, as presented in Figure 6c [14] The FTIR analysis of the prepared fresh samples is shown in Figure 7. The IR spec trum of calcined MgAl2O4 observed at 912 cm −1 , which corresponded to the stretching vi bration of Mg-O-Al falling into inorganic bands and Al-O bonds, was indicative of th crystal MgAl2O4 spinel [53][54][55]. With Co/TiO2-MgAl2O4, two peaks identified at 900 cm − corresponded to the vibration of Mg-O-Al, where the peak identified at 744 cm −1 corre sponded to O-Ti-O bond stretching vibrations depicting an anatase morphology [56,57 because the band range of 500-800 cm −1 followed the anatase crystal vibration modes [58] The presence of a small peak around 850 cm −1 corresponded to O-Co-O, which indicated the formation of Co3O4 [59]. In the IR spectrum of TiO2, the small peak was slightly shifted below 700 cm −1 , which could have been related to the stretching vibration of Ti-O bond ing.  [56,57] because the band range of 500-800 cm −1 followed the anatase crystal vibration modes [58]. The presence of a small peak around 850 cm −1 corresponded to O-Co-O, which indicated the formation of Co 3 O 4 [59]. In the IR spectrum of TiO 2 , the small peak was slightly shifted below 700 cm −1 , which could have been related to the stretching vibration of Ti-O bonding.

Dry Reforming of Methane (DRM)
The prepared fresh catalyst samples were loaded into a fixed bed reactor to conduct activity and stability tests during the DRM process. The feed gas ratio was set to 1, with a flow rate of 20 mL min −1 , and it was fed to reactor maintained at 750 °C with a catalyst loading of 0.5 g.

Catalyst Activity Test
DRM activity test results for the MgAl2O4 and composites with different Co loading are shown in Figure 8a-c. For MgAl2O4, the conversions of CH4 and CO2 were recorded at 61% and 68.5%, respectively. The selectivities of H2 and CO were observed at 17.5% and 26.5%, respectively, whereas the yields of H2 and CO were calculated at 11% and 17%, respectively. The CH4 conversion was observed to be less than the CO2 conversion, while the higher CO2 conversion could be attributed to the basic nature of the support that caused the activation of CO2 [60]. For the Co/TiO2-MgAl2O4, the increase in the conversion of CH4 and CO2 with Co loading could be associated with Co loading that showed a good interaction that formed CoTiO3 and CoAl2O4 with supports, as depicted by the XRD analysis. The higher activity results were due to the higher amount of reduced surface Co sites with increases in cobalt loading [61]. The higher activity results coming with increases in Co loading corresponded with the reduction of Co3O4 at lower temperatures, and XRD analysis confirmed the presence of Co3O4 species. The conversions of CH4 and CO2 were recorded at 68% and 73%, respectively for the composite of 2.5% depicted in Figure 8a. In contrast, 5% of Co loading resulted in conversions of 73% and 78% for CH4 and CO2, respectively, while 7.5% of Co loading resulted in 76% and 83% yields of CH4 and CO2, respectively. A similar trend of increase was shown for H2 and CO selectivity at 2.5-7.5% of Co loading. The highest H2 and CO selectivities recorded for 7.5% of Co loading were 43% and 46.5%, respectively, as presented in Figure 8b. The yields of H2 and CO remained 18% and 22%, respectively, for 2.5% of Co loading, and they were highest at 33% and 37%, respectively, for the 7.5% Co-loaded composite, as demonstrated in Figure 8b.

Dry Reforming of Methane (DRM)
The prepared fresh catalyst samples were loaded into a fixed bed reactor to conduct activity and stability tests during the DRM process. The feed gas ratio was set to 1, with a flow rate of 20 mL min −1 , and it was fed to reactor maintained at 750 • C with a catalyst loading of 0.5 g.

Catalyst Activity Test
DRM activity test results for the MgAl 2 O 4 and composites with different Co loading are shown in Figure 8a-c. For MgAl 2 O 4 , the conversions of CH 4 and CO 2 were recorded at 61% and 68.5%, respectively. The selectivities of H 2 and CO were observed at 17.5% and 26.5%, respectively, whereas the yields of H 2 and CO were calculated at 11% and 17%, respectively. The CH 4 conversion was observed to be less than the CO 2 conversion, while the higher CO 2 conversion could be attributed to the basic nature of the support that caused the activation of CO 2 [60]. For the Co/TiO 2 -MgAl 2 O 4 , the increase in the conversion of CH 4 and CO 2 with Co loading could be associated with Co loading that showed a good interaction that formed CoTiO 3 and CoAl 2 O 4 with supports, as depicted by the XRD analysis. The higher activity results were due to the higher amount of reduced surface Co sites with increases in cobalt loading [61]. The higher activity results coming with increases in Co loading corresponded with the reduction of Co 3 O 4 at lower temperatures, and XRD analysis confirmed the presence of Co 3 O 4 species. The conversions of CH 4 and CO 2 were recorded at 68% and 73%, respectively for the composite of 2.5% depicted in Figure 8a. In contrast, 5% of Co loading resulted in conversions of 73% and 78% for CH 4 and CO 2 , respectively, while 7.5% of Co loading resulted in 76% and 83% yields of CH 4 and CO 2 , respectively. A similar trend of increase was shown for H 2 and CO selectivity at 2.5-7.5% of Co loading. The highest H 2 and CO selectivities recorded for 7.5% of Co loading were 43% and 46.5%, respectively, as presented in Figure 8b. The yields of H 2 and CO remained 18% and 22%, respectively, for 2.5% of Co loading, and they were highest at 33% and 37%, respectively, for the 7.5% Co-loaded composite, as demonstrated in Figure 8b. For the low Co-loaded composite, relatively lower conversions of CH4 and CO2 were observed, primarily due to the presence of CoAl2O4 species, as observed in the XRD analysis, and these were reducible at higher temperatures. However, higher Co-loaded composites had comparatively more easily reducible species, thus resulting in higher activity [62]. Though the higher Co loading adhered with the issue of carbon deposition because the larger amount of available carbon species led to deactivation, in this case, the highest conversions of the 7.5% Co-loaded composite was because of the balance provided by the higher CO2 conversion that provided more oxygen species to the excess carbon formed by the CH4 decomposition [27]. The higher conversion rates of CO2 to CO and oxygen species due to fast reactions resulted in the reaction of the available oxygen species and the high amount of carbon produced with the CH4 decomposition for the highly loaded Co. The result was more CO produced in comparison to H2, as shown by the produced syngas ratio. The 7.5% Co-loaded composite, however, showed a gradual decrease in activity when tested for more extended-run stability tests. However, possible issues connected with high Co-loaded composites include the formation of cobalt clusters that affect support surface properties, and unfavourable gas adsorption conditions with the long-term stability analysis of the 7.5% Co-loaded composite led to the deactivation of the catalyst and, consequently, a lower catalytic activity with time [63]. Thus, considering the better activity and stability results, further tests for conversion, selectivity, yield, and a syngas For the low Co-loaded composite, relatively lower conversions of CH 4 and CO 2 were observed, primarily due to the presence of CoAl 2 O 4 species, as observed in the XRD analysis, and these were reducible at higher temperatures. However, higher Co-loaded composites had comparatively more easily reducible species, thus resulting in higher activity [62]. Though the higher Co loading adhered with the issue of carbon deposition because the larger amount of available carbon species led to deactivation, in this case, the highest conversions of the 7.5% Co-loaded composite was because of the balance provided by the higher CO 2 conversion that provided more oxygen species to the excess carbon formed by the CH 4 decomposition [27]. The higher conversion rates of CO 2 to CO and oxygen species due to fast reactions resulted in the reaction of the available oxygen species and the high amount of carbon produced with the CH 4 decomposition for the highly loaded Co. The result was more CO produced in comparison to H 2 , as shown by the produced syngas ratio. The 7.5% Co-loaded composite, however, showed a gradual decrease in activity when tested for more extended-run stability tests. However, possible issues connected with high Co-loaded composites include the formation of cobalt clusters that affect support surface properties, and unfavourable gas adsorption conditions with the long-term stability analysis of the 7.5% Co-loaded composite led to the deactivation of the catalyst and, consequently, a lower catalytic activity with time [63]. Thus, considering the better activity and stability results, further tests for conversion, selectivity, yield, and a syngas ratio of over 75 h longer-run stability tests were only conducted for the 5% Co-loaded composite.
To better understand the role of the supports and active metal, as well as their effects on a fully developed catalyst, different catalysts like MgAl 2 O 4 , TiO 2 , Co/MgAl 2 O 4 , Co/TiO 2 , and Co/TiO 2 -MgAl 2 O 4 were tested for 5 h TOS for CH 4 and CO 2 conversions, as shown in Figure 9a,b, respectively. Additionally, the H 2 and CO selectivity and the H 2 and CO yield for the prepared catalysts are presented in Figure 10a,b and Figure 11a,b, respectively. The results indicated that TiO 2 resulted in the lowest CH 4 and CO 2 conversion, and the lowest selectivity and yield of H 2 and CO compared to other catalysts because TiO 2 , in its pure form in the absence of metal, does not have a significant effect on conversion at higher temperatures but provides good metal-support interactions, as indicated by the formation of a CoTiO 3 intermediate while loading the Co and the provision of good stability [64]. For MgAl 2 O 4 support, CH 4 conversion averaged 61%, while CO 2 conversion averaged approximately 70% over-range during the 5 h TOS, which indicated that inherent basic nature of the MgAl 2 O 4 spinel provided good support and the combined use of Mg and Al could achieve good thermal stability [65]. The basic support aided the dissociation of CO 2 [66]. The results indicated that the use of an MgAl 2 O 4 support could provide decent catalytic results. The lower conversion, selectivity, and yield corresponding to Co/TiO 2 were due to the metallic Co oxidation because the oxidation species (CO 2 ) caused catalyst deactivation [67,68]. Co/MgAl 2 O 4 showed intermediate conversions of CH 4 and CO 2 , with average values of 65% and 72%, respectively, which could mainly be associated with the formation of CoAl 2 O 4 , as discussed in the XRD analysis of Co/MgAl 2 O 4 , and was the reason for the inhibition of carbon deposition [69]. The 5% Co/TiO 2 -MgAl 2 O 4 composite showed the maximum conversions of CH 4 and CO 2 , along with maximum H 2 and CO selectivity and yield. It is associated to good metal-support interactions that contributed to activity and provided good stability [39]. Due to its basic nature, the presence of the MgAl 2 O 4 support in the composite resulted in CO 2 adsorption and dissociation that produced CO, while the cobalt in the composite aided CH 4 activation to produce more H 2 and surface carbon. The TiO 2 support played role in the reaction of CoTiO 3 species with surface carbon to avoid coke deposition. ratio of over 75 hr longer-run stability tests were only conducted for the 5% Co-loaded composite.
To better understand the role of the supports and active metal, as well as their effects on a fully developed catalyst, different catalysts like MgAl2O4, TiO2, Co/MgAl2O4, Co/TiO2, and Co/TiO2ºMgAl2O4 were tested for 5 hr TOS for CH4 and CO2 conversions, as shown in Figure 9a,b, respectively. Additionally, the H2 and CO selectivity and the H2 and CO yield for the prepared catalysts are presented in Figures 10a,b and 11a,b, respectively. The results indicated that TiO2 resulted in the lowest CH4 and CO2 conversion, and the lowest selectivity and yield of H2 and CO compared to other catalysts because TiO2, in its pure form in the absence of metal, does not have a significant effect on conversion at higher temperatures but provides good metal-support interactions, as indicated by the formation of a CoTiO3 intermediate while loading the Co and the provision of good stability [64]. For MgAl2O4 support, CH4 conversion averaged 61%, while CO2 conversion averaged approximately 70% over-range during the 5 hr TOS, which indicated that inherent basic nature of the MgAl2O4 spinel provided good support and the combined use of Mg and Al could achieve good thermal stability [65]. The basic support aided the dissociation of CO2 [66]. The results indicated that the use of an MgAl2O4 support could provide decent catalytic results. The lower conversion, selectivity, and yield corresponding to Co/TiO2 were due to the metallic Co oxidation because the oxidation species (CO2) caused catalyst deactivation [67,68]. Co/MgAl2O4 showed intermediate conversions of CH4 and CO2, with average values of 65% and 72%, respectively, which could mainly be associated with the formation of CoAl2O4, as discussed in the XRD analysis of Co/MgAl2O4, and was the reason for the inhibition of carbon deposition [69]. The 5% Co/TiO2-MgAl2O4 composite showed the maximum conversions of CH4 and CO2, along with maximum H2 and CO selectivity and yield. It is associated to good metal-support interactions that contributed to activity and provided good stability [39]. Due to its basic nature, the presence of the MgAl2O4 support in the composite resulted in CO2 adsorption and dissociation that produced CO, while the cobalt in the composite aided CH4 activation to produce more H2 and surface carbon. The TiO2 support played role in the reaction of CoTiO3 species with surface carbon to avoid coke deposition.

Stability Analysis of Composite
The 5%Co/TiO2-MgAl2O4 was tested for stability analysis over the reaction time of 75 hrs under the same test conditions. Figure 12a demonstrates that the stability test showed a gradual increase in CO2 and CH4 conversion. The gradual rise was possibly due to the activation of some deposited carbon species. The selectivity of H2 and CO, as presented in Figure 12b, showed the same trend with a gradual rise and then stabilisation for the next phase. The stability in activity was noticed for 75 hrs on TOS.

Stability Analysis of Composite
The 5%Co/TiO2-MgAl2O4 was tested for stability analysis over the reaction time of 75 hrs under the same test conditions. Figure 12a demonstrates that the stability test showed a gradual increase in CO2 and CH4 conversion. The gradual rise was possibly due to the activation of some deposited carbon species. The selectivity of H2 and CO, as presented in Figure 12b, showed the same trend with a gradual rise and then stabilisation for the next phase. The stability in activity was noticed for 75 hrs on TOS.

Stability Analysis of Composite
The 5%Co/TiO 2 -MgAl 2 O 4 was tested for stability analysis over the reaction time of 75 h under the same test conditions. Figure 12a demonstrates that the stability test showed a gradual increase in CO 2 and CH 4 conversion. The gradual rise was possibly due to the activation of some deposited carbon species. The selectivity of H 2 and CO, as presented in Figure 12b, showed the same trend with a gradual rise and then stabilisation for the next phase. The stability in activity was noticed for 75 h on TOS. The yield of H2 and CO, as presented in Figure 13, showed similar trends to that of selectivity for the 75 hr TOS. The H2/CO ratio ( Figure 13) value below unity and close to 0.9 was estimated for the 75 hr TOS stability test. The H2/CO ratio below unity showed a lower amount of carbon formation and good stability, as a relatively greater amount of CO was produced. The reasons for greater CO production were the faster reaction of carbon with CO2 than CH4 dissociation and the reaction of intermediate CoTiO3 with carbon. Additionally, the possible reaction of H2O produced due to the RWGS reaction (though was not physically observed) with C produced more H2 and CO [70].  The yield of H 2 and CO, as presented in Figure 13, showed similar trends to that of selectivity for the 75 h TOS. The H 2 /CO ratio ( Figure 13) value below unity and close to 0.9 was estimated for the 75 h TOS stability test. The H 2 /CO ratio below unity showed a lower amount of carbon formation and good stability, as a relatively greater amount of CO was produced. The reasons for greater CO production were the faster reaction of carbon with CO 2 than CH 4 dissociation and the reaction of intermediate CoTiO 3 with carbon. Additionally, the possible reaction of H 2 O produced due to the RWGS reaction (though was not physically observed) with C produced more H 2 and CO [70]. The yield of H2 and CO, as presented in Figure 13, showed similar trends to that o selectivity for the 75 hr TOS. The H2/CO ratio ( Figure 13) value below unity and close to 0.9 was estimated for the 75 hr TOS stability test. The H2/CO ratio below unity showed a lower amount of carbon formation and good stability, as a relatively greater amount o CO was produced. The reasons for greater CO production were the faster reaction of car bon with CO2 than CH4 dissociation and the reaction of intermediate CoTiO3 with carbon Additionally, the possible reaction of H2O produced due to the RWGS reaction (though was not physically observed) with C produced more H2 and CO [70].

Characterisation of Spent Catalyst
The spent catalyst collected after 75 h of TOS was further analysed and characterised by XRD, TGA, SEM, and EDX. The results are shown in Figures 14 and 15 to investigate the carbon formation over the catalyst. Figure 14a shows that the XRD pattern of the spent catalyst confirmed the presence of the same crystalline phases as those observed in the fresh composite sample, having the same peaks of the MgAl 2 O 4 spinel (PDF#21-1152) with a main peak at 36.85 • (hkl; 311). TiO 2 (PDF#21-1276) was also observed as the same rutile phase with tetragonal geometry, and its main peak was slightly shifted at 51.8 • (hkl; 211). A slight shift in the main peak of CoAl 2 O 4 (PDF#44-0160) was observed at 36.0 • (hkl; 311), while Co 3 O 4 (PDF#43-1003) was identified in the fresh sample with peaks at 31.2 • (hkl; 220) and 36.8 • (hkl; 311). CoTiO 3 (PDF#15-0866) indicated a peak at 32.8 • (hkl; 104), as observed in the fresh sample. The graphite carbon (PDF#41-1487) with a hexagonal geometry confirmed the peak at 26.3 • (hkl; 002) with a d-spacing of 0.337 nm [71]. The TGA profile of the spent catalyst, as shown in Figure 14b, was analysed over three different temperature regions. Column I showed a total 4-5% weight loss up to 300 • C, which could be attributed to the moisture removal and other volatile species [72], where a significant weight loss of almost 14% was observed in column II in a temperature range of 300-500 • C due to the presence of reactive carbon species such as α-C and β-C [73]. Similarly, column III showed a 2-3% weight loss above 500 • C, which could be ascribed to a lower amount of filamentous carbon (γ-C) formation [3].

Characterisation of Spent Catalyst
The spent catalyst collected after 75 hrs of TOS was further analysed and characterised by XRD, TGA, SEM, and EDX. The results are shown in Figures 14 and 15 to investigate the carbon formation over the catalyst. Figure 14a shows that the XRD pattern of the spent catalyst confirmed the presence of the same crystalline phases as those observed in the fresh composite sample, having the same peaks of the MgAl2O4 spinel (PDF#21-1152) with a main peak at 36.85° (hkl; 311). TiO2 (PDF#21-1276) was also observed as the same rutile phase with tetragonal geometry, and its main peak was slightly shifted at 51.8° (hkl; 211). A slight shift in the main peak of CoAl2O4 (PDF#44-0160) was observed at 36.0° (hkl; 311), while Co3O4 (PDF#43-1003) was identified in the fresh sample with peaks at 31.2° (hkl; 220) and 36.8° (hkl; 311). CoTiO3 (PDF#15-0866) indicated a peak at 32.8° (hkl; 104), as observed in the fresh sample. The graphite carbon (PDF#41-1487) with a hexagonal geometry confirmed the peak at 26.3° (hkl; 002) with a d-spacing of 0.337 nm [71]. The TGA profile of the spent catalyst, as shown in Figure 14b, was analysed over three different temperature regions. Column I showed a total 4-5% weight loss up to 300 °C, which could be attributed to the moisture removal and other volatile species [72], where a significant weight loss of almost 14% was observed in column II in a temperature range of 300-500 °C due to the presence of reactive carbon species such as α-C and β-C [73]. Similarly, column III showed a 2-3% weight loss above 500 °C, which could be ascribed to a lower amount of filamentous carbon (γ-C) formation [3].   The SEM micrograph of the spent catalyst presented in Figure 14c,d shows the support surface that was modified after being exposed to the DRM for 75 hrs. The carbon presence was confirmed by the elemental analysis, as shown in Figure 15 that indicates the presence of almost 4.5% carbon in the spent catalyst. Though different carbonaceous species were formed due to the reactive phase during the reaction, a carbon gasification at high temperature was expected and produced CO that had a higher syngas ratio.

Reaction Mechanism
The overall possible reaction mechanism based on the product and spent catalyst analysis is presented in Figure 16. CH4 activation starts with the adsorption on the active Co surface to produce surface carbon and hydrogen species that then combine to produce hydrogen molecules in the gaseous phase, as represented by reactions (11) and (12) [74,75]. Furthermore, the adsorption of CO2 onto the supports of TiO2 and MgAl2O4 leads to the formation of CoTiO3 species, along with the dissociation of CO2 to CO, as represented by reactions ( (13)-(17)). However, the reaction of surface carbon with the CoTiO3 species may result in the formation of CO, as represented by reaction (18). 4 CH + 2Co C-Co + H-Co →  The SEM micrograph of the spent catalyst presented in Figure 14c,d shows the support surface that was modified after being exposed to the DRM for 75 h. The carbon presence was confirmed by the elemental analysis, as shown in Figure 15 that indicates the presence of almost 4.5% carbon in the spent catalyst. Though different carbonaceous species were formed due to the reactive phase during the reaction, a carbon gasification at high temperature was expected and produced CO that had a higher syngas ratio.

Reaction Mechanism
The overall possible reaction mechanism based on the product and spent catalyst analysis is presented in Figure 16. CH 4 activation starts with the adsorption on the active Co surface to produce surface carbon and hydrogen species that then combine to produce hydrogen molecules in the gaseous phase, as represented by reactions (11) and (12) [74,75]. Furthermore, the adsorption of CO 2 onto the supports of TiO 2 and MgAl 2 O 4 leads to the formation of CoTiO 3 species, along with the dissociation of CO 2 to CO, as represented by reactions ( (13)-(17)). However, the reaction of surface carbon with the CoTiO 3 species may result in the formation of CO, as represented by reaction (18).

Conclusions
This study investigated the synthesis of various Co-loaded, TiO2-MgAl2O4-supported catalysts for the DRM process in a thermally fixed bed reactor. The 5%Co/TiO2-MgAl2O4 showed the best catalytic performance due to its higher CH4 and CO2 conversions, improved selectivity, yield of H2 and CO, and higher stability for more than 75 hr TOS. The basic nature of MgAl2O4 helped the activation and dissociation of CO2, and the strong metal-support interaction while adding TiO2 as a co-support. This was evidenced by the formation of CoTiO3 and CoAl2O4 on the catalyst, which improved the conversion of CH4 and, consequently, the catalytic performance. Regardless of the highest CH4 and CO2 conversions of the 5%Co/TiO2-MgAl2O4 among the prepared composites, the H2/CO ratio was below the theoretical ratio. The reaction of CoTiO3 with the reactive carbon to form TiO2 and larger amounts of CO brought the H2/CO ratio to less than unity and improved stability for 75 hr TOS. The enhanced stability suggested that this catalyst has prospects for the up-gradation of industrial-scale syngas production when using the DRM process.

Conclusions
This study investigated the synthesis of various Co-loaded, TiO 2 -MgAl 2 O 4 -supported catalysts for the DRM process in a thermally fixed bed reactor. The 5%Co/TiO 2 -MgAl 2 O 4 showed the best catalytic performance due to its higher CH 4 and CO 2 conversions, improved selectivity, yield of H 2 and CO, and higher stability for more than 75 h TOS. The basic nature of MgAl 2 O 4 helped the activation and dissociation of CO 2, and the strong metal-support interaction while adding TiO 2 as a co-support. This was evidenced by the formation of CoTiO 3 and CoAl 2 O 4 on the catalyst, which improved the conversion of CH 4 and, consequently, the catalytic performance. Regardless of the highest CH 4 and CO 2 conversions of the 5%Co/TiO 2 -MgAl 2 O 4 among the prepared composites, the H 2 /CO ratio was below the theoretical ratio. The reaction of CoTiO 3 with the reactive carbon to form TiO 2 and larger amounts of CO brought the H 2 /CO ratio to less than unity and improved stability for 75 h TOS. The enhanced stability suggested that this catalyst has prospects for the up-gradation of industrial-scale syngas production when using the DRM process.