Catalytic Performance of Metal Oxides Promoted Nickel Catalysts Supported on Mesoporous γ-Alumina in Dry Reforming of Methane

Dry reforming of CH4 was conducted over promoted Ni catalysts, supported on mesoporous gamma-alumina. The Ni catalysts were promoted by various metal oxides (CuO, ZnO, Ga2O3, or Gd2O3) and were synthesized by the incipient wetness impregnation method. The influence of the promoters on the catalyst stability, coke deposition, and H2/CO mole ratio was investigated. Stability tests were carried out for 460 min. The H2 yield was 87% over 5Ni+1Gd/Al, while the CH4 and CO2 conversions were found to decrease in the following order: 5Ni+1Gd/Al > 5Ni+1Ga/Al > 5Ni+1Zn/Al > 5Ni/Al > 5Ni+1Cu/Al. The high catalytic performance of 5Ni+1Gd/Al, 5Ni+1Ga/Al, and 5Ni+1Zn/Al was found to be closely related to their contents of NiO species, which interacted moderately and strongly with the support, whereas free NiO in 5Ni+1Cu/Al made it catalytically inactive, even than 5Ni/Al. The 5Ni+1Gd/Al catalyst showed the highest CH4 conversion of 83% with H2/CO mole ratio of ~1.0.


Introduction
The combustion of paraffin hydrocarbons such as methane (CH 4 ) yields water and carbon dioxide, which is one of the main contributors to global warming. Efforts have been made for years for converting CH 4 and CO 2 into highly desired synthesis gas (syngas), composed of hydrogen and carbon monoxide H 2 /CO [1][2][3]. The reaction of CH 4 with CO 2 is also known as dry reforming of methane (DRM) [4,5], used along with other types of methane reforming such as steam and oxy-reforming reactions [6]. The usage of the DRM reaction to produce valuable chemicals helps to reduce the harmful impacts of CO 2 and CH 4 on the environment [7,8]. The CH 4 has a much higher global warming potential than carbon dioxide [9]. However, DRM is hampered by rapid catalyst deactivation due to carbon deposition and sintering of both the support and the active metal of catalyst [9][10][11]. The produced hydrogen is widely employed in various industries such as the production of raw chemicals (e.g., methanol and ammonia), fuel cell applications, hydrogenation agent in oil refineries, and reducing gases for the steel industry [4]. DRM includes two main reactions, the reforming of methane (Equation (1)) and the reverse water gas shift (Equation (2)): The conventional catalyst used in the DRM reaction is nickel supported on alumina (Ni/Al 2 O 3 ) because of its low cost and relatively high catalytic activity [12,13]. Shen et al. [14] investigated Ni-based catalysts supported on different morphologies of alumina (nanosheet, nanofiber, and particle) in the dry reforming of methane. The catalyst with nanosheet morphology displayed superb catalytic activity due to the best docking of Ni nanoparticle on the (110) plane of alumina nanosheet. However, alumina with nanofiber morphology exhibited the best stability due to the confinement of Ni particles within the nest-like structure of alumina nanofiber. Their results indicated that the catalytic performance of DRM was dependent on the morphology of alumina support. However, the Ni/Al 2 O 3 is known to deactivate due to the blockage of its active sites by carbon deposition, an issue widely reported in the literature [15,16]. To tackle this problem, coke resistant mesoporous materials are used as supports [8] because of their large surface areas and small or medium pores. Moreover, supports play an important role in the catalytic activity due to their probable chemical effect through their interaction with the active phase. Osman et al. [17] reported that using Al 2 O 3 support led to better interaction and synergetic effect among the active metal species in the catalyst composition. Mesoporous Al 2 O 3 -supported nickel catalysts exhibited the highest activity, stability, and excellent coke-resistance ability for DRM among several oxide-supported Ni catalysts [18][19][20]. The strong interaction between Ni and mesoporous Al 2 O 3 improved the dispersion of Ni, retarded its sintering, and improved the adsorption of CO 2 because highly dispersed Ni was very active for the reforming reaction and was less active for carbon deposition via CH 4 decomposition. Furthermore, mesoporous supported Ni catalysts were active for the direct activation of CH 4 on the surface of supported materials [16]. Al-Fatesh [21] investigated the promotional influence of gadolinium (Gd) on Ni/Y 2 O 3 catalyst, employed during the production of H 2 from carbon dioxide and methane. It was observed that a small amount of the promoter (1.0 wt. %) could decrease carbon deposition by 35% in comparison to the unpromoted one. Carbon dioxide temperature-programmed desorption (CO 2 -TPD) results showed that Gd-promoted catalysts possessed higher basicity than non-promoted Ni/Y 2 O 3 catalyst. In other work, Al-Fatesh et al. [22] investigated the effect of various loadings of gallium promoter of Ni supported on MCM-41 on the activity and stability for the production of syngas by DRM. The addition of gallium generated high surface areas, where 2.5 wt. % of Ga resulted in a surface area of 1036 m 2 /g, which was less than that of the unpromoted one (~1133 m 2 /g). The addition of Ga reduced both the density of basic sites and the amount of carbon deposition. Weight loss was not recorded for the 3.0 wt. % Ga + 5.0 wt. % Ni/MCM-41 sample. The 2.0 wt. % Ga sample displayed 88.2% conversion of methane and the best stability, while activity declined to only 1.6% after running the reaction for 400 min at 800 • C. The promoter increased the ratio of H 2 /CO mole ratio from 0.9 to 1.0. Alternatively, Al-Fatesh et al. [23] studied the influence of Gd promoter in Ni-supported on mesoporous silica (MCM-41) in DRM. The catalytic performance was attained to be significantly enhanced with approximately 0.1 wt. % Gd loading. Thus, Gd can act as a promoter for Ni-based catalysts in DRM reaction. This catalyst (i.e., 0.1 wt. % Gd + 5.0 wt. % Ni/MCM-41) resulted in >87% CH 4 and >91% CO 2 conversions and in the production of syngas with H 2 /CO mole ratio nearly equal to one. Mesoporous alumina supported catalyst was designed to allow for Ni dispersion [16]. The investigators decreased the carbon formation by promoting alumina with basic metal oxides. The Ni-based catalysts were able to retard the deactivation by promoting the catalyst supports. It was established that Ni/MCM-41 promoted by rhodium (Rh), prepared via the one-pot procedure, enhanced the stability and activity of the catalyst [24]. Copper (Cu) and cobalt (Co) addition to Ni/Al 2 O 3 -ZrO 2 catalyst enhanced the surface area, as reflected by BET surface area, especially by cobalt addition, while the Cu addition made the support particles more compact, and, in general, Cu and Co addition improved the conversion, the yields of hydrogen and carbon monoxide, and syngas ratio. The comparison of promoters revealed that the activity of Co was better than Cu [25]. The addition of zinc (Zn) to a Co/ZrO 2 catalyst, at a ratio up to 2.0 wt. %, had a positive effect on the catalyst activity and stability, whereas further addition above this limit had the opposite effect. The positive effect of zinc was due to the prevention of Co oxidation by the formation of a Co−Zn alloy [26].
The objective of this study was to determine the optimal promoter for Ni supported on mesoporous γ-alumina (meso-γ-Al 2 O 3 ) catalyst to maintain great stability and activity, while minimizing the carbon deposition during the DRM process. The effect of promoters such as Gd, Ga, Zn, and Cu on the activity, stability, and coke formation of Ni/meso-γ-Al 2 O 3 catalysts was examined. Various characterization techniques were employed to understand the catalytic performance.

Synthesis of Nickel Oxide Supported on Mesoporous-γ-Al 2 O 3 Doped with Metal Oxide
The catalysts were prepared by the incipient wetness impregnation method, also known as dry impregnation, and the procedure is described as follows. We used this method because when using nickel nitrate as a precursor for nickel, with high concentration, the solution pH is reduced, and the surface of alumina support is interrupted and is dissolved, which, in turn, creates docking sites for nickel and enables its efficient dispersion onto alumina [27]. On this basis, the required amounts of nickel nitrate hexahydrate to introduce nickel oxide with a weight percentage of 5.0 wt. %, of metal nitrate promoter source to introduce metal oxide promoter with a weight percentage of 1.0 wt. %, and of meso-γ-Al 2 O 3 were mixed and were ground together mechanically. Ultrapure water drops were added to the mixture to form a paste, which was stirred mechanically until it was completely dry at room temperature. Drying (at room temperature) and the addition of ultrapure water processes were performed three times to ensure homogeneous distribution within each sample. The samples were then calcined at 600 • C for three hours under static air, where the temperature was ramped at a rate of 3.0 • C·min −1 . Here and after, the catalysts will be referred as 5Ni/Al for non-promoted and 5Ni+1X/Al (X = Cu, Zn, Ga, or Gd) for promoted catalysts.

Catalytic Testing
DRM experiments were carried out under atmospheric pressure at a reaction temperature of 700 • C. A tubular stainless-steel reactor (i.d., 9.0 mm; length, 300 mm) was used. An amount of 0.1 g of the catalyst was placed between two quartz wool beds to perform the activity test. Temperature measurement, during the reaction, was measured by K-type thermocouple, which was placed axially at the center of the catalyst bed. Before the reaction, the catalysts were reduced at 600 • C with the flow of H 2 for an hour at a flow rate of 30.0 mL/min. During the experiments, the mixture of methane, carbon dioxide and nitrogen in a 3:3:1 volume ratio was used as feed gas, having space velocity of 2990 h −1 . The effluent gas was analyzed by an on-line GC (GC-2014 Shimadzu Corp., Kyoto, Japan), equipped with a thermal conductivity detector (TCD) and two columns: Porapak Q and Molecular Sieve 5A. This GC was connected in series/bypass connections in order to have a complete analysis of the reaction products. The methane conversion and hydrogen yield were calculated:

N 2 Physisorption
The Pore size distribution and the surface area of the catalysts were determined by physisorption of nitrogen at −196 • C by using a Micromeritics Tristar II 3020 surface area and porosity analyzer. The distribution of the pore size was calculated by Barrett, Joyner, and Halenda (BJH) method.

XRD
Rigaku (Miniflex) diffractometer, with a Cu Kα X-ray radiation working at 40 kV and 40 mA, was used to examine the structure of the catalysts before and after the reaction. A step of 0.02 • and scanning 2θ range of 10-85 • were used. X'Pert high score plus software was used to evaluate the data obtained from the instrument. Different phases were matched to the JCPDS data bank. The result of the XRD analysis ( Figure S1) is discussed in the supplementary file.

TPR
A sample of 70 mg was put in the TPR cell of a Micromeritics Auto Chem II apparatus. Then the furnace temperature was increased from room temperature to 1000 • C with heating rate of 10 • C·min −1 , under 40 mL·min −1 flow of 10 vol. % H 2 /Ar mixture. The H 2 consumption was monitored by a TCD unit.

Temperature-Programmed Oxidation (TPO)
TPO is a useful technique for describing the type of carbon deposit over the surface of the catalyst. With respect to the literature, formed carbon, over the surface of catalysts in CO 2 reforming of methane, occurs in various forms: graphitic, amorphous, and atomic [28]. Under oxidative medium and high temperature, deposited carbon can be oxidised to carbon dioxide. The carbon oxidation below 250 • C is called atomic carbon, while the carbon which oxidised in the range of 250-600 • C was categorized as amorphous, whereas graphitic carbon burned above 600 • C [28]. To this end, the TPO was performed under 10% O 2 /He atmosphere for identifying the form of deposited carbon on the surface of the catalyst by using a Micromeritics AutoChem II (Micromeritics, Atlanta, GA, USA). The analysis was performed in the temperature range of 50−800 • C, under a flow of 40 mL/min of 10% O 2 /He. The spent catalyst was pre-treated under argon atmosphere at 150 • C for half an hour and the temperature was then reduced to 25 • C.

Thermo-Gravimetric Analysis (TGA)
The TGA was carried out under air flow at 50 mL/min, using a TGA-15 SHIMADZU analyser, to determine the amount of carbon deposited on the catalyst surface after the reaction, where 10-15 mg of the spent catalyst was filled into a platinum pan of the thermo-balance. Then, the temperature was increased to 1000 • C at a heating rate of 20 • C·min −1 , and the weight reduction was determined.

Laser Raman (NMR-4500) Spectrometer
Laser Raman (NMR-4500) Spectrometer (JASCO, Japan) was used to record Raman spectra of the spent catalyst samples. The wavelength of the excitation beam was set to 532 nm, and objective lens of 100× magnification was used for the measurement. The laser intensity was adjusted to 1.6 mW. Each spectrum was received by averaging 3 exposures on 10 seconds. Spectra were recorded in the range 1200-3000 cm −1 (Raman shift) and were processed by using Spectra Manager Ver.2 software (JASCO, Japan).

BET (Brunauer-Emmett-Teller) and the BJH (Barrett, Joyner, and Halenda) analyses
These analyses provided the specific surface areas (S BET ), pore volume (V P ), and average pore diameter (d p ), depending on promoter identity. Table 1 displays the S BET , V P , and d p values for all the catalysts investigated in this work. Promoter loading of 1.0 wt. % did not affect V P and P d . The S BET parameter was not also affected by the promoter type except in the case of Gd promoter, which led to a slight increase in S BET in comparison with non-promoted catalyst. Nitrogen adsorption-desorption isotherms ( Figure 1) showed that all prepared catalysts had a mesoporous structure. The catalysts displayed type IV isotherm with a hysteresis loop of the H3-type, resulting from capillary condensation and evaporation at high relative pressures. This structural feature indicated the mesoporous structure of the γ-Al 2 O 3 support with hexagonal array. There was no significant change in the framework of the support due to the loading of the promoter. The isotherms of the catalysts had a strongly noticeable increase in the relative pressure region of 0.65-0.95 due to combination of N 2 capillary condensation in the mesopore and the condensation of N 2 in the interstitial cavities of the support.

Temperature-Programmed Reduction (TPR)
The H2-TPR was used to study the ease and extent of reducibility of the fresh catalysts. The reduction peaks as a function of temperature are shown in Figure 2. Reduction peaks were detected below 800 C. The different reduction temperature regions were due to the interaction strength between NiO and supports. The meso-γ-Al2O3 support showed no reduction peaks in TPR measurement [29]. Three reduction peaks were detected for promoted catalysts, whereas two peaks were detected for the non-promoted catalyst. The first peak, in the temperature range of 200300 C, was quite close to the reduction temperature of bulk NiO, which indicated that this kind of NiO had no interaction with the support. The second peak, in the temperature range of 500600 C, indicated moderate interaction with the support, while the third peak, in the temperature range of 700800 C, showed strong interaction of NiO with the mesoporous γ-Al2O3, as NiAl2O4 was being formed [30]. This strong interaction improved the dispersion of Ni and hindered its sintering during the DRM. The broadening of the last peak could be attributed to the reduction of Ni ion situated at different layers in the pore wall [25], thus emphasizing the intense interaction between the NiO particles and the support. The non-promoted catalyst 5Ni/Al had reduction peaks for NiO that strongly interacted with support in the temperature range of 700800 C. The 5Ni+1Cu/Al catalyst had reduction peaks for free NiO additionally at 300C. The rest of the catalysts (i.e., 5Ni+1Zn/Al, 5Ni+1Ga/Al, and 5Ni+1Gd/Al) had reduction peaks for NiO moderately and strongly interacted with the support. The 5Ni+1Gd/Al catalyst had a higher intensity of both types of NiO than 5Ni+1Zn/Al and 5Ni+1Ga/Al catalysts. The 5Ni/Al showed the highest peak at 700-800C for all catalysts and no apparent peaks at low temperatures, justifying its requirement for high reduction temperature and low tendency to react with hydrogen. The negative peaks below 200C in the TPR profiles are due to the hydrogen spillover into the mesopores of the catalysts, as we reported in our previous publication [31].

Temperature-Programmed Reduction (TPR)
The H 2 -TPR was used to study the ease and extent of reducibility of the fresh catalysts. The reduction peaks as a function of temperature are shown in Figure 2. Reduction peaks were detected below 800 • C. The different reduction temperature regions were due to the interaction strength between NiO and supports. The meso-γ-Al 2 O 3 support showed no reduction peaks in TPR measurement [29]. Three reduction peaks were detected for promoted catalysts, whereas two peaks were detected for the non-promoted catalyst. The first peak, in the temperature range of 200−300 • C, was quite close to the reduction temperature of bulk NiO, which indicated that this kind of NiO had no interaction with the support. The second peak, in the temperature range of 500−600 • C, indicated moderate interaction with the support, while the third peak, in the temperature range of 700−800 • C, showed strong interaction of NiO with the mesoporous γ-Al 2 O 3 , as NiAl 2 O 4 was being formed [30]. This strong interaction improved the dispersion of Ni and hindered its sintering during the DRM. The broadening of the last peak could be attributed to the reduction of Ni ion situated at different layers in the pore wall [25], thus emphasizing the intense interaction between the NiO particles and the support. The non-promoted catalyst 5Ni/Al had reduction peaks for NiO that strongly interacted with support in the temperature range of 700-800 • C. The 5Ni+1Cu/Al catalyst had reduction peaks for free NiO additionally at 300 • C. The rest of the catalysts (i.e., 5Ni+1Zn/Al, 5Ni+1Ga/Al, and 5Ni+1Gd/Al) had reduction peaks for NiO moderately and strongly interacted with the support. The 5Ni+1Gd/Al catalyst had a higher intensity of both types of NiO than 5Ni+1Zn/Al and 5Ni+1Ga/Al catalysts. The 5Ni/Al showed the highest peak at 700-800 • C for all catalysts and no apparent peaks at low temperatures, justifying its requirement for high reduction temperature and low tendency to react with hydrogen. The negative peaks below 200 • C in the TPR profiles are due to the hydrogen spillover into the mesopores of the catalysts, as we reported in our previous publication [31].  Figure 3A,B shows the conversions of CH4 and CO2 along the time-on-stream (TOS). The catalysts were highly stable at the reaction temperature of 700 C during the reaction time of 460 minutes, as shown in Figure 3. The CH4 and CO2 conversions of the catalyst system were found in order of 5Ni+1Gd/Al > 5Ni+1Ga/Al > 5Ni+1Zn/Al > 5Ni/Al > 5Ni+1Cu/Al. The catalytic performances of the first three catalysts were found to correlate with the content of NiO, which interacted moderately and strongly with the support, as shown in the H2-TPR result above. The H2/CO mole ratio over these catalysts were also close to 1.0 ( Table 2). It hinted towards suppression of reverse water gas shift reaction (CO2 + H2 → CO + H2O) over these catalyst systems. The 5Ni+1Gd/Al catalyst was found to be the most stable and resulted in the highest conversions of CH4 (~83%) and CO2 and (~89%) with the highest activity for producing syngas because of its higher oxygen storage capacity [23]. The calculated deactivation factors (DF) were shown in Table 2 below, with the lowest DF for the 5Ni+1Gd/Al catalyst. The H2/CO molar ratio versus TOS is shown in Figure 4C. The presence of free NiO (as shown in H2-TPR) in 5Ni+1Cu/Al caused less catalytic performance even than the nonpromoted catalyst 5Ni/Al. Low H2/CO ratio for both catalysts may be due to the dominance of reverse water gas shift reaction.   Figure 3A,B shows the conversions of CH 4 and CO 2 along the time-on-stream (TOS). The catalysts were highly stable at the reaction temperature of 700 • C during the reaction time of 460 min, as shown in Figure 3. The CH 4 and CO 2 conversions of the catalyst system were found in order of 5Ni+1Gd/Al > 5Ni+1Ga/Al > 5Ni+1Zn/Al > 5Ni/Al > 5Ni+1Cu/Al. The catalytic performances of the first three catalysts were found to correlate with the content of NiO, which interacted moderately and strongly with the support, as shown in the H 2 -TPR result above. The H 2 /CO mole ratio over these catalysts were also close to 1.0 ( Table 2). It hinted towards suppression of reverse water gas shift reaction (CO 2 + H 2 → CO + H 2 O) over these catalyst systems. The 5Ni+1Gd/Al catalyst was found to be the most stable and resulted in the highest conversions of CH 4 (~83%) and CO 2 and (~89%) with the highest activity for producing syngas because of its higher oxygen storage capacity [23]. The calculated deactivation factors (DF) were shown in Table 2 below, with the lowest DF for the 5Ni+1Gd/Al catalyst. The H 2 /CO molar ratio versus TOS is shown in Figure 3C. The presence of free NiO (as shown in H 2 -TPR) in 5Ni+1Cu/Al caused less catalytic performance even than the non-promoted catalyst 5Ni/Al. Low H 2 /CO ratio for both catalysts may be due to the dominance of reverse water gas shift reaction.

Temperature Programmed Oxidation (TPO)
TPO is a useful characterization technique for the analysis of the kind of carbon deposits on the surface of spent catalysts. As mentioned before, carbon deposits on the spent catalysts could be graphitic, amorphous, or atomic in nature, depending on the temperature of oxidation. It has been reported that carbon that oxidizes below 250 • C is atomic in nature, while carbon that oxidizes between 250-600 • C falls under the category of amorphous. Lastly, graphitic carbon oxidizes at a temperature above 600 • C [28].
The TPO showed peaks that correspond to the kind of carbon formed on the catalyst surface. The TPO curves for all the spent catalysts had higher intensity peaks appearing below 250 • C (Figure 4), corresponding to less ordered (more reactive) atomic carbon [32][33][34]. The negative peaks that appeared at around 400 • C indicate oxidation of the metals. A small positive peak can be seen at around 400 • C for all the used catalysts except 5Ni+1Zn/Al. This is an indication of the presence of amorphous carbon on these catalysts. Moreover, there appeared to be very small deposits of graphitic carbon on the spent 5Ni+1Ga/Al and 5Ni+1Cu/Al, which can be ascribed to the peaks appearing at above 600 • C.

Temperature Programmed Oxidation (TPO)
TPO is a useful characterization technique for the analysis of the kind of carbon deposits on the surface of spent catalysts. As mentioned before, carbon deposits on the spent catalysts could be graphitic, amorphous, or atomic in nature, depending on the temperature of oxidation. It has been reported that carbon that oxidizes below 250 °C is atomic in nature, while carbon that oxidizes between 250-600 °C falls under the category of amorphous. Lastly, graphitic carbon oxidizes at a temperature above 600 °C [28].
The TPO showed peaks that correspond to the kind of carbon formed on the catalyst surface. The TPO curves for all the spent catalysts had higher intensity peaks appearing below 250 C ( Figure  4), corresponding to less ordered (more reactive) atomic carbon [32][33][34]. The negative peaks that appeared at around 400 C indicate oxidation of the metals. A small positive peak can be seen at around 400 °C for all the used catalysts except 5Ni+1Zn/Al. This is an indication of the presence of amorphous carbon on these catalysts. Moreover, there appeared to be very small deposits of graphitic carbon on the spent 5Ni+1Ga/Al and 5Ni+1Cu/Al, which can be ascribed to the peaks appearing at above 600 °C.

Thermogravimetric analysis (TGA) of Spent Catalyst
The profiles of the five spent catalysts showed weight losses of 3.0 wt. %, 5.0 wt. %, 4.0 wt. %, 4.0 wt. %, and 2.0 wt. % for 5Ni+1Cu/Al, 5Ni+1Gd/Al, 5Ni+1Zn/Al, and 5Ni/Al, respectively ( Figure  5). All the spent catalysts were studied after catalytic cycle at 700 °C during 460 minutes on stream. The 5Ni+1Gd/Al had the highest carbon content due to high methane conversion into syngas and its decomposition into carbon. The catalyst probably did not do much in gasifying the carbon deposits, indicating its ability to accumulate carbon deposits compared to the other catalysts. On the other hand, 5Ni/Al showed the least carbon content.

Thermogravimetric analysis (TGA) of Spent Catalyst
The profiles of the five spent catalysts showed weight losses of 3.0 wt. %, 5.0 wt. %, 4.0 wt. %, 4.0 wt. %, and 2.0 wt. % for 5Ni+1Cu/Al, 5Ni+1Gd/Al, 5Ni+1Zn/Al, and 5Ni/Al, respectively ( Figure 5). All the spent catalysts were studied after catalytic cycle at 700 • C during 460 min on stream. The 5Ni+1Gd/Al had the highest carbon content due to high methane conversion into syngas and its decomposition into carbon. The catalyst probably did not do much in gasifying the carbon deposits, indicating its ability to accumulate carbon deposits compared to the other catalysts. On the other hand, 5Ni/Al showed the least carbon content.

Scanning Electron Microscope (SEM)
The SEM images of all samples are shown in Figure 6. The morphology of all the catalyst samples were more or less the same. All the catalysts showed agglomerated particles with undefined shapes, where the identity of the promoter had no effect on the morphology.

Scanning Electron Microscope (SEM)
The SEM images of all samples are shown in Figure 6. The morphology of all the catalyst samples were more or less the same. All the catalysts showed agglomerated particles with undefined shapes, where the identity of the promoter had no effect on the morphology.

Scanning Electron Microscope (SEM)
The SEM images of all samples are shown in Figure 6. The morphology of all the catalyst samples were more or less the same. All the catalysts showed agglomerated particles with undefined shapes, where the identity of the promoter had no effect on the morphology.

Raman Analysis
On the Raman spectra of all the spent catalysts (Figure 7), there are two bands with Raman shifts: 1474 ± 5 cm −1 and 1537 ± 10 cm −1 , which correspond to the D and G band, respectively. The D band can be ascribed to carbon deposits with imperfect structures that are disordered (amorphous), while the G band is a characteristic of well-ordered carbon structure (graphitic) [35][36][37]. The I D /I G ratio showed that the spent 5Ni/Al had the highest disordered (amorphous) carbon on its surface after the reaction, while this ratio decreased for the promoted catalysts, indicating that the presence of promoters caused a good order of carbon deposits (graphitic). This conclusion about the deposited carbon on the surface of the spent catalysts from Raman spectra was in parallel with the results of their corresponding TPO analysis.

Raman Analysis
On the Raman spectra of all the spent catalysts (Figure 7), there are two bands with Raman shifts: 1,474 ± 5 cm −1 and 1,537 ± 10 cm −1 , which correspond to the D and G band, respectively. The D band can be ascribed to carbon deposits with imperfect structures that are disordered (amorphous), while the G band is a characteristic of well-ordered carbon structure (graphitic) [35][36][37]. The ID/IG ratio showed that the spent 5Ni/Al had the highest disordered (amorphous) carbon on its surface after the reaction, while this ratio decreased for the promoted catalysts, indicating that the presence of promoters caused a good order of carbon deposits (graphitic). This conclusion about the deposited carbon on the surface of the spent catalysts from Raman spectra was in parallel with the results of their corresponding TPO analysis.

Conclusions
The N2 physisorption analysis confirmed the mesoporosity of the prepared catalysts. In the TPR analysis, an extra peak was observed owing to the promoting effects. In the XRD analysis, no peaks related to promoters were detected, and hence, they scattered well in the matrix of the catalyst. The TPO analysis depicted the presence of atomic carbon, some amorphous and graphitic carbon, as well as the oxidation of the metals. The TGA analysis indicated a similar amount of carbon deposition for all the spent catalyst. The SEM analysis displayed agglomerated particles with undefined shapes. Promoters with 1.0 wt. % loading (CuO, ZnO, Ga2O3, or Gd2O3) were used for enhancing the catalytic performance of 5Ni/Al in DRM. The CH4 and CO2 conversions increased in the order of 5Ni+1Cu/Al < 5Ni/Al < 5Ni+1Zn/Al < 5Ni+1Ga/Al < 5Ni+1Gd/Al. The catalytic performance of 5Ni+1Gd/Al, 5Ni+1Ga/Al, and 5Ni+1Zn/Al were found to correlate with the content of the NiO, which interacts moderately and strongly with support. Free NiO species in 5Ni+1Cu/Al made its catalytic performance even inferior to that of 5Ni/Al. 5Ni+1Gd/Al showed the highest conversions of CH4 (~83%) and CO2 (~89%) with H2/CO mole ratio of ~1.0.

Conclusions
The N 2 physisorption analysis confirmed the mesoporosity of the prepared catalysts. In the TPR analysis, an extra peak was observed owing to the promoting effects. In the XRD analysis, no peaks related to promoters were detected, and hence, they scattered well in the matrix of the catalyst. The TPO analysis depicted the presence of atomic carbon, some amorphous and graphitic carbon, as well as the oxidation of the metals. The TGA analysis indicated a similar amount of carbon deposition for all the spent catalyst. The SEM analysis displayed agglomerated particles with undefined shapes. Promoters with 1.0 wt. % loading (CuO, ZnO, Ga 2 O 3 , or Gd 2 O 3 ) were used for enhancing the catalytic performance of 5Ni/Al in DRM. The CH 4 and CO 2 conversions increased in the order of 5Ni+1Cu/Al < 5Ni/Al < 5Ni+1Zn/Al < 5Ni+1Ga/Al < 5Ni+1Gd/Al. The catalytic performance of 5Ni+1Gd/Al, 5Ni+1Ga/Al, and 5Ni+1Zn/Al were found to correlate with the content of the NiO, which interacts moderately and strongly with support. Free NiO species in 5Ni+1Cu/Al made its catalytic performance even inferior to that of 5Ni/Al. 5Ni+1Gd/Al showed the highest conversions of CH 4 (~83%) and CO 2 (~89%) with H 2 /CO mole ratio of~1.0.