Fe-Promoted Alumina-Supported Ni Catalyst Stabilized by Zirconia for Methane Dry Reforming

: The dry reforming of methane is a highly popular procedure since it can transform two of the most abundant greenhouse gases, methane and carbon dioxide, into useful syngases that can be further processed into valuable chemicals. To successfully achieve this conversion for the effective production of syngas, an optimal catalyst with advantageous physicochemical features must be developed. In this study, a variety of Ni-based catalysts supported by zirconia alumina (5Ni-10Zr + Al) were prepared by using the impregnation approach. Different loadings of Fe promoter were used, and the performances of the resulting catalysts in terms of activity and stability were investigated. The catalyst used in this study had an active metal component made of 5% Ni and x% Fe supported on 10ZrO 2 + Al 2 O 3 , where x = (1, 2, 3, and 4). The physicochemical characteristics of both freshly calcined and used catalysts were studied using a range of characterization techniques, such as: N 2 adsorption–desorption isotherms, XRD, H 2 -TPR, Raman spectroscopy, TGA, and TEM. An investigation of the effects of the Fe promoter on the catalytic activity of the catalyst (5Ni + xFe-10Zr + Al) was conducted. Amongst the studied catalysts, the 5Ni + 3Fe-10Zr + Al catalyst showed the best catalytic activity with CH 4 and CO 2 conversions of 87% and 90%, respectively, and had an H 2 /CO ratio of 0.98.


Introduction
To reduce greenhouse gas (GHG) emissions and slow down climate change, the global energy sector needs to move completely to a decarbonized system.The modern economy depends on energy, and its consumption is expected to increase for a very long time.Additionally, it is crucial to take into account fuel affordability and energy security.Global concerns over elevated carbon dioxide emissions released to the atmosphere from various sources have amplified over the previous decade.The excessive combustion of fossil fuels is believed to be a major contributor towards enhanced levels of carbon dioxide (CO 2 ), which is causing global climate change.Yet, with the widespread exploitation of natural gas resources, it has become extremely difficult to effectively utilize large amounts of methane (CH 4 ).Given that CO 2 is a fairly stable molecule, the generation of CO 2 via the oxidation of fossil fuels is an example of a so-called spontaneous process.Due to the continued combustion of fossil fuels, atmospheric CO 2 levels are predicted to rise even further in the future.The size of this rise is influenced by societal, technological, natural, and economic advancements, but it might ultimately be constrained by the supply of fossil fuels [1][2][3][4].The production of syngas (H 2 + CO) using greenhouse gases, CH 4 and CO 2 , which is also known as CH 4 reforming with CO 2 or the dry reforming of methane (DRM), has drawn significant attention in recent years.This makes DRM a potential viable alternative to steam reforming, which is frequently used in industrial applications to produce synthesis gas.DRM permits the conversion of both CO 2 and CH 4 into valuable fuels and chemicals as a perfect method to concurrently address the two challenges.Greenhouse gases (CH 4 and CO 2 ) were utilized as a source of carbon in syngas (CO and H 2 ) production, which serves as an industrially starting material for its conversion in an environmentally friendly and economically effective way.Hydrogen's reputation as an efficient and perfect energy transporter makes it even more significant, particularly in fuel cell technology [2][3][4][5][6].Numerous methane reforming techniques have been thoroughly studied, including autothermal reforming (ARM), dry reforming (DR), steam reforming (SRM), and partial oxidation (POM) [7][8][9].These processes use different oxidants, energetics, and ultimate H 2 /CO product ratios [9].Compared to conventional steam reforming, DRM produces H 2 /CO ratios suitable for Fischer-Tropsch synthesis and the production of methanol [10].While research into the direct conversion of methane into long-chain, higher-value products is still ongoing, typical industrial processes use the synthesis gas (syngas) as chemical feedstock for the synthesis of useful chemicals.In this regard, DRM, as shown in Equation (1), has the extra benefit of turning CO 2 into useful chemicals, while having the lowest operating cost among these processes [2,11]: The reverse water-gas shift is an important example of a side reaction: For every catalytic system, the dry reforming reaction is accompanied by quick side reactions that lessen selectivity, such as the reverse water-gas shift (RWGS) reaction in Equation (2).Due to the high C/H ratio of the feedstock, DRM catalysts frequently experience carbon deposition in addition to other more normal issues with high-temperature catalysis, such as active metal particle sintering.The two primary processes that create surface carbon are CO disproportionation (also known as the Boudouard reaction) in Equation (3) and CH 4 cracking in Equation (4).
Due to the larger rate of CO disproportionation than that of methane decomposition below 700 • C, a higher rate of carbon production is recorded at low temperatures.To avoid coking, the rate of carbon gasification must be higher than the sum of the rates in Equations ( 3) and (4).On the catalyst's surface, non-reactive carbon can take a variety of shapes, including nanotubes and graphite carbon, which can encapsulate the active particles.However, the drawback of dry reforming technologies is the quick catalyst deactivation brought on by carbon deposition, as well as agglomeration active metal particles at high temperatures [12].Nickel and noble metals such as Pt, Ru, and Rh have received the greatest attention as dry-reforming catalysts.Additionally, many catalysts made of noble metals exhibit great activity and durability against coking (because of the limited mobility and solubility of carbon in these metals), but their costs are considered to be too exorbitant for practical use in the DRM process.Although it has been demonstrated that nickel-based catalysts are quite active in the DRM process, they are often less resistant to carbon deposition.By preserving tiny supported nickel nanoparticles and utilizing metal-support interactions to lessen sintering, their stability can be increased [13].The aforementioned noble metals exhibit strong activity and anti-carbon forming properties.The majority of the research publications that have been published are devoted to exploring appropriate metals as dispersed phases on various supports and developing enhanced support materials.The selection of the support material has a big impact on the final stability of catalysts.Typically, during reforming processes, Al 2 O 3 serves as a support for Ni-based catalysts.By doing so, this catalyst series exhibits exceptional stability and catalytic activity at the same time.Under extreme methane dry reforming reaction conditions, altering the support with other active metal oxides promotes transition metal catalysts' activity and stability.Since the DRM commonly results in carbon deposition, it is essential to alter the catalysts with substances that might improve the adsorption and activation of CO 2 .These CO 2 sorbents would slow the build-up of stable carbon and speed up the gasification of deposited carbon on the catalyst's surface.Consequently, to improve CO 2 adsorption and carbon gasification, basic oxides, such as MgO, La 2 O 3 , ZrO 2 , and NaO, are added [6,14].There is consensus in the literature that support the idea that materials play a critical role in the DRM system [15][16][17][18].In the case of catalysts supported on inert substances such as SiO 2 , the mechanism follows a monofunctional pathway, in which the metal acts as the sole catalyst to activate both reactants [18].Once carbon is formed through the dehydrogenation of methane, subsequent CO 2 activation and carbon-carbon reactions are constrained, which results in catalyst deactivation [19][20][21][22][23][24][25].With an acidic (Al 2 O 3 ) or basic (La 2 O 3 , CeO 2 , and MgO) support, a bifunctional process occurs [26,27].CH 4 is activated on the metal, while CO 2 is activated on the support [22].The right supports must be able to maintain the metal's dispersion while operating and must be able to withstand high temperatures.Aluminum oxide is typically favored for use as support because of its mechanical strength, stability at high temperatures, and favorable textural qualities [23].Nickel-based catalysts with alumina supports, however, are more likely to cause carbon production [24].In catalysis, ZrO 2 supports have been widely used.This is a result of its exceptional acid-base characteristics, high-temperature stability, and surface O 2 mobility.It was determined that the catalytic performance of the supported catalysts was significantly impacted by the pore structure, crystalline phase, and crystalline size of the ZrO 2 support.It is crucial to specify the characteristics of the type of zirconia that was used because they come from diverse sources and have different morphologies, structural characteristics, and surface areas.Due to its redox behavior, capacity to behave as an acid-base bifunctional catalyst, reducibility, and excellent thermal stability, ZrO 2 is particularly promising as a catalyst and support in the catalysis disciplines.Zirconia is well recognized for slowing the rate of cerium deterioration at high temperatures [25].Zirconia doping in ceria is thought to have a positive effect by restricting heat sintering.It was noteworthy that ZrO 2 as a promoter (in Ni-ZrO 2 -Al 2 O 3 catalyst) demonstrated improved catalytic stability due to the enhancement of metal-support interaction, higher oxygen storage capacity, and subsequently, coke suppression [27].The dissociation of the CO 2 process, which converts into CO and O, was described as being improved by the addition of ZrO 2 and Ni/Al 2 O 3 , which led to a reduction in the generation of coke.There are a few investigations on the impact of Fe as a catalyst on ZrO 2 and Al 2 O 3 , jointly supporting the reforming catalytic activity [26][27][28].The authors of these studies looked at how Ni + Fe affected the creation of carbon on Zr + Al supports and evaluated their results, while methane was being dry-reformed, which tends to cause the production of coke.
In this study, the manifestation of the 5Ni-10Zr + Al catalyst promoted by iron (Fe) in a DRM reaction was assessed.There is, at present, a knowledge gap in the understanding of optimum Fe loading, as well as the role of Fe as a catalyst, and this forms the basis of our scientific motivation.To the best of our knowledge, the incorporation of Fe into the overall 5Ni-10Zr + Al catalytic system and its influence on the DRM process have not been fully studied by previous researchers.Characterizations of fresh and spent catalysts have also been investigated to elucidate the physicochemical properties of nickel dispersion and carbon deposition.In this study, wet impregnation was employed to generate 5Ni + xFe-10Zr + Al catalysts, which were then used in the dry reforming of methane.Catalysts with various iron loadings (1, 2, 3, and 4%) were synthesized in order to further examine the impact of Fe on the catalytic activity, and carbon retardation, and to establish the ideal loading.It became possible to understand carbon deposition and the impact of oxygen addition on the catalyst's performance by using a variety of characterization approaches.Catalysts for the methane reforming of carbon dioxide at 800 • C were used in experiments.BET, XRD, TPR, TEM, Raman spectra, and TGA-DTA were utilized to investigate both new and previously used catalysts.

BET Evaluation
N 2 adsorption-desorption experiments were conducted to look into the textural characteristics of the fresh catalysts, as illustrated in Figure 1.According to the IUPAC classification, Type IV isotherms with H2 hysteresis loops are shown in Figure 1.Typically, solids made up of masses of particles that create slit-shaped pores of various sizes and shapes exhibit these patterns, which are characteristic of mesoporous materials [28].Figure 1A demonstrates that nitrogen uptake began in a relative pressure range of 0.8-1.Specifically, the sample with the largest surface area (5Ni + 4Fe-10ZrO 2 + Al 2 O 3 ) adsorbed the most N 2 (126 cm 3 /g), while the sample with the smallest surface area (5Ni + 3Fe-10ZrO 2 + Al 2 O 3 ) adsorbed the least N 2 (118 cm 3 /g).Figure 1B shows the rate of change of pore volume with respect to the pore size.The textural properties of the studied catalysts are presented in Table 1.The specific surface area with nominal differences was recorded for all catalysts, where 5Ni + 4Fe-10Zr + Al has the largest surface area (126.3974m 2 /g).The unpromoted catalyst (5Ni-10Zr + Al) showed a slightly larger total pore volume (0.592 cm 3 /g) than those of the promoted catalysts.It is evident that the addition of the Fe promoter decreases the pore size, while the reduction of pore volume with the addition of Fe assumes a non-uniform trend.
addition on the catalyst's performance by using a variety of characterization approaches.Catalysts for the methane reforming of carbon dioxide at 800 °C were used in experiments.BET, XRD, TPR, TEM, Raman spectra, and TGA-DTA were utilized to investigate both new and previously used catalysts.

BET Evaluation
N2 adsorption-desorption experiments were conducted to look into the textural characteristics of the fresh catalysts, as illustrated in Figure 1.According to the IUPAC classification, Type IV isotherms with H2 hysteresis loops are shown in Figure 1.Typically, solids made up of masses of particles that create slit-shaped pores of various sizes and shapes exhibit these patterns, which are characteristic of mesoporous materials [28].Figure 1A demonstrates that nitrogen uptake began in a relative pressure range of 0.8-1.Specifically, the sample with the largest surface area (5Ni + 4Fe-10ZrO2 + Al2O3) adsorbed the most N2 (126 cm 3 /g), while the sample with the smallest surface area (5Ni + 3Fe-10ZrO2 + Al2O3) adsorbed the least N2 (118 cm 3 /g).Figure 1B shows the rate of change of pore volume with respect to the pore size.The textural properties of the studied catalysts are presented in Table 1.The specific surface area with nominal differences was recorded for all catalysts, where 5Ni + 4Fe-10Zr + Al has the largest surface area (126.3974m 2 /g).The unpromoted catalyst (5Ni-10Zr + Al) showed a slightly larger total pore volume (0.592 cm 3 /g) than those of the promoted catalysts.It is evident that the addition of the Fe promoter decreases the pore size, while the reduction of pore volume with the addition of Fe assumes a non-uniform trend.

Temperature-Programmed Reduction
H 2 temperature-programmed reduction was conducted to unravel the metal-support interaction of catalysts, as displayed in Figure 2. The H 2 -TPR profile can be divided into three regions of reduction peaks as different NiO species interacted with the support to varying degrees.The first zone, which is characterized by free NiO, is located in the lowtemperature range of 300-400 • C. The reduction maxima in the second zone is located in the 400-500 • C range, which is characterized by the NiO weakly interacting with the support.Large NiO particles aggregated on the external surface, interacting with the support.While in the third region, where the temperature is above 500-700 • C, NiO interacts moderately with the support, and the bulk-phase NiO species declined.The broad peak detected at around 800 • C is typically associated with the NiO species, which interacts strongly with the support.Reduction peaks for Fe 2 O 3 → Fe 3 O 4 → FeO → Fe should fall at about 350 • C, 550 • C, and 800 • C, respectively [27].In our catalyst system, these peaks were commonly merged with reduction peaks of NiO.It can be inferred from the figure that the addition of Fe significantly affected the intensity of the dominant peak that appeared beyond 800 • C. Fe-promoted catalysts, in particular, developed shoulder peaks in the second region where NiO interacted weakly and moderately with the support.Thus, the increased intensities and the presence of the shoulder peaks for the Fe-promoted catalysts increased the activity during the process.

Temperature-Programmed Reduction
H2 temperature-programmed reduction was conducted to unravel the metal-support interaction of catalysts, as displayed in Figure 2. The H2-TPR profile can be divided into three regions of reduction peaks as different NiO species interacted with the support to varying degrees.The first zone, which is characterized by free NiO, is located in the lowtemperature range of 300-400 °C.The reduction maxima in the second zone is located in the 400-500 °C range, which is characterized by the NiO weakly interacting with the support.Large NiO particles aggregated on the external surface, interacting with the support.While in the third region, where the temperature is above 500-700 °C, NiO interacts moderately with the support, and the bulk-phase NiO species declined.The broad peak detected at around 800 °C is typically associated with the NiO species, which interacts strongly with the support.Reduction peaks for Fe2O3 → Fe3O4 → FeO → Fe should fall at about 350 °C, 550 °C, and 800 °C, respectively [27].In our catalyst system, these peaks were commonly merged with reduction peaks of NiO.It can be inferred from the figure that the addition of Fe significantly affected the intensity of the dominant peak that appeared beyond 800 °C.Fe-promoted catalysts, in particular, developed shoulder peaks in the second region where NiO interacted weakly and moderately with the support.Thus, the increased intensities and the presence of the shoulder peaks for the Fe-promoted catalysts increased the activity during the process.planes of gamma phases of alumina, respectively (JCPDS card Nos.: 01-029-0063).Moreover, the tetragonal phases of zirconia were represented by peaks at 2 = 30°, 50°, and 60° (JCPDS reference no.01-079-1769).It appears that wt.% loadings of Fe upon the 5Ni-10Zr + Al catalyst have a negligible effect on the crystallinity of the overall catalyst structure.

Catalyst Performance
Prior to the evaluation of prepared catalysts, a blank experiment was carried out, where an empty stainless steel reactor was used without catalysts under the same feed ratio and the same reaction temperature of 800 °C.CH4 and CO2 conversions at 800 °C of the blank occurred at 1.55% and 0.25%, respectively, with an H2/CO ratio close to 0.11, denoting the negligibility effect of the metallic reactor.Figure 4 displays profiles of the H2/CO ratio, as well as CH4 and CO2 conversions.At 800 °C, the activity of each 5Ni-xFe-10Zr + Al catalyst (x = 0, 1, 2, 3, and 4 wt.%) was assessed.An induction phase of activation exists for all catalysts.The large Fe loading sample (5Ni + 4Fe-10Zr-Al) gave the worst DMR performance, with CH4 and CO2 conversions of 75% and 80%, respectively.Interestingly, the 5Ni + 3Fe-10Zr + Al catalyst exhibited an optimal DRM performance, with CH4 and CO2 conversions of 87% and 90%, respectively, and an H2/CO ratio of 0.98.The results clearly indicate that the incorporation of an Fe promoter clearly affects the performance of the 5Ni-10ZrAl catalyst in terms of both CH4 and CO2 conversion.By increasing the Fe loading from 1 to 3 wt.%,CH4 conversion increased from 75 to 78%.However, a further increase in Fe loading up to 4 wt.%negatively reflects CH4 conversion, which decreased down to 74%.It is important to notice that Fe-promoted catalysts are stable over time.In particular, the 5Ni + 3Fe-10Zr + Al catalyst showed the best performance in terms of activity and stability.CO2 conversion was always better than that of CH4, owing to the effect of the reverse water gas shift reaction (Equation ( 2)), in which CO2 reacted with the produced hydrogen.The CO2 conversion of the unpromoted 5Ni-10Zr + Al catalyst is less efficient

Catalyst Performance
Prior to the evaluation of prepared catalysts, a blank experiment was carried out, where an empty stainless steel reactor was used without catalysts under the same feed ratio and the same reaction temperature of 800 • C. CH 4 and CO 2 conversions at 800 • C of the blank occurred at 1.55% and 0.25%, respectively, with an H 2 /CO ratio close to 0.11, denoting the negligibility effect of the metallic reactor.Figure 4 displays profiles of the H 2 /CO ratio, as well as CH 4 and CO 2 conversions.At 800 • C, the activity of each 5Ni-xFe-10Zr + Al catalyst (x = 0, 1, 2, 3, and 4 wt.%) was assessed.An induction phase of activation exists for all catalysts.The large Fe loading sample (5Ni + 4Fe-10Zr-Al) gave the worst DMR performance, with CH 4 and CO 2 conversions of 75% and 80%, respectively.Interestingly, the 5Ni + 3Fe-10Zr + Al catalyst exhibited an optimal DRM performance, with CH 4 and CO 2 conversions of 87% and 90%, respectively, and an H 2 /CO ratio of 0.98.The results clearly indicate that the incorporation of an Fe promoter clearly affects the performance of the 5Ni-10ZrAl catalyst in terms of both CH 4 and CO 2 conversion.By increasing the Fe loading from 1 to 3 wt.%,CH 4 conversion increased from 75 to 78%.However, a further increase in Fe loading up to 4 wt.%negatively reflects CH 4 conversion, which decreased down to 74%.It is important to notice that Fe-promoted catalysts are stable over time.In particular, the 5Ni + 3Fe-10Zr + Al catalyst showed the best performance in terms of activity and stability.CO 2 conversion was always better than that of CH 4 , owing to the effect of the reverse water gas shift reaction (Equation ( 2)), in which CO 2 reacted with the produced hydrogen.The CO 2 conversion of the unpromoted 5Ni-10Zr + Al catalyst is less efficient than that of Fe-promoted catalysts.Hence, the addition of Fe acts on the reversed water gas shift reaction due to its basic nature; it favors the adsorption of CO 2 [29].
than that of Fe-promoted catalysts.Hence, the addition of Fe acts on the reversed water gas shift reaction due to its basic nature; it favors the adsorption of CO2 [29].Raman shift bands can be detected at 1350 cm −1 (D band) and 1580-1620 cm −1 (G band).The D band indicates structural imperfections of graphite, and the G band is attributed to in-plane carbon-carbon stretching vibrations of graphite.

Thermogravimetric Analysis (TG)
The weight loss of spent catalysts is exhibited in Figure 6.The TG curve presents three segments: weight loss at between 200 • C and 500 • C is predominantly brought on by the oxidation of amorphous carbon; weight loss at below 200 • C is principally relevant to the removal of H 2 O and other chemisorbed species.The gasification of graphite carbon is the key factor contributing to weight loss at above 600 • C. As shown in Figure 6A, the 5Ni-4Fe-10Zr + Al catalyst lost the least weight, while 5Ni + 2Fe-10Zr + Al lost the most weight.From Figure 6A, it is clear that the Fe promoter affected the coke deposition.The unpromoted catalyst and the catalyst with the highest Fe loading provided the least amount of coke formation.This could be accredited to the low activity of these catalysts.However, the high-activity catalysts generated more coke as a result of the rate of the reactions.

Thermogravimetric Analysis (TG)
The weight loss of spent catalysts is exhibited in Figure 6.The TG curve presents three segments: weight loss at between 200 °C and 500 °C is predominantly brought on by the oxidation of amorphous carbon; weight loss at below 200 °C is principally relevant to the removal of H2O and other chemisorbed species.The gasification of graphite carbon is the key factor contributing to weight loss at above 600 °C.As shown in Figure 6A, the 5Ni-4Fe-10Zr + Al catalyst lost the least weight, while 5Ni + 2Fe-10Zr + Al lost the most weight.From Figure 6A, it is clear that the Fe promoter affected the coke deposition.The un-promoted catalyst and the catalyst with the highest Fe loading provided the least amount of coke formation.This could be accredited to the low activity of these catalysts.However, the high-activity catalysts generated more coke as a result of the rate of the reactions.
Figure 6B displays the rate of the change in TG against the temperature (DTG).The 5Ni-10Zr + Al sample shows that its carbon gasification took place at around 350 °C, whereas the Fe-promoted samples gasified at above 650 °C.This phenomenon explains the variation of carbon type formed due to the Fe-promoter.

TEM
TEM images of 5Ni + xFe-ZrAl (x = 0 and 2 wt.%) catalysts at 100 and 200 nm scales are shown in Figure 7. Figure 7A,B shows fresh 5Ni-10Zr + Al and spent samples, respectively, where agglomerated catalyst particles of fresh and small sizes of spent 5Ni-10Zr + Al samples can be seen.Figure 7C,D shows fresh and spent samples (5Ni + 2Fe-10Zr + Al), respectively.The used catalyst demonstrates the growth of carbon nanotube filaments.The size of the nanotubes is improved by the Fe-promoter.

TEM
TEM images of 5Ni + xFe-ZrAl (x = 0 and 2 wt.%) catalysts at 100 and 200 nm scales are shown in Figure 7. Figure 7A,B shows fresh 5Ni-10Zr + Al and spent samples, respectively, where agglomerated catalyst particles of fresh and small sizes of spent 5Ni-10Zr + Al samples can be seen.Figure 7C,D shows fresh and spent samples (5Ni + 2Fe-10Zr + Al), respectively.The used catalyst demonstrates the growth of carbon nanotube filaments.The size of the nanotubes is improved by the Fe-promoter.

TEM
TEM images of 5Ni + xFe-ZrAl (x = 0 and 2 wt.%) catalysts at 100 and 200 nm scales are shown in Figure 7. Figure 7A,B shows fresh 5Ni-10Zr + Al and spent samples, respectively, where agglomerated catalyst particles of fresh and small sizes of spent 5Ni-10Zr + Al samples can be seen.Figure 7C,D shows fresh and spent samples (5Ni + 2Fe-10Zr + Al) respectively.The used catalyst demonstrates the growth of carbon nanotube filaments The size of the nanotubes is improved by the Fe-promoter.

Catalyst Preparation
In order to make 5Ni + xFe/10ZrO 2 -Al 2 O 3 , where x = (1, 2, 3, and 4), 10 mL of distilled water and 5% of nickel were combined and stirred at room temperature in an 80 mL glass crucible.The support, 10ZrO 2 -Al 2 O 3 , was then added.Prompters 1, 2, 3, and 4% Ferrite nitrate were added to the solution.The solution was then dried for 30 min at 80 • C without stirring.A 3 • C/h heating rate was used to calcine the catalysts in the air for up to 3 h at a temperature of 700 • C.

Catalytic Analysis
At 1 atm, the catalyst's activity (0.1 g) was assessed in a continuous-flow fixed-bed reactor (30 cm long and 0.94 cm in diameter).To measure the reaction temperature, a thermocouple was attached to the catalyst's middle bed.Prior to the reaction starting, catalysts were prepared by adding hydrogen at a flow rate of 30 mL/min for an hour at 700 • C. The remaining H 2 in the reactor was then removed by purging it with flowing nitrogen at a rate of 20 mL/min for 20 min at 700 • C. The reactor temperature was permitted to reach 800 • C before the N 2 purge was stopped.The input gas, CH 4 /CO 2 /N 2 , was supplied and the rate was maintained throughout the reaction at 70 mL/min at a space velocity of 42,000 mL/(h gcat).A 30/30/10 volume ratio was used.A thermal conductivity detector and an online GC (GC-Shimadzu 2014, Kyoto, Japan) equipped with two columns (Porapak Q and Molecular Sieve 5A) linked to the reactor were used to measure the concentration of the reactor's products and unconverted feed gases.(TCD).N 2 gas in the feed was used as an internal standard.Its composition remained constant.CH 4 , CO 2 conversion, and H 2 /CO (syngas ratio), respectively, were calculated using the following Equations: Methane conversion (%) = ((CH 4 ,in − CH 4 ,out)/CH 4 ,in) * 100 Carbon dioxide conversion (%) = ((CO 2 ,in − CO 2 ,out)/CO 2 ,in) * 100 Syngas Ratio = mole of H 2 produced/mole of CO produced

Characterization of the Catalyst
By employing nitrogen physisorption at 196 • C, the specific surface area of catalysts was determined.The surface area was with a Micromeritics Tristar II 3020 device-(Micromeritics, Atlanta, GA, USA) using the Brunauer-Emmett-Teller method (BET-(Micromeritics, Atlanta, GA, USA)).The amount of carbon deposition on the spent catalysts was assessed by thermal gravimetric analysis (TGA) in air with a Shimadzu TGA-51.(Shimadzu Corporation, Kyoto, OP, Japan) Using a laser Raman (NMR-4500-JASCO, Tokyo, Japan) spectrometer, the amount of graphitization and the type of carbon deposition over the catalysts were determined (JASCO, Tokyo, Japan).The excitation laser used was 532 nm in wavelength.Using a TEM (transmission electron microscope), "120 kV JEOL JEM-2100F-Akishima, Tokyo, Japan," the structure of the used samples was captured.At 120 kV, TEM micrographs were recorded.An X-ray diffractometer was used to study the crystal structure and phases of new catalysts.The experiment was carried out using a Miniflex Rigaku diffractometer (Rigaku, Bahrain, Saudi Arabia), which ran at 40 kV and 40 mA and generated Cu Ka X-ray radiation.Data were gathered with a step magnitude of 0.01 and a 2 h angle span of 10-85.The temperature-programmed reduction was carried out using a Micromeritics Auto Chem II 2920 instrument (H 2 -TPR) (Micromeritics, Atlanta, GA, USA).Prior to the experiments, the catalysts were heated for one hour at 200 • C in an argon atmosphere, and they were subsequently cooled to ambient temperature.For H 2 -TPR, 0.07 g of the sample was circulated through an H 2 /Ar (v/v, 10/90) gas mixture at a rate of 40 mL/min, while being heated to a temperature of 1000 • C at a rate of 10 • C/min.Using a thermal conductivity detector, the H 2 consumption signal was measured.

Conclusions
This study looked at how an Fe promoter affected a Ni catalyst supported over an alumina catalyst stabilized by ZrO 2 during the CO 2 reforming of CH 4 at 800 • C. The synthesis of all catalysts was performed using the dry impregnation technique.The characterization results showed that the process performance was significantly impacted by the inclusion of Fe.The activity and stability performances of the various catalysts, 5Ni + xFe-10Zr + Al, x = (1, 2, 3, and 4), employed for DRM at 800 • C varied.With an H 2 /CO ratio nearly equal to 1, the 5Ni + 3Fe-10Zr + Al sample had the highest CH 4 and CO 2 conversions of 87% and 90%, respectively.In contrast, the sample with the lowest CH 4 and CO 2 conversions was 5Ni + 4Fe-10Zr + Al.XRD patterns of the 5Ni-10Zr + Al showed the alpha phases of alumina and the tetragonal phases of zirconia.The BET result showed that the 5Ni + 4Fe-10Z + Al sample had the highest surface area, whereas the TGA examination exhibited that the 5Ni + 4Fe-10Z + Al sample lost the least weight.

Figure 1 .
Figure 1.(A) N2 measurements of adsorption and desorption on fresh Ni-supported catalysts, (B) shows the rate of change of pore volume with respect to the pore size.

Figure 1 .
Figure 1.(A) N 2 measurements of adsorption and desorption on fresh Ni-supported catalysts, (B) shows the rate of change of pore volume with respect to the pore size.

Figure 5
Figure 5 displays the Raman spectra of employed Ni-supported catalysts.All the samples display two visible peaks.With the exception of the 5Ni + 1Fe-10Zr + Al sample, the addition of Fe to 5Ni-10Zr + Al samples has a negligible effect on the molecular composition and molecular structure depicted by the position of the Raman shift bands.Raman shift bands can be detected at 1350 cm −1 (D band) and 1580-1620 cm −1 (G band).The D band indicates structural imperfections of graphite, and the G band is attributed to inplane carbon-carbon stretching vibrations of graphite.

Figure 5
Figure 5 displays the Raman spectra of employed Ni-supported catalysts.All the samples display two visible peaks.With the exception of the 5Ni + 1Fe-10Zr + Al sample, the addition of Fe to 5Ni-10Zr + Al samples has a negligible effect on the molecular composition and molecular structure depicted by the position of the Raman shift bands.Raman shift bands can be detected at 1350 cm −1 (D band) and 1580-1620 cm −1 (G band).The D band indicates structural imperfections of graphite, and the G band is attributed to in-plane carbon-carbon stretching vibrations of graphite.

Catalysts 2023, 13 , x 8 of 12 Figure 5 .
Figure 5.This shows the Raman spectra of the used Ni-supported catalysts at 800 °C.

Figure 6 .
Figure 6.(A) TGA curve.(B) DTGA for temperature gravimetric analysis of the used catalyst after 7 h.

Figure
Figure 6B displays the rate of the change in TG against the temperature (DTG).The 5Ni-10Zr + Al sample shows that its carbon gasification took place at around 350 • C, whereas the Fe-promoted samples gasified at above 650 • C.This phenomenon explains the variation of carbon type formed due to the Fe-promoter.

Figure 6 .
Figure 6.(A) TGA curve.(B) DTGA for temperature gravimetric analysis of the used catalyst after 7 h.

Author
Contributions: A.H.F.: project administration, investigation, conceptualization, formal analysis; A.S.A.-F.and A.A.I.: writing-original draft, supervision, data curation, methodology, validation, review, funding acquisition, editing; A.E.A. and Y.A.A.-B.: formal analysis, writing-original draft; F.S.A., A.B. and A.A.A.-Z.: Investigation, conceptualization, data curation, M.F.A. and Y.A.M.: formal analysis, writing-original draft All authors have read and agreed to the published version of the manuscript.Funding: The authors acknowledge the assistance of Deanship for Research & Innovation, Ministry of Education in Saudi Arabia for funding (IFKSURG-2-560), King Saud University, Riyadh, Saudi Arabia.

Table 1 .
Catalysts with particular surface areas, pore volumes, and pore diameters.

Table 1 .
Catalysts with particular surface areas, pore volumes, and pore diameters.