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Article

Impact of Ga, Sr, and Ce on Ni/DSZ95 Catalyst for Methane Partial Oxidation in Hydrogen Production

by
Salma A. Al-Zahrani
1,*,
Omer Bellahwel
2,
Ahmed Aidid Ibrahim
2,
Mohammed F. Alotibi
3,*,
Najat Masood
1,
Sahar Y. Rajeh
1,
Ahmed Al Otaibi
1,
Hessah Difallah A. Al-Enazy
1 and
Ahmed S. Al-Fatesh
2
1
Chemistry Department, Faculty of Science, University of Ha’il, P.O. Box 2440, Ha’il 81451, Saudi Arabia
2
Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
3
Institute of Refining and Petrochemicals Technologies, King Abdulaziz City for Science and Technology (KACST), P.O. Box 6086, Riyadh 11442, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(12), 851; https://doi.org/10.3390/catal14120851 (registering DOI)
Submission received: 30 October 2024 / Revised: 18 November 2024 / Accepted: 21 November 2024 / Published: 24 November 2024
(This article belongs to the Special Issue Advances in Catalytic Dry Reforming of Methane)
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Abstract

:
The greenhouse gas CH4 is more potent than CO2, although both these gases are solely responsible for global warming. The efficient catalytic conversion of CH4 into hydrogen-rich syngas, which also demonstrates economic viability, can deplete the concentration of CH4. This study examines the partial oxidation of methane (POM) prepared by the wetness impregnation process using 5% Ni supported over DSZ95 (93.3% ZrO2 + 6.7% Sc2O3) and promoted with 1% Ga (gallium), 1% Sr (strontium), and 1% Ce (cerium). These catalysts are characterized by surface area porosity, X-ray diffraction, FT-Infrared spectroscopy, Raman infrared spectroscopy, temperature programmed reduction, CO2 temperature-programmed techniques, desorption techniques, thermogravimetry, and transmission electron microscopy. The characterization results demonstrate that Ni is appropriate for the POM because of its crystalline structure, improved metal support contact, and increased thermal stability with Sr, Ce, and Ga promoters. The synthesized catalyst 5Ni+1Ga-DSZ95 maintained stability for 240 min on stream during the POM at 700 °C. Adding a 1% Ga promoter and active metal Ni to the DSZ95 improved the CH4 conversion from 70.00% to 75.90% and raised the H2 yield from 69.21% to 74.80%, while maintaining the reactants’ stoichiometric ratio of (CH4:O2 = 2:1). The 5Ni+1Ga-DSZ95 catalyst is superior to the other catalysts, given its rich catalyst surface, strong metal support interaction, high surface area and low amount of carbon deposit. The high H2/CO ratio (>2.6) and H2 yield close to 75% indicate that 5Ni+1Ga-DSZ95 is a potent industrial catalyst for hydrogen-rich syngas production through partial oxidation of methane.

1. Introduction

The main global energy source is fossil fuels, and the amount of energy consumed worldwide is steadily rising. The global catastrophe of climate change is being accelerated by the unrestrained growth of fossil fuels, which is strangling the atmosphere with greenhouse gases [1]. Methane, a potent greenhouse gas, can be transformed into valuable products through various reforming processes. This not only helps mitigate climate change but also provides a sustainable source of energy. Methane and steam combine in the steam reforming process with a catalyst, usually nickel-based, to create hydrogen-rich syngas [2,3].
C H 4 + H 2 O → C O + 3 H 2     Δ H 29 ° = 206   k J / m o l
This method produces a high ratio of hydrogen to carbon monoxide, which makes it appropriate for a variety of uses, including ammonia synthesis and fuel cells. Nevertheless, it necessitates high pressures and temperatures, which increases energy consumption [2]. In the dry reforming process, the methane reacts with carbon dioxide to produce carbon monoxide and hydrogen gas [3,4].
C H 4 + C O 2 → 2 C O + 2 H 2     Δ H 298 ° = 247   k J / m o l
The technique has the advantage of using carbon dioxide, a greenhouse gas, as a reactant, but it is prone to carbon accumulation on the catalyst, which can lead to deactivation [5,6]. In the partial oxidation of methane (POM), methane reacts with oxygen to produce carbon monoxide and hydrogen, and water [7,8].
C H 4 + 1 2 O 2 → C O + 2 H 2     Δ H 298 ° = − 36   k J / m o l
This method requires lower temperatures than steam reforming and a H2/CO ratio > 2 is also accomplished during the POM [9]. The product of reforming reaction, syngas, serves as a synthetic feedstock for synthesizing high-order hydrocarbon liquid fuel and methanol [10]. The partial oxidation of methane showed significant reactions in both noble metals and non-noble metals such as Ni [10]. Optimizing catalyst composition, shape, and support materials to increase catalytic activity and stability has been the focus of recent study. Moreover, significant advancements have been achieved in the process of partial oxidation of methane by utilizing supports of metals, such as MgO, CeO2, ZrO2, and FeO2. The MgO-supported Ni catalyst formed NiO-MgO solid solution, which resulted in less exposure of metallic Ni for the reaction and finally ended with lower catalytic activity towards PM. In the same way, the fast deactivation over the lanthana-supported Ni catalyst was due to the excessive deposit of carbon during the POM reaction [11]. Simplifying the design and size of reactors through the use of adiabatically operated vessels has led to a reduction in capital expenses [12,13]. Coking poses a serious risk to methane partial oxidation because it can deactivate and decrease the effectiveness of the catalyst. Using particular catalysts can assist in lessening this problem. In this way, Sr, Ga, and Ce have demonstrated potential [14,15]. The entire performance and characteristics of a catalyst are significantly influenced by the support material. It influences the dispersion, stability, and reactivity of the active catalytic components by offering a physical framework for their dispersion [16]. Additionally, CH4 can be oxidized by O2 to formate and formaldehyde, which then split into syngas, by stabilized zirconia (ZrO2+added oxides (e.g., yttria, magnesia, calcia)) and zirconia (pure ZrO2) [17]. Ni-dispersed zirconia may aid in controlling the oxidation of CH4 in the presence of O2 [18]. The ability of zirconia support to quickly release lattice O2 if the molecular oxygen supply is cut off is well known. It is a useful material in many catalytic applications, especially those requiring redox processes because of this special characteristic [19]. Significant phase transition problems with the ZrO2-based catalyst system occur at high temperatures, causing the active sites to sinter and ultimately deactivate. As a result, stabilized zirconia will boost confidence more than zirconia by itself. Recently, “metal-oxide stabilized zirconia” has drawn major attention for high-temperature applications [20,21,22,23]. Ni dispersed over 30 wt.% TiO2-70 wt.% ZrO2 (Ni/TiZr) achieved a 30% H2 yield with a 4.2 H2/CO ratio at a 600 °C reaction temperature [24]. The promotional addition of Ce promotors over Ni/TiZr was also found to be advantageous for DRM. The addition of ceria over Ni/TiZr increased the moderate level basic sites as well as the oxygen endowing capacity of the lattice, resulting in a 38–42% H2 yield with a 3.8 H2/CO ratio. Upon promotional addition of Sr, Ni/TiZr benefited from the higher degree of reducibility and enhanced concentration of active sites for POM, resulting in a >40% H2 yield with a 3.7 H2/CO ratio. Promoters like cerium improve oxygen mobility, facilitate carbon gasification, and improve metal dispersion. Strontium enhances support connections, increases Ni reducibility, and dynamically controls CO2 interaction. Gallium interacts with oxygen species, activates CO2, and prevents the creation of carbon, which may change the acidity of the catalyst.
Due of the identical radii of Sc3+ and Zr4+, scandia–zirconia, among other stabilized zirconia (such as lantana–zirconia, phosphate–zirconia, and scandia–zirconia), has the ability to form an oxide vacancy without altering the size imbalance [25]. Among all the zirconia base solid solutions, scandia stabilized zirconia has the highest electrical conductivity and is regarded as a promising solid electrolyte for solid oxide fuel cells intended to operate at moderate temperatures (700–900 °C) [17,18]. In the past, different amounts of ceria and scandia were used to add ZrO2 as electrolyte materials for solid oxide fuel cells operating at intermediate temperatures. The material 4Ce7ScZr was found to have the maximum ionic conductivity [26]. When it comes to conducting electricity at high temperatures, scandia–ceria-integrated zirconia (CSZ) is a proven industrial-grade material. This novel material is gaining popularity for high-temperature catalytic reactions similar to the POM after its successful application in solid oxide electrolytic materials. Ni dispersed over ceria–scandia-incorporated zirconia achieved (CSZ) a >45% H2 yield with a H2/CO ratio 3 in the POM reaction [27]. During the POM process, CSZ is anticipated to support the active site, stabilize the catalyst against high temperatures, and improve oxygen mobility for the oxidation reaction. The sufficient catalytic improvement with a promoted Ni/stabilized-ZrO2 system gives the encouragement to develop a high-performance catalyst for the POM reaction by using Ni/stabilized ZrO2. In this work, Ni is supported over a DSZ95 (Sc-modified zirconia) catalyst for the partial oxidation of methane in a fixed bed reactor at 700 °C. To acquire high catalytic activity and stability, the pristine catalyst is promoted with a 1% promoter of Sr to produce oxygen vacancies, Ce to enhance redox characteristics, and Ga to modify acid–base properties. TPR (temperature programmed reduction), CO2-TPD (temperature-programmed desorption), XRD (X-ray diffraction analysis), FTIR (Fourier Transform Infrared Spectroscopy), BET (Brunauer, Emmett and Telle), TEM (transmission electron microscopy), Raman, and other techniques were used to examine the properties of both new and spent catalysts. The goal was to identify the cause of their exceptional catalytic activity and stable performance.

2. Results

2.1. N2 Isothermal Adsorption–Desorption

Figure 1 shows that the catalysts display type IV Nitrogen adsorption–desorption isotherms. The hysteresis loops observed are of type H3, characteristic of mesoporous materials with slit-type pores [28]. Table 1 displays the BET surface area, pore volume, and pore size of the catalysts under study. When the promoters (Ce, and Ga) are added, the catalyst’s surface area usually increases slightly compared to the unpromoted catalyst. 5Ni+1Ga-DSZ95 shows the maximum surface area (8.9 m2/g), whereas 5Ni-DSZ95 has a 7.1 m2/g surface area. It indicates that the addition of Ce or Ga promoters results in a higher dispersion of Ni. The addition of Sr might lead to the blocking of existing pores within the catalyst structure. This reduces the accessible surface area. On the other hand, the catalyst 5Ni-DSZ95 has the smallest pore size (28.4 nm). When promoters, particularly Ce and Ga, are added, the pore size increases substantially to 33.8 nm to 33.4 nm. Based on the data provided in Table 1, 5Ni+1Ga-DSZ95 seems to be the most promising adsorbent in terms of surface area, pore volume and pore size, indicating it may have the highest adsorption capacity.

2.2. X-Ray Diffraction (XRD) Analysis

Figure 2 displays the XRD diffraction patterns for fresh 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 catalysts. All catalyst shows rhombohedral scandium zirconium oxide (Sc2Zr5O13) phases at Bragg’s angle at 30.4°, 35.3°, 50.7°, 60.3°, 63°, 74.7°, 82.3°, 85.1° and 95.5° (JCPDS reference number 00-035-1492) and cubic NiO at Bragg’s angle at 37.3° and 43.4° (JCPDS reference number 00-073-1519). Most of the diffraction patterns for the monoclinic ZrO2 phase are merged with the rhombohedral scandium zirconium oxide (Sc2Zr5O13) phase, except 28.3° (JCPDS reference number 01-078-0047). Based on the XRD pattern, it seems that the addition of different dopants (Ga, Ce, Sr) to the base material (5Ni-DSZ95) results in similarity in the crystal structure. The relative intensities of the peaks suggest that the dopants have different effects on the abundance of the various phases. Upon the addition of strontium, the diffraction patterns are shifted relatively towards a lower Bragg’s angle whereas Ga promotion addition brings a diffraction angle shift towards a relatively higher angle. It indicates lattice expansion and contraction upon Sr and Ga addition over 5Ni/MCM-41 respectively.

2.3. H2 Temperature Programmed Reduction

H2-TPR has been considered an effective technique for identifying the interaction between the support and the active metals. The reducibility of various catalysts was investigated by H2-TPR and the results are shown in Figure 3 and Table 2. 5Ni-DSZ95 showed two reduction peaks about 375 °C and 500 °C. The earlier reduction peak is attributed to the NiO species, which moderately interacted with the support, whereas the later reduction peak is ascribed to the NiO species, which strongly interacted with the support [29,30]. Upon promotional addition of 1 wt.% Sr over 5Ni-DSZ95, the reduction peak intensity of these peaks is increased and a diffuse reduction peak about 675 °C is observed. The diffuse peak is attributed to the reduction of SrCO3 in the presence of H2 [31]. The formation of SrCO3 by the reaction of Sr+2 and CO2 (from air) during calcination was reported earlier [32,33]. Overall, the 5Ni+1Sr-DSZ95 catalyst has a higher concentration of reducible NiO than the unpromoted catalyst. The reduction profile of the 1 wt.% Ce-promoted 5Ni-DSZ95 catalyst shifted towards a lower temperature, whereas for the 1 wt.% Ga-promoted 5Ni-DSZ95 catalyst, the reduction peaks shifted towards a higher temperature. It indicates the weak metal support interaction in the case of 5Ni+1Ce-dSZ05 and strong metal support interaction for 5Ni+1Ga-DSZ95. Table 2 displays the relative H2 consumptions during the TPR analysis. The reducibility values of the catalyst inserted in the last column of Table 2 indicate that the catalysts possess similar reducibility values.

2.4. Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR is a potent method for examining the functional groups in materials and learning about their composition and structure. Figure 4 displays the as-prepared materials’ FTIR spectroscopy, which was measured in the 500–4000 cm−1 range. All of the prepared samples displayed comparable FTIR spectra, as the image illustrates. The broad and strong absorption band centered at around 3437 cm−1 is ascribed to the stretching vibration of (OH−) groups and hydrogen-bonded water molecules [34]. The peak at 1470 cm−1 is probable due to the bending vibration of the C-H bond in a methyl group. The band centered at around 1635 cm−1 is due to the H–O–H bending vibration modes, confirming the presence of water molecules in the interlayer region [35]. The bands centered at 1525 cm−1 and 1470 cm−1 are linked to the carbonate species (CO3−2) in the interlayer spaces; the v3 antisymmetric stretching modes are responsible for the former, while the O–C–O asymmetric vibrations are responsible for the latter [36,37]. 5Ni+1Sr-DSZ95 is characterized by the presence of intense vibration peaks about 1525 cm−1 and 1470 cm−1 due to the presence of SrCO3. The presence of SrCO3 is also verified in H2-TPR.

2.5. Temperature Programmed Desorption (CO2-TPD)

In Figure 5, the CO2-TPD profile of the catalysts is shown. This profile helps to understand the CO2 adsorption capacity and basic profile of the catalyst. All four materials show a similar general trend, with the TCD signal increasing as the temperature increases. The addition of different elements (Sr, Ga, Ce) to the 5Ni-DSZ95 base material has a noticeable effect on the TCD signal. All the catalysts show CO2 desorption peaks at different temperature ranges. The peaks at about 100 °C are attributed to CO2 interacting with the hydroxyl weak Brønsted basic sites, while the peaks at 250 °C and around 500 °C are attributed to CO2 interacting with oxygen anion moderate strength basic sites. The peaks appearing beyond 500 °C are caused by the strong basic sites linked to adsorption on low-coordination oxygen anions acting as strong basic sites [38]. The Sr-promoted catalyst shows a sharp peak in the high-temperature region of CO2 desorption (at 800 °C), which is linked to highly dispersed SrCO3 species and thermally stable surface carbonate, respectively [39,40]. Table 3 exhibits the CO2 desorption profile of different catalysts. Due to presence of basic SrCO3, the Sr-promoted 5Ni-DSZ95 catalyst has the highest basic value (1.5533 cm3/g), while the Ga-promoted catalyst has the lowest basicity (0.4148 cm3/g).

2.6. Raman Spectra of the Fresh Catalysts

The Raman spectra of 5Ni-DSZ95 and Ga, Sr, and Ce-promoted 5Ni-DSZ95 catalysts are shown in Figure 6. The Raman bands at 150 cm−1, 290 cm−1, 460 cm−1, 570 cm−1, 700 cm−1, 800 cm−1 and 1095 cm−1 are observed over all catalysts. The Raman band at 150 cm−1 and 570 cm−1 were reported for the rhombohedral ScZrOx phase [41]. The band about 570 cm−1 and 460 cm−1 is related to the oxide vacancy in the solid solution [42,43,44]. The band at 700 cm−1 is related to the defect-induced peaks [45], whereas the Raman intense band about 1098 cm−1 is ascribed to lattice defects [46]. The vacancy or defects are known for decomposition sites for molecular oxygen, ozone, N2O, etc., which must influence the oxidation reaction [47,48,49]. The Raman intensity of 5Ni+1Ga-CSZ95 and 5Ni+1Sr-DSZ95 are similar and in the same way, the Raman intensities of 5Ni-DSZ95 and 5Ni+1Ce-DSZ95 catalysts are similar. In comparison, the Raman intensity of 5Ni+1M-CSZ95 (M = Ga, Sr) is higher than the other catalysts.

2.7. The Catalytic Activity Evaluation

The catalytic activity of the 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 catalysts towards partial oxidation of methane was tested at 700 °C. Figure 7A shows the CH4 conversion profiles of the catalysts. All catalysts exhibit a rapid increase in CH₄ conversion during the initial time on stream (TOS). The conversion assumes flat levels following the initial increase, indicating a stable catalytic performance. The conversion curves indicate that the catalysts are relatively stable under the reaction conditions. Interestingly, both 5Ni+1Ga-DSZ95 and 5Ni+1Sr-DSZ95 are defect rich, which provides sites for the adsorption and dissociation of oxygen molecules. The 5Ni+1Ga-DSZ95 shows the highest CH4 conversion and H2-yield throughout 240-min time on stream. The constantly high activity throughout the reaction over 5Ni+1Ga-DSZ95 is due to the strongest metal support interaction over the largest catalyst surface area. This indicates that this catalyst exposes most of the stable active sites on the largest surface. 5Ni+1Ga-DSZ95 shows a ~75% CH4 conversion, ~74% H2 yield and H2/CO ratio >2.6 constant up to 240-min time on stream. The mechanisms of the process are summarized in the supplementary information (Figure S3). The Ga-promoted catalysts enhance the performance of POM through pronounced interaction of CH4 at active sites under strong metal support interaction and enhanced dissociation of molecular oxygen at defect-rich sites. Defects act as sites for oxygen adsorption and activation, facilitating the dissociation of molecular oxygen into atomic oxygen, which promotes the oxidation of methane and intermediate species.
The metal–support interaction of Sr-promoted 5Ni-DSZ95 is somewhat weaker than the Ga-promoted 5Ni-DSZ95 catalyst. The pore blocking of existing pores also decreased the surface area of the catalyst 5Ni+1Sr-DSZ95 compared to 5Ni+1Ga-DSZ95. In addition, the catalytic activity of 5Ni+1Sr-DSZ95 is relatively inferior compared to the 5Ni+1Ga-DSZ95 catalyst. 5Ni+1Sr-DSZ95 shows a 73% CH4 conversion, 72% H2 yield and >2.6 H2/CO ratio. The metal–support interaction of the ceria-promoted 5Ni-DSZ95 catalyst is the lowest out of all the catalysts. With respect to the Ga- and Sr-promoted catalyst, the Ce-promoted catalyst also has a lower concentration of defects. Molecular oxygen is less interactive over 5Ni+1Ce-DSZ95 catalysts than 5Ni+1M-DSZ95 (M = Ga, Sr) catalysts. The catalytic activities for the POM over unpromoted catalysts and Ce-promoted 5Ni-DSZ95 are found to be comparable and less than 5Ni+1M-DSZ95 (M = Ga, Sr) catalysts. The 5Ni-DSZ95 catalysts maintain the highest H2/CO ratio throughout the reaction (2.74), while the promoted catalysts depicted slightly lower H2/CO ratios compared to the unpromoted catalyst (2.64–2.69).

2.8. Raman Spectra for the Spent Catalysts

The Raman analysis of the catalysts that were employed—5Ni-DSZ95, 5Ni+1Sr/DSZ95, 5Ni+1Ga/DSZ95, and 5Ni+1Ce-DSZ95—is shown in Figure 8. It is possible to see the D (deformation) and G (graphitic) bands at roughly 1345 and 1590 cm−1, respectively. On the other hand, the D and G bands can be seen at about 1341 and 11,588 cm−1, respectively, for expended 5Ni-DSZ95. There are carbon deposits in the wasted catalysts with different levels of graphitization. A higher degree of graphitization has been discovered to be indicated by carbon deposits with a low ID-to-IG ratio [50]. It is clear from the figure that 5Ni+1Sr-DSZ95 exhibits the highest degree of graphitization, with 5Ni+1Ga/DSZ95 ranking in second.

2.9. Thermo-Gravimetric Analysis (TGA)

Figure 9 depicts the TGA analysis of the catalysts. All samples show a gradual weight loss as the temperature increases. This is possible due to the decarburization or volatilization of components in the samples. The samples exhibit significant weight losses, particularly between 300 °C and 500 °C. The onset temperatures at which weight loss begins to accelerate appear to be around 300 °C for all samples. The end set point is the temperature at which the weight loss essentially ceases, which was expected to be between 500 °C and 600 °C.
The 5Ni+1Ga-DSZ95 sample exhibits a relatively stable weight loss compared to the others, suggesting that adding Ga might result in the lowest amount of carbon deposit. On the other hand, the 5Ni+1Sr-DSZ95 sample shows the highest overall weight loss, indicating that the addition of Sr must result in the highest amount of carbon deposit. The apparent increase in weights during TGA could be related to the Ni oxidation due to atmospheric oxygen [51,52]. By combining the Raman and TGA result of spent catalysts, it can be concluded that the excessive growth of graphitic carbon deposit over 5Ni+1Sr-DSZ95 mainly causes its inferior catalytic activity compared to the 5Ni+1Ga-DSZ95 catalyst.

2.10. TEM Images of Selected Catalysts

The TEM pictures of the 5Ni-DSZ95 and 5Ni+1Ga-DSZ95 catalysts are displayed in Figure 10. In TEM images, the promoters and Ni are finely dispersed over the support and the boundary for the particles cannot be recognized, except for the 5Ni+1Ga/DSZ95 catalyst. There is a significant amount of carbon on the 5Ni/DSZ95 catalyst, while the 5Ni+1Ga/DSZ95 catalyst shows a relatively clean surface with minimal carbon deposition. Upon comparing the catalytic activity of 5Ni+1Ga/DSZ95 with literature reports on the POM, it is evident that the activity of the current catalyst is at its maximum (Table 4).

3. Materials and Methods

Catalyst Preparation

In an 80 mL glass crucible, 10 mL of distilled water and 5% of Ni were mixed and agitated at room temperature to create 5Ni+x-DSZ95, where x = (0, 1Sr, 1Ce, and 1Ga). After that, DSZ95 (93.3% ZrO2 + 6.7% Sc2O3) was introduced as a support. The solution was supplemented with 1Sr, 1Ce, and 1Ga nitrate promoter. After that, the mixture was left to dry at 80 °C for 30 min without being stirred. The catalysts were calcined in the air for up to three hours at 700 °C using a heating rate of 3 °C/h. Daiichi Kigenso Kagaku Kogyo Co., Ltd., Saka, Japan, donated the DSZ95 assistance. The catalytic analysis (S1) and characterization techniques used in this partial oxidation investigation (S2) are thoroughly described in the Supplemental Material. These techniques closely resemble those that have already been documented in the literature [1,10]. The details of the catalyst compositions are presented in the Supplementary Information (Figure S2).

4. Conclusions

This work examined how different promoters (Sr, Ga, and Ce) affected the structural, textural, and catalytic characteristics of 5Ni-DSZ95 (93.3% ZrO2+6.7% Sc2O3) catalysts for the partial oxidation of methane. The addition of promoters affected the surface area, pore volume, CO2 adsorption capacity, basic sites, and catalytic activity, according to characterization procedures. It was found that the catalyst enhanced by Ga performed the best in terms of CH4 conversion (75.90%) and H2 yield (74.80%) due to the defect-rich catalyst surface, strong metal support interaction, maximum catalyst surface area and least amount of carbon deposit over the catalyst surface. The Sr-promoted 5Ni-DSZ95 catalyst is as defect rich as 5Ni+1Ga-DSZ95 but the 5Ni+1Sr-DSZ95 catalyst has a decreased surface area and decreased metal–support interaction compared to the 5Ni+1Ga-DSZ95 catalyst. The 5Ni+1Sr-DSZ95 catalyst also faces a high quantity of graphitic carbon deposit during the POM reaction. Relatively, the catalytic activity of 5Ni+1Sr-DSZ95 is inferior compared to the 5Ni+1Ga-DSZ95 catalyst. Overall, 5Ni+1Sr-DSZ95 acquires a 73% CH4 conversion, 72% H2 yield and >2.6 H2/CO ratio. In the same way, the Ce-promoted 5Ni-DSZ95 catalyst has the weakest metal support interaction and demonstrates lower catalytic activity. The catalytic activity of 5Ni+1Ce-DSZ95 is found to be comparable to unpromoted catalysts. This process faces limitations such as soot formation and catalyst deactivation, which is particularly evident in carbon deposition on the Sr-promoted catalyst. These findings highlight the crucial role of promoter selection in optimizing catalyst performance and stability, while also emphasizing the need for further research to address the challenges of carbon formation and long-term catalyst deactivation in the partial oxidation of the methane process. The process’s additional drawback is the possibility of an explosion brought on by the combination of oxygen with combustible gases, which restricts its widespread use. Additionally, pure oxygen must be supplied by an air separation facility, which raises the financial expense.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14120851/s1: Catalytic Analysis S1; Characterization of the Catalyst S2; Detailed description of catalyst synthesis S3; Figure S1. Schematic diagram of the exp. set-up for POM. Figure S2. EDX analysis for composition of the samples: (A) 5Ni+1Ce-DSZ95; (B) 5Ni+1Ga-DSZ95; (C) 5Ni+1Sr-DSZ95. Figure S3. Mechanistic representation of Ga- promoted for Ni catalyst in POM.

Author Contributions

S.A.A.-Z., A.A.I. and A.S.A.-F.: methodology, data curation, conceptualization, investigation, data curation, writing—review and editing; O.B., M.F.A. and N.M.: methodology, formal analysis, software; H.D.A.A.-E.: formal analysis, Funding acquisition, Validation, A.A.O. and S.Y.R.: data curation writing—original draft preparation. All authors have read and agreed to the published version of the manuscript.

Funding

Scientific Research Deanship at University of Ha’il—Saudi Arabia through project number (RG-24 019).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

This research has been funded by the Scientific Research Deanship at University of Ha’il—Saudi Arabia through project number (RG-24 019).

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00851 g001
Figure 1. The N2 adsorption isotherms for 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
Figure 1. The N2 adsorption isotherms for 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
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Catalysts 14 00851 g002
Figure 2. XRD diffraction patterns of fresh (A) pristine 5Ni-DSZ95 and 5Ni-DSZ95 doped with various elements (Ga, Ce, Sr); (B) zoomed-in different peak intensities for 5Ni-DSZ95 and its doped samples; (C) zoomed-in the shift between 5Ni-DSZ95 and 5Ni+1SrGa/DSZ95.
Figure 2. XRD diffraction patterns of fresh (A) pristine 5Ni-DSZ95 and 5Ni-DSZ95 doped with various elements (Ga, Ce, Sr); (B) zoomed-in different peak intensities for 5Ni-DSZ95 and its doped samples; (C) zoomed-in the shift between 5Ni-DSZ95 and 5Ni+1SrGa/DSZ95.
Catalysts 14 00851 g002
Catalysts 14 00851 g003
Figure 3. H2-TPR profiles of the promoted and pristine catalysts.
Figure 3. H2-TPR profiles of the promoted and pristine catalysts.
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Catalysts 14 00851 g004
Figure 4. Infrared spectra of fresh catalysts calcined at 600 °C: 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
Figure 4. Infrared spectra of fresh catalysts calcined at 600 °C: 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
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Catalysts 14 00851 g005
Figure 5. CO2-TPD of fresh catalysts calcined at 600 °C: 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
Figure 5. CO2-TPD of fresh catalysts calcined at 600 °C: 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95.
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Catalysts 14 00851 g006
Figure 6. Raman spectra of the fresh catalysts of (A) 5Ni+1Ga-DSZ95 and 5Ni+1Sr-DSZ95, (B) 5Ni-DSZ95 and 5Ni+1Ce-DSZ95.
Figure 6. Raman spectra of the fresh catalysts of (A) 5Ni+1Ga-DSZ95 and 5Ni+1Sr-DSZ95, (B) 5Ni-DSZ95 and 5Ni+1Ce-DSZ95.
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Catalysts 14 00851 g007
Figure 7. Catalytic activity results of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 catalysts versus time on stream (TOS): (A) CH4 conversion (%); (B) H2-yield (%) (C) CO yield (%); (D) H2/CO ratio.
Figure 7. Catalytic activity results of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 catalysts versus time on stream (TOS): (A) CH4 conversion (%); (B) H2-yield (%) (C) CO yield (%); (D) H2/CO ratio.
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Catalysts 14 00851 g008
Figure 8. Raman spectra for the spent catalysts of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95, showing the extent of graphitization of the carbon deposits.
Figure 8. Raman spectra for the spent catalysts of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95, showing the extent of graphitization of the carbon deposits.
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Catalysts 14 00851 g009
Figure 9. TGA profiles of spent catalysts of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 operated at 700 °C.
Figure 9. TGA profiles of spent catalysts of 5Ni-DSZ95, 5Ni+1Sr-DSZ95, 5Ni+1Ga-DSZ95, and 5Ni+1Ce-DSZ95 operated at 700 °C.
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Catalysts 14 00851 g010
Figure 10. (A,B) TEM image of fresh 5Ni-DSZ95 at 100 nm and 20 nm scales. (C) Particle size distribution of fresh 5Ni-DSZ95. (D,E) TEM image of spent 5Ni-DSZ95 at 100 nm and 20 nm scales. (F) Particle size distribution of spent 5Ni-DSZ95. (G,H) TEM image of fresh 5Ni+1Ga-DSZ95 at 100 nm and 20 nm scales. (I) Particle size distribution of fresh 5Ni+1Ga-DSZ95. (J,K) TEM image of spent 5Ni+1Ga-DSZ95 at 100 nm and 20 nm scales. (L) Particle size distribution of spent 5Ni+1Ga-DSZ95 catalysts.
Figure 10. (A,B) TEM image of fresh 5Ni-DSZ95 at 100 nm and 20 nm scales. (C) Particle size distribution of fresh 5Ni-DSZ95. (D,E) TEM image of spent 5Ni-DSZ95 at 100 nm and 20 nm scales. (F) Particle size distribution of spent 5Ni-DSZ95. (G,H) TEM image of fresh 5Ni+1Ga-DSZ95 at 100 nm and 20 nm scales. (I) Particle size distribution of fresh 5Ni+1Ga-DSZ95. (J,K) TEM image of spent 5Ni+1Ga-DSZ95 at 100 nm and 20 nm scales. (L) Particle size distribution of spent 5Ni+1Ga-DSZ95 catalysts.
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Table 1. Textural aspects of the catalysts.
Table 1. Textural aspects of the catalysts.
SampleBET Surface Area
(m2/g)		Pore Volume
(cm3/g)		Average Pore Size
(Å)
5Ni-DSZ957.10.04284
5N+1Sr-DSZ956.90.04306
5Ni+1Ce-DSZ958.10.06338
5Ni+1Ga-DSZ958.90.06334
Table 2. H2 consumption in the TPR analysis.
Table 2. H2 consumption in the TPR analysis.
SampleMax Peak
Temperature (°C)		Quantity
(cm3/g STP)		DR a (%)
5Ni-DSZ9549521.816.6
5Ni+1Sr-DSZ9548523.815.7
5Ni+1Ga-DSZ9556318.110.4
5Ni+1Ce-DSZ9544218.69.4
Degree of Reduction (DR) a (%) = (consumed H2 during H2-TPR/theoretical H2 essential to complete the reduction).
Table 3. CO2 desorption quantities in the TPD analysis.
Table 3. CO2 desorption quantities in the TPD analysis.
SampleTemperature
(°C)		Quantity
(cm3/g STP)		Total Quantity
(cm3/g STP)
5Ni-DSZ95101.50.15270.9447
346.10.6672
589.80.0706
767.20.0084
894.30.0458
5Ni+1Sr-DSZ95103.60.20261.5533
346.00.2887
413.60.0185
605.00.0743
802.70.9692
5Ni+1Ce-DSZ95105.80.26420.7247
329.90.3761
423.90.0165
608.60.0679
5Ni+1Ga-DSZ9596.70.15080.4148
318.80.2061
439.80.0216
591.60.0363
Table 4. A comparison of the present work activity with the results mentioned in previous studies.
Table 4. A comparison of the present work activity with the results mentioned in previous studies.
SampleCatalyst Weight (mg)CH4/O2Reaction Temperature (°C)CH4 Conversion (%)H2 Yield (%)Ref.
Ni/Ce–ZrO2152:180027.4-[19]
CexZr1–xO2102:150040.020[53]
YSZ *302:1400–47540.010[54]
14%Ni/ZrO2 70044.036[50]
10Ni/PO4+ZrO2102:160044.036[10]
2wt% Cs, Ce, Sr over Ni/TiZr102:160045.040[24]
Ni/ZrO2102:170060.045[55]
Ni/ZrO2103:3700--43[56]
Ni/ZrO21055:35800--10[57]
Ni/Y2O3-ZrO2103:3700--45[58]
Ni/La2O3-ZrO2-55:35800--11[57]
Ni/CeO2-ZrO2151:1700--34[59]
5Ni-DSZ951002:170069.270[R*]
[R*]
5Ni+1Ga-DSZ951002:170075.975
YSZ * = yttrium-stabilized zirconia; DSZ95 = 93.3% ZrO2+6.7% Sc2O3; R* = this work.
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Al-Zahrani, S.A.; Bellahwel, O.; Ibrahim, A.A.; Alotibi, M.F.; Masood, N.; Rajeh, S.Y.; Al Otaibi, A.; Al-Enazy, H.D.A.; Al-Fatesh, A.S. Impact of Ga, Sr, and Ce on Ni/DSZ95 Catalyst for Methane Partial Oxidation in Hydrogen Production. Catalysts 2024, 14, 851. https://doi.org/10.3390/catal14120851

AMA Style

Al-Zahrani SA, Bellahwel O, Ibrahim AA, Alotibi MF, Masood N, Rajeh SY, Al Otaibi A, Al-Enazy HDA, Al-Fatesh AS. Impact of Ga, Sr, and Ce on Ni/DSZ95 Catalyst for Methane Partial Oxidation in Hydrogen Production. Catalysts. 2024; 14(12):851. https://doi.org/10.3390/catal14120851

Chicago/Turabian Style

Al-Zahrani, Salma A., Omer Bellahwel, Ahmed Aidid Ibrahim, Mohammed F. Alotibi, Najat Masood, Sahar Y. Rajeh, Ahmed Al Otaibi, Hessah Difallah A. Al-Enazy, and Ahmed S. Al-Fatesh. 2024. "Impact of Ga, Sr, and Ce on Ni/DSZ95 Catalyst for Methane Partial Oxidation in Hydrogen Production" Catalysts 14, no. 12: 851. https://doi.org/10.3390/catal14120851

APA Style

Al-Zahrani, S. A., Bellahwel, O., Ibrahim, A. A., Alotibi, M. F., Masood, N., Rajeh, S. Y., Al Otaibi, A., Al-Enazy, H. D. A., & Al-Fatesh, A. S. (2024). Impact of Ga, Sr, and Ce on Ni/DSZ95 Catalyst for Methane Partial Oxidation in Hydrogen Production. Catalysts, 14(12), 851. https://doi.org/10.3390/catal14120851

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