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Article

Advancements in Methane Dry Reforming: Investigating Nickel–Zeolite Catalysts Enhanced by Promoter Integration

by
Anis H. Fakeeha
1,
Ahmed A. Ibrahim
1,
Ahmed I. Osman
2,*,
Ahmed E. Abasaeed
1,
Yousef M. Alanazi
1,
Fahad S. Almubaddel
1,* and
Ahmed S. Al-Fatesh
1
1
Chemical Engineering Department, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia
2
School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, UK
*
Authors to whom correspondence should be addressed.
Processes 2024, 12(9), 1826; https://doi.org/10.3390/pr12091826
Submission received: 8 July 2024 / Revised: 11 August 2024 / Accepted: 23 August 2024 / Published: 28 August 2024
(This article belongs to the Section Chemical Processes and Systems)
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Versions Notes

Abstract

:
A promising method for converting greenhouse gases such as CO2 and CH4 into useful syngas is the dry reformation of methane (DRM). 5Ni-ZSM-5 and 2 wt.% Ce, Cs, Sr, Fe, and Cu-promoted 5Ni-ZSM-5 catalysts are investigated for the DRM at 700 °C under atmospheric pressure. The characterization, including XRD, TPR, TPD, TPO, N2 adsorption–desorption, TGA, TEM, and Raman spectroscopy, revealed that the catalyst’s active sites are distributed throughout the pore channels and on the surface, contributing to the stability of the catalyst. Specifically, the CO2-TPO followed by the O2-TPO experiment using spent catalysts confirmed the oxidizing capacity of CO2 during the DRM reaction. The Ce-promoted catalyst showed the greatest increase in catalytic activity among other catalysts. The 5Ni+2Ce-ZSM-5 catalyst exhibited twice the concentration of acid sites compared to the Cs-promoted counterpart, even though both catalysts achieved similar quantities of active and basic sites. Without compromising H2 and CO selectivity, this finding underscores the crucial role of acid sites in enhancing CH4 and CO2 conversion. With a GHSV of 42,000 mL/(h.gcat), the 5Ni+2Ce-ZSM-5 catalyst demonstrated impressive CH4 conversion rates of 42% at 700 °C and 70% at 800 °C. The reactants spend more time over catalysts during the subsequent reduction of GHSV to 21,000 mL/(h.gcat), resulting in the best catalytic performance with 80% CH4 and 83% CO2 conversions.

1. Introduction

The increasing global population is driving up energy demand, primarily fueled by the use of fossil fuels [1]. This dependence, unfortunately, leads to increasing greenhouse gas emissions, especially CO2, which ultimately fuels global warming and puts our climate at risk [2]. The undeniable human influence on climate change is largely driven by greenhouse gases like CO2 and CH4 [3]. While CO2 plays a significant role, recent studies reveal that CH4 has around 84 times greater warming potential per molecule, making it a particularly problematic contributor to global warming [4]. Researchers are exploring ways to reduce greenhouse gas emissions from industry due to concerns about climate change. The dry reforming of methane is a promising catalytic solution for consuming both CH4 and CO2 together [5]. The production of the DRM involves syngas, which is an important feedstock in industries. These syngas can be further processed into cleaner burning liquid oils or even ultra-pure hydrogen, offering a sustainable path for our future energy needs [6,7]. The DRM’s energy intensity limits its ability to absorb the two most common greenhouse gases. As CO2 is so stable and unreactive, breaking its molecular bonds during the reaction process requires a large input of energy [8]. Traditional dry methane reformers require high temperatures (up to 1000 °C) to overcome reaction limitations and produce syngas. The DRM process utilizes a Ni catalyst to convert both CO2 and CH4 into valuable syngas (a mixture of CO + H2) [9,10,11]. Equation (1) shows the dry reforming reaction:
C H 4 + C O 2 ↔ 2 H 2 + 2 C O   Δ H °   298   K = + 247   kJ   mol
Dry reforming is a highly endothermic reaction. Although efficient, the DRM procedure has a significant challenge: the rapid deactivation of the catalyst [10,12,13], since two different carbon contributors in the feed stream create a compounded effect, intensifying the coking process on the catalyst [14]. The most desirable catalytic systems have generally been shown to be based on highly scattered metallic nickel [15] and frequently outperform noble metal catalysts. The low cost and abundance of nickel offers a compelling choice, which is further amplified by its exceptional ability to activate and cleave C-H and C-C bonds [16,17]. The focus shifts to designing catalysts that are both highly active and coking-resistant [18]. Fortunately, there are various approaches that can address this issue, such as catalyst promotion. Guo et al. looked into the use of a Ce-promoted Ni-based catalyst for the dry reforming of methane, exhibiting outstanding catalytic activity and stability [19]. Likewise, Abahussain et al. investigated the role that Cs played in promoting Ni-based catalyst activity, and their findings also indicated increased catalytic stability and activity [20]. Nguyen et al. explored how incorporating copper and iron (promoters) affects the performance of nickel-based catalysts supported on mesoporous silica. They found that the presence of these promoters significantly alters the nickel’s physicochemical properties, leading to substantial changes in both the catalytic activity and stability of the material [21]. The Sr promoter for a Ni-based catalyst utilized in the dry reforming of methane was studied by Ahmed S. Al-Fatesh et al. Their findings showed that carbon deposits were more effectively reduced and efficiently oxidized by CO2 [22]. Using Ni-based catalysts promoted with Cs, Ce, and Sr, Owgi et al. assessed the methane dry reforming process [23]. The promoters’ introduction had a beneficial effect on the catalyst’s performance and stability. Studies have demonstrated that, with the right supports, heterogeneous catalysts may effectively drive the DRM even under difficult circumstances. The DRM reaction employs various catalyst supports, encompassing oxides like SiO2, Al2O3, and MgO alongside crystalline structures like zeolites and perovskites [24]. Zeolites are used as catalyst supports in reforming reactions due to their microporous structure, large surface area, and strong CO2 adsorption, leading to both enhanced activity and product selectivity [25]. The successive employment of zeolites as catalyst supports in the DRM stems from their remarkable properties: exceptional metal dispersion for maximized active sites, inherent stability for extended catalyst life, minimal support–metal interaction for better catalytic function, and superior coke resistance for cleaner and more efficient reactions [26]. Using a one-pot hydrothermal method, Liu et al. prepared Ni@ZSM-5 catalysts for the steam reforming of naphthalene, as the tar model [27], where the naphthalene’s conversion efficiency was found to be 91.5%, was accompanied by a high yield of hydrogen (6.75%). The catalytic performance of the zeolite-supported Ni catalysts was assessed by Bizkarra et al. [28], where the zeolite L, which was modified by CeO2 and exchanged an alkaline metal (Cs or Na), was used as a support in the steam reforming of blends of bio-oil and bio glycerol in a variety of sizes and geometries, including nanocrystals and discs. It was discovered that the combination of disc-shaped zeolite and cesium inclusion improved Ni metal dispersion, leading to a higher hydrogen yield.
In this study, our objective was to find the most effective promotor with Ni-based catalysts over a ZSM support for producing syngas consistently. We accomplished this by selecting 2 wt.% equivalent promoters (Cu, Fe, Sr, Ce, and Cs), 5 wt.% equivalent nickel, and a ZSM-5 support by using the impregnation technique. We then thoroughly examined their performance under a standard conditions (700 °C, 42,000 mL/(h.gcat), 300 min) for methane reforming with CO2. The extensive characterization of the prepared catalysts using various analytical tools has been performed, providing detailed insights into their structural, morphological, and compositional features.

2. Experimental Materials and Methods

2.1. Materials

Nickel (II) nitrate hexahydrate, Ni (NO3)2.6H2O, 98% (Alfa Aesar, Ward Hill, MA, USA), and strontium nitrate, Sr (NO3), 98%, copper (II) nitrate trihydrate Cu (NO3)2.3H2O, 98%, iron (III) nitrate nonahydrate Fe (NO3)3(H2O)9, 99%, cerium nitrate hexahydrate Ce (NO3)3.6 H2O, 98%, CsNO3, 98% (Alfa Aesar, Ward Hill, MA, USA), were used as sources for the Ni active metal and for promoters, respectively. The ZSM-5 (Si/Al = 9, surface area 235.48 m2/g, pore volume 0.088 cm3/g) (Daiichi Kigenso Kagaku Kogyo Co., Ltd., Osaka, Japan) was used as a support.

2.2. Catalyst Preparation

ZSM-5-supported nickel catalysts were manufactured via wet impregnation. Nickel nitrate hexahydrate (giving 5% Ni) was dissolved in ultrapure water and stirred with the ZSM-5 support for uniform dispersion at 80 °C. Subsequent drying at 120 °C removed solvents, followed by calcination at 600 °C for solidification. The final product, a fine powder, was then ready for use in chemical reactions. Instead of solely using a nickel precursor, the promoted catalyst involves combining nickel and promoter precursors that contribute 2% to the final composition. The preparation process remains the same. Catalysts containing both nickel and an additional promotional element (Ce, Cs, Cu, Fe, or Sr) will be referred to as “5Ni+1x-ZSM-5”, where “x” signifies the specific promoter used. On the other hand, the catalyst containing only nickel will be designated as “5Ni-ZSM-5” for a clear distinction, as shown in Table 1. This naming scheme facilitates the easy identification and discussion of the catalysts throughout the paper. The Supplementary File contains detailed descriptions of the catalyst’s characterizations and performance evaluation under the S1 and S2 headings.

3. Results

3.1. Characterizations

The DRM reaction is carried out over a reduced catalyst. So, the surface area and porosity, X-ray diffraction, and Raman and infrared spectroscopy of reduced-5Ni-ZSM-5 and reduced-5Ni+2M-ZSM-5 (M = Ce, Cs, Cu, Fe, Sr) are carried out. Figure 1 displays the N2 adsorption–desorption isotherms and information regarding the reduced catalyst system’s surface area, pore volume, and pore diameter. The International Union of Pure and Applied Chemistry (IUPAC) classification suggests that analyzing the volume of nitrogen absorbed at different relative pressures can reveal the presence of mesopores in a material [29]. When the volume of absorbed nitrogen is high at high relative pressures, it indicates the presence of mesopores, which further display type IV isotherm behavior. The presence of a type H3 hysteresis loop at higher relative pressures and type IV isotherm behavior indicated non-rigid aggregates like pores with unlimited adsorption at a high P/P0 [30]. The incorporation of promoters (Ce, Cs, Cu, Fe, and Sr) decreases the pristine sample’s surface, indicating the promoters’ filling of the pores.
The powder X-ray diffraction patterns of ZSM-5, reduced-5Ni-ZSM-5, and reduced-5Ni+2M-ZSM-5 (M = Ce, Cs, Cu, Fe, Sr) are shown in Figure 2A,B. The XRD pattern in the wide-angle diffraction range displays the characteristic peaks of the ZSM-5 structure at 2θ = 7.9°, 8.8°, 23.2°, 23.96°, and 24.3°, which correspond to the crystallographic planes (101), (200), (501), (422), and (313) [ICDD reference number 00-044-0002]. On the other hand, the X-ray diffraction patterns of the cubic crystalline structure of the cubic Ni phase are also observed at 2θ = 44.5°, 51.8°, and 76.4° [ICDD reference number 00-004-0850], and these are corresponding to (111), (200), and (220) crystallographic planes. Attributed to their small quantities, the incorporation of the promoters in the zeolite matrix did not show any significant crystalline peaks.
Figure 2C,D shows the Raman spectra of the reduced catalysts. The translation Si/Si and Al/Al lattice modes are evident at 138 cm−1 [31,32]. The vibration mode of Al-O in the aluminosilicate is also evident at about 680 cm−1. The rest of the vibration modes, at about 283 cm−1, 395 cm−1, 921 cm−1, 1050 cm−1, and 1160 cm−1, are related to the Si-O vibration in the aluminosilicates [33,34]. It is noticeable that the polarizability of the electrons over the current catalyst system is highly dependent on the type of promoters used. In the case of the Sr-promoted catalyst, there is a high intense Raman peak, whereas in the case of ceria promotion, only the first few Raman bands are intense (Figure 2C). The intensity of the Raman peaks of the rest of the catalysts is decreased more than six times. Among the rest of the catalysts (5Ni-ZSM-5, 5Ni+2Cs-ZSM-5, 5Ni+2Cu-ZSM-5, and 5Ni+2Fe-ZSM-5), the Si-O-Al Raman vibration peak of the Cs-promoted catalyst is at a maximum (Figure 2D). Figure 2E displays the infrared vibration spectra of catalysts. The spectrum shows characteristic peaks for typical ZSM-5 at 450 cm−1, 550 cm−1, 800 cm−1, 1095 cm−1, and 1225 cm−1 [35], in which peaks at about 300–450 cm−1 and 500–650 cm−1 are related to the pore opening mode of “zeolite and double five rings” in the MFI zeolite. Next, the infrared vibration peaks at about 750–850 cm−1 and 1050–1200 cm−1 are characteristically symmetric and asymmetric TOT mode linkages in the zeolite. The vibration peak at 1630 cm−1 is ascribed to the bending vibration of O-H. Interestingly, a prominent peak of about 1730 cm−1 is also observed on every catalyst. In the literature, this peak is ascribed to bidentate CO2-adsorbed species [36]. The presence of CO2-adsorbed surface species indicates all catalysts are interacting quite well with CO2 even at normal temperature and pressure.
The H2 consumption profile (through H2-temperature-programmed reduction techniques), CO2-temperature-programmed desorption, and NH3-temperature-programmed desorption profiles of catalysts are shown in Figure 3 and Tables S1–S3. TPR is a crucial technique that helps in characterizing the catalyst’s reducibility. Additionally, it can be used to confirm the catalyst’s suitable reduction temperatures [37]. The actual hydrogen consumption is often higher than the theoretical value calculated based on the stoichiometry of the reduction reaction. Some of the hydrogen migrates from the metal surface to the support and reacts with oxygen species not directly associated with NiO, leading to a deviation from theoretical calculations. Additionally, some of the consumed hydrogen may react with reducible surface oxide. The concentration of reducible surface oxide is composed of the combined contribution of individual metal oxides matrixes, mixed oxide compositions, dopant concentration at the surface, and the dopant concentration in bulk [38,39]. These factors are not currently considered. From Figure 3A, the H2 consumption in H2-TPR was observed between the temperatures of 200 °C and 700 °C, indicating the completion of the reduction process before 700 °C. The 5Ni-ZSM-5 catalyst represents two reduction peaks, at about 350 °C and 500 °C, which are attributed to the reduction of NiO at the outer surface of ZSM-5 and in the pore of ZSM-5, respectively [36,40].
Notably, the nonpromoted 5Ni-ZSM-5 sample displayed the lowest hydrogen consumption (7.9 cm3/g STP), resulting in minimum active Ni sites during the DRM (Figure 3D). The addition of promoters to the pristine sample increases the H2 consumption. This indicates that promoters induce a higher reducibility of NiO over the catalyst. Promoters can improve the dispersion of NiO on the support, increasing the surface area available for reduction. Again, the incorporation of a promotor may modify the local structure of NiO by introducing strain or defects that aid in the reduction. During the reductive treatment, hydrogen gas reacts with NiO and forms metallic Ni [41]. Very soon, Ni metal nucleates into metal crystallite, which initiates the hydrogen dissociation. The dissociated hydrogen accelerates the reduction process further [41,42]. The Cu-promoted catalyst has no high-temperature reduction peak, indicating the absence of NiO in the pore channel of ZSM-5. The low-temperature reduction peak of the Cu-promoted catalyst is also extended below 250 °C due to copper oxide’s reduction in the low-temperature range [43,44]. Over the Sr-promoted catalyst, the high-temperature peak becomes more prominent. This indicates a higher population of reducible NiO into the pore channels. The Fe-promoted catalyst has the maximum H2 consumption (21.30 cm3/g STP), indicating the presence of the highest amount of reducible species (iron oxide and nickel oxide) [45,46]. The ceria-promoted catalyst and cesium-promoted catalyst have an equal hydrogen consumption (~18 cm3/g STP), indicating the presence of an equal amount of reducible species over the catalyst surface.
In Figure 3B, the CO2-temperature-programmed desorption profile of the catalysts is shown, which helps to understand the CO2 adsorption capacity and basic profile of the catalyst. All the catalysts show CO2 desorption peaks of about 250 °C and ~400 °C, which are attributed to weak and moderate strength basic sites [47]. The surface hydroxyl and surface oxide contribute to weak basicity and moderate strength basic sites, respectively. Nonpromoted catalysts, Cs-promoted catalysts, and Ce-promoted catalysts have sharp peaks of about 400 °C, in which the CO2-TPD profile of the ceria-promoted catalyst is most intense. This indicates the prominent presence of moderate strength basic sites over 5Ni-ZSM-5 and 5Ni+2Cs-ZSM-5 and the highest population of moderate strength basicity over the 5Ni+2Ce-ZSM-5 catalyst. The Sr-promoted catalyst shows additional peaks in the high-temperature region of CO2 desorption (at 650 °C and 750 °C), which is linked to highly dispersed SrCO3 species and thermally stable surface carbonate, respectively [47,48]. According to Table S2, the cerium-promoted catalyst exhibits the highest CO2 desorption within the intermediate temperature zone. This attribute likely contributes to its observed superior CO2 interaction capability. Likewise, the strontium-promoted catalyst stands out with the maximum CO2 desorption values across both the low- and high-temperature zones. To understand the acid profile of the catalyst, the NH3 temperature-programmed desorption of the catalysts is carried out and shown in Figure 3C and Table S3. This profile could be classified into three categories: weak, medium, and strong acidic sites of the catalysts <240 °C, 240–420 °C, and >420 °C, respectively [39] (Figure 3C). The ammonia desorption peak at higher temperatures proportionally indicates the strength of acidic sites over the catalyst surface. The Cs-promoted catalyst has a minimum concentration of acid sites. The Fe-promoted catalyst shows a very high intense peak for a strong acidic site, indicating the high population of acid sites on the catalyst surface. The Cu-promoted catalyst has a reduction peak at ~550 °C, and the Sr-promoted catalyst has an additional peak at ~630 °C, indicating the presence of the proportional strength of acidity on the catalyst surface. It was observed that an adequate amount of Bronsted acid sites over zeolite may help the heterolytic dissociation of C-H (of CH4) overactive sites (metallic Ni) [49]. However, for methane dissociation, the optimum ratio of acid sites and active sites is required because a high concentration of acid sites may further trigger coke deposition over the catalyst surface [50]. So, with acid sites, there is competition between methane decomposition and coke deposition [51]. An energy-dispersive X-ray spectroscopy (EDX) analysis was performed on the support ZSM-5 (Figure S1) and on a fresh sample of 5Ni+2Ce-ZSM-5 (Figure S2) catalyst to confirm their elemental composition. The analysis confirmed that all the expected elements were present, and the percentage loadings were very close to those intended in the catalyst preparation. This indicates that the catalysts were successfully synthesized with the desired composition. The Raman spectra of the spent 5Ni+2M-ZSM-5 (M = Cs, Ce, Sr, Cu, and Fe) catalyst are shown in Figure 4A. Raman spectroscopy was used to characterize the structure of samples for assessing the quality and crystallinity of the carbon nanomaterials that are formed on the catalyst surface [52]. All the Raman spectra display the presence of peaks related to carbon, which appears beyond 1200 cm−1. The 5Ni-ZSM-5, 5Ni+2Cu-ZSM-5, and 5Ni+2Ce-ZSM-5 catalysts show D bands at 1342 cm−1, 1345 cm−1, and 1344 cm−1 respectively. The D band indicates the existence of disordered carbon structures, including defects on carbon nanotube walls and the presence of amorphous carbon. They show G bands at 1614 cm−1, 1592 cm−1, and 1575 cm−1, respectively. These G bands are attributable to graphite due to the presence of highly organized, graphitic crystalline carbon structures [53]. The bands shift visibly beyond 1614 cm−1 for the 5Ni+2Fe-ZSM-5, 5Ni+2Cs-ZSM-5, and 5Ni+2Sr-ZSM-5 catalysts due to ramifications of D and G bands, and new Raman bands appeared at 1690–2150 cm−1. These bands are attributed to non-suspended and suspended graphene layers [54,55]. The amount of carbon that had been deposited was calculated using a thermogravimetric (TG) analysis. Figure 4B displays the weight loss of the carbon deposits on the catalyst samples as a fraction of the total weight loss. Initially, up to 200 °C, the weight loss is due to the evaporation of water from the spent catalyst. So, the weight loss (%) up to this temperature is not considered as a carbon loss (%). The nonpromoted catalyst sample 5Ni-ZSM-5 indicates the maximum weight loss of 11.7% at a reaction temperature of 700 °C and a reaction duration of 5 h, followed by 5Ni+2Fe-ZSM-5, 5Ni+2Sr-ZSM-5, 5Ni+2Ce-ZSM-5, 5Ni+2Ce-ZSM-5, and 5Ni+2Cu-ZSM-5 catalysts, which produced 7.3%, 6.9%, and 6.2%, 5.6%, and 4% weight loss, respectively. The weight loss (%) over the 5Ni+2Ce-ZSM-5 catalyst is half that of the unpromoted catalyst.
To understand the oxidizing potential of CO2, CO2-TPO, and CO2-TPO followed by O2-TPO for the spent 5Ni/ZSM-5 and spent 5Ni2Ce/ZSM-5 catalysts, their analyses are carried out (Figure 4C,D). For the overspent 5Ni/ZSM-5 catalyst, intense oxidizing peaks of about 500 °C and 800 °C are observed under CO2-TPO. The carbon deposit must be oxidized by CO2 at about 500 °C. There is still some carbon that is not oxidized by CO2 up to a reaction temperature of 700 °C, but it was oxidized at 800 °C. If O2-TPO is carried out just after CO2-TPO, a prominent peak of about 660 °C appears. This result indicates that some of the carbon deposit is not oxidized by CO2 but oxidized by O2. These carbon species can be termed inert carbon species. Also, in the literature, the O2-TPO peak beyond 600 °C was described as an inert carbon species [56]. In the case of the spent 5Ni2Ce/ZSM-5 catalyst, there are diffuse CO2-TPO peaks. TGA also shows less weight loss % over the 5Ni2Ce/ZSM-5 catalyst than the 5Ni/ZSM-5 catalyst. This observation indicates the oxidation of carbon deposits by lattice oxygen induced by ceria, resulting in a lower carbon deposit in total over the catalyst at the end of the reaction. However, if O2-TPO is carried out just after CO2-TPD over the 5Ni2Ce/ZSM-5 catalyst, a broad oxidation peak of about 650 °C is observed. This indicates that either ceria or CO2 is not able to oxidize the inert carbon deposit over the catalyst. The inert carbon can only be oxidized by O2.
The transmission electron microscopy image and particle size distribution of fresh and spent 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts are shown in Figure 5. The particle size of the fresh 5Ni-ZSM-5 catalyst is 27.51 nm, which is grown to 30.57 nm after the reaction. In the same way, the particle size of the 5Ni+2Ce-ZSM-5 catalyst grows from 27.7 nm to 28.0 nm after the reaction. During the growth of the carbon nanotube, the active Ni site is mounted on the tip of the carbon nanotube, where the diameter of Ni is equal to the diameter of the carbon nanotube. Some of the Ni sites encapsulated under the carbon nanotube are also evident over both 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts. The metallic Ni mounted on the tip of the carbon nanotube remains exposed for the DRM reaction, whereas the active sites encapsulated under the carbon nanotube become further inactive for the DRM.

3.2. Catalyst Activity

The DRM was performed on all the ZSM-5 catalysts using a fixed bed reactor at 700 °C and 1 atm. To ensure there were no interferences from the reactor or measurement system, a “blank test” was conducted first. This test copied the actual experiments but omitted the catalyst altogether. Pressure, temperature, feed, and gas hourly space velocity were all maintained at the intended operating conditions. The blank test showed negligible conversions, confirming that any observed conversions during the actual experiments could be attributed solely to the catalyst activity. Furthermore, to enhance the reliability of the results, the obtained data represent the average of three repeated experimental measurements. In Figure 6 and Figure S3, the catalytic activity of the 5Ni+2M-ZSM-5 (M = Cs, Ce, Sr, Fe, and Cu) catalysts in terms of CH4 conversion (%), CO2 conversion (%), H2 selectivity, and CO selectivity are observed. The “CO2-TPO followed by O2-TPO” experiment over the spent catalyst validates the role of CO2 in oxidizing the carbon deposit within the reaction temperature. However, the inert carbon deposit can only be oxidized by O2.
The nonpromoted 5Ni-ZSM-5, Fe, and Cu-promoted 5Ni-ZSM-5 catalysts do not exhibit significant activity as they offer CH4 conversions of less than 20% in 300 min on the stream at 700 °C. Nonpromoted catalysts have a minimum number of active sites, and these active sites are proportionally distributed at the outer surface as well as inside the pore channel of ZSM-5. The Cu-promoted catalyst has a larger number of reducible species, but copper oxide also contributes to this, which is also inactive for the DRM. The catalyst promoted with Cu lacks highly interacting active sites, as indicated by H2-TPR’s study. Previous investigations have shown that such regions are typically located within the pore structure of the support material. Notably, the pore channels of the 5Ni+2Cu-ZSM-5 catalyst do not exhibit the presence of active sites.
Altogether, the catalytic performance of the Cu-promoted 5Ni-ZSM-5 catalyst is even less than the nonpromoted catalyst. The presence of a maximum amount of strong acid sites over the Fe-promoted catalyst (Figure 3C) may induce a higher carbon deposition. Again, over the 5Ni+2Fe-ZSM-5 catalyst, there is a maximum concentration of reducible species, but these reducible species are also related to iron oxide, which is inactive for the DRM. Among the promoted catalysts, the maximum weight loss is observed over the spent 5Ni+2Fe-ZSM-5 catalyst. All total, the catalytic activity results over the 5Ni+2Fe-ZSM-5 catalyst are also inferior towards the DRM.
The 5Ni+2Sr-ZSM-5 catalyst has an intermediate population of active sites (between unpromoted catalyst and Ce or Cs-promoted catalyst), the highest population of basic sites, and the second highest population of acid sites. Although the 5Ni+2Sr-ZSM-5 catalyst has a lower concentration of active sites than the 5Ni+2Cs-ZSM-5 catalyst, it has a maximum population of highly stable Ni (in the pore channel). Additionally, the 5Ni+2Sr-ZSM-5 catalyst bears the population of strong basic sites and strong acid sites. Overall, the catalytic performance of the 5Ni+2Sr-ZSM-5 catalyst improves more than the 5Ni+2Cs-ZSM-5 catalysts. It acquires an average CH4 conversion of 26% and CO2 conversion of 35% at a 700 °C reaction during 300 min on the stream.
Cs-promoted and Ce-promoted catalysts have very similar concentrations of active sites and basic sites. The 5Ni+2Ce-ZSM-5 catalyst has about double the concentration of acid sites than the 5Ni+2Cs-ZSM-5 catalyst. The CH4 conversion and CO2 conversion over the ceria-promoted catalyst are also double that of the cesium-promoted catalyst. The 5Ni+2Cs-ZSM-5 catalyst shows an average of about 22% CH4 conversion and 26% CO2 conversion, whereas the 5Ni+2Ce-ZSM-5 catalyst achieves 42% CH4 conversion and 50% CO2 conversion at 700 °C, during a 6 h reaction. However, the 5Ni+2Ce-ZSM-5, 5Ni+2Cs-ZSM-5, and 5Ni-ZSM-5 catalysts show the same H2 selectivity (43%) and CO selectivity (57%) at the end of 300 min on the stream (Figure S3). This indicates the role of acid sites over the substrate conversion during the DRM. For optimum catalytic performance, a high concentration of active sites and an adequate proportion of acid-base sites are required. The 5Ni+2Ce-ZSM-5 catalyst achieves an average ~42% CH4 conversion and 50% CO2 conversion at 700 °C. As the DRM is an endothermic reaction, upon increasing the reaction temperature to 800 °C, the average CH4 conversion and CO2 conversion jump to 70.5% and 78%, respectively, over the 5Ni+2Ce-ZSM-5 catalyst (Figure 4C,D). Notably, the CO2 conversions surpass the corresponding CH4 conversions, which is attributed to the prevalence of the reverse water gas shift reaction (H2 + CO2  ⇌ H2O + CO). The result of the H2/CO ratio of the samples is exhibited in Figure S3. The H2/CO ratio of the unpromoted catalyst, ceria-promoted catalyst (5Ni+2Ce-ZSM-5), and cesium-promoted catalyst (5Ni+2Cs-ZSM-5) are about similar (~0.8), whereas it is relatively lower over the strontium-promoted catalyst (5Ni+2Sr-ZSM-5). This ratio fluctuates up to 210 min over the Cu-promoted catalyst (5Ni+2Cu-ZSM-5).
Figure 6E and Figures S4 and S5 depict the effect of reaction temperatures on CH4 conversion and CO2 conversion, as well as their related linear fits, using the Arrhenius equation over the best catalyst (5Ni+2Ce/ZSM-5). The trend denotes that the conversions increase as the reaction temperature increases due to the endothermic nature of the DRM reaction and the stability of the active site at such a high temperature. The apparent activation energy of CH4 conversion and CO2 conversion are 28.861 kJ/mol and 21.81 kJ/mol, respectively. The detailed calculation of apparent activation energy is shown in Supporting Information S3.
Figure 6F shows how the gas hourly space velocity (GHSV) affects the catalytic activity of the optimum 5Ni+2Ce/ZSM-5 catalyst when the CH4: CO2 feed ratio is held constant at 1 ATMs at an operating temperature of 800 °C. The gas hourly space velocities studied included 21,000 and 42,000 mL/(h.gcat). The results indicated that as the GHSV value decreased, both CH4 and CO2 conversions increased. It was found that the adsorption and reaction processes for CH4 and CO2 required sufficient time to be completed, as the DRM reaction begins when both gases are adsorbed from the inlet stream onto the catalyst surface [57]. When the gas hourly space velocity (GHSV) value was decreased from 42,000 to 21,000 mL/(h.gcat), this resulted in a rise in the contact time between the reactant components and the catalyst [58]. Consequently, there was an increase in the amount of reactant adsorption, leading to a rise in CH4 and CO2 conversions (up to 80% and from 83%, respectively) during the 300 min on the stream.
Now, the reaction mechanism over the best catalyst is presented in Figure 7. The ZSM-5 support provides its outer surface as well as a pore channel to stabilize the active Ni sites for the dry reforming of methane. Upon using promotors, reducibility, basicity, and acidity profiles are modified greatly. 2 wt.% ceria promotional addition over the 5Ni-ZSM-5 catalyst, the active sites are distributed homogenously on the outer surface as well as in the inner pore channel. This has the highest population of moderate-strength basic sites (contributed by surface oxide) and, more importantly, an adequate proportion of acid–base sites. The DRM reaction is carried out at the outer as well as in the inner pore channels of ZSM-5. The proportional extent of active sites and acidity catalyzes the rate-determining step of the DRM reaction, that is, the dissociation of the C-H bond (of CH4) into CH4−x (x = 1–4). The dissociated CH4−x (x = 1–4) is subsequently oxidized by surface-interacted CO2 (by basic sites) into syngas at the outer as well as inner pores of the ZSM-5 catalyst.

4. Conclusions

This study investigates the effect of 2 wt.% diverse promoters (copper, iron, strontium, cerium, cesium) over 5% nickel catalysts supported by a ZSM-5 zeolite towards the dry reforming of methane. The catalysts are evaluated at a 700 °C reaction temperature, 1 atm pressure, 42,000 mL/(hgcat) gas hourly space velocity, and 300 min on the stream. The “CO2-TPO followed by O2-TPO” experiment confirms the role of CO2 in oxidizing the carbon deposit. The unpromoted catalyst has a minimum concentration of active sites but the promoted catalyst bears a relatively higher concentration of it. The inferior catalytic performance of copper-promoted catalysts is due to a lack of stable active sites inside the pore channels.
The marginally improved catalytic performance of the Fe-promoted catalyst is due to the highest concentration of reducible species made up of major nickel oxide and minor iron oxide. In total, the Fe and Cu-promoted catalysts and unpromoted catalyst has a CH4 conversion of less than 20% up to 300 min. The Sr-promoted 5Ni-ZSM-5 catalyst has a relatively higher concentration of active sites than the nonpromoted catalyst, but this catalyst has the highest population of stable active sites and the maximum concentration of strong acid-basic sites. The catalytic performance of 5Ni2Sr-ZSM-5 is improved to a 26% CH4 conversion at a 700 °C reaction during 300 min on the stream. The concentration of active sites and basic sites over the Ce-promoted 5Ni-ZSM-5 and Cs-promoted 5Ni-ZSM-5 catalysts are about similar, but the concentration of acid sites over the former is about double that of the latter. The CH4 conversion and CO2 conversion over 5Ni+2Ce-ZSM-5 is also double that of the 5Ni+2Cs-ZSM-5 catalyst, but the H2 selectivity and CO selectivity remain the same during the 300 min on the stream. This indicates the role of acid sites on substrate conversion during the DRM. Good catalytic activity needs a high population of active sites as well as an adequate proportion of acid–base sites. The apparent activation energy for CH4 conversion and CO2 conversion over the 5Ni+2Ce-ZSM-5 catalyst is found at 28.86 kJ/mol and 21.81 kJ/mol. The 5Ni+2Ce-ZSM-5 catalyst achieves ~42% CH4 conversion at 700 °C and 70.5% CH4 conversion at 800 °C. Furthermore, upon decreasing the gas hourly space velocity, the contact time between reactants and the catalyst is increased, which results in a rise of CH4 conversion up to 80%. This study highlights the importance of tailoring the catalyst’s reducibility, the role of the pore architect for stabilizing active sites, and acidic–basic characters for optimal performance. Understanding the dynamic interplay between the catalyst pore model, stability of active sites, acidic–basic characters, and reaction mechanisms will pave the way for the rational design of next-generation reforming catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr12091826/s1, Paragraph S1: Catalyst Characterization; Paragraph S2: Activity Test; Table S1: The quantity of hydrogen consumption at different temperatures, total hydrogen consumption, and theoretical hydrogen consumption over different catalysts; Table S2: The quantity of CO2 desorption at different temperatures and the total quantity of CO2 desorption over different catalysts; Table S3: The quantity of NH3 desorption at different temperatures and the total quantity of NH3 desorption over different catalysts; Figure S1: EDX profile of support (ZSM-5); Figure S2: EDX of the 5Ni+2Ce-ZSM-5; Figure S3: (A) H2 selectivity vs. time on stream (B) CO selectivity vs. time on stream over 5Ni-ZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr) catalysts. (C) H2/CO ratio vs. TOS. Reaction conditions: Reaction temperature 700 °C, pressure atmosphere, gas hourly space velocity 42,000 ml/(h.gcat); Paragraph S3. Detailed calculation of apparent activation energy over 5Ni+2Ce-ZSM-5 catalyst; Figure S4: Ln(k) vs. 1/T plot and the slope of Plot for CH4; Table S4: CH4 conversion at different temperatures, intercept, slope, and apparent activation energy; Figure S5: Ln(k) vs. 1/T plot and the slope of Plot for CO2; Table S5: CO2 conversion at different temperatures, intercept, slope, and apparent activation energy.

Author Contributions

A.A.I. and A.S.A.-F.: methodology, conceptualization, investigation, data curation, and writing—original draft; A.H.F. and A.E.A.: methodology, and formal analysis; A.I.O.: Funding acquisition, conceptualization, investigation, data curation, writing—original draft, and validation, Y.M.A. and F.S.A.; data curation and methodology. All authors have read and agreed to the published version of the manuscript.

Funding

Deputyship for Research and Innovation, “Ministry of Education” in Saudi Arabia (project number: IFKSUDR_E116).

Data Availability Statement

This study did not generate or analyze any new data. This article does not qualify for data sharing.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research and Innovation, “Ministry of Education” in Saudi Arabia, for funding this research work through the project number (IFKSUDR_E116).

Conflicts of Interest

The authors affirm that their work in this study was not influenced by any known conflicting financial interests or personal relationships. The authors declare no conflicts of interest to disclose.

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Figure 1. (A–F) N2 adsorption–desorption isotherms of reduced-5Ni-ZSM-5 and reduced-5Ni+2M-ZSM-5 (M = Ce, Cs, Cu, Fe, Sr). S.A: surface area, P.V: pore volume, and P.D: pore diameter.
Figure 1. (A–F) N2 adsorption–desorption isotherms of reduced-5Ni-ZSM-5 and reduced-5Ni+2M-ZSM-5 (M = Ce, Cs, Cu, Fe, Sr). S.A: surface area, P.V: pore volume, and P.D: pore diameter.
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Figure 2. (A) XRD of ZSM-5. (B) XRD of reduced-5NiZSM-5 and reduced-5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr) for fresh catalysts calcined at 600 °C. (C) Raman spectra of reduced 5Ni+2Ce-ZSM-5, and reduced 5Ni+2Sr-ZSM-5 catalysts. (D) Raman spectra of reduced 5Ni-ZSM-5, reduced 5Ni+2Cs-ZSM-5, reduced 5Ni+2Cu-ZSM-5, and reduced 5Ni+2Fe-ZSM-5. (E) FTIR of 5NiZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr).
Figure 2. (A) XRD of ZSM-5. (B) XRD of reduced-5NiZSM-5 and reduced-5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr) for fresh catalysts calcined at 600 °C. (C) Raman spectra of reduced 5Ni+2Ce-ZSM-5, and reduced 5Ni+2Sr-ZSM-5 catalysts. (D) Raman spectra of reduced 5Ni-ZSM-5, reduced 5Ni+2Cs-ZSM-5, reduced 5Ni+2Cu-ZSM-5, and reduced 5Ni+2Fe-ZSM-5. (E) FTIR of 5NiZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr).
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Figure 3. (A) H2-TPR for fresh catalysts calcined at 600 °C of 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (B) CO2-TPD of fresh catalysts calcined at 600 °C: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (C) NH3-TPD of fresh catalysts calcined at 600 °C: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (D) A comparable bar diagram for representing the total H2 consumption amount, total NH3 desorption amount, and total CO2 desorption amount of each catalyst.
Figure 3. (A) H2-TPR for fresh catalysts calcined at 600 °C of 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (B) CO2-TPD of fresh catalysts calcined at 600 °C: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (C) NH3-TPD of fresh catalysts calcined at 600 °C: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5. (x = Ce, Cs, Cu, Fe, and Sr). (D) A comparable bar diagram for representing the total H2 consumption amount, total NH3 desorption amount, and total CO2 desorption amount of each catalyst.
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Figure 4. (A) Raman analysis isotherms of spent catalysts after 300 min, and (B) TGA of spent catalysts after 300 min: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr). (C) CO2-TPO and CO2-TPO followed by O2-TPO of spent 5Ni-ZSM-5. (D) CO2-TPO and CO2-TPO followed by O2-TPO of spent 5Ni+2Ce-ZSM-5.
Figure 4. (A) Raman analysis isotherms of spent catalysts after 300 min, and (B) TGA of spent catalysts after 300 min: 5Ni-ZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr). (C) CO2-TPO and CO2-TPO followed by O2-TPO of spent 5Ni-ZSM-5. (D) CO2-TPO and CO2-TPO followed by O2-TPO of spent 5Ni+2Ce-ZSM-5.
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Figure 5. TEM image of fresh 5Ni-ZSM-5 at different scales: (A) 100 nm, (B) 50 nm, (C) 20 nm, (D) 10 nm, and (E) 5 nm. (F) Particle size distribution graph of fresh 5Ni-ZSM-5. TEM image of spent 5Ni-ZSM-5 at different scales: (G) 100 nm, (H) 50 nm, (I) 20 nm, (J) 10 nm, and (K) 2 nm. (L) Particle size distribution graph of spent 5Ni-ZSM-5. TEM image of fresh 5Ni2Ce-ZSM-5 at different scales: (M) 100 nm, (N) 50 nm, (O) 20 nm, (P) 10 nm, and (Q) 5 nm. (R) Particle size distribution graph of fresh 5Ni2Ce-ZSM-5. TEM image of spent 5Ni2Ce-ZSM-5 at different scales: (S) 100 nm, (T) 50 nm, (U) 20 nm, (V) 10 nm, and (W) 5 nm. (X) Particle size distribution graph of spent 5Ni2Ce-ZSM-5.
Figure 5. TEM image of fresh 5Ni-ZSM-5 at different scales: (A) 100 nm, (B) 50 nm, (C) 20 nm, (D) 10 nm, and (E) 5 nm. (F) Particle size distribution graph of fresh 5Ni-ZSM-5. TEM image of spent 5Ni-ZSM-5 at different scales: (G) 100 nm, (H) 50 nm, (I) 20 nm, (J) 10 nm, and (K) 2 nm. (L) Particle size distribution graph of spent 5Ni-ZSM-5. TEM image of fresh 5Ni2Ce-ZSM-5 at different scales: (M) 100 nm, (N) 50 nm, (O) 20 nm, (P) 10 nm, and (Q) 5 nm. (R) Particle size distribution graph of fresh 5Ni2Ce-ZSM-5. TEM image of spent 5Ni2Ce-ZSM-5 at different scales: (S) 100 nm, (T) 50 nm, (U) 20 nm, (V) 10 nm, and (W) 5 nm. (X) Particle size distribution graph of spent 5Ni2Ce-ZSM-5.
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Figure 6. (A) CH4 conversion at 700 °C, 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (B) CO2 conversion at 700 °C, 1 atmosphere, space velocity of 42,000 mL/(h.gcat). (C) Effect of reaction temperature on the CO2 conversion over 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts at 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (D) Effect of reaction temperature on the CH4 conversion over 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts at 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (E) CH4 conversion (%) against 650 °C–800 °C reaction temperature over 5Ni+2Ce-ZSM-5. (F) CH4 conversion (%) and CO2 conversion (%) over 5Ni+2Ce-ZSM-5 at 21,000 and 42,000 space velocity.
Figure 6. (A) CH4 conversion at 700 °C, 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (B) CO2 conversion at 700 °C, 1 atmosphere, space velocity of 42,000 mL/(h.gcat). (C) Effect of reaction temperature on the CO2 conversion over 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts at 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (D) Effect of reaction temperature on the CH4 conversion over 5Ni-ZSM-5 and 5Ni+2Ce-ZSM-5 catalysts at 1 atmosphere, gas hourly space velocity of 42,000 mL/(h.gcat). (E) CH4 conversion (%) against 650 °C–800 °C reaction temperature over 5Ni+2Ce-ZSM-5. (F) CH4 conversion (%) and CO2 conversion (%) over 5Ni+2Ce-ZSM-5 at 21,000 and 42,000 space velocity.
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Figure 7. The DRM reaction scheme over 5Ni+2Ce-ZSM-5 zeolite.
Figure 7. The DRM reaction scheme over 5Ni+2Ce-ZSM-5 zeolite.
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Table 1. Textural aspects of 5Ni-ZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr).
Table 1. Textural aspects of 5Ni-ZSM-5 and 5Ni+2x-ZSM-5 (x = Ce, Cs, Cu, Fe, and Sr).
SamplesBET-Surface Area (m2/g)Pore Volume (cm3/g)Pore Size (Å)
5Ni-ZSM-52350.08854.2
5Ni+2Ce-ZSM-52260.09962.5
5Ni+2Cs-ZSM-52220.07171.5
5Ni+2Cu-ZSM-52170.08756.8
5Ni+2Fe-ZSM-52640.10352.3
5Ni+2Sr-ZSM-52410.09256.4
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Fakeeha, A.H.; Ibrahim, A.A.; Osman, A.I.; Abasaeed, A.E.; Alanazi, Y.M.; Almubaddel, F.S.; Al-Fatesh, A.S. Advancements in Methane Dry Reforming: Investigating Nickel–Zeolite Catalysts Enhanced by Promoter Integration. Processes 2024, 12, 1826. https://doi.org/10.3390/pr12091826

AMA Style

Fakeeha AH, Ibrahim AA, Osman AI, Abasaeed AE, Alanazi YM, Almubaddel FS, Al-Fatesh AS. Advancements in Methane Dry Reforming: Investigating Nickel–Zeolite Catalysts Enhanced by Promoter Integration. Processes. 2024; 12(9):1826. https://doi.org/10.3390/pr12091826

Chicago/Turabian Style

Fakeeha, Anis H., Ahmed A. Ibrahim, Ahmed I. Osman, Ahmed E. Abasaeed, Yousef M. Alanazi, Fahad S. Almubaddel, and Ahmed S. Al-Fatesh. 2024. "Advancements in Methane Dry Reforming: Investigating Nickel–Zeolite Catalysts Enhanced by Promoter Integration" Processes 12, no. 9: 1826. https://doi.org/10.3390/pr12091826

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