Investigation of Partial Oxidation of Methane at Different Reaction Parameters by Adding Ni to CeO₂ and ZrO₂ Supported Cordierite Monolith Catalyst

Abstract

The climate crisis, driven by increasing CO₂ levels in the atmosphere, has heightened the need for new, environmentally friendly energy sources. Hydrogen gas, which can meet our energy needs, has become a particularly intriguing topic. This study investigated the partial oxidation reaction of methane with cordierite monolith catalysts. The Ni-coated catalysts were supported with γ-Al₂O₃, CeO₂, ZrO₂, and CeO₂-ZrO₂. The catalysts were tested at temperatures of 750, 800, and 850 °C with different flow rates and methane feed concentrations (2%, 5%, and 10%). It was demonstrated that catalyst activity varies depending on these parameters. It has been found that high gas hourly space velocity (GHSV) and CH₄ feed rates decrease catalyst activity. The obtained reaction results indicated that the optimal reaction parameters were 800 °C, a GHSV of 1 × 10⁴ h⁻¹, and a CH₄ feed concentration of 2%. By optimizing these parameters, catalysts with high CH₄ conversion and selectivity for H₂ and CO were achieved. The prepared catalysts were characterized using scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and temperature-programmed reduction (TPR).

1. Introduction

Climate change, a global issue, has emerged as a significant problem today due to greenhouse gas emissions resulting from both natural processes over centuries and, concurrently, as a consequence of human activities. With the increasing population, fossil fuels are being used to meet the growing energy demands, leading to CO₂ and methane emissions [1]. Signed in 2015 to mitigate the problems caused by global warming, the Paris Agreement aims to reduce CO₂ emissions by 45% by 2030 and reach net zero by 2050 [2]. Transitioning to renewable sources in the energy sector, which constitutes a significant portion of greenhouse gas emissions, is of paramount importance.

Hydrogen is considered a unique energy source that will lead to profound changes in the energy sector and enable the maximum utilization of renewable resources [3]. In this context, it is expected that hydrogen will play a significant role in achieving international energy and climate goals. Currently, various strategies are being developed to facilitate the widespread adoption of hydrogen technologies and increase investments. Additionally, increasing demand for hydrogen and hydrogen-based fuels is at the forefront of these efforts [4].

Various methods have been developed to obtain hydrogen gas, and efforts are ongoing to reduce costs and increase efficiency simultaneously. Hydrogen gas, which can be produced by many different methods, is obtained from various sources such as natural gas, coal, biomass, and thermochemical methods. [5] Utilizing the existing infrastructure, hydrogen production from hydrocarbons is seen as the optimal choice for reducing greenhouse gas (GHG) emissions to a certain extent in the short term [6]. Due to its high H/C ratio and acceptable cost levels, methane gas plays a dominant role [7]. At the same time, although methane is a key component of non-renewable natural gas, it is also obtained through the decay of organic matter [8].

The production of hydrogen from methane involves four different reformations. These are partial oxidation (POX), steam methane reforming (SMR), autothermal reforming (ATR), and dry methane reforming (DRM) [9]. Catalytic partial oxidation of methane (CPOM) requires less energy compared to other reactions and is an exothermic reaction [10]. Hickman and Schmidt have determined the ideal H₂/CO ratio as 2 through the partial catalytic oxidation of methane. As a result, they have demonstrated that synthesis gas (H₂ + CO) can be obtained. As seen in Equation (1), methane converts to H₂ and CO gases through the POX reaction [11].

$${\mathrm{C}\mathrm{H}}_4+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2\to \mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}{\Delta H}_r^{°}=-36 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(1)

One benefit of the partial oxidation process is the creation of long-chain hydrocarbons and synthetic fuels using Fischer-Tropsch technology from the resulting syngas [12]. When reviewing the literature, a series of studies have been conducted on the catalytic partial oxidation of methane for hydrogen and syngas production. Noble metals such as Pt, Ru, and Rh are featured in the literature as the most active catalysts [13,14,15,16,17]. For instance, Hickman and Schmidt [11] have reported that in metal-coated ceramic monoliths, the reaction on Pt and Rh surfaces can rapidly convert methane and oxygen, producing hydrogen and carbon monoxide. However, the high cost and limited accessibility of noble metals have directed researchers towards transition metals such as Ni, Co, Fe, and Cu [18,19,20,21]. Ni-supported catalysts are preferred due to their particularly high activity, selectivity, and cost-effectiveness. The US Geological Survey estimates that in 2023, Cerium (Ce) will cost approximately 5 USD per kilogram, Zirconium (Zr) costs 28 USD per kilogram, and Nickel (Ni) will cost 22 USD per kilogram. Additionally, Platinum (Pt) costs approximately 32,150 USD per kilogram, and Ruthenium (Ru) costs 15,110 USD per kilogram. Noble metals, despite being highly useful structures for producing effective catalysts, are significantly more expensive than metal oxides and transition metals [22]. However, despite the many advantages of Ni-supported catalysts, they also have drawbacks, such as slowing down reaction rates due to carbon accumulation, leading to low stability [23]. Therefore, researchers have focused on catalysts supported by metal oxides to prevent carbon accumulation. A thorough examination of the impact of MgO addition on Fe-supported catalysts for the hydrogenation of CO₂ to create long-chain hydrocarbons was reported in a recent study. It was noted that MgO stimulates the development of oxygen vacancies and helps to reduce Fe oxides; nevertheless, MgO changes into inactive MgCO₃ during the CO₂ hydrogenation reaction, which reduces the selectivity of the reaction. The work highlights the important effects that metal oxide supports have on catalyst stability and selectivity [24,25]. Although zirconium dioxide (ZrO₂) is a frequently preferred support material in catalysts due to its good mechanical and thermal stability, it reduces activity by reducing the availability of oxygen in the synthesis gas. It has been reported that using CeO₂ together with metal oxides is advantageous to increase oxygen storage capacity [26]. Cerium oxide creates vacancies for oxygen involved in reactions occurring on the catalyst surface, providing a high oxygen storage capacity and thus preventing carbon accumulation on the catalyst surface [27]. Roh et al. [28] reported that the addition of ZrO₂ to CeO₂ leads to improvements in oxygen storage capacity, redox properties, thermal resistance, and metal dispersion support. As a result, better performance was achieved in CO oxidation, methane combustion, and NO reduction. They stated that the catalyst coated with Ni added to CeZrO₂ has a higher surface area compared to catalysts coated solely with CeO₂ or ZrO₂, and that the Ni-Ce-ZrO₂ catalyst achieved over 97% CH₄ conversion without deactivation for 100 h at 800 °C. Babakouhi et al. [21] studied with a %10 Ni/Al₂O₃-CeO₂ catalyst with varying CeO₂ ratios in a combination of POX and SMR. They noted a decrease in activity due to the increase in the CeO₂ ratio, attributed to a decrease in the catalyst’s surface area and pore volume. In a different study, the transformations of methane through steam reforming were investigated using Ni-Ce-supported powder and monolith catalysts. It was noted that carbon deposition was higher in catalysts prepared in powder form compared to monolith catalysts, resulting in lower performance. Additionally, they demonstrated that the addition of Ce to the catalysts significantly reduced coke formation [29]. The support metals that significantly influence the activity of the catalyst, as well as the suitability of the catalyst used for the reactor, are crucial. In the POX reaction conducted in a tubular reactor, powdered catalysts used can eventually clog the reactor, leading to a decrease in pressure [30,31].

In addition, cordierite monoliths (2MgO·2Al₂O₃·5SiO₂) consist of small parallel channels, allowing for controlled pressure drop. Simultaneously, they facilitate higher mass transfer, thereby enhancing the efficiency of the catalyst. Moreover, in industrial applications, the cleaning process of the reactor is much more advantageous compared to powdered catalysts [31,32]. Cordierite monolith is coated with the γ-Alumina wash coating method to increase surface area and porosity. This ensures the adhesion of metals and metal oxides to the monolith and enhances selectivity during the reaction stage [33]. Hickman and Schmidt investigated the effect of mass transfer in the POX reaction and demonstrated that the reaction rate has a significant impact on selectivity [34]. With the increase in flow rate, the diffusion effect occurring on the catalyst surface decreases, making it more difficult for molecules to engage in the reaction [32].

In this study, a comprehensive experimental study on hydrogen production from partial oxidation of methane using Ni-based monolithic catalysts is presented. The goal is to produce highly active catalysts without requiring noble metal supports, in contrast to earlier work. There is insufficient data in the literature currently accessible on Ni-based monolithic catalysts supported by CeO₂ and ZrO₂ for the partial oxidation of methane to produce hydrogen. In light of this, research has been conducted on increasing catalytic activity by generating oxygen vacancies on the surface of Ni-based monolith catalysts by including ZrO₂ and CeO₂ metal oxides. Numerous studies have been conducted on the impact of varying temperatures, flow rates, and CH₄% feed parameters on the partial oxidation process of methane. Using a cordierite monolith, the impact of these parameters on the efficiency of hydrogen production was ascertained. High methane conversions were achieved using low methane feeds (2%, 5%, 10%) in a CH₄/O₂/N₂ gas mixture. The results may help improve the sustainability and efficiency of hydrogen production methods on a large industrial scale.

2. Materials and Methods

2.1. Preparation of Catalysts

The cordierite monolith support (Rauschert GmbH, Veilsdorf, Germany) used in this study have dimensions of 48 × 48 × 150 mm and a geometry of 400 cpsi. It was prepared in cylinders with a diameter of 13 mm and a length of 20 mm, in accordance with the reactor dimensions. In order to ensure easy adhesion of metal and metal oxides to ceramic monolith surfaces, a 5 M Aluminum nitrate (Al(NO₃)₃·9H₂O, 98%, ABCR, Karlsruhe, Germany) solution was prepared and coated by the wash coating method. After calcination, the γ-Al₂O₃ structure was obtained on the support, and thus a large surface area was obtained on the cordierite monolith. Then, Ce(NO₃)₃.6H₂O (%99, MercK, Darmstadt, Germany) and H₂N₂O₈Zr (ABCR, Karlsruhe, Germany) were used to prepare 0.1 M Cerium nitrate, and Zirconium nitrate solutions. 1M Nickel nitrate solution was prepared by the sol-gel method using Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, MercK KGaA, Darmstadt, Germany), Citric acid monohydrate (C₆H₈O₇H₂O, purity: >99% from Carlo Erba Reagenti SPA, Milan, Italy), and Urea (NH₂CONH₂, 98%, Alfa Aeser, Karlsruhe, Germany). The pH value of the prepared solutions was adjusted to 6 with a 1 M Ammonia solution. Between each washcoating process, the catalysts were dried at 110 °C for 1 h and then calcined at 900 °C for 5 h. This procedure was repeated until the desired weight of catalyst deposited on the monolith walls was achieved. At the same time, an ultrasonic bath was used to ensure metal deposition on the monolith surface. To test the adhesion of the wash coating on the cordierite walls, the percentage weight loss of the dried monoliths was calculated according to the following equation:

$$\frac{W_f-W_u}{W_f-W_o}\times 100$$

(2)

where W_f is the weight of the monolith at the end of the wash-coating procedure, W_u is the weight of the monolith after calcination, and W₀ is the weight of the uncoated monolith [35].

2.2. Characterizations

The crystalline phases of all catalysts were evaluated via X-ray diffraction (XRD). Analyses was performed in the 2θ range of 10 to 80°. An Empyrean MultiCore diffractometer (Malvern Panalytical, Malvern, UK) with CuKα radiation (λ = 0.15418 nm) was used.

The surface morphology and elemental distribution of the catalysts were examined by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) using the Zeiss EVO LS 10 instrument (Zeiss, Oberkochen, Germany). To examine the surface morphology of the coated monoliths, the catalysts were divided in half, and the surface in the middle was observed. The specific surface areas (BET), pore volumes, and pore sizes of the catalysts were investigated using the N₂ adsorption technique on the Micromeritcs ASAP 2020 instrument (Micromeritics, Norcross, GA, USA).

Temperature-programmed reduction with hydrogen (H₂-TPR) analyses were carried out in a stainless-steel type reactor, (14 mm inner diameter and 1 m length). The monolith catalyst was placed in the middle of the reactor and the analyses were carried out under atmospheric pressure. The reactor is equipped with a tube furnace (Protherm, PTF 14/50/450, Alserteknik, Ankara, Turkey) and mass flow controls (Teledyne Hastings, HFC 202 series, Hampton, VA, USA). The temperature was monitored using a thermocouple. The flow rate was adjusted with a mass flow control station (TRL-MFCS, TRL Instruments, Ankara, Turkey), and the product flow was analyzed using a mass spectrometer (Hiden Analytical QGA, Warrington, UK). A gas mixture of 5% H₂/N₂ by volume was introduced into the reactor at a rate of 200 mL/min. During analysis, the temperature was increased to 1000 °C at a rate of 5 °C/min. The reduction temperatures of the catalysts were determined by calculating the amount of H₂ consumed during the reduction reaction according to the amount of H₂ released.

2.3. Catalytic Tests

Methane conversions and hydrogen selectivities of monolithic catalysts were studied at temperatures of 750 °C, 800 °C, and 850 °C under atmospheric pressure. Analyses were carried out by providing a gas flow with a volume ratio of CH₄/O₂ feed gases at a molar ratio of 2:1 and a gas hourly space velocity (GHSV) of 1 × 10⁴ h⁻¹ and 2 × 10⁴ h⁻¹. Before starting the partial oxidation reaction, the catalysts were subjected to reduction for 1 h at 5% H₂/N₂ flow at 3 °C/min to activate the catalyst according to the reduction temperatures obtained as a result of TPR. The reactor was supplied with a CH₄/O₂/N₂ gas mixture where the methane concentration was varied to 2%, 5%, and 10%. This notation clarifies the specific methane content within the gas mixture used during the catalytic performance evaluations. The reactor system used for our study is shown in Figure 1. Gases with purity levels of 99.95% for CH₄, 99.999% for N₂, and 99.9999% for O₂ were supplied to the reactor system by mass flow controllers. Before the test began, each MFC calibration was adjusted for the usage flow rate. For MFC calibration, a Gilian Gilibrator 2 (Sensidyne, St. Petersburg, FL, USA) was employed. The desired flow curves, were tested according to the calibration curve and the necessary definitions were made for the mass spectrometer device.

CH₄ conversions, H₂ selectivities, and CO selectivities are explained by Equations (3)–(5) below, respectively [36].

$${\mathrm{X}}_{{\mathrm{C}\mathrm{H}}_4}\left(\mathrm{\%}\right)=\frac{{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}-{\mathrm{C}\mathrm{H}}_{4,\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}}\times 100$$

(3)

$${\mathrm{S}}_{{\mathrm{H}}_2}(\mathrm{\%})=\frac{{\mathrm{H}}_{2,\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{H}}_{2,\mathrm{o}\mathrm{u}\mathrm{t}}+{\mathrm{C}\mathrm{H}}_{4,\mathrm{o}\mathrm{u}\mathrm{t}}}\times 100$$

(4)

$${\mathrm{S}}_{\mathrm{C}\mathrm{O}}(\mathrm{\%})=\frac{{\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}}+{\mathrm{C}\mathrm{O}}_{2,\mathrm{o}\mathrm{u}\mathrm{t}}}\times 100$$

(5)

Here, X represents conversion, and S represents selectivity.

3. Results and Discussion

3.1. Characterization of Catalyst

All ceramic-supported catalysts were prepared by the wash coating method. The results of the prepared monolithic catalysts were comparatively analyzed. XRD profiles of monolithic catalysts are shown in Figure 2. The FWHM values of catalysts were determined by XRD software, which is HighScore Plus (version 5.2) (Malvern Panalytical, Malvern, UK). The crystal sizes were calculated using the Scherrer equation [37] with FWHM values and are given in

Table 1. Crystal phase and sizes of cordierite monolithic catalysts.

Code	Catalyst	Crystal Phases	Crystal Size (nm)
S0	Monolith Support	Mg2Al4Si5O18 (01-076-6037)	38.33
S1	Ni	NiO (04-002-5335)	38.86
S2	γ-Al2O3/Ni	NiO (04-002-5335)	43.71
S3	γ-Al2O3/CeO2/Ni	NiO (04-002-5335) CeO2 (04-005-4553)	49.00 35.53
S4	γ-Al2O3/ZrO2/Ni	NiO (04-002-5335) ZrO2 (04-007-0953)	29.11 25.56
S5	γ-Al2O3/CeO2-ZrO2/Ni	NiO (04-002-5335) CeO2 (04-005-4553) ZrO2 (04-007-0953)	29.11 29.61 14.07

. It is observed that all catalysts have high crystallinity. The distinct peaks in the 2θ range of 1 to 30 contain the main components of the cordierite monolith.

The peaks of Magnesium Aluminum Silicate—Cordierite (Mg₂(Al₄Si₅O₁₈)) of the orthorhombic structured cordierite monolith are clearly seen in all catalysts. The major peaks of Mg₂(Al₄Si₅O₁₈) correspond to the (110), (310), (002), (112), (312), (222), and (421) planes, and are located at 2θ positions of 10.51°, 18.13°, 19.06°, 21.77°, 26.41°, 28.50°, and 29.65°, respectively.

The main peaks of NiO with cubic structure were found at 37.32°, 43.36°, and 62.99° in the 2θ position in the (111), (200), and (220) planes, respectively. CeO₂ crystal phases with cubic structure are represented by peaks at 28.5°, 33.1°, 47.5°, and 56.3° (2θ) in the (111), (200), (220), and (311) planes, respectively. ZrO₂ crystal phases with orthorhombic structure were found in the (111), (200), (202), and (113) planes at the 2θ position at 30.13, 35.33, 50.87, and 60.23°, respectively. The fact that the peaks related to NiO, CeO_2, and ZrO₂ are not obvious is due to their high distribution on the cordierite surface.

The morphology and elemental distribution of catalysts were investigated using the SEM technique in Figure 3 and Figure 4. The blank monolith is represented by the S0 catalyst. It displays images at scales of 50× Figure 3a, 50,000× Figure 3b, and 200× Figure 3c, respectively. The monolith catalyst’s thickness on one side was determined to be 198.64 μm. The catalyst images for S1–S5 are displayed at magnifications of 100× Figure 3a, 10,000× Figure 3b, and 30,000× Figure 3c, in that order. Details of the metal and metal oxide particles accumulated between the surface and pores of the cordierite monolith with a layered structure are observed. When compared with the empty cordierite monolith mentioned in the literature, SEM micrographs exhibit similarities [38].

The distribution of spherical NiO particles accumulated between the layers of the catalyst is also observed. Upon coating the cordierite monolith with high-surface-area alumina, a mosaic-like coating layer is formed on the surface due to its macroporous structure. These layers enable high loadings to be achieved. The difference between the images of S1 and other catalysts clearly demonstrates this. It is evident that NiO, CeO₂, and ZrO₂ metals uniformly adhere to the γ-Al₂O₃ particles. The elemental mapping images of the catalysts are given in Figure 4. It is seen that the Ni, CeO_2, and ZrO₂ elements are distributed homogeneously on the surface of the monolithic catalyst. The Al, Mg, and Si elemental contents in the blank monolith were proven by the mapping image.

The mass percentages of the surface elements are given in

Table 2. The mass weights of the surface elements of monolithic catalysts.

Catalyst	Surface Element Content (Mass Fraction %)
	O	Mg	Al	Si	Ni	CeO2	ZrO2
S1	40.42	9.27	21.38	25.49	3.44	-	-
S2	35.61	6.17	29.35	16.02	12.86	-	-
S3	39.74	6.11	32.51	16.46	1.30	3.87	-
S4	40.23	5.67	30.18	13.98	3.22	-	6.72
S5	37.55	5.32	29.84	12.08	3.56	6.59	5.06

. The presence of Mg, Al, and Si elements, which are the main components of the cordierite monolith, can be observed. The highest Ni presence was observed in the S2 catalyst. In this case, it can be said that the majority of nickel accumulates on the surface. Simultaneously, the augmentation of the surface area can be attributed to the support of the cordierite monolith with alumina. Active catalytic sites emerge with the accumulation of metals and metal oxides on the catalysts. This, in turn, influences the conversion, stability, and transformation processes of the catalyst [39]. As seen in Figure 5, since the loading rates of the support elements are low, Mg, Al, and Si elements continue to be the main components of the catalysts.

Table 3. The characteristics of monolithic catalysts.

Catalyst	SBET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
S1	0.9409	0.004721	20.068
S2	2.5726	0.008115	12.618
S3	2.4832	0.007582	12.231
S4	2.9475	0.007748	10.515
S5	1.0673	0.003538	13.259

shows the specific surface areas (BET), pore volumes, and pore sizes of the catalysts. The surface area of Ni-monolith catalysts (0.9409 m²/g) has been found to be lower than that of other catalysts. It has been proven that adding alumina to support the catalysts increases the materials’ surface area significantly. It was found that the surface area of the monolith catalyst increased when ZrO₂ metal oxide was used, despite the smaller pore size. With the smaller pore size, the S4 monolith catalyst works more selectively and also has a greater adsorption capacity due to the higher surface area. All of the catalysts had a mesoporous structure when their pore diameters were investigated. The reduced surface area of the S5 catalyst can be attributed to the loading of more metal and metal oxide.

In Figure 6, the hydrogen temperature-programmed reduction profiles of Ni-doped catalysts have been thoroughly examined. It has been demonstrated that the reduction properties of Ni-doped catalysts can vary when different supports are used. NiO types can be classified into three categories: amorphous NiO on the surface, weakly interacting with Al₂O₃, or strongly interacting with Al₂O₃ [39]. Below 300 °C, weak peaks transform into free NiO species. Uninteracted-free NiO species during the reaction cause the sintering of Ni, leading to coke formation [40]. Three reduction peaks have emerged in the S2 catalyst, the first occurring at 530 °C and the second at 660 °C, indicating NiO species strongly interacting with the Al₂O₃ support [21,41]. The peak of hydrogen consumption around 860 °C observed in the S2 catalyst can be attributed to highly dispersed NiAl₂O₄ species [42]. It can be seen that the CeO₂ doped S3 catalyst interacts strongly with NiO at 520 °C. As the temperature rises, CeO₂ begins to transform into Ce₂O₃ [43]. Weak peaks observed at 870 °C in the S3 catalyst and at 860 °C in the S5 catalyst indicate the interaction between the formed Ce₂O₃ and Al₂O₃ [44]. It has been observed that the CeO₂-supported catalyst is reduced at slightly higher temperatures compared to ZrO₂-doped catalysts. This indicates that the supported catalyst interacts more strongly with Ni [41]. NiO species appear at 504 °C in the S4 catalyst and 513 °C in the S5 catalyst. It was observed that the reduction shifted to lower temperatures with the addition of ZrO₂ support to the catalyst. Simultaneously, the addition of Al₂O₃ to the catalysts results in improved nickel distribution on the support.

3.2. Catalytic Performance

The effects of varying metal oxide contents in cordierite monolith catalysts on CH₄ conversions and CO and H₂ selectivities at different temperatures, flow rates, and CH₄ feed rates were examined. All reactions had a H₂/CO molar ratio of 2, indicating that a partial oxidation reaction occurred. Considering the TPR results, the reduction temperatures of the catalysts were determined.

In

Table 4. Methane conversion, hydrogen, and carbon monoxide selectivities at GHSV = 2 × 10⁴ h⁻¹.

		S1			S2			S3			S4			S5
T (°C)	CH4 feed (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)
750	2	98.5	99.0	94.2	98.5	99.0	93.4	96.5	97.4	92.2	97.0	97.0	76.5	94.0	95.8	89.4
	5	97.4	98.2	97.4	97.4	98.2	97.1	95.6	96.7	95.2	96.4	96.5	79.0	92.6	94.7	92.6
	10	95.3	96.4	96.0	95.2	96.4	95.7	92.8	94.2	93.4	95.5	95.4	79.1	90.8	92.9	90.5
800	2	98.0	98.6	94.1	98.5	98.9	92.1	98.5	98.9	94.9	95.5	96.8	91.5	97.0	97.8	91.8
	5	97.2	98.0	97.4	98.4	98.9	97.4	98.0	98.5	97.4	93.4	95.3	94.5	96.4	97.4	95.4
	10	95.3	96.4	96.7	97.2	97.9	97.2	96.1	97.0	96.7	90.7	92.8	92.7	95.0	96.2	95.2
850	2	97.0	97.9	93.5	98.5	99.0	94.1	98.5	98.9	96.9	90.5	93.1	89.0	96.0	97.2	91.0
	5	96.6	97.6	97.1	98.8	99.2	98.0	98.4	98.9	98.0	90.8	93.4	92.4	96.8	97.8	96.3
	10	95.2	96.3	96.7	98.1	98.6	98.2	97.3	97.9	97.8	89.1	91.6	91.5	96.1	97.1	96.8

, partial oxidation reactions of the catalysts were carried out with 2 × 10⁴ h⁻¹ GHSV. Their activities were examined with 2%, 5%, and 10% methane feed at temperatures of 750, 800, and 850 °C.

When the Ni-coated catalyst (S1) on the cordierite monolith was examined, a decrease in CH₄ conversion and H₂ and CO selectivities was observed as the temperature increased. While the H₂ selectivity was 99% with 2% CH₄ feeding at 750 °C, it decreased to 97.9% when the temperature reached 850 °C. Likewise, CH₄ conversion decreased from 98.5% to 97%.

In the reactions carried out with 5% CH₄ feed, CH₄ conversions were found to be 97.4%, 97.2%, and 96.6%, and H₂ selectivities were 98.2%, 98.0%, and 97.6%, respectively, at 750, 800, and 850 °C.

According to the reaction results performed with 10% CH₄ feed, CH₄ conversion and H₂ selectivity decreased compared to the reaction results with 2% CH₄ feed. This decrease may have been observed due to carbon accumulation and sintering in the catalyst as the temperature increased [8]. A similar situation was found in the studies conducted by Tezel et al. [45]. The decrease in catalyst activity as the CH₄ concentration increases during the reaction is due to the faster carbon accumulation on the metal surface [46].

By adding Alumina support to the catalyst, the catalyst became more stable. No decrease was observed with the increase in temperature for the S2 catalyst, and CH₄ conversions, H_2, and CO selectivities at 750 °C gave similar results to the S1 catalyst. In the reaction results with 2% CH₄ feed, no temperature-dependent change in the activity of the catalyst was observed; CH₄ conversion was found to be 98.5%, and H₂ selectivity was found to be ~99.0%. In addition, CO selectivity increased as the temperature increased to 850 °C. According to the reaction results performed with 5% CH₄ feed, CH₄ conversions were found to be 97.4% (750 °C), 98.4% (800 °C), and 98.8% (850 °C), respectively. CH₄ conversion rates increased with increasing temperatures. A similar situation is valid for H₂ selectivity, and it showed a very good H₂ selectivity of 99.2% with 5% CH₄ feeding at 850 °C. A decrease in catalyst activity was observed as the CH₄ feed rate increased to 10%.

When the reaction temperature increased from 750 °C to 850 °C, an increase in the CH₄ conversion and H₂ and CO selectivity of the S3 catalyst was observed. However, with increasing CH₄% feeding, conversion and selectivities showed a decreasing trend. As the CH₄% feed increased from 2% to 10% at 750 °C, there was a decrease in CH₄ conversion by 3.7% and H₂ selectivity by 3.2%. Accordingly, CO selectivity increased. The highest CH₄ conversion and H₂ selectivity were obtained with 2% CH₄ feeding at 800 °C, at 98.5% and 98.9%, respectively.

A decrease in conversion and selectivity performances was observed in the S4 catalyst due to the increase in temperature. According to TPR results, the reduction of NiO in ZrO₂-doped catalysts at lower temperatures shows that it is not well distributed on the surface [47]. The catalyst with the lowest CO% selectivity among all studies was ZrO₂ doped S4. In the reaction where methane is fed with different parameters from 2% to 10% at 750 °C, the CO selectivities are 76.5%, 79.0%, and 79.1%, respectively. The highest CH₄ and H₂ levels, 97%, were seen in the reaction performed with 2% CH₄ feed at 750 °C.

In the S5 catalyst, it is seen that optimum 97% CH₄ conversion and 97.8% H₂ selectivity are achieved with a 2% CH₄ feed at 800 °C. The reaction carried out with the combination of CeO₂ and ZrO₂ metal oxides is more stable compared to metal oxides used alone (S3 and S4).

Within the scope of the study, reactions were carried out at 1 × 10⁴ h⁻¹ GHSV, as seen in

Table 5. Methane conversion, hydrogen, and carbon monoxide selectivities at GHSV = 1 × 10⁴ h⁻¹.

		S1			S2			S3			S4			S5
T (°C)	CH4 feed (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)	XCH4 (%)	SH2 (%)	SCO (%)
750	2	99.0	99.2	95.0	99.0	99.2	94.2	99.0	99.2	93.5	98.5	98.3	76.8	98.0	98.4	91.0
	5	95.2	96.5	98.4	95.2	96.6	98.7	95.6	96.8	97.2	99.0	99.1	84.1	95.2	96.6	96.1
	10	96.0	96.9	95.7	96.4	97.3	95.6	96.0	96.8	94.1	98.9	98.9	80.0	95.9	96.9	93.3
800	2	99.0	99.2	94.1	99.0	99.2	91.9	99.0	99.2	96.0	97.5	98.0	92.3	99.0	99.2	91.8
	5	95.2	96.6	99.0	96.6	97.6	98.0	96.4	97.4	98.3	94.0	95.8	98.2	96.6	97.6	98.2
	10	97.4	98.0	97.3	98.1	98.6	96.6	98.0	98.4	96.7	96.1	97.1	95.5	98.0	98.5	96.3
850	2	98.5	98.8	93.4	99.0	99.2	92.7	99.0	99.2	97.5	97.5	98.0	92.3	99.0	99.2	93.3
	5	95.8	97.0	99.3	97.4	98.2	99.0	97.0	97.9	98.9	95.0	96.5	99.3	97.2	98.1	99.3
	10	98.7	99.0	98.1	99.0	99.3	97.9	98.9	99.2	97.9	96.5	97.4	96.0	99.0	99.3	97.9

. Catalysts S1 and S2 showed approximately similar activity. In the reactions carried out at 750, 800, and 850 °C with 2%, 5%, and 10% CH₄ feed, CH₄ conversion was found to be 99% and H₂ selectivity was 99.2%. CO selectivity was found to be around 93% on average. When the CH₄% feed was increased to 5% in the S1 catalyst, a 3% decrease in its activity was observed.

In the reactions carried out with 2% CH₄ feed on the S3 catalyst, the CH₄ conversion and H₂ selectivities were found to be 99.0% and 99.2%, respectively, without being affected by the temperature increase. However, with the increase in temperature, an increase in CO selectivity was observed at 93.5%, 96.0%, and 97.5%, respectively. As the CH₄ feed increased, the catalyst activity decreased.

When the activity of the S4 catalyst was examined, the highest results were obtained when 5% CH₄ was fed at 750 °C. Here, CH₄ conversion was found to be 99.0%, H₂ selectivity was 99.1%, and CO selectivity was 84.1%. It can be said that carbon accumulation occurs due to low CO selectivity [48,49]. The S4 catalyst, which had high activity at 750 °C, started to lose its activity as the temperature increased. Here, at 750 °C, while CH₄ conversion and H₂ selectivity increased at both GHSV values, CO selectivity decreased. Steam reforming may have occurred as a result of the reaction of the CH₄ fed into the reaction with the H₂O formed, as shown in Equations (6) and (7) [50].

$${2\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}{\Delta H}_r^{°}=-483 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(6)

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}{\Delta H}_r^{°}=210 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(7)

As shown in Equation (1), the first methane underwent partial oxidation to form CO and H₂. Then, the resulting H₂ reacted with oxygen to form water. CH₄, which reacted again with the water formed, resulted in the formation of more H₂.

CeO₂ and ZrO₂-supported S5 catalysts showed the same performance as S2 catalysts at 850 °C with 10% CH₄ feeding. 99.0% CH₄ conversion, 99.3% H₂ selectivity, and 97.9% CO selectivity were found. As the temperature increased, their conversion and selectivity increased. Within these parameters, the performances of the catalysts are S2 = S5 > S3 > S1 > S4, respectively.

Partial oxidation reaction tests were carried out with the same parameters as the cordierite monolith without supporting it with any metal or metal oxide (results are not shared in the table). It was observed that the unsupported monolith did not achieve CH₄ conversion and H₂ selectivity. Jin et al. [51] shared detailed information about the effect of Ni support on the activation of the partial oxidation reaction of methane. With the addition of Ni metal to the catalyst, the oxidation reaction of methane begins by converting Ni⁰ ions into NiO, and when it reaches high temperatures, the partial oxidation reaction of methane occurs by turning into Ni⁰. Ni—C and Ni^δ+—O^δ− species, which emerge from the transformation of Ni on the surface, constitute a step for the partial oxidation reaction of methane. The analysis of the XRD and TPR data in the investigation revealed a reduction in the abundance of Ni species. At the same time, it can be said that Ce and Zr support the reaction by oxidation and reduction.

A significant performance decrease was observed with increasing temperature in the reactions carried out on ZrO₂-supported monolith catalysts. The reason why CeO₂-supported catalysts have better activity than ZrO₂-supported catalysts is due to the high oxygen storage feature of CeO₂. Partial oxidation reactions were carried out by Larimi and Avali [52] with Ni-supported CeO₂ and ZrO₂ catalyst combinations, and they proved that the catalyst with the lowest catalytic activity was Ni/ZrO_2, and the catalyst with the highest catalytic activity was realized with the CeO₂-ZrO₂ combination. When the catalytic activities of the catalysts were examined depending on the flow rate, CH₄ feed, and temperature, a decrease in their conversion and selectivity occurred with an increase in the reaction flow rate. The studies conducted by Babakouhi and colleagues [21] with Ni/Al₂O₃-CeO₂ catalysts support this situation.

Table 6. Results of catalysts’ conversion and selectivity with the partial oxidation reaction of methane in the literature.

Catalyst	XCH4 (%)	SH2 (%)	SCO (%)	Temperature, °C	GHSV, h−1	Ref.
Ni monolith catalyst	81.49	82.72	78.75	800	1 × 104	[20]
Mo-Ni monolith catalyst	95.67	91.57	89.53	800	1 × 104	[53]
Ru-Mo-Ni monolith catalyst	95.31	92.72	91.21	800	1 × 104	[53]
Ni-Ni/MgAl2O4 monolith catalyst	85.30	91.50	93.00	700	1 × 105	[54]
Ce-Zr/Ni monolith catalyst	94.00	98.00	96.00	850	1 × 105	[55]
Ni/CeO2	98.00	93.00	98.00	800	53 × 103	[56]
Ni/ZrO2	96.00	96.00	98.00	800	53 × 103	[56]
Ni/CeZrO2	98.00	95.00	98.00	800	53 × 103	[56]
Pd–Ni/CeO2	75.80	66.10	22.80	550	12 × 103	[57]
Zr/Ni sponge catalyst	84.00	98.00	95.00	850	1 × 105	[58]
Ni/Al2O3	95.30	97.40	98.00	800	-	[59]
Rh/CeO2–ZrO2	97.00	87.00	96.00	750	99 × 103	[60]
Ru/CeO2	74.10	65.10	21.70	600	12 × 103	[61]
Ru–Ni/CeO2	76.40	63.70	20.60	600	12 × 103	[61]
S1	99.00	99.20	94.10	800	1 × 104	In this study
S2	99.00	99.20	91.90	800	1 × 104	In this study
S3	99.00	99.20	96.00	800	1 × 104	In this study
S4	97.50	98.00	92.30	800	1 × 104	In this study
S5	99.00	99.20	91.80	800	1 × 104	In this study

presents the CH₄ conversion, H₂, and CO selectivity data from the partial oxidation reaction of methane as reported in the literature. This comparison has enabled a clearer understanding of the influence of different parameters on catalyst performance. Compared to other catalysts documented in the literature, it has been noted that the cordierite monolithic catalysts utilized in our investigation exhibit noticeably excellent results for the partial oxidation of methane.

Based on a review of the literature, it was discovered that the CH₄ conversion at 800 °C for the Ni monolithic catalyst was 81.49% [20], for the Mo-Ni monolithic catalyst it was 95.67%, and for the Ru-Mo-Ni monolithic catalyst it was 95.31% [53]. Furthermore, 91.5% H₂ selectivity and 85.30% CH₄ conversion of the Ni-Ni/MgAl₂O₃ monolithic catalyst were reported based on reaction data carried out at 700 °C [54]. Another similar investigation reported that the Ce-Zr/Ni monolithic catalyst had a 94% CH₄ conversion and a 98% H₂ selectivity at 850 °C [55]. When Ni/CeO₂, Ni/ZrO₂ and Ni/CeZrO₂ catalysts prepared as powder catalysts were examined, CH₄ conversions were found to be between 98–96% [56]. It has also been shown how temperature affects the selectivity and conversion of the catalyst, even though it is a noble metal with superior properties, as seen in the Pd-Ni/CeO₂ catalyst [57]. This illustrates the degree to which the chosen parameters influence the outcome. When the results in the literature were compared with this study, remarkable results were obtained in terms of CH₄ conversion, H₂ and CO selectivity.

The optimal reaction parameters were determined to be 800 °C, GHSV of 1 × 10⁴ h⁻¹, and 2% CH₄ feed concentration based on the reaction findings that were obtained. The results are given in Figure 7. Consequently, partial oxidation processes were carried out for 200 min for each catalyst under these conditions in order to assess the stability of the catalysts. Table 4 and Table 5 show the initial methane conversion, hydrogen selectivity, and CO selectivity results of the developed catalysts at temperatures between 750 and 850 °C, with methane ratios of 2%, 5%, and 10% in the feed stream, respectively. These data belong to measurements taken for each parameter over a duration of 30 min. Stability tests were performed at an optimum reaction parameter duration of 200 min using a feed stream containing 2% CH4, as shown in Figure 7. Based on these tests, the methane conversion of the S5 catalyst, which contains CeO₂ and ZrO₂, remained consistent at 99% over a period of 200 min. A similar situation is valid for the S2-coded catalyst. Nevertheless, the S1, S3, and S4 catalysts exhibited a reduction of 0.5% in methane conversion. When analyzing the CO selectivity, the S5 catalyst exhibited a minimal loss of 1.71%, whereas the S1, S2, S3, and S4 catalysts resulted in decreases of 5.25%, 7.11%, 2.49%, and 5.50%, respectively.

When analyzing the selectivity for H₂, the S5 catalyst exhibited a minimal drop of only 0.04%. In contrast, the selectivity values for S1, S2, S3, and S4 were discovered to be 0.44%, 0.42%, 0.46%, and 0.43%, respectively. While both the S5 and S2 catalysts may seem equal in terms of methane conversion, the S5 catalyst exhibits significantly higher efficiency in terms of CO selectivity and H₂ selectivity. After analyzing the data, it was found that the γ-Al₂O₃/CeO₂-ZrO₂/Ni (S5) monolith catalyst was the most stable.

4. Conclusions

The study involves the preparation of Ni, γ-Al₂O₃/Ni, γ-Al₂O₃/CeO₂/Ni, γ-Al₂O₃/ZrO₂/Ni, and γ-Al₂O₃/CeO₂-ZrO₂/Ni catalysts on cordierite monolith using the wash coating method. Monolith coating methods significantly affect the activity of the catalyst, and ultrasonic treatment was applied to each catalyst during the coating stage to ensure a homogeneous distribution. It was revealed that when γ-Al₂O₃ support was added, a mosaic-like layer was formed on the catalyst surface, and Ni distribution increased as the surface area increased. The H₂-TPR profiles revealed a strong interaction between support metal oxides and NiO. Different parameters were used to perform partial oxidation reactions on the catalysts. The molar H₂/CO ratio of the catalysts was 2 in all reactions. We emphasized throughout the study how important it is to optimize GSHV and methane feed rates to increase CH₄ conversion, H_2, and CO selectivity. According to this study, the ideal methane feed rate at 1 × 10⁴ h⁻¹ GHSV is 2%. It is possible that the decrease in catalytic activity with increasing GHSV and methane feed rates is due to carbon deposits on the catalyst surface. Simultaneously, upon comparing our results with those of other studies, we observed that the Ni-coated catalysts demonstrated superior performance in CH₄ conversion, H₂, and CO selectivity, particularly when paired with -Al₂O₃, CeO₂, and ZrO₂ supports. These findings have significant potential for metal and metal oxide coatings, particularly for hydrogen production and the reduction of greenhouse gas emissions. It will therefore serve as a reference point for future research aimed at improving catalyst stability and performance in the partial oxidation processes of methane. Considering that cordierite monoliths are used in the automotive sector as catalytic converters and in industrial applications to separate unwanted particles from exhaust gases, the superior properties of the monolith offer a sustainable and economical option in industrial applications. It also plays an important role in ensuring energy efficiency and process safety. The monolithic catalysts obtained in the study showed high selectivity and activity. The hydrogen obtained can be used safely as fuel in the automotive sector.