A Novel Polymer-Derived Ni/SiOC Catalyst for the Dry Reforming of Methane

Abstract

Nickel (Ni)-based catalysts, prepared by pyrolyzing Ni-containing polydimethylsiloxane (Ni-PDMS), were evaluated for their activity in the dry reforming of methane (DRM) reaction. The pyrolyzed PDMS support was found to be largely microporous, and the active nickel particles were nano-sized but were not dispersed evenly in the resulting catalysts. The catalysts were prepared with 0 wt%, 2 wt%, 4 wt%, and 6 wt% Ni in PDMS prior to pyrolysis. The resulting catalysts demonstrated notable activity in the DRM reaction, comparable to many of those described in the published literature. The catalyst with 6 wt% Ni (prior to pyrolysis) displayed the highest conversion of methane (47%) and the lowest loss of activity (9.8%) over 11 h of continuous operation. This research was successful in exploring novel polymer-derived catalysts, specifically pyrolyzed Ni-PDMS catalysts, in the dry reforming of methane (DRM) reaction.

1. Introduction

The growing use of fossil fuels, agricultural waste, and landfills compound the amount of carbon dioxide (CO₂) and methane present in the Earth’s atmosphere [1,2]. Together, these greenhouse gases contribute largely to global warming issues. Methane is more potent than CO₂ as a greenhouse gas in the short term, and considering both its concentration in the atmosphere and its potency, it is responsible for about one-third of greenhouse gas effects that contribute to global warming [3]. The oil and natural gas sector is the largest industrial source of methane emissions in the US [3]. It currently flares approximately 165 billion cubic feet of methane gas per year on public lands, valued to be nearly USD 275M [4]. Due to the significant potential for harnessing the value of these waste gases, catalysis and reaction engineering have been leveraged to turn methane and CO₂ into value-added products.

One way to utilize methane and CO₂ is in through the dry reforming of methane. Methane reforming reactions produce carbon monoxide (CO) and hydrogen gas (H₂), collectively called syngas. Syngas is used in a variety of subsequent reactions. Some of the most prominent uses include the separation and retrieval of hydrogen gas to synthesize ammonia, the production of methanol, and the Fischer–Tropsch synthesis of fuels [5,6,7]. For some downstream processes, such as methanol and Fischer–Tropsch synthesis, a H₂:CO molar ratio of 2:1 is preferred, whereas higher ratios are preferred when the desired product is pure hydrogen [7].

The reforming of methane can be carried out through various methods. Partial oxidation of methane (POM) is an exothermic process in which methane is partially oxidized with oxygen to produce syngas. While this process is more energy efficient, POM is challenging due to the difficulty in controlling the reaction for syngas selectivity [6,8,9]. Steam reforming of methane (SRM) is an endothermic process that utilizes water vapor as the oxidizing agent for methane. SRM is predominantly used at a commercial scale, but it involves high energy consumption due to its high operational temperatures and pressures [6,10]. Autothermal reformation (ATR) is an exothermic process combining POM and SRM. The energy produced from POM is utilized for the second step of SRM, ultimately producing syngas. While this reaction is attractive with respect to energy efficiency, achieving the desired syngas ratio is dependent on the first step of the process. Adjacent fields of research are focused on optimizing the ATR process [1,6,11]. A fourth method is dry reforming (DRM), which is an endothermic process that utilizes CO₂ as the oxidizing agent in the formation of syngas [1,2,5,6,8,9]. While the energy input required to facilitate this reaction is even higher than SRM, the 1:1 molar ratio of methane to CO₂ as reactants is enticing for environmental benefits. Methane reforming has been completed through various combinations of these methods, each presenting its own unique challenges related to the efficiency and effectiveness of syngas production [1,2,5].

This study focuses on the dry reforming of methane, which utilizes both methane and CO₂ to produce syngas, as demonstrated by the primary DRM reaction presented in Equation (1):

CH_{4 (g)} + CO_{2 (g)} → 2 CO_(g) + 2 H_{2 (g)}    ∆H_298K = +247 kJ/mol

(1)

As demonstrated by the standard enthalpy of this reaction, DRM is highly endothermic, requiring temperatures of 650–1000 °C to overcome the high stability of methane and carbon dioxide [1]. In the DRM reaction, several side reactions occur. Some of the most prominent side reactions are listed below in Equations (2)–(5) [12,13,14,15]:

CO_{2 (g)} + H_{2 (g)}  ←→  CO _(g) + H₂O _(g)   ∆H_298K = +41 kJ/mol

(2)

2CO_(g) → CO_2(g) + C_(s)   ∆H_298K = −172.4 kJ/mol

(3)

CH_{4 (g)}→ 2H_{2 (g)} + C_(s)         ∆H_298K = +74.9 kJ/mol

(4)

CH_{4 (g)} + H₂O_(g) → CO_(g) + 3 H_{2 (g)}     ∆H_298K = +206.1 kJ/mol

(5)

At 650–1000 °C, these side reactions have several implications. The reverse water–gas shift (Equation (2)) alters the hydrogen-to-carbon monoxide ratio of the syngas by consuming hydrogen to produce water, which can also interfere with active sites of the catalyst [16]. Additionally, the methane cracking (Equation (4)) and Boudouard (Equation (3)) reactions contribute to the loss of carbon during the reaction due to the deposition of carbon on the catalyst. This results in rapid deactivation of the catalyst, as carbon builds up on the metallic active sites [1,2,6].

As implied by the above discussion, there are two predominant challenges to the DRM reaction: (1) the requirement for high temperatures to generate syngas and (2) catalyst deactivation by sintering and carbon deposition [12,13]. Therefore, catalysts and support materials involved in the dry reforming process must be able to maintain functionality at active sites in high-temperature environments and inhibit deactivation by carbon deposition [14,15]. Recent research in DRM has been conducted to address these challenges by developing new catalysts [12,13,14] and new support materials [14,17,18] and by coupling thermal catalysis with other processes [15], such as photocatalysis [19,20] and electrochemical catalysis [21,22].

Noble metals, such as platinum, iridium, ruthenium, and rhodium, demonstrate the highest activity and lowest deactivation rates in DRM reactions [2,12,14]. However, their high cost compared to cheaper and more widely available transition metals [5,6] makes them economically unattractive [14,15]. Therefore, reduced nickel and cobalt metal catalysts are most often researched for the DRM reaction [1,12,15,23], as they have excellent activity for the DRM reaction. However, the catalyst stability of Ni- and Co-based catalysts must be improved. Methods to improve these catalysts include the inclusion of bi-metallic and multi-metallic catalysts [12,13,15,17,23,24]; catalyst promoters, such as ceria, alkaline, alkaline earth, and transition metal oxides, which promote CO₂ activation [13,14,15]; and novel supports [13,14,15,17].

Metal particle size and dispersion on the catalyst support are two major factors found to optimize catalyst activity and longevity [2,12,25,26]. Active sites with smaller nickel particles, ideally below 10 nm in diameter, have demonstrated the most success in preventing the buildup of carbon that blocks active sites [2,25,27]; however, smaller nickel particles are also observed to have poorer thermal stability and are more susceptible to sintering [27]. In addition, uniform dispersion of the active metal throughout the support enables the availability of more active sites throughout the catalyst, contributing to higher syngas production rates [28].

The structure of the catalyst support material and its interaction with the active metal are important in developing catalysts that resist coke deposition and deactivation during the DRM reaction [12,13,17]. A review by Xu & Park, 2024 [17] states that the choice of catalyst support is pivotal for inhibiting carbon deposition on the catalyst. Support properties, such as oxygen mobility, oxygen storage capacity, and thermal stability, are important in the context of resisting coke formation on a catalyst in the DRM reaction [17]. It has been demonstrated that a uniform, mesoporous support material with a high BET surface area can enhance the catalyst–support interaction and dispersion of the catalyst metal on the support [29]. Mesopores, defined as having a diameter between 2 and 50 nm [28], allow for the formulation of nano-sized nickel particles without blocking the access of reactants to active sites [2]. The accessibility of active sites on the catalyst leads to high selectivity for syngas production on mesoporous supports [29]. In addition, highly dispersed metal active sites on a high-surface-area support can prevent water vapor from the RGWS reaction from blocking active sites [28].

The inhibition of catalyst deactivation by coking or carbon deposition has been investigated by many researchers [12,13,17]. For example, in a review by Nguyrn et al. [13], transition metal carbides are gaining interest in DRM reactions due to their thermal stability and inherent catalytic activity. Transition metal carbides undergo unique recarburization–oxidation cycles during the DRM reaction [13], which help prevent the formation of carbon deposits on the catalyst.

Polydimethylsiloxane (PDMS), a type of polymer with a Si-O-Si backbone, can be pyrolyzed to form silicon oxycarbon (SiOC), a porous ceramic material [30,31,32]. Many silica-based materials display mesoporosity and high BET surface areas, ranging from 400 to 900 m²/g [2], as well as thermal and chemical stability in harsh conditions [29]. Additionally, Ni-SiOC interactions have been observed where methyl groups present on the siloxane backbone reduce nickel particles during pyrolysis, which creates polymer-derived catalysts with active nickel metal within its structure [33]. Recently, polymer-derived catalysts (PDCs) are being considered as a support material in catalyzing the DRM reaction. While several synthesis methods explored have had some success, including vat-based photopolymerization [16] and Ni-incorporated membrane pyrolysis [33], more research must be carried out to explore the large variety of PDCs and their role in the DRM reaction.

This research explores pyrolyzed polydimethylsiloxane (PDMS) as a novel support material for a nickel catalyst for the dry reforming of methane reaction. Several Ni-PDMS catalysts were synthesized to investigate the effects of nickel on the physical and chemical characteristics of pyrolyzed PDMS, as well as to determine the effect of nickel loading, the reactant molar ratio, and reaction temperature on the performance of pyrolyzed Ni-PDMS in the DRM reaction. It was hypothesized that pyrolyzed Ni-containing PDMS would result in a high-surface-area, thermally stable, and coke-resistant catalyst with highly dispersed Ni nanoparticles within an Si-O-C support (following pyrolysis) that is active in the DRM reaction. Ultimately, improving the long-term activity levels of DRM catalysts is essential to make DRM a feasible form of syngas production at a commercial scale.

2. Results

2.1. Catalyst Synthesis and Characterization

A total of four catalysts were synthesized for this study by pyrolyzing PDMS membranes that contain 0 wt% to 6 wt% Ni. The catalyst composition data are summarized in

Table 1. Summary of catalyst composition before and after pyrolysis.

Catalyst ID	Ni-PDMS Before Pyrolysis		Ni-PDMS After Pyrolysis
	Nominal wt% Ni	Estimated Molar Ratio Ni/Si	Estimated wt% Ni 1	EDS Molar Ratio Ni/Si
0Ni-PDMS	0%	0	0%	0
2Ni-PDMS	1.85%	0.053	4.3%	0.147
4Ni-PDMS	3.44%	0.106	12.4%	0.202
6Ni-PDMS	5.94%	0.210	21.8%	0.190+/−0.105

¹ The estimated wt% Ni in the pyrolyzed catalysts was based upon the assumption of no loss of Ni during pyrolysis and measured total mass loss in TGAs at the pyrolysis temperature (750 °C).

.

Thermogravimetric analyses (TGAs) were conducted on Ni-PDMS polymers prior to pyrolysis, as shown in Figure 1. The percentages of mass lost from the Ni-PDMS samples as a function of temperature are shown in

Table 2. Summary of masses lost as a function of temperature from TGAs.

Catalyst ID	<300 °C	300–500 °C	>500 °C	Total
0Ni-PDMS	0.8%	13.1%	44.5%	52.1%
2Ni-PDMS	2.2%	16.9%	47.1%	57.0%
4Ni-PDMS	2.3%	15.9%	66.3%	72.3%
6Ni-PDMS	8.7%	18.3%	70.1%	77.7%

. Below 300 °C, mass losses can be attributed to the losses of water introduced by the nickel acetate and the escaping of light gases trapped within the membrane [16,34]. The Ni precursor (Ni acetate tetrahydrate) decomposes predominantly at temperatures between 300 °C and 500 °C [31]. In addition, PDMS also starts to thermally degrade in this range of temperatures to volatile cyclic siloxanes [35,36]. The mass lost at temperatures greater than 500 °C increases as the Ni content increases. This suggests that the Ni catalyzes the decomposition of the support into volatile siloxanes and hydrocarbons [16,37,38] at higher temperatures. Once 750 °C is reached, no more mass is lost from the materials.

The surface areas of all catalysts were evaluated by nitrogen adsorption at 77 K using the Brunauer, Emmet, and Teller (BET) method, with the results provided in

Table 3. Summary of catalyst characterization.

Catalyst	BET Surface Area	Pore Volume	Micropore Volume 1	Average Pore Width 2	Average Micropore Width 1	Ni Crystal Size 3
ID	(m2/g)	(cm3/g)	(cm3/g)	(Å)	(Å)	(Å)
0Ni-PDMS	520	0.222	0.198	17.2	16.2	N/A
2Ni-PDMS	409	0.195	0.153	19.3	16.56	334
4Ni-PDMS	337	0.168	0.123	20.2	16.63	362
6Ni-PDMS	417	0.231	0.172	22.8	17.82	306

¹ The micropore volume and width were calculated using t-plots and the Harkins and Jura method. ² Calculated using the Barrett–Joyner–Halenda (BJH) model on desorption data. ³ Based upon XRD profiles and the Scherrer Equation, Equation (6).

. The 2Ni-PDMS, 4Ni-PDMS, and 6Ni-PDMS adsorption isotherms in Figure 2 exhibit both microporous and mesoporous structures, while 0Ni-PDMS appears to be mainly microporous, according to the IUPAC classifications [28].

The presence of Ni in the pyrolyzed PDMS support generally appears to decrease the BET surface area of the material. The average pore width and micropore width increase with increasing nickel loading. All nickel crystals are nano-sized (<40 nm).

The crystallinity and crystal phases of the pyrolyzed Ni-PDMS catalysts were characterized using X-ray diffraction (XRD). The XRD patterns, as shown in Figure 3, demonstrate the presence of amorphous SiO₂ for 0Ni-PDMS and weakly crystalline SiO₂, as demonstrated by small, broad peaks at 2θ = 27° [38,39]. A fully reduced version of nickel metal can be seen at 2θ = 44.5°, 52°, and 77° [40]. The crystal size of the nickel for each catalyst was estimated using XRD data via the Scherrer’s equation, which is given by Equation (6) and summarized in Table 3.

$$Crystal Size (d)={\displaystyle \frac{K\lambda }{\beta cos\left(\theta \right)}}$$

(6)

In Equation (6), d is the crystallite diameter (nm), λ is the incident wavelength of 0.154 nm, θ is the angle in radians, FWHM = full width at half maximum (radians), and K is a shape factor that was taken to be 0.9 for this study.

When the predominant XRD peak of Ni at 2θ = 44.5° is investigated more closely, as shown in Figure 4, there is a shift in the peak toward lower Bragg angles as the Ni content increases in the Ni-PDMS catalysts. According to Kumar and Kar [41], when atoms with larger ionic radii substitute for smaller atoms in a crystal lattice, the lattice parameter increases due to bond stretching and an increase in the unit cell volume. In the context of this study, the shift toward lower Bragg angles in the predominant XRD peak of Ni gives evidence that the carbon content in the Ni metal species decreases as the Ni content in the catalyst increases.

Scanning electron microscopy (SEM) was used to gain a structural understanding of all catalysts. Figure 5 shows 4Ni-PDMS at 5000× magnification with a visual representation of the silica, nickel, oxygen, and carbon compositions.

Figure 5a shows two contrasting materials within the microstructure of the 4Ni-PDMS, with one appearing smooth and nonporous and the other appearing as clumps clustered on the surface. Rather than being dispersed in the micro- and mesopores of the support, nickel is contained in porous “debris “on the outside of an apparently smooth silica material, seen in Figure 5b,c. In Figure 5d, carbon appears to coincide with the nickel-containing materials, suggesting that carbon could have deposited on or within the nickel during the synthesis process. This is supported by the shift in the Bragg angle in the XRD profiles shown in Figure 5. The interaction between Ni and carbon during synthesis could be beneficial to coking resistance in the DRM reaction [42] but has also been shown to decrease the activity of the catalyst [43].

FTIR was performed on unused and used catalysts to understand the structural differences created in the catalysts during the DRM, with the results displayed in Figure 6.

Peaks contained in the range of 1250 to 800 cm⁻¹ are functional groups containing silica, oxygen, and carbon [16]. The peak, which is intensified in used catalysts, at approximately 800 cm⁻¹ is thought to represent Si-C bonds [41]. The small valleys at 2850 and 2900 cm⁻¹ are indicative of C-H bonds [44]. The peaks at 1050 and 1150 cm⁻¹ represent Si-O and Si-O-Si bonds, respectively [44,45], which may be more pronounced in the used catalysts due to the presence of quartz wool [46]. Quartz wool was used to secure the catalyst in the reaction tube and was difficult to separate from the used catalyst following the reaction.

EPR data were used to compare the oxidation state of the nickel in the 4Ni-PDMS unused and used samples, as shown in Figure 7.

The broad signal within the fresh catalyst indicates the existence of unpaired electrons [38,47,48]. However, the used catalysts give little to no evidence of unpaired electrons. The difference in EPR spectra between used and unused catalysts is evidence that the nickel active sites change during the DRM reaction. The changes in Ni during the reaction may be due to sintering of the Ni nanoparticles or changes in the Ni composition. Many nickel oxidation states are unregistered in EPR due to the lack of an unpaired electron or magnetic state of Ni(II) [49].

Thermogravimetric analysis (TGA) was used to qualitatively determine the carbon deposition on each catalyst after 11 h of reaction time, pictured in Figure 8.

The 0Ni-PDMS catalyst displays mass loss from about 100 to 300 °C, which can be attributed to water that has adsorbed onto its surface [16]. Qualitatively, carbon deposition on all Ni-PDMS catalysts can be observed, as evidenced by the mass loss from 600 to 700 °C [25,50]. The 0Ni-PDMS experiences the smallest loss, as it was largely inactive as a catalyst. The quartz wool used to secure the catalyst in the reaction tube could not be completely separated from the used catalyst following the reaction. Therefore, no quantitative amount of carbon deposition on the catalysts can be defensibly calculated from these data. Figure 8 also shows an increase in mass of the 6Ni-PDMS from 400 to 600 °C. It is possible that the catalyst was reacting with oxygen in this temperature range, causing the observed gain of mass through the formation of Ni or Si oxides.

2.2. Catalytic Activity

The Ni-PDMS catalysts were assessed for their performance in the DRM reaction over 11 h of continuous operation. In all cases, the carbon, hydrogen, and oxygen balances averaged >90%.

Figure 9a,b show the conversions of methane and carbon dioxide, respectively, on each of the catalysts. Figure 9c shows the H₂ selectivity, and Figure 9d shows H₂/CO ratio with time onstream. As the loading of nickel increases, the % CH₄ conversion, % CO₂ conversion, H₂ selectivity, and H₂/CO ratio increase. Based on these measurements of catalyst productivity, the 6Ni-PDMS is the most active catalyst of those studied in this research. These results are consistent with other nickel catalysts used in the DRM reaction, where the optimal nickel loading is between 7 and 10% [1]. The 6Ni-PDMS had the highest BET surface area, micropore volume, and pore size of the Ni-loaded catalyst in this study, which follows the activity trends observed in other studies [28,29].

Another trend common for catalysts in the DRM reaction is that as active metal loading increases, the carbon deposition and catalyst deactivation increase [27] because carbon deposits on the active metal sites of the catalyst [1]. However, a contradictory trend can be observed in this study. As demonstrated by the trends over time in Figure 9, later listed in

Table 4. Comparison of catalytic activity metrics for catalysts within and outside of this study.

Catalyst	Catalyst Synthesis Method	Operating Temperature	CH4 Conversion	CO2 Conversion	H2/CO Ratio	TOF 1	Activity Loss	Reference
ID		(°C)	(%)	(%)		(min−1)	(%)
6Ni-PDMS	Pyrolyzed Ni-PDMS	750	47	54	0.70	2.07 ± 0.079	9.8	This work
6Ni-PDMS	Pyrolyzed Ni-PDMS	700	22	34	0.51	0.70 ± 0.36	30	This work
6Ni-PDMS	Pyrolyzed Ni-PDMS	650	16	29	0.46	0.52 ± 0.05	19	This work
4Ni-PDMS	Pyrolyzed Ni-PDMS	750	37	51	0.53	3.11 ± 0.53	17	This work
2Ni-PDMS	Pyrolyzed Ni-PDMS	750	15	25	0.41	3.90 ± 1.06	49	This work
Ni@SiO2	SiO2 core–shell Ni nanoparticle catalyst	750	58	72	0.75	0.998	5.0	[51]
Pd/SiO2	Impregnation	750	65	70	-	75	66	[52]
Pt/Al2O3	Impregnation	750	60	76	0.73	26.1	6.7	[53]
Ni/Al2O3	Impregnation	750	75	76	0.75	2.95	14	[53]
Ni/Al2O3	Impregnation	750	50	65	0.80	11.0	-	[54]
Ni-CNTs, mesocellular silica	Chemical vapor deposition technique	650	50	58	0.62	2.62	7.5	[55]

¹ All TOF values were calculated based upon Equation (12) for comparison. Methane conversions after approximately 2 h online were used.

, the 6Ni-PDMS has the lowest activity loss, followed by the 4Ni-PDMS. Often, activity loss is attributed to coke buildup on the catalyst over the time of reaction [1,2].

Figure 10 demonstrates the effects of the reactant gas molar ratio on the DRM reaction. As the CH₄:CO₂ ratio increases, % CO₂ conversion, H₂ selectivity, and H₂/CO ratio increases, while the % CH₄ conversion decreases. This follows that as an excess of CH₄ is provided, the conversion of the limiting reagent CO₂ increases, and vice versa. The increase in H₂ selectivity and H₂/CO ratio is due to the shift in the RWGS reaction (see Equation (2)), where the lower concentrations of CO₂ push equilibrium in the RWGS toward H₂O and CO.

Figure 11 show catalyst activity metrics for the 6Ni-PDMS catalyst in this study when facilitating the DRM reaction at reaction temperatures of 650 °C, 700 °C, and 750 °C. As the reaction temperature increases, % CH₄ and % CO₂ conversions increase, as well as the H₂/CO ratio. However, the reaction temperature had no significant effect on H₂ selectivity. The DRM reaction is endothermic, so increasing the temperature results in an increase in the reaction rate, hence the conversion of CH₄ and CO₂. The increasing H₂/CO ratio with an increasing reaction temperature may be a result of increased conversion of CH₄ and CO₂ as well as side reactions, as represented by Equations (3)–(5).

3. Discussion

This research was successful in exploring a novel polymer-derived Ni-based catalyst in the dry reforming of methane (DRM) reaction. Polymer-derived catalysts have recently been a topic of research [50], including for DRM reactions [56]. When PDMS is pyrolyzed, it forms a ceramic material that consists of silicon oxycarbide (SiOC). SiOC is a thermally stable material that has potential applications in a wide range of fields, including catalysis.

Incorporating Ni into PDMS followed by pyrolysis was expected to result in catalysts with small nanostructured Ni particles that are well distributed within a SiOC ceramic structure. However, in this study, Ni-PDMS catalysts, prepared by pyrolyzing nickel acetate-containing PDMS, were not homogenous; the nickel-containing species were contained predominantly in porous grainy structures deposited onto microporous silicon oxide supports, which is evidenced by the elemental mapping shown in Figure 6. The microporosity of the catalysts is apparent by inspection of Table 3, which shows relatively high-surface-area materials (330–520 m²/g) with most of the pore volume (>70%) being contained in micropores. The nickel species are predominantly Ni(0), as indicated by XRD profiles. However, EPR spectra (Figure 8) show that there is a presence of unpaired electrons in the Ni species in the unused catalyst, giving evidence that the Ni species is reduced during the DRM reaction.

The Ni-PDMS catalysts demonstrate significant activity in the DRM reaction and comparable turnover frequency (TOF), CH₄ conversion (%), and activity loss to those reported in the literature. Of the catalysts used in this study, the 6Ni-PDMS displayed the highest CH₄ conversion and lowest activity loss in the DRM reaction after 11 h of continuous operation at 750 °C and a 1:1 molar ratio of CH₄ and CO₂. The reasons for observing a higher activity of 6Ni-PDMS compared to the other Ni-PDMS catalysts in this study include the higher concentration of Ni, hence, active sites, at the surface of the catalyst. The lower loss of activity in this same catalyst may be due to slight differences in the Ni dispersion within and on the catalyst and Ni–support–carbon interactions, which are evident by the shift in the Ni diffraction pattern in Figure 4. At a molar reactant ratio of 2:1 CH₄:CO₂ and a reaction temperature of 750 °C, the H₂/CO molar ratio was closest to 1.

A comparison of the catalyst results with those from the published literature is provided in Table 4. The catalysts prepared in this study have similar activities compared to other Ni-based catalysts for DRM. However, in comparing the TOF values, Pt- and Pd-based catalysts have higher TOF values than Ni-based catalysts in general.

4. Materials and Methods

4.1. Catalyst Preparation

Polydimethylsiloxane (PDMS) was prepared by following the Dow Corning Sylgard 184 Silicone Elastomer kit. After adding the curing agent to the polymer at the prescribed 1:10 mass ratio, hydrated nickel acetate (Sigma Aldrich, St. Louis, MO, USA) was added to separate batches and mixed vigorously by hand for several minutes. Initial slurries were prepared at 0, 2, 4, and 6 wt%, defined by Equation (7).

$$Ni\left(wt\%\right)={\displaystyle \frac{mass Ni}{(mass polymer)}}$$

(7)

The Ni-PDMS polymers were cured with heat. The Ni-PDMS samples were cut into ¼ inch cubes and pyrolyzed under an argon atmosphere at 750 °C for 2 h. Following pyrolysis, the catalysts were crushed and sieved to yield catalyst particles <0.234 mm.

4.2. Catalyst Characterization Techniques

The surface area analyses of the Ni-PDMS catalysts were conducted using nitrogen adsorption at −196 °C in a Micromeritics TriStar II 3020 Version 2.00. Before the analysis, the samples were degassed under vacuum at 125 °C for 30 min. Using the adsorption isotherm data, the surface areas were calculating using the Brunauer–Emmett–Teller (BET) method and the Rouquerol criteria [57], two of which are as follows:

🡺

The intercept of the BET plot must be positive;

🡺

$V_{ads}\left(1-{\displaystyle \frac{p}{p_0}}\right)$ increases with ${\displaystyle \frac{p}{p_0}}$.

Additionally, the Harkins–Jura method was used to determine the micropore volume and width [58].

Powder X-ray diffraction (XRD) measurements were completed with a Bruker D8 X-ray diffractometer (Billerica, MA, USA) at a 2θ range of 10 to 80° to determine the crystallinity and crystal phases in the catalyst. Data were collected with a step size of 0.01° at 0.1 s per step.

A TA Instruments Q500 thermogravimetric analyzer (TGA) (New Castle, DE, USA) was used to understand the thermal profiles of the Ni-PDMS samples before they were pyrolyzed. Mass loss was normalized over a temperature range of 0 to 800 °C at a ramp of 10 °C/min. Prior to pyrolysis, the TGAs were conducted under a nitrogen atmosphere. TGAs were also conducted on used and unused Ni-PDMS catalysts following pyrolysis. These analyses were conducted in air, otherwise, using the same parameters as described above.

Fourier Transform Infrared spectroscopy (FTIR) was performed using the Nicolet 670 FTIR (Thermo Fisher Scientific, Waltham, MA, USA) to compare the functional groups present in the fresh and used catalyst samples.

Electron paramagnetic resonance (EPR) was completed to understand the oxidation state of the nickel present in selected catalyst samples using the Bruker ELEXSYS E580 spectrometer (Billerica, MA, USA).

A Zeiss Supra 35 variable-pressure FEG SEM (Oberkochen, Germany) was used to investigate the morphologies of the Ni-PDMS catalysts. The SEM is equipped with an EDAX Genesis 2000 energy dispersive spectrometry (EDS) detector (MahWah, NJ, USA). EDS was used to provide information regarding the elemental composition of each catalyst. The spectra were taken at 15 KeV and 10 mm WD with an aperture size of 60 µm.

4.3. Catalytic Tests

Catalytic activity tests for the dry reforming of methane (DRM) reaction were conducted a ¼” OD stainless steel tube containing 50 mg of catalysts secured with quartz wool in its center. A Lindberg/Blue Mini-Mite tube furnace was used to control the reaction temperature at 650 °C, 700 °C, or 750 °C. Reactant gases (methane and carbon dioxide) along with an inert gas (nitrogen) were introduced to the catalyst under atmospheric pressure at a total flow of 60 ccpm.

Table 5. Experiments testing activity level of all Ni catalysts in this study.

Trials	Reaction Temperature	Feed Gas Molar Ratio	Catalysts
1–4	750 °C	1:1:1 CH4:CO2:N2	all Ni-PDMS catalysts
5	650 °C	1:1:1 CH4:CO2:N2	6Ni-PDMS
6	700 °C	1:1:1 CH4:CO2:N2	6Ni-PDMS
7	750 °C	1:2:1 CH4:CO2:N2	6Ni-PDMS
8	750 °C	2:1:1 CH4:CO2:N2	6Ni-PDMS

displays the experimental conditions applied to each catalyst. The reaction temperature and molar ratio were varied. The composition of the product stream was analyzed in 1 h intervals via gas chromatography using the SRI Model 8610C GC (Torrance, CA, USA), equipped with a TCD and FID detector and a methanator to facilitate the analyses of carbon monoxide and carbon dioxide. Two trials were run at each given set of conditions to demonstrate reproducibility, and the results provided are an average of the two trials.

Equations (8) and (9) were used to calculate the percent conversion of the reactants carbon dioxide and methane, respectively.

$${CO}_2 Conversion (\%)={\displaystyle \frac{({CO}_{2,in}-{CO}_{2,out})}{\left({CO}_{2,in}\right)}}\times 100\%$$

(8)

$${CH}_4 Conversion (\%)={\displaystyle \frac{({CH}_{4,in}-{CH}_{4,out})}{\left({CH}_{4,in}\right)}}\times 100\%$$

(9)

The selectivity of hydrogen gas and the H₂/CO ratio were determined using Equations (10) and (11).

$$H_2 Selectivity (\%)={\displaystyle \frac{ H_{2,out}/2}{\left({CH}_{4,in}-{CH}_{4,out}\right)}}\times 100\%$$

(10)

$${\displaystyle \frac{H_2}{CO}}ratio={\displaystyle \frac{ H_{2,out}}{\left({CO}_{out}\right)}}$$

(11)

The turnover frequency (TOF) of each catalyst is represented by Equation (12). In this study, the moles of active sites were assumed to equal the moles of Ni on the catalyst, which is supported by Alabi et al. [59].

$$TOF={\displaystyle \frac{ molecules converted}{time \ast active catalyst}}={\displaystyle \frac{ {CH}_{4,converted} \left(\frac{moles}{min}\right)}{\frac{moles Ni}{g pyrolyzed catalyst} \ast g catalyst used}}$$

(12)

Activity loss is measured as a percentage in Equation (13), which compares the difference between the initial and final CH₄ conversion, relative to the initial conversion. Due to coking of the catalyst, activity loss is a common issue in the DRM reaction.

$$Activity Loss \left(\%\right)={\displaystyle \frac{ Initial {CH}_{4,conversion}-Final {CH}_{4,conversion}}{Initial {CH}_{4,conversion} }}\times 100\%$$

(13)

5. Conclusions

In this study, novel Ni-containing polymer-derived catalysts (Ni-PDMS) were synthesized, characterized, and investigated as catalysts for the DRM reaction. The pyrolyzed PDMS support was found to be largely microporous. The nickel particles were nano-sized, but they did not disperse evenly across the pyrolyzed PDMS support. Overall, the catalysts are non-homogenous in composition and structure. The Ni-PDMS catalysts demonstrate significant activity in the DRM reaction and have comparable turnover frequencies (TOF), % CH₄ conversions, and activity losses to some catalysts reported in the literature. The 6Ni-PDMS displayed the highest % CH₄ conversion and lowest activity loss in the DRM reaction of the catalysts in this study. Additionally, a molar reactant ratio of 2:1 CH₄:CO₂ demonstrated a H₂/CO ratio closest to 1 of all ratios studied in this work. A reaction temperature of 750 °C demonstrated the highest % CH₄ conversion of all reaction temperatures studied in this work. This research was successful in exploring a novel polymer-derived Ni-based catalyst in the dry reforming of methane (DRM) reaction.