Mechanistic Behavior of Basicity of Bimetallic Ni/ZrO₂ Mixed Oxides for Stable Oxythermal Reforming of CH₄ with CO₂

Abstract

The mixed oxides of Ni/ZrO₂, Ni-Ca/ZrO₂, Ni-Ba/ZrO₂, and Ni-Ba-Ca/ZrO₂ were prepared using the co-precipitation method at a pH of precisely 8.3. The catalytic mixed oxides of Ni/ZrO₂, Ni-Ca/ZrO₂, Ni-Ba/ZrO₂, and Ni-Ba-Ca/ZrO₂ were characterized using x-ray diffraction XRD, Brunauer Emmett Teller (BET), scanning electron microscopy (SEM), and metal dispersion for the screening of phase purity, surface area, and morphology. The mixed oxides are subjected to CO₂-TPD to quantify the basicity of every composition. The mixed oxide catalysts of Ni/ZrO₂, Ni-Ca/ZrO₂, Ni-Ba/ZrO₂, and Ni-Ba-Ca/ZrO₂ were screened for oxythermal reforming of CH₄ with CO₂ in a fixed bed tubular reactor at 800 °C. Among all catalysts, the Ba- and Ca- loaded Ni-Ba-Ca/ZrO₂ showed high conversion by the decomposition of methane and CO₂ disproportionation throughout the time on stream of 29 h. The high activity with stability led to less coke formation over Ni-Ba-Ca/ZrO₂ over the surface. The stable syngas production with an active catalyst bed contributed to the improved bimetallic synergy. The high surface basicity of Ni-Ba-Ca/ZrO₂ may keep actively gasifying the formed soot and allow for further stable reforming reactions.

1. Introduction

Environmental concerns like the effect of greenhouse gases have prompted a focus on plentiful naturally available resources like methane. Thus, green catalytic technologies capable of converting methane and carbon dioxide, two main greenhouse gas contributors, are crucial to avoid their significant release into the atmosphere. The targeted CO₂ is used as one of the reactants (which may free resources from industrial flue gases), and another reactant CH₄, which is naturally abundant, is used to perform the oxythermal reforming (OTR) reaction [1,2,3]. OTR is a significant industrial process that produces syngas (synthesis gas H₂, CO) from a feedstock mixture (CO₂ and CH₄) with the dilution of nitrogen through a highly stable Ni-based mixed oxide catalyst [4,5]. Apart from the OTR reaction, highly crystalline mixed oxides over supported metallic materials have been well used for steam and dry reforming reactions. The significant use of syngas in various fine chemical and fuel cell industries is growing by 6 EJ produced globally each year, accounting for nearly 2% utilization [6,7,8,9]. The characteristics of syngas can impact combustion processes in internal combustion engines, with properties like the OTR process being blended with the two reactions of steam reforming and partial reforming in a single reactor. The reactor holds both endothermic and exothermic reactions; the produced energy will be utilized for consecutive reactions. Although this reaction has a significant industrial application, many stability issues need to be addressed to obtain the ideal syngas ratio.

Optimizing the catalyst parameters, like obtaining the highly crystalline, metal dispersion of mixed oxides with the optimization of the physical properties through one-pot synthesis is a challenging quest. Distribution, with the synergy of metallic particles over the surface, and the basicity of the surface are potential factors for stable syngas production. Research addresses the main challenge of soot formation on catalytic beds. Our article mainly focuses on catalyst synthesis through co-precipitation method to trigger soot formation and reactions leading to atomic economy. The basicity and high surface oxygen content help to oxidize the soot formation over the catalyst surface during a reforming reaction. A mixed oxide catalyst may be loaded with a rich oxygen element like a cerium oxide catalyst with reaction parameters resulting in an economical process [6,7,8,9]. The parameters of the catalyst must align with the composition of feed gases and the production H₂/CO ratio. The methane decomposition part of the reaction will go over the metallic sites like copper, Fe, Co, Ni-based catalysts, and Pt-, Rh-, and Ru-doped catalysts, while the use of promoters like Ce, Ca, and K metal formulations such as Fe, Co, and Ni [10,11,12,13,14,15,16] as bifunctional catalysts is suitable. Another part of CO₂ dissociation is processed through basic sites on the catalyst surface. More recently, mixed oxide catalysts with bifunctional properties that have emerged for partial oxidation (POX), dry reforming, and autothermal reforming (ATR) may facilitate the dehydrogenation and dissociation of methane and disproportionation of carbon oxide over metals like, e.g., CeO₂, ZrO₂, and Bi₂O₅ to provide selective oxidation functionality without deactivation, whereas optimizing the reaction and catalyst parameters is the quest.

This report is intended to showcase the effect of parameters like metal dispersion, basicity, crystallinity, and surface area on the screening results of the oxythermal reforming of CH₄ with CO₂ at 800 °C over metallic mixed oxide catalysts. The mono- and bimetallic catalysts, i.e., Ni/ZrO₂, Ni–Ba/ZrO_2, Ni–Ca/ZrO₂ and Ni–Ba–Ca/ZrO₂ are prepared using the coprecipitation method to understand the effect of the promoter (Ba and/or Ca) on the optimized reforming reaction.

2. Result and Discussion

2.1. Catalyst Surface Properties

The mixed oxide catalysts physicochemical properties are listed in

Table 1. Synthesis composition and elemental analysis of Ni-bimetallic/ZrO₂ mixed oxide catalyst materials.

Catalyst Code a	Metal Content wt.%				Elemental Analysis%
	Ni	Ba	Ca	Zr	Ni	Ba	Ca	Zr
Ni/ZrO2	20	0	0	80	19.97	0	0	78.98
Ni–Ba/ZrO2	20	10	0	70	19.91	9.89	0	69.91
Ni–Ca/ZrO2	20	0	10	70	19.97	0	9.91	69.89
Ni–Ba–Ca/ZrO2	20	5	5	70	19.98	4.82	4.81	69.87

^a Prepared using the co-precipitation method.

and

Table 2. Chemisorption properties of Ni–bimetallic/ZrO₂ mixed oxide catalysts.

Catalyst Code	Surface Area m2/g a	Pore Diameter (nm) a	Pore Volume (cm3/g) a	Metal Dispersion (%) b	Metallic Surface Area m2/g b	Coke Content wt.% c
Ni/ZrO2	58.71	2.09	0.02	0.1	0.01	19.46
Ni–Ba/ZrO2	131.41	7.1	0.57	4.8	2.1	3.57
Ni–Ca/ZrO2	145.18	7.6	0.61	5.2	2.6	1.68
Ni–Ba–Ca/ZrO2	157.15	8.3	0.67	7.4	3.1	0.90

^a Measured by N₂ physisorption; ^b measured from the Auto Chem program in CO pulse chemisorption; ^c coke content is measured after 3000 min of methanation by TGA analysis.

. In Table 1, the elemental analysis is found be close to the initial composition of the mixed oxide. The Ni, with and without metals (Ba, Ca), promoted by a ZrO₂-supported catalyst, shows a significant difference in surface area, which may make a difference in chemisorption activities. The Ni-Ba/ZrO₂ shows a surface area of 131.41 m²/g, whereas Ni–Ca/ZrO₂ was found to be 145.18 m²/g, respectively.

The addition of both metals, Ba and Ca, over Ni/ZrO₂ shows a remarkable improvement in surface area of 157.15 m²/g. Similarly, pore diameter and pore volume also increase in the order of Ni/ZrO₂ ˂ Ni–Ba/ZrO₂ ˂ Ni–Ca/ZrO₂ ˂ Ni–Ba–Ca/ZrO₂, respectively. The significant rise in surface area of the bimetallic catalyst (Ni–Ba–Ca/ZrO₂) is clear evidence of the influence of the promoter in the solid matrix. The Ni/ZrO₂ metal dispersion is 0.1%; Ni–Ba/ZrO₂ was found to be 4.8%; whereas Ni–Ca/ZrO₂ was found to be 5.2%, respectively. The addition of Ba and Ca showed a potential rise in metal dispersion of 7.4%. The crystallinity of the solid catalysts is analyzed by X-Ray Diffraction (XRD) for phase purity. Figure 1, for Ni–Ba/ZrO₂, Ni–Ca/ZrO₂, and Ni–Ba–Ca/ZrO₂, shows the typical cubic fluorite-type phase (PDF-ICDD 28-0271). The key ZrO₂ support lines of (111), (200), (220), (311), and (222) confirm that Ni, Ba, and Ca are incorporated in the lattice of the solid solution [16,17]. The sharper peak was found over 29° and, 30.97°, and the monoclinic phase was identified over 30.97°, 49.8°, and 58.6° angles. The peak intensity is sharper over Ni–Ba–Ca/ZrO₂ compared with Ni–Ba/ZrO₂ and Ni–Ca/ZrO₂.

2.2. Effect of Ba and Ca Metal Loading on Oxythermal Reforming

The present oxythermal reforming feedstock has a typical ratio of CH₄:CO:O₂ = 1:1:0.2, which is endothermic in nature. Partial oxidation of methane offers a substitute to reduce the operational budget, but it relies on oxygen, making oxygen production expenses approximately 50% of the total investment and posing a high explosion risk at elevated temperatures. Our earlier report confirms that the addition of second and third metals to Ni was found to be a very effective NiCoMn/ZrO₂ trimetallic catalyst for the autothermal reforming reaction. The addition of the metal Ce and K was found to have a positive result, with the stability of methane and CO₂ decomposition being 95.5% and 89.9%, respectively [18,19]. According to Figure 2, all the catalysts show comparable chemisorption over catalytic surfaces. Ni/ZrO₂ catalyst CH₄ conversion drops drastically from 43.2 wt.%, whereas CO₂ conversion drops from 33.2 wt.%.

The coke samples were tested after the 29 h spent with the catalyst using TGA. The Ni/ZrO₂ catalyst led to the highest coke formation of 19.46 wt.%, with the surface becoming inactive after 25 h of steam. The addition of Ba and Ca in the Ni–Ba–Ca/ZrO₂ catalyst had potentially stable activity, whereas the Ni–Ca/ZrO₂ and Ni–Ba/ZrO₂ showed a considerable decrease in conversion with coke formation of 1.68% and 3.57% at 800 °C. The better activity of Ni–Ba–Ca/ZrO₂ may be attributed to high crystallinity with surface area, which led to better metal dispersion. Takeguchi et al. [20] tested Ni/ZrO₂ for partial oxidation, steam reforming, and autothermal reforming reactions at 550 °C and 650 °C by varying the feed compositions. The CH₄ conversion was found higher in autothermal reforming and lower in partial oxidation over ZrO₂-supported bimetallic catalyst with better stability over the time.

Figure 3. H₂/CO ratio over bimetallic catalysts (■) Ni–Ba–Ca/ZrO₂, (●) Ni–Ca/ZrO₂, (▲) Ni–Ba/ZrO₂, (▼) Ni/ZrO₂ at 800 °C.

Figure 3. H₂/CO ratio over bimetallic catalysts (■) Ni–Ba–Ca/ZrO₂, (●) Ni–Ca/ZrO₂, (▲) Ni–Ba/ZrO₂, (▼) Ni/ZrO₂ at 800 °C.

The production of the H₂/CO ratio reached above 1:1 ratio throughout the oxythermal reforming of methane, which is the feedstock for oxygenate synthesis. The H₂/CO ratio for Ni/ZrO₂ was 0.7 and dropped significantly. Figure 3 shows the Ni–Ba–Ca/ZrO₂ reached 1:5 with considerable stability, whereas the H₂/CO ratio of Ni–Ca/ZrO₂ was 1:3 and lost stability after 14 h of time on stream. The Ni–Ca/ZrO₂ chemisorption showed H₂/CO ratio of 1:1 and a stability decrease with a time on stream of 7 h. Our previous report shows that the perovskite La_1−xBa_xMnO₃ used for the reforming reaction of CO₂ with CH₄ over varying ratios of the metals Ba/Mn achieves a high porosity and surface area for stable conversion and H₂/CO selectivity. We found that the optimum amount of Mn³⁺/Mn⁴⁺ to Mn²⁺ promoted oxygen availability to remove coke. The lattice oxygen is very effective for maintaining surface activity stability of [13]. The tri-reforming of methane was found to be cost effective due to the combination of steam and dry reforming with partial oxidation (CH₄ + O₂ + CO₂ + H₂O), which may be associated with the compensations and shortcomings of each process. Figure 4 shows the H₂/CO ratio of our previous report. The Ni/AlCeZrOx catalyst showed much potential and was shown to be an outstanding candidate by the addition of a second metal, i.e., La, Mg, Co, and Fe.

With the addition of the second metal, cobalt, manganese with Ni shows a promising improvement in conversion with remarkable stability and negligible amounts of coke formation [20,21,22,23,24]. This report also observes the basicity of the surface is quite critical for CO₂ dissociation. A well-known drawback of reforming reactions is rapid coke formation, which may lead to the deactivation of the catalyst surface, which may be attributed to coke deposition and catalyst sintering [25,26,27]. The tri-reforming of methane has the potential to significantly reduce carbon deposition. In addition, the presence of O₂ in the feed enables in-situ energy generation through exothermal methane oxidation, enhancing the energy efficiency of the process. Furthermore, the ability to adjust reactant compositions allows for versatile gas composition synthesis, suitable for various synthesis gas applications.

2.3. Effect of Basicity on Conversion and Selectivity

Basicity is very significant in influencing the soot oxidation process and CO₂ activation, which makes the catalyst surface active throughout the steam. The basic strength of catalyst was found for weak sites at temperatures of <200 °C, medium sites at temperatures of 200–350 °C, strong sites at temperatures of 350–600 °C, and very strong sites at temperatures of >600 °C [11]. For Ni/ZrO₂, Ni-Ba/ZrO₂, Ni-Ca/ZrO₂, and Ni-Ba-Ca/ZrO₂,

Table 3. CO₂–TPD analysis of Ni–bimetallic/ZrO₂ mixed oxide catalysts.

Catalyst Code	Basic Sites µmol/g
	Weak (<200 °C)	Medium (200–350 °C)	Strong (350–600 °C)	Very Strong (>600 °C)
Ni/ZrO2	12.41	5.1	0.31	2.6
Ni–Ba/ZrO2	5.39	0.1	0.41	19.6
Ni–Ca/ZrO2	11.24	0.8	1.97	39.8
Ni–Ba–Ca/ZrO2	15.91	1.3	2.7	51.8

shows two prominent levels of basicity at weak and very strong sites.

At high temperatures, the response of Ni/ZrO₂ was found to be 2.6 µmol/g; the response of Ni–Ba/ZrO₂ was found to be 19.6 µmol/g; the response of Ni–Ca/ZrO₂ was found to be 39.8 µmol/g; and the response of Ni–Ba–Ca/ZrO₂ found to be 51.8 µmol/g. The increase in basic sites clearly confirms the metal influence over the catalysts surface, which facilitates a significant improvement in reaction parameters [28,29,30]. The surface basicity may provide three different roles: (i) enhancement of CO₂ adsorption and activation, (ii) coke removal via gasification, and (iii) promoting CH₄ activation. The basicity of the catalyst is a potential characteristic for the activation and adsorption of CO₂ and H₂O, which are acidic in nature. The role of surface basicity is the enhancement of CO₂ adsorption and activation. The basic sites (electron-rich oxides) strongly adsorb the weakly acidic molecule. These basic sites promote CO₂ activation, enabling CO₂ dissociation [31,32,33]. The oxythermal reforming results in coke forming over the catalyst surface due to the low basic sites of Ni/ZrO₂. The high surface basic sites of Ni–Ba–Ca/ZrO₂ promote coke gasification by CO₂ activation and maintain the catalytic activity.

C_(S) + CO₂ → 2 CO

(1)

The strong adsorption of carbon precursors by basicity sites decreases the Boudouard reaction and methane cracking, which results in a higher syngas yield over time on stream.

C_(S) + H₂O → 2 CO + H₂

(2)

The perfect utilization of CO₂ (reaction 1) leading to a favorable shift in the product ratio (reaction 2) provides the H₂-rich syngas less atomic loss. Ni–Ca/ZrO₂ was found to have better stability. Throughout the 29 h of time on stream (Figure 2), there was low deactivation compared to Ni-Ba/ZrO₂. The basic sites may also synergize the metal sites dispersion as a dual function. Figure 5 shows the H₂/CO ratio of Ca as a second metal, and Ni–Ca/ZrO₂ was also found to be quite stable compared with monometallic Ni/ZrO₂.

Jin et al. [9] experimented with Mg addition with different amounts of MgO doped over Ni/Al₂O₃ for the dry reforming reaction and found it to be quite effective. The optimal addition of MgO enhances catalyst basicity, which facilitates CO₂ activation and adsorption [11]. The catalytic process leads to the gasification of coke precursors [34,35,36]. The basic strength of the catalyst may also neutralize the acid-based reaction, which affects the carbon economy. The H₂/CO ratio is 1.5 over Ni–Ba–Ca/ZrO₂, which seems to show that bimetallic synergy between the support and metal provides excellent physicochemical interactions by enhancing the basicity, and metal dispersion facilitates the reduction of carbon filament growth and leads to stable syngas production. The Ni–Ba–Ca/ZrO₂ catalyst shows negligible (0.98%) coke formation for 29 h on stream and is labeled as an efficacy catalyst due to the perfect combination of metallic sites and basicity of the catalyst surface leading to stability, selectivity, and longevity towards syngas (H₂ + CO).

3. Experimental Methods

3.1. Synthesis of Mixed Oxide Catalytic Materials

The nitrate based precursors of nickel, barium, and calcium are immersed in distilled water to completely dissolve and become precipitated with an aqueous solution of NaOH at room temperature at a pH of 8.3. The supported zirconium oxychloride is dissolved separately to precipitate with an aqueous solution of NaOH at room temperature at a pH of 8.3. Both precipitates were mixed, and the pH was adjusted to 8.3 and aged. The precipitate was aged for 6 h under continuous stirring. The mature precipitates were vacuum filtered to wash off all nitrates in the mixture. The filtrate was air-dried for 28 h at 110 °C. The dried mixed oxides are powdered with agate mortar before the firing (calcination) in a furnace. The resulting powdered mixed oxide materials Ni/ZrO₂, Ni-Ca/ZrO₂ Ni-Ba/ZrO₂, and Ni-Ba-Ca/ZrO₂ (Table 1) were stored in desiccator for characterization and the screening of oxythermal reforming.

3.2. Characterization and Catalytic Activity

The mixed oxide materials were characterized for crystallinity, surface area, volume, and basicity using X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), and CO₂-TPD using the procedure from our earlier reports [11]. The oxythermal reforming was carried out in a fixed-bed tubular reactor with a loading of 0.3 g of mixed oxides prior to reduction in the presence of H₂ at 530 °C for 2 h. The oxythermal reforming reaction was carried out at 800 °C using a mixture of CH₄:CO:O₂ = 1:1:0.2 ratios with Gas Hourly Space Velocity (GHSV) was 21,000⁻¹ with N₂ (1 mole) with dilution. The catalyst test was performed in a fixed bed reactor with stable feed gas composition, temperature control (±1 °C), and consistent flow rates. All the reactor conditions are verified thoroughly before starting the reactions. The product H₂ and CO (syngas) stream was evaluated by online gas chromatography Agilent Inc, India. The online gas chromatography is calibrated regularly with gas standards to ensure the accuracy of analysis composition. The CH₄, CO conversions, and H₂:CO ratios were analyzed after 1 h of steady state reaction, and conversion and selectivity calculations were provided in our earlier reports [11].

4. Summary

Oxythermal reforming is a well-known process for the production of syngas. The reforming catalyst is mainly composed of active metals like Ni, Ba, and Ca over a ZrO₂ support. The bimetallic catalyst is prepared using the coprecipitation method at a pH of 8.3 and characterized for crystallinity by X-ray diffraction, crystal morphology, surface area, pore volume, and metal dispersion. Oxythermal reforming over Ni/ZrO₂, Ni-Ba/ZrO₂, Ni-Ca/ZrO₂, and Ni-Ba-Ca/ZrO₂ is screened at 800 °C. The screening results for Ni/ZrO₂ show a drastic fall in conversion. The bimetallic Ni–Ba–Ca/ZrO₂ was found to be a stable catalyst that could withstand the time on steam. The Ni-Ba/ZrO₂ and Ni–Ca/ZrO₂ were found to have unstable catalytic activity with a lack of metal dispersion and crystallinity. The decomposition of CH₄ over metallic sites is more effective over Ni–Ba–Ca/ZrO₂ compared to Ni–Ba/ZrO₂ and Ni–Ca/ZrO₂. The disproportionation and effective activation of CO₂ due to the basic sites of the surface establish the interaction and withstand coke gasification.