The role of ceria in promoting Ni catalysts supported on phosphate‐modified zirconia for the partial oxidation of methane

Abstract

The catalytic partial oxidation of methane (POM) is aimed at the mitigation of CH₄ (a highly potent greenhouse gas) from the environment and the synthesis of syngas with a high H₂/CO ratio. Herein, to enhance the POM reaction, Ni‐supported phosphate‐modified‐zirconia were synthesized with promotor “Ce” to achieve high H₂/CO ratio (2.4–3.2). The catalysts were characterized by surface area and porosity, X‐ray diffraction, RAMAN, temperature‐programmed experiments (TPR, CO₂‐TPD, and TPO), and TEM. Increasing the ceria addition over 10Ni/PO₄ + ZrO₂ resulted in lower crystallinity, higher dispersion of active sites, and enhanced the surface area of catalyst. The unique and prominent reducibility and basicity of NiO‐species and surface oxide ions, respectively, are particularly notable at 4 wt.% ceria loading. At a reaction temperature of 600°C, the highest concentration of active sites and a unique concentration of moderate strength basic sites can be achieved with 4 wt.% ceria loading over 10Ni/PO₄ + ZrO₂. This leads to 44% conversion of CH₄, 36% yield of H₂, 35% yield of CO₂, and H₂/CO ratio of 3.16 for the POM reaction. The cyclic H₂TPR‐O₂TPO‐H₂TPR experiment confirms the reorganization of the active site towards high temperature under oxidizing gas O₂ and reducing gas H₂ gas stream during the POM reaction.

INTRODUCTION

The partial oxidation of methane (POM) by O₂ performs two aims together. The first one is the mitigation of a highly potent greenhouse gas, CH₄, from the environment and the second is the generation of syngas (H₂ and CO) with an H₂/CO ratio approximately 2 (CH₄ + 0.5O₂ → 2H₂ + CO; Δ$${H}_{298K}^{0}$$ = −36 kJ/mol).¹ Greenhouse gases are the primary cause of global warming, while syngas is a synthetic feedstock used for producing high-order hydrocarbon liquid fuel and methanol. Nobel metal and supported Nobel metal showed profound activity toward the partial oxidation of methane.^2,3 However, the high cost of noble metals is always a limitation for industrial applications. Metallic Ni is also an active site for POM but serious sintering against temperature is a major drawback. This time, proper selection of support becomes crucial which can hold the active sites long against high temperature. Alumina-supported Ni catalyst outperformed others towards POM⁴ due to strong metal-support interaction. The selection of proper support other than aluminium is also a matter of investigation. Ni supported over MgO was found more selective to total oxidation of methane than selective oxidation.⁵ La₂O₃ and TiO₂-like supports are found not suitable for holding Ni for long as both supports oxidize the active sites Ni.^5,6

The use of reducible redox support like CeO₂ and ZrO₂ may be recommendable as support for an oxidizing reaction where delay in oxygen transport can be minimized by lattice oxygen of redox support. H₂-chemosorption study showed that Ni supported over zirconia has a larger uptake of H₂ than Ni supported over ceria⁷ indicating less reducibility over Ni/CeO₂. In another study, the total mobile oxygen content of CeO₂ was estimated to be 2.16% (of total oxygen in support) whereas for ZrO₂ the total mobile oxygen content was 0.032%.⁸ A support favouring NiO reducibility should be more preferred and so Ni supported over ZrO₂ may have wide applicability towards POM reaction. The high oxygen storage capacity of ceria can be utilized as a promoter for Ni/ZrO₂ catalyst. Jing et al. observed a higher dispersion of Ni over zirconia than silica.⁹ However, there is a serious phase transition issue with ZrO₂ which weakens the metal support interaction and ultimately turns into inferior catalytic performance.

Nowadays the use of second metal/non-metal oxide (i.e., Y₂O₃, La₂O₃, WO₃, and PO₄²⁻) along with ZrO₂ was found to stabilized zirconia against phase transition due to the formation of stable cubic or tetragonal ZrO₂ phases.^10–13 In the presence of oxygen, iron phosphate was utilized in selective oxidation of methane into methanol.¹⁴ Partial oxidation of methane into syngas was also tested over Ni supported over calcium-hydroxy phosphate (Ca/PO₄ = 10/6), Ni supported over calcium-phosphate and Ni supported over strontium phosphate.^15–20 Upon promotional addition of Ce over Ni/Ca-apatite, coke deposition was suppressed effectively.²⁰ Previously, we studied that the addition of ceria over phosphate-zirconia supported Ni catalyst led to an increase in surface reducibility and additional CH₄ decomposition sites. This resulted in the formation of NiP and Ni(PO₃)₂ along with metallic Ni.¹³ Herein, we have used 8 wt.% phosphate-92%zirconia (PO₄ + ZrO₂) as support for holding active sites Ni for the POM reaction. In the goal of excelling catalytic activity further, 1–5 wt.% of ceria was added as a promoter over 10Ni/PO₄ + ZrO₂. The 10NixCe/PO₄ + ZrO₂ (x = 1–5 wt.%) catalysts are characterized by surface area and porosity, X-ray diffraction, RAMAN spectroscopy, H₂-temperature programmed reduction, and CO₂-temperature programmed reduction. The fine-tuning between characterization results and catalytic activity in accordance with the reaction mechanism justify the applicability of the catalyst for POM reaction.

EXPERIMENTAL

Materials

Ni (NO₃)₂.6H₂O (Purity 98%, Alfa Aesar), Ce (NO₃).6H₂O (ACROS Organic) and 8 wt.%PO₄–92 wt.%ZrO₂ (Daiichi Kigenso Kagaku Kogyo Co.). As per the specification of 8 wt.%PO₄–92 wt.%ZrO₂, it has a tetragonal-ZrO₂ phase. The surface area of the particle is 246 m²/g and 50% of particles in the catalyst are smaller than 4.73 μm (D₅₀ = 4.73 μm).

Catalyst preparation

A wet-impregnation method was employed to get the wanted catalysts for the partial oxidation of methane. Supports of zirconia (ZrO₂) and phosphate (PO₄) were impregnated in an aqueous solution of Ni nitrate precursor to obtain catalyst samples at a nominal Ce (Daichi Kigenso Kagaku Kogyo Co. Ltd.) loading of 1, 2, 3, 3.5, 4, and 5 wt.%. The solution was heated at 80°C under agitation for 5 h, then dried overnight at 120°C, and finally calcined at 600°C for 24 h. Supporting Information S1 file includes a detailed description of the characterization.

Catalyst characterization

Supporting Information S1 file includes a detailed description of the characterization.

Catalyst activity test

0.1 g of catalyst sample is packed in a tubular stainless steel fixed-bed reactor (PID Eng & Tech; 9 mm I.D) equipped with a furnace. The temperature of the catalyst bed is monitored by a K-type thermocouple (fitted axially at the centre of the catalyst bed). Before the POM reaction, reductive pretreatment of the catalyst is carried out under hydrogen (flow rate 30 mL/min) for 1 h at 800°C. Further to remove the hydrogen gas from the catalyst bed, the reactor is purged with N₂. Then, the temperature of the reactor is stabilized at 600°C for the POM reaction. A mixture of CH₄, O_2, and N₂ gases (volume ratio of feed gases CH₄/O₂ was set to 2) is allowed to pass through the packed catalyst bed at a total feed rate of 24 mL/min and space velocity of 14,400 mL/h/g_cat. The product is analysed by a gas chromatograph equipped with a propak Q column, molecular sieve columns, and a thermal conductivity detector. The composition of effluent gases is calculated by the normalization method, and the equations for the determination of CH₄ conversion and H₂ yield and H₂/CO ratio are used as follows: 1$${\text{CH}}_{4}\,\text{conversion}\,( \% )=\frac{{\text{CH}}_{4,\text{in}}-{\text{CH}}_{4,\text{out}}}{{\text{CH}}_{4,\text{in}}}\times 100$$ 2$${{\rm{H}}}_{2}\,\text{Yield}\,( \% )=\frac{{{\rm{H}}}_{2,\text{out}}}{2{x}\,{\text{CH}}_{4,\text{in}}}\times 100$$ 3$$\frac{{{\rm{H}}}_{2}}{\mathrm{CO}}=\frac{\mathrm{Mole}\,\mathrm{of}\,{{\rm{H}}}_{2}\,\mathrm{produced}}{\mathrm{Mole}\,\mathrm{of}\,\mathrm{CO}\,\mathrm{produced}}.$$

RESULT AND DISCUSSION

Characterization result and discussion

As the DRM reaction is carried out over a reduced catalyst, the X-ray diffraction pattern of the reduced catalyst system is carried out to understand the presence of phases during the DRM reaction (Figure 1A,B). As per the specification from Daichi Kigenso Kagaku Kogyo Co. Ltd., PO₄ + ZrO₂ support has only tetragonal ZrO₂ phase. Over the current reduced-catalyst system also, the most obvious diffraction patterns are appeared for the tetragonal ZrO₂ phase. The 10Ni/PO₄ + ZrO₂ catalyst has shown the most intense peak at about 2θ = 30.2 ̊ and low-intensity peaks at 50.2 ̊ and 60.2 ̊ for the tetragonal ZrO₂ phase (JCPDS reference number 01-079-1769). The ceria-related phases are not observed in the XRD indicating fine dispersion of ceria over the catalyst surface. The diffraction pattern for hexagonal Ni phase (at Bragg's angle 2θ = 39.1°, 41.41°, and 45.09°; JCPDS reference number: 00-045-1027), tetragonal Ni₁₂P₅ phase (at Bragg's angle 2θ = 38.4°, 41.7°, 47°, and 49°; JCPDS reference number: 01-074-1381) and cubic Ni₅Zr phases (at Bragg's angle 2θ = 27.27°, 38.35°, 45°, 47°, 60.2°, 62°, 73°, 81°, and 85°; JCPDS reference number: 00-010-0229) are observed (Figure 1A–E). It is interested to note that over fresh catalyst system, the cubic NiO phase (at Bragg's angle 2θ = 37.2 ̊, 43.2 ̊, 62.9 ̊, and 75.3 ̊; JCPDS reference number 01-073-1523) and monoclinic Ni₂P₂O₇ phase (at Bragg's angle 2θ = 30.2 ̊; JCPDS reference number 00-033-0950) are prominent where diffraction peak pattern for the monoclinic Ni₂P₂O₇ phase and tetragonal ZrO₂ phase are merged at approximately 30 ̊ Bragg's angle (Supporting Information: Figure S1). But in the reduced catalyst system, the diffuse peak of NiO and tetragonal Ni₁₂P₅ phase and cubic Ni₅Zr phase are observed. It indicates that upon reduction, the tetragonal phase of ZrO₂ remains present but the cubic NiO phase is reduced into cubic Ni phase and Ni₂P₂O₄ phase is reduced to tetragonal Ni₁₂P₅ phase. It is observed that upon the addition of 2 wt% Ceria over 10Ni/PO₄ + ZrO₂, the diffraction peak intensity of both zirconia and Ni-related phases are decreased indicating the higher dispersion of Ni over less crystalline support (Figure 1B–E). The diffraction pattern of 10Ni4Ce/PO₄ + ZrO₂ is unique. With respect to 10Ni2Ce/PO₄ + ZrO₂, the diffraction peak intensity of 10Ni4Ce/PO₄ + ZrO₂ is lower about most of the Bragg's angle except most intense plane (~30° Bragg's angle). Approximately 30° Bragg's angle is related to tetragonal zirconia phase. That means, 4 wt% ceria loading over 10Ni/PO₄ + ZrO₂ induces the lower crystallinity/higher dispersion of Ni-related phases only. It is reported that ceria has good interaction with NiO which induces finer dispersion of NiO.²¹ At 5 wt% ceria loading over 10Ni/PO₄ + ZrO₂, the crystallinity of catalyst is dropped to minimum.

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The Raman spectra of reduced-10Ni/PO₄ + ZrO₂ and reduced-10NixCe/PO₄ + ZrO₂ catalysts are shown in Figure 1F. The presence of tetragonal-ZrO₂ phase are also verified by Raman sharp peak at 146.6 cm⁻¹, 253.3 cm⁻¹, and 629 cm⁻¹.^22,23 The Ni-related Raman vibration band is also observed at 436.6 cm⁻¹.^23,24 Upon loading 2 wt% Ce, the intensity of these peaks are maximum. It indicates the involvement of zirconium with NiO and ceria.

N₂ adsorption isotherm, porosity distribution, and surface parameters (surface area, pore volume, and average pore diameter) of reduced-10Ni/PO₄ + ZrO₂ and reduced-10NixCe/PO₄ + ZrO₂ (x = 1–5 wt.%) catalysts are shown in Supporting Information: Figure S2 and Table S1. The adsorption type IV isotherms with H₂ hysteresis loop indicates the presence of ink-bottle type mesopores. Ni supported over phosphate-zirconia has just 21.43 m²/g surface area, 0.05 cm³/g pore volume, and 9 nm average pore diameter. Upon loading of ceria 2 wt.% over 10Ni/PO₄ + ZrO₂, the surface area is raised more than three times and pore volume to more than 2.5 times. In XRD diffraction results also, the crystalline peak intensity for Ni-related phases and tetragonal-ZrO₂ phase are declined upon the addition of 2 wt% Ce over 10Ni/PO₄ + ZrO₂ catalyst. It indicates higher dispersion of Ni-related phases over less crystalline support which commutatively results into a higher surface area.^21,25 The crystallinity and surface parameters (the surface area, pore volume, and average pore diameter) of 10Ni2Ce/PO₄ + ZrO₂ catalyst and 10Ni4Ce/PO₄ + ZrO₂ catalyst are comparable. The crystallinity of the 5 wt% ceria promoted 10Ni/PO₄ + ZrO₂ catalyst is minimum and so surface parameters of this catalyst is highest (Supporting Information: Figure S2 and Table S1). 10Ni2Ce/PO₄ + ZrO₂ catalyst has 82.23 m²/g surface area, 0.14 cm³/g pore volume and 5.6 nm average pore diameter. The dV/dlogW versus W plot (V is volume and W is pore width) also shows monomodal pore size distribution and pore of size 4.5–5.6 nm is exclusively present over Ni/PO₄ + ZrO₂ and NixCe/PO₄ + ZrO₂ (x = 2–5 wt.%) catalysts (Supporting Information: Inset figures in Figure S2).

H₂-TPR profiles and the quantity of H₂ consumed at different temperature of 10Ni/PO₄ + ZrO₂ and 10NixCe/PO₄ + ZrO₂ catalysts are shown in Figure 2 and Supporting Information: Table S2. A diffuse reducible peak of about 250°C is attributed to the reduction of surface ceria species.^23,24 Another low-temperature reduction peak of about 370°C signifies the reduction of “surface interacted-NiO” into Ni.^26,27 The high temperature reduction peaks about 630°C and 770°C are due to consecutive reduction of Ni₂P₂O₇ → Ni₁₂P₅ → Ni₂P.^11,28 The lower reduction temperature of “surface interacted NiO” indicates that the reduction of “surface interacted-NiO” is easier than the reduction of Ni₂P₂O₇. The total H₂ consumption over an unpromoted catalyst (10Ni/PO₄ + ZrO₂) is maximum (75.5 cm³/g). A general trend in the reduction profile is noticed upon increasing ceria loading over the 10Ni/PO₄ + ZrO₂ catalyst. The peak intensity of about 250°C is increased whereas the peak intensity of about 630°C and 770°C are decreased upon increasing ceria loading (Figure 2A,B). It seems that increasing the amount of surface ceria species (upon ceria loading) may hinder the NiO species toward the reduction.²⁹ The reduction behaviour of 10Ni/PO₄ + ZrO₂ catalyst upon 4 wt.% ceria loading is unique. At 4 wt.% ceria loading, the peak intensity at the intermediate temperature region (630°C) and high-temperature region (770°C) is suppressed but at about 370°C, the reduction peak is amplified. It indicates the presence of the highest population of firmly reducible NiO-species over 10Ni4Ce/PO₄ + ZrO₂ catalyst. It should be also noted that before the POM reaction, the catalyst is reduced at 700°C, and active sites are generated up to 700°C. So, in the H₂-TPR profile, the reduction peak of NiO below 700°C is significant in the partial oxidation of methane. The total H₂ consumption in this significant range (up to 700°C) is maximum (45.5 cm³/g) over 10Ni4Ce/PO₄ + ZrO₂ catalyst. Upon loading 5 wt.% of ceria over 10Ni/PO₄ + ZrO₂, the total concentration of reduced species (like NiO and Ni₂P₂O₇) over the catalyst surface becomes minimum.

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It should be understood that the prime aim for reductive pretreatment (before the POM reaction) is to make catalytic active sites (Ni) ready for the POM reaction. The H₂-TPR profile gives the idea of the relative presence of active sites at different temperatures. So, the concentration of these active sites regulates the initial activity over the catalyst. As O₂ is used as an oxidant, it should be noted that the oxygen molecule has the potential to oxidize the metallic Ni into NiO and again the product H₂ has the ability to reduce the NiO.^26,27 During the DRM reaction on the catalyst surface, the distribution of active sites may change due to oxidation–reduction cycles by oxygen and hydrogen respectively. To study this, we have carried out reductive treatment (by H₂-TPR) followed by oxidative treatment (by O₂-TPO) and finally a reductive treatment again (by H₂-TPR) over 10Ni/PO₄ + ZrO₂ and 10Ni4Ce/PO₄ + ZrO₂ respectively (Figure 2C,D). After a sequential reduction–oxidation–reduction cycle, a single reduction peak from 500°C to 850°C is appeared. It is previously discussed that this temperature range is related to the consecutive reduction of Ni₂P₂O₇. It indicates that active sites “metallic Ni” is not oxidize under O₂ but nickel phosphide phases (Ni₁₂P₅ and Ni₂P) are exclusively oxidized into Ni₂P₂O₇. Further under H₂ (which is one of the products of POM), Ni₂P₂O₇ is reduced again into nickel phosphide. The reduction peak profile of ceria-promoted catalyst (10Ni4Ce/PO₄ + ZrO₂) is shifted towards a relatively lower temperature than the 10Ni/PO₄ + ZrO₂ catalyst (Figure 2E). This suggests that there is a growing trend towards a higher edge of reducibility with ceria-promoted catalysts. The CO₂-TPD profile of the catalysts is shown in Figure 2F. The CO₂-desorption profile of the current catalyst system can be characterized by the presence of a broad peak around peak maxima 270°C which indicates the presence of weak basicity borne by surface hydroxyl over the catalyst surface. After ceria incorporation, an additional peak either at 470°C or 700°C is observed. The CO₂-desorption peak at around 470°C is caused by moderate strength basic sites of surface oxide ions, while the peak at about 700°C indicates the presence of stable surface carbonates that are strong basic sites.^26,30 Upon loading of 1–2 wt.% ceria over 10Ni/PO₄ + ZrO₂ catalyst, the total basicity of catalyst is increased. 4 wt.% ceria incorporation over 10Ni/PO₄ + ZrO₂ catalyst shows an additional intense peak of about 470°C whereas rest catalysts have additional peak about 700°C. It is interesting to note that the concentration of surface oxide ion (moderate strength basic sites) and “surface interacted NiO” are grown immensely over 4 wt% Ce promoted 10Ni/PO₄ + ZrO₂ catalyst. It indicates that at 4 wt% Ce loading, there is optimum interaction between surface oxide ion (influenced by ceria) and Ni-species resulting in the formation of more “surface interacted NiO.” The interaction of finely dispersed nickel oxide species in contact with CeO₂ was also studied previously.³¹ Upon loading 5 wt.% of ceria over 10Ni/PO₄ + ZrO₂, the population to basic sites over the catalyst surface is turned to minimum.

The TEM image and particle size distribution of reduced-10Ni/PO₄ + ZrO₂ and reduced-10Ni4Ce/PO₄ + ZrO₂ catalysts are shown in Figure 3. The effect of cerium loading on particle size is evident clearly. The reduced-10Ni4Ce/PO₄ + ZrO₂ catalyst has a smaller particle size of 4.37 nm (than 6.24 nm in reduced-10Ni/PO₄ + ZrO₂ catalyst).

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Catalytic activity result and discussion

Phosphate-modified zirconia has a stable zirconia phase which can hold the NiO and Ni₂P₂O₇ (as active site precursors) as well as the CeO₂ promoter against high-temperature POM. During reductive pretreatment, Ni-related phases (like hexagonal Ni, tetragonal Ni₁₂P₅) are organized over the catalyst surface from NiO and Ni₂P₂O₇ over 10Ni/PO₄ + ZrO₂ catalyst. The reduced-10Ni/PO₄ + ZrO₂ catalyst has 34 m²/g surface area, 0.06 cm³/g pore volume, 7.3 nm pore diameter. Cyclic H₂TPR-O₂TPO-H₂TPR showed that during the POM reaction under exposure of oxidizing gas O₂ (one of the gas feed of POM) and reducing gas H₂ (one of the product of POM), metallic Ni remains stable but Ni₂P₂O₇ ↔ Ni₁₂P₅ ↔ Ni₂P cycle is continued. The 10Ni/PO₄ + ZrO₂ catalyst converts more or less 34% CH₄ during POM reaction up to 300 min time on stream. However, the H₂ yield over the unpromoted catalyst is just 13%–11% during 300 min time on stream. The CO₂-yield and H₂/CO ratio over 10Ni/PO₄ + ZrO₂ are also minimum (31% CO₂-yield and 2.42 H₂/CO ratio) than the rest catalysts.

The catalytic activity of 10Ni/PO₄ + ZrO₂ and 1–5 wt.% ceria promoted 10Ni/PO₄ + ZrO₂ catalysts are shown in Figure 4 and the speculative reaction pathways of POM is shown in Figure 5. POM reaction takes place ideally by dissociation of CH₄ and O₂ over Ni, followed by a combination of dissociated species into CO and H₂ (Figure 5A).³² This pathway is known as a direct pathway and the stoichiometry ratio of H₂/CO in direct pathways is equal to 2. However, over the current catalyst system, an H₂/CO ratio between 2.4 and 3.6 is observed. It indicates that other reaction pathways are also existing than the direct pathway. The complete oxidation of a hydrocarbon into CO₂ and H₂O is a common oxidation reaction in the presence of oxygen (Figure 5B; Step 1). Further, the catalytic reaction between CO₂ and CH₄ over a supported-Ni catalyst may lead to dry reforming of methane (DRM) having stoichiometric H₂/CO ratio 1 (Figure 5B; Step 2a). Again, the catalytic reaction between H₂O and CH₄ may lead to steam reforming of methane (SRM) having stoichiometric H₂/CO ratio 3 (Figure 5B; Step 2b). This two-step mechanism is termed an indirect pathway and it can access H₂/CO ratio >2. Upon ≥3 wt% ceria loading over 10Ni/PO₄ + ZrO₂ catalyst, the H₂/CO ratio remains >3 which indicates the prominent presence of an indirect reaction pathway.³³

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Upon promotional addition of 2 wt.% ceria promoter over 10Ni/PO₄ + ZrO₂, reducible surface ceria species increases, active sites are more dispersed over less crystalline support, surface area is raised more than three times and the total basicity of the catalyst is raised. The increase in catalyst basicity promotes greater CO₂ species involvement in the second step of indirect pathways for POM. In the same way, the reduction of surface ceria species results in the instant release of lattice oxygen. When either CO₂ species or mobile lattice oxygen comes into contact with CH₄ and turns CH₄ into syngas further. These surface modifications result in enhanced CH₄ conversion (38%), H₂ yield (21%), CO₂ yield (33%), and H₂/CO ratio (3.05) over 10Ni2Ce/PO₄ + ZrO₂ catalyst respectively. Upon increasing ceria loading up to 3 wt.%, the amount of reducible surface ceria species as well as catalytic activity are growing continuously. The CH₄ conversion, H₂-yield, CO₂ yield and H₂/CO ratio over 10Ni3Ce/PO₄ + ZrO₂ catalyst are found 41%, 26%, 36%, and 3.2% respectively.

The catalytic activity correlation of 4 wt.% ceria promoted 10Ni/PO₄ + ZrO₂ catalyst requires a deliberated discussion. The surface area and porosity profile of 10Ni4Ce//PO₄ + ZrO₂ catalyst is not much different than 10Ni2Ce//PO₄ + ZrO₂ catalyst. The reducibility and basicity profile of 10Ni/PO₄ + ZrO₂ catalyst are modified greatly upon promotional addition of 4 wt% Ce. The population of easily reducible “surface interacted NiO-species” has grown abruptly over 10Ni4Ce/PO₄ + ZrO₂. The current catalyst is reduced up to 700°C, up to this temperature, the largest number of active sites are organized over 10Ni4Ce/PO₄ + ZrO₂ than the rest catalysts. Cyclic H₂TPR-O₂TPO-H₂TPR results of 10Ni4Ce/PO₄ + ZrO₂ is very similar to unpromoted catalyst, but it has a higher edge of reducibility than unpromoted catalyst. The CO₂-TPD profile shows the basicity enrichment over 10Ni4Ce/PO₄ + ZrO₂ catalyst by surface oxide ion (moderate strength basic sites). The highest number of active sites induces CH₄ decomposition and O₂ dissociation in the direct pathway as well as, and it can promote DRM and SRM reaction (Figure 5, Steps 2a and 2b) through the indirect reaction pathway of POM. In the same way, additional basicity incorporated by surface oxide ions in 10Ni4Ce/PO₄ + ZrO₂ may induce DRM reaction (Figure 5, Step 2a) through the indirect reaction pathway of POM. Altogether, the 10Ni4Ce/PO₄ + ZrO₂ catalyst shows the highest activity (44% CH₄ conversion, 36% H₂ yield, 35% CO₂ yield, and 3.16 H₂/CO ratio) for the POM reaction. 10Ni5Ce/PO₄ + ZrO₂ catalyst are enriched with the surface area and pore volume among rest catalyst but it had least concentration of basic sites as well as reducible species (like NiO and Ni₂P₂O₇). Overall, the POM activity over 10Ni5Ce/PO₄ + ZrO₂ catalyst is decremental. It shows 41% CH₄ conversion, 35% H₂ yield, 35% CO₂ yield, and 3.09 H₂/CO ratio.

CONCLUSION

The high H₂/CO ratio (2.4–3.2) in POM reaction over the current catalyst indicates the presence of direct and indirect pathways of POM reaction. 10Ni/PO₄ + ZrO₂ catalyst is made up of a stabilized tetragonal zirconia phase which carries an adequate number of active sites derived from NiO and Ni₂P₂O₇. Toward the POM reaction, 10Ni/PO₄ + ZrO₂ catalyst attains a low H₂ yield (11%) but 2.42 H₂/CO ratio during 300 min time on stream. During the POM reaction, metallic Ni remains stable under oxidizing gas O₂ (which is one of the feed gas of POM) and reducing gas H₂ (which is formed during the POM reaction). But the Ni₂P₂O₇ ↔ Ni₁₂P₅ ↔ Ni₂P cycle continues. Upon 2–4 wt% Ce addition, the active sites are more dispersed over less crystalline support resulting high surface area than unpromoted catalyst. 10Ni4Ce/PO₄ + ZrO₂ catalyst retains the highest concentration of moderate strength basic sites (surface oxide ion) and total reducible species up to reduction temperature 700°C (including the highest concentration of “surface interacted NiO-species”). Reducibility increases the concentration of active sites whereas basicity engages a higher concentration of CO₂ over the catalyst surface in the second step of the indirect reaction pathway. 10Ni4Ce/PO₄ + ZrO₂ catalyst conveyed 44% CH₄ conversion, 36% H₂ yield, and 3.16 H₂/CO ratio towards POM. When ceria loading exceeds 4 wt.% on 10Ni/PO₄ + ZrO₂ catalyst, the total concentration of reducible species (like NiO and Ni₂P₂O₇) as well as basic sites is depleted which adversely affects the catalytic performance for POM.

AUTHOR CONTRIBUTIONS

Abdulaziz A.M. Abahussain: Conceptualization; methodology; writing—original draft. Ahmed S. Al-Fatesh: Project administration; methodology; formal analysis; data curation; editing. Jehad Abu-Dahrieh: Project administration; methodology; formal analysis; writing—review. Dharmesh M. Vadodariya: Conceptualization; formal analysis; data curation; writing—original draft.; Khaled M. Banabdwin: Formal analysis; data curation. Naif Alarifi: Resources; visualizaion; formal analysis. Ahmed A. Ibrahim: Methodology; data curation; reviewing. Anis H. Fakeeha: Conceptualization; investigation. Ahmed E. Abaseed: Supervision; validation; editing. Rawesh Kumar: Data curation; formal analysis; writing—original draft, writing—review & editing.

ACKNOWLEDGEMENTS

The authors would like to extend their sincere appreciation to the Researchers Supporting Project number (RSP2024R368), King Saud University, Riyadh, Saudi Arabia. Jehad Abu-Dahrieh acknowledges Queen's University Belfast for supporting research. Rawesh Kumar and Dharmesh M. Vadodariya acknowledge Indus University, Ahmedabad, for supporting research.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

No data was used for the research described in the article.

References

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