Catalytic combustion of methane over a highly active and stable NiO/CeO₂ catalyst

Abstract

In the last decades, many reports dealing with technology for the catalytic combustion of methane (CH₄) have been published. Recently, attention has increasingly focused on the synthesis and catalytic activity of nickel oxides. In this paper, a NiO/CeO₂ catalyst with high catalytic performance in methane combustion was synthesized via a facile impregnation method, and its catalytic activity, stability, and water-resistance during CH₄ combustion were investigated. X-ray diffraction, low-temperature N₂ adsorption, thermogravimetric analysis, Fourier transform infrared spectroscopy, hydrogen temperature programmed reduction, methane temperature programmed surface reaction, Raman spectroscopy, electron paramagnetic resonance, and transmission electron microscope characterization of the catalyst were conducted to determine the origin of its high catalytic activity and stability in detail. The incorporation of NiO was found to enhance the concentration of oxygen vacancies, as well as the activity and amount of surface oxygen. As a result, the mobility of bulk oxygen in CeO₂ was increased. The presence of CeO₂ prevented the aggregation of NiO, enhanced reduction by NiO, and provided more oxygen species for the combustion of CH₄. The results of a kinetics study indicated that the reaction order was about 1.07 for CH₄ and about 0.10 for O₂ over the NiO/CeO₂ catalyst.

Key words： methane combustion; NiO/CeO₂ catalyst; interaction; oxygen vacancy; kinetic study

Introduction

In recent decades, a great deal of research into technology for the catalytic combustion of methane (CH₄) has been conducted because of the high efficiency, safety, and low emissions (NO_x, CO, etc.) of this technology [¹‒³]. CH₄ is the most inert hydrocarbon, and the synthesis of highly active and stable catalysts for the catalytic combustion of CH₄ remains challenging. Noble metal-based catalysts, especially supported Pt and Pd catalysts, have been extensively investigated for CH₄ combustion [⁴‒⁶]. For instance, 100% conversion of CH₄ over Pd-NiCo₂O₄/SiO₂ was achieved at 378°C [⁷]. However, the use of Pt and Pd catalysts in industrial applications is limited by their high cost, low resistance against poisoning, and decreased catalytic activity at high temperature. Transition metal-based catalysts (hexaaluminates, perovskites, and transition metal composite oxides) have also been widely investigated owing to their fairly good stability and low cost [⁸‒¹⁰]. Hexaaluminate catalysts are very stable during the combustion of methane, but their catalytic activity requires improvement. Similarly, perovskites exhibit desirable catalytic performance, but their Brunner-Emmett-Teller (BET) surface area decreases greatly at high temperatures, which negatively affects their activity [¹¹]. Cobalt oxide (generally Co₃O₄) catalysts have shown high catalytic activity in CH₄ combustion as cobalt is the one of the most active transition metal elements [¹²‒¹⁴]. High conversion (90%) [¹²] can be achieved at a low temperature of 345°C; this performance is similar to that of noble metal catalysts. Unfortunately, cobalt oxide catalysts are sensitive to moisture and easily deactivated. Compared to cobalt oxides [¹⁵,¹⁶], CuO-based catalysts exhibit higher stability and better water resistance in CH₄ combustion, but their catalytic performance is much lower. Therefore, it is necessary to develop a new, thermally stable low-cost catalyst for the combustion of methane.

Recently, attention has increasingly focused on the synthesis and catalytic activity of nickel oxides [¹⁷‒¹⁹]. For example, H₂ has been produced by ethanol steam reforming over NiO/CeO₂ with a selectivity value of 61.5 mol-% and an ethanol conversion of 95.0 mol-% [¹⁸]. However, NiO-based catalysts show low catalytic activity in the reforming of methane [²⁰‒²²]. To enhance its catalytic activity in CH₄ combustion, NiO must be loaded onto an appropriate support. CeO₂ exhibits unique redox properties and a high oxygen storage capacity (OSC) [²³,²⁴]. Thus, CeO₂-supported NiO catalysts have been considered for CH₄ combustion [²⁵‒²⁷]. The addition of CeO₂ to NiO has been observed to notably enhance its N₂O decomposition activity [²⁶]. However, to the best of our knowledge, almost no research into the thermal stability and water resistance of NiO-based catalysts has been reported. The mechanism for the improvement performance of NiO-based catalysts was also unclear. In this study, an effective NiO/CeO₂ catalyst was prepared using a facile impregnation method, and the catalytic activity, stability, and water resistance of the catalyst during CH₄ combustion were investigated. X-ray diffraction (XRD), low-temperature N₂ adsorption, thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FT-IR), hydrogen temperature programmed reduction (H₂-TPR), methane temperature programmed surface reaction (CH₄-TPSR), Raman spectroscopy, electron paramagnetic resonance (EPR), and transmission electron microscopy (TEM) characterization were carried out in order to explain the increased catalytic activity and stability in detail.

Experimental

Catalyst preparation

Typically, the CeO₂ support was obtained by the thermal decomposition method using Ce(NO₃)₃·6H₂O (Sinopharm Chemical Reagent Co., Ltd. (SCRC), 99.0%) as a precursor at 450°C for 4 h in an air atmosphere [²⁸]. The obtained CeO₂ support was then impregnated with Ni(NO₃)₂ (SCRC, 99.0%) solutions of various concentrations to achieve NiO wt-% loadings from 1 to 20 wt-%. The Ni/CeO₂ products were calcined at 450°C for an additional 4 h in an air atmosphere to fabricate NiO/CeO₂ catalysts with 1, 3, 5, 7, 10, 15 and 20 wt-% NiO.

NiO/Al₂O₃ (10 wt-%) was synthesized using the same method as the 10 wt-% NiO/CeO₂ catalyst (Al₂O₃ was purchased from BASF Co., Ltd.). The preparation methods for the SiO₂ and KIT-6 supports were described in previous reports [²⁹,³⁰]. 10 wt-% NiO/SiO₂ and 10 wt-% NiO/KIT-6 were obtained using the same impregnation method as for the NiO/CeO₂ catalysts.

Catalyst characterization

XRD patterns of all samples were recorded using a Bruker D8 Focus diffractometer with CuKα radiation (l = 1.541 Å, operated at 40 kV and 40 mA). The N₂ adsorption/desorption isotherms were measured at ‒196°C using a NOVA 4200e Surface Area and Pore Size Analyzer. Before the measurements, the samples were outgassed at 180°C under a vacuum for 4 h. The BET surface areas of the samples were calculated using the BET method, and the pore size distributions were calculated using the Barrett-Joyner-Halenda method from the adsorption branches of the isotherms. The total pore volumes (V_p) were estimated at a relative pressure of 0.985 (P/P₀). The combined thermogravimetric analysis and differential thermal analysis (TG-DTA) runs were performed using a PerkinElmer Pyris Diamond with a WCT-2 thermal analyzer at a heating rate of 10°C·min^‒1 from room temperature to 800°C. The Raman spectra were obtained using a Renishaw in Viat+ Reflex spectrometer equipped with a Charge Coupled Device detector at ambient temperature under moisture-free conditions. The emission line at 514.5 nm was produced by an Ar⁺ ion laser (Spectra Physics) and was focused on an analysis spot of about 1 mm on the sample under the microscope. FT-IR absorption spectra were recorded using a Nicolet NEXUS 670 FT-IR spectrometer, with 32 scans at an effective resolution of 4 cm^‒1. The sample to be measured was ground with KBr and pressed into a thin wafer for analysis. H₂-TPR was conducted using a thermal conductivity detector (TCD). The sample (50 mg) was loaded into a quartz tube reactor. A 5 vol-% H₂/N₂ gas mixture was used with a flow rate of 40 mL·min^‒1, and the reactor was heated from room temperature to 850°C at a heating rate of 10°C·min^‒1. CH₄-TPSR of the samples was performed in a quartz micro-reactor. The catalyst (200 mg) was pretreated in the reactor at 500°C for 30 min under a 50 mL·min^-1 flow of a 20 vol-% O₂/He gas mixture. After the sample was cooled, it was then heated to 800°C in 50 mL·min^‒1 of 1 vol-% CH₄/He at a heating rate of 10°C·min^-1. The outlet gas was analyzed using a quadrupole mass spectrometer (INFICON Transpecter 2). The signals of CH₄, H₂O, CO, O₂, CO₂ were recorded at mass-to-charge ratio (m/z) = 15, 18, 28, 32, and 44, respectively.

Catalytic activity tests and kinetic studies

The catalytic performance of the NiO/CeO₂ catalysts for CH₄ combustion was tested in a quartz tube reactor (ϕ 6 mm) at atmospheric pressure. The reactant gas mixture, which consisted of 1 vol-% CH₄ + 4 vol-% O₂ in Ar at a total flow rate of 50 mL·min^‒1, was passed through the catalyst (200 mg) bed. The reactants and products were analyzed online using a gas chromatograph equipped with TCD. The activities of the catalysts were measured at programmed temperatures from 200°C to 700°C at a heating rate of 5°C·min^‒1. The catalyst activity was characterized using the T₁₀, T₅₀, and T₉₀ values, which represent the temperatures at which CH₄ conversions of 10%, 50% and 90% were obtained, respectively.

In the measurement of kinetic parameters, the flow rate of the reactant gas was over 50 mL·min^‒1, eliminating external diffusion. Internal diffusion was eliminated by pelleting the catalyst to 20‒40 mesh. During the measurement of the reaction orders, the reaction temperature was 320°C and the reactant gas consisted of 0.5‒4 vol-% CH₄ and 2‒10 vol-% O_2, balanced with Ar. During the measurement of the activation energy, the reactant gas consisted of 1 vol-% CH₄ and 4 vol-% O₂, balanced with Ar, and the weight hourly space velocity (WHSV) ranged from 15000 to 60000 mL·g^‒1·h^‒1. The conversion of CH₄ was adjusted to below 15%. The reaction reached a steady state after 30 min, and the reaction rates were obtained.

Results and discussion

Effect of the support

In order to investigate the influence of the catalytic support on the activity of the NiO catalyst in CH₄ combustion, SiO₂, Al₂O₃ and KIT-6 supports with BET surface areas of 387, 208 and 708 m²·g^‒1, respectively, were compared with the CeO₂ support. Figure 1 shows the catalytic activities of the four catalysts, NiO/CeO₂, NiO/SiO₂, NiO/Al₂O₃ and NiO/KIT-6. All the catalysts contained 10 wt-% NiO. Although SiO₂, Al₂O₃, and KIT-6 had much higher BET surface areas than CeO₂, they exhibited lower catalytic performance. The T₁₀ values of 10 wt-% NiO/SiO₂, 10 wt-% NiO/Al₂O₃, and 10 wt-% NiO/KIT-6 were 407°C, 439°C and 389°C, respectively. That is, they were 74°C, 106°C and 56°C higher than that of the 10 wt-% NiO/CeO₂ catalyst.

Fig.1 Effect of the supports on the activity of the supported NiO catalysts (gas space velocity= 60000 mL·g^‒1·h^‒1).

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The XRD patterns of the four samples are displayed in Fig. 2(a). Diffraction peaks corresponding to cubic phase NiO were observed for all samples except the NiO/Al₂O₃ catalyst. Due to the high surface area of the supports, the NiO nanoparticles were better dispersed on the SiO₂ and KIT-6 supports than on the CeO₂ support. Only diffraction peaks corresponding to Al₂O₃ were observed on the NiO/Al₂O₃ catalyst due to the strong interaction between NiO and Al₂O₃. The reduction behaviors of the 10 wt-% NiO/CeO₂, 10 wt-% NiO/SiO₂, 10 wt-% NiO/Al₂O₃, and 10 wt-% NiO/KIT-6 samples are presented in Fig. 2(b). For the NiO/CeO₂ sample, the peak centered at ~330°C was assigned to the reduction of NiO. The intensity of the reduction peak at 330°C was very low for the NiO/Al₂O₃ catalyst, and an additional strong reduction peak at 545°C was observed. The higher temperature reduction peak was assigned to the reduction of NiO due to its strong interaction with Al₂O₃ [³¹,³²]. The reduction temperature of NiO in NiO/SiO₂ was ~350°C. Two reduction peaks at 355°C and 500°C were observed on NiO/KIT-6, which corresponded to the reduction of NiO via weak and strong interactions with KIT-6, respectively. These results demonstrated that the intensity of the interaction between NiO and the support decreased in the order Al₂O₃>CeO₂>KIT-6 ≈ SiO₂. The different reduction properties of NiO depended on the intensity of the interaction between NiO and the support rather than the BET surface area or the NiO dispersion. The high activity of the NiO/CeO₂ catalyst in CH₄ combustion resulted from the appropriate intensity of the interaction between NiO and CeO₂, which allowed for higher oxygen mobility and OSC than SiO₂, Al₂O₃, or KIT-6 [³³].

Fig.2 (a) XRD patterns and (b) H₂-TPR profiles of the different NiO catalysts.

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Effect of the NiO loading

The light-off curves of CH₄ conversion over the CeO₂-supported NiO samples with different NiO loadings are shown in Fig. 3.

Fig.3 Light-off curves of the catalytic combustion of methane over the NiO/CeO₂ catalysts (gas space velocity= 60000 mL·g^‒1·h^‒1).

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During the CH₄ combustion reaction, CH₄ was completely oxidized to CO₂ and H₂O (as determined by analysis using the INFICON Transpecter 2 quadruple mass spectrometer). Pure CeO₂ had a low catalytic activity with high T₁₀, T₅₀, and T₉₀ values of 456°C, 540°C and 615°C, respectively. However, the catalytic activity of NiO/CeO₂ improved with increasing NiO content. The 10 wt-% NiO/CeO₂ showed the highest catalytic activity, with low T₁₀, T₅₀, and T₉₀ values of 333°C, 415°C, and 467°C, respectively. No improvement in the catalytic activity was observed when the NiO loading was further increased to 15% and 20%, as the NiO nanoparticles became aggregated and the number of reactive sites did not increase. As shown in Table 1, the increase in the reaction rate of CH₄ with increasing NiO loading (from 1‒10 wt-%) was due to the increase in the number of NiO reactive sites. The CH₄ reaction rate did not increase further for NiO loadings of over 10%, which was in agreement with the results of the light-off curves.

Tab.1 CH₄ reaction rate over NiO/CeO₂ catalysts at 360°C

Samples	Cat./mg	Flow rate/(mL·min‒1)	Concentration of CH4 /vol-%	Conversion of CH4 /%	CH4 reaction rate /(×10‒4 mmol·g‒1·s‒1)
1 wt-% NiO/CeO2	200	50	1	6.4	1.09
3 wt-% NiO/CeO2	200	50	1	10.4	1.77
5 wt-% NiO/CeO2	200	50	1	14.6	2.49
7 wt-% NiO/CeO2	200	100	1	9.0	3.07
10 wt-% NiO/CeO2	200	100	1	10.7	3.63
15 wt-% NiO/CeO2	200	100	1	10.3	3.51
20 wt-% NiO/CeO2	200	100	1	10.9	3.70

Effect of water vapor in the feed gas

In order to investigate the effect of water vapor on the catalytic performance of the synthesized NiO/CeO₂ during CH₄ combustion, 3.1% water vapor (25°C) was added to the feed gas; the catalytic performance of 10 wt-% NiO/CeO₂ in the absence and presence of the water vapor is shown in Fig. 4. When 3.1% water vapor was added to the reaction stream, the catalytic performance of the 10 wt-% NiO/CeO₂ catalyst declined slightly; T₁₀, T₅₀, and T₉₀ increased to 355°C, 425°C and 480°C, respectively. It was concluded that the addition of water resulted in competition between water and CH₄ molecules for adsorption on the active sites of the NiO/CeO₂ catalyst, resulting in some of the active oxygen sites on the catalyst being blocked by water. The result also showed that CH₄ was still totally oxidized to CO₂ and H₂O over the NiO/CeO₂ catalyst without steam reforming reactions or partial oxidation to generate CO and H₂, even in the presence of 3.1% water vapor. This finding was not in agreement with the previous work [³⁴]. Therefore, the 10 wt-% NiO/CeO₂ catalyst exhibited a degree of water resistance during CH₄ combustion.

Fig.4 Effect of the addition of water vapor in CH₄ combustion over 10 wt-% NiO/CeO₂: (a) without H₂O and (b) with 3.1% water vapor (25°C; gas space velocity= 60000 mL·g^‒1·h^‒1).

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Determination of the kinetic parameters of CH₄ oxidation

The empirical kinetic rate equation of CH₄ combustion was as follows:

(1)r=k{P}_{{\mathrm{C}\mathrm{H}}_4}^{\alpha }{P}_{{\mathrm{O}}_2}^{\beta }{P}_{{\mathrm{C}\mathrm{O}}_2}^{\gamma }{P}_{{\mathrm{H}}_2\mathrm{O}}^{\delta }\mathrm{.}

When the conversion of CH₄ at 320°C was less than 10%,P_{{\mathrm{CO}}_{\mathrm{2}}}^{\gamma }andP_{{\mathrm{H}}_2\mathrm{O}}^{\delta }were assumed to be constant, allowing Eq. (1) to be simplified as follows:

(2)r=k{P}_{{\mathrm{C}\mathrm{H}}_4}^{\alpha }{P}_{{\mathrm{O}}_2}^{\beta }\mathrm{.}

By taking the logarithm of Eq. (2), Eq. (3) can be obtained:

(3)\ln r=\alpha \ln P_{C{H}_{\mathrm{4}}}+\beta \ln P_{O_{\mathrm{2}}}+\ln k.

Under the conditions of 0.5‒4 vol-% CH₄ and 4% O₂ at a WHSV of 30000 mL·g^‒1·h^‒1 and 320°C, Eq. (3) was simplified to\ln r=\alpha \ln P_{{\mathrm{CH}}_{\mathrm{4}}}+C. A plot of \ln r against\ln P_{{\mathrm{CH}}_{\mathrm{4}}}for the 10 wt-% NiO/CeO₂ catalyst at 320°C is shown in Fig. 5(a). The reaction order of CH₄ (a) was equal to the slope of the resulting straight line, which was about 1.07. The reaction order of O₂ (b) was determined to be about 0.10 by calculation. Thus, the reaction rates for CH₄ and O₂ were nearly first order and zero order, respectively (Figs. 5(a,b)). These findings were in agreement with those reported for a NiO/Ce_0.75Zr_0.25O₂ catalyst and Pd-based catalyst [²⁵,³⁵].

Fig.5 Dependence of the reaction rates on the CH₄ and O₂ concentration over the 10 wt-% NiO/CeO₂ catalyst at 320°C: (a) 0.5‒4 vol-% CH₄, 4 vol-% O₂, in Ar; (b) 2‒10 vol-% O₂, 1 vol-% CH₄, in Ar. (P_{{\mathrm{CH}}_{\mathrm{4}}}and P_{{\mathrm{O}}_{\mathrm{2}}}, partial pressures; gas space velocity= 30000 mL·g^‒1·h^‒1).

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The Arrhenius equation is given below as Eq. (4):

(4)k=A\cdot \exp (-{\displaystyle \frac{E_a}{\mathrm{R}T}}).

Taking the logarithm of Eq. (4), Eq. (5) can be obtained:

(5)\ln r-\ln P_{{\mathrm{C}\mathrm{H}}_4}^{\alpha }-\ln P_{{\mathrm{O}}_2}^{\beta }=\ln A-\frac{Ea}{\mathrm{R}T}\mathrm{.}

The components of the reactant feed gas did not change during the kinetic tests, and the conversion of CH₄ at 320°C was<15%. Thus, \ln P_{{\mathrm{CH}}_{\mathrm{4}}}^{\alpha }and \ln P_{{\mathrm{O}}_2}^{\beta } were assumed to be approximately constant, and Eq. (5) was simplified to \ln \mathrm{\hspace{0.17em}}r=-E\mathrm{a}/(\mathrm{R}T)+C. The plots of \ln \mathrm{\hspace{0.17em}}r against \ln P_{{\mathrm{CH}}_{\mathrm{4}}}over the 10 wt-% NiO/CeO₂ and CeO₂ catalysts are shown in Fig. 6. The activation energy for 10 wt-% NiO/CeO₂ was determined to be 61.7 kJ∙mol^‒1, which was much lower than that of CeO₂ (114.6 kJ∙mol^‒1). Additionally, the activation energy for 10 wt-% NiO/CeO₂ fell within the range of values reported for other transition oxides [³⁶,³⁷].

Fig.6 Arrhenius plots of the reaction rate of CH₄ combustion (activation energy, E_a) over (a) pure CeO₂ and 10 wt-% (b) NiO/CeO₂ (gas space velocity= 30000 mL·g^‒1·h^‒1).

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Catalyst characterization

XRD

The XRD patterns of the CeO₂-supported NiO catalysts are shown in Fig. 7. The CeO₂ fluorite structure (JCPDS file No. 34-0394) was observed in all the samples. Weak diffraction peaks of cubic phase NiO (JCPDS file No. 65-2901) were observed for the NiO/CeO₂ catalysts with loadings of 7 wt-% or greater. The NiO particles were highly dispersed on the support surface, and thus NiO peaks could not be observed at the lower loading amounts (NiO<7 wt-%). When the concentration of NiO was greater than 7 wt-%, the NiO nanoparticles tended to aggregate. However, the diffraction peaks of the NiO crystal phase were still weak even in the XRD pattern of the 20 wt-% NiO/CeO₂ catalyst, which indicated that CeO₂ improved the dispersion of NiO. For all the NiO/CeO₂ samples, there was no evident shift in diffraction peaks with CeO₂, which revealed that most of the Ni²⁺ ions formed NiO nanoparticles dispersed on the CeO₂ surface. Only a small amount of Ni²⁺ ions was incorporated into the CeO₂ lattice by the impregnation method.

Fig.7 XRD patterns of the NiO/CeO₂ samples with various NiO loadings.

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N₂ adsorption/desorption

The textural properties and catalytic activity of all the samples are shown in Table 2. No obvious changes in the crystallite size or pore volume of NiO/CeO₂ were observed with different NiO loadings. CeO₂ had a BET surface area of 76.4 m²·g^‒1, and the BET surface area decreased with increasing NiO wt-%. The BET surface area of the 10 wt-% NiO/CeO₂ catalyst was 58.1 m²·g^‒1, which was lower than that of CeO₂. Unexpectedly, the catalytic performance of NiO/CeO₂ was much higher than that of CeO₂. This suggested that the BET surface area was not the key factor in the catalytic performance in CH₄ combustion.

Tab.2 Textural properties and catalytic activities of CeO₂ and NiO/CeO₂ samples for CH₄ combustion

Samples	Crystallite size /(na·ma)	Surface area /(m2·g‒1)	Pore volume /(cm3·g‒1)	Catalytic activity /°C
				T10	T50	T90
CeO2	8	76.4	0.206	456	540	615
1 wt-% NiO/CeO2	8.1	70.4	0.197	381	471	575
3 wt-% NiO/CeO2	8.3	68.8	0.195	362	442	516
5 wt-% NiO/CeO2	8.5	65.3	0.185	352	430	493
7 wt-% NiO/CeO2	8.8	61.1	0.183	349	424	487
10 wt-% NiO/CeO2	8.9	58.1	0.173	333	415	467
15 wt-% NiO/CeO2	8.9	57.2	0.171	350	416	472
20 wt-% NiO/CeO2	9	55.5	0.169	355	410	467
10 wt-% NiO/CeO2 (used)b	10.6	45.5	0.154	–	–	–

a) Crystallite sizes were calculated from the line broadenings of the (111) plane of CeO2 using the Scherrer equation; b) 10% NiO/CeO2 catalyst collected after reaction at 600°C for 150 h.

H₂-TPR

Figure 8 shows the H₂-TPR profiles of CeO₂, NiO, and all the NiO/CeO₂ catalysts. A weak reduction peak was observed at ~780°C for CeO₂ and all the NiO/CeO₂ samples, which was attributed to the reduction of bulk oxygen. Three hydrogen consumption peaks, a₁, a₂, and b, were observed for the NiO/CeO₂ catalysts. When Ni²⁺ is incorporated into the lattice of CeO₂ to replace some Ce⁴⁺ cations, charge unbalance and lattice distortion occur within the structure of CeO₂ due to the smaller radius of Ni²⁺. Oxygen vacancies were then generated, which could easily adsorb oxygen to form oxygen species with high activity and reducibility [²¹]. Thus, the a₁ and a₂ peaks were ascribed to the reduction of surface-adsorbed oxygen [¹⁷]. As the NiO loading was increased from 3 wt-% to 20 wt-%, the areas of the a₁ and a₂ peaks did not change remarkably, but the reduction temperatures of a₁ and a₂ shifted to higher temperature.

Fig.8 H₂-TPR profiles of the CeO₂, NiO, and NiO/CeO₂ samples.

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The reduction peak centered at ~330°C (b peak) was assigned to the reduction of NiO. The temperature for the reduction of NiO in NiO/CeO₂ was much lower than that of pure NiO (~416°C) due to the interaction between CeO₂ and NiO, which facilitated the reduction of Ni²⁺. For the 1 wt-% NiO/CeO₂ sample, only one main reduction peak at 248°C was observed; the reduction peak at ~330°C was absent. NiO was easily reduced in this sample because it was highly dispersed on the surface of CeO₂.

Raman spectroscopy

The Raman spectra of CeO₂ and the CeO₂-supported NiO samples are shown in Fig. 9. The spectra were obtained using a 514.5 nm laser as the excitation source. As shown in Fig. 9, a strong band was observed at ~462 cm^‒1 in the CeO₂ spectrum; this band was assigned to the F_2g Raman active mode of the fluorite structure of CeO₂ [³⁸]. No evident shifts of the band at 462 cm^‒1 were observed for any of the NiO/CeO₂ samples, which indicated that the NiO/CeO₂ samples did not experience any evident lattice expansion or contraction in comparison with CeO₂. All samples also exhibited another broad weak band at ~575 cm^‒1 that was attributed to the oxygen vacancy [³⁹,⁴⁰] mentioned in the discussion of the H₂-TPR results. The bands at ~620 cm^-1 in the NiO/CeO₂ samples were ascribed to NiO species [⁴¹], which suggested that highly dispersed NiO was present on the surface of the samples. The areas of the bands at 462, 575, and 620 cm^‒1 were denoted as A₄₆₂, A₅₇₅, and A₆₂₀, respectively. The A₅₇₅/A₄₆₂ ratio in Raman spectra of CeO₂-based catalysts is typically used to determine the concentration of oxygen vacancies, while A₆₂₀/A₄₆₂ represents the concentration of highly dispersed NiO [⁴²]. Increasing the oxygen mobility of CeO₂-based catalysts by introducing defective sites is known to be effective in promoting hydrocarbon oxidation reactions [⁴³]. Moreover, highly dispersed NiO exhibits high catalytic activity in CH₄ combustion due to its good redox properties [¹⁷]. To investigate the influence of the concentrations of oxygen vacancies and highly dispersed NiO on the catalytic activity in greater detail, the correlations between the special activity (CH₄ conversion per unit surface area) at 460°C and the (A₅₇₅+A₆₂₀)/A₄₆₂ ratio are shown in Fig. 10. With increasing (A₅₇₅+A₆₂₀)/A₄₆₂ ratio, the concentration of oxygen vacancies and highly dispersed NiO increased, and the special activity of the catalyst also increased.

Fig.9 Raman spectra of NiO/CeO₂ with NiO loadings of (a) 0, (b) 1 wt-%, (c) 3 wt-%, (d) 5 wt-%, (e) 7 wt-%, (f) 10 wt-%, (g) 15 wt-%, and (h) 20 wt-%. Inset: Raman spectrum of 10 wt-% NiO/CeO₂.

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Fig.10 Special conversion of CH₄ per unit surface area at 460°C vs. (A₅₇₅+A₆₂₀)/A₄₆₂ in the Raman spectra of CeO₂-supported NiO with NiO loadings of 0, 1, 3, 5, 7, and 10 wt-% (gas space velocity= 60000 mL·g^‒1·h^‒1).

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CH₄-TPSR

To determine the origin of the high performance of the NiO-based catalysts, CH₄-TPSR was also performed, and the results are shown in Fig. 11 and Fig. 12. Two CO₂ generation peaks were observed in the CH₄-TPSR profile of pure CeO₂ (Fig. 11), which corresponded to the oxidation of methane by surface oxygen and bulk oxygen [⁴⁴]. The CH₄-TPSR profiles of the NiO/CeO₂ samples were similar to that of CeO₂; CO₂ desorption was observed as a weak broad plateau at 320°C‒520°C in the former and as a sharp peak at 580°C‒680°C in the latter. As the NiO loading was increased from 1 wt-% to 10 wt-%, the intensity of the latter peak increased. This indicated that the activity of bulk oxygen increased and the amount of bulk oxygen was enhanced, and thus, that the incorporation of NiO improved the bulk oxygen mobility of CeO₂ [⁴⁵]. Generally, oxygen transport is a key step in CH₄ combustion over NiO-based catalysts [⁴⁶]. Therefore, the incorporation of NiO was an effective way to improve the catalytic performance in CH₄ combustion.

Fig.11 MS signals of CO₂ (m/z = 44) from CH₄-TPSR on NiO/CeO₂ with various NiO loadings.

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Fig.12 MS signals from CH₄-TPSR on CeO₂ and 10 wt-% NiO/CeO₂ (H₂ (m/z = 2), CH₄ (m/z = 16), CO (m/z = 28), and CO₂ (m/z = 44)).

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Figure 12 shows the details of the reaction during the CH₄-TPSR of the CeO₂ and NiO/CeO₂ samples. CH₄ was completely oxidized to CO₂ and H₂O over pure CeO₂ when the temperature reached 410°C, which was very similar to the T₁₀ of CeO₂. However, H₂ was generated when the temperature was above 496°C, revealing that partial oxidation of CH₄ occurred because the amount of surface oxygen in CeO₂ was insufficient to achieve the complete oxidization of CH₄. When NiO was incorporated onto CeO₂, both the activity and the amount of surface oxygen in CeO₂ were improved. As a result, CH₄ was completely oxidized to CO₂ and H₂O by the surface oxygen at 320°C‒520°C.

Electron paramagnetic resonance (EPR)

EPR is a sensitive and efficient method for the direct investigation and detection of paramagnetic materials, but it can only characterize magnetically active materials or substances with a single electron. According to the principles of EPR detection, the unpaired 4f electrons of Ce³⁺ should produce a paramagnetic signal under the external magnetic field. However, as shown in Fig. 13, the paramagnetic signal of pure CeO₂ was weak and it was difficult to distinguish the signals of the unpaired electrons. This occurred because the Ce³⁺ signal is broadened by rapid spin-lattice relaxation at room temperature, resulting in undetectable signals [⁴⁶]. It is believed that the paramagnetic signal of Ce³⁺ can only be detected at low temperature (close to or even below the liquid helium temperature), as the rapid spin-lattice relaxation effect is avoided at such low temperatures. After the CeO₂ was loaded with NiO, a weak paramagnetic signal appeared at g = 2.009, corresponding to the oxygen radical (O^‒); the paramagnetic signal appearing near g = 2.029 corresponded to the Ni²⁺ ion or to oxygen ions (O₂^‒) absorbed on the surface of CeO₂. It was difficult to observe the paramagnetic signal of Ce³⁺ on pure CeO₂. The signal of Ce³⁺ was still difficult to observe after NiO loading. The 10 wt-% NiO/CeO₂ sample exhibited broad paramagnetic signals at g = 2.384 and g = 2.474, which were signals of the unpaired electrons in the Ni⁺ ion, indicating that Ce promoted the reduction of Ni.

Fig.13 EPR signals of (a) CeO₂ and (b) 10 wt-% NiO/CeO₂ (inset: magnification of the EPR spectrum).

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TEM characterization

Figure 14 shows the change in the particle size before and after loading NiO on the surface of CeO₂. Since the combustion of the precursor in the direct thermal decomposition process was intense, and no agents such as surfactants were added to control the morphology, the prepared CeO₂ (D-CeO₂) had no regular morphology, and its particle size was not uniform. After loading NiO, namely, in the 10 wt-% NiO/CeO₂ sample, most of the NiO was well-dispersed, but a few large NiO flakes were present. These results indicated that some of the NiO aggregated, which was consistent with the XRD results.

Fig.14 TEM images of (a) CeO₂ and (b) 10 wt-% NiO/C.

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Long term stability of the NiO/CeO₂ catalyst

The long-term stability of the NiO/CeO₂ catalyst was also investigated. The stability test of the NiO/CeO₂ catalyst was conducted at 600°C for 150 h with a WHSV of 15000 mL·g^‒1·h^‒1. As shown in Fig. 15, the CH₄ conversion of the 10 wt-% NiO/CeO₂ catalyst remained stable at 100% without any decrease, which suggested that the synthesized NiO/CeO₂ had good stability in CH₄ combustion.

Fig.15 Methane conversion over 10 wt-% NiO/CeO₂ vs. reaction time at 600°C (gas space velocity= 15000 mL·g^‒1·h^‒1).

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The XRD patterns of fresh and used 10 wt-% NiO/CeO₂ catalysts are presented in Fig. 16(a). For the fresh sample, the diffraction peaks of the cubic CeO₂ fluorite structure and a weak diffraction peak of the cubic NiO phase were observed. After 150 h of reaction, no distinct changes to the diffraction peaks of CeO₂ and NiO were observed. This indicated that the NiO nanoparticles were still highly dispersed on the CeO₂. Generally speaking, pure CeO₂ and NiO can be sintered easily at high temperature [²⁵], which would result in deactivation during CH₄ combustion. The sintering between CeO₂ and NiO at high temperature was partly prevented in the 10 wt-% NiO/CeO₂ catalyst by the appropriate interactions between NiO and CeO₂. Thus, the presence of NiO enhanced the stability of CeO₂ in CH₄ combustion. H₂-TPR profiles of fresh and used samples are shown in Fig. 16(b). After 150 h of reaction, the reduction temperature and the intensity of the b peak (corresponding to the reduction of NiO) of the used catalyst were similar to those of the fresh sample, which indicated that NiO was very stable when supported on CeO₂. Evident changes in the a₁ and a₂ peaks (corresponding to reduction of adsorbed oxygen) in the used catalyst were compared to those of the fresh sample, revealing that the amount of adsorbed oxygen decreased due to the consumption of oxygen during long-term reaction. In addition, the decrease in the amount of adsorbed oxygen was also caused by the decrease in the BET surface area (shown in Table 2) of NiO/CeO₂ after reaction at 600°C for 150 h.

Fig.16 (a) XRD patterns of CeO₂, fresh 10 wt-% NiO/CeO₂, and used 10 wt-% NiO/CeO₂; (b) H₂-TPR profiles of fresh and used 10 wt-% NiO/CeO₂ catalyst.

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As shown in Fig. 17(a), a weak adsorption band was observed in the FT-IR spectra of fresh and used 10 wt-% NiO/CeO₂ at ~1640 cm^‒1, which corresponded to the O–H vibration. Adsorption bands at 1065, 1350, and 1510 cm^‒1 were also observed in both the fresh and used samples, and were ascribed to unidentate carbonates [⁴⁷]. Due to surface reconstruction or the basicity of the surface, the formation of carbonates on the surface of solids containing CeO₂ is favorable, and can lead to a decline in catalytic activity [⁴⁸]. The intensity of these peaks did not change remarkably after the reaction, which indicated that new carbonates were not formed. The TG-DTA curves of fresh and used 10 wt-% NiO/CeO₂ catalyst are presented in Fig. 17(b). No evident weight loss was observed above 200°C (<2%). This indicated that no coke was deposited on the surface of the used catalyst.

Fig.17 (a) FT-IR spectra of fresh and used 10 wt-% NiO/CeO₂; (b) TG-DTA curves of used NiO/CeO₂ catalyst.

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Conclusions

In summary, a series of NiO/CeO₂ catalysts were prepared via a facile impregnation method. When the NiO loading amount was 10 wt-%, the NiO/CeO₂ exhibited high catalytic performance in CH₄ combustion, with low T₁₀, T₅₀, and T₉₀ values of 333°C, 415°C and 467°C, respectively. Moreover, the 10 wt-% NiO/CeO₂ catalyst also demonstrated high stability and good water resistance during CH₄ combustion, as evidenced by the fact that it maintained a CH₄ conversion of 100% after 150 h of reaction with a 600°C stream.

The interaction between CeO₂ and NiO was also investigated. The oxygen vacancy concentration, activity, and amount of surface oxygen were found to be enhanced in the NiO/CeO₂ catalyst due to the incorporation of Ni²⁺ into the CeO₂ lattice. As a result, the mobility of bulk oxygen in CeO₂ was increased, and the activation energy of CH₄ combustion decreased. NiO was well-dispersed on the surface of CeO₂, which prevented the aggregation of NiO. In this way, CeO₂ improved the reduction properties of NiO and provided a much greater amount of oxygen species for CH₄ combustion. Therefore, the synthesized 10 wt-% NiO/CeO₂ catalyst is an excellent catalyst for CH₄ combustion.

Further work will be focused on the catalytic mechanism of CH₄ combustion over NiO/CeO₂. Additionally, the development of a more active NiO/CeO₂ catalyst should be feasible through optimization of the catalyst parameters.

Acknowledgements

This work was supported financially by Shanghai Sailing Program (17YF1413100) and Shanghai Rising-Star Program (19QB1401700).

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