Heterosturcture NiO/Ce_1−xNi_xO₂: synthesis and synergistic effect of simultaneous surface modification and internal doping for superior catalytic performance

Abstract

In this work, a two-step method that involves a hydrothermal reaction and a subsequent calcination was initiated to prepare a novel heterostructure (1 − x)CeO₂·xNiO. All samples were systematically characterized by X-ray diffraction, Raman spectra, Transmission electron microscopy, In situ DRIFT spectra, Brunauer–Emmett–Teller technique, H₂-temperature-programmed-reduction, and O₂-temperature-programmed-desorption. Distinct from the conventional sol–gel or co-precipitation methods, the present methodology allows Ni²⁺ to be simultaneously doped in the CeO₂ lattice and dispersed onto the surfaces of CeO₂ nanoparticles by simply varying the calcination temperature from 400 to 700 °C. When calcined at an optimized temperature of 500 °C, the heterostructure at x = 0.1 showed a mean grain size of 18 nm, specific surface area of 69.3 m² g⁻¹, the smallest lattice parameter, and the largest amount of surface adsorbed and desorped oxygen species, which has led to an excellent catalytic performance towards CO and CH₄ oxidation. Such a superior performance benefits from the unique synergistic effects of the finely dispersed NiO and Ni–O–Ce solid solution species detected in the heterostructure.

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1. Introduction

Catalytic oxidation reactions have drawn great attention in recent decades due to the energy utilization and environmental requirements.^1–3 Many types of catalysts such as pure oxides,⁴ mixed oxides,⁵ and supported systerms⁶ have been studied for catalytic oxidations. CeO₂ has a relatively high thermal stability and redox behavior, and therefore is widely used as an important support in CO oxidation reactions.⁷ It is well established that noble metals supported on CeO₂ usually exhibit excellent catalytic performance, while these supported noble metals are expensive. It is hence necessary to develop cheap substitution catalysts for catalytic oxidation reactions. Transition metal oxides are promising alternatives because of the low price. Recent reports have shown that MO_X (M = transition metal ions, like Co³⁺/Co²⁺, Cu²⁺, Ni²⁺)^8–10 supported on CeO₂ exhibits excellent activity of catalytic oxidation since addition of transition oxide enables the promotion of oxygen storage capacity and redox properties of CeO₂. As a consequence, there is a strong demand for development of composite CeO₂–MO_X catalysts with low cost and relatively high activity.

Catalytic performance enhancement of CeO₂–MO_X catalysts is not only due to the interactions between MO_X and CeO₂ that can further modulate the intrinsic surface properties of supported phases, but also due to the internal doping of transition metal species for creating more defects (oxygen vacancies).^11–16 Besides these, CeO₂ shows the merits in preventing the sintering of transition metal ions, as indicated for transition-metal oxide supported systems (e.g., CeO₂–Fe₂O₃, CeO₂–NiO, CeO₂–CuO)^17–19 when applying in catalytic oxidation reactions. Among all transition metal oxide supported systems, CeO₂–NiO materials could be a highly promising catalyst for various reactions. Many methodologies have been reported in literature to prepare CeO₂–NiO mixed oxide catalysts with different Ni contents, which include co-precipitation,²⁰ citrate acid method,²¹ or impregnation method.^22,23 However, there still exists certain controversies in these studies. For instance, it is reported that Ni species are doped in CeO₂ lattice, which favors to form oxygen vacancies and improves reducibility of CeO₂.^24,25 While, others thought that NiO particles are highly dispersed on CeO₂ surfaces, providing strong metal-support interactions between NiO particles and CeO₂.^26,27 One may wonder if it is possible to achieve heterostructure (1 − x)CeO₂·xNiO that simultaneously involves the doped Ni species and surface modification of NiO nanocrystals, since such heterostructure expects to show the synergistic effects in producing excellent catalytic performance essential for applications.

In this work, we adopted a two-step method that involves a hydrothermal reaction and a subsequent calcination to prepare a novel heterostructure (1 − x)CeO₂·xNiO. The sample synthesis is based on the following considerations: (i) hydrothermal method is effective to synthesize the precusors of CeO₂ and beta-Ni(OH)₂,²⁸ and (ii) both NiO and CeO₂ have the similar cubic structures, while Ni²⁺ is far smaller than Ce⁴⁺. When calcined at high temperatures, part of Ni ions could be doped in CeO₂ lattice, while part of Ni species would be highly dispersed onto CeO₂ grains to form heterostructure (1 − x)CeO₂·xNiO. With the successful sample synthesis, the catalytic oxidation behaviors of the heterostructure were investigated, which have shown an excellent activity towards oxidation of CO and CH₄. Further, the impacts of calcination temperature on surface structure, internal defect structure, and furthermore the catalytic activities were discussed.

2. Experimental section

2.1. Sample syntheses

(1 − x)CeO₂·xNiO (x = 0.1). All samples were prepared by a hydrothermal method followed with calcination treatment. The synthetic procedure can be briefly described as follows. A mixed solution that contains 0.225 M Ce(NO₃)₃·6H₂O and Ni(NO₃)₂·6H₂O at a molar ratio of Ce : Ni = 9 : 1 was sufficiently stirred, into which 5 M NaOH solution was added till pH = 13. The mixed solution was then transferred to 100 mL Teflon-lined stainless steel autoclave, which was allowed to react at 220 °C for 24 h. After naturally cooling down to room temperature, the precipitates were filtered, washed with distilled water till pH = 7, and dried at 80 °C in air. Eventually, the dried precipitates were calcined at 400, 500, 600, and 700 °C, respectively, in air for 4 h.

(1 − x)CeO₂·xNiO (0.05≤x ≤ 0.7). Using the procedures for x = 0.1, the samples (1 − x)CeO₂·xNiO with different nickel contents were prepared, when the calcination temperature was fixed at 600 °C.

Pure-phases of the end compounds (1 − x)CeO₂·xNiO (x = 0 and 1) were also prepared for comparison.

x = 0. The synthetic procedure for CeO₂ was similar to that for x = 0.1, while no Ni(NO₃)₂·6H₂O was involved during hydrothermal reactions. The precipitates by hydrothermal method were also calcined at 400, 500, 600, and 700 °C in air for 4 h, respectively.

x = 1. The synthetic procedure for NiO was similar to that for x = 0.1, while no Ce(NO₃)₃·6H₂O was involved during the hydrothermal reactions. The precipitate thus obtained was calcined at 500 and 600 °C in air for 4 h.

2.2. Sample characterization

All samples were characterized by X-ray diffraction (XRD) using a Rigaku Miniflex apparatus equipped with Cu Kα radiation (λ = 0.15418 nm). The scanning rate was set at 0.3° min⁻¹, and the step size was 0.02°. Aluminum powders were added into the samples (amounting 10–15 wt%) to serve as an internal standard for peak positions calibration. Raman spectra of the samples were obtained on a Renishaw, UV-vis raman system 1000 with an excitation line of 785 nm. Morphologies of the samples were observed by scanning electron microscopy (Phenom G₂), and transmission electron microscopy (TEM) (JEM-2010). Sample was dispersed in anhydrous ethanol and then treated under ultrasound for 30 minutes to get a well-dispersed solution before TEM measurements. The molar ratios of Ce to Ni and weight proportions for the samples were measured by Inductively Coupled Plasma OES spectroscopy (Ultima2).

Specific surface areas were determined from nitrogen adsorption data measured at liquid nitrogen temperature using Brunauer–Emmett–Teller (BET) technique on a Micromeritics ASAP 2000 Surface Area and Porosity Analyzer. The samples were pre-treated in N₂ at 120 °C before N₂ absorption analysis. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed on a Nicolet 6700 apparatus equipped with diffuse reflectance accessory (Harrick CHC-CHA-3). All sample powders were pre-treated in situ at 400 °C in pure He (99.99%) with a flow rate of 60 mL min⁻¹ to eliminate water traces. Then, all powders were cooled to 160 °C in flowing He. Subsequently, 1% CO and 99% He at a total flow rate of 40 mL min⁻¹ were introduced into the in situ chamber, and the spectra were recorded in 10 min.

H₂ temperature-programmed-reduction (TPR) for all samples were measured. Prior to H₂-TPR experiments, 50 mg samples were treated at 200 °C for 2 h in Ar. After cooling down to 30 °C, samples were heated to 900 °C at a rate of 5° min⁻¹ in 10 vol%H₂ in Ar at a flow rate of 30 mL min⁻¹. H₂ consumption of the samples was calculated by comparing to CuO reference.

To examine the mobility of lattice oxygen, O₂ temperature-programmed-desorption (TPD) for all samples were measured using a conventional chemisorption analyzer (Finesorb 3010C). The measurements were performed under a flow of pure He (30 ml min⁻¹) over 100 mg catalyst at a heating rate of 10 °C min⁻¹ from room temperature to 900 °C. All samples were pre-treated in He at 120 °C for 40 min, and then in pure O₂ at 120 °C for 60 min before TPD test.

2.3. Catalytic activity test

CO catalytic oxidation. The catalytic test was carried out on a fixed-bed quartz microreactor equipped with an online gas chromatograph (Shimadzu GC-2014) and a quadrupole mass spectrometer (Pfeiffer Prisma Plus QMG 220 M2). All measurements were done at atmospheric pressure with 50 mg catalyst (50–70 mesh) without any pre-treatment. The reagent gas consisted of 1.0% CO, 20% O₂, and 79% Ar with a flow rate of 20 mL min⁻¹. The specific velocity was set at 24 000 mL g⁻¹ h⁻¹. The conversion of CO was calculated from the difference in CO concentration between the inlet and outlet gases.

CH₄ catalytic oxidation. CH₄ oxidation test was performed on the same reactor for CO catalytic oxidation, but with a catalytst mass of 100 mg. 1% CH₄ and 4% O₂ balanced with Ar at a flow rate of 50 mL min⁻¹ were set as the reagent gas, and the specific velocity was set at 30 000 mL g⁻¹ h⁻¹.

3. Results and discussion

3.1. CO catalytic oxidation activity of the samples

All samples (1 − x)CeO₂·xNiO (0.05 ≤ x ≤ 0.7) calcined at 600 °C showed a pronounced catalytic performance towards CO oxidation. As indicated in ESI (Fig. S1†), NiO content showed a significant influence on the catalytic performance. The catalyst (1 − x)CeO₂·xNiO at x = 0.05 has a relatively poor activity and the complete temperature of CO conversion (T_100%) was 200 °C. As NiO content increased to x = 0.1, CO conversion rate increased obviously, showing a decrease in T_100% to 180 °C. However, the activity of catalysts were not remarkably improved when varying the NiO content in the range of 0.1 ≤ x ≤ 0.5. Further increasing NiO content results in a decrease in catalytic activity. Especially for x = 0.6 and 0.7, T_100% started to increase to 190 and 200 °C. In the following, we will address the sample at x = 0.1 due to its greatly improved oxidation activity.

Fig. 1 compares the CO catalytic oxidation activity of the sample (1 − x)CeO₂·xNiO at x = 0.1 with those for the end compositions (x = 0, 1). All these samples were prepared after calcination at 600 °C. It is seen that the sample at x = 0.1 performs more excellent catalytic performance when comparing to the end compounds at x = 0 and 1. For the latter cases, the sample at x = 0 was least active, almost showing no conversion at temperatures <200 °C. Meanwhile, the sample at x = 1 gave an intermediate catalytic activity with 100% conversion around 210 °C.

Fig. 1 CO oxidation activities of the samples (1 − x)CeO₂·xNiO at (a) x = 0.1, (b) x = 1, and (c) x = 0 prepared after calcination at 600 °C.

In order to further improve the catalyst properties, calcination temperature for x = 0.1 was optimized. As shown in Fig. 2, when the calcination temperature was set at 400 °C, the sample at x = 0.1 showed a complete CO conversion around 190 °C. Increasing calcination temperature to 500 °C led to an optimum catalytic performance by reducing the complete conversion temperature to 160 °C. Further increasing calcination temperature beyond 500 °C, catalytic activity decreased readily. For instance, after calcination at 700 °C, the complete conversion needed a higher temperature of 210 °C. Therefore, the optimized calcination temperature for x = 0.1 that shows the excellent catalytic activity was determined to be 500 °C.

Fig. 2 CO oxidation activities of the samples (1 − x)CeO₂·xNiO at x = 0.1 prepared after calcination at (a) 500, (b) 600, (c) 400, and (d) 700 °C.

Reproducibility of the sample at x = 0.1 calcined at 500 °C were investigated. As shown in Fig. 3, the catalytic performance for x = 0.1 after calcination at 500 °C was reproducible for four runs, no matter what the run sequence was. In addition, the stability of the catalytic performance was further examined at a conversion rate of 93–94% for x = 0.1 and x = 1. As indicated in inset of Fig. 3, for x = 0.1, CO conversion showed no sign of deactivation at 155 °C, even when the catalytic reaction periods were prolonged beyond 100 h. CO conversion for x = 1 slightly fluctuate from 25 h to 40 h. Therefore, synergistic effect of NiO and CeO₂ could improve the stability of the catalyst compared with pure oxide.

Fig. 3 CO oxidation reproducibility of the samples (1 − x)CeO₂ ·xNiO at x = 0.1 prepared after calcination at 500 °C. All catalytic tests were run for four times. Inset shows the long-term catalytic durability at a 93–94% conversion for a long period of 100 h for the samples at x = 0.1 (black dot) and 1 (red dot) after calcinations at 500 °C.

To get insights into the optimum catalytic activity for x = 0.1, we systematically characterized our samples by XRD, Raman spectra, TEM, BET, H₂-TPR, and O₂-TPD as below. All relevant results were compared with those reported in literature for similar systems.

3.2. XRD

Fig. 4 shows XRD patterns of the samples (1 − x)CeO₂·xNiO (x = 0.1) prepared after calcinations at different temperatures. To assign the peaks correctly, Al powder (space group Fm3̄m, JCPDS, no. 04-0787) was used as an internal standard. For comparison, XRD data for the end compounds (x = 0, 1) were also given. From Fig. 4, it is seen that both the end compounds showed the diffractions that can be well fitted in a cubic fluorite phase of CeO₂ (space group Fm3̄m, JCPDS, no. 65-5923) at x = 0 and NiO (space group Fm3̄m, JCPDS, no. 47-1049) at x = 1. Comparatively, for x = 0.1, majority of the intensive diffraction peaks can be assigned to CeO₂, while several weak diffraction peaks for NiO were also observed, as indicated by the enlarged diffraction patterns in ESI (Fig. S2†). One may thus conclude that traces of NiO co-existed with the dominant phase CeO₂. This observation is completely different from a previous report by Wrobel et al.,²⁹ who concluded that Ni tends to dope in CeO₂ lattice to form a solid solution at Ni/Ce < 0.5. For both component phases in x = 0.1, the crystalline degrees were strongly influenced by calcination. As indicated in Fig. 4, with increasing the calcination temperature, diffraction intensities for components CeO₂ and NiO gradually increased, while the relevant diffraction width became narrowed, which suggests an increased crystalline degree of the component phases.

Fig. 4 XRD patterns of the samples (1 − x)CeO₂·xNiO at x = 0 after calcination at 600 °C (a), x = 0.1 after calcinations at (b) 400, (c) 500, (d) 600, (e) 700 °C, and (f) x = 1 after calcination at 600 °C. Al was used as an internal standard for peak positions determination.

Table 1 shows the mean crystallite size, lattice constant, and lattice strain for samples (x = 0, 0.1) prepared after calcinations at different temperatures. It is well documented that either particle size reduction or lattice strain effect could contribute to the peak broadening of XRD patterns. To figure out what is the main reason for diffraction broadening, we employed Williams³⁰ and Scherrer relation^31,32 and calculated the mean crystallite size and lattice strain for the dominant cubic fluorite phases of x = 0 and 0.1 prepared after calcinations at different temperatures using the following equation,³³

where β is the full width at half maximum (FWHM), θ is the diffraction angle, λ is the X-ray wavelength, D is the mean particle size, and η is the lattice strain. Four dominant diffraction peaks (111), (200), (220), and (311) of CeO₂ fluorite structures were used for all calculations. Since crystallite sizes and dopants can have great impacts on lattice dimension of fluorite structures, the lattice parameters for x = 0 and 0.1 were calculated using a profile fitting by a least-squares method employing the computer program GSAS implemented with EXPGUI.³⁴

Table 1 Mean particle size (D), lattice parameter (nm), and lattice strain (η) of (1 − x) CeO₂·xNiO at x = 0 and 0.1 after calcinations at different temperatures, and molar ratios of Ce to Ni for x = 0.1 after calcinations at different temperatures

Calcination temperature (° C)	Samples (1 − x)CeO2·xNiO
	Mean particle size (nm)		Lattice parameters (nm)		Lattice strain (η)		Molar ratios of Ce to Ni
	x = 0	x = 0.1	x = 0	x = 0.1	x = 0	x = 0.1	x = 0.1
400	17.6	17.1	0.5414	0.5410	0.18%	0.68%	0.92 : 0.11
500	18.5	17.7	0.5414	0.5405	0.26%	0.56%	0.91 : 0.11
600	18.5	17.9	0.5413	0.5412	0.32%	0.51%	0.91 : 0.11
700	26.3	25.8	0.5414	0.5413	0.25%	0.34%	0.92 : 0.12

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From Table 1, it is seen that the crystallite sizes for a doping of 10% Ni²⁺ strongly depend on calcination temperatures. When calcination temperature was lower than 700 °C, mean crystallite size for x = 0 and 0.1 were roughly the same, around 18 nm. However, increasing the calcination temperature to 700 °C, there occurred an obvious growth of the crystallite size. For example, the crystallite size for x = 0 grew up to approximately 26 nm. Similar growth of crystallite size was observed for x = 0.1. Such observations could be attributed to the kinetics factors of the nanosized CeO₂. It is well established that crystallite growth of CeO₂ is very slow when calcination temperature is lower than 600 °C. Above this temperature, there appears an exponential increase in crystallite size.^35,36 As indicated in Table 1, the molar ratios of Ce to Ni for x = 0.1 measured using ICP were closer to the initial ones, the stoichiometric atomic concentration was retained when varying calcination temperature.

The lattice parameters for x = 0 and 0.1 were comparatively studied to comprehend the doping behavior of Ni²⁺ in fluorite CeO₂ lattice. As indicated in Table 1, with increasing calcination temperature, the lattice parameters for x = 0 did not show apparent changes, but retaining around 0.5414 nm, consistent with the bulk nature for big crystallite size. Alternatively, for x = 0.1, the lattice parameters slightly decreased from 0.5410 to 0.5405 nm, as the calcination temperature increased from 400 to 500 °C. Further increasing calcination temperature led to an increase in lattice parameter. It means that the sample at x = 0.1 calcined at 500 °C might have more Ni ions doped in CeO₂ lattice. This is because Ni²⁺ with an ionic radius of 0.072 nm is smaller than that of 0.101 nm for Ce⁴⁺. When Ni²⁺ ions are incorporated in ceria lattice to form a solid solution Ce_1−xNi_xO₂, one may expect a decrease in lattice parameter, as reported elsewhere.³⁷

To further confirm the incorporation of part Ni²⁺ in CeO₂ to form solid solutions after calcinations, we compared the diffraction peak (111) for fluorite structure of all samples (Fig. S3†). When fixing the intensive diffraction line of internal standard (Al) at two theta of 38.47° for the standard diffraction data for Al, one can clearly see that the (111) diffraction peak for x = 0.1 shifted towards higher diffraction angles, relative to x = 0. This observation is similar to those previously reported.^25,38 For x = 0.1, the calcination at 500 °C had led to a largest shifts towards higher diffraction angles, which confirms that more Ni species were incorporated in fluorite lattice and formed a solid solution.

The lattice strains for all samples were also highly dependent on the calcination temperatures. As indicated in Table 1, the lattice strain for x = 0 was all much smaller, being distributed around η = 0.2–0.3% when varying the calcination temperature from 400 to 700 °C. However, the lattice strain for x = 0.1 after calcination at 400 °C was 0.7%, at least double that for x = 0, which is a good proof of the dislocation in CeO₂ structure as a result of nickel ion incorporation.^33,39 As calcination temperature increased, lattice strain for x = 0.1 decreased continuously. Further increasing the calcination temperature up to 700 °C, the lattice strain for x = 0.1 droped down to 0.3%, which is probably due to the significantly increased crystallite size.

It appears that doping part of low-valence Ni²⁺ in CeO₂ after calcination at temperatures <600 °C has produced a smaller lattice parameter (Table 1). In the following, Raman spectra were used to study the impacts of calcination temperature on the lattice strain and oxygen vacancies.

3.3. Raman spectra

Fig. 5 shows Raman spectra of the samples (1−x)CeO₂·xNiO (x = 0.1) prepared after calcination at given temperatures. For comparison, the Raman data for x = 0 were also given. For x = 0.1 prepared after calcination at 400 °C, a most intensive signal at 461 cm⁻¹ and four weak signals at 225, 260, 530, and 630 cm⁻¹ were observed. The most intensive Raman signal at 461 cm⁻¹ is characteristic of the intrinsic vibration mode F_2g of ceria,⁴⁰ while the weaker one at 260 cm⁻¹ is associated with the second-order transverse acoustic mode of ceria.⁴¹ This set of Raman bands is different from a previous report.^42,43 For the latter cases, no Raman signal at 260 cm⁻¹ is observed, but only one band at 225 cm⁻¹ for CeO₂–NiO systems. It is noted that the most intensive Raman signals for x = 0.1 after calcination at 400 and 500 °C are much broadened when comparing to that after calcination at 700 °C. Such a Raman signal broadening could be explained by the inhomogeneous strain effects (Table 1) associated with dispersion in particle size and Ni²⁺ incorporation.

Fig. 5 Raman spectra of the samples (1 − x) CeO₂·xNiO at x = 0.1 after calcination at (a) 400, (b) 500, (c) 700 °C, and (d) x = 0 after calcinations at 600 °C.

Doping of smaller ions (like Ni²⁺ in CeO₂) can give a lattice constriction and therefore a decrease in lattice spacing and bond length, leading to a red shift in Raman mode. To examine the impacts of the calcination on the doping behavior, the most intensive Raman signal at 461 cm⁻¹ was carefully studied, as indicated in ESI (Fig. S4†). It is seen that when calcined at lower temperatures of 400 and 500 °C, the Raman signal at 461 cm⁻¹ for x = 0.1 shifted towards lower frequencies, comparing to that for x = 0. This phenomenon could be attributed to the formation of Ce_1−xNi_xO₂ solid solution with Ni²⁺ ions replacing for Ce⁴⁺ ions in CeO₂ lattice. This assumption is further confirmed by the appearance of well-defined weak Raman signals at 225 and 550 cm⁻¹, since these weak signals are closely linked to the lattice defects originated from the oxygen vacancies as reported elesewhere.⁴³ Therefore, the solid solutions Ce_1–xNi_xO₂ were formed when calcination temperature was lower than 600 °C. Another important feature for Ni²⁺ doping is the modification of oxygen sublattice,⁴² which is demonstrated by the weak Raman signal observed at 630 cm⁻¹ in Fig. 5(a) and (b). When the calcination temperature was further increased to 700 °C, the intensities of Raman signals at 225, 550, and 630 cm⁻¹ decreased, while that of the most intensive one at 461 cm⁻¹ increased, which indicates a sharply decrease in the concentration of lattice defects when calcined at 700 °C. One may thus imagine that dopant Ni²⁺ and the relevant defects might be expelled from the bulk lattice to the surface after high-temperature calcination.

3.4. TEM

As stated above, syntheses of the samples (1 − x)CeO₂·xNiO involved two key processes: The first step is hydrothermal reactions between Ce(NO₃)₃·6H₂O and Ni(NO₃)₂·6H₂O aqueous solution at 220 °C, and the second one is the calcination at different temperatures. As indicated in Fig. 6a, morphologies for x = 0.1 before calcination were primarily nanocuboids and nanorods. Fig. 6b shows the inter-planar spacings of nanorods, which were about 0.31 nm, in good agreement with that for (111) facet of CeO₂. Besides, the samples obtained before calcination contained a small amount of β-Ni(OH)₂ (shown in Fig. S5†). It is difficult to detect β-Ni(OH)₂ nanoparticles in HRTEM, since its low concentration and poor crystalline degree. To certicificate the uniformity of Ni element in the sample, EDX mapping was also taken in Fig. 6c, which revealed that Ni element is uniformly distributed in the sample before calcination.

Fig. 6 (a) TEM, (b) HRTEM, and (c) TEM-EDX mapping for x = 0.1 before calcinations, (d) TEM and (e) HRTEM images of x = 0.1 after calcination at 500 °C. Insert shows the SAED pattern of NiO nanoparticle.

After high-temperature calcinations, the phase compositions and the relevant morphologies were changed, as indicated by SEM images of the sample (1 − x)CeO₂·xNiO at x = 0.1 after calcination at different temperatures in ESI (Fig. S6†). Fig. 6(c and d) show the HRTEM images of the sample (1 − x)CeO₂·xNiO at x = 0.1 after calcination at 500 °C. It is seen that most of the particles exhibited a subspheroidal morphology, which is completely different from the shape of nanoclusters when NiO/CeO₂ catalyst was prepared by solution combustion synthesis using citric acid as the oxidant.⁶ Despite of very low concentration of Ni²⁺, the sample after calcination was still observed to be composed of two types of particles: roundish CeO₂ particles with surfaces attached by small NiO particles. CeO₂ particles is proved by calculation of the lattice spacing of 0.27 nm for facet of CeO₂ (200). As shown in Fig. 6(e), the CeO₂ nanoparticle was estimated to about 13 nm. NiO particles (6 nm) attached on CeO₂ surface is evidenced by TEM and SAED results (insert of Fig. 6e). According to the fast Fourier transform (FFT) analysis of the lattice fringes, SAED pattern can be indexed to be the [12̄ 1] zone axis of cubic phase NiO, and these pattern spots are consistent with XRD results. The angles between the facets of (111) and (101̄), (210) were about 90.32° and 139.59°, respectively, in good agreement with the theoretical values of 90.82° and 140.80°.

3.5. BET

We compared the CO conversion at a reaction temperature of 150 °C and BET surface area for x = 0.1 and 0 as a function of calcination temperature. CO oxidation curve of (1 − x)CeO₂·xNiO catalysts at x = 0 calcined at different temperatures was shown in Fig. S8.† As indicated in Fig. 7 and S8,† for pure CeO₂ (x = 0) calcined at different temperatures, there was no activity when reaction temperature was increased to 150 °C, while the sample calcined at 600 °C showed the best activity among the samples for x = 0. However, for x = 0.1, the maximum activity is obtained after calcination at 500 °C. This indicates that addition of Ni has a great effect on CeO₂ activity and structure, and that the catalytic activity miximun for x = 0.1 calcined at 500 °C is not simply caused by the state of CeO₂ but probably the distribution of Ni species. Furthermore, when calcined at temperature <600 °C, the BET surface area for x = 0.1 was larger than that for x = 0, though the mean particle size for both samples did not show apparent changes. One may deduce that the increased BET surface area may be related to the change of catalyst structure. At lower calcination temperature, part of Ni ions were doped in CeO₂ lattice, while other parts of Ni ions were highly dispersed as segregation at surfaces for (1 − x)CeO₂·xNiO at x = 0.1. Alternatively, higher calcination temperatures >700 °C may distruct the structure of solid solution and cause sintering and growth of the particles, resulting in the gradual loss of surface area.

Fig. 7 Relationships among CO conversion at 150 °C, specific surface area, and calciniation temperature for samples (1 − x)CeO₂·xNiO at x = 0.1 and 0 after calcination at different temperatures from 400 to 700 °C.

As indicated in Table 1, when calcined at 500 °C, the lattice parameters for the dominant fluorite phase were smallest. It means that more Ni²⁺ ions could be doped in CeO₂ lattice, which expects to give more oxygen vacancies essential for high activity. In the following, H₂-TPR spectra were used to study the reduction behaviour of catalyst surface introduced by Ni species doped into CeO₂ lattice and surface NiO species for the samples at x = 0.1 calcined at different temperature.

3.6. H₂-TPR

Generally, large surface area enables more surface active centers exposed to the reactants, and therefore is helpful to enhance the catalytic activity. The BET surface area for x = 0.1 after calcination at 500 °C is not the largest, which however gave a relatively high catalytic activity among the samples at x = 0.1. To uncover the structural nature behind the abnormal catalytic performance, H₂-TPR datas of the samples (1 − x)CeO₂·xNiO at x = 0, 0.1, and 1 were comparatively studied. For x = 0 (Fig. 8f), there appeared two reduction peaks. Both peaks centered at 400 °C and 750 °C were assigned to the reduction of surface and bulk oxygen species of CeO₂, respectively.⁴⁴ For x = 1, there were two reduction peaks at 368 and 480 °C, which correspond to the stepwise reduction of NiO, respectively, as represented by the symbols “β” and “γ”²⁰ (Fig. 8e).

Fig. 8 H₂-TPR data of the samples (1 − x)CeO₂·xNiO at x = 0.1 after calcinations at (a) 400, (b) 500, (c) 600, (d) 700 °C, and those at (e) x = 1 and (f) x = 0 after calcinations at 600 °C.

As shown in Fig. 8a–d, the samples at x = 0.1 calcined at different temperature showed four hydrogen consumption peaks, α₁, α₂, β, and δ. The first two peaks could be ascribed to the reduction of adsorbed oxygen species, which is easily accessible by H₂ at low temperatures.⁴⁵ When Ni²⁺ was incorporated in CeO₂ lattice to form solid solution, oxygen vacancies were formed which favor to surface oxygen adsorption. The β and δ peaks are ascribed to NiO reduction and bulk oxygen reduction of crystalline CeO₂, respectively. The lower reduction temperature of β peaks for x = 0.1 when compared to that of x = 1 indicates that NiO nanoparticles have a high dispersion degree on surfaces of catalysts and simultaneously proved the interactions existing between NiO and support CeO₂.

As indicated in Table 2, the calculated H₂ consumption ratios of the β peak to α₂ peak associated with Ni species distributed just followed such a sequence: sample at x = 0.1 after calcinations at 500 °C < 600 °C < 400 °C < 700 °C. For x = 0.1 calcined at 500 °C, H₂ consumption for peak α₂ was highest, and that for β peak was the lowest among the samples for x = 0.1. This phenomenon further certifies that more Ni²⁺ ions were doped in CeO₂ lattice to form solid solution after 500 °C calcination, which led to a drop in β peak intensity caused by the reduction of dispersed NiO species. When calcination temperature was higher than 500 °C, the intensities for peaks α₁ and α₂ became weak, while that for β peak became strong, which implies that high temperature calcinations above 500 °C led to the destruction of solid solution structure and the increase of NiO amount distributed onto CeO₂ surface.

Table 2 Reduction peak positions, H₂ consumption, desorption peak positions, and desorption areas for x = 0.1 after calcinations at different temperatures from 400 to 700 °C

Calcination temperature(°C)	(1 − x)CeO2·xNiO (x = 0.1)
	Peak position (°C)				H2 consumption(mmolg^−1)				Desorptionpeak position (°C)			Desorption area
	T1 (α1)	T2 (α2)	T3 (β)	T4 (γ)	n2 (α2)	n3 (β)	n4 (γ)	n3/n2 β/α2	T1	T2	T3	n1	n2	n3	n1 + n2 + n3
400	157	222	293	733	0.089	0.477	0.357	5.4	120	271	591	535	104	295	934
500	156	212	298	740	0.104	0.135	0.337	1.3	147 (α peak)			3034			3034
600	156	206	297	746	0.048	0.157	0.288	3.3	112 (α peak)			2285			2285
700	166		251	715	0.027	0.317	0.320	11.7	101	337		212	186		398

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3.7. O₂-TPD

O₂-TPD was used to examine the mobility of the lattice oxygen. Fig. 9 shows the O₂-TPD of (1 − x)CeO₂·xNiO samples at x = 0, x = 0.1 and x = 1. It is seen that calcination of x = 0 at 600 °C (Fig. 9e) gave three obvious oxygen desorption signals at 112 °C, 250 °C and 500 °C, which are related to the weakly adsorbed molecular oxygen, the chemically adsorbed oxygen on surface, and the chemically adsorbed oxygen adjacent to the oxygen vacancies.⁴⁶ The sample at x = 1 gave two weak desorptions at about 180 and 400 °C and one strong signal at 800 °C. (Fig. 9f), which are attributed to the desorption of non-stoichiometric oxygen (O₂ and O⁻) and lattice oxygen (O²⁻) of NiO, respectively.⁴⁷

Fig. 9 O₂-TPD of the samples (1 − x)CeO₂·xNiO at x = 0.1 after calcination at (a) 400, (b) 500, (c) 600, (d) 700 °C, and those at (e) x = 0 and (f) x = 1 after calcinations at 600 °C.

For all samples at x = 0.1, the desorption peaks were related to surface oxygen species desorption, since bulk oxygen is difficult to be desorbed. The sample at x = 0.1 calcined at 400 °C showed three obvious desorption signals at 120°, 271°, and 591 °C, the shape of which is quite similar to that of pure CeO₂. It is thus indicated that incorporation of Ni ions in CeO₂ has little effect on the desorption of surface oxygen species after calcination at 400 °C. With increasing the calcination temperature to 500 °C, O₂-TPD profile for x = 0.1 (Fig. 9b) showed a strong and broad α peak in a wider temperature range (80–430 °C), which is attributed to the desorption of nonstoichiometric oxygen species (O₂⁻), indicating an enhanced surface oxygen mobility. As the calcination temperature increases to 600 and 700 °C, α peak became slightly smaller and divided in two signals at 101 and 337 °C.

Desorption area of each peak was also calculated and integrated (shown in Table 2). It is observed that area summation of desorption peaks for different samples follows such a sequence: the sample (1 − x)CeO₂·xNiO catalysts at x = 0.1 calcined at 500 °C > 600 °C > 400 °C > 700 °C. Sample calcined at 500 °C has a larger surface oxygen desorption amount, which would give rise to a better activity. When calcination temperature increases, the desorption ability for surface oxygen species drops, which accounts for the decrease in CO oxidation activity.

3.8. CH₄ oxidation

The above experiments have demonstrated that the sample at x = 0.1 after calcination at 500 °C showed the best catalytic activity towards CO oxidation. In the following, this sample was further tested for oxidation of CH₄. For comparison, pure CeO₂ (x = 0) calcined at 500 °C was also tested under the similar conditions. As monitored by mass spectra, CO₂ and H₂O were the only detectable products during complete CH₄ oxidation. As shown in Fig. 10, for x = 0.1 after calcination at 500 °C, there occurred a CH₄ conversion of 50% at 478 °C, which appears to be superior to that of 12% for x = 0 at the same temperature. Samples at x = 0.1 calcined at different temperatures were tested for CH₄ oxidation and the results are shown in ESI (Fig. S9†). It is seen that the samples calcined at 400 and 500 °C showed a respectively high CH₄ oxidation activity, among which there were no obviously difference in CH₄ oxidation activity. However, after higher temperature calcinations at 600 and 700 °C, the activity was dramatically suppressed, with a complete CH₄ conversion of 50% at 570 and 750 °C, respectively. The activity variation order for CO and CH₄ oxidation are different as calcination temperature increases from 400 to 700 °C, which are caused by the dinstinct active species in CO and CH₄ oxidation reactions. In a word, sample at x = 0.1 after calcination at 500 °C can act as an excellent catalyst for CO oxidation, but also for CH₄ catalytic oxidation.

Fig. 10 CH₄ oxidation activities of the samples (1 − x)CeO₂·xNiO at (a) x = 0.1 and (b) x = 0 prepared after calcinations at 500 °C.

Conclusions

A series of samples (1 − x)CeO₂·xNiO were prepared by a hydrothermal method combined with a subsequent high-temperature calcination. Calcination of the sample at x = 0.1 in a temperature range from 400 to 700 °C is applied to tune the distribution of Ni²⁺ ions in internal lattice and surface segregation. Depending on the distributions of Ni²⁺, the calcined samples showed the different catalytic activities towards CO oxidation. The sample after calcination of x = 0.1 at 500 °C yielded the optimum catalytic activities towards CO oxidation and CH₄ oxidation, which is the consequence of the incorporation of more Ni²⁺ in CeO₂ lattice, high concentration of oxygen vacancies, and the associated adsorption/desorption abilities of surface oxygen species. High temperature calcination may damage the solid solution structure, by showing aggregation of surface NiO nanoparticles and the decreased adsorption/desorption abilities of surface oxygen species. The findings reported in this work are fundamentally important, which may help to find more advanced catalytic materials for technological applications.

Acknowledgements

This work was financially supported by NSFC (21025104 and 21271171), and NBRP of China (no. 2011CBA00501 and 2013CB632405).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, XRD, SEM, Raman, CO oxidation activity and pseudocapacitance performance. See DOI: 10.1039/c3ra44186e

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