Mesoporous NiO–CeO2 catalysts for CO oxidation: Nickel content effect and mechanism aspect

Applied Catalysis A: General 494 (2015) 77–86

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Mesoporous NiO–CeO2 catalysts for CO oxidation: Nickel content effect and mechanism aspect Changjin Tang a,b , Jianchao Li a , Xiaojiang Yao c , Jingfang Sun b , Yuan Cao a,b , Lei Zhang a,b , Fei Gao b , Yu Deng b , Lin Dong a,b,∗ a

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China Jiangsu Key Laboratory of Vehicle Emission Control, Center of Modern Analysis, Nanjing University, Nanjing 210093, PR China c Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China b

a r t i c l e

i n f o

Article history: Received 11 December 2014 Received in revised form 20 January 2015 Accepted 26 January 2015 Available online 2 February 2015 Keywords: Mesoporous catalysts NiO–CeO2 CO oxidation Physicochemical characterization Reaction mechanism

a b s t r a c t Mesoporous NiO–CeO2 catalysts with different nickel contents (meso-Nix Ce, x = 0.05, 0.1, 0.2) were prepared by a KIT-6-templating method and characterized by XRD, N2 physisorption, TEM, Raman, XPS and H2 -TPR to understand their catalytic performances in CO oxidation. The introduction of nickel induced an obvious modification of the pore system of ceria, but the pore size and distribution were hardly affected by nickel content. HRTEM, XRD and Raman results clearly depicted free NiO (both clustered and amorphous) and doped nickel species, while XPS suggested the presence of another kind of nickel species (interfacial NiO). Probably owing to the unique mesoporous structures, the interfacial NiO decreased with increasing nickel content, which was coincident with CO oxidation activity. Besides, the comparable apparent activation energy (∼55 kJ/mol) for all samples indicated similar reaction pathway was followed. By correlating the catalytic performances with textural and compositional properties, it was found the latter dominated catalytic performance, and interfacial NiO was deduced to be the main active species for CO oxidation. Lastly, a synergetic interaction between atomically neighboring nickel oxide and ceria on the adsorbed reactants was tentatively proposed to understand the reaction mechanism. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Catalytic oxidation of carbon monoxide (CO) is a prototype reaction in heterogeneous catalysis and has received tremendous attention for decades, primarily for two reasons. Technologically, it is a major reaction in vehicle exhaust control, CO2 lasers, and other industrial processes [1]; Theoretically, it is relatively simple and can be used as a model system to study the mechanism of heterogeneous catalysis process [2]. Traditionally, CO oxidation is conducted with noble metals supported on reducible or irreducible supports and distinct activities are displayed [3,4]. Nevertheless, by taking into the fact that noble metals have limited reserves and are expensive, there is an urgent need to develop low cost alternatives, and recent studies have witnessed great development of ceria-based

∗ Corresponding author at: Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China. Tel.: +86 25 83592290; fax: +86 25 83317761. E-mail address: [email protected] (L. Dong). http://dx.doi.org/10.1016/j.apcata.2015.01.037 0926-860X/© 2015 Elsevier B.V. All rights reserved.

transition metal oxide catalysts for CO oxidation owing to their unique redox properties [5–7]. Nickel oxide, with rich reserves and excellent redox property, has been attempted to combine with ceria forming supported or mixed oxide catalysts. The addition of nickel into ceria could generally increase oxygen vacancies and improve oxygen diffusion of ceria, and thus contributes to favorable performances in many reactions. Wang et al. [8] employed NiO/CeO2 as a novel catalyst for NO removal by CO, and it was observed that NO could be totally converted to N2 at temperature as low as 210 ◦ C. Qiao et al. [9] tested MOx –CeO2 (M = Cu, Mn, Fe, Co, and Ni) catalysts in CH4 wet combustion. They found with respect to other mixed oxide catalysts, NiO–CeO2 catalyst showed the best durability. Additionally, nickel–ceria catalysts were also reported to be active for hydrodechlorination, steam reforming of hydrocarbons, N2 O decomposition and CO oxidation [10–14]. With the development of material science, ever increasing interest has been attracted on ceria-based nanostructures with regular shape, hollow or porous structures, which are in close relation with their catalytic properties [15–17]. Especially, much attention has been paid to mesoporous structures owing to the merits of

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high surface areas and unique confined environment [18–20]. It is well documented that the performances of ceria-based mesoporous catalysts are associated with multiple parameters. From the aspect of textual properties, Luo et al. [21] and Sun et al. [22], respectively, demonstrated that the surface area and symmetry of mesoporous framework greatly influenced the activity in CO oxidation. In addition, the performances of mesoporous catalysts are closely dependent on their compositional properties [18]. Of these diverse factors, the contributions of each and the predominant contributor to the enhanced activity are still not clear. Moreover, in consideration of the fact that preparation of ordered mesoporous catalysts is always operated via hard template method, the confined environment offered by the porous template is bound to bring some differences to the compositional and spatial distribution of the species [23]. Unfortunately, this is not sufficiently studied and little information is available, particularly with respect to the textural and compositional properties. Thus, a thorough investigation of the physicochemical properties of mesoporous catalysts is much needed and significant to understand their catalytic behaviors. In the present study, by employing NiO–CeO2 as a representative, we study in detail the textural properties and chemical compositions of the mesoporous catalysts and then correlate them with the catalytic performances in CO oxidation. The choosing of NiO–CeO2 is mainly based on the following considerations: (1) ordered mesoporous NiO–CeO2 has not been reported in previous studies; (2) NiO–CeO2 catalyst is a good candidate for low temperature CO oxidation but the structure–activity correlation is still not very clear. With the aid of a series of characterization techniques (XRD, N2 sorption, TEM, Raman, H2 -TPR and XPS), it is shown that the pore system of ordered mesoporous ceria is distinctly modified due to the introduction of nickel oxide. Moreover, three kinds of nickel species (i.e., doped Ni2+ in ceria lattice, interfacial NiO in strong interaction with ceria and free NiO particles) are confirmed in the catalysts, and the quantity of interfacial NiO decreases with increasing nickel content, which is coincident with the catalytic performance. Combined with the kinetic study results, some powerful evidences are obtained for the discrimination of active species in CO oxidation on NiO–CeO2 catalysts and a possible reaction mechanism concerning synergetic interaction between atomically neighboring nickel oxide and ceria on the adsorbed reactants is thus proposed. 2. Experimental 2.1. Preparation of catalysts 2.1.1. Preparation of hard template (KIT-6) KIT-6 was prepared according to reference [24]. In a typical process, 2.67 g P123 was dissolved in a mixed solution of 2.67 g 1-butanol, 4.5 ml concentrated HCl and 94 ml H2 O at 35 ◦ C. 5.67 g TOES was slowly dropped into the solution under vigorous stirring and hydrolyzed at 35 ◦ C for 24 h. Then the suspension was hydrothermally aged at 100 ◦ C in an autoclave for 24 h. After filtering and washing, as-prepared KIT-6 was obtained. The removal of surfactant was realized by air calcination at 550 ◦ C for 5 h with a ramping rate of 1 ◦ C/min. 2.1.2. Preparation of mesoporous nickel–ceria catalysts A mixed solution of nickel and cerium salts (totally 0.8 M) was prepared by dissolving Ni(NO3 )2 ·6H2 O and Ce(NO3 )3 ·6H2 O in ethanol. Typically, 20 ml of above-prepared solution was co-impregnated into 1 g of KIT-6 template. After ethanol was evaporated, the obtained composite was calcined at 550 ◦ C for 6 h. The silica template was removed by treating the composite in a 2 M hot

(50 ◦ C) NaOH solution. The template-free product was collected by centrifugation, washed with distilled water, dried and named as meso-Nix Ce, in which x represents the molar ratio of nickel in the composite. Mesoporous pure CeO2 was prepared in a similar way but without the addition of nickel nitrate and denoted as mesoCeO2 . 2.2. Characterization of catalysts X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert Pro diffractometer, equipped with a Ni-filtered Cu K␣ radiation ( = 0.15418 nm). The X-ray tube was operated at 40 kV and 40 mA. The average grain size was determined from XRD line broadening measurement using the Scherrer equation, d = K/ˇcos, where  is the X-ray wavelength,  is the diffraction angle, K is the particle shape factor, usually taken as 0.89, and ˇ is full width at half maximum in radians. Transmission electron microscopy (TEM) images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The sample was dispersed in A.R. grade ethanol with ultrasonic treatment and the resulting suspension was allowed to dry on carbon film supported on copper grids. Nitrogen sorption isotherms were measured at −196 ◦ C using a Micromeritics ASAP 2020 system. The samples were degassed for 160 min at 300 ◦ C in the degas port of the adsorption analyzer. The pore size distributions were calculated from the adsorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) algorithm. Raman spectra were collected on a Renishaw inVia Laser Raman spectrometer using Ar+ laser beam. The Raman spectra were recorded with an excitation wavelength of 514 nm and the laser power of 20 mW. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 Versaprobe system, using monochromatic Al K␣ radiation (1486.6 eV) operating at an accelerating power of 150 W. The samples were outgassed at room temperature in a UHV chamber (<5×10−7 Pa). All binding energies (B.E.) were referenced to the C 1s peak at 284.6 eV. The experimental errors were within ±0.1 eV. H2 temperature-programmed reduction (H2 -TPR) measurement was carried out in a quartz U-tube reactor. Before reduction, the samples were pretreated in N2 stream at 150 ◦ C for 1 h and then cooled to room temperature. After that, a H2 -Ar mixture (7% H2 by volume) with a flow rate of 70 ml/min was switched on and the temperature was increased linearly at a rate of 10 ◦ C/min. A thermal conductivity cell was used to detect the consumption of H2 on stream. 2.3. Activity and kinetic studies The activities in CO oxidation were measured in a flow, fixedbed micro-reactor at atmospheric pressure. The reactor was a 3 mm i.d. (5 mm o.d., length 40 cm) quartz tube housed in a furnace. The reaction temperature was measured with a K-type thermocouple inserted in the middle of the catalyst bed. The reaction gas composition was 1.6 vol% CO, 20.8 vol% O2 and 77.6 vol% N2 . The total flow rate of the reactants was kept constant at 25 ml/min and the used amount of catalyst was 50 mg. The catalysts were pretreated in a N2 stream at 150 ◦ C for 1 h before switched to the reaction gas stream and the experimental data were collected after steady state was achieved. They were collected in single measurement. The reactor inlet and outlet streams were measured using an online gas chromatograph (Shimadzu GC-14C). CO, O2 and N2 were separated by a 5A molecular sieve column and detected using TCD. The CO conversion (XCO ) was calculated as follows: XCO (%) =

[CO]in − [CO]out × 100 [CO]in

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Fig. 1. The small angle XRD patterns of KIT-6 and mesoporous metal oxide (inset) samples.

Kinetic studies to ascertain the apparent activation energy (Ea ) of the catalysts were performed under differential reaction conditions, with typically 5–15 mg catalyst powder diluted with chemically inert quartz sand to ensure operation in the kinetic regime (<20% conversion of CO). 3. Results and discussion 3.1. Textural properties of mesoporous NiO–CeO2 samples (small angle XRD, N2 physisorption, TEM) The mesostructures of NiO–CeO2 catalysts are firstly studied by XRD technique. Fig. 1 shows the small angle XRD patterns of the KIT-6 template and mesoporous NiO–CeO2 samples. The quality of hard template KIT-6 is revealed by the appearance of an intense (211) peak together with a (220) shoulder peak, indicating the formation of highly ordered mesostructure with cubic Ia3d symmetry [24]. This ordered structure is inversely replicated after introduction of nickel and cerium salts and the subsequent removal of silica. From inset of Fig. 1, we can find that all samples display a peak around 2 = 1◦ , implying the formation of mesoporous structures for nickel–cerium binary mixed oxides. Moreover, the peak intensity decreases with the increase of nickel content. These results indicate that in comparison with pure ceria, the presence of a second element provokes partial disturbing of the ordered mesoporous ceria framework. Fig. 2 shows the N2 sorption isotherms and pore size distribution (PSD) curves of the mesoporous pure CeO2 and NiO–CeO2 samples. It can be seen that all samples display type IV isotherms with hysteresis loops, indicative of their mesoporous structures. This is consistent with small angle XRD result. The sorption isotherm of mesoporous pure ceria displays a pronounced capillary condensation step starting at p/p0 = 0.6, while it shifts to lower relative pressure (p/p0 = 0.45) for the mesoporous NiO–CeO2 samples. The jumps at p/p0 = 0.45(0.6)−0.9 are attributed to the capillary condensation of the mesopores produced by removal of silica walls, and the steeper jump at p/p0 = 0.9−1.0 can be ascribed to the interstitial pores between particles. The corresponding PSD curves are shown in Fig. 2b. In line with previous report [25], mesoporous pure ceria exhibits a bimodal pore size distribution with diameters of 3.4 and 9.1 nm, in addition to the interstitial pores with broad size range (30–100 nm). Accordingly, the formation of two mesopores is related to the unique pore structures of KIT-6. As shown in Scheme 1, KIT-6 is composed of two sets of interpenetrating mesopores. If the introduced guest species fill both sets of pores with

Fig. 2. The (a) N2 physisorption isotherm and (b) corresponding pore size distribution of meso-CeO2 and meso-Nix Ce samples.

the connection of micropores (case I in Scheme 1), then the resulting mesoporous materials will have a pore diameter equivalent to the KIT-6 wall thickness (∼3.5 nm). If on the other hand, the guest species grow within only one set of KIT-6 mesopores (case II in Scheme 1), the resulting pore diameter will be equivalent to the dimensions of two walls plus a pore of KIT-6 and is certainly greater than that in the first case. Thus, the appearance of bimodal pore size distribution of mesoporous pure ceria indicates the random filling of metal precursors in one set and two sets of pores of KIT-6. Interestingly, after introduction of nickel species, the pore system is distinctly changed. In contrast to the bimodal pore size distribution for mesoporous pure ceria, it is observable that irrespective of nickel contents, the mesoporous NiO–CeO2 samples display only a unimodal pore size distribution, with mesopores centered at ca. 3.5 nm preserved while the larger mesopores at 9 nm disappeared. Since the larger mesopores are originated from the filling of only one set of mesopores in KIT-6, the disappearance of the larger pores indicates that the bridged micropores are stuffed by ceria species with the assistance of nickel species, resulting in full connection between the two sets of mesopores in KIT-6. The BET specific surface areas and BJH pore volumes of samples are summarized in Table 1. In principle, the surface areas (81–111 m2 /g) are higher than reported data for common nickel–ceria catalysts. Subrahmanyam et al. had prepared NiO/CeO2 catalysts by combustion method and found the surface areas were in the range of 30–50 m2 /g [14]. Additionally, the surface areas of catalysts do not vary monotonically with the compositions. From Table 1, it is observable that by increasing NiO, the surface areas of mesoporous NiO–CeO2 undergo a volcano type

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Scheme 1. Schematic illustration of the formation of two kinds of mesopores for mesoporous metal oxide templated from KIT-6.

Table 1 The physicochemical properties of mesoporous meso-CeO2 and meso-Nix Ce samples. Sample

S (m2 /g)a

meso-CeO2 81 meso-Ni0.05 Ce 111 meso-Ni0.1 Ce 111 98 meso-Ni0.2 Ce a b c d e f

V (cm3 /g)b

A (Å)c

D (nm)d

A600 /A463 e

Ce3+ (%)f

0.21 0.13 0.15 0.10

5.420 5.401 5.399 5.384

8.6 7.1 6.9 5.8

0.04 0.12 0.13 0.18

15.1 18.2 20.7 21.8

Specific surface area. Pore volume. Lattice parameter of ceria. Average grain size of ceria determined by XRD. Area ratio between Raman peak at 600 and 463 cm−1 . Surface Ce3+ concentration determined by XPS.

change. Meso-CeO2 shows the lowest surface area (81 m2 /g), and meso-Ni0.05 Ce and meso-Ni0.1 Ce samples possess the identical and highest surface area of 111 m2 /g. Then the surface area begins to decline. We supposed two factors might be related to such a change. One is the ordered mesoporous structure and the other being ceria grain size. With increasing of nickel content, the periodic degree of mesopores is decreased, which induces partial collapse of mesoporous ceria framework and causes the reduction of surface area. On the other hand, the grain size of ceria particles decreases with nickel content (Table 1, as shown by the following XRD result), which contributes to the increase of surface area. Notably, in comparison with the influence of ceria grain size on surface area, it seems that the effect of ordered mesoporous framework is more pronounced, as reflected by the fact that meso-Ni0.2 Ce sample has the smallest ceria grain size but shows the lowest surface area. To further ascertain the textural characteristics of the prepared samples, TEM characterization is carried out. Fig. 3 shows the TEM images of the prepared samples. Numerous well-arranged pores were found in mesoporous pure CeO2 and NiO–CeO2 samples (Fig. 3a–d), and the absence of bulk solid particles implies high efficiency of this method in preparing enriched mesoporous materials. These periodic mesopores can be viewed as a kind of intra-particle pores [26]. In addition to the small intra-particle pores, interstitial pores with broad size distributions are also visible (circled parts in Fig. 3b–d). The sizes of these interstitial pores are in the range of 30–100 nm and in good agreement with N2 sorption result. By careful inspection, differences can be observed among various samples. For the three mesoporous NiO–CeO2 samples, the

intra-particle pores are uniform in size. The information of small mesopores is represented by the inset in Fig. 3c. It can be seen that the pore diameter is 3–4 nm. In contrast, for mesoporous pure ceria, it is found that in addition to the small mesopores with size of around 3.5 nm, larger mesopores with size of 9 nm also exist, verifying the bimodal distribution of mesopores. Moreover, in comparison with meso-CeO2 , meso-Ni0.05 Ce and meso-Ni0.1 Ce, distinctions can be also found for meso-Ni0.2 Ce sample. In general, the mesoporous ordering of meso-Ni0.2 Ce is relatively poor with respect to other samples. Although well-arranged pores can still be observed, their scale is much limited. This is in line with the small angle XRD result. Besides, apart from bulk particles, small separated particles with size of 30–50 nm are also visible as illustrated in Fig. 3d. The HRTEM image (Fig. 3e) confirms that they are crystalline CeO2 instead of NiO, which excludes the possibility of isolated NiO particles as a result of increased NiO. Thus, it is highly possible that the fine particles are fractures of bulk particles, which provides additional evidence for the disturbed integrity of mesoporous ceria by introducing sufficient nickel species. 3.2. The chemical compositions of mesoporous NiO–CeO2 samples (HRTEM, XRD, Raman, XPS and H2 -TPR) In order to identify the detailed chemical compositions of the mesoporous NiO–CeO2 catalysts, HRTEM, XRD, Raman, XPS and H2 TPR characterizations are employed in combination. The signals of cerium and nickel species are revealed by high resolution TEM. Both bulk ceria crystallites and tiny NiO clusters are visible in Fig. 3f, as evidenced by the interplanar spacings of 0.31 and 0.21 nm that are compatible with the expected distance between the (111) plane of cubic ceria and (200) plane of NiO, respectively. Besides, similar with previous reports, we also find certain amorphous species, which can be reasonably attributed to nickel species [27]. Fig. 4 shows the wide angle XRD patterns of mesoporous pure ceria and NiO–CeO2 samples. The characteristic peaks of cubic fluorite CeO2 [JCPDS #43-1002] at 2 = 28.7◦ , 33.2◦ , 47.7◦ and 56.6◦ appear in all patterns, demonstrating the formation of mesoporous framework with crystalline ceria walls and the maintaining of cubic fluorite structure after nickel introduction. This is also supported by electron diffraction result (inset in Fig. 3d). Moreover, the absence of any diffraction peaks of nickel species indicates they are highly dispersed or/and existed as small particles that are not sensitive to

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Fig. 3. Typical TEM images of (a) meso-CeO2 , (b) meso-Ni0.05 Ce, (c) meso-Ni0.1 Ce, (d) meso-Ni0.2 Ce samples with low magnification and of (e, f) meso-Ni0.2 Ce with high magnification. The SAED pattern is inset in (c), and the circled and arrowed symbols represent the interstitial pores and fractured ceria, respectively.

Fig. 4. The wide angle XRD patterns of meso-CeO2 and meso-Nix Ce samples.

XRD [28], and the absence of bulk NiO particles is in good accordance with TEM result. Previously, it was reported for NiO–CeO2 catalysts, NiO diffraction peaks were discernable when its content reached 20% in molar percentage [8,29]. Hence, the present result shows that by taking advantage of the mesoporous structure with large surface area, the dispersion of NiO is improved.

The crystalline phase of ceria is independent of nickel content, but the grain size and lattice parameter of ceria are noticeably influenced. With increasing of NiO, the peak intensity of ceria is weakened, suggesting a gradual decline in ceria sizes is occurred. They are calculated by Scherrer equation from (220) peak and the results are shown in Table 1. A significant decrease of 33% (from 8.6 to 5.8 nm) in ceria size is found when 20 at.% nickel is introduced. Combining with TEM result, it is deduced that the nickel oxide species are sandwiched in ceria and act as blocks to inhibit the growth of crystalline ceria. On the other hand, the lattice parameter of ceria is decreased with NiO content (Table 1). Considering the smaller radius of Ni2+ (69 pm) in comparison with Ce4+ (92 pm), the shrinkage of ceria lattice indicates some Ni2+ have substituted Ce4+ and doped into the lattice of CeO2 . This doping of foreign metal ions in ceria is probably another factor for the decreasing of ceria grain size [30]. From the lattice parameter of the mixed oxides, one can determine the magnitude of the modification of the CeO2 structure by nickel ions, and the smallest lattice parameter of meso-Ni0.2 Ce sample conveys the largest amount of doped Ni2+ in ceria lattice. The doping of nickel ions in ceria will consequently induce change of ceria interior properties (e.g., bulk defects) [31] and this is further explored by Raman characterization. As shown in Fig. 5, in addition to a main peak characteristic of the F2g vibration mode of cubic fluorite CeO2 (463 cm−1 ) and a shoulder peak representing oxygen vacancy (ca. 600 cm−1 ), another weak peak at 831 cm−1 is observed for mesoporous NiO–CeO2 samples, which is not observed

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Fig. 5. The Raman spectra of (a) meso-Ni0.05 Ce, (b) meso-Ni0.1 Ce, (c) meso-Ni0.2 Ce samples and (d) meso-CeO2 .

in mesoporous pure ceria. According to literatures [32–34], the Raman peak at around 830 cm−1 was assigned to peroxide (O2 2− ) species adsorbed on isolated two-electron surface defects. With the doping of Ni2+ , the following process probably takes place: O2− − Ce4+ − O2− − Ce4+

Ni2+ doping 2−

−→

O

− Ce4+ −  − Ni2+ + 2e + O

To compensate the charge imbalance between Ce4+ and Ni2+ , an oxygen vacancy (represented by ) together with two electrons will be formed. These newly generated electrons are commonly supposed to be transferred to next Ce4+ forming Ce3+ . However, in the present case, Raman result clearly shows they can also combine with oxygen vacancy to form anionic vacancy, which functions as reservoir for molecular oxygen activation and O2 2− species generation [35]. From Fig. 5, it can be found meso-Ni0.2 Ce displays the most intensive O2 2− signal, which is in line with the concentration of oxygen vacancy calculated by the area ratio of peaks at 463 and 600 nm (A600 /A463 , Table 1) [36], indicating a close relationship of doped nickel ions, oxygen vacancy and activated oxygen species. XPS was performed in order to further illuminate the surface oxygen species as well as the chemical state of the elements existing in catalysts. Fig. 6a shows the O 1s spectra of mesoporous NiO–CeO2 catalysts. In addition to the presence of the main peak O’ at 529.3 eV attributed to the lattice oxygen of the metal oxides, an apparent shoulder peak O” at 531.9 eV representing the absorbed oxygen and oxygen in carbonates, hydroxyl groups is observed [37]. The relative percentage of these two kinds of oxygen species (i.e., lattice oxygen and chemisorbed oxygen) is quantified based on the area of O and O , and the ratio of SO /SO for these samples follows the order: meso-CeO2 < meso-Ni0.05 Ce < meso-Ni0.1 Ce < meso-Ni0.2 Ce, which is in accordance with the order of the oxygen vacancy concentration and Raman signal at 831 cm−1 . Moreover, it is also notable that in comparison with previously reported O 1s signal of ceria-based catalysts (O /O < 0.8) [14,38], the O /O value is apparently higher, which is supposed to be related with the unique mesoporous structure and relatively large surface area that provide sufficient space for molecular oxygen activation.

Fig. 6. The XPS spectra of (a) O 1s, (b) Ce 3d and (c) Ni 2p of mesoporous pure ceria and NiO–CeO2 samples.

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Fig. 7. H2 -TPR profiles of mesoporous NiO–CeO2 samples.

The concentration of Ce3+ is reflected by Ce 3d spectra. It can be seen that the signals are slightly decreased with nickel content, in agreement with the reduced portion of cerium after nickel introduction. The complex spectrum of Ce 3d is decomposed into eight components with the assignment defined in Fig. 6b. According to the literatures, the bands labeled u and ␯ represent 3d10 4f1 initial electronic state corresponding to Ce3+ , while the other six bands labeled u and  , u and  , u0 and 0 are related to Ce4+ [37–39]. Moreover, from the area of these peaks, the contents of Ce3+ relative to the total Ce concentration of these samples are calculated by the following equation: Ce3+ (%) =

S  + Sv × 100 (Su + Sv )

u

The results are listed in Table 1. It is shown that for meso-CeO2 , the percentage of Ce3+ is 15.1% and comparable to the reported value [39]. Enlargement of Ce3+ concentration is observed when nickel is introduced. This observation is in good accordance with the Raman and O 1s results and demonstrates that the chemisorbed oxygen species have a close relation with Ce3+ concentration, which is in turn linked to nickel content. As for nickel species, the spectra are shown in Fig. 6c. It can be safely concluded that most Ni are in +2 oxidation state, which is substantiated by the main peaks at ca. 855.0 eV for all the catalysts. In addition, a broad peak centered at ca. 861.5 eV is seen and can be assigned to the shake-up satellite peak of Ni3+ (Ni2 O3 ) [14]. It is obvious that the satellite peak is almost negligible for mesoNi0.05 Ce, suggesting limited Ni3+ in meso-Ni0.05 Ce in comparison with the other two samples. This reflects intimate electronic interaction between nickel and ceria is probably taken place, which can be expressed as follows: Ce3+ + Ni3+ → Ce4+ + Ni2+ The above deduction is further revealed by the shift of the main peak of meso-Ni0.05 Ce. As a direct result of electronic transfer from Ce3+ to Ni3+ , the binding energy of meso-Ni0.05 Ce slightly shifts to lower value, as can be observed in Fig. 6c. The XRD, Raman and XPS results have given clear evidences of NiO clusters and bulk doped nickel species in ceria lattice. However, the information of surface dispersed nickel species is still limited. By taking into consideration of the potential discrimination of varied nickel species from their different reduction profiles [29,40], H2 -TPR characterization is carried out and the results are shown in Fig. 7. Irrespective of nickel contents, mesoporous NiO–CeO2 samples show similar reduction features and three H2 consumption

83

Fig. 8. The catalytic performance of meso-CeO2 and meso-Nix Ce samples in CO oxidation reaction.

peaks (˛, ˇ and ) are observed. From Raman result, it is known that adsorbed oxygen species (i.e., O2 2− ) is present on the surface of nickel–cerium mixed oxides. The adsorbed oxygen species are well acknowledged to be easily reduced by H2 [41]. Thus, in line with literatures [8,42], the ˛ peak with the lowest reduction temperature (262 ◦ C) is ascribed to the reduction of surface peroxide species. Previous studies demonstrated that reduction of pure NiO particles usually took place at 300–400 ◦ C [43], a temperature scale close to the ˇ peak. Thus, it is reasonable to attribute ␤ peak the reduction of free NiO particles. In combination with the HRTEM result (Fig. 3f), we suppose both clustered NiO and amorphous NiO species accounting for peak ˇ. In addition to free NiO, it is widely reported that another kind of NiO, which has strong interaction with ceria, is present in nickel–ceria system [8,42,44]. Due to the strong interaction, the reduction of NiO is retarded in comparison with free NiO [40]. Hence, in the present study, peak  represents the reduction of the interfacial NiO in strong interaction with ceria. By further comparing the peak intensity, it is known that the doped nickel in ceria and free NiO particles are increased with NiO contents. However, the interfacial NiO exhibits an opposite trend. The intensity of peak  decreases with nickel content, suggesting aggregation of surface dispersed nickel species is probably happened. Moreover, in contrast to the first two peaks, the peak of  shifts gradually to lower value after increasing NiO content, indicating weakened interaction between NiO and ceria at higher NiO content. This is also in accordance with the decreased quantity of interfacial NiO. Furthermore, combined with the XPS result of Ni 2p, we suppose the restriction of nickel aggregation may also be associated with the electronic interaction [45]. 3.3. The catalytic performance of mesoporous NiO–CeO2 samples in CO oxidation and kinetic study The activities of the prepared samples in CO oxidation are displayed in Fig. 8. In the whole operation temperature range (125–225 ◦ C), it can be seen that the CO conversions of mesoporous pure CeO2 is very poor (lower than 20%), and increases slowly with the elevation of temperature. Upon NiO introduction, the activity is greatly enhanced. More than 70% CO are converted to CO2 at the temperature of 225 ◦ C on mesoporous NiO–CeO2 catalysts, suggesting nickel species constitute the main active species in CO oxidation. Interestingly, for mesoporous NiO–CeO2 catalysts, their activities are not increased with the amount of reactive surface oxygen species or nickel content, but follow the sequence of meso-Ni0.05 Ce > 0meso-Ni0.1 Ce  meso-Ni0.2 Ce. For the worst

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On the other hand, for the compositional properties of mesoporous nickel–ceria catalysts, the decreased activity with nickel content suggests the varied nickel species are not identically active for CO oxidation. Moreover, the opposite evolution behaviors of the interfacial NiO species and other two nickel species with nickel content present a good chance to ascertain the active species. In fact, the activity is coincident with quantity of interfacial NiO species. Since the textural properties (pore structure, surface area and pore volume) are not much varied for the three catalysts, it is reasonable to conclude that interfacial NiO are the main active species for CO oxidation in the present mesoporous NiO–CeO2 catalyst system. 3.5. The possible reaction mechanism of mesoporous NiO–CeO2 catalysts for CO oxidation

Fig. 9. Arrhenius plots for CO oxidation over indicated catalysts.

meso-Ni0.2 Ce catalyst, it only gets the CO conversion of 12% at 200 ◦ C, which is merely about one seventh of the values of mesoNi0.05 Ce and meso-Ni0.1 Ce. To explore the kinetic parameter of CO oxidation over mesoporous NiO–CeO2 catalysts, the apparent activation energy (Ea ) was calculated using the Arrhenius function. In the past years, CO oxidation over ceria-based catalysts is extensively studied and the kinetics was reported to be first-order toward CO pressure and zero toward oxygen pressure [46]. Therefore, it is reasonable to suppose that the CO oxidation on nickel–ceria catalysts would obey a firstorder reaction mechanism with respect to CO concentration in the presence of excessive oxygen (CO/O2 molar ratio = 1/13). The corresponding Arrhenius plots are displayed in Fig. 9. It can be found that although with different activities, all catalysts display the similar apparent activation energies (Ea = 55 kJ/mol), suggests the similar reaction route is probably followed. 3.4. Correlation of the textural and compositional properties with catalytic performances Both the textural and compositional properties of mesoporous NiO–CeO2 catalysts are characterized to explore the major factor linked to activity change. It should be mentioned that for ceriabased mixed oxide catalysts, it is still controversial whether textual or compositional properties are key to CO oxidation. For example, Luo et al. [21] suggested the excellent properties of mesoporous CuO–CeO2 catalysts was ascribed to its large surface area, while our recent study [47] indicated that instead of textual properties, the Cu–O–Ce entity of mesoporous CuO–CeO2 determined its activity in CO oxidation. In the present study, from the small angle XRD result, we know that the mesoporous ordering is decreased with nickel content. However, N2 physisorption and TEM results reveal that pore structures are independent of nickel content, and similar pore size distributions are obtained for the three mesoporous NiO–CeO2 samples. Thus, we suppose the influence of pore structure on the activity change is minor. On the other hand, for the textural parameters of surface area and pore volume, we can find the although the surface area and pore volume of meso-Ni0.2 Ce display the lowest values, the gap value between meso-Ni0.2 Ce and the other two samples is not comparable with the activity difference. Besides, the meso-Ni0.1 shows the best textural property among the three samples but not obtains the best activity. Based on these experimental facts, we suppose that the textural properties of mesoporous NiO–CeO2 catalysts are not the predominant factor influencing the catalytic activities.

The study of structure–activity correlation is a central topic for catalysis research and vital for design of excellent catalysts [38,48]. Considering the reaction mechanism of CO oxidation on nickel–ceria catalysts, limited information is available. Subrahmanyam et al. carried out an investigation on NiO/CeO2 catalysts for CO oxidation and correlated the activity with the oxygen vacancy concentration, dispersion of NiO and metal-support interaction between NiO and CeO2 [14]. Similar conclusion was obtained in our previous study [49]. Nevertheless, these factors are still entangled. Recently, Luo et al. prepared a series of MOy /Ce0.9 M0.1−x O2−␦ catalysts (M = Cu, Ni, Co, Fe) by a sol–gel method and rinsed the surface MOy species by an acid treatment to study the roles of MOy and Mn+ species in CO oxidation [50]. A synergetic effect between MOy species and Ce–M–O solid solution in activation of reactants was proposed to interpret the catalytic performance. However, one of the important things we should bear in mind is that there are generally several kinds of surface MOy species in MOy /CeO2 catalysts. For example, in NiO/CeO2 catalyst, the surface NiO species such as highly dispersed NiO and free NiO particles are well documented in literatures and confirmed in the present study. In their work, the role of different surface MOy species is not discriminated, which leads to the lack of a clear picture of synergetic interaction. By recalling the relevant catalyst system with NiO–CeO2 , there were studies showing that for lower valent cations, e.g., Ni2+ and Zn2+ , their doping in CeO2 was beneficial to lower the formation energy of oxygen vacancy, which increased the density of oxygen vacancy defects and consequently promoted CO oxidation activity [51–53]. Moreover, free NiO particles were reported to be active for CO oxidation [54], and FT-IR investigation showed CO molecules could be activated on Ni2+ in the form of Ni2+ (CO) and Ni2+ (CO)2 [55], despite the signal is sometimes too weak to be detected [50]. As revealed by Raman and H2 -TPR results in the present study, meso-Ni0.2 Ce catalyst exhibits the largest amounts of oxygen vacancies and free NiO particles. The former can induce the generation of defects and prompt molecular oxygen activation, while the latter can play the role of CO activation. Thus, it is properly supposed the meso-Ni0.2 Ce catalyst be most active for CO oxidation. However, it does not exhibit the best activity. It is well accepted the activation of reactants is prerequisite for catalytic reactions. However, to accomplish the entire process, effective interaction between reactants should also be fulfilled [56]. For certain activated reactant, for example CO, it can react with O2 in two forms, i.e., gaseous and adsorbed, corresponding to the classic reaction mechanisms of Eley–Rideal (E–R) [57,58] and Langmuir–Hinshelwood (L–H) [57,59], respectively. From the activity test result, it is deduced the reaction between activated CO and gaseous O2 (or activate O2 and gaseous CO) is hard to proceed or contribute little to activity enhancement, since meso-Ni0.2 Ce catalyst with the largest amount of O2 and CO activation sites does not have the best activity. Thus, the reaction should mainly take place between adsorbed reactants (CO and O2 ). As illustrated in Scheme 2,

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Scheme 2. Possible reaction mechanism of CO oxidation over mesoporous NiO–CeO2 catalysts.

we suppose the reason for the poor activity of meso-Ni0.2 Ce catalyst is the lack of effective interaction between adsorbed reactants (CO and O2 ). Although with the most favorable ability in activation of O2 and CO, the distance between adsorbed sites is not in sufficient vicinity (e.g., atomic range). Meanwhile, it is highly possible that the innatively existed diffusion energy barrier is not easy to be overcome [60] and thus the migration of the adsorbed species is not so facile, which in turn inhibits the efficient interaction between adsorbed reactants. In other words, the synergetic interaction of separated nickel oxide and ceria on the reactants can not be easily realized. Instead, as for interfacial NiO species, the case changes greatly. Owing to the intimate contact between NiO and ceria, the steric gap between CO and O2 adsorption sites no longer exists. Therefore, after adsorption of O2 and CO, the activated reactants facilely interacted with each other, making the reaction proceed efficiently. 4. Conclusion In this work, ordered mesoporous nickel–cerium mixed oxides with various NiO contents were prepared by using KIT-6 as a hard template and tested in the reaction of CO oxidation. It was found that with introduction of nickel species, the pore system of mesoporous ceria was distinctly modified. Moreover, the integrity of mesopores was disturbed when sufficient nickel was introduced, whereas the size and distribution of mesopores were hardly affected. The interaction between nickel and ceria weakened with increasing nickel content, resulting in reduced interfacial NiO species. This was coincident with the catalytic activities in CO oxidation. As such, interfacial NiO in strong interaction with ceria was supposed to be the main active species for CO oxidation and a possible reaction mechanism concerning on synergetic interaction between atomically neighboring nickel oxide and ceria on the adsorbed reactants was tentatively proposed. Acknowledgements The work was financially supported by the National Nature Science Foundation of China (21273110, 21303082), Jiangsu Province Science and Technology Support Program (Industrial, BE2014130), the Doctoral Fund of Ministry of Education of China (2013009111005) and Open Project Program of the State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University (201207). Kind assistances offered by Dr Yuhai Hu and Dr Xi Hong are also gratefully acknowledged. References [1] X.W. Xie, Y. Li, Z.Q. Liu, M. Haruta, W.J. Shen, Nature 458 (2009) 746–749. [2] A.A. Herzing, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Science 321 (2008) 1331–1335. [3] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301–309.

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