Enhanced catalytic performance for CO preferential oxidation over CuO catalysts supported on highly defective CeO2 nanocrystals

Enhanced catalytic performance for CO preferential oxidation over CuO catalysts supported on highly defective CeO2 nanocrystals

Applied Surface Science 422 (2017) 932–943 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 422 (2017) 932–943

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Enhanced catalytic performance for CO preferential oxidation over CuO catalysts supported on highly defective CeO2 nanocrystals Cheng Wang a , Qingpeng Cheng a , Xinlei Wang a , Kui Ma a , Xueqin Bai a , Sirui Tan b , Ye Tian a,∗ , Tong Ding a , Lirong Zheng c , Jing Zhang c , Xingang Li a,∗ a Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science and Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China b School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, PR China c Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 12 March 2017 Received in revised form 26 May 2017 Accepted 3 June 2017 Available online 7 June 2017 Keywords: CuO/CeO2 catalysts CO PROX Ce(OH)3 nanowires CeO2 nanocrystals

a b s t r a c t Herein, we synthesized CeO2 nanocrystals (Ce250) with a large number of surface lattice defects by calcining the superfine Ce(OH)3 nanowires at 250 ◦ C. They were then utilized as the support of the CuO catalysts for CO preferential oxidation (CO PROX). The strong interaction between CuO and CeO2 is the key factor to improve the catalytic activity, and our results show that the surface lattice defects on the CeO2 support can strengthen the interaction between CuO and CeO2 during the catalyst preparation procedure. With the increase of the calcination temperature, the surface areas of the CuO/CeO2 catalysts gradually drops, but the interaction between CuO and CeO2 is strengthened. Their catalytic activities present a “volcano” type, and the CuCe500 calcined at 500 ◦ C shows the highest catalytic activity. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The proton exchange membrane fuel cells (PEMFCs) with hydrogen as the clean energy have many advantages such as quick start, low operation temperature, high-efficiency and so on [1,2]. Now, the PEMFCs have attracted much attention in the application in the residential power generations. The reforming hydrogen is the typical source of hydrogen for PEMFCs. However, the reforming hydrogen contains 0.5–1% carbon monoxide, which can poison the Pt-based anode catalysts. Before the reforming hydrogen is used in the PEMFCs, CO must be removed below 10 ppm [3]. The preferential oxidation of CO (CO PROX) is a cost-effective and practical method for the deep removal of CO in H2 -rich stream [4]. Efficient catalysts for CO PROX must have high activity to remove CO and selectivity of O2 to CO2 at the working temperature (80–120 ◦ C) of PEMFCs [1]. There are many catalysts used for CO PROX, including Pt-group metal catalysts [5–10], Au nanocatalysts [11–14] and none-noble metal catalysts [15,16]. The Pt-group metal catalysts, which include Pt [5,6], Pd [7], Ru [8,9] and Rh [10], exhibit high activity for CO PROX. However, H2 can easily be dissociated on the catalysts, which reduces the selectivity of O2 to CO2 of the catalysts. Au nanocatalysts [11–14] have excellent low temper-

∗ Corresponding authors. E-mail addresses: [email protected] (Y. Tian), xingang [email protected] (X. Li). http://dx.doi.org/10.1016/j.apsusc.2017.06.017 0169-4332/© 2017 Elsevier B.V. All rights reserved.

ature CO oxidation activity, but the selectivity of O2 to CO2 is low for its high adsorption capacity of H2 . Moreover, Au nano-particles are easily aggregated during the reaction [11], so their catalytic stability is poor. The none-noble metal catalysts including the CuO and Co3 O4 catalysts show excellent activity with cheap price [15,16]. The CuO/CeO2 catalysts with the advantage of high selectivity of O2 to CO2 and high activity have attracted much attention [17–20]. Generally, the increase of the specific surface area of the support can improve the dispersion of CuO and increase the catalytic activity of the catalysts [21,22]. Luo et al. [23] prepared the CuO-CeO2 catalysts with a high specific surface area (more than 91 m2 g−1 ) by surfactant-templated method for low-temperature CO oxidation. They found that compared with the catalysts with small surface area, this catalyst presented the high CO oxidation activity with the lowest T90 at 80 ◦ C. Li et al. [24] prepared four kinds of CuO/CeO2 catalysts using metal-organic-framework- driven and self-template route by changing the organic ligands. The specific surface areas of the catalysts were 97, 88, 53 and 72 m2 g−1 , respectively, corresponding to CO full conversion temperatures of 110, 140, 190 and 290 ◦ C. Thus, they concluded that the CO oxidation activity of the catalysts increased with the increase of specific surface areas. However, the specific surface area of the CuO/CeO2 catalyst prepared by the conventional method was generally small. Avgouropoulos et al. [25] prepared the CuO/CeO2 catalysts by co-precipitation method, urea-combustion method, citrate-hydrothermal method and impregnation method, and their

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specific surface areas were 17, 38, 66 and 23 m2 g−1 , respectively. They found that the best catalyst (prepared by urea-combustion method) reached the CO full conversion point at 170 ◦ C, and the CO PROX activity of the catalysts prepared by urea-combustion method and citrate-hydrothermal method was much higher than that of the catalysts prepared by co-precipitation method and impregnation method. The high performance of the CuO/CeO2 catalysts is generally attributed to the strong interaction between CuO and CeO2 and the excellent oxygen storage capacity of CeO2 [18]. It was reported that the CO PROX activity of the CuO/CeO2 catalysts was much better than that of the pure CuO or CeO2 catalysts [26]. Yao et al. demonstrated the existence of the interaction between the CuO and CeO2 support by comparing the reduction of CuO and CuO/CeO2 in CO atmosphere [27]. For the CuO/CeO2 catalysts, a reversible reaction of Ce3+ + Cu2+ ↔ Ce4+ + Cu+ occurs on the surface of the catalysts [28–30]. Through the in-situ studies, Wang et al. found that the interaction between CuO and CeO2 was actually the interaction between CuO and oxygen vacancies [31]. Many studies have indicated that for the CuO/CeO2 catalysts, the surface defects and oxygen vacancies on the CeO2 support have obvious effect on the dispersion of CuO and the interaction between CuO and CeO2 . Si et al. [32] reported that the surface defects of the CeO2 support were favorable to the strong binding of CuO and the formation of the active sites (Cu-[Ox ]-Ce). Mock et al. [33] found that the rough surfaces with significant surface defects were much more efficient for the interaction between Cu species and CeO2 compared with the smooth one. Yao et al. [34] reported that the amount of defects and imperfections in the ceria appeared to determine the dispersion of CuO. In this work, in order to enhance the interaction between CuO and CeO2 and increase the specific surface area of the catalyst, the CeO2 nanocrystals with a large amount of surface lattice defects were prepared by calcining superfine Ce(OH)3 nanowires. The as-synthesized defective CeO2 nanocrystals were utilized as the support of the CuO catalysts to evaluate their catalytic activity for CO PROX. The thermogravimetry (TG) and derivative thermogravimetry (DTG) results were used to determine the calcination temperature of the precursor. The extended X-ray absorption fine structure (EXAFS), X-ray Diffraction (XRD) and N2 adsorptiondesorption were employed to study the physical property and structure of the catalysts. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to observe the morphology and surface structure information. The temperature-programmed reduction by hydrogen (H2 -TPR), UVRaman and X-ray photoelectron spectroscopy (XPS) were used to determine the chemical property of the catalysts. Thereafter, the effects of the interaction between CuO and CeO2 and the specific surface area on the catalytic activity for CO PROX were discussed.

2. Experimental 2.1. Catalysts preparation The Ce(OH)3 nanowires were adopted as the precursor of the CeO2 support, and they were prepared according to the literature [35]. Typically, 0.634 g of cerium acetate was dissolved in 80 mL of mixed solution of water and ethanol (water: alcohol = 1: 1). The mixed solution was placed in an oil bath at 140 ◦ C under mechanical stirring. 20 mL of 30% aqueous ammonia was added into the mixed solution, and a suspension liquid was formed. The oil bath was held at 140 ◦ C, and the suspension liquid was stirred and refluxed for 12 h. The obtained precipitate was centrifuged and washed with water and ethanol for several times. The Ce(OH)3 nanowire precursor was obtained by drying the precipitate in vac-

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uum at room temperature for 3 h. The precursor was calcined at 250 ◦ C for 3 h, and the obtained powder was denoted as Ce250. Then, 10% CuO in weight was loaded on the Ce250 support by an equal volume impregnation with a solution of Cu(NO3 )2 . The catalysts were calcined at 250, 400, 500 and 600 ◦ C for 3 h, and marked as CuCeX, X = 250, 400, 500 and 600, respectively. The catalyst was also calcined at 500 ◦ C for 6 h, marked as CuCe500–6 h. The Ce(OH)3 nanowire precursor was also calcined at 500 ◦ C, marked as Ce500. 10% CuO in weight was loaded on the Ce500 by equal volume impregnation with a solution of Cu(NO3 )2 . The catalyst was calcined at 500 ◦ C for 3 h, and marked as CuCe500-500. For comparison, the conventional CeO2 prepared by calcining Ce2 (CO3 )3 precursor were also adopted as the support [36]. Typically, Na2 CO3 solution (0.5 mol L−1 ) was dropwise added to an aqueous solution of cerium nitrate with a concentration of 0.4 mol L−1 to adjust the pH = 10 under magnetic stirring. The pH of the solution was monitored by pH meter, and a solution of Na2 CO3 was dropwise added to maintain the pH value at 10 for 2 h under magnetic stirring. The obtained Ce2 (CO3 )3 precipitate was aged at 60 ◦ C for 2 h. The Ce2 (CO3 )3 precipitate was filtered and washed with water and alcohol for several times. The Ce2 (CO3 )3 precipitate was dried at 60 ◦ C for 12 h and then calcined at 500 ◦ C for 3 h to obtain the CeO2 support, which was marked as Ce500-P. Finally, 10% CuO in weight was loaded on Ce500-P by an equal volume impregnation with a solution of Cu(NO3 )2 . The obtained catalyst was calcined at 500 ◦ C (marked as CuCe500-P). 2.2. Catalysts characterizations The TG experiments were performed on a Perkin Elmer Diamond thermo gravimetric analyzer from ambient temperature to 600 ◦ C with the heating rate of 10 ◦ C min−1 . The test atmosphere was air. The H2 -TPR experiments with the catalyst dosage of 30 mg were carried out on a TP-5079 TPDRO equipment (Xianquan Co. Ltd.), and the heating rate was 10 ◦ C min−1 from ambient temperature to 900 ◦ C. The gas composition was 8% H2 /N2 mixture, and the gas flow rate was 30 mL min−1 . The XRD experiments were performed on a Rigaku D/Max-2500 polycrystalline powder diffractometer using Cu K␣ (␭ = 0.15418 nm) as the radiation source. The N2 adsorption-desorption experiments were carried out on a Quantachrome QuadraSorb SI apparatus at −197 ◦ C. The sample was degassed at 200 ◦ C for 3 h prior to testing, and the specific surface area (SBET ) was calculated using the Brunauer-Emmertt-Teller (BET) method. The pore size distribution was calculated using the Barret-Joyner-Halenda (BJH) model based on the adsorption branch of the isotherm. The TEM and high resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100F system which was operated at 200 KV. The SEM image was obtained on a Hitachi S-4800 to observe the overall morphology of the sample. The UV Raman spectra were recorded on a DXR Microscope laser Raman spectrometer with a laser wavelength of 325 nm and a measurement range of 100–2000 cm−1 . The XPS experiments were performed on a PHI-1600 ESCA SYSTEM spectrometer using Al-K␣ as a radioactive source, and the energy calibration was performed using contaminated carbon (C 1s, BE = 284.6 eV) as standard. The EXAFS tests were conducted at the Beijing Synchrotron Radiation Center. The energy calibration was performed using Cu-foil as the standard. The Cu-K side absorption spectra of all the catalyst samples were collected at room temperature using fluorescence mode, and the data were analyzed by Athena software. The insitu diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) was carried out at the Nicolet Nexus spectrometer equipped with a MCT detector. The catalyst was firstly purging in N2 with flow rate of 40 mL min−1 at 200 ◦ C for 1 h. The reaction gas was 5% CO + 5% O2 + 70% H2 + balanced N2 with the flow rate of 40 mL min−1 .

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Fig. 1. SEM (a), TEM (b) and HRTEM (c, d) images of the Ce(OH)3 nanowires.

2.3. CO preferential oxidation (CO PROX) The CO PROX reactions were carried out in a fixed bed reactor. The particle size of the catalysts was between 40 and 60 meshes. The amount of the catalyst was 500 mg. The molar composition of the feeding gas was CO: O2 : H2 : N2 = 1: 1: 50: 48. The weight hourly space velocity (WHSV) was 12,000 mL g−1 h−1 . All of the components in exhaust were monitored and analyzed on-line by Agilent 7890A Gas Chromatography. Before the activity test started, the feeding gas firstly passed through the bypass line to get a stable content. Then, the activity tests were carried out in the temperature range of 50–200 ◦ C. The CO conversion (X) and O2 to CO2 selectivity (S) were calculated by the following equations: X = ([CO]in -[CO]out )/[CO]in

(1)

S = ([CO]in -[CO]out )/2([O2 ]in -[O2 ]out )

(2)

[CO] in and [CO] out represent the inlet and outlet concentration of CO, respectively. [O2 ] in and [O2 ] out represent the inlet and outlet oxygen concentration, respectively. 3. Results 3.1. SEM and TEM result of the supports The SEM image (Fig. 1a) and the TEM images (Fig. 1b–d) of the precursor of the as-synthesized CeO2 supports show that the precursor is the superfine nanowire with the length of several hundred nanometers and the diameter of 2–5 nm. The HRTEM image (Fig. 1d) clearly displays the interplanar spacing values of 0.185 nm and 0.320 nm, corresponding to (101) and (300) planes of Ce(OH)3 , respectively. Fig. 2 displays the TEM (a, e, i) and HRTEM (b, f, j) images of the Ce250, Ce500 and Ce500-P supports. Clearly, the

Ce(OH)3 nanowires were broken into the CeO2 nanocrystals when they were calcined at 250 ◦ C or 500 ◦ C. The crystallite size of the support is in the order of Ce250 < Ce500 < Ce500-P. The magnified images of the HRTEM in Fig. 2c, d, g, h, k, l show the surface lattice defects. A large amount of lattice defects are formed on the surface of the Ce250 support. The increase of the calcination temperature lessens the amount of the surface lattice defects of the Ce500 support. No obvious surface lattice defect is observed on the Ce500-P support prepared by the Ce2 (CO3 )3 precursor.

3.2. TG analysis of the precursor of the CeO2 support In order to determine the calcination temperature of the support, the TG analysis of the precursors of the CeO2 supports was carried out. Fig. 3a shows the TG and DTG curves of the Ce(OH)3 precursor. The peak of the DTG curve below 150 ◦ C belongs to the dehydration of the precursor, and the strong peak in the range of 150 − 300 ◦ C is attributed to the decomposition of Ce(OH)3 (Ce(OH)3 + 1/4O2 → CeO2 + 3/2H2 O) [35]. Moreover, the TG analysis of the Ce250 support was also performed. TG curve in Fig. S1 shows that the weight loss of the Ce250 support is less than 3%, which may be attributed to the desorption of the adsorbed species on the surface. This result indicates that the Ce(OH)3 precursor has completely decomposed after being calcined at 250 ◦ C for 3 h. Therefore, 250 ◦ C is chosen to calcine the Ce(OH)3 precursor to prepare CeO2 support. Fig. 3b shows the TG and DTG curves of the Ce2 (CO3 )3 precursor. The peak of the DTG curve below 200 ◦ C is assigned to the dehydration. The DTG peaks between 200 and 400 ◦ C are attributed to the decomposition of the surface and bulk Ce2 (CO3 )3 (Ce2 (CO3 )3 + 1/2O2 → 2CeO2 + 3CO2 ) [37,38]. The complete decomposition temperature of Ce2 (CO3 )3 exceeds 330 ◦ C. Therefore, the Ce2 (CO3 )3 precursor need higher calcination temperature to pre-

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Fig. 2. TEM and HRTEM images of the Ce250 (a, b, c, d), Ce500 (e, f, g, h) and Ce500-P (i, j, k, l) supports.

pare the CeO2 support than the Ce(OH)3 precursor. We choose 500 ◦ C to calcine the Ce2 (CO3 )3 to prepare CeO2 support. A part of the Ce(OH)3 precursor was also calcined at 500 ◦ C for comparison. 3.3. XRD Fig. 4 shows the XRD patterns of the CeO2 supports and the CuO/CeO2 catalysts. Table 1 lists the crystallite size calculated according to Full width at Half maximum of the diffraction peak of the CeO2 (111) plane (at 2␪ = 28.5◦ ) with Scherrer equation. The diffraction peaks of the supports agree well with the fluorite-type cubic CeO2 phase (JCPDS 89–8436). In Table 1, the crystallite size of the CeO2 support follows the order of Ce250 < Ce500 < Ce500-P, indicating that the calcination temperature and the preparation method can affect the crystallite size of the CeO2 support. The main diffraction peaks of the catalysts also match with the fluorite-type cubic CeO2 phase. The full width at half maximum of the CeO2 diffraction peaks of the CuCeX

Table 1 The physical properties of the samples. Catalyst

SBET (m2 g−1 )

Pore volume (cm3 g−1 )

crystallite size of CeO2 a (nm)

Ce250 Ce500 Ce500-P CuCe250 CuCe400 CuCe500 CuCe600 CuCe500-500 CuCe500-P CeCe500-6h

166 92 66 142 115 97 56 60 24 92

0.371 0.205 0.082 0.188 0.230 0.159 0.168 0.094 0.069 0.126

4.6 6.1 9.2 4.8 4.9 6.0 7.4 7.9 12.0 6.3

a

Determined from the XRD results in Fig. 4 with Scherrer equation.

catalysts gradually narrows with the increase of the calcination temperature of the catalyst, indicating the gradual enlarged crystallite size of the CeO2 support in the catalyst. The crystal-

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Fig. 4. XRD patterns of the (1) Ce250, (2) Ce500, (3) Ce500-P, (4) CuCe250, (5) CuCe400, (6) CuCe500, (7) CuCe600, (8) CuCe500-500 and (9) CuCe500-P samples.

Fig. 3. TG-DTG curves of (a) the Ce(OH)3 nanowire and (b) the Ce2 (CO3 )3 precursor.

lite size of the CeO2 support in the catalyst follows the order of CuCe250 < CuCe400 < CuCe500 < CuCe600 < CuCe500–500 < CuCe500P (Table 1). The inset in Fig. 4 displays that two weak diffraction peaks belonging to CuO (JCPDS 80–1268) at 2␪ of 35.6◦ and 38.7◦ appear in the XRD patterns of the CuCe500, CuCe600, CuCe500-500 and CuCe500-P catalysts. There is no obvious CuO diffraction peak in the XRD patterns of the CuCe250 and CuCe400 catalysts. The XRD results indicate that a high calcination temperature will also lead to the growth of the CuO crystallites. 3.4. Nitrogen adsorption-desorption Table 1 gives the specific surface areas of the samples. The Ce250 support shows the largest specific surface areas of 166 m2 g−1 . Compared with the Ce250 support, the higher calcination temperature of 500 ◦ C decreases the specific surface area of the Ce500 support (92 m2 g−1 ). The Ce500-P support has a minimum specific surface area of 66 m2 g−1 . In Table 1, the specific surface areas of the catalysts with the same calcination temperature at 500 ◦ C decreases with the decrease of the specific surface area of the CeO2 support (CuCe500 > CuCe500–500 > CuCe500-P). Moreover, with the increase of the calcination temperature, the specific surface areas of the CuCeX catalysts gradually decreases from 142 (CuCe250) to 57 (CuCe600) m2 g−1 .

Fig. S2 and S3 show the nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distribution curves of the supports and the catalysts, respectively. The hysteresis loops of the N2 adsorption-desorption isotherms indicate the mesoporous feature of the samples. Fig. S2b shows that there are a lot of 1.5–30 nm of porous in the Ce250 support. After calcination at 500 ◦ C, the pores in the Ce500 support are reduced. There are few pores in the Ce500-P support. Fig. S3b shows that few pores exist in the CuCe500-P catalyst. With the increase of the calcination temperature, in the CeCuX catalysts, the pores with the size smaller than 5 nm gradually decreased, and some pores with the size larger than 10 nm are formed. 3.5. TEM of the catalyst The TEM characterization was carried out to observe the morphology of the CuO/CeO2 catalysts. Fig. 5 and Fig. S4 show the HRTEM and TEM images of the catalysts, respectively. Most of the interplanar spacing of the crystallites in the catalysts corresponds to the (111) and (110) planes of CeO2 . For the catalysts using the Ce250 as support, the size of the CeO2 crystallite in the catalysts enlarges with the increase of the calcination temperature of the catalysts. The CeO2 crystallite size of the catalysts calcined at 500 ◦ C is in the order of CuCe500 < CuCe500–500 < CuCe500-P. The crystallite size is consistent with the results calculated by XRD. TEM-EDS elemental mapping images of the CuCe500 catalyst are inset in Fig. 5c. Both the Cu and Ce element are homogeneously distributed in the catalysts. No CuO crystallite is found in all of the TEM images, indicating that copper species in the catalysts are highly dispersed on the surface of the CeO2 support or/and may be partly incorporated into the CeO2 lattice to form solid solution [39,40]. Fig. 5a–d shows that there are a large amount of lattice defects on the surface of the CuCeX catalysts. It may be because the Ce250 support has a lot of the surface lattice defects. The lattice defects on the surface of the CuCeX catalyst gradually decrease with the increase of the calcination temperature of the catalyst. The CuCe500-500

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Fig. 5. HRTEM images of the (a) CuCe250, (b) CuCe400, (c) CuCe500, (d) CuCe600, (e) CuCe500-500 and (f) CuCe500-P catalysts.

catalyst has fewer surface lattice defects than the CuCeX catalysts. It is probably because of the higher calcination temperature of the Ce500 support than that of the Ce250 support. The CuCe500-P catalyst has few surface lattice defects compared with the CuCeX and CuCe500-500 catalysts. 3.6. EXAFS In order to obtain the local coordination structural information of Cu species in the catalysts, all of the catalysts were characterized by EXAFS. Fig. 6 shows the radial structure functions (RSFs) obtained from the EXAFS spectra of Cu K-edge. In Fig. 6, the main coordination peaks of the catalysts are consistent with that of the standard CuO reference, indicating that the Cu species in the cata-

lysts are mainly CuO. The coordination peaks at 0.15 and 0.25 nm correspond to the first Cu-O shell and Cu–Cu shell of CuO, respectively [41–43]. The peak of the CuCeX catalysts corresponding to Cu–Cu shell at 0.25 nm gradually increases with the increase of the calcination temperature of the catalyst, indicating the increase of the Cu–Cu coordination number. Therefore, the RSFs results are the explicit proof that the increase of the calcination temperature leads to the growth of the CuO crystallites in the CuCeX catalysts. This result is consistent with the previous XRD result in Fig. 4. 3.7. CO PROX performance of the catalysts Fig. 7 and Table S1 present the catalytic performance of the catalysts for CO PROX reaction in the temperature range of 50–200 ◦ C.

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Fig. 6. RSFs of Cu K-edge of the (1) CuCe500-P, (2) CuCe500-500, (3) CuCe600, (4) CuCe500, (5) CuCe400, (6) CuCe250, (7) CuO and (8) Cu2 O samples.

Fig. 8. H2 -TPR profiles of the (1) CuCe500-P, (2) CuCe250, (3) CuCe400, (4) CuCe500500, (5) CuCe500 and (6) CuCe600 catalysts.

The CO conversion over the catalysts at 60 ◦ C is lower than 20%. The specific reaction rate in surface area at 60 ◦ C (denoted as r60 ) is calculated, and listed in Table S1. The r60 over the CuCeX catalysts gradually increase with the increase of the calcination temperature. The r60 over the catalysts with the same calcination temperature at 500 ◦ C is in the order of CuCe500 > CuCe500–500 > CuCe500-P. 3.8. H2 -TPR

Fig. 7. CO conversion (䊏) and O2 to CO2 selectivity (䊉) during CO PROX over the (1) CuCe500-P, (2) CuCe500-500, (3) CuCe250, (4) CuCe400, (5) CuCe500 and (6) CuCe600 catalysts.

Compared with the CuCe500-P catalyst using the conventional CeO2 as support, the CuCeX and CuCe500-500 catalysts using the CeO2 nanocrystals derived from nanowire precursor as support obviously enhance the catalytic performance. The temperature window to match 100% CO conversion is defined as T100-100 in Table S1. T50 and T100 represent the temperature of 50 and 100% CO conversion, respectively, which are also listed in Table S1. T100-100 and T100 are adopted to evaluate the performance of the catalysts. The CuCe500 has the widest T100-100 . For the CuCeX catalysts, T100 over the CuCeX catalysts decrease and then elevate with the increase of the calcination temperature of the catalysts. The CuCe500 catalyst presents the best activity with the lowest T50 and T100 at 70 and 100 ◦ C, respectively. Compared with the CuCe500 catalyst, the CuCe500-500 catalyst with a higher calcination temperature of the CeO2 support at 500 ◦ C shows the lower activity with higher T50 and T100 . Moreover, we find an interesting result that the O2 to CO2 selectivity over the CuCeX and CuCe500-500 catalysts is very similar, and it begins to decrease at nearly the same temperature (about 130 ◦ C).

The redox properties of the catalysts were analyzed by H2 TPR. The reduction peaks of the CuO/CeO2 catalysts can be divided into two regions. The reduction peaks below 300 ◦ C belong to the reduction of CuO, while the peak above 300 ◦ C is attributed to the reduction of CeO2 [17,18]. Fig. 8 shows the H2 -TPR profiles of the catalysts in the range of 100–300 ◦ C. The reduction profiles of the CuCeX and CuCe500-500 catalysts can be divided into two peaks, denoted as ␣ and ␤, respectively. Three reduction peaks can be observed in the H2 -TPR profile of the CuCe500-P catalyst, denoted as ␣, ␤ and ␥, respectively. Generally, the peak ␣ is attributed to the reduction of the highly dispersed CuO with a strong interaction with the CeO2 support [44,45], marked as CuO-␣. As suggested by XRD and TEM results in Figs. 4 and 5, most of Cu species are highly dispersed on the catalysts. Therefore, we attribute the peak ␤ to the reduction of the highly dispersed CuO which weakly interacts with CeO2 [44,45], marked as CuO-␤. The peak ␥ is attributed to the reduction of bulk CuO which does not interact with CeO2 [45], marked as CuO-␥. The position of the peak ␣ in the H2 -TPR profile can evaluate the strength of the interaction between CuO and CeO2 in the catalysts, the lower reduction temperature of the peak ␣ means the stronger interaction between CuO and CeO2 . The temperature of peak ␣ of the CuCeX catalyst gradually decreases with the increase of the calcination temperature, indicating that the interaction between CuO and CeO2 is gradually enhanced. The strength of the interaction between CuO and CeO2 are consistent with the result of the r60 , which indicates that the intrinsic activity is obviously affected by the strength of the interaction between CuO and CeO2 . After comparison of the position of the peak ␣, we can conclude that the strength of the interaction between CuO and CeO2 in the catalysts calcined at 500 ◦ C is in the order of CuCe500 > CuCe500–500 > CuCe500-P. Table S2 gives the results of the quantitative analysis of H2 -TPR. For the CuCeX catalysts, the area of peak ␣ gradually decreases when the calcination temperature increases. It indicates

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Table 2 The UV-Raman, XPS of O 1s, Auger of Cu LMM, XPS of Ce 3d and surface Cu content results of the catalysts. Catalyst

I581 /I450 a

Olatt (%)b

Cu+ (%)c

Ce3+ (%)d

Cu/(Cu + Ce)e

CuCe250 CuCe400 CuCe500 CuCe600 CuCe500-500 CuCe500-P

0.61 0.61 0.47 0.39 0.47 0.33

49.1 50.7 54.8 45.9 48.2 38.3

63.6 75.3 92.5 88.3 75.7 86.8

14.3 11.6 9.7 7.9 9.4 6.1

0.423 0.273 0.293 0.315 0.247 0.234

a b c d e

The intensity ratio of the Raman peak at 581 and 450 cm−1 . The lattice oxygen content determined according to the XPS of O 1s result. The surface Cu+ content determined according to the Auger of Cu LMM result. The surface Ce3+ content determined according to the XPS of Ce 3d result. The surface Cu content determined according to the XPS result.

the decrease of CuO-␣. Probably, the increase of the calcination temperature induces the aggregation of a part of CuO-␣. Additionally, the total hydrogen consumption for the reduction from Cu2+ to Cu0 is higher than the theoretical value, indicating that part of surface CeO2 has also been reduced due to the H2 spilling effects [45]. Moreover, the amount of H2 uptake also decreases with the increase of the calcination temperature. We deduce that the increase of the calcination temperature leads to the decreased amount of the defects in the ceria support which makes the reduction of the ceria support more difficult. For the catalysts with different supports, the area of the peak ␣ is in the order of CuCe500 > CuCe500–500 > CuCe500-P.

3.9. UV-Raman analysis In order to obtain the surface information of the CuO/CeO2 catalysts, the UV-Raman characterization was performed for all of the catalysts. As shown in Fig. 9a, the band centered at 450 cm−1 in the Raman spectra of the catalysts is attributed to the triply degenerate F2g symmetric vibration (Ce O Ce stretching). Generally, it locates at near 464 cm−1 for the pure cubic fluorite CeO2 [17]. The shift indicates the change of the surface crystal lattice parameter of the CeO2 support [46]. Two broad Raman peaks centered at 581 cm−1 and 1171 cm−1 indicate that a large amount of oxygen vacancies are formed on the surface of the catalysts [47]. The relative amount of oxygen vacancies can be expressed as intensity ratio of the bands locating at 581 and 450 cm−1 (noted as I581 /I450 ) [48], which can also indicate the relative amount of the surface lattice defect, and the results are summarized in Table 2. The I581 /I450 ratios of the CuCe250 and CuCe400 catalysts are very close, indicating that the amount of the oxygen vacancies in the two catalysts is nearly the same. The I581 /I450 ratios suggest that the amount of the oxygen vacancies on the surface of the catalysts using the Ce250 as support gradually decreases with the increase of the calcination temperature when the calcination temperature exceeds 400 ◦ C. This result indicates that the surface lattice defects of the CuCeX catalysts decrease with the increase of the calcination temperature, which is consistent with the result of the TEM (Fig. 5). As suggested by the I581 /I450 ratio, both the CuCe500 and CuCe500500 catalysts with the same calcination temperature and the same precursor of the CeO2 support have the similar amount of oxygen vacancies. The amount of oxygen vacancies in the CuCe500-P catalyst is less than that in the CuCe500 and CuCe500-500 catalysts, indicating that the catalysts prepared by the nanowire precursor have much more surface lattice defects than the catalyst prepared by the Ce2 (CO3 )3 precursor. The surface lattice defects of the CeO2 supports also were characterized by UV-Raman (Fig. 9b), corresponding to the I593 /I455 ratios. According to the I593 /I455 ratios in Table 3, the amount of the surface lattice defects on the surface of the support follows the

Fig. 9. UV-Raman spectra of (a) the (1) CuCe500-P, (2) CuCe500-500, (3) CuCe250, (4) CuCe400, (5) CuCe500 and (6) CuCe600 catalysts and (b) the (1) Ce500-P, (2) Ce500 and (3) Ce250 supports.

Table 3 The UV-Raman results of the supports. Support

I593 /I455 a

Ce250 Ce500 Ce500-P

0.96 0.58 0.37

a

The intensity ratio of the Raman peak at 593 and 455 cm−1 .

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Fig. 10. XPS spectra of O 1s of the (1) CuCe500-P, (2) CuCe500-500, (3) CuCe250, (4) CuCe400, (5) CuCe500 and (6) CuCe600 catalysts.

order of Ce250 > Ce500 > Ce500-P. This result is consistent with the TEM result. 3.10. XPS and Auger spectra analysis The XPS characterization was performed to obtain the chemical state and composition information on the surface of the catalysts. The XPS spectra in the O 1s region can be deconvoluted into two peaks (peak ␣ and ␤) (Fig. 10). The binding energy and the full width at half maximum (FWHM) of each peak is listed in Table S3. The peak ␣ with high binding energy (HBE) corresponds to the oxygen species apart from lattice oxygen, and the peak ␤ with low binding energy (LBE) corresponds to the lattice oxygen (denoted as Olatt ) [48]. Wang and Holgado et al. reported that the binding energy of the lattice oxygen in Ce (IV) oxide was about 529.6 eV [49,50]. The binding energy of the other oxygen species on the surface of ceria, such as hydroxyl groups on the surface, oxygen chemisorbed on the surface, grain-boundary impurities and oxide ions in the defective CeOx (x < 2) was about 531–533 eV [51]. Some researcher reported that the binding energy of the lattice oxygen in CuO was about 529.3-529.6 eV [52,53], and the binding energy of the lattice oxygen in Cu2 O was about 530.3 eV. [54] The binding energy of the oxygen adsorbed on the surface of CuO or Cu2 O was about 531.5 eV. [55,52] In this work, the binging energy of the peak ␤ is higher than that of lattice oxygen in CuO and ceria, indicating that Cu2 O which has higher binging energy of the lattice oxygen than CuO and ceria is formed. The FWHM of the peak ␣ of the CuCe500-P catalyst is larger than that of the other samples, indicating that the CuCe500P catalyst probably has more kinds of surface adsorbed oxygen. The Olatt species are generally considered to be the active oxygen [18]. The content of the Olatt is concluded according to the peak area and is listed in Table 2. The results show that the content of the Olatt in the CuCeX and CuCe500-500 catalyst is about 50%, and the CuCe500-P catalyst has the lowest Olatt content. Fig. 11a shows the XPS spectra in the Cu 2p region of the catalysts in which the peak at binding energy of 933.5 eV corresponds to Cu 2p3/2 , and the peak at binding energy of 953.5 eV corresponds to Cu 2p1/2 [25,56–58]. The shake-up peaks in the range of 936–950 eV suggest the existence of Cu2+ ions [56,57]. According to the previous reports, the binding energy of well-dispersed Cu2+ ions is at about 934.9 eV, and the lower binding energy generally indicates the presence of Cu+ ions and/or Cu0 species [25,58]. All of the catalysts have the weak shake-up peaks, indicating that there is a small amount of Cu2+ ions on the surface of the catalyst. Compared with other cata-

Fig. 11. XPS spectra of Cu 2p (a) and Auger spectra of Cu LMM (b) of the (1) CuCe500P, (2) CuCe500-500, (3) CuCe250, (4) CuCe400, (5) CuCe500 and (6) CuCe600 catalysts.

lysts, the Cu 2p3/2 peak of the CeCu250 catalyst shifts toward higher binding energy and the shake-up peak of the CeCu250 catalyst is more obvious, indicating that more Cu2+ ions exist on the surface of the CeCu250 catalyst. The binding energy of Cu 2p3/2 peak of the catalysts excluding the CuCe250 catalyst is very close, indicating that the Cu species on the surface of the catalyst are similar. The Cu2+ and Cu+ ions can not be distinguished in the XPS spectra in the Cu 2p region in this work [58,59]. In order to further understand the chemical state of Cu species on the surface of the catalyst, the Auger lines of Cu LMM (Fig. 11b) were measured to distinguish Cu2+ and Cu+ ions. Generally, the peak at 917.5 and 915.0 eV correspond to Cu2+ ions and Cu+ ions, respectively [60], and the Cu LLM peak of Cu0 is at about 918.6 eV [61]. So there is no Cu0 on the surface of the catalysts. Moreover, the result of in-situ DRIFTS also indicates that Cu+ ions exist in the catalyst (in Fig. S5). The peaks at 910.3 and 923.0 eV correspond to different Auger transition levels [60]. The relative content of Cu+ ions was calculated according to the ratio of the peak area of Cu+ ions to the whole peak area, and

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Fig. 12. XPS spectra of Ce 3d of the (1) CuCe500-P, (2) CuCe500-500, (3) CuCe250, (4) CuCe400, (5) CuCe500 and (6) CuCe600 catalysts.

the results were listed in Table 2. The results indicate that most of Cu species on the surface of the catalysts are Cu+ ions (from 64% to 92%). The high concentration of Cu+ ions on the surface mainly results from the reduction of Cu2+ under the XPS measurement condition (high vacuum, strong excitation due to X-ray) [61]. Additionally, a reversible reaction of Ce3+ + Cu2+ ↔ Ce4+ + Cu+ probably occurs on the surface of the CuO/CeO2 catalyst, as well [28–30]. Fig. 12 shows the Ce 3d XPS spectra of the samples, which can be divided into eight peaks corresponding to four pairs of Ce3d spin-orbit doublets. The u (u-u”’) represents Ce 3d3/2 spin-orbit components and v (v-v”’) represents Ce 3d5/2 spin-orbit components. The peaks of u’ and v’ correspond to the 3d3/2 level and 3d5/2 level of Ce3+ respectively, and other peaks are assigned to Ce4+ species [62]. The relative content of Ce3+ can be calculated by the ratio of the peak area of u’ and v’ to the total peak area of Ce 3d and the result are listed in Table 2 [18,63].The surface Ce3+ is resulted from the surface defects. The surface Ce3+ content decreases with the increase of the calcination temperature of the CuCeX catalysts and is in the order of CuCe500 > CuCe500–500 > CuCe500-P for the catalysts with the different supports, which may be due to the decrease of the surface defect. Moreover, the surface atomic ratio of Cu:Ce is calculated from the XPS result and listed in Table 2. The surface Cu content is not in consistent with the result of the CO PROX activity, and thus we can conclude that the surface Cu content may not the key factor to affect the performance of the catalysts. 4. Discussion With the increase of the calcination temperature of the CuCeX catalysts, the nitrogen adsorption-desorption results indicate that the specific surface area of the CuCeX catalyst gradually decreases from 142 (CuCe250) to 56 (CuCe600) m2 g−1 ; the XRD and EXAFS results confirm that the size of the CuO crystallites in the catalysts gradually increases; and the UV-Raman results indicate that the oxygen vacancies on the surface of the catalysts, which is favorable to the mobility of the lattice oxygen and the enhancement of the catalytic activity [18], gradually decrease. Apparently, the change of these factors is negative to the activity of the catalyst for CO PROX. The H2 -TPR results suggest that with the increase of the calcination temperature of the catalyst, the strength of the interaction between CuO and CeO2 is enhanced. Although some of the factors

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is negative, the apparent catalytic activity for CO PROX over the CuCeX catalysts increases with the increase of the calcination temperature of the catalysts until 500 ◦ C. Therefore, we conclude that the effect of the strength of the interaction between CuO and CeO2 on the catalytic activity is more significant than those of the other factors. The apparent catalytic activity of the CuCe500 catalyst is higher than that of the CuCe600 catalyst. It is attributed that the increase of the size of the CuO nanocrystal and the decrease of the specific surface area and the amount of the oxygen vacancies on the surface of the CuCe600 catalyst lower the catalytic activity. In this work, the activity and selectivity of the catalyst using the different CeO2 supports are obviously affected by both of the strength of the interaction between CuO and CeO2 and the specific surface area of the CeO2 support. The r60 , which eliminate interference of the specific surface area of the catalyst, is in the order of CuCe500 > CuCe500–500 > CuCe500-P. It is consistent with the strength of the interaction between CuO and CeO2 as indicated by the H2 -TPR results. The HRTEM images of the support in Fig. 2 shows that the surface lattice defects of the CeO2 support prepared by calcining the Ce(OH)3 nanowire precursor are much more than that on the surface of the CeO2 support prepared by calcining the Ce2 (CO3 )3 precursor. Additionally, both the Ce250 and Ce500 supports was prepared from the precursor of the Ce(OH)3 nanowires, but the CuCe500 and CuCe500-500 catalysts, both of which were calcined at 500 ◦ C, showed the different strength of the interaction between CuO and CeO2 , as well as the specific reaction rate of r60 . In Fig. 2, compared with the Ce500, a larger amount of surface lattice defects exists on the Ce250 support. Thus, it indicates that the presence of the surface lattice defects on the CeO2 support can strengthen the interaction between CuO and CeO2 during the catalyst preparation procedure, resulting in an improved catalytic activity. Compared with the Ce500 support, the lower calcination temperature of the Ce250 support leads to the larger specific surface area of the support. The larger specific surface area is beneficial for the dispersion of CuO, and it is favorable to the formation of the strong interaction between CuO and CeO2 . Moreover, the specific surface area of the CuCe500 catalyst (97 m2 g−1 ) is larger than that of CuCe500-500 (60 m2 g−1 ). In order to determine the reason for the difference of the specific surface area, we extend the calcination time of the CuCe500 catalyst to 6 h at 500 ◦ C, and the catalyst is denoted as CuCe500–6 h. Although the CuCe500–6 h and CuCe500-500 catalysts have the same catalyst calcination temperature and time for CeO2 , the specific surface area of the CuCe500–6 h catalyst (92 m2 g−1 ) is still larger than that of CuCe500-500 (60 m2 g−1 ). It indicates that the strong interaction between CuO and CeO2 can inhibit the sintering of the CeO2 support. Generally, smaller nanocrystals have a larger growth trend during the calcination process. In this work, a different trend is observed. In Table 1, the CeO2 nanocrystals (about 9.2 nm) in Ce500-P support is larger than that (about 6.1 nm) in the Ce500 support. However, the CeO2 nanocrystals in the Ce500 support, which grow from 6.1 (Ce500) to 7.9 nm (CuCe500-500), does not show more obvious growth trend than that in the Ce500-P support, which grow from 9.2 (Ce500-P) to 12.0 nm (CuCe500-P). The change of the specific surface area can give the same conclusion. The decrease of specific surface area from 66 (Ce500-P) to 24 m2 g−1 (CuCe500-P) is more obvious than that from 92 (Ce500) to 60 m2 g−1 (CuCe500-500). We deduce that the more amounts of pores and the stronger interaction between CuO and CeO2 in the CuCe500-500 catalyst than that in CuCe500-P catalyst may inhibit the sintering of the CeO2 support.

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5. Conclusion The Ce250 supports prepared by calcining the superfine Ce(OH)3 nanowire precursor at 250 ◦ C displays much more surface lattice defects and higher specific surface area than the Ce500-P support prepared by calcining the Ce2 (CO3 )3 precursor. The lattice defects on the surface of the support obviously enhance the strength of the interaction between CuO and CeO2 in the CuO/CeO2 catalysts during the catalyst preparation. The increase of the calcination temperature of the CuCeX catalysts using Ce250 as the support enhances the strength of the interaction between CuO and CeO2 . The catalytic results for CO PROX over the catalysts indicate that the strength of the interaction between CuO and CeO2 is the key factor to affect the catalytic activity and selectivity of O2 to CO2 . The CuCeX and CuCe500-500 catalysts show obviously higher catalytic activity than the CuCe500-P catalyst. The activity of the CuCeX catalysts elevates and then decreases with the increase of the calcination temperature of the catalysts. The CuCe500 catalyst shows the highest apparent catalytic activity with the widest T100-100 and the lowest CO full convention temperature, mainly resulting from the strong interaction between CuO and defective CeO2 support and the relativity large surface area. Acknowledgements This work is financially supported by National Natural science foundation of China (No. 21476159, 21476160), the Natural Science Foundation of Tianjin (No. 15JCZDJC37400, 15JCYBJC23000), the 973 program (2014CB932403), and the Program for Introducing Talents of Discipline to Universities of China (No.B06006). We gratefully acknowledge Beijing Synchrotron Radiation Facility (BSRF) for the assistance in the XAFS experiment. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.06. 017

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