Metal-organic-framework derived controllable synthesis of mesoporous copper-cerium oxide composite catalysts for the preferential oxidation of carbon monoxide

Fuel 229 (2018) 217–226

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Full Length Article

Metal-organic-framework derived controllable synthesis of mesoporous copper-cerium oxide composite catalysts for the preferential oxidation of carbon monoxide

T

Xia Gonga, Wei-Wei Wangc, Xin-Pu Fuc, Shuai Weic, Wen-Zhu Yuc, Baocang Liua,b, ⁎ ⁎ Chun-Jiang Jiac, , Jun Zhanga,b, a

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, PR China Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China b c

G R A P H I C A L A B S T R A C T

A facile MOFs-derived controllable strategy was developed to construct highly active CuxCe1−xO2 catalysts through directly annealing CuxCe1−x-BTC MOFs under different temperatures for CO-PROX reaction.

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-organic-frameworks Surface active species Copper-cerium oxide composite CO-PROX

Among currently studied catalysts, CuO-CeO2 based materials hold the greatest promise for the preferential oxidation of CO (CO-PROX). Recently, many efforts have been concentrated on developing the original nanostructures inherited from metal-organic-frameworks (MOFs), which are considered to be excellent sacrificial templates or precursors to achieve metal oxide (or metal) nanoparticles with unique structure. In this paper, we synthesized CuO-CeO2 catalysts using an efficient and general strategy derived from CuxCe1−x-BTC MOFs after high temperature treatment. The as-prepared CuO-CeO2 catalysts display variable morphologies, crystal structures, and specific surface areas based on different ratios of Cu/Ce and calcination temperature. The catalytic performance shows that all CuO-CeO2 composite catalysts derived from the CuxCe1−x-BTC MOFs via heat treatment exhibit excellent catalytic performance for the CO-PROX reaction, and the Cu0.3Ce0.7O2 is the most active catalyst obtained under high calcination temperature at 650 °C for 4 h, demonstrating that the increase of Cu content and high temperature treatment can create more highly dispersed CuO clusters, which is in favor of

⁎

Corresponding authors at: College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China (J. Zhang). E-mail addresses: [email protected] (C.-J. Jia), [email protected] (J. Zhang).

https://doi.org/10.1016/j.fuel.2018.04.071 Received 15 January 2018; Received in revised form 20 March 2018; Accepted 14 April 2018 0016-2361/ © 2018 Published by Elsevier Ltd.

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the CO-PROX reaction. Meanwhile, the in-situ DRIFTS results show that the Cu0.3Ce0.7O2 catalyst displays the super CO adsorption capability, which induces the difference of catalytic performance for the CO-PROX reaction.

1. Introduction

2. Experimental section

Cerium dioxide (CeO2) is often used to support transition or noble metals and provides distinctive catalytic functionality due to its typical capacity of shifting between Ce4+ and Ce3+ and the production of oxygen vacancy defects simultaneously, which are considered to be critical factors for its application in heterogeneous catalysis [1–4]. As we all know that the catalytic activity can be dramatically increased with Au and Pt species supported on CeO2 for preferential oxidation of CO (CO-PROX), water-gas shift, CO oxidation and oxidation of toluene [5–7]. However, the high cost, poor durability, and low resistance to toxicity of Au and Pt species significantly hinder its large-scale commercial application. In recent years, the development of non-noble metal catalysts with excellent performance has received extensive attention. Among the reported non-noble metal catalysts for various catalytic reactions, ceria-supported CuO catalysts have been regarded as promising candidates due to their low cost, environmental friendliness, and outstanding thermal stability. Landi et al. synthesized CeO2/ CuO wash coated monoliths catalysts by a modified dip coating procedure [8], which presented a good catalytic performance for the CO–PROX reaction. Guo et al. prepared CuO/CeO2 catalysts via an ethanol-thermal method for the CO-PROX reaction and the catalysts exhibited the excellent catalytic performance. Meanwhile, they investigated the synergistic effect between copper and ceria [9]. Metal–organic frameworks (MOFs) are constituted by the self-assembly of inorganic metal and organic linkers and have drawn much attention in recent years [10–12]. To date, thousands of MOFs and their diverse assemblies formed by a large variety of metal ions and organic linkers have been reported and their structures are not only abundant and intriguing but also designable and tailorable.[13–15]. In addition to their diverse structures and compositions, MOFs possess high surface area, uniform and tunable porosity, and multi-functionality compared with traditional microporous and mesoporous materials. These attractive features make MOFs very promising to be used as precursors and templates for synthesizing hierarchical porous materials for many applications in electrocatalytic oxygen reduction reaction (ORR) [16], Fischer–Tropsch reaction [17], and CO oxidation [18]. Maiti et al. reported a facile method to prepare the nanostructured CeO2 using [Ce (1,3,5-BTC)(H2O)6] MOFs as a precursor [19]. Zhang et al. synthesized an efficient CuO/Cu2O catalyst with specific morphology, crystalline phase, and composition derived from Cu–BTC MOFs [20], which exhibited a high performance for CO oxidation. In this work, we constructed a series of surface state-finely controlled CuO-CeO2 catalysts using CuxCe1−x-BTC MOFs (BTC = 1,3,5benzenetricarboxylic acid) as precursors. The interface of CuO-CeO2 catalysts is maximized by using MOFs as templates, which is in favor of enhancing the catalytic performance for CuO-CeO2 catalysts [21]. The porous structured CuO-CeO2 catalysts composed of uniformly dispersed CuO and CeO2 nanoparticles are achieved by thermolysis of CuxCe1−xBTC MOFs. The synthetic approach is surfactant-free and scalable at a low cost. The optimum porous structured Cu0.3Ce0.7O2 catalysts show favorable catalytic performance for the CO-PROX reaction. Through calcining the Cu0.3Ce0.7-BTC MOFs precursor under different temperatures from 450 °C to 750 °C, the optimum Cu0.3Ce0.7O2-650 catalyst with the highest CO conversion (85% at 80 °C) for the CO-PROX reaction can be obtained. The H2-TPR, Raman, and in situ DRIFTS results suggest that the reducibility, oxygen vacancy, and Cu+ active site of CuO-CeO2 catalysts have greatly influence on the catalytic performance for CO-PROX reaction.

2.1. Preparation of CuO-CeO2 catalysts A low temperature solvothermal method was used to synthesize CuxCe1−x-BTC frameworks. Briefly, 2 mmol of Cu(NO3)2·3H2O and Ce (NO3)3·6H2O were added into 8 mL of deionized water and 12 mL of ethanol. Then, 2 mmol of benzene-1,3,5-tricarboxylic acid (H3BTC) were added into 12 mL of dimethylformamide (DMF) and subsequently poured into the above mixed solution, continuously stirred for several minutes to form the uniform solution. Then, the mixed solution was transferred into stainless steel reactor with a polytetrafluoroethylene liner at 80 °C for 24 h. Finally, the precipitate was collected and washed using ethanol and DMF, and then dried under oven for 3 days at 130 °C. The powder was calcined at 550 °C for 4 h to obtain CuO-CeO2 catalysts.

2.2. Characterization of CuO-CeO2 catalyst The nitrogen adsorption-desorption test was carried out by a Builder SSA-4200 instrument. XRD was performed on a PANalytical X’Pert3 diffractometer. Raman spectra were measured by Raman microscope system. XPS measurements were finished using VG Scientific ESCALAB Mark II spectrometer. H2-TPR was performed in a Builder PCSA-1000 instrument. N2O chemisorption was performed to determine the dispersion of CuO for CuO-CeO2 samples. (The detail information was listed in Supporting Information) The in-situ DRIFTS experiments were carried out using a Bruker Vertex 70 FT-IR spectrometer. “CO adsorption” experiment was performed to investigate the process of CO adsorption over CuO-CeO2 catalysts. Prior to experiment, the catalysts (10 mg) were activated by in situ calcination at 300 °C for 30 min under synthetic air and then cooled to a certain temperature (80 °C) under pure N2 (30 mL·min−1). Then the background spectrum was collected via 32 scans at 4 cm−1 resolution. Finally, the catalyst was exposed to the feed gas (1% CO + 20% O2 in N2) inside the DRIFTS cell, and then the DRIFTS spectra were continuously collected for 30 min at 80 °C. DRIFT spectra were collected by using OPUS software.

2.3. Catalytic tests of CuO-CeO2 catalysts The catalytic tests were performed in a plug-flow reactor under the CO PROX reaction. Then, the catalyst (50 mg) was loaded into the reactor under the feed stream (1.0% CO, 1.0% O2 and 50% H2 (N2 balanced)) with the gas flow rate of 50 mL min−1. The catalyst was pretreated under synthetic air (21% O2/79% N2) at 300 °C for 30 min for activation. The compositions of the effluent streams were analyzed employing an online gas chromatograph (GC9160 series) with a thermal conductivity detector (TCD) and the sensitivity for CO is about 3840 mV·mL/mg. The CO conversion and CO2 selectivity were calculated according to the following equations:

CO conversion(%): CCO = (nCO,in−nCO,out)/nCO,in × 100

CO2 selectivity(%): SCO2 = 1/2 × (nCO,in−nCO,out)/(nO2,in−nO2,out) × 100 where nCO, and nO2, are represented peak area values of GC response CO and O2, respectively, before and after reaction.

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3. Results and discussion

the preparation process, the dispersed CuO may be separated out from the bulk CuO-CeO2 catalysts to the surface of the catalysts under the high temperature calcination, which is in favor of the CO-PROX reaction. And the peak at 178 °C can be ascribed to the reduction of CuO species incorporated into the CeO2 lattice (β) (Cu-O-Ce species) [35–37], suggesting that the copper species are partially introduced into CeO2 lattice due to the heat treatment at high temperatures when the content of copper species is more than 20% in the catalysts, which is in agreement with the XRD and Raman results. While the third sharp peak at higher temperature can be assigned to the reduction of bulk CuO (γ) [38], according to the results of XRD analysis. The α peak positions of Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 composite catalysts show no big difference, suggesting that the well-dispersed copper species are quite similar. However, when the Cu content is decreased, the α peak positions are shifted to the higher temperature and are overlapped with the β peaks at 180 °C simultaneously for Cu0.1Ce0.9O2 and Cu0.2Ce0.8O2 catalysts. And the γ peak positions are almost similar for all catalysts and the peak areas increase with the increase of copper content.

3.1. Catalyst characterization Fig. 1 displays the XRD patterns of CuO-CeO2 catalysts generated using CuxCe1−x-BTC MOF as a precursor. The XRD pattern of the CuxCe1−x-BTC MOF is shown in Fig. S1. It is clear all CuO-CeO2 catalysts show diffraction peaks of fluorite CeO2 (JCPDS #43-1002) and CuO species (JCPDS #80-1268). It is noticed that the crystallite size of CeO2 decreases from 10.5 to 8.8 nm with the increase of Cu content based on the estimation from the (111) diffraction peak (2θ = 28.549°) using Scherrer equation (Table 1). On the other hand, the intensities of CuO diffraction peaks become stronger progressively with the increase of Cu contents, indicating the formation of numerous CuO particles. Additionally, compared with other catalysts, the location of diffraction peaks of Cu0.3Ce0.7O2 catalyst can be observed to shift to lower 2θ angles (Fig. S2), suggesting the incorporation of Cu ions into CeO2 lattice by partially substituting Ce4+ [22–24]. Fig. 2a shows the morphology of the as-synthesized Cu0.3Ce0.7O2 catalyst. The Cu0.3Ce0.7O2 catalyst inherits the morphology of CuxCe1−x-BTC precursor (Fig. S3). Well-defined aligned nanobars are formed with sharp edges. A magnified TEM image (the inset of Fig. 2a) shows the rough surface of the nanobars, which may be attributed to the formation of mesoporosity during the thermal treatment. Additionally, the TEM images in Fig. 2b displayed that the nanobars are constituted by multiple small nanoparticles. This type of structure would generate numerous voids and endow high surface area. The unique structural characteristics would also make the small molecules easily accessible to the surfaces of sample, leading to high catalytic activity. In addition, it can be seen that the morphologies of all catalysts have little change with the ratio of Cu/Ce increased from 1:9 to 1:1 (Fig. S4). From the HRTEM image (Fig. 2c), it can be observed that the lattice fringes is corresponding to the (111) crystal plane of CuO and CeO2. In order to confirm the presence of both Ce and Cu species in the nanobars of Cu0.3Ce0.7O2 catalyst, the element mapping analysis was performed (Fig. 2d-f). The results confirm that the elements of Cu and Ce are distributed homogeneouly in Cu0.3Ce0.7O2 catalyst. N2 adsorption–desorption isotherms and BJH for CuO-CeO2 catalysts obtained under different molar ratios of Cu/Ce MOFs precursors are similar and demonstrate the mesoporous nature (Fig. S5) [25,26]. The broad capillary condensation step at high relative pressure is related to the textural pores between the particles [27]. BET surface area values are calculated to be 45.7 to 58.5 m2·g−1, respectively (Table 1). The relatively high surface area of CuO-CeO2 composite catalysts is caused by the unique brick-upon-tile morphology containing vast voids [28,29]. UV-Raman spectra of CuO-CeO2 composite catalysts afford the information about the oxygen lattice vibrations (Fig. 3). Generally, the main band at near 463 cm−1 corresponds to triply degenerated F2g mode of fluorite CeO2 [26,30], and the other band at 584 cm−1 is correlated with the oxygen vacancies in CeO2 lattice and attributed to the presence of defective structure in CeO2 [31,32]. The position of the main F2g mode band shift slightly with the increase of Cu content, indicating the incorporation of copper into fluorite lattice of CeO2 [31,33]. The oxygen vacancies in CuO-CeO2 catalysts are generated from intrinsic defects due to the conversion of Ce4+ to Ce3+ [25,34]. The ratio of peaks at 584 and 464 cm−1 (noted as A584/A464), which represents the relative concentration of oxygen vacancies [32], are shown in Table S1. The redox properties of the catalysts greatly influence the CO oxidation reaction. Therefore, the H2-TPR experiment was carried out to get the information about the reducibility of copper species in CuOCeO2 catalysts (Fig. 4). Three typical peaks over the Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 composite catalysts can be observed. The broad one at 151 °C can be attributed to the strong interaction of well-dispersed CuO clusters with CeO2 (α) [22,35]. With the Cu content increased during

3.2. Catalytic properties The catalytic activity of CuO-CeO2 catalysts with different Cu contents was evaluated for the CO-PROX reaction in the temperature range of 40–200 °C. The catalytic performance for CO conversion and selectivity towards CO2 is shown in Fig. 5a and 5b. The CO conversion is increased with the increase of Cu content from 10% to 30%. Then, further increasing CuO content does not change the catalytic activity, and in turn, results in a decrease of catalytic activity in the following order: Cu0.3Ce0.7O2 ≈ Cu0.5Ce0.5O2 > Cu0.2Ce0.8O2 > Cu0.1Ce0.9O2. Turnover frequencies (TOFs) based on dispersion of the CuO particles for all CuO-CeO2 catalysts are calculated (Table 2) according to the formula as reported in the literature: [39]

TOF(s−1) = XCO FCO NAV

mCuO,C 1 mcat XCuO 8πd

where XCO is the CO conversion at 80 °C, FCO is the flow rate of CO in mol s−1, NAV is Avogadro’s constant, mCat is the amount of catalyst, XCuO is the CuO loading in the catalyst. mCuO,c means the weight of a single CuO crystallite depending on its size. The value could be derived from its volume obtained under the assumption of its semispherical shape and density of CuO (6.45 g cm−3). TPR-N2O chemisorption technique was used to evaluate the dispersion (D) of CuO [40]. It is obviously that the dispersion of CuO (DCuO) is decreased with the increase of Cu content and the TOF is increased with the increase of Cu content. Although the high Cu loading amount may have negative influence on the dispersion of CuO for CuOCeO2 catalysts, more active sites (CuO species) may exist in the catalyst at high Cu loading amount due to the increase of total Cu content. CuO

(e)

Intensity (a.u.)

(d) (c) (b) (a)

10

20

30

40

50

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of CuO-CeO2 catalysts with different molar ratios of Cu/ Ce: (a) 0:1, (b) 1:9, (c) 2:8, (d) 3:7, and (e) 1:1. 219

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Table 1 Physicochemical properties of various CuO-CeO2 catalysts synthesized under different molar ratios of Cu/Ce. SBET (m2/g)

Vp (cm3/g)

Dp (nm)

0:1 1:9 2:8 3:7 1:1

12.1 10.5 9.5 8.9 8.8

51.0 45.7 54.7 58.5 52.1

0.23 0.35 0.25 0.22 0.21

8.9 15.2 9.3 7.4 7.9

Intensity (a.u.)

dCeO2 (nm)

Intensity (a.u.)

CuO-CeO2 catalyst (Cu/Ce)

(e)

(e) (d) (c) (b)

(d) (c) (b)

(a)

(a)

200

400

600

800

1000

420

Raman shift (cm-1)

Meanwhile, the increased Cu loading amount can increase the numbers of CuO-CeO2 interface, which is beneficial for boosting the synergetic interaction between CuO and CeO2 and further improving the catalytic performance [41]. We performed Raman, XPS, and H2-TPR measurements to reveal the in-depth understanding for the superior catalytic performance of Cu0.3Ce0.7O2 catalyst. The Raman result shows that the relative concentration of oxygen vacancies (A584/A464) in Cu0.3Ce0.7O2 is 39.7%, which is almost the same as that in Cu0.5Ce0.5O2 (40.3%), but higher than those in Cu0.1Ce0.9O2 (19.2%) and Cu0.2Ce0.8O2 (33.6%) catalysts (Fig. 3). Meanwhile, the molar ratios of Ce3+/(Ce3+ + Ce4+) increases in the order of Cu0.1Ce0.9O2 < Cu0.2Ce0.8O2 < Cu0.3Ce0.7O2 ≈ Cu0.5Ce0.5O2, which are calculated according to the XPS results (Fig. S6 and Table S1), indicating that Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 catalysts have more oxygen vacancies. The catalytic activity of CuO-CeO2 catalysts may largerly rely on the concentrations of oxygen vacancies and Ce3+, and the catalyts with more oxygen vacancies and Ce3+ may possess superior CO PROX catalytic activity. Additionally, the reduction peaks of Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 catalysts shifts to lower temperature with the increase of the Cu/Ce molar ratio, as it can be seen in H2-TPR spectra (Fig. 4). The lower reduction temperature indicates a stronger interaction between CuO and CeO2 in Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 catalysts and results in the enhanced catalytic activity [42,43]. Moreover, the reduction peak attributed to welldispersed CuO clusters gradually becomes stronger with the increase of Cu content in Cu0.3Ce0.7O2 and Cu0.5Ce0.5O2 catalysts, implying that more well-dispersed CuO clusters are existed in these catalysts, which is indeed beneficial for improving the catalytic activity. It is concluded that the well dispersed CuO is crucial for CO oxidation while the bulk CuO is spectator. As for the selectivity to CO2, it lies on the competition between CO and H2 oxidation. Since the activation energy of H2 oxidation is higher,

480

510

Fig. 3. Raman spectra of CuO-CeO2 catalysts with various molar ratios of Cu/Ce at (a) 0:1, (b) 1:9, (c) 2:8, (d) 3:7, and (e) 1:1.

TCD signal

γ

1:9 2:8 3:7 1:1

β α

50

100

150

200

250

300

350

Temperature (oC) Fig. 4. H2-TPR spectra of CuO-CeO2 catalysts with various molar ratios of Cu/ Ce.

the rate of H2 oxidation increases faster than that of CO oxidation when improving the temperature, which is in agreement with the results of catalytic activity test. The selectivity of all samples is similar, and rapidly decreases with the increasing of reaction temperature on account of occurrence the competitive oxidation of H2 [44]. It is well known that a suitable annealing temperature is significant for the formation of more active sites and the improvement of the

(b)

(a)

450

Raman shift (cm-1)

(c)

500 nm

d=0.31 nm CeO2 (111)

1 um

50 nm

10 nm

(d) ))(

(e) ))(

(f)) )(

10 um

7 um

7 um

d=0.23 nm CuO (111)

Fig. 2. SEM (a, d), TEM (b), and HRTEM (c) images and the element mappings of (e) Ce and (f) Cu of Cu0.3Ce0.7O2 catalyst. The inset in (a) shows the magnified TEM image of Cu0.3Ce0.7O2 catalyst. 220

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100

(a)

100

CO2 Selectivity (%)

CO Conversion (%)

X. Gong et al.

80 60

1:9 2:8 3:7 1:1

40 20 0

(b)

80 60 1:9 2:8 3:7 1:1

40 20 0

40 60 80 100 120 140 160 180 200

40 60 80 100 120 140 160 180 200

Temperature (oC)

Temperature (oC)

Fig. 5. (a) Conversion of CO and (b) selectivity of CO2 over different CuO-CeO2 catalysts at different molar ratios of Cu/Ce for the CO-PROX with a GHSV set at 60000 mL·h−1·gcat−1.

the literatures (Table 3) [22,27,31,41,45]. The high activity of our obtained CuO-CeO2 (3:7) catalyst may be derived from the use of unique Cu0.3Ce0.7−BTC MOFs precursor. During the calcination, the confinement effect of MOF frameworks and the highly dispersed Cu and Ce in atomic level in Cu0.3Ce0.7-BTC MOFs precursor lead to the small size of CeO2 and CuO nanoparticles uniformly dispersed in Cu0.3Ce0.7O2 catalyst, which is in favor of forming more CeO2 and CuO interface and facilitates the interaction between Ce4+/Ce3+ and Cu2+/Cu+, further improving the catalytic performance of Cu0.3Ce0.7O2 catalyst. In addition, the catalytic performance of Cu0.3Ce0.7O2 catalysts with different calcination temperatures has been performed under simulated real conditions with CO2 (15%) and water vapor (10%) in the feed gas (Fig. S7). The addition of CO2 and H2O causes the decrease of the catalytic activity to a certain extent for Cu0.3Ce0.7O2-450, Cu0.3Ce0.7O2550, Cu0.3Ce0.7O2-650, and Cu0.3Ce0.7O2-750 catalysts. The temperature of maximum conversion of CO for Cu0.3Ce0.7O2-650 catalyst shifts from 120 °C to 160 °C. The catalytic activity of Cu0.3Ce0.7O2-650 catalyst remains the best among all catalysts. The deserved deactivation may be attributed to the competitive adsorption between CO and CO2 by the active sites located on the interface CuO-CeO2 sites and the blockage of the adsorbed molecular water on active sites, resulting in poor catalytic performance of Cu0.3Ce0.7O2 for the CO-PROX [9,44].

Table 2 Dispersion and crystallite size of CuO and CO conversion and TOF values for CuxCe1−xO2 catalysts with various molar ratios of Cu/Ce. Catalyst

DCuOa (%)

dCuOb (nm)

CO Conversion (%)c

TOFc (×10−3 s−1)

Cu0.1Ce0.9O2 Cu0.2Ce0.8O2 Cu0.3Ce0.7O2 Cu0.5Ce0.5O2

87.5 69.4 48.3 31.5

1.1 1.4 2.1 3.2

6.2 46.8 71.4 72.2

0.2 1.4 3.2 4.4

a b c

Cu dispersion (DCu) was determined by TPR-N2O chemisorption; [40] dCuO calculated according to the Cu dispersion (DCu): dCuO≈ 1/DCu; [39] Reaction temperature is 80 °C.

100

(a)

40 20

From the XRD patterns (Fig. S8), it can be observed that the shape of the characteristic peak is gradually sharpen and narrowed because of the increased crystallinity and the occurrence of big particle size [22]. It is clearly evidenced from the change of the crystallite size of Cu0.3Ce0.7O2 catalysts after being calcined under different temperatures (Table 4). Simultaneously, the crystallite size of Cu0.3Ce0.7O2 catalysts has little discrepancy between the fresh and the used catalysts (Fig. S8

100

80 60

3.3. Structure/activity relationships in Cu0.3Ce0.7O2 catalysts

450 oC 550 oC 650 oC 750 oC

0

CO2 Selectivity (%)

CO Conversion (%)

catalytic activity of CuO-CeO2 catalysts. Thus, the as-prepared Cu0.3Ce0.7-BTC MOFs precursor was calcined under different temperatures from 450 °C to 750 °C and the catalytic performance of the resulting Cu0.3Ce0.7O2 catalyst was tested for the CO-PROX reaction. The catalysts obtained under different temperatures were denoted as Cu0.3Ce0.7O2-X, where X is the calcination temperature. From the Fig. 6, it can be seen that Cu0.3Ce0.7O2-450 shows the lowest catalytic activity among all the Cu0.3Ce0.7O2-X catalysts. Then a progressive increase of CO conversion is observed for Cu0.3Ce0.7O2-X catalysts when rising the calcination temperature to 650 °C. Further rising the calcination temperature to 750 °C would reduce the catalytic activity with the conversion of CO similar to Cu0.3Ce0.7O2-550 catalyst. The catalytic activity follows the trend: Cu0.3Ce0.7O2-450 < Cu0.3Ce0.7O2550 ≈ Cu0.3Ce0.7O2-750 < Cu0.3Ce0.7O2-650. Interestingly, the selectivity to CO2 has no distinct change for all Cu0.3Ce0.7O2-X catalysts. The CuO-CeO2 catalysts derived from the Cu0.3Ce0.7-BTC MOFs precursor after high temperature treatment display the superior CO conversion in contrast with the CuO-CeO2 catalysts previously reported in

40 60 80 100 120 140 160 180 200

(b)

80 60 40 20

450 oC 550 oC 650 oC 750 oC

0 40 60 80 100 120 140 160 180 200

o

Temperature (oC)

Temperature ( C)

Fig. 6. (a) Conversion of CO and (b) selectivity of CO2 over Cu0.3Ce0.7O2 catalysts with different calcination temperatures for the CO-PROX with a GHSV set at 60000 mL·h−1·gcat−1. 221

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Table 3 Comparison of the catalytic activity of CuO-CeO2 catalysts for the CO-PROX in the presence of H2 at 80 °C. Catalyst

Conv. (%)

Select. (%)

Reaction gas component

WHSV (mL·h−1·gcat−1)

Ref.

Cu0.3Ce0.7O2 Cu0.05CeMn0.01 Meso-Cu0.2Ce0.8O CuO0.01/CeO2 CuO0.2−CeO2 Ce0.86-Zr0.07-Cu0.07

87 63 30 50 60 54

94 100 100 70 100 100

1.0%CO/1.0%O2/50%H2/N2 1.0%CO/1.0%O2/50%H2/Ar 1.0%CO/1.0%O2/50%H2/N2 1.0%CO/1.25%O2/50%H2/Ar 1.0%CO/1.0%O2/50%H2/N2 1.25%CO/1.25%O2/50%H2/He

60 60 60 40 60 22

This work [22] [27] [31] [41] [45]

000 000 000 000 000 000

intensity of the strong band increases apparently with the increase of calcination temperature. The area ratio of peaks at 584 and 464 cm−1 (noted as A584/A464) are shown in Table 4. The results suggest that the calcination temperature strongly affects the amounts of oxygen vacancies in CuO-CeO2 catalysts. Oxygen vacancy defects play a favorable role in the CO-PROX reaction. In addition, it can be observed that the peak position about 464 cm−1 for the Cu0.3Ce0.7O2-750 catalysts slightly shifts to the higher wavenumber, indicating the increase of their particle size and the crystallinity degree [22,46], which is in agreement with the XRD and TEM results. Fig. 9 shows the Ce 3d and Cu 2p XPS spectra of Cu0.3Ce0.7O2 catalysts treatment with different calcination temperature. The Ce 3d spectrum is fitted into eight peaks as displayed in Fig. 9 (A). The peaks denoted “V” represent the Ce 3d5/2 spin-orbit components and those labeled as “U” are attributed to the Ce 3d3/2 spin-orbit components. XPS spectra of all CuO-CeO2 catalysts exhibit similar profiles, V, V”, V”’, U, U” and U”’ represented the 4f configuration of Ce4+ ion, V’ and U’ represent the peaks of Ce3+ species, providing an evidence of the presence of Ce3+, indicating the redox nature of the catalysts [25,47,48]. The percentages of Ce3+ in the studied catalysts are determined according to the ratio of integrated Ce3+ peaks to the total Ce (Ce3+ + Ce4+) peaks. It is seen that the amount of present Ce3+ oxidation state in CeO2 is different (Table 4), which originates from different calcination temperatures. The degree of mutual effect between CuO and CeO2 is increased with the increase of the temperature, facilitating the conversion of Ce4+ to Ce3+ on the catalyst surface, which results in higher Ce3+ content. It is well known that the transformation of Ce4+ to Ce3+ can create the charge imbalance and the oxygen vacancy on the surface. The presence of ample oxygen vacancies and higher numbers of Ce3+ ions is highly beneficial for the CO-PROX reaction. In order to identify the surface oxidation state of Cu, the Cu 2p XPS spectra were carried out as shown in Fig. 9B. It is revealed that different Cu ions are existed in Cu0.3Ce0.7O2 catalysts. The peak located at 935.6–934.1 eV is ascribed to the Cu2+ and the other peak at 933.0–932.3 eV is the characteristic peak of Cu+ or Cu0 species [9]. The presence of shakeup satellite peaks at 942.3 eV also reveals that the CuO is present in the catalysts [9,49]. It can be concluded that both Cu2+ and Cu+/Cu0 species are present on the surface of Cu0.3Ce0.7O2 catalysts. It is worthy noticing that the main peak shifts to lower binding energy with the increase of calcination temperature over

and Table 4). The morphology and textural structure of Cu0.3Ce0.7O2 catalyst show little difference, maintaining their original morphology after being subjected to the calcination treatment in different temperatures (Fig. S9). However, the degree of crystallinity and the particle size are obviously increased. The results suggest that the CuO-CeO2 catalysts derived from CuxCe1−x-BTC MOFs have higher thermal and structure stability. As mentioned above that the reducibility of a catalyst is highly related to its catalytic performance. The N2O-chemisorption test for thermally treated catalysts was performed to determine the quantity of Cu species on the surface of Cu0.3Ce0.7O2-650, Cu0.3Ce0.7O2450, Cu0.3Ce0.7O2-550, and Cu0.3Ce0.7O2-750 catalysts. As shown in Table 4, obviously, the Cu0.3Ce0.7O2-650 catalyst possesses the most surface Cu species among these four catalysts. From H2-TPR measurements (Fig. 7), it can be observed that the analyzed catalysts display quite complex profiles. For Cu0.30Ce0.70O2-450 catalyst, the H2-TPR profile comprises two reduction peaks at lower temperature attributed to the reduction of incorporated Cu in CeO2 lattice (Cu-O-Ce species) as a result of high temperature calcination and another ascribed to the reduction of bulk CuO located in CeO2 framework. It is worthy noticing that a new reduction peak appears over Cu0.3Ce0.7O2-550 and Cu0.3Ce0.7O2-650 catalysts at lower temperature, which is attributed to highly dispersed CuO clusters on surface of CeO2. Moreover, the peak area (α/β) of Cu0.3Ce0.7O2-650 catalyst is higher than others, claiming that large amounts of CuO clusters are produced on the surface of CeO2. It’s may be the reason why the Cu0.3Ce0.7O2-650 catalyst exhibits the best catalytic performance among all compared catalysts. However, when the calcined temperature is risen up to 750 °C, the α and β peaks are coalesced as the peaks become sharper. According to the above argument, it can be deduced that the Cu-O-Ce species within Cu-Ce-BTC precursor migrate towards the surface forming the highly dispersed CuO clusters during the heat treatment, and the migration of Cu species from the bulk to the surface may lead to the decrease of oxygen vacancies in bulk and the increasing of oxygen vacancies in surface simultaneously, which are in favor of CO-PROX reaction. Furthermore, the calcination temperature is a key factor for the formation of active copper species and influences the interaction between CuO and CeO2. However, the exceedingly high calcination temperature may result in the aggregation of CuO species. From the above result, we draw a conclusion that the excessive calcination temperature indeed leads to the formation of large size of CeO2 and CuO nanoparticles and reduces the dispersion of Cu species, which is harmful for enhancing the catalytic performance. But low calcination temperature adverses to the migration Cu species from bulk to catalyst surface and leads to a low Cu dispersion, which also has negative influence on the catalytic activity. Therefore, a suitable calcination temperature is more favorable for the migration Cu species from bulk to catalyst surface forming highly dispersed Cu species and enhancing the interaction between CuO and CeO2 nanoparticles, which can effectively promote catalytic performance. Thus, the Cu0.3Ce0.7O2 catalyst with more CuO-CeO2 interface and highly dispersed Cu species exhibits the best CO PROX activity among all catalysts. The UV-Raman spectra of CuO-CeO2 catalysts obtained under different calcination temperatures are displayed in Fig. 8. A main peak at ∼464 cm−1 is found in all catalysts, corresponding to the F2g Raman vibration mode of fluorite CeO2, in line with the XRD results [25]. The

Table 4 Physicochemical properties of the Cu0.3Ce0.7O2 catalyst obtained under different calcination temperatures. Catalyst

Cu0.3Ce0.7O2-450 Cu0.3Ce0.7O2-550 Cu0.3Ce0.7O2-650 Cu0.3Ce0.7O2-750 a

222

DCuOa (%)

44.4 48.2 64.2 26.4

dCeO2 (nm)

XPS

Raman

Fresh

Used

Ce3+/Ce3++Ce4+ (%)

A585/A464 (%)

6.5 8.6 11.5 21.9

6.5 8.3 11.8 22.0

11.3 13.6 15.9 11.6

33.9 39.7 44.5 20.1

Cu dispersion (DCu) was determined by TPR-N2O chemisorption.

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TCD signal

γ

the CO adsorption center at the catalyst surface. Monte et al. [53] stated the high activity and selectivity of catalysts related to the Cu+ carbonyls from the higher stability of the Cu species. Fig. 10 presents the in situ DRIFTS spectra under CO reaction stream on Cu0.3Ce0.7O2 catalysts with different time intervals of adsorption at 80 °C in CO/N2 atmosphere, which the same amount of catalysts (about 10 mg) is loaded in the in-situ cell of this instrument. The region of 2400–2000 cm−1 is mainly consist of composed of vibrations of CO2 (g) and Cun+-CO species [32]. The generation of CO2 (g) is usually be related to the catalytic performance of CO oxidation [32,54]. It is well known that the bands at 2140–2100 cm−1 are regarded as the most significant region because they reflect the adsorption of CO on copper surfaces related to the linear or bridge-bonded CO species interacting with CuO, Cu2O, or Cu0 sites [55,56]. The CO adsorption behaviors on all Cu0.3Ce0.7O2 catalysts are almost the same. It can be seen that the band intensities at 2140–2100 cm−1 increase gradually from 45 s to 135 s and reach a maximum and then keep constant for further prolonged time to 1800 s, indicating the excellent CO adsorption capability for Cu0.3Ce0.7O2 catalysts. Typically, carbonyl bands around 2110 cm−1 appear upon the interaction of the catalysts with CO and are attributed to Cu+-CO carbonyls [55,57,58]. But more interestingly, the band intensities corresponding to Cu+-CO species on CuO0.3Ce0.7O2-650 catalyst are obviously higher than those of Cu0.3Ce0.7O2-450, CuO0.3Ce0.7O2-550 and CuO0.3Ce0.7O2-750 catalysts (Fig. 11), which have the same trend for catalytic activity of CuO0.3Ce0.7O2 catalysts. According to the above discussion, it can be concluded that the Cu+-CO carbonyls species would make predominant contribution to the CO-PROX reaction.

450 oC 550 oC 650 oC 750 oC

β α

50

100

150

200

250

300

350

Temperature (oC)

Intensity (a.u.)

Fig. 7. H2-TPR profiles over Cu0.3Ce0.7O2 catalysts under various calcination temperatures.

450 oC 550 oC 650 oC 750 oC

420

450

480

510

3.4. Catalytic stability test

200

400

600

800 To evaluate the stability of as-prepared CuO-CeO2 catalysts, the catalytic performance of Cu0.3Ce0.7O2 sample with different calcination temperatures was carried out at 120 °C for 24 h under PROX reaction conditions, since this temperature is close to the maximum CO conversion. As the results are depicted in Fig. 12, the conversion of CO even maintains closely to 100% except the Cu0.3Ce0.7O2-450 (90%) after 24 h. The selectivity of CO2 is 93%, 89%, 75%, and 80% initially for the Cu0.3Ce0.7O2 catalyst with different calcination temperatures at 450, 550, 650, and 750 °C, respectively, and retains almost constant after 24 h stability test. Additionally, the stability test with CO2 (15%) and H2O (10%) for Cu0.3Ce0.7O2-650 catalyst was carried out at 160 °C for 12 h under PROX reaction conditions and the results were displayed in Fig. S10. The conversion of CO and the selectivity of CO2 maintain 100% and 74% after 12 h, respectively, indicating the Cu0.3Ce0.7O2-650 catalyst has excellent stability under the presence of CO2 and H2O.

Raman shift (cm-1) Fig. 8. Raman spectra over Cu0.3Ce0.7O2 catalysts obtained under different calcination temperatures.

Cu0.3Ce0.7O2 catalysts. This peak shifting phenomenon is ascribed to the partial reduction of Cu2+ to Cu+ species [50]. The presence of Cu+ species might be due to the synergic effect between Cu and Ce, which leads to some electronic interaction. The mixing of Cu and Ce together can cause a redox reaction such as Cu2+ + Ce3+ ↔ Cu+ + Ce4+. According to previous works in the literature [32,51–54], the Cu+ play a critical role and Cu+-carbonyl species is identified as the active center for CO-PROX reaction. Yang et al. [40] considered that Cu+ is the main active site for CO oxidation. Bera et al. [52] found the Cu+ is

A

Intensity (a.u.)

u''

u' u v'''

Ce 3d v''

B

v' v

(c) (b) (a)

920

910

900

890

Cu 2p1/2

(d)

Intensity (a.u.)

u'''

(d)

880

Binding energy (eV)

Cu 2p

Cu 2p3/2 Cu2+ Cu+

shake up

(c) (b) (a)

955

950

945

940

935

930

Binding energy (eV)

Fig. 9. XPS spectra of Ce 3d (A) and Cu 2p (B) over Cu0.3Ce0.7O2 catalysts obtained under various calcination temperatures at (a) 450 °C, (b) 550 °C, (c) 650 °C, and (d) 750 °C. 223

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CO adsorbtion

0.1

a

b

Intensity (a.u.)

1800 s

Intensity (a.u.)

CO adsorbtion

0.1

1350 s 675 s 450 s 225 s 135 s 90 s

1800 s 1350 s 675 s 450 s 225 s 135 s 90 s 45 s

45 s

2400

2300

2200

2100

2000

1900

2400

2300

Wavenumber (cm-1) c

CO adsorbtion

2100

2000

1900

d

CO adsorbtion

0.1

Intensity (a.u.)

Intensity (a.u.)

0.2

1800 s 1350 s 675 s 450 s 225 s 135 s 90 s 45 s

2400

2200

Wavenumber (cm-1)

2300

2200

2100

2000

1900

1800 s 1350 s 675 s 450 s 225 s 135 s 90 s 45 s

2400

2300

Wavenumber (cm-1)

2200

2100

2000

1900

Wavenumber (cm-1)

Fig. 10. In-situ DRIFT spectra of CO adsorption at 80 °C over Cu0.3Ce0.7O2 catalysts obtained under various calcination temperatures: (a) 450 °C, (b) 550 °C, (c) 650 °C, and (d) 750 °C.

Intensity (a.u.)

1.0 0.8

4. Conclusion

CO adsorbtion

450 oC 550 oC 650 oC 750 oC

In summary, a facile MOFs-derived controllable strategy was developed to construct highly active CuxCe1−xO2 catalysts through directly annealing CuxCe1−x-BTC MOFs precursors under different temperatures. The unique morphological feature of CuxCe1−x-BTC MOFs can be mimicked and endow advantages for the CO-PROX reaction. Benefiting from the interaction between CuO and CeO2, the Cu0.3Ce0.7O2 composite catalyst exhibits rather high activity for the COPROX reaction. Simultaneously, we studied the catalytic performance of various Cu0.3Ce0.7O2 catalysts synthesized with different treatment temperatures. The catalytic performance of Cu0.3Ce0.7O2 catalysts is enhanced with the calcination temperature reaching the maximum up to 650 °C, proving that a suitable calcination temperature may enhance the interaction and the reducibility between CuO and CeO2. Moreover, the suitable calcination temperature is beneficial for producing more highly dispersed CuO cluster species and oxygen vacancies in CuxCe1−xO2 catalysts as verified by H2-TPR, Raman spectra, and in situ DRIFTS analysis. However, exceedingly high calcination temperature (750 °C) may impair the interaction between CuO and CeO2, which

0.6 0.4 0.2 0.0 2400

2300

2200

2100

2000

1900

-1

Wavenumber (cm )

100

(a)

100

CO2 Selectivity (%)

CO Conversion (%)

Fig. 11. In situ-DRIFT spectra on Cu0.3Ce0.7O2 catalysts obtained under different calcination temperatures tested at 80 °C for 135 s in CO/N2 atmosphere.

90 80

450 oC 550 oC 650 oC 750 oC

70 60 50 0

5

10

15

20

25

(b)

90 80 70

450 oC 550 oC 650 oC 750 oC

60 50 0

5

10

15

20

25

Time (h)

Time (h)

Fig. 12. Time-on-stream (a) conversion of CO and (b) selectivity to CO2 on Cu0.3Ce0.7O2 catalysts obtained with different calcination temperatures at 120 °C for 24 h. 224

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results in the decrease of well dispersed content and cripples catalytic performance over Cu0.3Ce0.7O2 catalysts for the CO-PROX reaction.

[21]

Notes [22]

The authors declare no competing financial interest.

[23]

Acknowledgements

[24]

This work was supported by Nature Science Foundation of China (NSFC) (21261011, 21661023, 21601096 and 21771117), the Excellent Young Scientists Fund from NSFC (21622106), Application Program from Inner Mongolia Science and Technology Department (2016), Program of Higher-level Talents of IMU (21300-5155105), the Taishan Scholar Project of Shandong Province (China), and Cooperation Project of State Key Laboratory of Baiyun Obo Rare Earth Resource Researches and Comprehensive Utilization (2017Z1950).

[25]

[26]

[27]

Appendix A. Supplementary data

[28]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2018.04.071.

[29]

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