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Optimum Preferential Oxidation Performance of CeO2CuOx-RGO Composites through Interfacial Regulating Junfang Ding, Liping Li, Huixia Li, Shaoqing Chen, Shaofan Fang, Tao Feng, and Guangshe Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15549 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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ACS Applied Materials & Interfaces

Optimum Preferential Oxidation Performance of CeO2-CuOx-RGO Composites through Interfacial Regulating Junfang Ding†, Liping Li†, Huixia Li†, Shaoqing Chen‡, Shaofan Fang‡, Tao Feng†, and Guangshe Li*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P.R. China ‡

Fujian Institute of Research in Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002,

P.R. China

KEYWORDS: interfacial regulating, reduced graphene oxide (RGO), copper-cerium based catalysts, synergistic effect, CO-PROX

ABSTRACT: Interfacial regulating offers a promising route to rationally and effectively design advanced materials for CO preferential oxidation. Herein, in this work, we initiated an interfacial regulating of CeO2-CuOx-RGO composites through adjusting the addition sequence of components during the support formation. The presence of RGO along with the sequence tuning of the components are confirmed to survey the changes of the oxidation state of copper species, the content and distribution of Cu+ site and the synergistic interactions between Cu–Ce mixed oxides and RGO over the catalysts. These catalysts were systematically characterized by ICP, XRD, TEM/HRTEM, H2-TPR, XPS, TGA, Raman spectra and in situ DRIFTS measurements. The results show that RGO is favorable for the generation of Cu+ and the dispersion of copper-cerium species in the as-prepared catalysts. Further, by multi-interfacial regulating of the CeO2-CuOx-RGO composites, the catalyst 1

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CeO2/CuOx-RGO exhibits a strikingly high catalytic oxidation activity at low-temperature coupled with a broader operation temperature window (i.e. CO conversion > 99.0 %, 140-220 oC) in CO selective oxidation reaction, which has been attributed to the high content of the active species Cu+ enriched on the surface, highly dispersed copper oxide clusters subjected to a strong interaction with ceria, and synergistic interactions between Cu–Ce mixed oxides and RGO.

1. INTRODUCTION

Hydrogen, a feasible power source to substitute the traditional fossil fuels, is a potential candidate to address energy and environmental issues.1-4 H2-rich gas is primarily produced by steam reforming of hydrocarbons and the subsequent water gas shift reactions (WGS), which however typically consists of 50-70 vol.% H2, 10-20 vol.% CO2, 5-10 vol.% H2O and 0.5-1 vol.% CO.5-7 The presence of small amount of CO would poison Pt-based electrocatalysts in Proton Exchange Membrane Fuel Cells (PEMFCs).8-10 Thus, industrial hydrogen requires a further purification treatment to reduce CO concentration to a trace level. Among various possible purification methods, preferential oxidation of carbon monoxide (CO-PROX) is considered as a most effective and straightforward method to reduce CO concentration below 10 ppm.11-15

CuO-CeO2 based composites, an alternative to conventional noble metal catalysts, play a vital role in preferential oxidation of CO owing to their strong ability in selectively accelerating a desirable CO oxidation from a H2-rich streams.16 One has witnessed great efforts in comprehending the surface catalytic reaction mechanism for CuO-CeO2 based catalysts, which helps to a great 2


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extent rationally design advanced catalysts for CO-PROX reactions.17 For instance, it is well established that CO adsorbs on coordination-unsaturated Cu+ sites and becomes activated. Then, the adsorbed CO undergoes surface reactions to form the intermediate species Cu+-CO that subsequently contact with oxygen species and occur CO oxidation.18-20 Furthermore, both Cu+ species and the synergistic effect of copper-cerium species show the positive effects on CO oxidation since synergistic interaction of CuO-CeO2 interfacial sites favors the electron transfer between Cu2+/Cu+ and Ce4+/Ce3+, that promotes catalytic activity of CuO-CeO2 catalysts.19,20 Thus, it is most likely that the performance of CuO-CeO2 based catalysts could be significantly improved by increasing the content of Cu+ species and enhancing the synergistic effect between copper and cerium. Unfortunately, Cu+ is easily oxidized to Cu2+, and thus it is infeasible to directly synthesize cuprous oxide species for promoted CO-PROX reaction,21 thus imposing serious restraints on the development of Proton Exchange Membrane Fuel Cells and their wide applications. For example, the catalytic performance at low temperatures ever reported is still poor, and the operation temperature window is not broad enough, which would seriously increase the thermal management cost of the fuel cell application. As a consequence, it is very important, but also highly challenging, to rationally design and integrate the active sites on CuO-CeO2 based catalysts. Integrating the active Cu+ sites on CuO-CeO2 based catalysts could be achieved through interfacial regulating with the help of graphene. This is because graphene has unique characteristics of high electronic mobility, tremendous specific surface area, and most importantly because the presence of graphene is favorable for the reduction of metal oxide to generate more reductive species.22-26 Wang and coworkers have obtained RGO/metal (Cu, Ni, Co) nano hybrid composites 3



by a facile thermal reduction method. 27 For copper oxide/RGO nanocomposite, as reported by K. Dolui group,28 the integration of nanoparticles into RGO leads to a synergetic effects between RGO nanosheets and the nanoparticles, which improves the catalytic activity for the reduction of 4-nitrophenol and functionality of the nanocomposites. Having these literature data in mind, we proposed that (i) when decoration with a low content of RGO, one may develop a new facile path to increase the content of active Cu+ species with an enhanced synergetic effect in CuO-CeO2 system for CO-PROX reaction, and moreover (ii) during the interfacial regulating, CuO species located at the interface of CuO-CeO2 system generally exhibit different types of coordination environments, leading to a promoted synergetic effect of the components. The strengthened interaction between copper and cerium species by regulating the interface would be essential for increasing the low-temperature catalytic activity and broadening the operation temperature window.

In this work, we initiated an interface regulating of a series of CeO2-CuOx-RGO composites through adjusting the addition sequence of the components during the support formation. Through a systematic study on the impacts of RGO addition and the sequence of components addition, we obtained the optimum conditions for changing the oxidation state of interfacial copper species, the content and distribution of Cu+ site, and strengthened the synergistic interactions between Cu–Ce mixed oxides and RGO. The composites with effective interfacial regulating have shown a strikingly high catalytic oxidation activity at low-temperatures coupled with a broader operation temperature window (140-220 oC) in CO selective oxidation reaction. The interfacial regulating methodology reported in this work may help to find more effective routes for advanced catalysts in CO selective oxidation reaction, and might greatly promote the development of Proton Exchange 4


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Membrane Fuel Cells and their applications.

2. EXPERIMENTAL SECTION

2.1. Materials.

Copper nitrate (Cu(NO3)2·3H2O, 98.0−102.0 wt%) and urea (CO(NH2)2, 99 wt%) were purchased from Sinopharm Chemical Reagent Corp (P. R. China), and cerium nitrate (Ce(NO3)3·6H2O, 99.5 wt%) from aladdin. Graphitic oxide (GO) was synthesized according to a modified Hummers’ method.29 All chemicals and solvents were used without further purification.

2.2. Sample Syntheses.

Sample CeO2/CuOx-RGO: Support CuOx-RGO was first synthesized according to the following procedure. Namely, 0.0524 g of GO was dispersed into 15 mL deionized water and sonicated for 1 h to form a GO aqueous suspension, and 0.9644 g of Cu(NO3)2·3H2O was dissolved in 15 mL deionized water. Then, given amounts of Cu(NO3)2.3H2O solution are dropwise added into GO suspension, and further stirred for 1 h to form a homogeneous suspension. pH value of the mixture solution was adjusted about to 7 using urea. The resulting mixture was sealed into a Teflon-lined steel autoclave and reacted at 130 oC for 24 h. The obtained precipitates were separated by centrifugation, washed with deionized water several times, and then dried at 60 oC over night in vacuum. The support thus obtained was named as CuOx-RGO. Support CuOx-RGO was impregnated in 30 mL 0.025 g/mL cerium nitrate solution. After stirring for 12 h, the power was 5



collected and dried over night at 60 oC. Finally, the powders were calcined at 400 oC for 4 h in N2 atmosphere at a heating rate of 2 oC·min-1, which yields the sample CeO2/CuOx-RGO.

Sample CuOx/CeO2-RGO: The preparation procedure is similar to that for CeO2/CuOx-RGO with the exception that CeO2-RGO support was first synthesized using Ce(NO3)3·6H2O and GO. Then, the obtained support was impregnated in 30ml 0.032 g/mL copper nitrate solution, which is followed by a similar calcination procedure to that for sample CeO2/CuOx-RGO.

Sample RGO/CeO2-CuOx: The preparation procedure is similar to that of sample CeO2/CuOx-RGO, with the exception that CeO2-CuOx support was first synthesized using Ce(NO3)3·6H2O and Cu(NO3)2.3H2O. Then, the obtained support was impregnated in GO aqueous suspension and suffered a similar calcination procedure to that for sample CeO2/CuOx-RGO.

Sample CeO2-CuOx-RGO: The sample was prepared using one pot synthesis method with a similar calcination procedure to that for sample CeO2/CuOx-RGO.

Sample CeO2/CuOx: The sample was synthesized following a similar method to that of CeO2/CuOx-RGO except that CeO2/CuOx was prepared without adding RGO.

The loading amount of reduced graphene (RGO) loading is set at 8 wt% and the molar ratio of Cu to Ce is 7:3 for all samples.

2.3. Characterization Methods.

The molar ratios of Cu to Ce in the samples were determined by inductively coupled plasma

6


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atomic emission spectroscopy (ICP-AES) on an OPTIMA 3300DV instrument (Perkin Elmer). Powder X-ray diffraction (XRD) patterns were recorded in a D/MAX2550 diffractometer equipped with Cu-Kα radiation (λ=0.15418 nm) at 40 kV and 15 mA in the range of 2θ between 10o and 80o. High purity nickel was used as internal standard for the correction of diffraction angle. Nitrogen adsorption-desorption measurements were carried out on an asap 2020 instrument. Prior to the measurements, all samples were degassed at 120 oC under vacuum for 6 h. BET specific surface area was obtained from the adsorption data. The scanning electron microscopy images (SEM) were examined using a Hitachi S-4800 scanning electron microscope. For the corresponding elemental mapping acquisition, the energy dispersive spectroscopy (EDS) was performed under the scanning electron microscopy (SEM) mode. Transmission electron microscopy images (TEM) were taken on a Tecnai G2S-TwinF20 apparatus.

Thermal gravimetric analysis (TGA) was carried out using a Netisch STA-449F5 system in a flow of air from room temperature to 900

o

C at a heating rate of 10

o

C·min-1. Hydrogen

temperature-programmed reduction experiments (H2-TPR) were performed in a Micromeritics AutoChem II 2920 Apparatus equipped with Thermal Conductivity Detector (TCD). The as-prepared samples (30 mg) were heated from 30 to 800 °C in a 5 % H2/Ar (50 mL·min−1) mixed gas at a heating rate of 10 °C·min-1. Prior to the measurements, the catalyst was treated at 200 oC for 1 h in an Ar stream to remove the contaminants. X-ray photoelectron spectra (XPS) of the samples were obtained with an ESCALAB250 apparatus using a monochromatic Al Kα X-ray source. The charging shift was calibrated using C 1s photoemission line at 284.8 eV.

7



Raman data were collected on an INVIA Raman system under an excitation of 488 nm. In-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) characterization was carried out in a Nicolet is50 FT-IR spectrometer equipped with a MCT detector and cooled by liquid nitrogen. Prior to the measurement, the samples (about 10 mg) were treated in-situ at 200 oC in pure N2 at a flow rate of 40 mL·min-1 to remove the contaminants from the catalyst surface. A background spectrum was collected for spectral correction after cooling to 30 oC. Then, the reaction gas (1% CO, 1.25% O2, 50% H2, and He as the balance gas), was introduced into the in-situ chamber at a flow rate of 10 mL·min-1. The spectra were averaged over 48 scans with a resolution of 4 cm−1.

2.4. Kinetic Measurements.

The reaction rates of the samples were carried out under CO-PROX conditions (1 % CO, 1.25 % O2 and 50 % H2 in He balance) with a space velocity of 200, 000 ml·gcat-1·h-1 at given temperatures of 50, 140, and 240 oC. The reaction rates were calculated according to the following equations,

where r is the reaction rate of the samples (mol•S-1•gcat-1); CCO represents the concentration of CO in the feed gas (%); XCO, the conversion rate of CO (%); V, the total flow rate (m3/s); Patm, atmospheric pressure (=1.01•105 Pa); mcat, the mass of sample (g); R, molar gas constant (=8.314 Pa•m3•mol-1•K-1); and T, temperature (K).

2.5. Catalytic Activity Study. 8


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Catalytic activity tests were carried out in a quartz reactor with a space velocity of 80, 000 ml·gcat-1·h-1 at atmospheric pressure. The reaction mixture consisted of 1% CO, 1.25% O2, and 50% H2 (volume fraction) with He as the balance gas. The activity tests were performed using 50 mg of the catalysts without any pretreatment. Samples were heated from 30 to 240 oC and were stabilized at each temperature plateau under the same reaction conditions for 10 min to reach the equilibrium. The inlet and outlet gas mixture were analyzed with an on-line GC-2014C gas chromatograph equipped with a thermal conductivity. 5A molecular sieve column was used to separate CO and O2. CO2 was determined by a TDX-01 column. CO conversion and CO2 selectivity were calculated according to the following equations,10

CCO(%) = ([CO]in - [CO]out) / [CO]in × 100%

(1)

Sco2(%)= 0.5([CO]in - [CO]out) / ([O2]in - [O2]out) × 100%

(2)

where [CO]in and [O2]in are the corresponding GC response peak area values of the inlet gas of CO and O2 at room temperature, respectively. [CO]out and [O2]out correspond, respectively, to GC response peak area values of CO and O2 after the reactor.

3. RESULTS AND DISSCUSION

3.1. Structures of the Composites during the Interfacial Regulating

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Figure 1. XRD patterns of the samples: (a) CeO2-CuOx-RGO; (b) RGO/CeO2-CuOx; (c) CuOx/CeO2-RGO; (d) CeO2/CuOx-RGO, and (e) CeO2/CuOx obtained through adjusting the addition sequence of components during the support formation.

X-ray diffraction patterns of the as-prepared samples are presented in Figure 1. For all samples, the diffraction peaks at two theta of 28.6o, 33.2o, 47.5o, and 56.7o are attributed to the planes (111), (002), (022), and (113) of CeO2 in a cubic ﬂuorite structure (JCPDS, No. 98-001-1748), respectively. Two diffraction peaks for CuO in a monoclinic structure at two theta of 35.6º and 38.4º were found for all samples. In addition, the characteristic diffraction peaks of Cu2O in a cubic-phase structure were also observed for all samples except for the sample CeO2/CuOx, which suggests that addition of RGO is favorable for the reduction of CuO to the active component of Cu2O. It is worth noting 10


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that for sample RGO/CeO2-CuOx, the diffraction peaks for Cu2O and metallic copper are the weakest, when comparing to other samples, which is attributed to the low content of reduced copper species in the sample. XRD data were refined using GSAS software as shown in Figure S1. The weight fractions of identified phases are listed in Table S1. The structural refinement results showed that the content of reduced copper species is in the order: CuOx/CeO2-RGO > CeO2/CuOx-RGO > CeO2-CuOx-RGO > RGO/CeO2-CuOx > CeO2/CuOx. Thus, we can deduce that the reduction of copper oxide is dependent on the sequence of the support formation. In addition, the characteristic diffraction peak of GO at two theta of 11º could not be observed for all samples, most likely because GO was transferred to RGO during the preparation process, as demonstrated by FT-IR (Figure S2). The absence of typical diffraction peak for RGO at two theta of 24.3º in the samples should be related to the low content of RGO ((Figure S3 and Table 1), the more disordered stacking and less agglomeration of RGO sheets in the samples, as reported elsewhere.30 Table 1 shows the structural properties of the samples. The average crystallite sizes of CuO and CeO2 in all samples were calculated by Scherrer formula based on the breadths of their main diffraction (111). As shown in Table 1, for as-prepared samples, the average crystallite sizes of CuO and CeO2 were about 19 and 5 nm, respectively. There are two striking changes that have to be mentioned when comparing the samples to those after addition of RGO to CeO2/CuOx: The first one is that the average crystallite sizes of CuO slightly decreased, and the surface areas increased obviously after the addition of RGO in comparison with CeO2/CuOx sample, suggesting that RGO was conducive to the dispersion of nano-particles. Typically, sample CeO2/CuOx-RGO with CuOx-RGO as support owns the largest BET surface area among all as-prepared samples, which is 11



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favorable for exposing more active sites and improving catalytic performance. The second one is that the lattice parameters for CeO2 component in the samples are all smaller than that of 3.124 Å for pure bulk CeO2. This striking observation cannot be explained in terms of small size effect, since our CeO2 component has a particle size of only 5 nm, which can give rise to a larger lattice parameter, as demonstrated in literature for smaller CeO2 nanoparticles. Instead, there are two primary reasons for this observation: (i) Cu ions have an ionic size much smaller than those for Ce ions, and therefore partial Cu ions might enter into the crystal lattice of CeO2; and (ii) when Cu2+ is partially reduced to Cu+, part of surface Ce3+ would be oxidized into Ce4+, which could lead to the lattice contraction of CeO2 lattice. 31,32 Table 1. Structural properties of the samples d-spacing (Å)

Particle size (nm)

Cu/Ce (molar ratio)a

Cu/Ce (molar ratio)b

RGO Content (%)c

CeO2

CuO

CuO

CeO2/CuOx-RGO

7:3

7.2 : 3

4.8

3.0962

2.3098

19.36

5.38

76.53

CuOx/CeO2-RGO

7:3

7.4 : 3

6.3

3.0978

2.3107

23.81

6.01

53.26

RGO/CeO2-CuOx

7:3

6.7 : 3

9.1

3.0934

2.3103

11.13

5.11

46.62

CeO2-CuOx-RGO

7:3

7.4 : 3

5.8

3.1045

2.3154

15.81

5.21

52.71

CeO2/CuOx

7:3

6.6 : 3

------

3.1019

2.3110

27.14

5.42

29.82

Samples

a

d

CeO2

e

SBET (m2/g)

Cu/Ce molar ratio according to the nominal composition. b Cu/Ce molar ratio determined by ICP-AES. c The content of RGO determined by TGA. d Calculated particle sizes from Scherrer equation using (111) diffraction peaks of CuO. e Calculated particle sizes from Scherrer equation using (111) diffraction peaks of CeO2.

3.2. Morphologies of the Composites during the Interfacial Regulating

12


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Figure 2 ． TEM images and the HRTEM images of the samples: (a) CeO2/CuOx; (b) CeO2/CuOx-RGO; (c) RGO/CeO2-CuOx.

Morphological features of the as-prepared samples were detected by SEM, TEM, HRTEM, and SEM-EDX mapping. Figure 2 presents TEM and HRTEM images of the typical samples CeO2/CuOx, CeO2/CuOx-RGO, and RGO/CeO2-CuOx, while those for other two samples CeO2-CuOx-RGO and CuOx/CeO2-RGO are given in Figure S5. As shown in Figure 2a, the particles for sample CeO2/CuOx are mainly agglomerated, in agreement with SEM observation (Figure S4). Further HRTEM image analyses indicate that the lattice spacing for the fringes of CuO and CeO2 were 0.23 and 0.31 nm, respectively, which are be attributed to the (200) crystal plane of CuO and (111) crystal plane of cubic fluorite CeO2. Comparatively, for the samples containing RGO (Figures 2, Figure S5), the particles of copper-cerium components supported on RGO sheets became much smaller and less agglomerated, demonstrating that RGO is favorable for the dispersion of 13



copper-cerium species, but also efficiently protects nanoparticles from agglomeration. Further, the lattice spacing for components CuO and CeO2 can be clearly observed, as shown Figures 2 and S5. Simultaneously, for sample CeO2/CuOx-RGO, a portion of CuO nanoparticles were successfully reduced into active cuprous oxide species by graphene, as indicated by the lattice spacing of Cu2O in HRTEM image, in good agreement with our XRD data analysis.

Figure 3 compares SEM-EDX mapping of the samples RGO/CeO2-CuOx, CeO2/CuOx-RGO, and CeO2/CuOx. For RGO/CeO2-CuOx, Ce and Cu species are individually dispersed onto RGO sheets with a poor synergistic effect. Comparatively, for CeO2/CuOx-RGO, highly dispersed Ce and Cu species are closely anchored onto the surface of RGO sheets, indicating the presence of a strong interaction between Ce and Cu species. Differing from those of samples RGO/CeO2-CuOx and CeO2/CuOx-RGO, Ce and Cu species for CeO2/CuOx were coexistent and irregularly agglomerated. Based on the above analyses, it can be concluded that the sequence of the support formation and the presence of RGO could play an important role in the synergistic effect of interfacial copper and cerium species essential for the advanced catalytic performance.

14


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Figure 3. SEM-EDX mapping images of the samples: (a) RGO/CeO2-CuOx; (b) CeO2/CuOx-RGO; and (c) CeO2/CuOx.

3.3. TPR Measurements

Figure 4. H2-TPR profiles of the samples CeO2-CuOx-RGO, RGO/CeO2-CuOx, CuOx/CeO2-RGO, CeO2/CuOx-RGO, and CeO2/CuOx: (a) In a wide temperature range from room temperature to 800 o

C, (b) enlarged TPR spectra from 50 to 300 oC, and (c) enlarged TPR spectra from 300 to 800 oC

It is well known that CO oxidation reaction is closely related to the redox properties of the samples. H2-TPR analysis was thus carried out to obtain the information about the reducibility and the state of copper species in the as-prepared samples. Figure 4a shows the TPR profiles recorded in the temperature range from room temperature to 800 oC. Figures 4 (b, c) are the enlarged TPR spectra for all samples in temperature ranges of 50 to 300 oC and 300 to 800 oC, respectively. For all 15



samples, two strong multiple-step reduction peaks were observed below 300 oC. The presence of multiple-step at low temperature reduction process (LTRP) is attributed to the existence of different copper species in the prepared samples: The first one is associated with the most easily reducible CuOx species that are highly dispersed on the surface of nanocrystalline CeO2 with a strong interaction with ceria, which is indicated by the low-temperature reduction peak (α) in Figure 4b. While the second one is assigned to the reduction of isolated CuOx that weakly interacts with ceria or the two- or three-dimensional copper clusters at small sizes, as shown by the reduction peak (β) at relatively a higher temperature.33-35 The copper species have smaller particle size and a good dispersion, since the reduction peak of bulk copper species, denoted as γpeak in reference,36 was not observed in H2-TPR profiles of our samples. The experimental H2 consumption (based on the area of the reduction peak) was compared with the theoretical H2 consumption (calculated for the complete reduction of the copper species according to the phase contents of XRD refinement results), as shown in Table 2. The experimental H2 consumption surpasses the amount needed to totally reduce Cu +/2+ species into Cu0 except for RGO/CeO2-CuOx, which may be due to the partial reduction of Ce4+ ions at lower temperature along with reduction of Cu+/2+, as reported elsewhere.37-39 When comparing all samples, CeO2/CuOx-RGO has the lowest reduction temperatures for both peaks (α, β) even though the particle size of component CuO is not the smallest one. This observation suggests that a relatively strong interaction between CuOx and RGO could result in more active oxygen species in CuOx-RGO support, i.e. RGO is favorable for promoting the reduction of CuOx. Further data comparison for those samples containing RGO indicates that the reduction temperatures vary with the order of addition of 16


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components. Thus, it is reasonable that the addition sequence of the precursors can affect the interfacial interactions between copper species and cerium components, thereby affecting the redox behavior of the samples. In addition to the strong reduction peaks below 300 oC, three peaks are also found in a temperature range from 415 to 655 oC for sample CeO2-CuOx-RGO. These peaks centered at 407, 505, and 657 oC can be attributed to the reduction of surface ceria, carbon species on RGO sheets, and bulk ceria, respectively.40,41 Comparatively, only two reduction peaks at about 415 and 680 oC were observed over the sample CuOx/CeO2-RGO, and only one peak at about 700 oC for samples CeO2/CuOx-RGO, CeO2/CuOx and RGO/CeO2-CuOx. Previous studies41 have shown that pure ceria can give two reduction peaks at 480 and 750 oC, corresponding to the reduction of surface ceria and bulk ceria, respectively. The reduction peaks of ceria species for current samples shifted towards lower temperatures when comparing to pure ceria. Therefore, CuOx species and RGO facilitate the reduction of ceria species.

Table 2. Reduction temperature and XPS results of the samples

Samples

H2 consumption(LTRP) ( µmol H2 /gcat)

Theoretical H2 consumption(LTPR)a ( µmol H2 /gcat)

Isat/Imp

Ce3+ (%)

CeO2/CuOx-RGO

5.66×103

3.80×103

0.25

20.84

CuOx/CeO2-RGO

6.13×103

3.42×103

0.36

25.81

RGO/CeO2-CuOx

4.49×103

5.10×103

0.42

--

CeO2-CuOx-RGO

6.04×103

4.51×103

0.43

13.04

CeO2/CuOx

7.08×103

5.68×103

0.42

16.73

a

Theoretical H2 consumption was calculated according to the phase contents of XRD refinement 17



results.

3.4. XPS Measurements of Composites

Figure 5. XPS spectra of (a) Cu 2p and (b) Ce 3d for given samples.

XPS is used to detect the surface compositions and chemical states of the samples. Core level spectra of Cu 2p and Ce 3d are presented in Figure 5. Cu 2p spectra consisted of two groups of peaks (Figure 5a and Table S2), which are assigned to Cu 2p3/2 and Cu 2p1/2, respectively. According to the previous reports for bulk copper oxides,42-44 the position of Cu 2p3/2 peak and the shake-up satellite peak could be used to distinguish Cu2+ from the reduced copper species, such as Cu+ and Cu0, because the binding energy of Cu+ or Cu0 is in a range of 932.8-933.5 eV and does not show shake-up peak at the higher binding energy sides. Comparatively, for Cu2+ ion, 2p3/2 peak locates at 933.8 eV with an intense shake-up satellite peak. As shown in Figure 5a, the spectra exhibited satellite shake-up peaks, while their intensities are much weaker than that for CuO, representing the presence of more reduced copper species. And it could be estimated by the ratio of the intensities of 18


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the satellite peaks to those of the main peak (Isat /Imp). As shown in Table 2, all catalysts show a Isat/Imp value lower than that of 0.55 for CuO, further indicating the presence of partially reduced copper species.41,42

Cu+ and Cu0 are hard to be distinguished since their 2p photoelectron peaks have almost the same binding energies. Instead, there is a big separation of Auger peak (Cu LMM) for Cu+ and Cu0, which could help to determine the valence state of copper species on the surface.45 As shown in Figure S6, all samples gave a broad Auger line at about 917 eV, as originated from an overlap between signals of Cu+ and Cu2+. The kinetic energy of CuOx/CeO2-RGO and CeO2/CuOx-RGO shifted towards the lower kinetic energy side when comparing to that for sample CeO2/CuOx, suggesting the existence of a large number of Cu+ species on the surface of CuOx/CeO2-RGO and CeO2/CuOx-RGO. Furthermore, for CeO2/CuOx-RGO, the ratio of Isat/Imp is much smaller than that for other catalysts (Table 2), though the content of the reduced copper species by XRD refinement is not the highest. This observation should be associated with a surface enrichment effect of reduced copper species for CeO2/CuOx-RGO. Combined with Cu LMM, it can be inferred that the main component of the reduced copper species is Cu+, and that the surface with enriched Cu+ species is favorable for CO adsorption and oxidation.

Figure 5b shows Ce 3d XPS spectra of the samples with different supports. Owing to the many-body effects, Ce 3d core level of Ce4+ containing compounds consists of three pairs of photoelectron peaks, usually labeled as v, u (Ce 3d94f2O 2p4); v′, u′ (Ce 3d9 4f1 O 2p5 ) and v′′ , u′′(final state of Ce 3d9 4f0 O 2p6 ). While 3d photoelectron spectrum of Ce3+ shows two pairs of 19



peaks, denoted as α, α′ (Ce 3d9 4f1 O 2p6) and β, β′ (Ce 3d9 4f2 O 2p5), respectively (Table S3).7,46,47 Ce 3d spectra shown in Figure 5b can be resolved in 5 pairs of peaks except for one of RGO/CeO2-CuOx that is deconvoluted into three pairs of peaks due to a small amount of Ce3+ on the surface of RGO/CeO2-CuOx. The concentration of surface Ce3+ in the samples are calculated as follows,7 Ce3+ (%) = A( Ce3+ )/A( Ce3+ + Ce4+ ) × 100 % The values of Ce3+ concentration in the samples are presented in Table 2. CeO2/CuOx-RGO exhibits a concentration of Ce3+ higher than that of CeO2/CuOx. It means that the presence of RGO in the support of CuOx can result in more Ce3+, which may be related to the electron rich and reducible properties of RGO. On the other hand, Ce3+ concentration in the samples containing RGO is highly dependent on the type of interface. The presence of Ce3+ is usually accompanied by the formation of oxygen vacancies that are favorable for the electron transfer and CO adsorption to enhance the catalytic performance in CO oxidation. Therefore, one may expect to improve the catalytic performance of CeO2-CuOx-RGO composite by interfacial regulating, i.e. altering the formation sequence of support and adjusting oxygen vacancies.

3.5. Raman Spectra of the Composites

20


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Figure 6. Raman spectra for given samples using 488 nm excitation.

Raman spectrum analyses were performed to further examine the crystalline phases in the samples. The obtained spectra are presented in Figure 6. Pure CeO2 possesses a characteristic band centered at 460 cm−1, attributing to the F2g Raman vibration mode of fluorite-type cubic structure.7,48 This mode represents the stretching vibrations of oxygen atoms around Ce4+ ions. A distinct red shift of F2g band, as shown in Figure 6, is observed for all samples when comparing to pure CeO2, which can be ascribed to the incorporation of copper ions into CeO2 lattice, in good agreement with the decrease of CeO2 lattice parameter as shown in Table 1. Additionally, two broad bands at 587 and 1171 cm-1 are also observed for all samples except for RGO/CeO2-CuOx. Both bands are respectively assigned to the defect-induced (D) mode and second-order longitudinal optical (2LO) 21



mode, which is associated with the generation of oxygen vacancies in CeO2 lattice.49-51 However, the typical vibration peak of RGO was not observed, possibly due to its low content. We have used TG and successfully determined the RGO content of only about 6 wt% (Figure S3 and Table 1). FT-IR spectrum was measured to clarify the existence of added graphene. As shown in Figure S2, all samples showed a peak at 3143 cm-1 in FT-IR spectra except for CeO2/CuOx. This peak is attributed to the hydroxyl stretching vibrations of RGO. Furthermore, a peak at about 1390 cm-1 is associated with the stretching vibration of CO-H, which is also caused by the presence of graphene.52-54

3.6. In Situ DRIFTS Measurements of the Composites

22


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Figure 7. DRIFTS spectra obtained under CO-PROX conditions (1 % CO, 1.25 % O2 and 50 % H2 in He balance ) over the samples: (a) CeO2/CuOx-RGO; (b) RGO/CeO2-CuOx; (c) CeO2/CuOx; and (d) CuOx/CeO2-RGO.

In order to comprehend the processes taking place over the samples during the course of CO-PROX reaction, we carried out in situ DRIFT spectrum measurement under CO-PROX conditions. Figure 7 presents the DRIFTS spectra recorded in a reaction stream (1 % CO, 1.25 % O2 and 50 % H2 in He balance) over the samples CeO2/CuOx-RGO, CeO2/CuOx, RGO/CeO2-CuOx and CuOx/CeO2-RGO. DRIFTS spectra are mainly composed of three characteristic zones. Bands appeared in the 3800-2800 cm−1 zone correspond to hydroxyl species, including isolated hydroxyls and associated species.55-58 A peak at 2907 cm-1, clearly observed only for CeO2/CuOx-RGO catalyst, is attributed to the associated hydroxyls, which should be related to the heavy oxidization of graphene. The spectral zone below 1700 cm−1 is ascribed to the contribution of carbonate, carboxylate or formate species.57-59 The bands at 1436 and 1110 cm-1 can be attributed to C-O vibration in carbonate.57 The bands at 1546 and 1338 cm-1 belong to the bidentate carbonates and mono- or tri-dentate carbonate species, respectively.56 Sets of peaks located at 1510, 1371, 2949, and 2846 cm-1 characteristic of formate species should be observed to represent the onset of a residual WGS process,59 while these peaks were hardly seen for all samples, possibly due to a high CO2 selectivity for our samples. Furthermore, in situ DRIFT spectrum confirms that CO is oxidized mainly by reaction with oxygen atoms to form a carbonate intermediate species in CeO2-CuOx-RGO

23



composites. The third spectral zone is in a range 2400-2000 cm−1, a finger-print region of CO-PROX reaction that is mainly consisted of the vibrations of two species, CO2 (g) and Cun+-CO species.13 A band at 2359 and 2339 cm−1 that are addressed to the generation of CO2 (g) could be correlated to the catalytic performance of CO oxidation.60 The formation of carbon dioxide at a temperature of 30 oC for our catalysts is attributed to the reaction of the unsaturated coordination Cun+ and CO. Moreover, for all CeO2-CuOx-RGO samples (Figure S7), the intensity of band for CO2 increased with the reaction temperatures until the complete conversion of CO, owing to the redox reaction between the adsorbed CO and the Cu+-CO species on the surface of the samples. The vibrations of Cun+-CO species mainly located at about 2200-2140 cm−1, 2140-2100 cm−1 and 2100-2000 cm−1 have been assigned to the adsorptions of CO on Cu2+, Cu+ and Cu0 sites, respectively, according to the results in literature.61-63 In general, the band at 2140 cm−1 was attributed to CO adsorption at Cu+ owing to the fact that Cu2+-CO species are unstable and could not exist at room temperature.53,55 In our experiment, a strong characteristic peak at 2104 cm-1 , 2140 cm-1 or 2100cm-1 was observed for samples CeO2/CuOx-RGO, CuOx/CeO2-RGO and CeO2-CuOx-RGO when exposed to the reaction gases for 10 min at 30 oC (Figure 7), indicating that CO can be adsorbed on Cu+ active sites at 30 oC to form Cu+−CO species. In principle, the intensity of Cu+-carbonyl species at room temperature gives a measure of the amount of CO oxidation active sites formed at the interface upon reductive interaction with CO-PROX mixture. Thus, active interfacial Cu+ shows different CO catalytic performance as a function of the interfacial regulating of CeO2-CuOx-RGO composites. It is clear from Figures 7 and S7 that the intensity of this band for samples CeO2/CuOx-RGO, 24


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CuOx/CeO2-RGO and CeO2-CuOx-RGO is distinctly higher than that for RGO/CeO2-CuOx. Such a difference could be primarily caused by a high Cu+ content in samples CeO2/CuOx-RGO, CuOx/CeO2-RGO and CeO2-CuOx-RGO, as demonstrated by XRD and XPS. All these explain the nature for its better catalytic performance at low temperature. It is worth noting that the intensity of the band for Cu+-carbonyl species increases with the reaction temperature and reaches the maximum at 80 oC, owing to the enhanced interaction between copper ions and the chemisorbed CO molecules. Further increasing the reaction temperature, CO oxidation is largely accelerated, and the intensity of Cu+-carbonyl band decreases. The bands in the zones of 2140-2100 cm−1 can be divided into two characteristic regions, presenting two kinds of copper species that possess different interaction strength with ceria.64 The bands at low frequencies are associated with the smaller copper oxide clusters that have a strong interaction with ceria due to the presence of a stronger M-C bond and a weaker C-O bond.13 In contrast, other bands at higher frequencies represent the larger copper oxide clusters that usually show a weak interaction with ceria. Thus, it can be deduced that the strengths of synergistic effects, based on the peak position of Cu+–CO band, follow the sequence of CeO2-CuOx-RGO < CeO2/CuOx-RGO < RGO/CeO2-CuOx < CeO2/CuOx = CuOx/CeO2-RGO. The relatively low wavenumber of the IR adsorption band for Cu+–CO species and stronger synergistic effects on CeO2-CuOx-RGO and CeO2/CuOx-RGO are responsible for their low reduction temperature in H2-TPR even though the particle size of component CuO is not the smallest one. In addition, the red shift of the Cu+-carbonyl species peak, caused by the reduction of Cu+ to Cu0 in H2 riched, as reported in literature,65 was not observed during the heating process. This means that Cu+ is stable in 25



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these samples. Moreover, it is generally considered that Cu0 facilitates H2 dissociation and oxidation.66 Thus, there is no red shift of Cu+-carbonyl species peak, indicating a high CO2 selectivity for these samples. On the basis of all the above results, CeO2/CuOx-RGO sample is expected to give the best catalytic activity among all samples. 3.7. Catalytic Performance of the Composites

Figure 8. CO conversion (a) and CO2 selectivity (b) over the catalysts ( [CO]in=1%, [O2]in=1.25%, [H2]in=50%, He balance; T=30–240

o

C, GHSV=80,000 mL·gcat−1 ·h

−1

, room temperature,

atmospheric pressure).

To evaluate the catalytic performance of the as-prepared samples, CO-PROX reaction was carried out with a space velocity of 80, 000 mL·gcat−1·h−1. In this experiment, CO oxidation (CO+ O2→CO2) is the target reaction; while H2 oxidation (H2+O2→H2O) is the side reaction to be minimized. Thus, both CO conversion and CO2 selectivity under the operation temperatures are very important to evaluate a PROX catalyst. Figure 8(a, b) shows the variations of CO conversion and CO2 selectivity for as-prepared samples with temperature in hydrogen-rich streams. According to T50% and the 26


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temperature window, CeO2/CuOx-RGO sample exhibited a higher low-temperature oxidation activity and wider temperature window for CO-PROX, relative to sample CeO2/CuOx, confirming the important role of RGO in the samples. Due to the existence of a large amount of Cu+ species on the surface of CeO2/CuOx-RGO sample, as demonstrated by XPS analysis, introduction of RGO facilitates the dispersion and reduction of copper-cerium species, improves the formation of Ce3+ and oxygen vacancies, and enhances the interaction between copper and cerium species. Both Cu+ species and the synergistic effect of copper-cerium species show the positive effects on CO oxidation, since the surface enriched Cu+ is beneficial for activating CO and since synergistic interaction of CuO-CeO2 interfacial sites favors the electron transfer between Cu2+/Cu+ and Ce4+/Ce3+. All these promote the catalytic activity of CuO-CeO2 catalysts. The best CO conversion observed for multi-interface catalyst CeO2/CuOx-RGO should be related to the efficient synergistic effect given by these factors. The interfacial regulating reported here could change and optimize the surface structure of CeO2-CuOx-RGO composites, which may explain why CeO2/CuOx-RGO catalyst exhibits a strikingly high catalytic oxidation activity at low-temperature coupled with a broader operation temperature window when comparing to the traditional CuO-CeO2-based catalysts reported in literature (Table S4).

As shown in Figure 8b, CO2 selectivity of all samples remained more than 90 % from 50 to 100 o

C. When the reaction temperature reached 120 oC, competitive oxidation reaction of H2 and CO

occurs, and O2 selectivity for CO steeply decreases. Therefore, CO2 selectivity is determined by the competitive oxidation reactions of H2 and CO, as governed by oxidation state of copper species, the content and distribution of Cu+ site, and the synergistic interactions between Cu–Ce mixed oxides 27



and RGO. The ratios of reduced copper species and Ce3+ are similar for both samples CeO2/CuOx-RGO and CuOx/CeO2-RGO, while the CO2 selectivity is different. When comparing to the sample CuOx/CeO2-RGO, the sample CeO2/CuOx-RGO is merited by (i) a high-concentration of active species Cu+ enriched on surface (Table 2), (ii) highly dispersed copper oxide clusters showing a strong interaction with ceria species, and (iii) synergistic interactions between Cu–Ce mixed oxides and RGO (Figures 4, 7). All these account for the differences in CO conversion at high temperature and CO2 selectivity for both samples.

Figure S8 shows the relationship between CO conversion and CO2 selectivity for all samples. CeO2/CuOx-RGO sample shows a relatively higher performance than others, no matter from the viewpoint of CO conversion or CO2 selectivity. Most importantly, among all catalysts, only CeO2/CuOx-RGO can work around 200 oC, superior to other samples. The reaction rates of the as-synthesized catalysts were measured under a reaction condition with a weight hourly space velocity of 200, 000 ml·gcat-1·h-1 at given temperatures. The reaction rates determined for catalyst CeO2/CuOx-RGO were 9.08×10-7 mol•S-1•gcat-1 at 50 oC, 1.51×10-5 mol•S-1•gcat-1 at 140 oC, and 1.01×10-5 mol•S-1•gcat-1 at 240 oC, respectively, which are all apparently higher than those for other samples (Figure S9). These results demonstrate that CeO2/CuOx-RGO exhibits an excellent catalytic performance because of its optimal interface/surface structure.

4. CONCLUSION

We have provided a novel strategy to synthesize CeO2-CuOx-RGO composites with tunable multi-interfaces. Addition of RGO and the sequence of support formation are found to play the vital 28


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roles in interfacial regulating necessary for the promoted CO-PROX reaction. RGO is favorable for the generation of a large amount of Cu+ and the dispersion of copper-cerium species, while the sequence of support formation directly governs the multi-interface among the components, thereby regulating the catalyst properties. In CeO2-CuOx-RGO composites, CeO2/CuOx-RGO catalyst exhibits the best catalytic performance, which could realize 100% CO conversion at 140 oC, showing a broad operation window up to 220 oC. The excellent catalytic performance of this sample is beneficial from the high-concentration active species Cu+ enriched on surface, highly dispersed copper oxide clusters subjected to a strong interaction with ceria species, and the synergistic interactions between Cu–Ce mixed oxides and RGO. The interfacial regulating strategy reported in this work may show a great potential to prepare excellent low-valence metal ions enriched nano-composites with the optimum multi-interfaces and catalytic performance for many important applications.

■ASSOCIATED CONTENT s

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns and the data refinements, FT-IR spectra, TGA curves, SEM images, TEM images of CeO2-CuOx-RGO and CuOx/CeO2-RGO, Auger spectra, DRIFTS spectra of CeO2-CuOx-RGO, Comparison of catalytic performance in CO-PROX over CeO2/CuOx-RGO and those catalysts reported in literature, Selectivity to CO2 as a function of CO conversion for samples and reaction rates of the samples. ■AUTHOR INFORMATION 29



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Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS This work is financially supported by NSFC (Grants 21671077, 21771171, 21571176, 21611530688, and 21025104).

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