Catalysis Communications 51 (2014) 95–99

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Catalysis Communications

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Short Communication

Improving the dispersion of CeO2 on γ-Al2O3 to enhance the catalyticperformances of CuO/CeO2/γ-Al2O3 catalysts for NO removal by CO

Chengyan Ge a,1, Lichen Liu a,1, Zhuotong Liu a, Xiaojiang Yao a, Yuan Cao a, Changjin Tang a,⁎,Fei Gao b, Lin Dong a,b,⁎a Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR Chinab Modern Analysis Center, Nanjing University, Nanjing 210093, PR China

⁎ Corresponding authors at: Hankou Road 22#, NanjinChina. Tel.: +86 25 83592290; fax: +86 25 83317761.

E-mail addresses: tangcj@nju.edu.cn (C. Tang), donglin1 These authors contribute equally.

http://dx.doi.org/10.1016/j.catcom.2014.03.0321566-7367/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 December 2013Received in revised form 27 March 2014Accepted 28 March 2014Available online 4 April 2014

Keywords:HAcSize effectNO+ COCuO/CeO2/γ-Al2O3

Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO2-modified γ-Al2O3 sup-port.With the help of HAc, the dispersion of CeO2 on γ-Al2O3 is significantly improved and the size of CeO2 nano-particles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that thepercentages of Ce3+ in CeO2 nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO2-modified γ-Al2O3 support and choose NO reduction with CO as a probe reaction to investigate the influencesof impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO2/γ-Al2O3 preparedin the solvent with volume ratio of 20:1 (H2O:HAc) has the highest activity in NO + CO reaction. Combing thestructural characterizations and catalytic performances, we think that the size of the CeO2 nanoparticles shouldbe a key factor that affects the activities of CuO/CeO2/γ-Al2O3. Furthermore, CuO dispersed on CeO2 nanoparticleswith an average size of ca. 5 nm should be the highest active sites for NO + CO reaction.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nitrogen oxides (NOx) and carbon monoxide (CO) emitted fromvehicle exhausts as some of the main atmospheric pollutants havecaused acute health problems to human beings [1–3]. Catalyticelimination is considered as the most efficient approach to handlethese pollutants. The reaction of NO reduction by CO has attractedmore attention due to the simultaneous transformation of NO and COto non-toxic N2 and CO2.

It is widely accepted that the properties of supports have great ef-fects on the catalytic behaviors of active components in the supportedcatalysts [4]. Surface modifications on the supports are proved to be ef-fective methods to enhance the catalytic performances. CeO2-modifiedγ-Al2O3 is widely used in catalysis because of its combination of thelarge surface area and stability of γ-Al2O3 and the oxygen storage andrelease capability of CeO2. Many kinds of metal nanoparticles are sup-ported on CeO2/γ-Al2O3 for catalytic applications. For example, Pd andNi nanoparticles have been loaded on CeO2/γ-Al2O3 for steamreformingofmethane [5,6]. Au/CeO2/γ-Al2O3 catalysts are used for the total oxida-tion of propene [7]. Metal oxides can also be dispersed on CeO2/γ-Al2O3

to enhance their catalytic performances. V2O5 has been dispersed on

g, Jiangsu Province 210093, PR

@nju.edu.cn (L. Dong).

CeO2/γ-Al2O3 for oxidation of dichloromethane [8]. CuO/CeO2/Al2O3 isa superior catalyst for the total oxidation of toluene [9].

Notably, in these previous studies, the content of CeO2 in CeO2/γ-Al2O3 supports usually has an optimized amount [10,11]. When theamount of CeO2 is low, the enhancement of catalytic activities is not ob-vious. However, when the amount of CeO2 is high, the catalytic perfor-mances begin to drop. As we have known, CeO2 is hard to disperse onγ-Al2O3 compared with other metal oxides (like TiO2), and can showan obvious XRD peak of CeO2 even though in low content (b5 wt.%)[11]. When the content of CeO2 increases, the size of CeO2 will growlarger quickly. Considering that the oxygen vacancy distributions andoxygen storage and release capability (OSR) of CeO2 are related withits particle size [12,13], the catalytic properties of CeO2/γ-Al2O3 sup-ports may have relationships with the size of CeO2 nanoparticles dis-persed on γ-Al2O3. In addition, the active components supported onCeO2/γ-Al2O3 are usually active when they contact with CeO2. Thepoor dispersion property of CeO2 on γ-Al2O3 is not favorable to increasethe contact of active components and CeO2. Therefore, improving thedispersion of CeO2 and controlling the size of CeO2 nanoparticles canbe alternative strategies to improve the catalytic performances ofCeO2/γ-Al2O3.

In thiswork,we develop amethod to improve the dispersion of CeO2

on γ-Al2O3. Acetic acid is addedwhenwe load CeO2 on γ-Al2O3 throughimpregnation. TEM images show that sizes of CeO2 nanoparticles aredramatically decreased with the assistant of acetic acid. Furthermore,the size of CeO2 nanoparticles can be tuned by controlling the concen-tration of acetic acid. CuO is loaded on the CeO2/γ-Al2O3 supports with

10 20 30 40 50 60 70 80

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♣−

♠− CeO 2

γ -Al2O3

Ce/Al (H2O)

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Ce/Al (50-1 )

Ce/Al (20-1 )

Ce/Al (1-1)

Ce/Al (HA c)

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10 20 30 40 50 60 70 80

Cu/Ce/Al (H2O)

Cu/Ce/Al (50-1)

Cu/Ce/Al (20-1)

Cu/Ce/Al (1-1)

Cu/Ce/Al (HA c)

♣

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♣−

♠

♠

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CeO 2

γ -Al2O3

b

Fig. 1. XRD patterns of (a) Ce/Al(n-m) and (b) Cu/Ce/Al(n-m).

96 C. Ge et al. / Catalysis Communications 51 (2014) 95–99

different CeO2 particle sizes for NO+CO reaction.Wefind that the CuO/CeO2/γ-Al2O3 catalysts with CeO2 nanoparticles of 5 nm show the bestactivity. These results show that more active CeO2/γ-Al2O3 supportscan be obtained through the acetic acid-assisted method, and the sizeof CeO2 nanoparticles has a significant influence on their activities.

2. Experimental

2.1. Catalyst preparation

2.1.1. The preparation of CeO2/γ-Al2O3 supportsγ-Al2O3 was purchased from the Fushun Petrochemical Institute in

China and was calcined at 750 °C for 7 h before being used for catalystpreparation. Its BET surface area was 150 m2 g−1.

CeO2/γ-Al2O3 samples were prepared by wet impregnation ofγ-Al2O3 with the mixed solution containing different volume ratiosof H2O and HAc (VH2O:VHAc = n-m) containing a required amount ofCe(NO3)3, The samples were dried at 110 °C overnight and thencalcined at 500 °C in flowing air for 4 h. For simplicity, samplesCeO2/γ-Al2O3(n-m) will be hereafter referred to as Ce/Al(H2O),Ce/Al(HAc), Ce/Al(1-1), Ce/Al(20-1), and Ce/Al(50-1), respectively.

2.1.2. The preparation of CuO/CeO2/γ-Al2O3 catalystsCuO/CeO2/γ-Al2O3 catalysts were prepared by wet impregnation of

CeO2/γ-Al2O3(n-m) supports with an aqueous solution containingCu(NO3)2 (CuO loading amount is 2.7 wt.%). The samples were driedat 110 °C overnight and then calcined at 500 °C in flowing air for 4 h.The resultant samples were denoted as CuO/CeO2/γ-Al2O3(n-m). Forsimplicity, samples CuO/CeO2/γ-Al2O3(n-m) will be hereafter referredto as Cu/Ce/Al(H2O), Cu/Ce/Al(HAc), Cu/Ce/Al(1-1), Cu/Ce/Al(20-1)and Cu/Ce/Al(50-1), respectively.

2.2. Catalyst characterization

XRD patterns were recorded on a Philips X'pert Pro diffractometerusing Ni-filtered Cu Kα radiation (λ = 0.15418 nm). The X-ray tubewas operated at 40 kV and 40 mA.

X-ray photoelectron spectroscopy (XPS) analysis was performed ona PHI 5000 Versaprobe system, using Al Kα radiation (1486.6 eV) oper-ating at an accelerating power of 150W. All binding energies (BEs)werereferenced to the C 1s peak at 284.6 eV. This reference gave BE valueswith an accuracy at ±0.1 eV.

H2-temperature-programmed reduction (H2-TPR) was carried outin a quartz U-tube reactor connected to a thermal conduction detectorwith H2–Ar mixture (7.3% H2 by volume) as a reducing agent. 50.0 mgof sample was used for each measurement. Before switching to the

H2–Ar stream, the sample was pretreated in a N2 stream at 300 °C for1 h. TPR started from room temperature at a rate of 10 °C min−1.

Transmission electronic microscope (TEM) images were recordedon a JEM-2100 (JEOL, Ltd.) transmission electronic microscope withthe accelerating voltage of 200 kV. The samples were crushed anddispersed in AR grade ethanol and the resulting suspensions wereallowed to dry on carbon film supported on copper grids.

2.3. Catalytic activity tests

The activities of the catalysts were determined under a light-off pro-cedure, involving a feed steamwith a fixed composition, 5% NO, 10% COand 85% He by volume as diluents. The fresh catalysts (50 mg) werepretreated in N2 stream at 100 °C for 1 h, and then the mixed gaseswere switched on. The reactions were carried out at different tempera-tureswith GHSV= 12,000 h−1. The reactionwas started from 150 °C to400 °C in the same procedure on the treated catalysts. Gas chromato-graph (GC) equipped with a thermal conductivity detector was usedfor analyzing the products, column A with Porapak Q was chosen forseparating N2O and CO2, and column B with 5 A and 13× molecularsieve (40–60 M) was used for separating N2, NO and CO.

3. Results and discussion

Firstly, in order to study the effect of solvent in the wet impregna-tion,we investigated the phase structures of as-prepared Ce/Al supportsand Cu/Ce/Al(n-m) catalysts. Fig. 1a shows the XRD patterns of a seriesof Ce/Al(n-m) catalysts prepared through HAc aqueous with differentconcentrations. Only diffraction peaks corresponding to CeO2 and γ-Al2O3 can be observed in all the samples. The intensities of peaks corre-sponding to γ-Al2O3 are similar in each sample, while the intensitiescorresponding to CeO2 are different. Notably, the intensities of peaksfor CeO2 show a negative relationship to the concentration of HAc aque-ous. For supported crystalline oxide, particle size can be estimatedthrough the Scherrer's equation [14]. Therefore, it indicates that thesize of CeO2 particles supported on γ-Al2O3 in Ce/Al(n-m) supportsin the following order: Ce/Al(H2O) N Ce/Al(50-1) N Ce/Al(20-1) N Ce/Al(1-1) N Ce/Al(HAc). From the XRD results, it impliesthat HAc can decrease the size of CeO2 nanoparticles, which leadsto the improvement of dispersion of CeO2 on Al2O3. The XRD pat-terns of Cu/Ce/Al(n-m) catalysts are displayed in Fig. 1b. Comparedwith bare CeO2 and γ-Al2O3 supports, no additional diffractionpeaks for crystalline copper oxide can be found in Cu/Ce/Al(n-m)catalysts except for CeO2 and γ-Al2O3, indicating that copperoxide was dispersed well or below the detection limit of XRD[15,16].

97C. Ge et al. / Catalysis Communications 51 (2014) 95–99

Structural details of Ce/Al supports were investigated by TEM(Fig. 2). Low-magnitude TEM images of Ce/Al supports showed thatthey exhibited similar microscopic morphologies. CeO2 nanoparticlesdispersed on γ-Al2O3 could be seen in HRTEM images and HAADF-STEM and EDX elementmapping are operated to get amore clear visionof dispersion of copper species and ceria onγ-Al2O3 (see Fig. S1–Fig. S3).The spacing of lattice fringes around 0.31 nmwas ascribed to the (111)planes of CeO2. Comparing the size of CeO2 nanoparticles in differentsamples, we could find that the size of CeO2 nanoparticles would de-crease with the increasing of the concentration of HAc aqueous. Thesize of CeO2 nanoparticles in Ce/Al(H2O) is about 10 nm while the sizeof CeO2 nanoparticles in Ce/Al(HAc) is only ca. 3 nm. These resultswere consistent with the XRD patterns, which confirm that the concen-tration of HAc aqueous would have a significant influence on the parti-cle size of CeO2 dispersed on γ-Al2O3. Previous studies had proved thatthe concentration of Ce3+ and oxygen vacancies in CeO2 nanoparticleswere associated with particle size of CeO2. Since oxygen vacancies andCe3+play important roles in theCeO2-based catalysts for redox catalytic

Fig. 2. The TEM images of (a) Cu/Ce/Al(H2O), (c) Cu/Ce/Al(50-1), (e) Cu/Ce/Al(20-1), (g) Cu/Ce/1), (f) Cu/Ce/Al(20-1), (h) Cu/Ce/Al(1-1), and (j) Cu/Ce/Al(HAc).

reactions, Ce/Al supports may have different catalytic properties due tothe different sizes of CeO2.

H2-TPR was conducted to investigate the influence of HAc on theredox properties of copper species supported on different Ce/Al sup-ports. As displayed in Fig. 3, the H2-TPR profiles of five Cu/Ce/Al(n-m)catalysts showed similar shapes. Three reduction peaks could be ob-served in all the samples, which could be ascribed to the reduction ofwell dispersed copper species on CeO2 (α peak), copper incorporatedinto CeO2 lattice (β peak) and the typical CuO interacting with γ-Al2O3 (γ peak), respectively. Furthermore, for the Cu/Ce/Al(20-1) sam-ple, theα peak,β peak andγpeak are 172 °C, 210 °C and 247 °C, respec-tively, which were lower than the other catalysts, suggesting thatcopper species were more easily to be reduced. To explain this moreclearly, the H2-TPR profiles of CuO/CeO2 (denoted as Cu/Ce) and CuO/γ-Al2O3 (Cu/Al) catalysts with the same Cu content to that of CuO/CeO2/γ-Al2O3 are shown in Fig. S4. It is observed that there are twomajor peaks for Cu/Ce and only one peak for Cu/Al. Moreover, thepeak temperature of Cu/Ce is lower than that of Cu/Al. These are in

Al(1-1), and (i) Cu/Ce/Al(HAc) and HRTEM images of (b) Cu/Ce/Al(H2O), (d) Cu/Ce/Al(50-

100 200 300 400 500

Cu/Ce/Al(HAc)

Cu/Ce/Al(1-1)

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αβ

γ

247172

210

Fig. 3. H2-TPR profiles of Cu/Ce/Al(H2O), Cu/Ce/Al(50-1), Cu/Ce/Al(20-1), Cu/Ce/Al(1-1)and Cu/Ce/Al(HAc).

98 C. Ge et al. / Catalysis Communications 51 (2014) 95–99

good agreement with our assignment of the three peaks shown in CuO/CeO2/γ-Al2O3 catalysts. That is, the first two peaks are arrived from cop-per–ceria entity and the last peak is attributed to Cu/Al.

The chemical states of copper species supported on Ce/Al are inves-tigated by XPS in Fig. 4a. Peaks located at 934.6 eV and 932.7 eV can beascribed to Cu2+ and Cu+, respectively. The percentages of Cu2+ andCu+ can be calculated according to the areas of peaks, which are listedin Table 1. Cu/Ce/Al(20-1) shows the highest percentage of Cu+ com-pared with Cu/Ce/Al(H2O) and Cu/Ce/Al(HAc). Besides, the chemicalstates of Ce in CeO2 nanoparticles are related with their sizes [13,17].The percentages of Ce3+ on the surface of catalysts are also calculatedbased on the Ce 3d XPS spectra in Fig. 4b. For smaller CeO2 nanoparti-cles, there is more Ce3+ especially when the size is below 3 nm. Whilethe size of CeO2 nanoparticles is as large as 10 nm, Ce3+ mainly locateson the surface and the percentage is low. Therefore, the percentages

950 945 940 935 930 925

Cu/Ce/Al(HAc)

Cu/Ce/Al(H2O)

Cu/Ce/Al(20-1)

Inte

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ty(a

.u)

Binding Energy(eV)

a

Fig. 4. (a) XPS spectra of Cu2p region of Cu/Ce/Al(H2O), Cu/Ce/Al(20-1) and Cu/Ce/Al(HAc)

of Ce3+ in different Cu/Ce/Al samples follow this order: Cu/Ce/Al(HAc) N Cu/Ce/Al(20-1) N Cu/Ce/Al(H2O).

The NO conversion and N2 selectivity at different operation temper-atures for Cu/Ce/Al(n-m) catalysts are shown in Fig. 5. When the im-pregnation solvent has a small amount of HAc, the corresponding Cu/Ce/Al(50-1) and Cu/Ce/Al(20-1) samples show enhanced activitiesthan Cu/Ce/Al(H2O). However, when large amount of HAc or pure HAcis used as the solvent, Cu/Ce/Al(1-1) and Cu/Ce/Al(HAc) catalystsshow lower activities than Cu/Ce/Al(H2O). Previousworks of our groupsshow that reduced copper species are favored for NO + CO reac-tion [18,19]. In this work, Cu species are supported on CeO2 nanoparti-cles or γ-Al2O3. The particle size of CeO2 can be controlled through theconcentration of impregnation solvent. TPR results show that more Cuspecies will contact with CeO2 nanoparticles in Cu/Ce/Al(HAc) and Cu/Ce/Al(20-1) than in Cu/Ce/Al(H2O) because HAc can improve the dis-persion of CeO2. Furthermore, the chemical states of Cu species arealso affected by the redox properties of CeO2 nanoparticles [12]. ForCeO2 nanoparticles around 4 nm, their oxygen storage and release abil-ity is superior to smaller CeO2 (b3 nm) or large CeO2 nanoparticles.CeO2 with particle sizes of 4–5 nm contributes to the excellent catalyticperformances in NO+CO reaction. For the possible reasons, two pointsare proposed. On one hand, the superior oxygen storage capacity linkedto oxygen vacancies provides the binding and activation sites for NOmolecules [20–22], which is conducive to the enhancement of NO activ-ity. On the other hand, the dispersion and size of CeO2 also affect thestate of the active species of copper [12], thereby affecting the catalyticactivity for NO + CO. When copper species are dispersed on 4–5 nmCeO2 nanoparticles, they may be more easily to be reduced and moreCu+ species can be found. Therefore, the Cu/Ce/Al(20-1) shows thehighest activity. When CuO are supported on different Ce/Al supports,more Cu+ species can be stabilized by CeO2 around 4 nm. Combingthe higher content of Cu+ (see Fig. S5 and Table 1) and better oxygenstorage and release ability, Cu/Ce/Al(20-1) sample shows the bestperformance in NO+ CO reaction.

4. Conclusions

In this work, we develop a facile method to improve the dispersionof CeO2 on γ-Al2O3 by using aqueous HAc as the impregnation solvent.The particle size of CeO2 can be controlled from 3 nm to 10 nm. CuO isloaded on CeO2/γ-Al2O3 with different CeO2 particle sizes for NO+ CO reaction. There are more Cu+ species in Cu/Ce/Al(20-1) whichare active for NO + CO reaction. Furthermore, CeO2 nanoparticles

920 910 900 890 880

Cu/Ce/Al(HAc)

Cu/Ce/Al(20-1)

Cu/Ce/Al(H2O)

Binding Energy (eV)

Inte

nsi

ty(a

.u)

b

. (b) XPS spectra of Ce3d region of Cu/Ce/Al(H2O), Cu/Ce/Al(20-1) and Cu/Ce/Al(HAc).

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Cu/Ce/Al(H2O) Cu/Ce/Al(50-1) Cu/Ce/Al(20-1) Cu/Ce/Al(1-1) Cu/Ce/Al(HAc)

b

Fig. 5. (a) NO conversion and (b) N2 selectivity (%) over the Cu/Ce/Al(n-m).

Table 1Atomic concentration of elements in Cu/Ce/Al(n-m) obtained from XPS.

Cu/Ce/Al(n-m) Cu (at.%) O (at.%) Ce (at.%) The proportion of Ce/Cu on surface Cu+/Cu2+ Ce3+%

Cu/Ce/Al(20-1) 0.55 56.36 0.84 1.5 2 17%Cu/Ce/Al(H2O) 0.81 57.25 0.57 0.7 1.7 12%Cu/Ce/Al(HAc) 1.51 55.4 0.43 0.3 1 20%

99C. Ge et al. / Catalysis Communications 51 (2014) 95–99

around 4 nm have better oxygen storage and release capacity. Thus,Cu/Ce/Al(20-1) shows the best catalytic performance because ofthe proper size of CeO2 nanoparticles. We believe that this workcan contribute to the synthesis of more active CeO2–Al2O3 compos-ite oxides for heterogeneous catalysis.

Acknowledgment

Financial support from the National Natural Science Foundation ofChina (21273110) and the National Basic Research Program of China(No. 2010CB732300) is greatly acknowledged. Kind assistance in TEMcharacterization offered by Dr. Xihong Lu from Sun Yat-Sen Universityis also gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2014.03.032.

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