Applied Catalysis A: General 242 (2003) 151–159

Redox behavior and catalytic properties ofCuO/Ce0.8Zr0.2O2 catalysts

Lei Maa,∗, Meng-Fei Luob, Song-Ying Chenaa Institute of Catalysis, Zhejiang University, Xixi Campus, Hangzhou 310028, PR Chinab Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China

Received 27 June 2002; received in revised form 6 September 2002; accepted 9 September 2002

Abstract

In this paper, the CuO/Ce0.8Zr0.2O2 catalysts were prepared for the catalytic reduction of NO by CO. The structure andthe redox behavior of these catalysts were investigated by using XRD and H2-TPR. Three peaks (�, � and�) are observedwhen H2 is used as reducing agent. The� peak is attributed to the dispersed copper oxide, the� peak is attributed to themoderate size bulk CuO and the� peak is attributed to the large size bulk CuO. The Cu loading and calcination temperaturehave effects on the catalytic activity for NO–CO reaction and the N2O selectivity. We conclude that the catalytic activity isrelated to the moderate size bulk CuO particles, and the N2O selectivity is related to the large size bulk CuO particles. Thedispersed copper oxide does not contribute to the catalytic activity and N2O selectivity.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ce0.8Zr0.2O2 solid solution; CuO; NO–CO reaction

1. Introduction

Recently, the selectivity catalytic reduction (SCR)of NO by CO over supported noble metal (Pt, Pdor Rh) catalysts has been intensively studied[1–6].Because of high prices and scarcity of noble metal,it is important to find an alternative catalytic com-ponent to reduce even replace the noble metal. A lotof research reveals that the transition-metal oxideshave high catalytic activity of reduction NO by CO[7–15]. Copper oxide has been demonstrated to be avery active species among the transition-metal oxidesfor this reaction[8,16–21].

∗ Corresponding author. Tel.:+86-571-88273283;fax: +86-571-88273283.E-mail address: leiman2001@hotmail.com (L. Ma).

Cerium oxide has been widely used in the automo-tive three-way catalytic converter. The most importantfunction of cerium oxide is high oxygen storage ca-pacity (OSC). The OSC provided by the redox cou-ple Ce4+/Ce3+ plays also a crucial role in enhancingthe activity in reducing conditions, making more oxy-gen available for the oxidation processes[22]. Further-more, the oxygen vacancies associated with reducedceria in the proximity of noble-metal particles havebeen suggested as promoting sites for NO and COconversion[22,23]. The structural modification andimperfection of the CeO2 lattice by insertion of othercation cause the enhancement of redox properties.CeO2–ZrO2 solid solution has higher thermally sta-ble, redox properties and catalytic activity than that ofCeO2 [24–26]. However, little study has been reportedthat the Ce0.8Zr0.2O2 solid solution supported copperoxide acted as a catalyst for reduction of NO by CO.

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(02)00509-4

152 L. Ma et al. / Applied Catalysis A: General 242 (2003) 151–159

The present work is concerned with redox behaviorof the CuO/Ce0.8Zr0.2O2 catalysts and also with thecatalytic activity for NO–CO reaction and selectivityof N2O in products. The attention is focused on therelation between the redox behavior and the catalyticactivity and selectivity.

2. Experimental

2.1. Preparation Ce0.8Zr0.2O2 solid solution

The Ce0.8Zr0.2O2 solid solution was prepared byevaporating an aqueous solution of the mixed metalnitrates containing an equivalent amount of citric acidto obtain a gel, followed by decomposition at 950◦Cfor 4 h [27]. The BET surface area of Ce0.8Zr0.2O2solid solution is 9.2 m2/g.

2.2. Preparation of catalysts

The CuO/Ce0.8Zr0.2O2 catalysts were prepared bythe conventional wet impregnation method using anaqueous solution of Cu(NO3)2. Prepared sampleswere dried at 120◦C, and then calcined at 650◦Cin air for 4 h. The CuO/Ce0.8Zr0.2O2 catalyst is do-nated as CuO/CZ. The loading of Cu is indicatedas CuO(x%)/CZ, where x represent the nominalCu loading (wt.%). In order to know the effect ofcalcination temperature on the CuO/CZ catalyst,a series of CuO(10%)/CZ catalysts were preparedat different calcination temperatures. The calcina-tion temperatures were 450, 650, 850 and 950◦C.The catalyst is donated as, for example CuO(10%)/CZ(450◦C).

2.3. X-ray diffraction

XRD data were obtained on a Rigaku D/max-IIIB powder diffractometer, using Cu K� radiation. Theintensity data were collected at 25◦C over a 2θ rangeof 20–80◦ with a step interval of 0.02◦.

2.4. Activity measurements

Catalytic activity measurements were carried out ina fixed bed reactor (i.d. 6 mm) using a 150 mg of cat-alyst. The total gas flow rate was set at 80 ml/min.

For NO–CO reaction, the gas consists of 6% NOand 6.5% CO in He. The catalysts were directly ex-posed to the reaction gas as the reactor temperaturewas stabilized without any pretreatment. The productswere analyzed by gas chromatography with MolecularSieves 13X and Porapak Q columns, both operating at60◦C.

2.5. H2-temperature-programmedreduction (H2-TPR)

The reduction properties of catalysts were measuredby means of the temperature-programmed reduction(TPR). TPR measurements were made in a flow sys-tem. The 20 mg catalyst was placed in a TPR cell(6 mm i.d.), into which H2:N2 (6:94) mixed gas wasintroduced. The temperature of the sample was pro-grammed to rise at a constant rate of 20◦C/min and theamount of H2 uptake during the reduction was mea-sured by a thermal conductivity detector (TCD). Thewater produced in TPR was trapped on a 5 Å molec-ular sieve.

3. Results and discussion

3.1. Catalysts characterization

Fig. 1 shows the XRD patterns of CuO/CZ cat-alysts with various Cu loadings. The pattern ofCe0.8Zr0.2O2 support is also showed for comparing.Only cubic phase is observed for Ce0.8Zr0.2O2 solidsolution. The weak peaks attributed to CuO crystalphase can be observed on CuO(1%)/CZ catalyst. Itindicates that the CuO bulk has already formed on theCuO(1%)/CZ catalyst. This may be related to smallsurface area of the Ce0.8Zr0.2O2 support. The CuOcrystal peaks become strong with increasing the Culoading.

Fig. 2 shows the XRD patterns of CuO(10%)/CZcatalysts which calcined at different temperatures.Table 1 lists the average size of CuO particles onCuO/CZ catalysts calcined at different temperatures.It shows the trend that the particle size of CuO crystalincreases with the calcination temperature increasing.It indicates that the CuO particles are sintered easilyat high temperature (>650◦C). But the average par-ticle size of CuO crystal on CuO(10%)/CZ(450◦C)

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Fig. 1. XRD patterns of CuO/CZ catalysts with various Cu loadings.

catalyst is close to that of CuO(10%)/CZ(650◦C)catalyst. It implies that calcining catalyst at low tem-perature (<650◦C) is not benefit for the dispersingof CuO particles obviously.

Fig. 2. XRD patterns of CuO(10%)/CZ catalysts calcined at different temperatures.

3.2. Reduction properties of CuO/CZ catalysts

Fig. 3 shows the TPR profiles of CuO/CZ catalystswith various Cu loadings. The reduction profile of the

154 L. Ma et al. / Applied Catalysis A: General 242 (2003) 151–159

Table 1Particle size of CuO crystala

Catalyst CuO(1 1 1) crystalparticle size (nm)

CuO(10%)/CZ(450◦C) 43.3± 1.9CuO(10%)/CZ(650◦C) 44.9± 2.1CuO(10%)/CZ(850◦C) 206.4± 6.3CuO(10%)/CZ(950◦C) 272.3± 8.6

a The data of the crystal particle sizes were the calculationresults using the Rietveld analysis. Every data is the average crystalparticle size.

Ce0.8Zr0.2O2 support is characterized by a single peakat about 640◦C. All CuO/CZ catalysts also have thepeaks at adjacent temperature. We believe that thesepeaks are attributed to the Ce0.8Zr0.2O2 support. TheCuO(1%)/CZ catalyst has two other peaks, however,a third peak is observed on other catalysts with higherCu loadings. These three peaks are designated by�, �and� in Fig. 3. They are attributed to the reduction ofCuO. The experimental H2 consumptions of�, �, �and Ce0.8Zr0.2O2 peaks are listed inTable 2. The the-oretical H2 consumption (CuO+ H2 → Cu+ H2O)is also listed in it. We find that the experimental H2consumption is close to theoretical H2 consumption.It means that the support and the CuO species are not

Fig. 3. H2-TPR profiles of CuO/CZ catalysts with various Cu loadings.

simultaneously reduced and the CuO is not in stronginteraction with the surface of support. It can be seenthat the H2 consumption and the position of the�peak remain unchanged with increasing Cu loading.However, the H2 consumption of the� and � peaksdepends on the Cu loadings strongly, and the posi-tions of the� and� peaks shift to higher temperaturewith increasing Cu loading. The XRD result (Fig. 1)reveals that CuO particles form crystalline CuO eas-ily even when the Cu loading is 1%. Zheng et al.[28]and La et al.[29] reported that CuO on the supportsurface existed in both crystalline and non-crystallineforms (dispersed metal oxide). So, we believe that the� peak is the dispersed copper oxide, while the� and� peaks are the bulk CuO[30]. The intensity of�and� peaks increase when CuO loading increases. Itmeans that the reduction of� and � peaks becomesmore difficult with increasing CuO loading. So the po-sitions of � and � peaks shift to higher temperaturewith increasing CuO loading.

Fig. 4shows the TPR profiles of CuO(10%)/CZ cat-alysts which calcined at different temperatures. The�,� and� peaks can also be observed on these profiles.The H2 consumptions of the�, � and� peaks are listedin Table 3. As the calcination temperature increases,the H2 consumption of the� peak decreases, even

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Table 2The H2 consumption of TPR peaks over CuO/CZ catalysts with various Cu loadings

Catalyst Experimental H2 consumption (10−6 mol/g) Theoretical H2 consumptionto reduce CuO (10−6 mol/g)

� peak � peak � peak CZ

CZ – – – 1.56± 0.08 –CuO(1%)/CZ 0.50± 0.03 3.04± 0.17 0 1.43± 0.07 1.86CuO(5%)/CZ 0.50± 0.03 9.33± 0.48 0.95± 0.05 1.47± 0.7 8.91CuO(10%)/CZ 0.50± 0.03 15.02± 0.77 2.17± 0.11 1.46± 0.08 16.90CuO(15%)/CZ 0.50± 0.03 20.32± 1.03 4.39± 0.24 1.50± 0.07 24.25

Fig. 4. H2-TPR profiles of CuO(10%)/CZ catalysts calcined at different temperatures.

disappears at 950◦C. The reason is dispersed cop-per oxide sintered easily to formed bulk CuO at hightemperature. As the calcination temperature increases(from 650 to 950◦C), the position of the� peak shifts

Table 3The H2 consumption of TPR peaks over CuO(10%)/CZ catalysts which calcined at different temperatures

Catalysts H2 consumption (10−6 mol/g)

� peak � peak � peak � + � peak

CuO(10%)/CZ(450◦C) 0.51± 0.03 17.24± 0.88 0 17.24± 0.88CuO(10%)/CZ(650◦C) 0.50± 0.03 15.02± 0.78 2.17± 0.13 17.19± 0.91CuO(10%)/CZ(850◦C) 0.28± 0.16 10.96± 0.59 6.07± 0.31 17.03± 0.90CuO(10%)/CZ(950◦C) 0 5.71± 0.30 11.10± 0.53 16.82± 0.83

to higher temperature and overlaps with� peak. Onthe same time, the H2 consumption of the� peak de-creases. However, the variety of the� peak is opposite.We find that the H2 consumptions of the� + � peak

156 L. Ma et al. / Applied Catalysis A: General 242 (2003) 151–159

are very close. It indicates that the� peak converts tothe� peak with increasing the calcination temperature.We suppose that the� peak is the moderate size bulkCuO, while the� peak is the large size bulk CuO. Themoderate size bulk CuO is sintered to form the largesize bulk CuO at higher temperature. The� peak tem-perature of CuO(10%)/CZ (450◦C) catalyst is higherthan that of CuO(10%)/CZ(650◦C) catalyst, we sup-pose that the main reason is the average particle sizeof CuO crystal on CuO(10%)/CZ(450◦C) catalyst isclose to that of CuO(10%)/CZ(650◦C) catalyst.

3.3. Catalytic activity for NO reduction by CO

Fig. 5 shows the catalytic activity of CuO/CZ cat-alysts with various Cu loadings. The Ce0.8Zr0.2O2support is also showed for comparing. The catalyticactivity of Ce0.8Zr0.2O2 is the lowest. The catalyticactivity of CuO/CZ catalysts enhanced with increas-ing the Cu loading. But the catalytic activity doesnot enhance obviously when the Cu loading exceeds10%. The catalytic activity of CuO(20%)/CZ catalystis very close to that of CuO(15%)/CZ catalyst. It in-dicates that increasing the Cu loading can not alwayspromote the catalytic activity. There must be a max-imum activity. The catalytic activity of the CuO/CZcatalyst which the Cu loading exceeds 10%, maybeclose to the maximum activity. According to the result

Fig. 5. Catalytic activity of CuO/CZ catalysts with various Cu loadings.

of H2-TPR (Fig. 3), the order of catalytic activity isconsistent with the varieties of� and � peaks. Wesuppose that the catalytic activity of CuO/CZ catalystis related to the bulk CuO.

Fig. 6 shows the catalytic activity of CuO/CZ cata-lysts calcined at different temperatures. As the calci-nation temperature increasing (from 650 to 950◦C),the catalytic activity decreases gradually. Accord-ing to the result of H2-TPR (Fig. 4), the order ofcatalytic activity is consistent with the varieties of� peak. On the basis of all above results, we be-lieve that the catalytic activity of CuO/CZ catalyst ischiefly related to the moderate size bulk CuO. For theCuO(10%)/CZ(450◦C) catalyst, its catalytic activityis the lowest among these four catalysts when thereaction temperature is below about 300◦C, but itscatalytic activity exceeds some other catalysts whenthe reaction temperature is above about 300◦C. Whenthe range of reaction temperature is from about 300to 400◦C, the NO–CO reaction will create a lot ofN2O on the CuO(10%)/CZ(450◦C) catalyst[31–33].The N2O selectivity of CuO(10%)/CZ(450◦C) cat-alyst is much higher than other three catalysts atthis reaction temperature range (Section 4). Wethink this can explain that the catalytic activity ofCuO(10%)/CZ(450◦C) catalyst is higher than othercatalysts at this reaction temperature range. On ba-sis of above results, we believe that 650◦C is a

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Fig. 6. Catalytic activity of CuO(10%)/CZ catalysts calcined at different temperatures.

comparatively appropriate calcinations temperature.The catalyst calcined at this temperature not onlyhas high catalytic activity but also has high N2selectivity.

Fig. 7. N2O selectivity of CuO/CZ catalysts with various Cu loadings.

3.4. N2O selectivity in products

Fig. 7 shows the N2O selectivity in products ofCuO/CZ catalysts with various Cu loadings. The N2O

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Fig. 8. N2O selectivity of CuO(10%)/CZ catalysts calcined at different temperatures.

selectivity in products decreases with the Cu load-ing increasing. According to the result of H2-TPR(Fig. 3), the order of N2O selectivity is opposite tothe varieties of� and� peaks.Fig. 8 shows the N2Oselectivity of CuO/CZ catalysts calcined at differenttemperatures. The N2O selectivity decreases with thecalcination temperature increasing. According to theresult of H2-TPR (Fig. 4), the order of N2O selec-tivity is opposite to the varieties of� peaks. On thebasis of all above results, we believe that the N2Oselectivity is determined by the large size bulk CuO.The CuO(1%)/CZ and CuO(10%)/CZ(450◦C) cat-alysts have no large size bulk CuO, therefore theyproduce a mass of N2O gas in the NO–CO reaction.On the contrary, there are many large size bulk CuOparticles on the CuO(10%)/CZ(950◦C) catalyst, theyrestrain strongly the formation of N2O in the NO–COreaction. So on the CuO(10%)/CZ(950◦C) catalyst,its N2O selectivity always keeps a very low propor-tion, approaches a constant at the range of reactiontemperature.

4. Conclusions

1. Because the BET surface area of Ce0.8Zr0.2O2 sup-port is very small, the XRD peaks attributed to CuO

crystal can be observed on all CuO/CZ catalystswith various Cu loading, even on the CuO(1%)/CZcatalyst.

2. The size of CuO particles increases with the calci-nation temperature increasing on the CuO(10%)/CZcatalysts.

3. The CuO/CZ catalysts have three reduction peaks(�, � and�) when use H2 as reduction agent. The� peak is attributed to the dispersed copper oxide,the � peak is attributed to the moderate size bulkCuO and the� peak is attributed to the large sizebulk CuO.

4. The Cu loadings and calcination temperature effectthe catalytic activity and the N2O selectivity. In-creasing the Cu loading will promote the catalyticactivity but decrease the N2O selectivity. However,increasing the Cu loading can not always promotethe catalytic activity. The catalytic activity does notenhance obviously when the Cu loading exceeds10%. Increasing the calcination temperature willdecrease both the catalytic activity and the N2O se-lectivity. We believe that 650◦C is a comparativelyappropriate calcination temperature. The catalystcalcined at this temperature not only has high cat-alytic activity but also has high N2 selectivity.

5. The dispersed copper oxide does not contributeto the catalytic activity and N2O selectivity. The

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catalytic activity is mainly related to the moderatesize bulk CuO and the N2O selectivity is mainlyrelated to the large size bulk CuO.

Acknowledgements

This work was supported by Zhejiang ProvincialNatural Science Foundation of China.

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