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Influence of crystallite size and interface on the catalytic performance over the CeO2/CuO catalysts Shanghong Zeng, Kewei Liu, Tianjia Chen, Haiquan Su* Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China
article info
abstract
Article history:
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like
Received 10 March 2013
morphology in order to obtain the CeO2/CuO catalysts with high BET surface area. The
Received in revised form
catalysts were characterized via SEM, XRD, H2-TPR, ICP, HRTEM and N2 adsorptione
31 August 2013
desorption techniques. The study shows that CeO2 and CuO interact on the contact
Accepted 7 September 2013
interface. The interaction of oxides switches on CO oxidation at 55 C and the synergistic
Available online 5 October 2013
effect of interaction also improves H2 oxidation at 95 C. CO oxidation takes place at the
Keywords:
catalysts can be more helpful for the presence of accumulated long periphery at interface
Hydrogen
of CeO2 and CuO than the larger CeO2 particles when most of CeO2 particles pile into the
contact interface of CeO2 and CuO. The high BET surface area and good dispersion of
CeO2/CuO
small clusters and distribute on the bulk CuO. The CeO2/CuO catalyst with 1:2 Ce/Cu molar
Size
ratio has the highest BET surface area and better dispersion of CeO2 among the catalysts,
Interface
therefore it display good catalytic activity, selectivity and stability.
Preferential CO oxidation
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Production of H2 for the proton-exchange membrane fuel cell (PEMFC) is usually accomplished by the multistep procedures which include catalytic reforming of hydrocarbons followed by water gas-shift (WGS) process [1,2]. PEMFC anode is highly sensitive to the presence of even trace amount of CO [3]. Therefore, one of the major problems is the removal of CO from the hydrogen-rich gas streams for the PEMFC practical use. Preferential oxidation of CO (PROX) has recently attracted more and more attention as the most efficient catalytic method to reduce CO content [4]. Over the past decade, many researches have been devoted to looking for the catalyst which is suitable to preferential oxidation of CO. Such a catalyst must be inactive for the oxidation of H2, and active for the oxidation of CO in the
80e250 C temperature range [5,6]. Some noble metal catalysts have shown good activity for the process, such as platinum or gold catalysts, and the former is available in the commercial systems [7e15]. It is found that CuO/CeO2 catalyst can be used as an alternative because of low cost and high performance compared with noble metal catalysts [16e21]. For the mechanism of CO oxidation over CuOeCeO2 catalyst, the studies show that both CuO and CeO2 play important roles in the reaction and the reaction occurs at the interface of CuO and CeO2 [22e24]. In 2010 Hornes et al. [8] reported an inverse CeO2/CuO catalyst with wider CO conversion window and higher CO2 selectivity in comparison with the classical CuO/ CeO2 catalyst, which is assigned to the maintenance of amount and properties of copper-ceria interfacial sites in this inverse system. Jia et al. [22] reported that the CO oxidation may occur at the interface between CuO and CeO2 by
* Corresponding author. Tel.: þ86 471 4995006; fax: þ86 471 4992981. E-mail address: [email protected] (H. Su). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.023
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comparison of the CuO/CeO2 catalyst with 4.1 nm CuO crystallite size and the CeO2/CuO catalyst with 4.0 nm CeO2 crystallite size. It is obvious that there is a synergistic effect occurred at the interface of the CuOeCeO2 catalyst and the particle sizes of CuO and CeO2 directly determine the perimeter and area of contact interface [22e24]. The influence of particle size and interface over the CuOeCeO2 catalyst is still in progress though these previous studies give considerable insight into the nature of CO oxidation. In this work, a series of inverse CeO2/CuO catalysts with different CeO2 loadings were prepared by the impregnation method in order to elucidate the effect of particle size and interface on catalytic performance in the CeO2/CuO system.
2.
Experimental
2.1.
Catalyst preparation
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology [25]. The Cu(CH3COO)2$H2O and urea were dissolved in 100 ml ethylene glycol, and the solution was vigorously stirred at room temperature. Then the solution were transferred into a stainless steel autoclave with 120 ml capacity Teflon liner and heated for 12 h at 140 C. After cooling to room temperature naturally, the precipitate was washed with distilled water and absolute ethyl alcohol, respectively. Further, the precipitate was dried at 60 C in the air for 24 h to obtain the yellowish CuO precursor. The as-prepared CuO precursor was impregnated with Ce(NO3)3 aqueous solution by equal volume impregnation method [26]. After impregnation, the samples were heated at 80 C for 24 h and calcined at 400 C for 1 h in the air. The obtained catalysts were named as A B, C and D according to the molar ratio of copper and cerium. The detail was listed in Table 1.
2.2.
Catalyst characterization
Scanning electron microscopy (SEM) images of the catalysts were taken on a Hitachi S-4800 scanning electron microscope, using secondary electrons to form the images. The samples were coated with a thin layer of Pt before scanning.
Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a PANalytical X’pert PRO diffractometer with Cu Ka source (l ¼ 0.15406 nm) and a power setting of 40 kV and 100 mA in the range of 2q between 0 and 90 . The average crystallite sizes were estimated from the line broadening using the Scherrer’s equation. N2 adsorptionedesorption measurements were performed at liquid nitrogen temperature (196 C) using a Micrometrics ASAP2020 adsorption apparatus. Before each measurement, the samples were outgassed for 12 h. The surface area and pore size distribution were determined by the BrunauereEmmetteTeller (BET) and the BarretteeJoynereeHalenda (BJH) methods, respectively. H2 temperature-programmed reduction (H2-TPR) experiments were conducted on a Micromeritics Apparatus (AutoChemⅡ2920). The reduction profiles were collected in the 10% H2/Ar gas mixture from room temperature to 900 C. The flow rate of gas and heating rate were 50 ml/min and 10 C/ min, respectively. The reactions were operated in a quartz reactor and the amount of H2 consumption was analyzed by a thermal conductivity detector (TCD). High resolution transmission electron microscope (HRTEM) images were carried out using an FEI tecnai F20 instrument. The samples were dispersed into ethanol with ultrasonic treatment for 10 min, and a drop of the suspension was placed on a copper grid for TEM observation. The content of copper in the samples was measured by inductively coupled plasma spectroscopy (ICP-OES) using a spectrometer from American Varian Corporation.
2.3.
Catalytic performance test
The preferential oxidation of CO in the hydrogen-rich gasses was carried out in a fixed-bed reactor under atmospheric pressure. The 100 mg catalyst diluted with quartz sands was loaded in quartz tubular reactor and a K-type thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The reaction gasses consisted of 1% O2, 1% CO, 50% H2 and N2 balance. The space velocity was 40,000 ml g1 h1 and the reaction was operated from 35 C to 215 C. The outlet gases were analyzed online with a set of gas chromatograph (GC-2014) equipped with a thermal conductivity detector (TCD). 5A molecular sieve column was used to separate CO, O2 and N2. CO2 was determined by a TDX column. Water was trapped before the gasses entering the GC and CO2 was absorbed before
Table 1 e Structure and textural properties of the CeO2/CuO catalysts. Sample
CeO2/CuO-A CeO2/CuO-B CeO2/CuO-C CeO2/CuO-D a b c
Ce/Cu molar ratioa 1:2 1:3 1:4 1:5
CuO cell parameter ( A)
4.692 4.682 4.687 4.694
3.409 3.416 3.420 3.428
5.128 5.132 5.130 5.139
CeO2 cell parameter ( A) 5.403 5.403 5.415 5.409
Preparation condition. Calculated from the Scherrer equation according to the (111) reflection peaks of CuO. Calculated from the Scherrer equation according to the (111) reflection peaks of CeO2.
Particle size(nm) d(CuO) 12.0 13.6 16.3 17.4
b
d(CeO2) 5.0 4.3 10.4 9.7
c
SBET (m2 g1)
Pore volume (cm3 g1)
103.7 99.4 11.5 8.5
0.241 0.239 0.038 0.033
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entering 5A molecular sieve. The conversion of CO (CCO) and the selectivity for CO oxidation (Sco2) were calculated according to the following equations (1) and (2), respectively [26,27]. CCO ¼ ½COin ½COout
½COin 100%
Sco2 ¼ 0:5 ½COin ½COout
½O2 in ½O2 out 100%
3.
Results and discussion
3.1.
Characterization of catalyst structure
(1) (2)
The structure of CeO2/CuO catalysts was characterized via complementary multi-techniques. Fig. 1 shows the SEM micrographs of the CuO precursor, which is composed of the heterogeneous microspheres. The microspheres consist of nanofibers and present three-dimensional, porous and cotton-ball-like structure. The CuO precursor can transfer into CuO support after subsequent calcination. Fig. 2 presents the SEM images of the CeO2/CuO catalysts. It is obvious that the CeO2/CuO-A and CeO2/CuO-B catalysts are porous and fluffy structure. However, the CeO2/CuO-C and CeO2/CuO-D catalysts display the aggregative morphology. It can be seen that the aggregation becomes more and more serious with the increase of copper content. It indicates that the molar ratio of two-kind oxides has a great influence on the morphology of the CeO2/CuO catalysts. The X-ray diffraction patterns of the CeO2/CuO catalysts are presented in Fig. 3. It can be observed that the CuO is monoclinic symmetry and CeO2 is cubic structure [28]. It is clear that the diffraction peaks of CeO2 and CuO all become sharper and sharper with the increase of copper content, suggesting that the crystallization degree of CeO2 and CuO increases accordingly. Table 1 lists the average crystallite sizes of CuO and CeO2, calculated from the line broadening of the most intense XRD reflection peaks according to Scherrer’s equation. It is obvious that the average crystallite sizes of CuO increase with the increase of copper content, indicating that the crystallite sizes of CuO are closely associated with the copper content in this system. For CeO2, the average crystallite sizes are 5.0 nm and 4.3 nm over the CeO2/CuO-A and CeO2/CuO-B catalysts,
Fig. 1 e SEM images of the as-prepared CuO precursor.
respectively. Also, the average crystallite sizes increase to 10.4 nm over the CeO2/CuO-C and 9.7 nm over the CeO2/CuO-D catalysts. The increase of CeO2 crystallite sizes is from the particle aggregation on the surface of catalysts. As shown in Table, the CuO supports with the smaller crystallite sizes are favorable for the dispersion of CeO2. In addition, the cell parameter of CeO2 over the CeO2/CuO catalysts exhibits little A) and pure CuO change in comparison with pure CeO2 (5.411 (4.685 A) except CeO2/CuO-A, indicating that the Cu2þ does not enter into the crystal lattice of CeO2 in the CeO2/CuO-B, CeO2/ CuO-C and CeO2/CuO-D catalysts. CeO2 and CuO interact on the contact interface [29,30]. It can be observed from Table 1 that the BET surface area of CeO2/CuO catalysts decreases with the increase of copper content. The CeO2/CuO-A and CeO2/CuO-B catalysts have higher BET surface area compared with the CeO2/CuO-C and CeO2/CuO-D catalysts. The reason is that CeO2 is much easier to obtain larger specific surface area than CuO [16]. In addition, the pore volume of CeO2/CuO catalysts also decreases with the increase of copper content, and it is related to the textural properties of CuO and CeO2 [31]. Fig. 4 displays the pore size distribution curves of CeO2/CuO catalysts. It is obvious that the most probable peaks shift to smaller pore direction with the increase of copper content. It is possible reason that the pore volume of CeO2/CuO catalysts decreases from CeO2/CuO-A to CeO2/CuO-D. Fig. 5 illustrates the H2-TPR profiles of CeO2/CuO catalysts. There are two overlapping reduction peaks on the H2-TPR profiles before 200 C (a and b peaks), respectively corresponding to the reduction of the highly dispersed CuO (about 150 C) and bulk CuO (from 160 C to 195 C). The reduction peaks at about 500 C and 650 C are attributed to the reduction of surface and bulk ceria. As shown in Fig. 5, CuO usually is reduced at about 380 C and pure CeO2 has the reduction peaks of surface and bulk ceria appeared at 480 C and 750 C, respectively [32]. It is noteworthy that the reduction temperature of CuO and bulk ceria all decreases in comparison with pure CuO and CeO2 because of their interaction on the interface [33,34]. This synergistic effect can produce the surface active oxygen and further improve the oxidation of CO. As can be seen in Fig. 5, the reduction peaks of CuO shift to higher temperature with the increase of copper content over the CeO2/CuO catalysts. The order of reducibility is CeO2/CuOA > CeO2/CuO-B > CeO2/CuO-C > CeO2/CuO-D, and it also is sequence of the synergistic effect of interaction. Fig. 6 shows HRTEM images of the CeO2/CuO-A and CeO2/ CuO-D catalysts. The CeO2/CuO-A catalyst consists of the small particle CeO2 and bulk CuO. The particle size of CuO is in the range from 12 nm to 25 nm. The CeO2 particles are about 5 nm, which are dispersed on the outer surface of bulk CuO. A small amount of CeO2 exists in the form of individual particle, and most of CeO2 particles pile into the small clusters. It is obvious that the good dispersion of CeO2 is favorable for the interaction between CeO2 and CuO. As mentioned in the TPR measurements, the interaction can improve the reducibility of CeO2 and CuO and promote the catalytic performance. For CeO2/CuO-D catalyst, the CuO particles gather together and form the block structure, leading to the decrease of BET surface area, which influence the dispersion of CeO2 on the block CuO. As shown in Fig. 6, the CeO2 particles are about 10 nm
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B A
10
20
30
40
50
60
70
80
2 Theta (deg.) Fig. 3 e XRD patterns of the catalysts: (A) CeO2/CuO-A; (B) CeO2/CuO-B; (C) CeO2/CuO-C; (D) CeO2/CuO-D.
0.0025
CeO2/CuO-A
0.0020
CeO2/CuO-C
dV/dw(cm3/g.Å)
CeO2/CuO-B CeO2/CuO-D
0.0015
0.0010
0.0005
0.0000 10
100
3.2.
Catalytic performance
The catalytic performance was tested between 35 and 215 C in the CO-PROX reaction, using a synthetic gas (1% CO, 1% O2, 50% H2 and N2 balance). The obtained CO conversion and selectivity towards CO2 are shown in Fig. 7. It is clear that all catalysts present a similar catalytic behavior. The CO conversion increases with the increase of reaction temperature until the reaction temperature reaches 155 C. After 155 C, the CO conversion begins to decrease with the increase of reaction
Hydrogen consumption (a.u.)
(331) (420)
(311) (222)
(400) (-311) (113)
(311)
C
(-202)
(110)
D
(202) (-113)
(200)
Intensity (a.u.)
CuO
and gather into some clusters on the block CuO. The particle sizes are compatible with the results of XRD measurements. (220)
(111)
CeO2
(002) (111)
Fig. 2 e SEM images of the CeO2/CuO catalysts: (A) CeO2/CuO-A; (B) CeO2/CuO-B; (C) CeO2/CuO-C; (D) CeO2/CuO-D.
CeO2/CuO-D CeO2/CuO-C CeO2/CuO-B CeO2/CuO-A
1000
Pore Width (Å) Fig. 4 e Pore size distribution curves of the CeO2/CuO catalysts.
100
200
300
400
500
600
700
800
900
Temperature (oC) Fig. 5 e H2-TPR profiles of CeO2/CuO catalysts.
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Fig. 6 e HRTEM images of CeO2/CuO-A and CeO2/CuO-D catalysts.
temperature because of the H2 competitive oxidation. It can be seen that the temperature window of CO total conversion becomes narrower with the increase of copper content. The CeO2/CuO-A catalyst presents the highest catalytic activity and widest temperature window of CO total conversion, and it can totally convert CO to CO2 from 115 C to 155 C. It is reported that the CuOeCeO2 catalyst also presents good activity for H2 oxidation while it has high activity for CO
oxidation, which influence its selectivity towards CO2 in the CO-PROX reaction [16,17]. Namely, the high activity generally corresponds to the low selectivity for the CuOeCeO2 catalyst. As shown in Fig. 7, the CeO2/CuO-A and CeO2/CuO-B catalysts have lower selectivity due to their higher low-temperature activity below 155 C compared with the CeO2/CuO-C and CeO2/CuO-D catalysts. However, the CeO2/CuO-A and CeO2/ CuO-B catalysts present higher high-temperature selectivity
o CO2selectivity/ C CO conversion/ C
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100 75 50 25 0 100
A B C D
75 50 25 0 20
40
60
80 100 120 140 160 180 200 220
Temperature/oC
Fig. 7 e CO conversion and selectivity to CO2 over the CeO2/ CuO catalysts ([CO]in [ 1%, [O2]in [ 1%, [H2]in [ 50%, N2 L1 balance; T [ 35e215 C, GHSV [ 40,000 ml gL1 , cat h P [ atmospheric pressure).
above 155 C in spite of higher high-temperature activity than the latter two catalysts. It is different from the reports about the selectivity [21,26]. It can be seen from Fig. 7 that all catalysts at 35 C have no activity for CO and H2 oxidation. The catalysts begin to oxidize CO when the temperature increases to 55 C, and the sequence of activity decreases with the increase of copper content, indicating that the CeO2 amount has a crucial role from the beginning. Jia et al. [22] thought that the CuOeCeO2 catalyst for CO oxidation is structure-sensitive and the activity is much higher on the larger CuO crystallite, which is attributed to a higher density of chemisorbed CO on the active sites on the larger CuO crystallite. It is clear that the beginning of CO oxidation at 55 C is not only related to the density of chemisorbed CO, but more important is the interaction between the copper and cerium because the CeO2/CuO-D catalyst can provide more adsorption sites for CO chemisorption due to the higher copper content compared with the other catalysts. It is reason that pure CuO and CeO2 can not present low-temperature activity for CO oxidation although they have high-temperature activity for this reaction [10], suggesting that the interaction switches on CO oxidation at the contact interface of CuO and CeO2. The CeO2/CuO-A always displays the highest activity than the other catalysts from 55 C, and its CO conversion reaches 95.9% at 95 C. However, the selectivity to CO2 begins to decline from 95 C, suggesting that H2 oxidation starts at this temperature. The CeO2/CuO-A catalyst preferentially achieves 100% CO conversion at 115 C, followed by the CeO2/CuO-B, CeO2/CuO-C and CeO2/CuO-D catalysts, and the sequence is consistent with the cerium content in the samples, which further suggests that CeO2 amount is a decisive factor for CO total conversion over the CeO2/CuO catalysts. After 155 C, CO conversion starts to decrease over the CeO2/CuO catalysts, and simultaneously the selectivity to CO2 promptly declines accordingly due to the presence of H2 competitive oxidation. The CeO2/CuO-A catalyst still presents the best catalytic performance after 155 C. Hornes et al. [8] reported that the inverse CeO2/CuO catalyst has wider full CO conversion window and higher CO2 selectivity, which is related to the limited reducibility of larger
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size CuO particles. In this work, as shown in Table 1 and Fig. 5, the CeO2/CuO-A catalyst has the smallest CuO particle size and displays the best reducibility compared with the other catalysts, but it still presents the widest full CO conversion window and highest CO2 selectivity. Why are the results contradictory with the report of Hornes et al. [8]? The reason is that they used the classical CuO/CeO2 as a contrast catalyst when the inverse CeO2/CuO was reported, however the CeO2/ CuO catalysts with the larger CuO particle sizes (CeO2/CuO-B, CeO2/CuO-C and CeO2/CuO-D) are employed as the contrast catalysts for the CeO2/CuO-A catalyst, indicating that the limited reducibility of larger size CuO particles is not the only condition for wide full CO conversion window and high CO2 selectivity, and the contact interface of CeO2 and CuO is a more important factor over the CeO2/CuO catalysts.
3.3.
Effect of crystallite size and interface on the activity
The turnover frequencies (TOFs) of the CeO2/CuO catalysts were investigated in order to understand the effect of crystallite size and interface on the catalytic activity during the preferential CO oxidation. The calculation of TOFa and TOFb was based on the following equations (3) and (4) reported by Meng-Fei Luo [22]. TOFa s1 ¼ XCO FCO NAV
mCeO2 ;c mCat XCeO2
TOFa mCeO2 ;c 1 ¼ XCO FCO NAV TOFb s1 ¼ 8pd mCat XCeO2 8pd
(3)
(4)
where XCO is CO conversion at 95 C, FCO is flow rate of CO in mol s1, NAV is Avogadro’s constant, mCeO2 ;c is weight of a single CeO2 particle, mCat is amount of catalyst, XCeO2 is CeO2 loading in the catalyst, and d is crystallite size of CeO2. The more details were described in the literature [22]. Table 2 lists the crystallite size, CeO2 loading, CO conversion, and TOF values of CeO2/CuO catalysts at 95 C. It is obvious that CO conversion increase along with the increase of CeO2 loading, indicating that the catalytic activity is closely related to the interaction between CeO2 and CuO. The TOFa corresponds to the reactivity of the collective active sites on the periphery of the CeO2eCuO interface if the reaction happens at the interface. It can be seen that the TOFa values increase with the increase of CeO2 particle size, indicating that the collective periphery increases accordingly. The TOFb reflects the activity of a single active site. It is interesting that the trend of TOFb values is same with that of TOFa values, suggesting that the bigger CeO2 particle could supply the longer contact interface between CeO2 and CuO for a single active site. As seen in Table 2, there is no linear relation between the crystallite sizes and TOF values, which is related to the distribution of CeO2. However, the general trend is that the TOFa and TOFb values increase with increasing the crystallite sizes of CeO2. These conclusions prove that CO oxidation takes place at the contact interface of CeO2 and CuO. It is consistent with the results of literature [22e24]. As shown in Table 2 and Fig. 7, the CeO2/CuO-A catalyst has not the highest TOFa and TOFb values, but it presents the best catalytic activity. The reason is that CeO2 is not distributed on the bulk CuO in the form of a single particle. As shown in the
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Table 2 e Composition, CO conversion and TOF values of the catalysts. Catalyst Crystallite CeO2/ CO TOFa TOFb size (nm) (CuO þ CeO2) conv. (s1)c (103 s1)c d(CeO2)a (%)c (%)b CeO2/ CuO-A CeO2/ CuO-B CeO2/ CuO-C CeO2/ CuO-D a b c
5.0
53
95.9
1.17
9.31
4.3
44
86.9
0.81
7.50
10.4
37
48.9
7.68
29.4
9.7
31
22.7
3.45
14.2
Determined by rietveld refinement. Calculated from the ICP analysis. Reaction temperature is 95 C.
HRTEM measurements, the CeO2 particles usually gather into some small clusters to distribute on the bulk CuO. In addition, the CeO2 shape also can not satisfy the assumption of its semispherical shape because of this distribution. Therefore, the results of TOFa and TOFb values only prove that CO oxidation might occur at the contact interface of CeO2 and CuO. Although the larger CeO2 particles with semispherical shape are favorable for the formation of longer contact periphery, the high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO2 and CuO under the condition of the above distribution. The CeO2/CuO-A catalyst has the highest BET surface area and better dispersion of CeO2 among the catalysts, therefore it display the best catalytic activity.
3.4.
Stability of catalyst
The stability of CeO2/CuO-A catalyst was tested at 115 C in the CO-PROX reaction in order to detect its life. As shown in Fig. 8, the CeO2/CuO-A catalyst presents good stability during 372 h operation time. CO conversion keeps 100% all the time. The selectivity towards CO2 is about 80% at the beginning of reaction, and the selectivity increases after 120 h, suggesting that the CeO2/CuO-A catalyst reaches stable operation period. However, the CeO2/CuO-A catalyst displays an obvious 105 102
90
99
80
96
70
CO2-selectivity
60 50
93
CO-conversion
0
50
100
150
200
250
300
350
CO-conversion (%)
CO2-selectivity (%)
100
90 400
Time on stream (h) Fig. 8 e CO conversion and selectivity to CO2 over the CeO2/ CuO-A catalyst ([CO]in [ 1%, [O2]in [ 1%, [H2]in [ 50%, N2 L1 balance; T [ 115 C, GHSV [ 40,000 ml gL1 , cat h P [ atmospheric pressure).
inactivation after 372 h. At the same time, the selectivity declines rapidly. The possible reason is that the CuO over the CeO2/CuO-A catalyst is reduced in the hydrogen-rich reaction gasses.
4.
Conclusions
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology in order to obtain the CeO2/CuO catalysts with high BET surface area for preferential oxidation of CO. An effect study of crystallite size and interface on the catalytic performance was performed with the following major conclusions: SEM images show that the molar ratio of CeO2 and CuO has a great influence on the structure and morphology of the CeO2/CuO catalysts. Powder XRD measurements indicate that CeO2 and CuO interact on the contact interface. TPR measurements indicate that the order of reducibility is CeO2/CuOA > CeO2/CuO-B > CeO2/CuO-C > CeO2/CuO-D, and it also is sequence of the synergistic effect of interaction. HRTEM micrographs show that most of CeO2 particles pile into the small clusters and distribute on the bulk CuO. The performance tests present that the interaction of oxides at 55 C switches on CO oxidation at the contact interface of CuO and CeO2 and the synergistic effect of interaction also improves H2 oxidation at 95 C over the CeO2/CuO-A catalyst. The limited reducibility of larger size CuO particles is not the only condition for wide full CO conversion window and high CO2 selectivity, and the contact interface of CeO2 and CuO is a more important factor over the CeO2/CuO catalysts. The research of crystallite size and interface shows that CO oxidation takes place at the contact interface of CeO2 and CuO. The high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO2 and CuO. The CeO2/CuO-A catalyst has the highest BET surface area and better dispersion of CeO2 among the catalysts, therefore it display good catalytic activity, selectivity and stability.
Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (grant no. 21066004, 21061008) and Natural Science Foundation of Inner Mongolia (grant no. 2010ZD01).
references
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Influence of crystallite size and interface on the catalytic performance over the CeO2/CuO catalysts Shanghong Zeng, Kewei Liu, Tianjia Chen, Haiquan Su* Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China
article info
abstract
Article history:
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like
Received 10 March 2013
morphology in order to obtain the CeO2/CuO catalysts with high BET surface area. The
Received in revised form
catalysts were characterized via SEM, XRD, H2-TPR, ICP, HRTEM and N2 adsorptione
31 August 2013
desorption techniques. The study shows that CeO2 and CuO interact on the contact
Accepted 7 September 2013
interface. The interaction of oxides switches on CO oxidation at 55 C and the synergistic
Available online 5 October 2013
effect of interaction also improves H2 oxidation at 95 C. CO oxidation takes place at the
Keywords:
catalysts can be more helpful for the presence of accumulated long periphery at interface
Hydrogen
of CeO2 and CuO than the larger CeO2 particles when most of CeO2 particles pile into the
contact interface of CeO2 and CuO. The high BET surface area and good dispersion of
CeO2/CuO
small clusters and distribute on the bulk CuO. The CeO2/CuO catalyst with 1:2 Ce/Cu molar
Size
ratio has the highest BET surface area and better dispersion of CeO2 among the catalysts,
Interface
therefore it display good catalytic activity, selectivity and stability.
Preferential CO oxidation
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Production of H2 for the proton-exchange membrane fuel cell (PEMFC) is usually accomplished by the multistep procedures which include catalytic reforming of hydrocarbons followed by water gas-shift (WGS) process [1,2]. PEMFC anode is highly sensitive to the presence of even trace amount of CO [3]. Therefore, one of the major problems is the removal of CO from the hydrogen-rich gas streams for the PEMFC practical use. Preferential oxidation of CO (PROX) has recently attracted more and more attention as the most efficient catalytic method to reduce CO content [4]. Over the past decade, many researches have been devoted to looking for the catalyst which is suitable to preferential oxidation of CO. Such a catalyst must be inactive for the oxidation of H2, and active for the oxidation of CO in the
80e250 C temperature range [5,6]. Some noble metal catalysts have shown good activity for the process, such as platinum or gold catalysts, and the former is available in the commercial systems [7e15]. It is found that CuO/CeO2 catalyst can be used as an alternative because of low cost and high performance compared with noble metal catalysts [16e21]. For the mechanism of CO oxidation over CuOeCeO2 catalyst, the studies show that both CuO and CeO2 play important roles in the reaction and the reaction occurs at the interface of CuO and CeO2 [22e24]. In 2010 Hornes et al. [8] reported an inverse CeO2/CuO catalyst with wider CO conversion window and higher CO2 selectivity in comparison with the classical CuO/ CeO2 catalyst, which is assigned to the maintenance of amount and properties of copper-ceria interfacial sites in this inverse system. Jia et al. [22] reported that the CO oxidation may occur at the interface between CuO and CeO2 by
* Corresponding author. Tel.: þ86 471 4995006; fax: þ86 471 4992981. E-mail address: [email protected] (H. Su). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.023
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comparison of the CuO/CeO2 catalyst with 4.1 nm CuO crystallite size and the CeO2/CuO catalyst with 4.0 nm CeO2 crystallite size. It is obvious that there is a synergistic effect occurred at the interface of the CuOeCeO2 catalyst and the particle sizes of CuO and CeO2 directly determine the perimeter and area of contact interface [22e24]. The influence of particle size and interface over the CuOeCeO2 catalyst is still in progress though these previous studies give considerable insight into the nature of CO oxidation. In this work, a series of inverse CeO2/CuO catalysts with different CeO2 loadings were prepared by the impregnation method in order to elucidate the effect of particle size and interface on catalytic performance in the CeO2/CuO system.
2.
Experimental
2.1.
Catalyst preparation
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology [25]. The Cu(CH3COO)2$H2O and urea were dissolved in 100 ml ethylene glycol, and the solution was vigorously stirred at room temperature. Then the solution were transferred into a stainless steel autoclave with 120 ml capacity Teflon liner and heated for 12 h at 140 C. After cooling to room temperature naturally, the precipitate was washed with distilled water and absolute ethyl alcohol, respectively. Further, the precipitate was dried at 60 C in the air for 24 h to obtain the yellowish CuO precursor. The as-prepared CuO precursor was impregnated with Ce(NO3)3 aqueous solution by equal volume impregnation method [26]. After impregnation, the samples were heated at 80 C for 24 h and calcined at 400 C for 1 h in the air. The obtained catalysts were named as A B, C and D according to the molar ratio of copper and cerium. The detail was listed in Table 1.
2.2.
Catalyst characterization
Scanning electron microscopy (SEM) images of the catalysts were taken on a Hitachi S-4800 scanning electron microscope, using secondary electrons to form the images. The samples were coated with a thin layer of Pt before scanning.
Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a PANalytical X’pert PRO diffractometer with Cu Ka source (l ¼ 0.15406 nm) and a power setting of 40 kV and 100 mA in the range of 2q between 0 and 90 . The average crystallite sizes were estimated from the line broadening using the Scherrer’s equation. N2 adsorptionedesorption measurements were performed at liquid nitrogen temperature (196 C) using a Micrometrics ASAP2020 adsorption apparatus. Before each measurement, the samples were outgassed for 12 h. The surface area and pore size distribution were determined by the BrunauereEmmetteTeller (BET) and the BarretteeJoynereeHalenda (BJH) methods, respectively. H2 temperature-programmed reduction (H2-TPR) experiments were conducted on a Micromeritics Apparatus (AutoChemⅡ2920). The reduction profiles were collected in the 10% H2/Ar gas mixture from room temperature to 900 C. The flow rate of gas and heating rate were 50 ml/min and 10 C/ min, respectively. The reactions were operated in a quartz reactor and the amount of H2 consumption was analyzed by a thermal conductivity detector (TCD). High resolution transmission electron microscope (HRTEM) images were carried out using an FEI tecnai F20 instrument. The samples were dispersed into ethanol with ultrasonic treatment for 10 min, and a drop of the suspension was placed on a copper grid for TEM observation. The content of copper in the samples was measured by inductively coupled plasma spectroscopy (ICP-OES) using a spectrometer from American Varian Corporation.
2.3.
Catalytic performance test
The preferential oxidation of CO in the hydrogen-rich gasses was carried out in a fixed-bed reactor under atmospheric pressure. The 100 mg catalyst diluted with quartz sands was loaded in quartz tubular reactor and a K-type thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The reaction gasses consisted of 1% O2, 1% CO, 50% H2 and N2 balance. The space velocity was 40,000 ml g1 h1 and the reaction was operated from 35 C to 215 C. The outlet gases were analyzed online with a set of gas chromatograph (GC-2014) equipped with a thermal conductivity detector (TCD). 5A molecular sieve column was used to separate CO, O2 and N2. CO2 was determined by a TDX column. Water was trapped before the gasses entering the GC and CO2 was absorbed before
Table 1 e Structure and textural properties of the CeO2/CuO catalysts. Sample
CeO2/CuO-A CeO2/CuO-B CeO2/CuO-C CeO2/CuO-D a b c
Ce/Cu molar ratioa 1:2 1:3 1:4 1:5
CuO cell parameter ( A)
4.692 4.682 4.687 4.694
3.409 3.416 3.420 3.428
5.128 5.132 5.130 5.139
CeO2 cell parameter ( A) 5.403 5.403 5.415 5.409
Preparation condition. Calculated from the Scherrer equation according to the (111) reflection peaks of CuO. Calculated from the Scherrer equation according to the (111) reflection peaks of CeO2.
Particle size(nm) d(CuO) 12.0 13.6 16.3 17.4
b
d(CeO2) 5.0 4.3 10.4 9.7
c
SBET (m2 g1)
Pore volume (cm3 g1)
103.7 99.4 11.5 8.5
0.241 0.239 0.038 0.033
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entering 5A molecular sieve. The conversion of CO (CCO) and the selectivity for CO oxidation (Sco2) were calculated according to the following equations (1) and (2), respectively [26,27]. CCO ¼ ½COin ½COout
½COin 100%
Sco2 ¼ 0:5 ½COin ½COout
½O2 in ½O2 out 100%
3.
Results and discussion
3.1.
Characterization of catalyst structure
(1) (2)
The structure of CeO2/CuO catalysts was characterized via complementary multi-techniques. Fig. 1 shows the SEM micrographs of the CuO precursor, which is composed of the heterogeneous microspheres. The microspheres consist of nanofibers and present three-dimensional, porous and cotton-ball-like structure. The CuO precursor can transfer into CuO support after subsequent calcination. Fig. 2 presents the SEM images of the CeO2/CuO catalysts. It is obvious that the CeO2/CuO-A and CeO2/CuO-B catalysts are porous and fluffy structure. However, the CeO2/CuO-C and CeO2/CuO-D catalysts display the aggregative morphology. It can be seen that the aggregation becomes more and more serious with the increase of copper content. It indicates that the molar ratio of two-kind oxides has a great influence on the morphology of the CeO2/CuO catalysts. The X-ray diffraction patterns of the CeO2/CuO catalysts are presented in Fig. 3. It can be observed that the CuO is monoclinic symmetry and CeO2 is cubic structure [28]. It is clear that the diffraction peaks of CeO2 and CuO all become sharper and sharper with the increase of copper content, suggesting that the crystallization degree of CeO2 and CuO increases accordingly. Table 1 lists the average crystallite sizes of CuO and CeO2, calculated from the line broadening of the most intense XRD reflection peaks according to Scherrer’s equation. It is obvious that the average crystallite sizes of CuO increase with the increase of copper content, indicating that the crystallite sizes of CuO are closely associated with the copper content in this system. For CeO2, the average crystallite sizes are 5.0 nm and 4.3 nm over the CeO2/CuO-A and CeO2/CuO-B catalysts,
Fig. 1 e SEM images of the as-prepared CuO precursor.
respectively. Also, the average crystallite sizes increase to 10.4 nm over the CeO2/CuO-C and 9.7 nm over the CeO2/CuO-D catalysts. The increase of CeO2 crystallite sizes is from the particle aggregation on the surface of catalysts. As shown in Table, the CuO supports with the smaller crystallite sizes are favorable for the dispersion of CeO2. In addition, the cell parameter of CeO2 over the CeO2/CuO catalysts exhibits little A) and pure CuO change in comparison with pure CeO2 (5.411 (4.685 A) except CeO2/CuO-A, indicating that the Cu2þ does not enter into the crystal lattice of CeO2 in the CeO2/CuO-B, CeO2/ CuO-C and CeO2/CuO-D catalysts. CeO2 and CuO interact on the contact interface [29,30]. It can be observed from Table 1 that the BET surface area of CeO2/CuO catalysts decreases with the increase of copper content. The CeO2/CuO-A and CeO2/CuO-B catalysts have higher BET surface area compared with the CeO2/CuO-C and CeO2/CuO-D catalysts. The reason is that CeO2 is much easier to obtain larger specific surface area than CuO [16]. In addition, the pore volume of CeO2/CuO catalysts also decreases with the increase of copper content, and it is related to the textural properties of CuO and CeO2 [31]. Fig. 4 displays the pore size distribution curves of CeO2/CuO catalysts. It is obvious that the most probable peaks shift to smaller pore direction with the increase of copper content. It is possible reason that the pore volume of CeO2/CuO catalysts decreases from CeO2/CuO-A to CeO2/CuO-D. Fig. 5 illustrates the H2-TPR profiles of CeO2/CuO catalysts. There are two overlapping reduction peaks on the H2-TPR profiles before 200 C (a and b peaks), respectively corresponding to the reduction of the highly dispersed CuO (about 150 C) and bulk CuO (from 160 C to 195 C). The reduction peaks at about 500 C and 650 C are attributed to the reduction of surface and bulk ceria. As shown in Fig. 5, CuO usually is reduced at about 380 C and pure CeO2 has the reduction peaks of surface and bulk ceria appeared at 480 C and 750 C, respectively [32]. It is noteworthy that the reduction temperature of CuO and bulk ceria all decreases in comparison with pure CuO and CeO2 because of their interaction on the interface [33,34]. This synergistic effect can produce the surface active oxygen and further improve the oxidation of CO. As can be seen in Fig. 5, the reduction peaks of CuO shift to higher temperature with the increase of copper content over the CeO2/CuO catalysts. The order of reducibility is CeO2/CuOA > CeO2/CuO-B > CeO2/CuO-C > CeO2/CuO-D, and it also is sequence of the synergistic effect of interaction. Fig. 6 shows HRTEM images of the CeO2/CuO-A and CeO2/ CuO-D catalysts. The CeO2/CuO-A catalyst consists of the small particle CeO2 and bulk CuO. The particle size of CuO is in the range from 12 nm to 25 nm. The CeO2 particles are about 5 nm, which are dispersed on the outer surface of bulk CuO. A small amount of CeO2 exists in the form of individual particle, and most of CeO2 particles pile into the small clusters. It is obvious that the good dispersion of CeO2 is favorable for the interaction between CeO2 and CuO. As mentioned in the TPR measurements, the interaction can improve the reducibility of CeO2 and CuO and promote the catalytic performance. For CeO2/CuO-D catalyst, the CuO particles gather together and form the block structure, leading to the decrease of BET surface area, which influence the dispersion of CeO2 on the block CuO. As shown in Fig. 6, the CeO2 particles are about 10 nm
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B A
10
20
30
40
50
60
70
80
2 Theta (deg.) Fig. 3 e XRD patterns of the catalysts: (A) CeO2/CuO-A; (B) CeO2/CuO-B; (C) CeO2/CuO-C; (D) CeO2/CuO-D.
0.0025
CeO2/CuO-A
0.0020
CeO2/CuO-C
dV/dw(cm3/g.Å)
CeO2/CuO-B CeO2/CuO-D
0.0015
0.0010
0.0005
0.0000 10
100
3.2.
Catalytic performance
The catalytic performance was tested between 35 and 215 C in the CO-PROX reaction, using a synthetic gas (1% CO, 1% O2, 50% H2 and N2 balance). The obtained CO conversion and selectivity towards CO2 are shown in Fig. 7. It is clear that all catalysts present a similar catalytic behavior. The CO conversion increases with the increase of reaction temperature until the reaction temperature reaches 155 C. After 155 C, the CO conversion begins to decrease with the increase of reaction
Hydrogen consumption (a.u.)
(331) (420)
(311) (222)
(400) (-311) (113)
(311)
C
(-202)
(110)
D
(202) (-113)
(200)
Intensity (a.u.)
CuO
and gather into some clusters on the block CuO. The particle sizes are compatible with the results of XRD measurements. (220)
(111)
CeO2
(002) (111)
Fig. 2 e SEM images of the CeO2/CuO catalysts: (A) CeO2/CuO-A; (B) CeO2/CuO-B; (C) CeO2/CuO-C; (D) CeO2/CuO-D.
CeO2/CuO-D CeO2/CuO-C CeO2/CuO-B CeO2/CuO-A
1000
Pore Width (Å) Fig. 4 e Pore size distribution curves of the CeO2/CuO catalysts.
100
200
300
400
500
600
700
800
900
Temperature (oC) Fig. 5 e H2-TPR profiles of CeO2/CuO catalysts.
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Fig. 6 e HRTEM images of CeO2/CuO-A and CeO2/CuO-D catalysts.
temperature because of the H2 competitive oxidation. It can be seen that the temperature window of CO total conversion becomes narrower with the increase of copper content. The CeO2/CuO-A catalyst presents the highest catalytic activity and widest temperature window of CO total conversion, and it can totally convert CO to CO2 from 115 C to 155 C. It is reported that the CuOeCeO2 catalyst also presents good activity for H2 oxidation while it has high activity for CO
oxidation, which influence its selectivity towards CO2 in the CO-PROX reaction [16,17]. Namely, the high activity generally corresponds to the low selectivity for the CuOeCeO2 catalyst. As shown in Fig. 7, the CeO2/CuO-A and CeO2/CuO-B catalysts have lower selectivity due to their higher low-temperature activity below 155 C compared with the CeO2/CuO-C and CeO2/CuO-D catalysts. However, the CeO2/CuO-A and CeO2/ CuO-B catalysts present higher high-temperature selectivity
o CO2selectivity/ C CO conversion/ C
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100 75 50 25 0 100
A B C D
75 50 25 0 20
40
60
80 100 120 140 160 180 200 220
Temperature/oC
Fig. 7 e CO conversion and selectivity to CO2 over the CeO2/ CuO catalysts ([CO]in [ 1%, [O2]in [ 1%, [H2]in [ 50%, N2 L1 balance; T [ 35e215 C, GHSV [ 40,000 ml gL1 , cat h P [ atmospheric pressure).
above 155 C in spite of higher high-temperature activity than the latter two catalysts. It is different from the reports about the selectivity [21,26]. It can be seen from Fig. 7 that all catalysts at 35 C have no activity for CO and H2 oxidation. The catalysts begin to oxidize CO when the temperature increases to 55 C, and the sequence of activity decreases with the increase of copper content, indicating that the CeO2 amount has a crucial role from the beginning. Jia et al. [22] thought that the CuOeCeO2 catalyst for CO oxidation is structure-sensitive and the activity is much higher on the larger CuO crystallite, which is attributed to a higher density of chemisorbed CO on the active sites on the larger CuO crystallite. It is clear that the beginning of CO oxidation at 55 C is not only related to the density of chemisorbed CO, but more important is the interaction between the copper and cerium because the CeO2/CuO-D catalyst can provide more adsorption sites for CO chemisorption due to the higher copper content compared with the other catalysts. It is reason that pure CuO and CeO2 can not present low-temperature activity for CO oxidation although they have high-temperature activity for this reaction [10], suggesting that the interaction switches on CO oxidation at the contact interface of CuO and CeO2. The CeO2/CuO-A always displays the highest activity than the other catalysts from 55 C, and its CO conversion reaches 95.9% at 95 C. However, the selectivity to CO2 begins to decline from 95 C, suggesting that H2 oxidation starts at this temperature. The CeO2/CuO-A catalyst preferentially achieves 100% CO conversion at 115 C, followed by the CeO2/CuO-B, CeO2/CuO-C and CeO2/CuO-D catalysts, and the sequence is consistent with the cerium content in the samples, which further suggests that CeO2 amount is a decisive factor for CO total conversion over the CeO2/CuO catalysts. After 155 C, CO conversion starts to decrease over the CeO2/CuO catalysts, and simultaneously the selectivity to CO2 promptly declines accordingly due to the presence of H2 competitive oxidation. The CeO2/CuO-A catalyst still presents the best catalytic performance after 155 C. Hornes et al. [8] reported that the inverse CeO2/CuO catalyst has wider full CO conversion window and higher CO2 selectivity, which is related to the limited reducibility of larger
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size CuO particles. In this work, as shown in Table 1 and Fig. 5, the CeO2/CuO-A catalyst has the smallest CuO particle size and displays the best reducibility compared with the other catalysts, but it still presents the widest full CO conversion window and highest CO2 selectivity. Why are the results contradictory with the report of Hornes et al. [8]? The reason is that they used the classical CuO/CeO2 as a contrast catalyst when the inverse CeO2/CuO was reported, however the CeO2/ CuO catalysts with the larger CuO particle sizes (CeO2/CuO-B, CeO2/CuO-C and CeO2/CuO-D) are employed as the contrast catalysts for the CeO2/CuO-A catalyst, indicating that the limited reducibility of larger size CuO particles is not the only condition for wide full CO conversion window and high CO2 selectivity, and the contact interface of CeO2 and CuO is a more important factor over the CeO2/CuO catalysts.
3.3.
Effect of crystallite size and interface on the activity
The turnover frequencies (TOFs) of the CeO2/CuO catalysts were investigated in order to understand the effect of crystallite size and interface on the catalytic activity during the preferential CO oxidation. The calculation of TOFa and TOFb was based on the following equations (3) and (4) reported by Meng-Fei Luo [22]. TOFa s1 ¼ XCO FCO NAV
mCeO2 ;c mCat XCeO2
TOFa mCeO2 ;c 1 ¼ XCO FCO NAV TOFb s1 ¼ 8pd mCat XCeO2 8pd
(3)
(4)
where XCO is CO conversion at 95 C, FCO is flow rate of CO in mol s1, NAV is Avogadro’s constant, mCeO2 ;c is weight of a single CeO2 particle, mCat is amount of catalyst, XCeO2 is CeO2 loading in the catalyst, and d is crystallite size of CeO2. The more details were described in the literature [22]. Table 2 lists the crystallite size, CeO2 loading, CO conversion, and TOF values of CeO2/CuO catalysts at 95 C. It is obvious that CO conversion increase along with the increase of CeO2 loading, indicating that the catalytic activity is closely related to the interaction between CeO2 and CuO. The TOFa corresponds to the reactivity of the collective active sites on the periphery of the CeO2eCuO interface if the reaction happens at the interface. It can be seen that the TOFa values increase with the increase of CeO2 particle size, indicating that the collective periphery increases accordingly. The TOFb reflects the activity of a single active site. It is interesting that the trend of TOFb values is same with that of TOFa values, suggesting that the bigger CeO2 particle could supply the longer contact interface between CeO2 and CuO for a single active site. As seen in Table 2, there is no linear relation between the crystallite sizes and TOF values, which is related to the distribution of CeO2. However, the general trend is that the TOFa and TOFb values increase with increasing the crystallite sizes of CeO2. These conclusions prove that CO oxidation takes place at the contact interface of CeO2 and CuO. It is consistent with the results of literature [22e24]. As shown in Table 2 and Fig. 7, the CeO2/CuO-A catalyst has not the highest TOFa and TOFb values, but it presents the best catalytic activity. The reason is that CeO2 is not distributed on the bulk CuO in the form of a single particle. As shown in the
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Table 2 e Composition, CO conversion and TOF values of the catalysts. Catalyst Crystallite CeO2/ CO TOFa TOFb size (nm) (CuO þ CeO2) conv. (s1)c (103 s1)c d(CeO2)a (%)c (%)b CeO2/ CuO-A CeO2/ CuO-B CeO2/ CuO-C CeO2/ CuO-D a b c
5.0
53
95.9
1.17
9.31
4.3
44
86.9
0.81
7.50
10.4
37
48.9
7.68
29.4
9.7
31
22.7
3.45
14.2
Determined by rietveld refinement. Calculated from the ICP analysis. Reaction temperature is 95 C.
HRTEM measurements, the CeO2 particles usually gather into some small clusters to distribute on the bulk CuO. In addition, the CeO2 shape also can not satisfy the assumption of its semispherical shape because of this distribution. Therefore, the results of TOFa and TOFb values only prove that CO oxidation might occur at the contact interface of CeO2 and CuO. Although the larger CeO2 particles with semispherical shape are favorable for the formation of longer contact periphery, the high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO2 and CuO under the condition of the above distribution. The CeO2/CuO-A catalyst has the highest BET surface area and better dispersion of CeO2 among the catalysts, therefore it display the best catalytic activity.
3.4.
Stability of catalyst
The stability of CeO2/CuO-A catalyst was tested at 115 C in the CO-PROX reaction in order to detect its life. As shown in Fig. 8, the CeO2/CuO-A catalyst presents good stability during 372 h operation time. CO conversion keeps 100% all the time. The selectivity towards CO2 is about 80% at the beginning of reaction, and the selectivity increases after 120 h, suggesting that the CeO2/CuO-A catalyst reaches stable operation period. However, the CeO2/CuO-A catalyst displays an obvious 105 102
90
99
80
96
70
CO2-selectivity
60 50
93
CO-conversion
0
50
100
150
200
250
300
350
CO-conversion (%)
CO2-selectivity (%)
100
90 400
Time on stream (h) Fig. 8 e CO conversion and selectivity to CO2 over the CeO2/ CuO-A catalyst ([CO]in [ 1%, [O2]in [ 1%, [H2]in [ 50%, N2 L1 balance; T [ 115 C, GHSV [ 40,000 ml gL1 , cat h P [ atmospheric pressure).
inactivation after 372 h. At the same time, the selectivity declines rapidly. The possible reason is that the CuO over the CeO2/CuO-A catalyst is reduced in the hydrogen-rich reaction gasses.
4.
Conclusions
The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology in order to obtain the CeO2/CuO catalysts with high BET surface area for preferential oxidation of CO. An effect study of crystallite size and interface on the catalytic performance was performed with the following major conclusions: SEM images show that the molar ratio of CeO2 and CuO has a great influence on the structure and morphology of the CeO2/CuO catalysts. Powder XRD measurements indicate that CeO2 and CuO interact on the contact interface. TPR measurements indicate that the order of reducibility is CeO2/CuOA > CeO2/CuO-B > CeO2/CuO-C > CeO2/CuO-D, and it also is sequence of the synergistic effect of interaction. HRTEM micrographs show that most of CeO2 particles pile into the small clusters and distribute on the bulk CuO. The performance tests present that the interaction of oxides at 55 C switches on CO oxidation at the contact interface of CuO and CeO2 and the synergistic effect of interaction also improves H2 oxidation at 95 C over the CeO2/CuO-A catalyst. The limited reducibility of larger size CuO particles is not the only condition for wide full CO conversion window and high CO2 selectivity, and the contact interface of CeO2 and CuO is a more important factor over the CeO2/CuO catalysts. The research of crystallite size and interface shows that CO oxidation takes place at the contact interface of CeO2 and CuO. The high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO2 and CuO. The CeO2/CuO-A catalyst has the highest BET surface area and better dispersion of CeO2 among the catalysts, therefore it display good catalytic activity, selectivity and stability.
Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (grant no. 21066004, 21061008) and Natural Science Foundation of Inner Mongolia (grant no. 2010ZD01).
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