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Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2 Keizo Nakagawa a,b,c,∗ , Takuya Ohshima c , Yoshiki Tezuka c , Megumi Katayama c , Masahiro Katoh a,c , Sigeru Sugiyama a,b,c a Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan b Department of Resource Circulation Engineering, Center for Frontier Research of Engineering, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan c Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan
a r t i c l e
i n f o
Article history: Received 2 May 2014 Received in revised form 29 June 2014 Accepted 10 August 2014 Available online xxx Keywords: CeO2 nanostructures CuO/CeO2 Morphology Self-assembly Soot combustion
a b s t r a c t Rod and ellipsoid shaped CeO2 nanostructures were prepared using self-assemblies of amine surfactants as templates through the decomposition of cerium carbonate hydroxide. The morphologies of the products were greatly influenced by amine surfactants with different alkyl chain lengths. Different shaped CeO2 nanostructures supported CuO (CuO/CeO2 ) catalysts were prepared by a conventional impregnation method. CuO/CeO2 nanorods showed high reducibility at lower temperature, larger amount of oxygen species from H2 -TPR and possessed larger external surface area and mesopore volume from N2 adsorption as compared with CuO/CeO2 with uncontrolled morphology. These catalysts showed effective soot combustion at lower temperature under tight contact condition using carbon black (CB) as a model of soot particle, as compared with CuO/CeO2 with uncontrolled morphology. Larger external surface area and mesopore volume of CuO/CeO2 catalysts contributed to the improved physical contact condition between CeO2 catalysts and aggregated CBs. Surface reducibility and the improved physical contact condition affected greatly their catalytic activity for soot combustion. © 2014 Elsevier B.V. All rights reserved.
1. Introduction CeO2 and CeO2 -based materials have attracted much attention in environmental and energy-related application. They are widely used as three-way catalysts for auto-exhaust gases [1], water–gas shift reaction [2], fuel cells [3,4], soot combustion [5–11] and so on. The features of CeO2 in these applications are mainly due to the unique combination of its elevated oxygen transport capacity, coupled with its ability to shift easily between the reduced and oxidized states (Ce3+ ↔ Ce4+ ). The improvement of oxygen storage capacity of CeO2 has been achieved by the morphology control such as the formation of cubic and rod CeO2 crystals [2,12–14]. Cerium carbonate hydroxide is a useful precursor to synthesize CeO2 nanostructure through thermal decomposition because
∗ Corresponding author at: Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan. Tel.: +81 88 656 7430; fax: +81 88 656 7430. E-mail address: [email protected] (K. Nakagawa).
cerium carbonate hydroxides have a potential to possess different unique morphologies such as rod, flower, triangular microplate, spindle, shuttle, fiber, sticks and flakes [8,9,15–21]. The obtained CeO2 nanostructures from cerium carbonate hydroxide precursors have advantages of higher porosity and more oxygen vacancies which are derived from decomposition reaction of the cerium carbonate hydroxide. Some prior works regarding the application for oxidation with CeO2 catalysts prepared from cerium carbonate hydroxide has been reported [8,9,16,21]. Among them, soot combustion proceeds at the three-phase boundary between gas (O2 )–solid (soot)–solid (catalyst) and the catalytic performance would be related to oxygen storage capacity of CeO2 , the diffusion of O2 as well as the physical contact condition with soot [10]. From this point of view, CeO2 nanostructures prepared form cerium carbonate hydroxide precursors can be alternative for soot combustion because of the advantages of intrinsic property and morphological effect on contact condition with soot. We believe that such effective structural design based on catalysts preparation closely in connection with catalytic reaction system would lead to great improvement for their performance.
http://dx.doi.org/10.1016/j.cattod.2014.08.005 0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Nakagawa, et al., Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2 , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.005
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Consequently, in this study, we have developed different shaped CeO2 nanostructures such as rod and ellipsoid shape using selfassemblies of amine surfactants as templates. The morphologies of the products were greatly influenced by amine surfactants with different alkyl chain length. The addition of metal oxide to CeO2 significantly modifies the oxygen storage capacity [6]. Thus different shaped CeO2 nanostructures supported CuO (CuO/CeO2 ) catalysts were prepared by a conventional impregnation method. As a result, these catalysts showed effective soot combustion at lower temperature compared with CuO/CeO2 prepared without surfactant template under tight contact condition. Effects of morphologies of CeO2 nanostructures for the reducibility and catalytic soot combustion were examined. 2. Experimental The synthesis consisted of self-assembly based on the interactions between cerium carbonate hydroxide and amine surfactant molecules in hydrothermal solution. A 2.17 g of Ce(NO3 )3 • 6H2 O was mixed with 50 ml of 1M-(NH4 )2 CO3 solution. An 80 ml of 0.375 M-laurylamine (LA) was added into the above mixed solution. The pH was adjusted to 9.5 by adding aqueous NH3 . The mixed solution was then transferred to Teflon autoclaves, aged at 100 ◦ C for 24 h then kept at 140 ◦ C for 96 h. After washing with distilled water and 2-propanol, the resulting products were centrifuged and then dried at 60 ◦ C in air. Cerium carbonate hydroxides with different morphologies were prepared with same method using different amine surfactants such as propylamine (PA) and dodecanediamine (DDA). The obtained dried samples are designated Ce-PA, Ce-LA and Ce-DDA, respectively. For comparison, CeO2 particles were prepared without amine surfactant, and were designated as Ce-no amine. CuO/CeO2 nanostructures were prepared by a conventional impregnation method. The dried cerium carbonate hydroxide were dispersed in 50 ml of acetone containing Cu(NO3 )2 • 3H2 O. The suspension was stirred and dried at 333 K. The sample thus obtained was calcined at 550 ◦ C for 5 h. Finally, the obtained samples are designated CuO/Ce-PA, CuO/Ce-LA, CuO/Ce-DDA and CuO/Ce-no amine, respectively. The content of CuO in the CuO/CeO2 was controlled to ca. 8 wt%. Soot combustion was determined using commercially available CB powder. The catalytic test was carried out with gravimetric thermal analysis (TG). CB and the calcined sample with weight ratio of 1/4 [6,11] were ground in an agate mortar and pestle for 10 min to obtain a tight contact mixture. The mixture of CB and the calcined sample was treated at 300 ◦ C for 1 h under the nitrogen atmosphere and then heated in the temperature range from 200 ◦ C to 700 ◦ C in air at a constant rate of 5 ◦ C min−1 . The characteristic temperatures at which the weight loss proceeded at the highest rate (Tmax ) were determined from DTA curves. 3. Results and discussion CeO2 nanostructures with different morphologies were prepared using amine surfactants with different alkyl chain length. Structural analysis was carried out by XRD, SEM and TEM. Fig. 1 shows XRD patterns of the dried samples prepared with different amine surfactants. The diffraction peaks of Ce-no amine and Ce-PA are identified to be a hexagonal phase of CeCO3 OH (JCPDS 32-0189). In contrast, the peaks of Ce-LA can be indexed to a orthorhombic Ce2 O(CO3 )2 • H2 O (JCPDS 43-0602). The diffraction peaks of Ce-DDA do not seem to correspond to any known pure cerium carbonate hydroxide related phases. Whilst the main diffraction peaks follow the diffraction patterns of orthorhombic Ce2 O(CO3 )2 • H2 O (JCPDS 43-0602), some diffraction peaks at 21.8, and 22.7◦ are observed and these peaks would be derived from cerium dicarbonate
Fig. 1. XRD patterns of the obtained samples prepared with different amine surfactants, (a) dried samples, (b) calcined samples supported CuO.
succinate hydrate (Ce2 (CO3 )2 (CH2 )2 • (CO2 )2 • 2H2 O) (PDF 20-1571). In addition, in the lower angle region of 2 < 20◦ , some peaks at 4.5 and 9.0◦ were observed for Ce-DDA. These peaks indicate the formation of a strictly ordered lamellar phase with an interlayer spacing of 1.96 nm. Fig. 2 shows SEM images of the dried samples prepared with different amine surfactants. As shown in Fig. 2a, aggregated large particles were observed in the synthesis without amine surfactant. In the case of Ce-PA, irregular ellipsoid shape particles were observed. In contrast, rod-like morphologies were clearly observed for Ce-LA and Ce-DDA. In the HRTEM image of Ce-LA, lattice images were clearly observed and it was found that the nanorod was composed of tiny nanoparticles with diameters of 2–5 nm as shown in Fig. A.1 in supplementary file. The diameter and length of the rodlike morphologies were ca. 40 nm and 1 m for Ce-LA and ca. 70 nm and 2 m for Ce-DDA, respectively. Remarkable differences on the morphology and crystalline structure of the dried samples were found. For the synthesis of Ce2 O(CO3 )2 • H2 O and CeCO3 OH, the reaction processes may be summarized by the following reactions [15,20]: 2[Ce(H2 O)n ]3+ + 3CO3 2− → Ce2 O(CO3 )2 • H2 O + CO2 + (2n−1)H2 O
[Ce(H2 O)n ]3+ + H2 O → [Ce(OH)(H2 O)n−1 ]2+ + H3 O+
(1)
(2)
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According to the previous studies, crystal phase of cerium carbonate hydroxide is related to synthesis condition such as concentration ratio of carbonate/cerium ion and hydrothermal temperature [15,20]. In this synthesis, orthorhombic Ce2 O(CO3 )2 • H2 O formed in the presence of LA or DDA because these hydrophobic surfactants adsorbed on the cerium carbonate hydroxide seeds, which would suppress the reaction with carbonate ion and change the reaction condition as compared with the synthesis with PA or without amine surfactant. The potential for 1D nanostructures of cerium carbonate hydroxide in hydrothermal solution would arise from self-assembling of such tiny particles into nanorod during the aging process. In the presence of amine surfactant as a capping reagent, the amine surfactant molecules would adsorb on the cerium carbonate hydroxide seeds. The long alkyl chains of the LA and DDA were adjusted by rotation to form the rod shape. The growth of rod-like shape crystals would reduce the curvature imposed on the surface-anchored molecules, such that favorable changes in the inorganic lattice and organic bending energies can occur by cooperative mesoscale transformations [22,23]. In the case of Ce-DDA, the formation of lamellar phase of DDA on the cerium carbonate hydroxide would accelerate the elongation of rod. Thus the nanorod morphology composed of tiny nanoparticles were formed in the surfactant self-assembling system and amine surfactant molecules play an important role for the formation of different shaped nanostructures of cerium carbonate hydroxide. After calcination with Cu species, the diffraction pattern of all the catalysts corresponded to CeO2 with cubic fluorite structure. The heat treatment led to the conversion of each cerium carbonate hydroxide to CeO2 although tiny diffraction peaks derived from CuO were observed at around 2 = 35 and 39◦ , implying that Cu species would be highly dispersed on the surface of CeO2 nanostructures (Fig. 1b). Fig. 3 shows SEM images of the calcined samples prepared with different amine surfactants. It can be found that the morphologies of obtained CeO2 are similar with each cerium carbonate hydroxide before calcination. Very small CuO nanoparticles were supported on the surface of CeO2 nanorods although aggregated CuO nanoparticles were also observed (Fig. A.2).
Fig. 4 shows N2 adsorption isotherms at 77 K for the different CuO/CeO2 catalysts after calcination. The BET specific surface area, external surface area determined by t-plot method and mesopore volume estimated by DH method of the CuO/CeO2 catalysts are listed in Table 1. Larger amount of N2 adsorption below P/P0 = 0.1 was observed for CuO/Ce-no amine and CuO/Ce-LA compared with other catalysts, which leads to the larger specific BET surface areas. It is noted that the larger external surface area and mesopore volume were obtained in the order of CuO/Ce-DDA > CuO/CeLA > CuO/Ce-PA > CuO/Ce-no amine. We assume that the reason of these large values for the external surface area and mesopore volume was derived from the formation of CeO2 nanorods (Ce-LA and Ce-DDA) after calcinations and it is also related to the diameter and length of nanorods. Fig. 5 shows the results of H2 -TPR profiles and the reduction peak temperatures are summarized in Table 1. According to the 60 CuO/Ce-no amine PA CuO/Ce-P CuO/Ce-L LA CuO/Ce-DDA -1
(3)
40
3
[Ce(OH)(H2 O)n−1 ]2+ + CO3 2− → CeCO3 OH + (n−1)H2 O
Fig. 3. SEM images the calcined samples supported CuO, (a) CuO/Ce-no amine, (b) CuO/Ce-PA, (c) CuO/Ce-LA, (d) CuO/Ce-DDA.
Va / cm (STP)g
Fig. 2. SEM images of the dried samples prepared with different amine surfactants, (a) Ce-no amine, (b) Ce-PA, (c) Ce-LA, (d) Ce-DDA.
3
20
0 0.0
0.2
0.4
0.6
0. 8
1.0
p/p0 / Fig. 4. Nitrogen adsorption–desorption isotherm of different CuO/CeO2 catalysts. Closed and opened circles express adsorption and desorption branches, respectively.
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Table 1 Morphology, CuO loading, SBET , SEXT , Vmeso , HTPR and Tmax of CuO/CeO2 catalysts. Sample
Morphology of CeO2
CuO loading (wt%)
SBET a (m2 g−1 )
SEXT b (m2 g−1 )
Vmeso c (cm3 g−1 )
HTPR (mmol g−1 )
Tmax (◦ C)
CuO/Ce-no amine CuO/Ce-PA CuO/Ce-LA CuO/Ce-DDA
Aggregate Ellipsoid Rod Rod
7.6 7.8 8.1 7.8
55 42 54 43
13 17 29 36
0.027 0.054 0.073 0.084
1.47 1.42 1.65 1.55
500 454 432 413
a b c
BET specific surface area. External surface area was estimated by t-plot method. Mesopore volume was estimated by DH method.
previous studies [6,24], TPR profile exhibits two reduction peaks at around 500 and 900 ◦ C for pure CeO2 , which were derived from the reduction of surface and bulk oxygen species. The reduction profile of bulk CuO was observed at 321 ◦ C (not shown), which indicates the reduction of Cu2+ to Cu0 . Strong metal oxide–support interactions (SMSI) between CuO and CeO2 are believed to lead to a decrease of their individual reduction temperatures [24]. For the CuO/CeO2 catalysts, two reduction peaks were observed at around 170–180 and 200 ◦ C and there was no reduction profile over 300 ◦ C. These two peaks are attributed to the reduction of well-dispersed cupper species (denoted as ␣) and copper incorporated into CeO2 lattice (denoted as ), respectively [24,25]. H2 consumption during the reduction for all CuO/CeO2 catalysts was higher than that needed for complete Cu2+ to Cu0 reduction (around 1.1 mmol g−1 for the catalysts with CuO loading of 7.6–8.1 wt%), which can be associated with the reduction of copper species and CeO2 surface oxygen. These results indicate that interaction between Ce and Cu species enhanced the reducibility of CeO2 . It was found that CuO/CeO2 catalysts with rod morphology (CuO/Ce-LA and CuO/CeDDA) possess larger amount of H2 consumption as compared with the other catalysts. Raman spectra as shown in Fig. A.3 and Table A.1 in supplementary file proved the existence of more oxygen vacancies [25,26] for CuO/Ce-LA and CuO/Ce-DDA than CuO/Ce-no amine and CuO/Ce-PA. We consider that Cu2+ prefers to occupy the substituted Ce4+ site in the CeO2 nanorods structure, and thus the concentration of oxygen vacancy increase. The mobility of oxygen ions via oxygen vacancies occurs readily, which suggests that the redox cycles between Ce4+ and Ce3+ can promote the reactivity of CuO. In H2 -TPR profiles, interestingly, it was found that both two reduction peaks shifted to lower temperature for CuO/Ce-DDA. The reason of these result for CuO/CeO2 catalysts with rod morphology may be due to the enhancement of the reducibility by
the highly dispersed CuO on the CeO2 surface and the formation of Cu–O–Ce species. During the impregnation method for the synthesis of CuO/CeO2 nanostructures, Cu2+ would replace with cationic amine molecules adsorbed on the surface of cerium carbonate hydroxide by cation exchange. Thus Cu2+ ions are expected to be highly dispersed onto the surface of cerium carbonate hydroxide, which results in the formation of small CuO nanoparticles on CeO2 . It is assumed that higher dispersion of CuO was achieved due to the longer rod morphology and the interaction between CuO and CeO2 would be enhanced especially for CuO/Ce-DDA, leading to the lower reduction profiles. Another reason can be considered by the preferential formation of Cu–O–Ce species because of the strong interaction between highly dispersed Cu2+ and CeO2 from the results of Raman spectra. Further investigations about chemical state of Cu and Ce species are needed to clarify the relationship between morphology and reduction profiles. Fig. 6 shows DTA curves of soot combustion over CuO/CeO2 catalysts. The values of Tmax determined from the curves are summarized in Table 1. The soot combustion performance of CuO/CeO2 catalysts strongly depends on the morphology of CeO2 . The DTA curves for CuO/CeO2 catalysts with rod morphology (CeLA and Ce-DDA) showed maximum value at lower temperature compared with other catalysts with ellipsoid shaped morphology, which indicate that CuO/CeO2 catalysts with rod morphology showed higher activity for soot combustion than other catalysts. It is noted that lower temperature soot combustion were observed in the order of CuO/Ce-DDA > CuO/Ce-LA > CuO/Ce-PA > CuO/Ce-no amine. It seems that the catalytic activity of soot combustion for the obtained CuO/CeO2 catalysts depends on external surface area and mesopore volume rather than BET specific surface area. Soot combustion supposedly takes place at the three-phase boundary between gas (O2 )–solid (soot)–solid (catalyst), therefore the catalytic performance should be related to the following factors, oxygen storage capacity of CeO2 , the diffusion of O2 and the physical contact condition between CeO2 and soot [10]. The high reducibility at lower temperature and the larger amount of oxygen species in
Fig. 5. H2 -TPR profiles of CuO/CeO2 catalysts.
Fig. 6. Normailized DTA carves of the mixture of CB and CuO/CeO2 catalysts.
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catalyst with rod morphology available for the oxidation reaction as shown in the results of H2 -TPR is one of the important factors for the soot combustion. On the other hand, it is considered that the surface area of catalysts strongly affects the activity of soot combustion in the CeO2 –CuO catalysts [6]. However, the other research group had reported that specific surface area of CeO2 fiber did not affect the soot combustion [8]. This discrepancy would come from the difference of catalysts morphology. Thus the value of external surface area or mesopore volume would be another indication rather than specific surface area in the case of solid reactant. Our results imply that higher external surface area and mesopore volume of catalysts contributed to the improved physical contact condition between CeO2 catalysts and aggregated CBs and this affected greatly their catalytic activity for soot combustion. Rod particles were obtained for CuO/Ce-LA and CuO/Ce-DDA and this would accelerate the all factors that are necessary for the improvement of the soot combustion. CuO/Ce-DDA had higher reducibility at low temperature than the other catalysts and these are reason why CuO/Ce-DDA showed the superior catalytic property of soot combustion. N2 adsorption isotherms measured at 77 K, BET specific surface area, external surface area and mesopore volume of CuO/Ce-LA and CuO/Ce-no amine were compared before and after reaction (Fig. A.4 and Table A.2). These values of both catalysts intensely decreased after reaction because soot combustion was performed in the temperature range from 200 ◦ C to 700 ◦ C. However, higher values of external surface area and mesopore volume for CuO/Ce-LA were obtained as compared with CuO/Ce-no amine. These results imply that CuO/CeO2 catalysts with rod morphology have a possibility to show better activity of soot combustion than CuO/CeO2 with uncontrolled morphology even after reaction although the decrease of their catalytic performance would be inevitable during reaction at this present. 4. Conclusions We have demonstrated that different shaped CeO2 nanostructures such as rod and ellipsoid shape using self-assemblies of amine surfactants as templates can be effectively prepared by through the decomposition of cerium carbonate hydroxide. These different shaped CeO2 nanostructures supported CuO catalysts give effective soot combustion at lower temperature compared with CuO/CeO2 prepared without surfactant template due to their surface reducibility and larger external surface area and mesopore volume. These novel nanorod composites are demonstrated to be highly capable of trapping for effective soot combustion, which is an important reaction relating to air purification. The morphology control of such composite may open up valuable perspectives for wider catalytic applications. Acknowledgements This work was funded by a Grant-in-Aid for Young Scientists (B) (Grant No. 20750167) for the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors gratefully acknowledge Mr. T. Ueki (Center for Technical Support, Institute of Technology and Science, The University of Tokushima) for his technical support with TEM experiments.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.08.005.
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Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2 Keizo Nakagawa a,b,c,∗ , Takuya Ohshima c , Yoshiki Tezuka c , Megumi Katayama c , Masahiro Katoh a,c , Sigeru Sugiyama a,b,c a Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan b Department of Resource Circulation Engineering, Center for Frontier Research of Engineering, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan c Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan
a r t i c l e
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Article history: Received 2 May 2014 Received in revised form 29 June 2014 Accepted 10 August 2014 Available online xxx Keywords: CeO2 nanostructures CuO/CeO2 Morphology Self-assembly Soot combustion
a b s t r a c t Rod and ellipsoid shaped CeO2 nanostructures were prepared using self-assemblies of amine surfactants as templates through the decomposition of cerium carbonate hydroxide. The morphologies of the products were greatly influenced by amine surfactants with different alkyl chain lengths. Different shaped CeO2 nanostructures supported CuO (CuO/CeO2 ) catalysts were prepared by a conventional impregnation method. CuO/CeO2 nanorods showed high reducibility at lower temperature, larger amount of oxygen species from H2 -TPR and possessed larger external surface area and mesopore volume from N2 adsorption as compared with CuO/CeO2 with uncontrolled morphology. These catalysts showed effective soot combustion at lower temperature under tight contact condition using carbon black (CB) as a model of soot particle, as compared with CuO/CeO2 with uncontrolled morphology. Larger external surface area and mesopore volume of CuO/CeO2 catalysts contributed to the improved physical contact condition between CeO2 catalysts and aggregated CBs. Surface reducibility and the improved physical contact condition affected greatly their catalytic activity for soot combustion. © 2014 Elsevier B.V. All rights reserved.
1. Introduction CeO2 and CeO2 -based materials have attracted much attention in environmental and energy-related application. They are widely used as three-way catalysts for auto-exhaust gases [1], water–gas shift reaction [2], fuel cells [3,4], soot combustion [5–11] and so on. The features of CeO2 in these applications are mainly due to the unique combination of its elevated oxygen transport capacity, coupled with its ability to shift easily between the reduced and oxidized states (Ce3+ ↔ Ce4+ ). The improvement of oxygen storage capacity of CeO2 has been achieved by the morphology control such as the formation of cubic and rod CeO2 crystals [2,12–14]. Cerium carbonate hydroxide is a useful precursor to synthesize CeO2 nanostructure through thermal decomposition because
∗ Corresponding author at: Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjimacho, Tokushima 770-8506, Japan. Tel.: +81 88 656 7430; fax: +81 88 656 7430. E-mail address: [email protected] (K. Nakagawa).
cerium carbonate hydroxides have a potential to possess different unique morphologies such as rod, flower, triangular microplate, spindle, shuttle, fiber, sticks and flakes [8,9,15–21]. The obtained CeO2 nanostructures from cerium carbonate hydroxide precursors have advantages of higher porosity and more oxygen vacancies which are derived from decomposition reaction of the cerium carbonate hydroxide. Some prior works regarding the application for oxidation with CeO2 catalysts prepared from cerium carbonate hydroxide has been reported [8,9,16,21]. Among them, soot combustion proceeds at the three-phase boundary between gas (O2 )–solid (soot)–solid (catalyst) and the catalytic performance would be related to oxygen storage capacity of CeO2 , the diffusion of O2 as well as the physical contact condition with soot [10]. From this point of view, CeO2 nanostructures prepared form cerium carbonate hydroxide precursors can be alternative for soot combustion because of the advantages of intrinsic property and morphological effect on contact condition with soot. We believe that such effective structural design based on catalysts preparation closely in connection with catalytic reaction system would lead to great improvement for their performance.
http://dx.doi.org/10.1016/j.cattod.2014.08.005 0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Nakagawa, et al., Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2 , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.005
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Consequently, in this study, we have developed different shaped CeO2 nanostructures such as rod and ellipsoid shape using selfassemblies of amine surfactants as templates. The morphologies of the products were greatly influenced by amine surfactants with different alkyl chain length. The addition of metal oxide to CeO2 significantly modifies the oxygen storage capacity [6]. Thus different shaped CeO2 nanostructures supported CuO (CuO/CeO2 ) catalysts were prepared by a conventional impregnation method. As a result, these catalysts showed effective soot combustion at lower temperature compared with CuO/CeO2 prepared without surfactant template under tight contact condition. Effects of morphologies of CeO2 nanostructures for the reducibility and catalytic soot combustion were examined. 2. Experimental The synthesis consisted of self-assembly based on the interactions between cerium carbonate hydroxide and amine surfactant molecules in hydrothermal solution. A 2.17 g of Ce(NO3 )3 • 6H2 O was mixed with 50 ml of 1M-(NH4 )2 CO3 solution. An 80 ml of 0.375 M-laurylamine (LA) was added into the above mixed solution. The pH was adjusted to 9.5 by adding aqueous NH3 . The mixed solution was then transferred to Teflon autoclaves, aged at 100 ◦ C for 24 h then kept at 140 ◦ C for 96 h. After washing with distilled water and 2-propanol, the resulting products were centrifuged and then dried at 60 ◦ C in air. Cerium carbonate hydroxides with different morphologies were prepared with same method using different amine surfactants such as propylamine (PA) and dodecanediamine (DDA). The obtained dried samples are designated Ce-PA, Ce-LA and Ce-DDA, respectively. For comparison, CeO2 particles were prepared without amine surfactant, and were designated as Ce-no amine. CuO/CeO2 nanostructures were prepared by a conventional impregnation method. The dried cerium carbonate hydroxide were dispersed in 50 ml of acetone containing Cu(NO3 )2 • 3H2 O. The suspension was stirred and dried at 333 K. The sample thus obtained was calcined at 550 ◦ C for 5 h. Finally, the obtained samples are designated CuO/Ce-PA, CuO/Ce-LA, CuO/Ce-DDA and CuO/Ce-no amine, respectively. The content of CuO in the CuO/CeO2 was controlled to ca. 8 wt%. Soot combustion was determined using commercially available CB powder. The catalytic test was carried out with gravimetric thermal analysis (TG). CB and the calcined sample with weight ratio of 1/4 [6,11] were ground in an agate mortar and pestle for 10 min to obtain a tight contact mixture. The mixture of CB and the calcined sample was treated at 300 ◦ C for 1 h under the nitrogen atmosphere and then heated in the temperature range from 200 ◦ C to 700 ◦ C in air at a constant rate of 5 ◦ C min−1 . The characteristic temperatures at which the weight loss proceeded at the highest rate (Tmax ) were determined from DTA curves. 3. Results and discussion CeO2 nanostructures with different morphologies were prepared using amine surfactants with different alkyl chain length. Structural analysis was carried out by XRD, SEM and TEM. Fig. 1 shows XRD patterns of the dried samples prepared with different amine surfactants. The diffraction peaks of Ce-no amine and Ce-PA are identified to be a hexagonal phase of CeCO3 OH (JCPDS 32-0189). In contrast, the peaks of Ce-LA can be indexed to a orthorhombic Ce2 O(CO3 )2 • H2 O (JCPDS 43-0602). The diffraction peaks of Ce-DDA do not seem to correspond to any known pure cerium carbonate hydroxide related phases. Whilst the main diffraction peaks follow the diffraction patterns of orthorhombic Ce2 O(CO3 )2 • H2 O (JCPDS 43-0602), some diffraction peaks at 21.8, and 22.7◦ are observed and these peaks would be derived from cerium dicarbonate
Fig. 1. XRD patterns of the obtained samples prepared with different amine surfactants, (a) dried samples, (b) calcined samples supported CuO.
succinate hydrate (Ce2 (CO3 )2 (CH2 )2 • (CO2 )2 • 2H2 O) (PDF 20-1571). In addition, in the lower angle region of 2 < 20◦ , some peaks at 4.5 and 9.0◦ were observed for Ce-DDA. These peaks indicate the formation of a strictly ordered lamellar phase with an interlayer spacing of 1.96 nm. Fig. 2 shows SEM images of the dried samples prepared with different amine surfactants. As shown in Fig. 2a, aggregated large particles were observed in the synthesis without amine surfactant. In the case of Ce-PA, irregular ellipsoid shape particles were observed. In contrast, rod-like morphologies were clearly observed for Ce-LA and Ce-DDA. In the HRTEM image of Ce-LA, lattice images were clearly observed and it was found that the nanorod was composed of tiny nanoparticles with diameters of 2–5 nm as shown in Fig. A.1 in supplementary file. The diameter and length of the rodlike morphologies were ca. 40 nm and 1 m for Ce-LA and ca. 70 nm and 2 m for Ce-DDA, respectively. Remarkable differences on the morphology and crystalline structure of the dried samples were found. For the synthesis of Ce2 O(CO3 )2 • H2 O and CeCO3 OH, the reaction processes may be summarized by the following reactions [15,20]: 2[Ce(H2 O)n ]3+ + 3CO3 2− → Ce2 O(CO3 )2 • H2 O + CO2 + (2n−1)H2 O
[Ce(H2 O)n ]3+ + H2 O → [Ce(OH)(H2 O)n−1 ]2+ + H3 O+
(1)
(2)
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According to the previous studies, crystal phase of cerium carbonate hydroxide is related to synthesis condition such as concentration ratio of carbonate/cerium ion and hydrothermal temperature [15,20]. In this synthesis, orthorhombic Ce2 O(CO3 )2 • H2 O formed in the presence of LA or DDA because these hydrophobic surfactants adsorbed on the cerium carbonate hydroxide seeds, which would suppress the reaction with carbonate ion and change the reaction condition as compared with the synthesis with PA or without amine surfactant. The potential for 1D nanostructures of cerium carbonate hydroxide in hydrothermal solution would arise from self-assembling of such tiny particles into nanorod during the aging process. In the presence of amine surfactant as a capping reagent, the amine surfactant molecules would adsorb on the cerium carbonate hydroxide seeds. The long alkyl chains of the LA and DDA were adjusted by rotation to form the rod shape. The growth of rod-like shape crystals would reduce the curvature imposed on the surface-anchored molecules, such that favorable changes in the inorganic lattice and organic bending energies can occur by cooperative mesoscale transformations [22,23]. In the case of Ce-DDA, the formation of lamellar phase of DDA on the cerium carbonate hydroxide would accelerate the elongation of rod. Thus the nanorod morphology composed of tiny nanoparticles were formed in the surfactant self-assembling system and amine surfactant molecules play an important role for the formation of different shaped nanostructures of cerium carbonate hydroxide. After calcination with Cu species, the diffraction pattern of all the catalysts corresponded to CeO2 with cubic fluorite structure. The heat treatment led to the conversion of each cerium carbonate hydroxide to CeO2 although tiny diffraction peaks derived from CuO were observed at around 2 = 35 and 39◦ , implying that Cu species would be highly dispersed on the surface of CeO2 nanostructures (Fig. 1b). Fig. 3 shows SEM images of the calcined samples prepared with different amine surfactants. It can be found that the morphologies of obtained CeO2 are similar with each cerium carbonate hydroxide before calcination. Very small CuO nanoparticles were supported on the surface of CeO2 nanorods although aggregated CuO nanoparticles were also observed (Fig. A.2).
Fig. 4 shows N2 adsorption isotherms at 77 K for the different CuO/CeO2 catalysts after calcination. The BET specific surface area, external surface area determined by t-plot method and mesopore volume estimated by DH method of the CuO/CeO2 catalysts are listed in Table 1. Larger amount of N2 adsorption below P/P0 = 0.1 was observed for CuO/Ce-no amine and CuO/Ce-LA compared with other catalysts, which leads to the larger specific BET surface areas. It is noted that the larger external surface area and mesopore volume were obtained in the order of CuO/Ce-DDA > CuO/CeLA > CuO/Ce-PA > CuO/Ce-no amine. We assume that the reason of these large values for the external surface area and mesopore volume was derived from the formation of CeO2 nanorods (Ce-LA and Ce-DDA) after calcinations and it is also related to the diameter and length of nanorods. Fig. 5 shows the results of H2 -TPR profiles and the reduction peak temperatures are summarized in Table 1. According to the 60 CuO/Ce-no amine PA CuO/Ce-P CuO/Ce-L LA CuO/Ce-DDA -1
(3)
40
3
[Ce(OH)(H2 O)n−1 ]2+ + CO3 2− → CeCO3 OH + (n−1)H2 O
Fig. 3. SEM images the calcined samples supported CuO, (a) CuO/Ce-no amine, (b) CuO/Ce-PA, (c) CuO/Ce-LA, (d) CuO/Ce-DDA.
Va / cm (STP)g
Fig. 2. SEM images of the dried samples prepared with different amine surfactants, (a) Ce-no amine, (b) Ce-PA, (c) Ce-LA, (d) Ce-DDA.
3
20
0 0.0
0.2
0.4
0.6
0. 8
1.0
p/p0 / Fig. 4. Nitrogen adsorption–desorption isotherm of different CuO/CeO2 catalysts. Closed and opened circles express adsorption and desorption branches, respectively.
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Table 1 Morphology, CuO loading, SBET , SEXT , Vmeso , HTPR and Tmax of CuO/CeO2 catalysts. Sample
Morphology of CeO2
CuO loading (wt%)
SBET a (m2 g−1 )
SEXT b (m2 g−1 )
Vmeso c (cm3 g−1 )
HTPR (mmol g−1 )
Tmax (◦ C)
CuO/Ce-no amine CuO/Ce-PA CuO/Ce-LA CuO/Ce-DDA
Aggregate Ellipsoid Rod Rod
7.6 7.8 8.1 7.8
55 42 54 43
13 17 29 36
0.027 0.054 0.073 0.084
1.47 1.42 1.65 1.55
500 454 432 413
a b c
BET specific surface area. External surface area was estimated by t-plot method. Mesopore volume was estimated by DH method.
previous studies [6,24], TPR profile exhibits two reduction peaks at around 500 and 900 ◦ C for pure CeO2 , which were derived from the reduction of surface and bulk oxygen species. The reduction profile of bulk CuO was observed at 321 ◦ C (not shown), which indicates the reduction of Cu2+ to Cu0 . Strong metal oxide–support interactions (SMSI) between CuO and CeO2 are believed to lead to a decrease of their individual reduction temperatures [24]. For the CuO/CeO2 catalysts, two reduction peaks were observed at around 170–180 and 200 ◦ C and there was no reduction profile over 300 ◦ C. These two peaks are attributed to the reduction of well-dispersed cupper species (denoted as ␣) and copper incorporated into CeO2 lattice (denoted as ), respectively [24,25]. H2 consumption during the reduction for all CuO/CeO2 catalysts was higher than that needed for complete Cu2+ to Cu0 reduction (around 1.1 mmol g−1 for the catalysts with CuO loading of 7.6–8.1 wt%), which can be associated with the reduction of copper species and CeO2 surface oxygen. These results indicate that interaction between Ce and Cu species enhanced the reducibility of CeO2 . It was found that CuO/CeO2 catalysts with rod morphology (CuO/Ce-LA and CuO/CeDDA) possess larger amount of H2 consumption as compared with the other catalysts. Raman spectra as shown in Fig. A.3 and Table A.1 in supplementary file proved the existence of more oxygen vacancies [25,26] for CuO/Ce-LA and CuO/Ce-DDA than CuO/Ce-no amine and CuO/Ce-PA. We consider that Cu2+ prefers to occupy the substituted Ce4+ site in the CeO2 nanorods structure, and thus the concentration of oxygen vacancy increase. The mobility of oxygen ions via oxygen vacancies occurs readily, which suggests that the redox cycles between Ce4+ and Ce3+ can promote the reactivity of CuO. In H2 -TPR profiles, interestingly, it was found that both two reduction peaks shifted to lower temperature for CuO/Ce-DDA. The reason of these result for CuO/CeO2 catalysts with rod morphology may be due to the enhancement of the reducibility by
the highly dispersed CuO on the CeO2 surface and the formation of Cu–O–Ce species. During the impregnation method for the synthesis of CuO/CeO2 nanostructures, Cu2+ would replace with cationic amine molecules adsorbed on the surface of cerium carbonate hydroxide by cation exchange. Thus Cu2+ ions are expected to be highly dispersed onto the surface of cerium carbonate hydroxide, which results in the formation of small CuO nanoparticles on CeO2 . It is assumed that higher dispersion of CuO was achieved due to the longer rod morphology and the interaction between CuO and CeO2 would be enhanced especially for CuO/Ce-DDA, leading to the lower reduction profiles. Another reason can be considered by the preferential formation of Cu–O–Ce species because of the strong interaction between highly dispersed Cu2+ and CeO2 from the results of Raman spectra. Further investigations about chemical state of Cu and Ce species are needed to clarify the relationship between morphology and reduction profiles. Fig. 6 shows DTA curves of soot combustion over CuO/CeO2 catalysts. The values of Tmax determined from the curves are summarized in Table 1. The soot combustion performance of CuO/CeO2 catalysts strongly depends on the morphology of CeO2 . The DTA curves for CuO/CeO2 catalysts with rod morphology (CeLA and Ce-DDA) showed maximum value at lower temperature compared with other catalysts with ellipsoid shaped morphology, which indicate that CuO/CeO2 catalysts with rod morphology showed higher activity for soot combustion than other catalysts. It is noted that lower temperature soot combustion were observed in the order of CuO/Ce-DDA > CuO/Ce-LA > CuO/Ce-PA > CuO/Ce-no amine. It seems that the catalytic activity of soot combustion for the obtained CuO/CeO2 catalysts depends on external surface area and mesopore volume rather than BET specific surface area. Soot combustion supposedly takes place at the three-phase boundary between gas (O2 )–solid (soot)–solid (catalyst), therefore the catalytic performance should be related to the following factors, oxygen storage capacity of CeO2 , the diffusion of O2 and the physical contact condition between CeO2 and soot [10]. The high reducibility at lower temperature and the larger amount of oxygen species in
Fig. 5. H2 -TPR profiles of CuO/CeO2 catalysts.
Fig. 6. Normailized DTA carves of the mixture of CB and CuO/CeO2 catalysts.
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catalyst with rod morphology available for the oxidation reaction as shown in the results of H2 -TPR is one of the important factors for the soot combustion. On the other hand, it is considered that the surface area of catalysts strongly affects the activity of soot combustion in the CeO2 –CuO catalysts [6]. However, the other research group had reported that specific surface area of CeO2 fiber did not affect the soot combustion [8]. This discrepancy would come from the difference of catalysts morphology. Thus the value of external surface area or mesopore volume would be another indication rather than specific surface area in the case of solid reactant. Our results imply that higher external surface area and mesopore volume of catalysts contributed to the improved physical contact condition between CeO2 catalysts and aggregated CBs and this affected greatly their catalytic activity for soot combustion. Rod particles were obtained for CuO/Ce-LA and CuO/Ce-DDA and this would accelerate the all factors that are necessary for the improvement of the soot combustion. CuO/Ce-DDA had higher reducibility at low temperature than the other catalysts and these are reason why CuO/Ce-DDA showed the superior catalytic property of soot combustion. N2 adsorption isotherms measured at 77 K, BET specific surface area, external surface area and mesopore volume of CuO/Ce-LA and CuO/Ce-no amine were compared before and after reaction (Fig. A.4 and Table A.2). These values of both catalysts intensely decreased after reaction because soot combustion was performed in the temperature range from 200 ◦ C to 700 ◦ C. However, higher values of external surface area and mesopore volume for CuO/Ce-LA were obtained as compared with CuO/Ce-no amine. These results imply that CuO/CeO2 catalysts with rod morphology have a possibility to show better activity of soot combustion than CuO/CeO2 with uncontrolled morphology even after reaction although the decrease of their catalytic performance would be inevitable during reaction at this present. 4. Conclusions We have demonstrated that different shaped CeO2 nanostructures such as rod and ellipsoid shape using self-assemblies of amine surfactants as templates can be effectively prepared by through the decomposition of cerium carbonate hydroxide. These different shaped CeO2 nanostructures supported CuO catalysts give effective soot combustion at lower temperature compared with CuO/CeO2 prepared without surfactant template due to their surface reducibility and larger external surface area and mesopore volume. These novel nanorod composites are demonstrated to be highly capable of trapping for effective soot combustion, which is an important reaction relating to air purification. The morphology control of such composite may open up valuable perspectives for wider catalytic applications. Acknowledgements This work was funded by a Grant-in-Aid for Young Scientists (B) (Grant No. 20750167) for the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors gratefully acknowledge Mr. T. Ueki (Center for Technical Support, Institute of Technology and Science, The University of Tokushima) for his technical support with TEM experiments.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.08.005.
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Please cite this article in press as: K. Nakagawa, et al., Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2 , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.005