Hollow nano-particles formation for CuO-CeO2-ZrO2 via a supercritical anti-solvent process

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016, P. 1126

Hollow nano-particles formation for CuO-CeO2-ZrO2 via a supercritical anti-solvent process JIANG Haoxi (姜浩锡)1,2, ZHOU Jiali (周佳丽)1,2, SUN Huanhua (孙焕花)1,2, LI Yonghui (李永辉)1,2,*, ZHANG Minhua (张敏华)1,2 (1. Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China; 2. Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China) Received 28 March 2016; revised 20 June 2016

Abstract: Hollow CuO-CeO2-ZrO2 nano-particles were prepared with supercritical anti-solvent apparatus by using methanol as solvent and supercritical carbon dioxide as anti-solvent. Two key factors (i.e., pressure and temperature) were investigated to explore the effects of catalyst structure and physic-chemical properties (i.e., morphology, reducing property, oxygen storage capacity and specific surface area). The resulting materials were characterized with X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET), hydrogen temperature programmed reduction (H2-TPR) and oxygen storage capacity (OSC) measurement, respectively. The experimental results showed that lower temperatures promoted production of hollow structure nano-particulates. The particle morphology also changed significantly, i.e. the solid construction was first transferred to hollow structure then back to solid construction. The optimal conditions for obtaining hollow nano-particles were determined at 45 °C, 18.0–24.0 MPa. Keywords: supercritical anti-solvent (SAS); CuO-CeO2-ZrO2; nano-particulate; hollow structure; rare earths

In recent years, ceria-based (CeO2) catalysts have been widely investigated as their broad application in threeway catalysts (TWC). In particular, ceria-zirconia composite oxides (CeO2-ZrO2) have received considerable attention for numerous catalytic applications, such as enhancing the removal of the emissions of CO, HC and NOx from automobile exhaust[1,2], because it presents high temperature stability, low reduction temperature and excellent oxygen storage capacity (OSC)[3–5]. Nevertheless, pure ceria itself does not perform well in catalyst because when CeO2 was exposed to high temperatures, the specific surface area decreases sharply, which causes a decrease of crucial redox properties, oxygen storage and release capacity[6]. However, it was reported that the reducibility of Ce4+ and the mobility of lattice oxygen can be strongly improved after being incorporated with Zr4+ [7]. Moreover many reports have shown that the addition of some metal additives, such as noble metals (Pt, Pd, Rh)[8,9] and non-precious metals (Cu)[10], increases the reactivity of low-temperature oxygen species formed on ceria. Guo et al.[11] have reported that CuO-CeO2 catalyst performed very well in low temperature selective catalytic reduction (SCR), in which Cu species was responsible for the SCR catalyst activity. Dong et al.[12] have prepared CuO-CeO2-ZrO2 catalysts by four methods, and

found that the high activity and selectivity of CuO-CeO2ZrO2 catalysts were obtained from the high dispersion of copper and the strong interaction between CuO and CeO2. In the past decade, supercritical CO2 (sc-CO2) is the most commonly used supercritical fluid due to advantages of non-flammability, non-toxicity, low price, good reproducibility and relatively mild critical conditions (TC=31.1 ºC, PC=7.38 MPa)[13]. As a novel environmental-friendly preparation route, supercritical antisolvent technology (SAS) has aroused wide attention to produce nano- and micro-sized particles[14]. The most attractive advantage of the SAS process is the ability to control particles size and morphology[15–17]. In our previous work, the MnOx-CeO2 hollow nano-spheres prepared via the SAS technique showed excellent oxygen mobility and rich surface active oxygen species[18,19]. Both temperature and pressure have a complex effect on those nano-particles with special physic-chemical structures[20]. Tang et al.[21] have prepared CeO2 with different morphologies under various pressures or temperatures. In this paper, hollow CuO-CeO2-ZrO2 nano-particles were processed by SAS, using supercritical CO2 as antisolvent medium and methanol as solvent under different operating pressures and temperatures. The influences of different pressures and temperatures on the morphology

Foundation item: Project supported by the National Natural Science Foundation of China (20976120) and Natural Science Foundation of Tianjin (09JCYBJC06200) * Corresponding author: LI Yonghui (E-mail: [email protected]; Tel.: +86-22-27406119) DOI: 10.1016/S1002-0721(16)60144-8

JIANG Haoxi et al., Hollow nano-particles formation for CuO-CeO2-ZrO2 via a supercritical anti-solvent process

of the nano-composite oxides were characterized by means of HR-TEM, and the specific surface areas calculated by BET. Besides, H2-TPR and OSC were used to study the reducibility and oxygen storage performance, respectively.

1 Experimental 1.1 Materials Copper acetate (Cu(CH3COO)2) with a purity of 99.9% and methanol (analytical grade) were bought from Kermel (Tianjin, China). Cerium acetylacetonate (Ce(acac)3) and zirconium acetylacetonate (Zr(acac)4) were prepared based on the procedure reported previously[22]. Carbon dioxide (CO2) was purchased by Hexagon (Tianjin, China) with the purity of 99.9%. 1.2 Preparation In this study, the precursor nano-particle CuO-CeO2ZrO2 was produced by an SAS50 apparatus (Thar Technologies Co., USA) with sc-CO2 as the anti-solvent. The flow chart of SAS experimental apparatus is shown in Fig. 1. SAS process was started by continuously introducing CO2 into the precipitator at a constant flow rate of 45 g/min. To study the effect of the pressure and temperature on the particles pressure was set to be 15, 18, 21, 24 and 28 MPa (denoted as CZCu-P-1, CZCu-P-2, CZCu-P-3, CZCu-P-4, CZCu-P-5, respectively) and temperatures 35, 40 and 45 ºC（denoted as CZCu-T-1, CZCu-T-2, CZCu-P-1, respectively. After steady state was achieved, a mixed solution of cerium acetylacetonate (0.25 wt.%), zirconium acetylacetonate (0.25 wt.%) (molar ratio Ce to Zr 1:1.24 ) and copper acetate (0.25 wt.%) with methanol as solvent was injected into the precipitator at 1 mL/min. The CuO-CeO2-ZrO2 nano-particles can be obtained after the solution droplets contacting with sc-CO2. After SAS process, sc-CO2 was supposed to flow for another 30 min with a flow rate of 20 g/min to

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remove the residual methanol. Finally, the hollow CuOCeO2-ZrO2 nano-particles were obtained after the prepared precursor was calcined in a muffle furnace at 600 ºC for 2 h. 1.3 Characterization The morphologies of the samples were characterized by high resolution transmission electron microscopy (HRTEM) in a Tecnai G2 F20 type instrument (Philips, Holland). The crystal phases of the samples were carried out by X-ray diffraction technique (XRD, Rigaku D/Max 2500) with Cu Kα radiation. Temperature-programmed reduction (TPR) experiments were carried out on an Autochem II2920 (Micromeritic, USA) under 10% H2/Ar (50 mL/min) with temperature ramped from 25 to 900 ºC at the rate of 10 ºC/min. The apparatus of OSC measurements was the same as TPR. The catalysts were first tested by TPR device and then cooled down to 560 ºC. Afterward the samples were purged under N2 condition of (20 mL/min) at this temperature for another 30 min. Finally, 5% O2/N2 (10 mL/min) was injected in the pulse at 560 ºC, and the OSC was calculated by the TCD signal. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method by nitrogen adsorption (Micromeritics Tristar 3000) at –196 ºC.

2 Results and discussion 2.1 Effects of third component on MCP (mixture critical point) curves The MCP curves of the ternary CO2-methanol-solute system were measured at 45 ºC and the results are shown in Fig. 2. As seen in Fig. 2, the MCP curves of the system moved slightly toward higher pressures after adding 0.25 wt.% Ce(acac)3 into the CO2-methanol system. In addition, the movement of MCP curves became significant after adding 0.25 wt.% Cu(CH3COO)2 or 0.25 wt.% Ce(acac)3, 0.25 wt.% Zr(acac)3, 0.25 wt.% Cu(CH3COO)2. The Cu content in precursor was 9.59

Fig. 1 Flow chart of SAS experimental apparatus 1–CO2 cylinder; 2–Cooler; 3–Mass flow meter; 4–CO2 pump; 5–Pre-heater; 6–Liquid solution container; 7-Liquid pump; 8, 12, 13, 18– Valve; 9, 15–Pressure gauge; 10, 16–Temperature indicator controller; 11–Precipitator; 14, 19–Back pressure regulator; 17–Separator

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Reverchon et al.[23] also found that: (1) the MCP curves of the ternary systems moved toward higher pressures; (2) the offset degree of the MCP curves was associated with the solute, and the stronger interaction between the solute and the solvent, for the binary system, the greater the impact is. Combined with the results of this work it can be seen that the addition of Cu(CH3COO)2 has a greater influence on the binary CO2-methanol system than those of Ce(acac)3. Because the interaction between Cu(CH3COO)2 and methanol is stronger than that between Ce(acac)3 and methanol. This may be the reason why the precursor of CuO-CeO2-ZrO2 transforms from hollow structures into solid. Fig. 2 MCP of methanol-carbon dioxide added with the third component at 45 ºC (1) CO2-methanol; (2) CO2-methanol-Ce(acac)3 (0.25 wt.%); (3) CO2-methanol-Cu(CH3COO)2 (0.25 wt.%); (4) CO2-methanolCu (CH3COO)2 (0.25 wt.%), Ce(acac)3 (0.25 wt.%), Zr(acac)4 (0.25 wt.%)

mol.% (according to ICP analysis). Precursors of Cu-Ce-Zr samples consisted of acetylacetonate of Cu, Ce and Zr, which transformed to their oxides after calcination. Therefore, the contents of Cu in the catalyst are the same as that in the precursors. The above results show that: (1) the addition of the Cu species has an effect on the MCP curve of the binary CO2-methanol system; (2) the degree of influence on the MCP curve varies from composition and quantity of the solute.

2.2 Effects of pressure on CuO-CeO2-ZrO2 particles 2.2.1 Effects of pressure on morphology and structure of CuO-CeO2-ZrO2 particles The prepared CuO-CeO2-ZrO2 particles under different pressures were characterized by HRTEM (shown in Fig. 3). As can be seen from Fig. 3, the morphology of the CuO-CeO2-ZrO2 particles was significantly influenced by the operating pressure. Intriguingly, the particles were solid solution under 15 MPa, but they showed an obvious hollow structure at 18 MPa. Then the hollow structure disappeared gradually with the increasing pressure until it turned into solid solution again at 28 MPa. Fig. 4 shows the XRD patterns of the Cu-Ce-Zr-O composite oxides prepared by SAS under different pressures. The un-modified CeO2-ZrO2 was denoted as

Fig. 3 HRTEM images of CuO-CeO2-ZrO2 composite oxide prepared by SAS under different pressures (a) CZCu-P-1; (b) CZCu-P-2; (c) CZCu-P-3; (d) CZCu-P-4; (e) CZCu-P-5

JIANG Haoxi et al., Hollow nano-particles formation for CuO-CeO2-ZrO2 via a supercritical anti-solvent process

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Fig. 4 XRD patterns of the CuO-CeO2-ZrO2 composite oxides prepared by SAS under different pressures

CZ-SAS. According to previous work[24], we can conclude that the ZrO2 could enter into the lattice of CeO2, forming a solid solution. In addition, the absence of CuO (2θ=35.5º and 38.7º) peaks may be one of the reasons that Cu2+ ions incorporated into the CeO2 lattice or CuO particles dispersed greatly on the surface of CeO2. Further proof is that a lattice shift was observed after adding Cu species, as shown in the magnified XRD result in Fig. 4(b). Since the ionic radius of Cu2+ (0.072 nm) is smaller than that of the Ce4+ (0.092 nm), diffraction peaks of the samples with Cu add-in moves towards larger angle. So it was  suggested that Cu2+ ions incorporated into the CeO2 lattice to form solid solution structure[25–27]. The surface area, pore volume, pore diameter, lattice parameters and average crystallite size of the samples prepared under different pressures are listed in Table 1. The lattice parameters and average crystallite size, which were calculated by Cohen and Debye-Scherrer methods, respectively, are shown in Table 1. They were both calculated from the (111)T diffraction peaks. It can be seen that the lattice parameters and the average crystallite size both decrease at first then grow with the increase of the

pressures. The trend of lattice parameters and average crystallite size correspond to the change of OSC (shown in the Table 2). It is obvious that the less lattice parameters (more solid solution formed) and average crystallite size (better dispersion of Cu) induced to higher OSC. Therefore, it can be concluded that OSC is related to the formation of solid solution (expressed as lattice parameters) and crystallite size. 2.2.2 Effects of pressure on reduction performance and OSC of CuO-CeO2-ZrO2 particles To study the reduction ability of CuO-CeO2-ZrO2 composite oxides under different pressures, we did the H2-TPR analyses. Fig. 5 shows the H2-TPR profiles of the CuO-CeO2-ZrO2 composite oxides. Two reduction peaks were obviously observed at the low temperature region from 130 to 150 ºC (donated as LT) and the high temperature region from 150 to 200 ºC (donated as HT). The first peak might be ascribed to the reduction of CuO which dispersed finely in close interaction with CeO2 matrix. The second reduction peak might be attributed to bulk copper oxide species which are less associated with ceria[28] . It can be inferred that Cu species exist in at least

Table 1 Textural properties of CuO-CeO2-ZrO2 composite oxide prepared by SAS under different pressures Pressure/

Specific surface area/

Pore volume/

Mean pore size/

Lattice parameters

Average crystal-

MPa

(m2/g)

(cm3/g)

nm

a=b=c/nm

lite size/nm

CZCu-P-1

15.0

12.70

0.0212

6.68

0.53829

10.6

CZCu-P-2

18.0

19.98

0.0354

7.95

0.53683

10.0

CZCu-P-3

21.0

17.76

0.0321

7.24

0.53613

9.9

CZCu-P-4

24.0

9.32

0.0224

9.63

0.53832

10.6

CZCu-P-5

28.0

9.05

0.0207

9.17

0.54165

12.8

Sample

Table 2 TPR quantitative analysis and OSC measurements results of CuO-CeO2-ZrO2 prepared by SAS under different pressures Sample

Pressure/ MPa

Peak 1 LT/°C

Peak 2

H2 consumption/(cm3/g)

HT/°C

OSC/

H2 consumption/(cm3/g)

(O2 µmol/g)

CZCu-P-1

15.0

142.7

11.02

188.5

18.27

549.5

CZCu-P-2

18.0

144.9

10.76

188.2

17.62

596.9

CZCu-P-3

21.0

141.6

9.96

190.5

20.40

612.1

CZCu-P-4

24.0

148.3

11.54

192.7

16.94

565.8

CZCu-P-5

28.0

145.2

9.40

185.8

17.21

531.1

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Fig. 5 H2-TPR profiles of CuO-CeO2-ZrO2 composite oxide prepared by SAS under different pressures

two forms in the composite oxides according to Hu’s report[29]. As for the attribution of the third peak, it is supposed to belong to the species showing the synergic effects with Cu and Ce, which we cannot identify yet upon current experiment skills. The quantitative analysis and the OSC measurements results are shown in Table 2. The data for hydrogen consumption during the TPR (Table 2) show that there are more actual hydrogen consumptions of the samples prepared under different pressures than the theoretical hydrogen consumptions required by Cu2+ conversion to Cu, may be because there is a synergistic effect between CeO2 and CuO in the composite oxide samples leading to reduction of CeO2, which is consistent with our previous

work[19]. It can also be found that the effect of pressure on hydrogen consumption of composite oxides is relatively weak. It has been reported that oxygen storage capacity is one of the most important indexes of Ce-based catalysts[5,30]. As can be seen from Table 2, the oxygen storage capacity is considerably influenced by the operating pressure. In addition, compared to the OSC value of un-modified CeO2-ZrO2, 506.0 µmol/g, the Cu-Ce-Zr-O composite oxides perform better oxygen storage capacity. It is obvious that the OSC value of the Cu-Ce-Zr-O composite oxides increases to a maximum firstly with increasing of pressure and then decreases. The highest OSC value of 612.1 μmol/g was obtained at 21.0 MPa. 2.3 Effects of temperature on CuO-CeO2-ZrO2 particles 2.3.1 Effects of temperature on morphology and structure of CuO-CeO2-ZrO2 particles As shown in Fig. 6, the HRTEM images of samples demonstrate that an obvious hollow structure appeared at the lower temperature, because as the temperature decreases, the MCP curves of the system moved toward lower pressure and further away from the operating pressure line, resulting in greater pressure difference, which promotes the formation of hollow structure nano-particles. The results of low temperature nitrogen adsorption (shown in Table 3) reveal that the specific surface area

Fig. 6 HRTEM images of CuO-CeO2-ZrO2 composite oxide prepared by SAS under different temperatures (a,d) CZCu-T-1; (b,e) CZCu-T-2; (c,f) CZCu-P-1

JIANG Haoxi et al., Hollow nano-particles formation for CuO-CeO2-ZrO2 via a supercritical anti-solvent process Table 3 Specific surface area and pore structure data of CuO-CeO2-ZrO2 prepared by SAS under different temperatures Specific surface

Pore volume/

Mean pore

Temper-

area/(m2/g)

/(cm3/g)

size/nm

ature/°C

CZCu-T-1

19.65

0.0507

10.33

35

CZCu-T-2

21.07

0.0473

8.98

40

CZCu-P-1

12.70

0.0212

6.68

45

Sample

and pore volume of the composite oxide increased with the decrease of the temperature, which is due to the formation of the hollow structure of the three component composite oxide particles. 2.3.2 Effects of temperature on reduction performance and OSC of CuO-CeO2-ZrO2 particles From the H2-TPR profiles of the samples, apparently there are two reduction peaks (donated as LT (low temperature) and HT (high temperature)) at low temperature region (130–200 ºC). The temperature of the reduction peak increased first and then decreased as the temperature increased. A small and gentle reduction peak appears at high temperature region (>500 ºC). It can be inferred that Cu species exist in at least two forms in the composite oxides as we discussed above. The quantitative analysis and the OSC measurements results are shown in Table 4. As can be seen from Table 4, with the preparation temperature decreasing from 45°C to 35°C, the H2 consumption decreased at first, and then increased while the reduction peak temperature first increased and then decreased. The interpretation of this complex phenomenon needs further study. Interestingly, the OSC of the samples did not increase, but decreased with decreasing temperature. Since redox behavior plays an important role in enhancing the OSC of the ceria solid solution, it is possible that CZCu-P-1 has the highest oxygen storage capacity due to its best redox behavior. 2.4 Formation mechanism of hollow structure nanoparticles The preparation of composite oxide precursors by SAS

Fig. 7 H2-TPR profiles of CuO-CeO2-ZrO2 composite oxide prepared at different temperatures

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Table 4 TPR quantitative analysis and OSC measurements results of CuO-CeO2-ZrO2 prepared by SAS under different temperatures Peak 1

TemperSample

ature/

LT/

°C

°C

H2 consumption/ (cm3/g)

Peak 2 HT/ °C

OSC/ H2 consumption/ (O2 µmol/g) (cm3/g)

CZCu-T-1

35

148.4

9.76

192.9

17.42

452.3

CZCu-T-2

40

152.2

7.38

196.1

16.03

478.4

CZCu-P-1

45

142.7

11.02

188.5

18.27

549.5

method mainly consists of two processes. One is the process of mutual diffusion between anti-solvent sc-CO2 and solvent, and the other is the process of the solute reaching saturation and precipitating as nucleation. Dukhin et al.[31] found that there is a relative rate in these two processes. The diffusion time and the nucleation time were recorded as µD and µN respectively. When the nucleation process is fast, i.e., when μD/μN>>1, it is a diffusion-controlled process. Correspondingly, it is a nucleation-controlled process when μD/μN <<1. The diffusion coefficient of sc-CO2 is larger than that of methanol. Hence, the diffusion of CO2 into methanol is the control step when methanol droplets contact with sc-CO2. The density of CO2 increases with increasing pressure, as a result, the solubility of the solvent methanol increases, and the driving force of the internal diffusion of the liquid drops enhances, which is beneficial to the diffusion. Nevertheless, diffusion coefficient is inversely proportional to the pressure; therefore, in a certain pressure range, the increase of pressure will slower the diffusion of sc-CO2, then the formation process of the particle is diffusion-controlled. The concentration of the solute in the droplet will be different, resulting in the formation of solid spherical shell and hollow structure. The mechanism of the process is shown in Fig. 8 route B. As the pressure rises, the anti-solvent ability of CO2 and the expansion rate of methanol became stronger[32]. When the pressure increases to a certain value, the density of sc-CO2 increases rapidly and the ability to dissolve the solvent is greatly increased. Therefore, the influence of the pressure on the solvent resistance dominates, which leads to the relatively slow nucleation proc-

Fig. 8 Schematic mechanism of nucleation process

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ess. At this point, the components in the internal of droplet are well-distributed and the supersaturation can be achieved simultaneously. The nucleation rate in the internal of liquid droplet is uniform, so that the solute can precipitate to form a uniform spherical particle without hollow structure. The process is shown in Fig. 8 route A. It can be seen that this is consistent with the above TEM characterization results.

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3 Conclusions In our study, the MCP curves of the ternary CO2methanol-solute system were determined by phase equilibrium[14]. It was observed that the addition of solute (Ce(acac)3, Zr(acac)3, Cu(CH3COO)2) made the MCP curves move toward higher pressures. The degree of the movement was related to the properties of the solute. The result showed that the interaction between copper acetate and methanol was strong, which had a relatively great effect on the MCP curves. In summary, our study on changing the operating pressure and temperature of the composite oxide nano-particles by SAS method showed that the low temperature was beneficial to the generation of hollow structure. With the increase of pressure, the morphology of the samples turned from solid construction to hollow structure and solid structure finally appeared. This phenomenon might be caused by the change of the diffusion time and nucleation time due to the difference in the solvent-resistant ability of CO2. Compared with the samples prepared under different pressures, CZCu-P-3 with the smallest average crystallite size (9.9 nm) showed the highest OSC value of 612.1 μmol/g. Besides, CZCu-P-1 preformed the best redox property among the CuO-CeO2-ZrO2 composite oxides prepared at various temperatures, which played a major role in raising its OSC value (549.5 μmol/g). For the CuO-CeO2-ZrO2 composite oxide, the suitable conditions for the generation of hollow structure nano-particles were at 45 ºC and between 18.0 and 24.0 MPa.

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