Pore structure of γ-Ga2O3–Al2O3 particles prepared by spray pyrolysis

Microporous and Mesoporous Materials 145 (2011) 131–140

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Pore structure of c-Ga2O3–Al2O3 particles prepared by spray pyrolysis Tsunenori Watanabe a,b, Yoshihisa Miki a, Takeo Masuda a, Hiroyoshi Kanai a, Saburo Hosokawa a, Kenji Wada a, Masashi Inoue a,⇑ a b

Department of Energy and Hydrocarbon Chemistry, Graduated School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan Power Engineering R&D Center, The Kansai Electric Power Company, Inc., 3-11-20, Nakoji, Amagasaki 661-0974, Japan

a r t i c l e

i n f o

Article history: Received 18 March 2011 Received in revised form 4 May 2011 Accepted 4 May 2011 Available online 10 May 2011 Keywords: c-Ga2O3–Al2O3 Solid solution Spray pyrolysis

a b s t r a c t Calcination of the precursor particles obtained by the spray pyrolysis of an aqueous solution of Ga(NO3)3 and Al(NO3)3 in the presence of HNO3 gave c-Ga2O3–Al2O3 solid solutions and their pore structures were examined. The temperatures of the sequential ovens and flow rates of air as an entrainer of the mists for spray pyrolysis affected the pore structure of the products, although the pore systems were developed during the subsequent calcination stage. These products had two types of pore structures and origin of these pore structures was discussed on the basis of the core–shell structure of the spherical particles of the products. Effects of the concentrations of metal nitrates and HNO3, and of the addition of urea to the starting solution in place of HNO3 upon that pore structure of the products were also examined. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Spray pyrolysis using an ultrasonic atomizer is one of the useful methods to directly produce fine ceramic powders from metal salt solutions [1]. Some papers have reported the synthesis of Al2O3 by spray pyrolysis. Roy et al. investigated a-Al2O3 formation using Al(NO3)3
9H2O as a starting reagent [2]: amorphous Al2O3 was formed by spray pyrolysis through the oven set at >900 °C, and a-Al2O3 was obtained after 1 h heating of the amorphous Al2O3 at 900 °C. Jokanovic´ et al. investigated the synthesis of submicrometer spherical alumina powders by the spray pyrolysis [3]: they reported the influence of the process parameters on the formation mechanisms of the particles and on their morphologies. Kato et al. prepared a mixture of hollow and solid a-Al2O3 microspheres, and found that the yield of hollow microspheres was influenced by the mist-supply systems [4]. Vallet-Regí et al. investigated the method to control the structure of alumina synthesized by the spray pyrolysis using various aluminum compounds (AlCl3
6H2O, Al2(SO4)3
18H2O, and Al(NO3)3
9H2O) [5]. Ogi et al. reported the synthesis of c-Ga2O3 using a Ga(NO3)3 aqueous solution [6]. They synthesized GaN from the thus-prepared c-Ga2O3 particles by the reaction with ammonia. They also reported that highly crystalline Ga2O3 nanoparticles with an average diameter of 10 nm were obtained when a flux salt (LiCl) was added to the starting solution. Synthesis of MgAl2O4 spinel powders by the spray pyrolysis was also reported [7,8]. Kim et al. characterized Al2O3 particles prepared by spray pyrolysis using an Al(NO3)3 aqueous solu⇑ Corresponding author. Tel.: +81 75 383 2478; fax: +81 75 383 2479. E-mail address: [email protected] (M. Inoue). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.05.002

tion in the presence of both urea and cetyltrimethylammonium bromide and reported their pore structures [9]. We have reported that c-Ga2O3–Al2O3 solid solutions exhibited high performance for selective catalytic reduction (SCR) of NO with CH4. These solid solutions have been prepared by co-precipitation method [10], sol–gel method [11], and solvothermal method [12]; however, their catalytic activities strongly depended on the preparation methods. So far as the authors know, however, synthesis of Ga2O3–Al2O3 solid solutions by the spray pyrolysis method has never been reported. Recently, we synthesized the Ga2O3–Al2O3 mixed oxides by the spray pyrolysis and found that these samples had fairly high catalytic activities for CH4-SCR of NO [13]. In this work, pore structures of these samples were examined in relation to the preparation conditions, and mechanisms for the pore structure are discussed. 2. Experimental section 2.1. Preparation of c-Ga2O3–Al2O3 solid solution An aqueous solution containing gallium nitrate hydrate (Ga(NO3)3
nH2O, Mitsuwa Chemical; 0.05 mol L 1), aluminum nitrate nonahydrate (Al(NO3)3
9H2O, Nacalai Tesque; 0.172 mol L 1; Ga/ (Ga + Al) = 0.225), and nitric acid (HNO3, Wako Pure Chemical; 1.89 mol L 1) were used for spray pyrolysis unless otherwise mentioned. Spray pyrolysis (ACP-U3-H4-KD10, ON Sogo Denki) consists of the following three processes. First, mists were generated from the starting solution (or suspension) using three ultrasonic vibrators (1.6–1.75 MHz). The second step was the firing of the mists.

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Air was introduced to allow the mists to flow through an alumina tube (20 mm i.d. and 1.2 m long) set inside four sequential ovens. The air flow rate was adjusted in a range of 0.1–5 L min 1 to control the residence time of the mists in the heating zone (5–90 s) and the temperatures of the four ovens could be adjusted separatedly at 50–1000 °C. In this paper, six different temperature profiles were used as shown in Table 1. For the profiles (b)–(f), the temperatures of the four ovens were set, so that the temperature of mists linealy increases during the pyrolysis. The temperature of the first oven (200 °C) was selected because the TG–DTA profiles of Ga(NO3)3
nH2O and Al(NO3)3
9H2O showed that these reagents decomposed at 200 °C (data not shown). The temperature of the final oven (700 °C) was selected to avoid the formation of b-Ga2O3–hAl2O3 phases. The third step is the collection of the fired particles. The particles were collected on a membrane-filter heated at 130 °C. In this paper, the as-synthesized particles before calcination are designated as SP(x), and solid solutions calcined at 800 °C for 3 h are denoted as SP(x)-cal, where x stands for the Ga/(Ga + Al) molar ratio in the starting mixture. 2.2. Characterization of Ga2O3–Al2O3 solid solution Nitrogen adsorption isotherms were obtained using a volumetric gas-sorption system (Quantachrome Autosorb-1). The samples were dried in vacuo at 300 °C for 30 min prior to measurement. The BET surface areas were calculated from the isotherms using the BET equation. Total, micropore, mesopore, and outer surface areas as well as total, micropore, and mesopore volumes were calculated from the V–t plot derived from the isotherm. Morphologies of the samples were observed with a field emission scanning electron microscope (JSM-7000F, JEOL) operated at 30 kV. The cross-sectional images of the samples were taken with a transmission electron microscope (JEM2000FX, JEOL) operated at 200 kV, after the samples were embedded in epoxy resin and microtomed to 80–90 nm. Powder X-ray diffraction (XRD) patterns were measured on a Rigaku RINT 2500 diffractometer using Cu Ka radiation (40 kV, 4 kW) and a carbon monochromator. The crystallite size of the sample was calculated from the half-height width of the 440 diffraction peak (around 66° 2h) of the spinel structure using Scherrer equation. To examine the effect of ultrasonic treatment on the pore structure, SP(0.0) and SP(0.0)-cal were suspended in deionized water and sonicated at 20 kHz for 30 min using an Astrason XL2020 ultrasonic processor (Misonix). The suspensions were dried at 80 °C overnight.

that SP(0.225) synthesized using the temperature profile (e) was c-Ga2O3–Al2O3, while the other temperature profiles gave amorphous products. When these samples were calcined at 800 °C for 3 h, c-Ga2O3–Al2O3 was crystallized. The SEM image of SP(0.225)-cal prepared under the condition (c) is shown in Fig. 1 as a representative datum. The particles were spherical with smooth surface, and their diameters were distributed in a range of 0.1–1.5 lm. Fig. 2 shows the N2 adsorption isotherms of the samples prepared under the spray pyrolysis conditions (a)–(f). The V–t plots derived from the isotherms are shown in Supporting information Fig. S1, which indicate that all the samples are essentially mesoporous. Since the first segments of the V–t plots for samples (a) and (e) do not go through the origin, these samples may have micropores; however, contribution of these micropores to the total pore volume must be small. The shapes of the isotherms can be classified into three groups; these groups are correlated roughly with the amounts of heat supplied in the spray pyrolysis stage. The samples prepared by the heating profiles of (a), (d), and (e) were extensively heated in the spray pyrolysis stage; their isotherms are shown in Fig. 2a. c-Ga2O3– Al2O3 as-synthesized by heating profile (e) had a crystallite size of 4.7 nm, while the crystallite size of the calcined sample was only 5.3 nm, indicating that the crystallites formed at the spray pyrolysis stage scarcely grew by subsequent calcination (Table 2, No. 5). On the other hand, the isotherm of the SP(0.225)-cal sample (c) exhibits the smallest hysteresis loop (Fig. 2b). The shape of the isotherm of this sample exhibits meandering curves especially in the desorption branch. The SP(0.225) samples (b) and (f) were synthesized with insufficient thermal energy supplied during the spray pyrolysis. These samples showed larger hysteresis loops than those exhibited by the other samples (Fig. 2c). Table 2 summarized crystallite size, the BET surface area, and pore characteristics of the samples examined in this work. As shown in Table 2, the BET surface areas of the products (SP(0.225)) as synthesized by spray pyrolysis were quite small, while those of the calcined samples (SP(0.225)-cal) were relatively large. 3.2. Pore structures of Ga2O3 and Al2O3 The SP(0.0) and SP(1.0) samples were prepared under condition (c) and the pore structures were examined. The N2 adsorption

3. Results 3.1. Effect of temperature and flow rate during spray pyrolysis The effects of the temperature profiles of spray pyrolysis to synthesize SP(0.225) were investigated. The XRD patterns revealed Table 1 Conditions of spray pyrolysis to prepare SP(0.225).a Condition

(a) (b) (c) (d) (e) (f) a

Temperature of the oven (°C)

Residence time of the mists in the ovens (s)

1st

2nd

3rd

4th

700 200 200 200 200 200

700 366 366 366 566 266

700 533 533 533 733 433

700 700 700 700 1000 400

23 5 23 90 23 23

The SP(0.225) samples were calcined at 800 °C for 3 h to give SP(0.225)-cal.

Fig. 1. SEM image of SP(0.225)-cal obtained under spray pyrolysis condition (c) shown in Table 1.

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b

150

100

Adsorption

Desorption

Desorption

100

-1

(e)

3

3

Va / cm (STP) g

-1

150

Adsorption

Va / cm (STP) g

a

50

50

(c)

(a) (d) 0

0

0

0.2

0.4

0.6

0.8

0

1

0.2

0.4

0.6

0.8

1

P/P 0

P/P

0

c

150

Adsorption

Va / cm3 (STP) g

-1

Desorption

(b)

100

50

(f)

0

0

0.2

0.4

0.6

0.8

1

P/P

0

Fig. 2. N2 adsorption isotherms of SP(0.225)-cal. Samples (a)–(f) were prepared under the corresponding spray pyrolysis conditions shown in Table 1.

isotherms for SP(0.0) and SP(1.0) are shown in Fig. 3, and the V–t plots derived from the isotherms are shown in the Supporting information Fig. S2. As shown in Table 2, No.7, SP(0.0) was amorphous while c-Al2O3 was crystallized by calcination. On the other hand, c-Ga2O3 was directly crystallized under the spray pyrolysis conditions and it transformed into b-Ga2O3 by calcination. As shown in Figs. 3 and S2, the isotherm and V–t plot for SP(1.0) (cGa2O3) strongly suggests that it is mesoporous. This sample had a BET surface area of 94 m2 g 1 (Table 2, No. 8), and transformation into b phase caused a large decrease in surface area (38 m2 g 1). Fig. 3b shows the pore-size distributions of the samples calculated from the desorption branches of the isotherms. The cGa2O3 sample (SP(1.0)) had pores in a range of ca. 4.4–5.8 nm, while pore diameter of b-Ga2O3 (SP(1.0)-cal) was much larger (ca. 6.2–12.5 nm). The BET surface areas of SP(0.0) and SP(0.0)-cal were 2 and 93 m2 g 1, respectively (Table 2, No. 7). Calcination increased the BET surface area; similar results were obtained for SP(0.225) as mentioned above.

The isotherm of SP(0.0)-cal (Fig. 3a) was similar to that of SP(0.225)-cal (c) (Fig. 2b) as well as those of SP(0.1)-cal, SP(0.35)-cal, and SP(0.5)-cal (Fig. S3). These results indicated that the addition of the Ga component (Ga/(Ga + Al) 6 0.5) did not influence the pore structure of the product. Fig. 4a shows a cross-sectional TEM image of a relatively large particle of SP(0.0)-cal. This image clearly shows that the particle has a core–shell structure. Another important point is that the particles in the core part had relatively large sizes, while the shell (crust) comprised of intimate aggregate of fine primary particles. To obtain better understanding of the pore structure and of the development of the pore structure by calcination, SP(0.0) and SP(0.0)-cal were sonicated in deionized water, and the TEM images (Fig. 4d and b, respectively) and the isotherms (Fig. 5) after ultrasonic treatment were examined. Table 3 summarizes the effect of ultrasonic treatment on the N2 adsorption characteristics of the samples. The BET surface area and total pore volume significantly increased by the ultrasonic treatment of the as-synthesized

134

Table 2 Characterization of samples prepared by spray pyrolysis. No.

Synthesis condition for assyn sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 a b c d e f g h i j k l

0.225 0.225 0.225 0.225 0.225 0.225 0.0 1.0 0.225 0.225 0.225 0.225 0.225 0.225

Conditions of spray pyrolysisa

(a) (b) (c) (d) (e) (f) (c) (c) (c) (c) (c) (c) (c) (c)

Characterization of sample Calcinedb

Concentration in starting solution (mol L 1)

As-synthesized

HNO3

Ga(NO3)3 + Al(NO3)3

Structurec

Crystallite sized (nm)

BET surface areae (m2 g 1)

Structurec

Crystallite sized (nm)

BET surface area (m2 g 1)

Total surface areaf (m2 g 1)

Outer surface areag (m2 g 1)

Mesopore surface areah (m2 g 1)

Mesopore volumei (cm3 g 1)

Total pore volumej (cm3 g 1)

Average pore diameterk (nm)

1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 0 0.9 1.9 1.9 0.45l 1.11l

0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.56 1.11 0.22 0.22

AM AM AM AM

– – – – 4.7 – – 3.8 – – – – – –

3 3 3 3 41 3 2 94 4 4 3 2 4 14

c c c c c c c

8.0 8.0 7.4 7.2 5.3 8.0 7.2 – 7.5 7.6 7.7 8.4 5.8 8.6

85 99 109 72 121 85 93 38 59 59 78 87 73 17

97 100 – 75 121 84 97 39 60 59 81 91 73 16

14 7 30 – 8 5 – 10 37 – 14 8 6 11

83 93 – – 113 79 – 29 23 – 67 83 67 –

0.108 0.131 – – 0.171 0.115 – 0.081 0.064 – 0.129 0.144 0.092 –

0.146 0.158 0.190 0.175 0.210 0.145 0.170 0.103 0.154 0.173 0.173 0.170 0.115 0.027

6.9 6.4 7.0 9.7 6.9 6.8 7.3 10.9 10.4 11.7 8.9 7.8 6.3 6.3

c AM AM

c AM AM AM AM AM AM

b

c c c c c c

Conditions were shown in Table 1. Samples were obtained by calcination at 800 °C for 3 h. AM, amorphous; c, c-type structure; b, b-type structure determined by XRD analysis. Crystallite size of c-phase calculated by XRD line-broadening technique. Pretreatment for BET measurement was carried at 120 °C for 1 h in vacuo to avoid the development of pore structure by thermal treatment. Calculated from the slope of the first segment of the V–t plot. Calculated from the slope of the third segment of the V–t plot. Calculated by subtraction of outer surface area from total surface area. Estimated by extrapolation of the third segment to y-axis: because limited numbers of data points are available for the third segments, the values shown in this column should be considered to certain larger error limits. Calculated from the adsorption volume at P/P0 = 0.944. Calculated from BET surface area and total pore volume assuming that the sample have cylindrical pores with a constant diameter. Concentration of (NH2)2CO in the starting solution in stead of HNO3.

T. Watanabe et al. / Microporous and Mesoporous Materials 145 (2011) 131–140

Ga/ (Ga + Al) charged ratio

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T. Watanabe et al. / Microporous and Mesoporous Materials 145 (2011) 131–140

150

b

Adsorption Desorption

SP(0.0)-cal

ΔVa /Δlog(d /nm) / cm g

(γ -Al2 O3 ) SP(1.0) ( γ -Ga2 O3)

3

V a / cm (STP) g

-1

SP(0.0)-cal 100

2

(γ -Al 2 O3 )

3 -1

a

50

SP(1.0) (γ -Ga 2O3 )

1

SP(1.0)-cal (β -Ga 2 O3)

SP(1.0)-cal 0

( β -Ga 2 O 3 ) 0

0.2

0.4

0.6

0.8

1

P/P 0

0

1

10

100

d /nm

Fig. 3. N2 adsorption isotherms of SP(0.0)-cal (c-Al2O3), SP(1.0) (c-Ga2O3), and SP(1.0)-cal (b-Ga2O3) (a) and pore-size distributions calculated from the desorption branches of the isotherms (b). These samples were synthesized under the condition (c) shown in Table 1.

Fig. 4. TEM images of: (a) and (b), SP(0.0)-cal; (c) and (d), SP(0.0); (a) and (c), before; (b) and (d), after ultrasonic treatment.

product. The V–t plot of the sonicated SP(0.0) sample, shown in the Supporting information Fig. S4, clearly indicates that the sample is microporous.

The TEM image of SP(0.0) before ultrasonic treatment shown in Fig. 4c indicates that each sphere is homogeneously composed of fine particles having a pseudoboehmite-like morphology. This

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a 150

b 150

Adsorption

Adsorption Desorption

SP(0.0)-cal with ultrasonic -1

100

100

3

Va / cm (STP) g

Va / cm 3 (STP) g-1

Desorption

50

SP(0.0) with ultrasonic 50

SP(0.0)-cal without ultrasonic

SP(0.0) without ultrasonic

0

0

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P /P 0

0.6

0.8

1

P /P0

Fig. 5. N2 adsorption isotherms of (a) SP(0.0) and (b) SP(0.0)-cal with/without ultrasonic treatment.

Table 3 Surface area of SP(0.0) and SP(0.0)-cal with/without ultrasonic treatment. Sample

Ultrasonic treatment

BET surface area (m2 g 1)

Total surface areaf (m2 g 1)

Outer surface areag (m2 g 1)

Mesopore surface areah (m2 g 1)

Mesopore volumei (cm3 g 1)

Total pore volumej (cm3 g 1)

Average pore diameterk (nm)

SP(0.0)a

Without Withc Without

2d 68d 93e

– 68 97

– 21 –

– 47 –

– 0.038 0.056

0.003 0.071 0.170

– 4.2 7.3

Withc

130e

126

40

86

0.121

0.203

6.2

SP(0.0)calb a

Samples were synthesized under the condition (c) shown in Table 1. b Samples were obtained by subsequent calcination at 800 °C for 3 h. c Samples were suspended in deionized water and sonicated at 20 kHz for 30 min. The suspensions were dried at 80 °C overnight. d Pretreatment for BET measurement was carried at 120 °C for 1 h in vacuo to avoid the development of pore structure by thermal treatment. e Samples were evacuated at 300 °C for 30 min prior to the measurement. f Calculated from the slope of the first segment of the V–t plot. g Calculated from the slope of the third segment of the V–t plot. h Calculated by subtraction of outer surface area from the total surface area. i Estimated by extrapolation of the third segment to y-axis: because limited numbers of data points are available for the third segments, the values shown in this column should be considered to certain larger error limits. j Calculated from the adsorption volume at P/P0 = 0.944. k Calculated from BET surface area and total pore volume assuming that the sample have cylindrical pores with a constant diameter.

result indicates that the core–shell structure as observed in Fig. 4a is not formed in the pyrolysis stage but created during calcination. In the TEM image of the sample after ultrasonic treatment (Fig. 4d), completely pulverized particles were observed besides spherical particles. This pulverization seems to have taken place during microtoming, because the debris were localized in a limited area in the specimen. Presumably, ultrasonic treatment cracked the crusts of the spheres, but large surface energy of fine primary particles prevented complete pulverization of the primary particles; thus the sample had a microporous structure. For SP(0.0)-cal, we could not find any significant change in morphology caused by the ultrasonic treatment, while significant change was found in the isotherm. These results will be discussed in Section 4.4. 3.3. Effects of the concentrations of the starting materials on the pore structure of the SP(0.225) samples The N2 adsorption isotherms and V–t plots for SP(0.225)-cal synthesized with varying the concentration of HNO3 in a range of 0–1.9 mol L 1 are shown in Fig. S5, which suggests that the essential feature of the pore structure was not affected by the concentration of HNO3. As shown in Table 2 (Nos. 3, 9, and 10), the XRD

patterns revealed that all the SP(0.225)-cal samples were cGa2O3–Al2O3. Use of a high concentration of HNO3 (1.9 mol L 1) caused a large surface area. This result was presumably caused by the evaporation and decomposition of a large amount of HNO3 present in the mists during spray pyrolysis. The N2 adsorption isotherms and V–t plots of SP(0.225)-cal synthesized with varying the total concentration of the metal nitrates in a range of 0.22–1.11 mol L 1 with the ratio of Ga/(Ga + Al) fixed at 0.225 are shown in Fig. S6. The isotherms were essentially identical with each other, indicating that the concentration of the metal nitrates has little effect on the pore-structure of the product. Evaporation of water took place at the early stage of spray pyrolysis, and therefore water contents in the mists have negligible effect on the development of pores. As shown in Table 2 (Nos. 3, 11, and 12), all the samples were c-Ga2O3–Al2O3 and no correlation between the surface area and total concentration of the metal nitrates was found. 3.4. Effect of the addition of urea to the starting solution We investigated the effect of the addition of urea [(NH2)2CO] to the starting solution (without HNO3) on the physical properties of the products. The SP(0.225)-cal samples were synthesized without

T. Watanabe et al. / Microporous and Mesoporous Materials 145 (2011) 131–140

137

Fig. 6. SEM images of: (a)–(c), SP(0.225); (d)–(f), SP(0.225)-cal synthesized by spray pyrolysis of the solutions containing (NH2)2CO with the concentration of: (a) and (d), 0; (b) and (e), 0.45; (c) and (f), 1.1 mol L 1; under the condition (c) shown in Table 1.

150

Adsorption Desorption

Concentration of -1 (NH 2 )2 CO: 0 mol L

Va / cm (STP) g

-1

100

-1

3

0.45 mol L

50

-1

1.1 mol L

0 0

0.2

0.4

0.6

0.8

1

Fig. 6 shows the SEM images of SP(0.225) and SP(0.225)-cal obtained by calcination thereof. The shape of the particles synthesized in the absence of urea was similar to that of the particles synthesized with HNO3 as shown in Fig. 1. The particles synthesized in the presence of 0.45 mol L 1 urea had dimples on the surface. The particles synthesized using 1.1 mol L 1 urea had irregular shapes, presumably because a large number of dimples were formed on spherical particles. These morphologies suggest that mists composed of viscous liquid were formed during spray pyrolysis. Escape of the bubbles formed in the mists would be origin for the formation of the dimples [14,15]. Fig. 7 shows the effect of concentration of (NH2)2CO on the N2 adsorption isotherms of SP(0.225)-cal. The isotherm had a large hysteresis loop. Table 2 (Nos. 9, 13, and 14) shows the crystallite sizes calculated from XRD patterns and surface areas and pore volumes calculated from the V–t plots (Supporting information Fig. S7). The BET, total, and mesopore surface areas of SP(0.225)cal synthesized in the presence of 0.45 mol L 1 (NH2)2CO were larger than the corresponding values of SP(0.225)-cal synthesized without (NH2)2CO, probably because of the small crystallite size of the former sample. By the addition of (NH2)2CO, the total pore volume decreased, and the average pore diameter also decreased. The use of a large amount of (NH2)2CO produces very dense particles and the SP(0.225)-cal sample synthesized with 1.1 mol L 1 (NH2)2CO had a poorly-developed micropore system.

P /P0 4. Discussion Fig. 7. Effect of urea on the N2 adsorption isotherm of the SP(0.225)-cal.

4.1. Development of the pore structure urea and in the presence of 0.45 and 1.1 mol L 1 of urea with the ratio of Ga/(Ga + Al) fixed at 0.225. The XRD patterns revealed that all the samples obtained were c-Ga2O3–Al2O3.

The particles obtained by spray pyrolysis had low surface areas but ultrasonic treatment increased the surface area and total pore

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volume. These results suggest that the crusts of the samples were composed of so fine particles that nitrogen molecules cannot diffuse into the core part of the spheres. The shape of the isotherm and V–t plot of SP(0.0) clearly indicated that the sample was microporous, whereas the calcined sample was mesoporous. Therefore, pore structure was completely altered by calcination. Nevertheless, the pore structure of the products was affected by the spray pyrolysis conditions. When the SP(0.225) samples prepared with the temperature gradient were compared, the products prepared with insufficient thermal energy had large crystallite sizes (8.0 nm), while the products prepared with extensive thermal energy had small crystallite sizes. This result indicates that nuclei of cGa2O3–Al2O3 crystals were formed at the spray pyrolysis stage and suggests that a lesser number of nuclei were formed when thermal energy supplied at the spray pyrolysis stage was small, resulting in formation of c-Ga2O3–Al2O3 with large crystallite size at the subsequent calcination stage. 4.2. Pore structures of the products Some of the present products exhibited typical hysteresis loop (Fig. 2c) of type H2 in the IUPAC classification [16], while other products showed rather unique hysteresis loops (Fig. 2a and b). The isotherms of SP(0.225)-cal (Fig. 3a) and SP(0.0)-cal (Fig. 2b) exhibit meandering curves in the desorption branch. Similar isotherms have been previously reported and were explained by bi-modal distribution of pore size [17,18]. These shapes can be explained by the combination of two H2 type hysteresis loops as shown in Fig. 8; When a sample has two types of pore structures, which give the hysteresis loops <1> and <2>, the desorption branch of the isotherm of the sample would be as shown in a gray line in Fig. 8. The samples which exhibit this type of isotherms have the

core–shell structure as shown in Fig. 4a. Therefore, the two types of hysteresis loops can be attributed to the core and crust parts of the particles. Since the mode pore diameter calculated from the assumed hysteresis loop <2> (6.2 nm) is in reasonable agreement with the average pore diameter (7.3 nm, Table 2), hysteresis loop <2> can be attributed to the core part of the spheres. Although the pore diameter is much smaller than that the void width between the particles observed by TEM, the calculated pore diameter is determined by the narrow constrictions of the pores with varying width. Hysteresis loop <1> is due to the crust parts of the particles, and since the pore size in the crust part is small, the voids in a particle can be regarded as an ink-bottle shaped pore with narrow openings. However, the sharp decrease in adsorption volume observed at P/P0  0.45 in the desorption branch is caused by disappearance of capillary condensation at this relative pressure range; therefore, the pore-size distribution calculated by using the desorption branch of the isotherm does not have physical meanings [19,20], although it exhibits a sharp distribution peak at 3.7 nm. In the other words, the spherical particles are filled with nitrogen until relative pressure of the adsorptive reached 0.45. When capillary condensation at the crust part disappears, all the condensed nitrogen molecules held in the core part of the sphere also evaporates; thus, the isotherm has quite a large hysteresis loop. The isotherms shown in Fig. 2c and the isotherm of the sample prepared by the addition of 0.45 mol L 1 urea can be explained along the same line as hysteresis loop <1>, and this is presumably because the crusts of these particles have not cracks and are composed of dense compact of fine primary particles. The isotherms shown in Fig. 2a can be explained by the intermediate case between the isotherms shown in Fig. 2b and c, and for the samples shown in Fig. 2a the pore size determined by the void between the primary particles in the core part is smaller than that for the SP(0.225) sample shown in Fig. 2b. 4.3. Pore structure of gallia For the c-Ga2O3 (SP(1.0)) and b-Ga2O3 (SP(1.0)-cal) samples, the hysteresis loops can be explained by the pores with varying width with narrow constrictions formed between the primary particles, just as the explanation for hysteresis loop <2>. Calcination decreased the surface area and enlarged pore size, and these results are caused by the increase in the crystallite size of gallia. The mode pore diameter (9.0 nm) of the SP(1.0)-cal sample is in good agreement with the average pore diameter (10.8 nm, Table 2). This result supports the argument on the origin of the hysteresis loop <2>. As compared with the reported pore structure of the b-Ga2O3 sample prepared by the addition of aqueous ammonia (diluted in ethanol, 50 vol.%) to an ethanol solution of Ga(NO3)3 and subsequent calcination at 800 °C [21], the present product had a narrow pore size, low pore volume, small outer surface area, and small mode pore diameter, although both the samples (b phase) had essentially identical surface areas; 38 m2 g 1 for SP(1.0)-cal (Table 2, No. 8) and 40 m2 g 1 for the previously reported sample. These results indicate that the primary particles of Ga2O3 in SP(1.0)-cal were closely compacted during the spray pyrolysis stage.

Va

<1>

<2>

4.4. Effect of ultrasonic treatment of the SP(0.0)-cal sample 0

0.2

0.4

0.6

0.8

1

P/P0 Fig. 8. Imaginary isotherm for SP(0.0)-cal. Gray line corresponding to the desorption branch of SP(0.0)-cal is combination of hypothetical desorption branch <1> and <2>.

The results obtained from Fig. 5 can be summarized as follows. (1) Total surface area increased by ultrasonic treatment. (2) Outer surface area of the spheres seems not to be affected as judged from the slope of the V–t plot at the large thickness region. (3) Total accessible pore volume increased. (4) The mesopore volume which

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seems to be due to the spheres increased. (5) Pore size as judged from the decrease in the adsorption volume from the plateau region decreased. These results suggest that the most of spheres were not crushed by ultrasonic treatment, but result (3) suggests that some spheres were crushed contributing to the increase in the adsorption volume at the high relative pressure region. The result (5) may be explained by partial pulverization of the aggregated particles in the core part of the spheres. This was not proven by the TEM images because the particle-size distribution in the core part is quite large. However, pulverization of aggregated particles creates new surfaces (result (1)) and narrows the void between the particles. However, the increase in the pore volume due to the spheres (result (4)) cannot be explained by the process mentioned above, and it may be explained by the following scenario: some particles have so dense crust that the inner part of the spheres cannot be assessed by nitrogen. The ultrasonic treatment creates cracks in the crusts; thus the inner part the spheres can be assessed by nitrogen. This process increases both surface area (result (1)) and pore volume of spheres (result (4)).

4.5. Effect of urea Kim et al. characterized Al2O3 particles prepared by spray pyrolysis using an Al(NO3)3 aqueous solution in the presence of both urea and cetyltrimethylammonium bromide (CTAB) [9]. They reported that the BET surface area of alumina increased with the increase in the concentration of (NH2)2CO from 0.2 to 0.6 mol L 1 when CTAB/Al ratio was fixed; further increase in (NH2)2CO concentration up to 1.0 mol L 1 decreased the BET surface area. They concluded that an appropriate amount of (NH2)2CO help for increasing the surface area of Al2O3 because of the formation of NH3 and CO2 gases from urea when the temperature of droplet goes over 75 °C (this explanation seems to be caused by confusion between hydrolysis and pyrolysis of urea: if hydrolysis of urea takes place, urea gives CO2 and NH3. However, hydrolysis of urea is a slow process and recent research on the urea-SCR system, in which aqueous solutions of urea are fed to the exhaust from diesel engines, suggests that thermal decomposition of urea takes place after evaporation of water, yielding ammonia and isocyanic acid [22,23]). In the present case, the c-Ga2O3–Al2O3 solid solution having a large BET surface area was obtained by the use of 0.45 mol L 1 (NH2)2CO (No. 13 in Table 2), although it showed a large hysteresis loop in the N2 adsorption isotherm. Increase in pH of the mists by NH3 formed by pyrolysis of (NH2)2CO might cause a large number of nuclei created in the spray pyrolysis stage as Kim et al. [9] suggested; the large number of nuclei resulted in formation of small crystallites thus leading to the product with a large surface area. Thermal decomposition behavior of urea has been examined by many researchers [22–26]. It is generally accepted that thermal decomposition of urea starts after urea melts (133–135 °C). However, decomposition behavior strongly depends on the reaction conditions. Schaber et al. [24] reported that when urea was decomposed in an open vessel it gave biuret (NH2CONHCONH2) and then a mixture of cyanuric acid (C3H3N3O3) and ammelide (C3H4N4O2) with traces of ammeline (C3H5N5O) and melamine (C3H6N6). However, urea remained until 250 °C. Although triazine compounds (cyanuric acid, ammelide, ammeline, and melamine) have high melting points (>300 °C), a complex mixture of partially decomposed products gives viscous liquid since biuret also melts at 196–197 °C. This viscous liquid seems to be the origin of the morphology of the products. When 1.1 mol L 1 (NH2)2CO was added to the starting solution, surface tension of the viscous liquid causes the coagulation of metal species, thus leading to the extremely dense particles having a micropore system.

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5. Conclusions The spray pyrolysis of an aqueous solution of Ga(NO3)3 and Al(NO3)3 in the presence of HNO3 formed spherical particles. These particles were amorphous and had low surface areas because the crusts of the particles were dense. Calcination of the amorphous particles at 800 °C increased the surface area and the calcined products had mesoporous structures. However, the pore structure is determined not only by calcination but also by the spray pyrolysis conditions because nuclei of c-Ga2O3–Al2O3 crystals are formed at the spray pyrolysis stage. The solid solutions had two types of pore structures: one is composed of the pores with varying width with narrow constrictions, and this pore structure is formed in the core part of spherical particles. The other is characterized by ‘‘inkbottle shaped pores’’, and pore opening of the ink-bottle is formed by the crust part of the spherical particles. The total concentration of the metal nitrates and HNO3 concentration did not affect the pore structure of the c-Ga2O3–Al2O3 solid solutions, because water is evaporated at the early stage of spray pyrolysis. When urea was added to the starting solution in place of HNO3 the pore structure of the product was completely altered: When a small amount (0.45 mol L 1) was added, ink-bottle shaped pores were formed, and a large amount of urea (1.11 mol L 1) caused a large decrease in surface area. These pore structures are formed by viscous liquid generated by partial decomposition of urea at the spray pyrolysis stage. Acknowledgments The authors are grateful to Dr. Seiichiro Imamura of Kyoto University for his kind help. The authors also thank Dr. Toru Inagaki, Dr. Hiroyuki Yoshida, and Dr. Mitsunobu Kawano of the Kansai Electric Power Co., Inc. for their kind directions for the spray pyrolysis experiment. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2011.05.002. References [1] G.L. Messing, S.-C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. [2] D.M. Roy, R.R. Neurgaonkar, T.P. O’Holleran, R. Roy, Am. Ceram. Soc. Bull. 56 (1977) 1023. [3] V. Jokanovic´, Dj. Janac´kovic´, A.M. Spasic´, D. Uskokovic´, Mater. Trans. JIM 37 (1996) 627. [4] T. Kato, M. Tashiro, K. Sugimura, T. Hyodo, Y. Shimizu, M. Egashira, J. Ceram. Soc. Jpn. 110 (2002) 146. [5] M. Vallet-Regí, L.M. Rodríguez-Lorenzo, C.V. Ragel, A.J. Salinas, J.M. GonzálezCalbet, Solid State Ionics 101 (1997) 197. [6] T. Ogi, Y. Kaihatsu, F. Iskandar, E. Tanabe, K. Okuyama, Adv. Powder Technol. 20 (2009) 29. [7] Y. Suyama, A. Kato, Ceram. Int. 8 (1982) 17. [8] Dj. Janac´kovic´, V. Jokanovic´, Lj. Kostic-Gvozdenovic, R. Cirjakovic, I. PetrovicPrelevic, D. Uskokovic´, Key Eng. Mater. 132 (1997) 197. [9] J.H. Kim, K.Y. Jung, K.Y. Park, S.B. Cho, Micropor. Mesopor. Mater. 128 (2010) 85. [10] T. Masuda, T. Watanabe, Y. Miyahara, H. Kanai, M. Inoue, Top. Catal. 52 (2009) 699. [11] M. Haneda, N. Bion, M. Daturi, J. Saussey, J.-C. Lavalley, D. Duprez, H. Hamada, J. Catal. 206 (2002) 114. [12] M. Takahashi, T. Nakatani, S. Iwamoto, T. Watanabe, M. Inoue, Appl. Catal. B: Environ. 70 (2007) 73. [13] T. Watanabe, Y. Miki, T. Masuda, H. Kanai, S. Hosokawa, K. Wada, M. Inoue, Appl. Catal. A: General 396 (2011) 140. [14] S.-C. Zhang, G.L. Messing, M. Borden, J. Am. Ceram. Soc. 73 (1990) 61. [15] D.W. Sproson, G.L. Messing, T.J. Garder, Ceram. Int. 12 (1986) 3. [16] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [17] N. Andersson, P.C.A. Alberius, J.S. Pedersen, L. Bergström, Micropor. Mesopor. Mater. 72 (2004) 175. [18] S.H. Ye, S.-G. Kim, Korean J. Chem. Eng. 27 (2010) 1316.

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