Controllable Synthesis, Structure, and Catalytic Activity of Highly

Article pubs.acs.org/IECR

Controllable Synthesis, Structure, and Catalytic Activity of Highly Dispersed Pd Catalyst Supported on Whisker-Modified Spherical Alumina Yang Li, Junting Feng, Yufei He, David G. Evans, and Dianqing Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: A new route was introduced to synthesize novel mesoporous spherical alumina supports by in situ growth of alumina whiskers on the surface, and in the pores, of conventional spherical alumina using urea as an OH− donor and a surfactant as a structure-directing agent. BET results indicate that the modified spherical alumina exhibited much higher surface area and more regular mesoporous structure than the unmodified alumina. The whisker-modified spherical aluminas with a flowerlike arrangement of whiskers and higher surface area were then used as support to prepare highly dispersed Pd catalysts. Catalytic performances of the catalysts were studied in the catalytic hydrogenation/oxidation of 2-ethylanthraquinone (EAQ). Compared with a conventional Pd/alumina catalyst, novel Pd/whisker-modified alumina catalysts exhibited much higher Pd dispersion which resulted in more catalytically active sites and therefore significantly higher hydrogenation efficiency. Moreover, shorter diffusion distance reduced the deep hydrogenation of EAQ and consequently achieved higher selectivity and structural stability.

1. INTRODUCTION Hydrogen peroxide (H2O2) is being increasingly used in many green chemical processes because of its environmentally friendly properties.1,2 Catalytic hydrogenation/oxidation of 2ethylanthraquinone (EAQ) in the liquid phase is the most popular route for industrial scale production of H2O2.3−5 In this well-established method, EAQ is catalytically hydrogenated to 2-ethylanthrahydroquinone (EAQH2). Oxidation of EAQH2 results in the formation of H2O2 and regenerates the starting compound, EAQ. H2O2 is then extracted with water to obtain an aqueous solution of H2O2.6,7 However EAQH2, the primary product of EAQ reduction, is partly hydrogenated to various products, among which only 2-ethyltetrahydroanthrahydroquinone (H4EAQH2) is desired because its oxidation leads to H2O2 formation. EAQ and H4EAQ are therefore termed “active quinones”, while all the other products (2-ethylanthrone, 2ethyloxoanthrone, 2-ethylanthracene, and dimers) which are not capable of H2O2 formation are termed “degradation products”.8−10 Palladium-based catalysts have been extensively employed in the anthraquinone method of the H2O2 production process by virtue of their high activity, their easy separation, and the mild reaction conditions required.11,12 There are several factors which can affect the catalytic performance of supported Pd catalysts, such as nature of the support, metal dispersion, catalyst shape, and the metal−support interaction.13 Alumina is a well-known catalyst support, and compared with alumina particles of irregular shape, spherical alumina has the advantage that abrasion of the particles is lower as a result of their uniform size and smooth surfaces.14 When used in fixed beds, spherical alumina facilitates uniform packing of the bed, whereby variations in the pressure drop through the bed are minimized and channeling of the feed stream of reaction components is substantially reduced. Although spherical δ-aluminas are commonly used as catalyst supports in the hydrogenation/ oxidation of EAQ, they suffer from a broad pore size © 2012 American Chemical Society

distribution and low surface area, which results in a low metal dispersion and reduced catalytic activity and selectivity. Therefore, it is desirable to produce spherical mesoporous δalumina with uniform pore size and high surface area, especially external surface area, in order to maximize the dispersion of the active sites, shorten the diffusion path of reactants, and consequently enhance the catalytic performance. Many synthesis routes have been developed for the preparation of mesoporous alumina nanoparticles.15−19 Among them, organic−inorganic assemblies involving sol−gel processes using surfactants as structure-directing agents are regarded as one of the most promising approaches.20,21 For example, Pinnavaia and co-workers have reported the preparation of mesoporous alumina nanoparticles by condensation of aluminum alkoxide in the presence of neutral poly(ethylene oxide) surfactants.20 Yan and co-workers synthesized a series of highly ordered mesoporous aluminas with a two-dimensional (2-D) hexagonal structure through a sol−gel route with block copolymers as the soft templates.22 Yada et al. have reported the morphologically controlled synthesis of alumina mesostructures by a sodium dodecyl sulfate (SDS) templating route coupled with the homogeneous precipitation method using urea.23 Zhu and co-workers synthesized alumina nanofibers in the presence of poly(ethylene oxide) surfactant as a template.24 Bai and co-workers reported the synthesis of mesoporous aluminas with a uniform fibrous morphology by using the copolymer-controlled homogeneous precipitation method under hydrothermal conditions.25 However, fewer studies for synthesizing spherical alumina support with mesoporous structure have been reported. Received: Revised: Accepted: Published: 11083

February 13, 2012 July 12, 2012 July 19, 2012 July 19, 2012 dx.doi.org/10.1021/ie300385h | Ind. Eng. Chem. Res. 2012, 51, 11083−11090

Industrial & Engineering Chemistry Research

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heated in air with a ramping rate of 10 °C·min−1 to 450 °C and calcined at that temperature for 4 h, followed by slow cooling to room temperature to give the catalyst precursors. 2.4. Analysis and Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-600 X-ray powder diffractometer (Cu Kα radiation, λ = 0.154 06 nm) between 3 and 70° 2θ, with a scan speed of 10 deg·min−1. Low temperature N2 adsorption−desorption experiments were carried out using a Quantachrome Autosorb-1 system. The Barrett−Joyner−Halenda (BJH) method was used to calculate the pore volume and the pore size distribution. Elemental analysis was performed using a Shimadzu ICPS75000 inductively coupled plasma emission spectrometer (ICPES) and an Elementar Vario EL elemental analyzer. The morphology of the samples was examined using a Hitachi S4700 scanning electron microscope (SEM). The morphology and grain size of the samples were examined using a JEOL J2100 high-resolution transmission electron microscope (HRTEM). Temperature programmed reduction (TPR) of the catalysts was conducted on a Tianjin XQ TP-5000 chemisorption instrument with a thermal conductivity detector (TCD). About 100 mg of catalyst was loaded in a quartz reactor and heated at 400 °C in argon with a temperature ramp of 10 °C·min−1. TPR was carried out with a heating ramp rate of 5 °C·min−1 in a stream of 10% H2 in Ar, with a flow rate of 40 mL·min−1. The outlet gas was passed through a cold trap to remove the moisture produced during reduction. H2−O2 titration was carried out at 120 °C in a stream of N2. Before titration, the sample was saturated with hydrogen and then purged by flushing argon for 30 min. The same flushing under argon was performed between each titration cycle. After pretreatment, pulses of oxygen were introduced until full saturation of the catalyst was achieved. The chemisorbed oxygen was then titrated by hydrogen. Afterward a second oxygen titration was carried to confirm the volume of H2. The dispersion of Pd on the catalysts was calculated from the volume of H2 used for the titration of O2 by the following simplified equation:

In this work, we describe a new route to synthesize novel spherical mesoporous alumina by an in situ growth method. One-dimensional (1-D) mesoporous alumina whiskers were in situ constructed on the surface, and in the pores, of spherical γaluminas prepared by the oil-column method to eliminate macropores and enhance the surface area using a surfactanttemplating strategy. After calcination at 960 °C, whiskermodified spherical mesoporous δ-alumina was obtained. Highly dispersed Pd catalysts were then obtained by a conventional impregnation method, followed by calcination and reduction. The structure and properties of the Pd catalysts supported on the modified alumina, including their catalytic performance in the catalytic hydrogenation/oxidation of EAQ, have also been investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Urea, cetytrimethylammonium bromide (CTAB), poly(ethylene glycol) (PEG) with molecular weight of 17 000, hydrochloric acid, PdCl2, NaCl, and trimethylbenzene, were all AR grade and were used without further purification. The deionized water used in all experiments had a conductivity of less than 10−6 S·cm−1. EAQ and trioctyl phosphate (TOP) were supplied by Shandong Gaomi Taihong Chemical Co. Ltd. 2.2. In Situ Synthesis of Alumina Whiskers on Spherical γ-Al2O3 Particles. Spherical γ-Al2O3 particles with an average diameter of 2.5 mm were synthesized by the oilcolumn method according to our previous report.26 In a typical synthesis, 16.80 g of urea and 2.23 g of PEG were dissolved in 80.0 mL of deionized water, and the resulting solution was then added to an autoclave with 10.00 g of spherical γ-Al2O3 particles and aged at 120 °C for 24 h. The particles were then thoroughly washed with deionized water until the pH value of the washings reached 7. Whisker-modified spherical δAl2O3 was obtained after drying and calcination at 960 °C for 4 h. The molar ratio of the synthesis precursors was optimized by changing the amount of one component while maintaining the others constant as detailed in Table 1. Samples obtained by using PEG and CTAB as templating agents are denoted PEGand CTAB-whisker-modified spherical Al2O3, respectively.

D (%) =

Table 1. Synthesis Conditions for Different WhiskerModified Spherical Al2O3 Samples whisker-modified spherical alumina

CTAB (g)

PEG (g)

urea (g)

PEG-1 PEG-2 PEG-3 PEG-4 CTAB-1 CTAB-2 CTAB-3

− − − − 1.28 2.56 5.12

2.23 2.23 3.35 4.46 − − −

11.2 16.8 16.8 16.8 16.8 16.8 16.8

2VT HM(10−3) ·100 (3)(22.4)WP

(1)

where D (%) = dispersion of Pd, VTH = volume of H2 used for the titration of O2 (mL), M = relative molecular mass of Pd, W = mass of catalyst (g), and P = Pd mass fraction of the catalyst (wt %). A high performance liquid chromatograph (HPLC) equipped with a C18 separation column and UV200 detector was used to analyze the concentrations of EAQ and H4EAQ. The mobile phase was a mixture of methanol and water with a volume ratio of 70:30. The ultraviolet absorbance was 240 nm. 2.5. Catalytic Performance Tests. The hydrogenation experiments were carried out in a glass fixed-bed reactor. A 400 mL volume of TOP and 600 mL of trimethylbenzene were mixed together to obtain a mixed solvent. The working solution was then prepared by dissolving 100.0 g of solid EAQ in 1 L of the obtained mixed solvent. Then 50 mL of this working solution was added to the reactor with 10.0 g of catalyst at 50 °C under hydrogen. After 30 min, 5 mL of catalyst-free hydrogenation products was oxidized with O2 at room temperature. H2O2 was then extracted with deionized water to obtain a solution of H2O2. The content of H2O2 was

2.3. Preparation of Pd Catalysts. Pd catalysts were prepared by the impregnation method. The metal loading was 0.3 wt % in each case. A 0.0500 g sample of PdCl2 and 0.0360 g of NaCl were dissolved in deionized water to make a Na2PdCl4 solution (0.014 M) with a pH value of 3.3−3.7. Then 10.00 g of alumina support was added to 20.0 mL of the Na2PdCl4 solution. After impregnating for 2 h, the particles were washed with deionized water until there were no Cl− ions in the washings. After being dried for 5 h at 120 °C, the particles were 11084

dx.doi.org/10.1021/ie300385h | Ind. Eng. Chem. Res. 2012, 51, 11083−11090


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Figure 1. SEM images of (a) spherical alumina and (b) PEG-2-, (c) PEG-3-, and (d) PEG-4-whisker-modified alumina samples calcined at 960 °C. (e, f) Magnified images of PEG-4 whiskers.

spherical Al2O3. The length of PEG-2 whiskers (Figure 1b) was about 400 nm. With increasing PEG concentration, bundles of flaky alumina whiskers were observed (Figure 1c) and the quantity of whiskers increased. On further increasing the PEG concentration, rodlike whiskers were obtained and most of the whisker rods were packed into a flowerlike arrangement. The magnified SEM images of PEG-4 whiskers depicted in Figure 1e,f show that the PEG whiskers are apparently formed from thin strips through a layer-by-layer packing and that all the ends of whiskers are slightly shrunken. In order to further investigate the effect of the surfactant on the morphology of the whiskers, whisker-modified alumina samples were prepared using different concentrations of CTAB (see Table 1) and the SEM images are shown in Figure 2. Figure 2a shows that CTAB-1 whiskers formed with the lowest CTAB concentration also exhibited a regular needlelike morphology. The length of CTAB-1 whiskers was approximately 350 nm. With increasing CTAB concentration, the morphology of the whiskers transformed into flakes formed by layer-by-layer self-assembly and some flakes were packed into a flowerlike arrangement. On further increasing the CTAB concentration, the whiskers still displayed a flowerlike morphology. These flowerlike alumina materials are composed of luxuriant rodlike whiskers. The magnified SEM image depicted in Figure 2d shows that the tops of the CTAB-3 whiskers had a square shape with a length of about 100 nm. The above results indicate that the surfactant plays a key role in determining the quantity and morphology of whiskers by directing the growth of the aluminum species along a specific crystallographic direction.

analyzed by titration with KMnO4 solution. Before the titration, 10 mL of 10 wt % sulfuric acid solution was added to the H2O2 solution. The concentrations of active quinones (EAQ and H4EAQ) were determined by HPLC. The hydrogenation efficiency and selectivity toward active quinones are expressed by the following simplified equations:12 B=

5 CV0M 2 V

S=

n(EAQ) + n(H4EAQ) ·100% n0(EAQ)

(2)

(3) −1

where B = hydrogenation efficiency (g·L ), C = KMnO4 solution concentration (mol·L−1), V0 = volume of KMnO4 solution (mL), V = H2O2 solution volume (mL), M = molar mass of H2O2 (g·mol−1), and S = selectivity toward active quinones. n0 and n represent the number of moles of the composition in the initial working solution and in the solution after a cycle process, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphology of Whisker-Modified Spherical δAl2O3. Alumina whiskers were prepared in situ through a hydrothermal reaction using spherical alumina as the sole source of Al3+ and urea as the pH adjusting regent. The surfactants used in the synthesis were CTAB or PEG. SEM images of as-synthesized spherical alumina and PEG-whiskermodified alumina with different PEG concentrations (see Table 1) calcined at 960 °C are shown in Figure 1. Comparison of parts a and b of Figure 1 shows that alumina whiskers with a needle structure grew homogenously on the surface of the 11085



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In situ growth of the alumina whiskers on spherical alumina requires two conditions: the presence of a surfactant and an appropriate pH which is controlled by urea. In the homogeneous precipitation method, urea hydrolyzes slowly to release ammonia, thus acting as an OH− donor.27−29 In a solution with an appropriate pH, some of the spherical γ-Al2O3 dissolved followed by the adsorption of surfactants on the surface of spherical γ-Al2O3. In the alkaline solution, the dissociated Al3+ cations then formed alumina whiskers on the surface and in the pores of spherical γ-Al2O3 with the growth directed by the adsorbed surfactants. The amount of urea in the hydrothermal system determines the system pH and the amount of Al3+, thus controlling the morphology and amount of alumina whiskers formed. A proposed mechanism for the in situ formation of alumina whiskers on the surface of spherical alumina is represented in more detail in Scheme 1. Initially, alumina crystallites on the surface of spherical alumina were actived by OH− derived from the decomposition of urea. Activated Al3+ reacted with NH3 and CO2 in aqueous solution to form NH4[Al(OOH)HCO3] (AACH) nanocrystals. The resulting AACH nanocrystals were then adsorbed onto the surfactant micelle. The presence of hydrogen bonding reduces the free energy of the AACH crystallites, which allows the AACH crystallites to grow into AACH/surfactant nanowhiskers.25 The resulting AACH/ surfactant nanowhiskers then aggregate into AACH/surfactant microwhiskers through a layer-by-layer self-assembly mechanism. After further heat treatment, AACH precursor microwhiskers were transformed into alumina microwhiskers. 3.2. Structure and Porosity of Whisker-Modified Spherical δ-Al2O3. The XRD patterns of the powder scraped from the surface of CTAB-3- and PEG-4-modified alumina dried at 100 °C are shown in Figure 4a. Two patterns of the powder scraped from the surface of uncalcined samples can be indexed to crystalline AACH (JCPDS Card No. 42-0250),30,31 demonstrating the formation of AACH on the surface of spherical alumina. The XRD patterns of spherical alumina, CTAB-3-whisker-modified alumina, and PEG-4-whisker-modified alumina calcined at 960 °C are presented in Figure 4b. The patterns of spherical Al2O3 and whisker-modified Al2O3 calcination at 960 °C are roughly the same and are similar to those reported in the literature for δ-Al2O3,32 suggesting that the in situ growth of whiskers has little impact on the structure

Figure 2. SEM images of (a) CTAB-1-, (b) CTAB-2-, and (c) CTAB3-whisker-modified alumina samples calcined at 960 °C. (d) Magnified image of CTAB-3 whiskers.

The effect of varing the molar ratio of urea to Al2O3 on the morphology of the whisker-modified alumina samples was also studied, and the results are presented in Figure 3. It can be seen

Figure 3. SEM images of calcined (a) PEG-1- and (b) PEG-2-whiskermodified aluminas synthesized with different urea concentrations.

that the quantity and length of PEG-modified whiskers on the surface of spherical alumina gradually increased when the amount of urea was increased. For PEG-1 whiskers, a large portion of the whiskers had a length of less than 300 nm with the smallest being around 100 nm (see Figure 3a). For sample PEG-2, the whiskers were much larger with a typical length of about 400 nm. The above results indicate the importance of urea in controlling the amount and dimensions of the whiskers.

Scheme 1. Proposed Mechanism of the in Situ Formation of Alumina Whiskers on the Surface of Spherical Alumina

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Table 2. Textural Properties of the Samples sample spherical Al2O3 CTAB-2-whiskermodified Al2O3 CTAB-3-whiskermodified Al2O3 PEG-3-whisker-modified Al2O3 PEG-4-whisker-modified Al2O3

sp surf. area (m2·g−1)

av pore size (nm)

total pore vol (cm3·g−1)

92 107

23.4 15.7

0.72 0.76

120

15.2

0.80

114

19.2

0.78

127

18.5

0.81

the in situ formation of whiskers with regular size and welldeveloped pores on the surfaces and in the pores of spherical alumina. However, the pore size distributions are much narrower and the average pore sizes are smaller, with the reason being that the presence of whiskers inhibits the formation of macropores in spherical alumina during the subsequent heating process. As the quantity of alumina whiskers increased, the specific surface areas and total pore volumes of the resulting whisker-modified alumina materials (CTAB-3 and PEG-4) were further enhanced. CTAB-3- and PEG-4-whisker-modified aluminas were therefore selected as the supports for the synthesis of Pd catalysts in the subsequent work. 3.3. TPR and H2−O2 Titration of Pd Catalysts. TPR profiles obtained for the Pd-impregnated catalysts are shown in Figure 6. The peak below 100 °C in each curve can be

Figure 4. XRD patterns of (a) the powder scraped from the surface of modified alumina dried at 100 °C and (b) spherical Al2O3 and whisker-modified Al2O3 calcined at 960 °C.

of the calcined spherical alumina. The low intensity of the XRD peaks indicates that the alumina samples calcined at this temperature have low crystallinity. The N2 adsorption−desorption isotherms of spherical Al2O3, CTAB-whisker-modified Al2O3, and PEG-whisker-modified Al2O3 calcined at 960 °C were all of type II with an obvious hysteresis loop.33 The shape of the hysteresis loop in each case was a superposition of types H1 and H3. This is generally taken to indicate that samples have both tubular and parallel slitshaped capillary pores which are caused by the gas escaping during calcination and the stacking of alumina microcrystallites.34 The corresponding pore size distributions are shown in Figure 5, and the textural properties are listed in Table 2. The specific surface areas and total pore volumes of CTAB-2- and PEG-3-whisker-modified aluminas were higher than those of spherical Al2O3 itself; this may be attributed to

Figure 6. TPR profiles of PdO/Al2O3, PdO/CTAB-3-whiskermodified Al2O3, and PdO/PEG-4-whisker-modified Al2O3.

attributed to the reduction of PdO microcrystallites.35,36 The peak areas of PdO supported on whisker-modified Al2O3 supports are higher than that of PdO supported on unmodified alumina, which could be due to the larger crystallite size in the latter leading to incomplete reduction of PdO particles. H2−O2 titrations were performed on the Pd catalysts, since this technique is a good indication of the degree of metal dispersion. The corresponding results, along with the Pd loading as determined by ICP, are summarized in Table 3. The extents of metal dispersion in the Pd/whisker-modified Al2O3 catalysts were much higher than that in the Pd/Al2O3 catalyst, although the Pd loadings are essentially identical. These results confirm that in situ growth of alumina whiskers on the surface and in the pores of alumina support can significantly improve the distribution of Pd particles in Pd catalysts since the growth

Figure 5. Plots of pore size distribution of (a) spherical Al2O3 and (b) CTAB-2-, (c) CTAB-3-, (d) PEG-3-, and (e) PEG-4-whisker-modified Al2O3. 11087



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Pd.37 This gives direct evidence for the presence of highly dispersed Pd nanoparticles with small particle size in the Pd/ whisker-modified alumina catalysts. 3.5. Catalytic Performance of the Pd Catalysts. In order to test the catalytic performance, the prepared catalysts were packed in a fixed-bed reactor for the hydrogenation of EAQ. The hydrogenation efficiency of Pd catalysts with similar Pd loadings at different reaction times is shown in Figure 8A. Each catalyst exhibited low initial hydrogenation efficiency because the reduction of PdO crystallites on the catalyst is required at the beginning of the reaction. In comparison with the Pd catalyst prepared by impregnation of the unmodified alumina support (Figure 8A(a)), the catalysts supported on whiskermodified Al2O3 (Figure 8A(b,c)) exhibit distinctly higher efficiencies for the hydrogenation of EAQ. This can be attributed to the in situ growth of whiskers, which leads to an increase in surface area of the alumina spheres and thus a decrease in Pd particle size, resulting in the formation of more active centers in the Pd/whisker-modified Al2O3. With the extension of reaction time, a slight decrease in hydrogenation efficiency is observed over the catalysts supported on whiskermodified Al2O3, which can be ascribed to the accumulation of the degradation products.38 However, in case of the Pd catalyst supported on the unmodified alumina support, an obvious deactivation was presented in Figure 8A(a), which is attributed to the agglomeration of Pd particles, resulting in a decrease in Pd dispersion expect for the formation of the degradation products. In the anthraquinone method of the H2O2 production process, hydrogenation efficiency of a catalyst is not the only important parameter. The problem of EAQ degradation is technologically important since the degradation products accumulated in the solution and consequently a regeneration step could lead to the loss of active quinones. Thus, selectivity toward active quinones of the obtained Pd catalysts is shown in Figure 8B. Each catalyst exhibited high initial selectivity due to the incomplete hydrogenation of EAQ caused by the reduction of PdO particles at the beginning of the reaction. It is worth noting that the Pd/whisker-modified Al2O3 catalysts displayed much higher selectivity toward active quinones than the conventional Pd/alumina catalyst, indicating that fewer degradation products were formed in the cyclic hydrogenation/oxidation process. Higher selectivity of the Pd/ whisker-modified Al2O3 catalysts can be ascribed to the in situ growth of alumina whiskers on the surface of pristine alumina support, which significantly enhances the external surface area of support and accessibility of active sites, while shortening the diffusion distance and thus reducing the deep hydrogenation of EAQ.39 With the reaction time increasing, no obvious change of the selectivity can be observed over the Pd/ whisker-modified Al2O3 catalysts. The selectivities toward active quinones of Pd/CTAB-3-whisker-modified Al2O3 and Pd/ PEG-4-whisker-modified Al2O3 catalysts remained 95.0 and 96.5%, respectively, after 48 h. However, a slight decrease in selectivity toward active quinones is observed over the conventional Pd/alumina catalyst, indicating that the Pd/ whisker-modified Al2O3 catalysts possess preferable structural stability compared with the conventional Pd/alumina catalyst. Figure 9 shows representative HRTEM images of the Pd/ Al2O3 catalyst and Pd/whisker-modified Al2O3 catalysts after 48 h reaction. The Pd particle sizes of the used catalyst are listed in Table 3. The mean size of Pd particles in the used Pd/Al2O3 catalyst was larger than that in the fresh catalyst. However, the

Table 3. Properties of Pd Catalysts catalyst Pd/Al2O3 Pd/CTAB-3whiskermodified Al2O3 Pd/PEG-4whiskermodified Al2O3

Pd loadinga (wt %)

Pd dispersionb (%)

mean size of fresh Pd particlesc (nm)

mean size of used Pd particlesc (nm)

0.27 0.28

26.4 33.6

8 5

12 7

0.28

35.8

4

6

Determined by ICP analysis. bDetermined by H2−O2 titration analysis. cDetermined by HRTEM.

a

of whiskers can prevent the migration and aggregation of Pd particles. 3.4. Pd Particle Size in the Catalysts. The HRTEM images of the Pd/Al2O3 catalyst and Pd/whisker-modified Al2O3 catalysts (Figure 7) clearly reveal that the supported Pd

Figure 7. HRTEM images of (a) Pd particles in Pd/Al2O3 catalyst, (b) Pd/CTAB-whisker-modified Al2O3 catalyst, and (c) Pd/PEG-whiskermodified Al2O3 catalyst.

nanoparticles have significantly different particle sizes on the different supports. Pd/CTAB- and PEG-whisker-modified alumina have smaller mean Pd particle sizes than the Pd/ Al2O3 catalyst. The values of Pd particle sizes are listed in Table 3. The Pd particles in Figure 7 exhibit a lattice fringe at 0.23 nm, which is ascribed to the (111) facets of face-centered-cubic 11088



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Figure 8. (A) Catalytic activity and (B) selectivity of (a) Pd/Al2O3, (b) Pd/CTAB-3-whisker-modified Al2O3, and (c) Pd/PEG-4-whisker-modified Al2O3.

attributed to the larger surface area of the whisker-modified alumina which results in the uniform distribution of Pd particles and the availability of more active catalytic sites. Higher selectivity toward active quinones can be ascribed to a shorter diffusion distance over Pd/whisker-modified alumina catalysts, which reduced the deep hydrogenation of EAQ. The preferable catalytic performance of the Pd/modified alumina catalysts indicates that whisker-modified spherical alumina has a strong application potentiality of being a catalyst support. This work is believed to be of great significance for the design and synthesis of novel metal catalysts in the field of heterogeneous catalysis.

Figure 9. HRTEM images of Pd particles in the (a) used Pd/Al2O3 catalyst, (b) used Pd/CTAB-whisker-modified Al2O3 catalyst, and (c) used Pd/PEG-whisker-modified Al2O3 catalyst.

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Pd particles in Pd/whisker-modified Al2O3 catalysts remained highly dispersed with an average particle size of about 6 nm after reaction, comparable to those in the unused catalyst. This suggests that the presence of alumina whiskers can prevent the aggregation of Pd particles during the calcination and reduction processes. The observed high catalytic activities and stabilities of Pd/whisker-modified Al2O3 samples are consistent with a high and stable dispersion of Pd particles with small nanoparticle size on the modified supports.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 10 64425385. Tel.: +86 10 64436992. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the 973 Project (2011CBA00506), the Support Plan Project (2012BAE06B08), and the Doctoral Program of the Ministry of Education.

4. CONCLUSIONS One-dimensional alumina nanowhiskers have been synthesized in situ on the surface and in the pores of spherical alumina using urea as an OH− donor by a surfactant-templating strategy. Due to the in situ growth of whiskers, the modified spherical alumina exhibited a much higher surface area and more regular mesoporous structure than unmodified alumina. SEM images showed that the resulting alumina whiskers were highly dispersed on the surface of spherical alumina and the amount of surfactant and urea played a key role in determining the quantity and morphology of the whiskers. A proposed mechanism for the formation of alumina whiskers on the surface of spherical alumina was presented. The modified spherical aluminas with flowerlike arrangements of whiskers and higher surface areas were then used as supports to prepare highly dispersed Pd catalysts using a conventional impregnation method. The performances of the resulting Pd catalysts were studied in the hydrogenation/ oxidation of EAQ. Both Pd/PEG-whisker-modified-Al2O3 and Pd/CTAB-whisker-modified-Al2O3 catalysts exhibited higher hydrogenation efficiency, selectivity toward active quinones, and structural stability than the unmodified Pd/spherical Al2O3 catalyst. The enhanced hydrogenation efficiency can be

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REFERENCES

(1) Lunsford, J. H. The direct formation of H2O2 from H2 and O2 over palladium catalysts. J. Catal. 2003, 216, 455. (2) Samanta, C.; Choudhary, V. R. Direct synthesis of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in an aqueous acidic medium over halide-modified Pd/Al2O3 catalysts. Appl. Catal., A: Gen. 2007, 330, 23. (3) Choudhary, V. R.; Samanta, C. Role of chloride or bromide anions and protons for promoting the selective oxidation of H2 by O2 to H2O2 over supported Pd catalysts in an aqueous medium. J. Catal. 2006, 238, 28. (4) Fang, J.; Chen, X. Y.; Liu, B. S.; Yan, R.; Qiao, M. H.; Li, H. X.; He, H. Y.; Fan, K. N. Liquid-phase chemoselective hydrogenation of 2ethylanthraquinone over chromium-modified nanosized amorphous Ni-B catalysts. J. Catal. 2005, 229, 97. (5) Zhang, J. G.; Li, D. F.; Zhao, Y. J.; Kong, Q. D.; Wang, S. D. A Pd/Al2O3/cordierite monolithic catalyst for hydrogenation of 2ethylanthraquinone. Catal. Commun. 2008, 9, 2565. (6) Halder, R.; Lawal, A. Experimental studies on hydrogenation of anthraquinone derivative in a microreactor. Catal. Today 2007, 125, 48. 11089



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(7) Samanta, C. Direct synthesis of hydrogen peroxide from hydrogen and oxygen: An overview of recent developments in the process. Appl. Catal., A: Gen. 2008, 350, 133. (8) Drelinkiewicz, A.; Laitinen, R.; Kangas, R.; Pursiainen, J. 2Ethylanthraquinone hydrogenation on Pd/Al2O3. The effect of water and NaOH on the degradation process. Appl. Catal., A: Gen. 2005, 284, 59. (9) Drelinkiewicz, A.; Waksmundzka-Góra, A.; Sobczak, J. W.; Stejskal, J. Hydrogenation of 2-ethyl-9,10-anthraquinone on Pdpolyaniline(SiO2) composite catalyst. The effect of humidity. Appl. Catal., A: Gen. 2007, 333, 219. (10) Kosydar, R.; Drelinkiewicz, A.; Lalik, E.; Gurgul, J. The role of alkali modifiers (Li, Na, K, Cs) in activity of 2%Pd/Al2O3 catalysts for 2-ethyl-9,10-anthraquione hydrogenation. Appl. Catal., A: Gen. 2011, 402, 121. (11) Drelinkiewicz, A.; Pukkinen, A.; Kangas, R.; Laitinen, R. Hydrogenation of 2-ethylanthraquinone over Pd/SiO2 and Pd/Al2O3 in the fixed-bed reactor: The effect of the type of support. Catal. Lett. 2004, 94, 157−170. (12) Feng, J. T.; Wang, H. Y.; Evans, D. G.; Duan, X.; Li, D. Q. Catalytic hydrogenation of ethylanthraquinone over highly dispersed eggshell Pd/SiO2-Al2O3 spherical catalysts. Appl. Catal., A: Gen. 2010, 382, 240. (13) Augustyn, W. G.; McCrindle, R. I.; Coville, N. J. The selective hydrogenation of acetylene on palladium-carbon nanostructured catalysts. Appl. Catal., A: Gen. 2010, 388, 1. (14) Feng, J. T.; Lin, Y. J.; Li, F.; Evans, D. G.; Li, D. Q. Preparation, structure and properties of micro-spherical alumina with magnetic spinel ferrite cores. Appl. Catal., A: Gen. 2007, 329, 112. (15) Farag, H. K.; Endres, F. Studies on the synthesis of nanoalumina in air and water stable ionic liquids. J. Mater. Chem. 2008, 18, 442. (16) Li, Z. X.; Shi, F. B.; Li, L. L.; Zhang, T.; Yan, C. H. A facile route to ordered mesoporous-alumina-supported catalysts, and their catalytic activities for CO oxidation. Phys. Chem. Chem. Phys. 2011, 13, 2488. (17) Morris, S. M.; Fulvio, P. F.; Jaroniec, M. Ordered mesoporous alumina-supported metal oxides. J. Am. Chem. Soc. 2008, 130, 15210− 15216. (18) Proost, J.; Van Boxe, L. S. Large-scale synthesis of high-purity, one-dimensional α-Al2O3 structures. J. Mater. Chem. 2004, 14, 3058. (19) Pu, L.; Bao, X.; Zou, J.; Feng, D. Individual alumina nanotubes. Angew. Chem., Int. Ed. 2001, 40, 1490. (20) Zhang, Z.; Pinnavaia, T. J. Mesostructured γ-Al2O3 with a lathlike framework morphology. J. Am. Chem. Soc. 2002, 124, 12294. (21) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Self-adjusted synthesis of ordered stable mesoporous minerals by acid−base pairs. Nat. Mater. 2003, 2, 159. (22) Yuan, Q.; Yin, A. X.; Luo, C.; Sun, L. D.; Zhang, Y. W.; Duan, W. T.; Liu, H. C.; Yan, C. H. Facile synthesis for ordered mesoporous γ-aluminas with high thermal stability. J. Am. Chem. Soc. 2008, 130, 3465. (23) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Synthesis of aluminum-based surfactant mesophases morphologically controlled through a layer to hexagonal transition. Inorg. Chem. 1997, 36, 5565. (24) Zhu, H. Y.; Riches, J. D.; Barry, J. C. γ-Alumina nanofibers prepared from aluminum hydrate with poly(ethylene oxide) surfactant. Chem. Mater. 2002, 14, 2086. (25) Bai, P.; Su, F. B.; Wu, P. P.; Wang, L. K.; Lee, F. Y.; Lv, L.; Yan, Z. F.; Zhao, X. S. Copolymer-controlled homogeneous precipitation for the synthesis of porous microfibers of alumina. Langmuir 2007, 23, 4599. (26) Feng, J. T.; Lin, Y. J.; Evans, D. G.; Duan, X.; Li, D. Q. Enhanced metal dispersion and hydrodechlorination properties of a Ni/Al2O3 catalyst derived from layered double hydroxides. J. Catal. 2009, 266, 351. (27) Leggat, R. B.; Taylor, S. A.; Taylor, S. R. Adhesion of epoxy to hydrotalcite conversion coatings: I. Correlation with wettability and electrokinetic measurements. Colloids Surf., A 2002, 210, 69.

(28) Gao, Y. F.; Nagai, M.; Masuda, Y.; Sato, F.; Seo, W. S.; Koumoto, K. Surface precipitation of highly porous hydrotalcite-like film on Al from a zinc aqueous solution. Langmuir 2006, 22, 3521. (29) Chen, H. Y.; Zhang, F. Z.; Fu, S. S.; Duan, X. Angew. Chem., Int. Ed. 2008, 47, 2466−2469. (30) Zhu, Z. F.; Liu, H.; Sun, H. J.; Yang, D. Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum. Chem. Eng. J. 2009, 155, 925. (31) Ma, C.; Zhou, X.; Xu, X.; Zhu, T. Synthesis and thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). Mater. Chem. Phys. 2001, 72, 374. (32) Cesteros, Y.; Salagre, P.; Medina, F.; Sueiras, J. E. Effect of the alumina phase and its modification on Ni/Al2O3 catalysts for the hydrodechlorination of 1, 2, 4-trichlorobenzene. Appl. Catal., B: Environ. 1999, 22, 135. (33) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982; p 41. (34) Ma, X. Y.; Chai, Y. Y.; Evans, D. G.; Li, D. Q.; Feng, J. T. Preparation and selective acetylene hydrogenation catalytic properties of supported Pd catalyst by the in situ precipitation-reduction method. J. Phys. Chem. C 2011, 115, 8693. (35) Komhoma, S.; Mekasuwandumrong, O.; Praserthdama, P.; Panpranot, J. Improvement of Pd/Al2O3 catalyst performance in selective acetylene hydrogenation using mixed phases Al2O3 support. Catal. Commun. 2008, 10, 86. (36) Ahn, I. Y.; Kim, W. J.; Moon, S. H. Performance of La2O3- or Nb2O5-added Pd/SiO2 catalysts in acetylene hydrogenation. Appl. Catal., A: Gen. 2006, 308, 75. (37) Xiong, Y. J.; Cai, H. G.; Wiley, B. J.; Wang, J. G.; Kim, M. J.; Xia, Y. N. Synthesis and mechanistic study of palladium nanobars and nanorods. J. Am. Chem. Soc. 2007, 129, 3665. (38) Pierdomenico, B.; Federica, M.; Francesco, P.; Kari, E.; Paolo, C.; Tapio, O. S. Hydrogen Peroxide Direct Synthesis: Selectivity Enhancement in a Trickle Bed Reactor. Ind. Eng. Chem. Res. 2010, 49, 10627. (39) Drelinkiewicz, A.; Waksmundzka-Góra, A. Hydrogenation of 2ethyl-9,10-anthraquinone on Pd/SiO2 catalysts. The role of humidity in the transformation of hydroquinone form. J. Mol. Catal. A: Chem. 2006, 258, 1.

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