Nanotubular Gamma Alumina with High-Energy ... - ACS Publications

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Nanotubular gamma alumina with high energy external surfaces: synthesis and high performance for catalysis Weimeng Cai, Shengen Zhang, Jiangang Lv, Junchao Chen, Jie Yang, Yibo Wang, Xuefeng Guo, Luming Peng, Weiping Ding, Yi Chen, Yanhua Lei, Zhaoxu Chen, Weimin Yang, and Zaiku Xie ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Catalysis

Nanotubular gamma alumina with high energy external surfaces: synthesis and high performance for catalysis

Weimeng Cai†1, Shengen Zhang†1, Jiangang Lv1, Junchao Chen1, Jie Yang1, Yibo Wang1, Xuefeng Guo1, Luming Peng1, Weiping Ding*1, Yi Chen1, Yanhua Lei1,3, Zhaoxu Chen*1, Weimin Yang2, Zaiku Xie2 1

Lab of Mesoscopic Chemistry, Department of Chemistry, Nanjing University, Nanjing 210093,

China 2 3

Sinopec Shanghai Research Institute Petrolchemical Technology, Shanghai 201208, China Department of Physics, South University of Science and Technology of China, Shenzhen 518055,

China †

These authors contribute equally to this research.

*Correspondence author. E-mail: [email protected]; [email protected]

ABSTRACT: Inorganic nano crystals catalysts with high proportion of single and high energy surface can bring about high performance for catalysis and has been an important research topic in the past decades. Gamma alumina is one of the most important inorganic oxides used as solid acids or catalytic support for many more industrial catalysts. However, the preparation of gamma alumina mainly with high energy external surfaces has never been reported due to its complicated crystal structure. We demonstrate here in depth a new-type γ-alumina material from a systematic investigation, which is controllably synthesized as regular nanotubes with high energy {111} facets as main external surfaces. The new-type material shows much better performance as acid catalyst or catalytic support for metals, comparing with common γ-alumina whose main exposed surface is stable {100} or {110} facets in irregular morphology. As an example, palladium loaded on the new-type γ-alumina is easily prepared in higher dispersion and unique electronic states upon the stronger interaction with the support, giving rise to better catalytic performance for semi-hydrogenation of alkynes, without any assistance of other metals. The systematic investigation should open opportunities of catalyst innovation for new chemical reactions. Keywords：Crystal planes; Gamma alumina; Nanotubes; Palladium and support interaction; Solid acids; Hydrogenation; DFT calculations 1

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INTRODUCTION The investigation on catalysis of inorganic nano crystals has been attractive topics in the recent years.1-5 The unique shapes and regular external facets of nano crystals would cause them highly reactive and/or selective for certain catalytic reactions due to the match among the configurations of reactants and the facets of nanocrystals.6-8 In recent decades, there are many excellent reports on the crystal facet effect on catalysis about the catalytic nano materials of noble metals9,10 and metal oxides11-14. Gamma alumina is one of the most important catalytic materials and widely used as catalyst or catalyst support.15-18 Interestingly, despite its various applications, the precise bulk structure of γ-alumina has not been well recognized yet, because of its intricate crystal structure and some models have been suggested to represent bulk γ-alumina.19-25 Among these proposals, Digne et al.’s nonspinel structure model that contains 8 Al2O3 units is the most popular one currently and seems to adequately explain more facts about gamma alumina.26-28 Based on this structure model，the mainly exposed surfaces have been considered as {110} facets which also agree with the experimental results.29,30 Some other experimental results show that {100} surface with the lowest surface energy is the dominant plane.31 The {111} planes, with higher surface energy, however, are thermodynamically unstable, so it is difficult to synthesize γ-alumina mainly exposed their {111} surfaces. Based on theoretical calculation results and the use of correct reagent for protection of specific surface structure, the well-crystallized nanotubular γ-alumina is synthesized in current research, of which the exposed surfaces should be mainly {111} facets. To the best of our knowledge, such nanotubular gamma alumina with relatively explicit (111) surface planes has never been published in the literature. Our strategy includes the following points: 1) to find an adequate reagent for protection of {101} planes of boehmite (γ-AlOOH) by theoretical calculations; 2) to assemble very tiny particles of boehmite to nanotubes, still in boehmite structure, in presence of the protection reagent under hydrothermal conditions; 3) to convert the boehmite to γ-alumina at elevated temperatures by dehydration and the {101} planes of boehmite accordingly convert to {111} planes of γ-alumina; 4) to characterize the surface property of the novel γ-alumina by HRTEM, D/H exchange, NH3 temperature programmed desorption, solid state NMR and etc.; 5) to load palladium on the novel γ-alumina and to investigate the interactions between the palladium and 2


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the γ-alumina. The new-type material shows better performance as a high-efficiency catalyst and a catalytic support with unique properties for acid catalyzed and/or hydrogenation reactions. The systematic researches on the fundamentally important material should be very useful.

EXPERIMENTAL SECTION Preparation of samples γ-Al2O3: The typical procedure to synthesize nanotube alumina was as follows: 7.5g of oleic acid (Aldrich), 90.0 g boehmite colloidal particulates (obtained from Zhejiang Yuda Chemical Co., Ltd. China and prepared from the hydrolysis of isopropanol aluminum under acidic conditions) were dispersed in deionized water under magnetic stirring at 353 K for 30 min. Then an appropriate amount of ammonia aqueous solution was added into the mixture to adjust the pH value to ~8.5. After stirring for 8 h, the mixture was transferred into a teflon-lined stainless steel autoclave, heated to 453 K and aged for 72 h. After cooling to room temperature, the white solid precipitate was collected by centrifugation, rinsed with absolute ethanol for several times, and finally dried in air at 353 K followed by grinding. The precursor powder was then heated at 823 K for 8 h under air atmosphere, with a heating rate of 2 K·min-1. The final product was denoted as γ-Al2O3-nt. Traditional γ-Al2O3 with irregular morphology was purchased from Aluminum Corporation of China Limited and denoted as γ-Al2O3-c. An industry practical catalyst of gamma alumina was supplied by Sinopec, China and was named as γ-Al2O3-i. Pd/γ-Al2O3: Catalysts containing (0.05-1) wt% Pd/γ-Al2O3 were prepared by an incipient wetness impregnation method using Pd(NO3)2 (Aldrich) as a Pd precursor and γ-Al2O3-nt or γ-Al2O3-c as the support. The impregnated catalysts were dried at 353 K overnight, followed by calcined in 5% H2/N2 at 573 K for 2 h. Catalyst characterization XRD: The X-ray diffraction (XRD) patterns are collected in a Phillips X’Pro diffractometer using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 25 mA with 2θ values between 10 and 90°. Surface area: The specific surface area of the catalysts are measured on an ASAP 2020 apparatus at liquid nitrogen temperature, which are calculated according to the Brunauer–Emmett– Teller (BET) method based on the adsorption isotherm. The Barrett–Joyner–Halenda (BJH) 3


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method was used to calculate the pore volume and the pore size distribution. Zeta potential: The surface charge (zeta potential) of the boehmite colloidal particulates (suspension pH ~ 4) was determined by a zeta potentiometer (Zetasizer Nano ZS system, Malvern Instruments). The Nano ZS uses a 4 mW He-Ne laser operating at a wavelength of 633 nm. TEM: Transmission electron microscopy (TEM) measurements are taken on a JEOL JEM-200CX instrument at an accelerating voltage of 200 kV. NH3-TPD: The amount of NH3 chemisorbed on each catalyst was measured on a Micromeritics 2900 TPD/TPR analyser. Prior to the adsorption of the NH3, the catalysts (100 mg) are heated at 873 K in argon stream (20mL·min−1) for 2 h and cooling down in Ar to 303K. The catalysts are then saturated with a NH3/Ar (5% in Ar) stream (40 mL·min−1) at 303K for 1h. Subsequently, a pure Ar stream (20 mL· min−1) is passed at 373K for 1h in order to remove any physisorbed molecules. Once a stable line had been obtained, chemisorbed NH3 is desorbed by heating from 373K up to 857K at a rate of 10 K·min−1. The final temperature is kept for 20 min. H/D exchange: To detect the surface hydrogen qualitatively and quantitatively, the H/D exchange measurement was carried out. The as-prepared samples (100 mg) were pretreated under Ar (40 mL·min-1) at a heating rate of 10 K·min-1 to 873 K and kept at this temperature for 2 h. Then the sample is cooled under the same atmosphere to room temperature. Deuterium exchanged with protonium presented in the sample was measured by increasing the temperature to 873 K at a heating rate of 10 K·min-1. The signal of HD is monitored by mass spectrometry. XPS: The surface elemental analysis was performed using a PHI 5000 Versa Probe X-ray photoelectron spectroscopy (XPS) equipped with Mg Kα radiation. C1s peak at 284.6 eV was used as a calibration peak. In-situ CO-IR: In situ Fourier-transformed infrared absorption spectroscopy of CO experiments was carried out in a quartz cell equipped with CaF2 windows allowing sample activation and successive measurements in the range of 298-873 K. The catalysts were pressed into a disk located at the center of the cell and activated in the same cell used for the measurement. FT-IR spectra was collected with Bruker Tensor 27 infrared spectrophotometer at a spectra resolution of 4 cm-1 and accumulation of 64 scans. After H2 pre-treatment at 573 K for 2 h and He treatment at 303K for 1h, the catalyst is scanned to get a back ground record in 5 mL·min-1 He flow at 303K. And then, the catalyst was exposed to a CO flow at 303 K for 1h. IR spectra were 4


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recorded at 5 min intervals in 5 mL·min-1 He flow at 303 K, until there was no signal of gas phase CO while the cell was evacuated. Solid-State

31

P MAS NMR Experiments: Solid magic angle spinning (MAS) NMR

experiments were carried out using a Bruker Avance III 400 spectrometers equipped with 89 mm wide-bore 9.4 tesla superconducting magnets in 4 mm rotors at Larmor frequencies of 161.9 MHz. Single pulse 31P MAS NMR spectra were acquired at a spinning speed of 14 kHz. Considering the long relaxation time of 31P nuclei in NMR experiment, we used pulse with the width of 2.6s, 0.5s delay time. The 31P chemical shifts were reported relative to 85% aqueous solution of H3PO4, with NH4H2PO4 as a secondary standard (0.81 ppm). Before each

31

P MAS NMR experiment, the

γ-Al2O3 sample was placed in glass tubes and evacuated at 623 K under a pressure below 1 Pa for 3h on a vacuum line. 40 Torr of trimethylphosphine (TMP) was introduced into the activated catalysts and equilibrated for 1 h, then degassed for 1 h at room temperature. Finally, the samples were flame sealed. The sealed samples were transferred into a ZrO2 rotor (tightly sealed by a Kel-F cap) under a dry nitrogen atmosphere in a glove box. The data were processed with NUTs software. Catalytic Tests Ethanol dehydration: Catalytic experiments were carried out in a quartz fixed-bed reactor. 100 mg samples (40-60 mesh) were dispersed onto a quartz frit with an inner diameter of 3 mm. Ethanol was injection with a syringe at a constant rate using He as carrier gas. The reaction temperature was from 530 K to 700 K, and the reactant and product concentrations were determined by GC equipped with a HP-530 m × 0.32 mm column. Semi-hydrogenation of acetylene: The semi hydrogenation of acetylene in an ethylene-rich stream was investigated in a heated fixed-bed reactor under continuous flow conditions. The mixed gas was composed of 0.5% acetylene and 9.5% ethylene, with the balance being Ar. By modulating the ratio of H2 and the mixed gas, the total flow rate and the ratio of the reactant mixture, the H2/C2H2 ratio and gas hourly space velocity (GHSV) was varied. 0.1 g catalyst in size of 20-40 mesh was loaded in the reactor. Prior to the test, the samples were activated by hydrogen in a flow rate of 20 mL·min−1 at 573 K for 2 h. The gas stream at the reactor outlet was analyzed by online gas chromatography using 9560 gas chromatography. Before acquisition of the data 5


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(points related to the studied catalysts under different experimental conditions), at least five tests for point were executed in order to obtain reproducible values and carbon balance determined from the effluent gas was 100 ± 0.5%. Acetylene conversion and ethylene selectivity were defined as follows: Acetylene conversion% = Ethylene selectivity% =

C H inlet − C H outlet × 100% C H inlet

C H outlet − C H inlet × 100% C H inlet − C H outlet

Semi-hydrogenation of phenylacetylene: The semi hydrogenation of phenylacetylene was carried out in a bath-type autoclave reactor. In a typical reaction, 0.5 mL phenylacetylene and 49.5 mL phenylethylene were well mixed in the autoclave at room temperature. Then 100 mg of catalyst was added to the reaction mixture. The mixture was heated to reaction temperature under stirring. The reaction time was recorded as soon as the hydrogen was introduced into the reactor. A little of the reaction mixture was taken out from the system by a sampler in fixed interval of time. The mixture composition was analyzed by a GC.

THEORETICAL CALCULATIONS Method of calculation on adsorption of acetic acid on different surfaces of AlOOH All the DFT calculations were performed using the Vienna ab initio simulation package (VASP).32,33 The projector augmented wave (PAW)34,35 method was used to describe the ion-electron interactions. The Perdew, Burke, and Ernzerhof (PBE) functional36 of generalized gradient approximation (GGA) was employed to describe electron exchange and correlation. The Kohn-Sham equations were solved using a plane-wave basis set with a cutoff energy of 400 eV. Each atom was fully relaxed until the forces acting on each atom are smaller than 0.02 eV/Å. All systems were relaxed without symmetry or spin constraints. The surface energy for the slab in vacuum is obtained by the formula:

γ=

1 bulk ( E slab ( n) − nEAlOOH ) 2A

where A is the surface area of the unit cell, Eslab(n) is calculated total energy of the slab with n repeat units, and Ebulk is the bulk binding enrgy of one repeat unit. After adsorption of m acetic acid molecules on the slab, the surface energy is obtained by: 6


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γ=

1 bulk ( E slab ( n) − nEAlOOH − mE Ac ) 2A

Where EAc is the energy of one acetic acid molecule in vacuum. The bulk energy is defined as: bulk E AlOOH = E slab ( n ) − E slab ( n − 1)

At sufficiently large n, this value converges, apart from oscillations arising from quantum size effects (QSE), thus ensuring the convergence of surface energy.37, 38 The Monkhorst−Pack scheme K-point grid sampling is set as 7 × 3 × 7 for the bulk calculation. The optimized cell parameters are a = 2.86 Å, b = 11.90 Å and c = 3.70 Å. They are in good agreement with the experimental value: a = 2.88 Å, b = 12.24 Å and c = 3.71 Å.39 For the slab calculation, (1×1) unit cell is used for (001), (100) and (101) surface. (2×2) unit cell is used for (010) surface to accommodate the acetic acid molecule. The K-point grid sampling is (7×3×1) for (001), (3×5×1) for (010), (3×7×1) for (100) and (5×3×1) for (101) surface, respectively. Two Al and two oxygen atoms are exposed on (001), (100) and (101) and then two acetic acid molecules can be adsorbed on these surfaces. Four hydroxyl group are exposed on the (010) surface. After the adsorption of acetic acid, two hydroxyl group are displaced by acetic acid. Models of γ-Al2O3 and Computational Details for Pd4/γ-Al2O3 Despite its various applications, precise bulk structure of γ-alumina has not been obtained yet, because of its intricate crystal structure and many models have been suggested to represent bulk γ-alumina. Among these models, Digne’s non-spinel model that contains 8 Al2O3 units is the most popular one. Here we adopted Digne’s model24 to mimic γ-alumina. Our optimized volume value (46.27 Å3/γ-Al2O3 unit) agrees with the experimental (46.39 Å3/Al2O3 unit)40 better than the previous theoretical result (47.40 Å3/γ-Al2O3 unit)24. The optimized bulk parameters are: a = 5.542 Å, b = 8.346 Å, c = 8.003 Å and β = 90.59°. The γ-Al2O3 (110) and γ-Al2O3 (111) surfaces were constructed based on the above optimized bulk structure. Both the dehydrated and the hydrated surfaces with the OH density of 8.9 nm-2 for (110) and 6.7 nm-2 for (111) were built. The (110) surfaces were modeled using a p (2×2) supercell (a = 16.69 Å, b = 16.01 Å), the (111) surfaces using a p (2×1) supercell (a = 19.56 Å, b = 8.35 Å). For both the (110) and (111) surfaces, a five Al-O-layer slab was employed with the bottom three layers fixed during the geometry optimization. A 12 Å thick vacuum spacing was placed along the perpendicular direction to 7


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separate the periodic slabs. 2×2×1 and 2×4×1 k-point grids were used for the Brillouin zone integration for the (110) and (111) surfaces, respectively. The adsorption energy (Eads-Pd4) of Pd4 cluster on the surfaces and the adsorption energy (Eads-ene) of ethylene and acetylene were calculated with equation (1) and (2) respectively. Eads-Pd4= EPd4/slab – Eslab –EPd4

(1)

Eads-ene=Eene/ Pd4/slab –EPd4/slab – Eene

(2)

where EPd4/slab and Eslab refer to the DFT calculated total energies of the slab with and without Pd4 cluster adsorbed on it. EPd4 is the calculated energy of isolated Pd4 cluster. Eene/Pd4/slab denotes the total energy of ethylene or acetylene adsorbed slab, and Eene is the total energy of isolated ethylene or acetylene. All the periodic density-functional (DFT) calculations were performed with the Vienna ab initio simulation package (VASP) code,32,41,42 with the generalized gradient approximation of Perdew-Wang 9143,44 exchange-correlation functional to solve the Kohn-Sham equations. The interaction between valence electrons and the core was described by the projector augmented wave (PAW) method.45 The Monkhorst-Pack scheme was used for Brillouin zone sampling.46 A cutoff energy of 400 eV was adopted for all the calculations and a convergence criterion of 1.0×10-4 eV was set for the electronic self-consistent iterative criteria. Conjugate gradient scheme was used for geometry optimization, with a convergence condition of forces on each atom ≤ 0.02 eV·Å-1. Spin polarization was considered where necessary.

RESULTS AND DISCUSSION Synthesis of the nanotubular γ-alumina with high energy external surfaces The structural conversion of boehmite to γ-alumina has been thoroughly investigated in the past decades.47,48 Considering the Digne et al.’s model, the energies of typical planes of boehmite, i.e., (010), (001), (100) and (101), are calculated by DFT methods under various conditions and the results are listed in Table 1. The optimized structures after acetic acid adsorbed on different γ-AlOOH surfaces are summarized in Figure 1. In calculation, acetic acid is used as protection reagent. The results show clearly the protection of carboxylic acid adsorption on (101) plane of boehmite, by which the surface energy of (101) plane turns to the lowest one among the main four 8


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planes of low miller indexes. This means that the (101) plane of boehmite will gradually show up during the hydrothermal treatment of boehmite nanoparticles in assembly.

Figure 1. Optimized structures after acetic acid adsorbed on different γ-AlOOH surfaces. (a) 010 surface, (b) 001 surface, (c) 100 surface, (d) 101 surface. ΓAc means the surface energy upon adsorption of acetic acid. (Yellow sphere: Al, red sphere: O, white sphere: H, gray sphere: C.)

Table 1. Surface energy (Γ) of the γ-AlOOH surfaces in vacuum and after acetic acid adsorption Evac(a)

Area(b)

E Ac(c)

Γvac(d)

Ead(e)

ΓAc(f)

(eV)

(Å2)

(eV)

(meV/ Å2)

(eV)

(meV/ Å2)

010

-418.90

42.67

-605.39

23

-0.08

19

001

-415.64

35.25

-604.79

74

-0.75

31

100

-410.27

45.41

-605.00

116

-2.14

22

101

-409.20

57.41

-605.30

101

-2.49

15

Surface

* All the DFT calculations were performed using the Vienna ab initio simulation package (VASP) and the details of calculation can be found in experiment section. (a)

The energy of slab surface in vacuum condition; (b)Surface area; (c)The energy of slab surface after

acetic acid adsorption;

(d)

The surface energy in vacuum condition;

(e)

Adsorption energy of acetic

acid; (f)The surface energy after acetic acid adsorption. 9


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In our experimental preparation, the boehmite colloidal particulates (obtained from Zhejiang Yuda Chemical Co., Ltd, China) are prepared from the hydrolysis of isopropanol aluminum under controlled conditions, the suspension pH of this boehmite colloidal is about 4. The BET surface of the boehmite colloidal particulates (dried at 353 K) is ~183 m2/g. The XRD pattern of the dried

boehmite colloidal particulates (Figure S1) shows that all the diffraction peaks are indexed to γ-AlOOH (JCPDS, 21-1307). The TEM image (Figure 2a) shows that the boehmite colloidal particulates are consist of very tiny colloidal boehmite particles in the size of 2~3 nm (right inset of Figure 2a). These nanoparticles are found positively charged with very high Zeta potential (left inset of Figure 2a). By co-assembly with micelles of anionic surfactant (oleic acid, and ammonia or sodium hydroxide are used as base to adjust pH values of the solution) under hydrothermal conditions at 453 K for 72 h, nanotubular products are obtained with the average lengths and diameters of about 90.5 nm and 6.6 nm respectively (Figure 2b). As a result of DFT calculation, the adsorption of carboxyl acid offers large adsorption energy and stabilizes the (101) plane of boehmite. The oleic acid behaves as a bifunctional additive. The positively charged AlOOH particles assembled with the negatively charged micelles of oleic acid and, at the same time, some of the carboxyl head of oleic acid adsorbed on the AlOOH to protect the (101) planes stay behind the hydrothermal treatment. The inner diameters of the nanotubular products are mostly in several nanometers (Figure 2b). It appears that the alumina colloidal nanoparticles undergo a process of amalgamation within the nanotube walls, giving rise to compact structure during hydrothermal treatment. All the X-ray diffraction peaks of the sample (inset of Figure 2b) are indexed to

γ-AlOOH (JCPDS, 21-1307). By heating the sample at high temperatures, the γ-AlOOH transforms to γ-alumina, reserving the morphology of nanotubes through dehydration. The TEM image of a sample after calcination at 823 K (Figure 2c) show that the nanotubular morphology is remained and the average length and diameters of the as-prepared γ-alumina nanotube are about 68 nm and 6 nm respectively. The BET surface area of the as-prepared γ-alumina nanotube is ~140 m2/g. The XRD pattern of the calcined sample (inset of Figure 2c) shows that all the diffraction peaks are indexed to γ-alumina according to Digne et al.’s model.49,50 The highly magnified TEM image of a γ-alumina nanotube is displayed in Figure 2d. The wall of the nanotube is single crystal-like γ-alumina and the lattice fringes of planes with a d-spacing of 0.460 nm, corresponding to {110} planes of γ-alumina, are observed clearly. 10


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By the knowledge of γ-alumina crystal structure, the exposed lateral surface of the nanotube could parallel to {111} planes, in agreement with the results of the model analysis (Figures 3a,b). Figure 3a presents the unit cell of the γ-alumina bulk and the corresponding crystal facets according to Digne et al.’s model, the angle between the {111} planes at different two directions is 87.6°, and the angle between the {111} and {100} crystal planes is ~135°. The γ-alumina is an octagonal structure in air47 and the exposure ratio of {111} planes is enlarged due to the protection of oleic acid. On the basis of above theory and calculation results, a reasonable three-dimensional structure mode of our prepared alumina is constructed, with {111} crystal facets and {100} facets alternated to form an octagonal structure at cross section and the {111} crystal facts as main exposure planes (Figure 3b). The cutout picture of this three-dimensional structural model is also shown in Figure 3b, which reflects the structure given by the HRTEM image listed in Figure 2d, the exposed surface of the as-prepared nanotube could parallel to {111} planes. The synthesis process and the structural evolution of the alumina product are summarized in Figure 3c. The figures show the possible structure of the nanotube with {111} planes as main external surface based on the analysis of crystal structure of boehmite and γ-alumina, all the results appear coincident with each other.

Figure 2. Morphologies of the samples. a, The TEM image of the boehmite colloidal particulates, of which the Zeta potential measured by light scattering is shown as the left inset. The right inset shows a HRTEM image of the colloidal nanoparticles. b, The TEM image of the AlOOH nanotubes as synthesized at 453 K (Ammonia was used 11


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as base) and their corresponding XRD and electron diffraction patterns (inset). c, The TEM image of the γ-alumina nanotubes obtained by heating the AlOOH nanotubes at 823 K in air (Sodium hydroxide was used as base in assembly, which gave better morphology. Inset shows the corresponding XRD pattern.). d, The HRTEM images of a nanotubular γ-alumina (inset).

Figure 3. Illustration of the preparation of γ-Al2O3-nt. a, Unit cell of the gamma alumina bulk and corresponding crystal face. b, The corresponding three-dimensional structure model as well as cutout picture of this three-dimensional structural model. c, Schematic show of the process of synthesis and the structural evolution of the alumina during synthesis and heating.

Difference in surface properties between nanotubular γ-alumina and traditional γ-alumina The exposed surfaces of γ-alumina are commonly covered by hydroxyls, some of them functioned as Brönsted acid sites, which are extremely important to determine the surface property and the catalytic performance.51 According to the surface structure of alumina, there are many kinds of hydroxyls attached to differently coordinated aluminum ions on different surfaces, which determine the charge and bond strength of the OH groups. By and large, these OH groups can be assigned to two categories, i.e., acidic or nonacidic, according to their charge status. The more the net positive charge of the OH group, the weaker the O-H chemical bond is and the stronger the acidity of the OHδ+. The different hydroxyls would be detected by D/H exchange quantitatively. 52-54

The activation energy for the exchange reaction, i.e., D2 + OH OD + HD, could be

correlated to the O-H bond energy and the exchange is easier with a weaker O-H bond, as a result of DFT calculation,55 implying the stronger acidic OH groups exchanging with D2 at lower 12


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temperatures. Figure 4a shows the D/H exchange profiles of the new-type γ-alumina (γ-Al2O3-nt) and the D/H exchange results measured on the other two γ-alumina samples are also listed for comparison. One is traditional gamma alumina purchased from Aluminum Corporation of China Limited with irregular morphology and denoted as γ-Al2O3-c. The other is an industry practical catalyst from the Sinopec, China (γ-Al2O3-i). There are two HD evolution peaks observed for all the three samples. From the above analysis, the peak at the lower temperatures corresponds to the D2 exchange with the acidic hydroxyls and the peak at the higher temperatures is the D2 exchange with the nonacidic hydroxyls. The new-type γ-alumina possesses the largest number of acidic hydroxyls among the three samples, in accordance with the above-mentioned structure analysis that the main exposed surfaces of the γ-Al2O3-nt should be {111} planes. The hydroxyl densities of the three samples quantitatively measured by the D/H exchange are 0.81×1015, 0.49 × 1015 and 0.36 × 1015 cm-2 for γ-Al2O3-nt, γ-Al2O3-c and γ-Al2O3-i, respectively. NH3 temperature-programmed desorption (NH3-TPD) is employed to characterize the acidities of the samples and the results are depicted in Figure 4b. The γ-Al2O3-nt has the highest acid density and their acidity is also stronger than the other samples, considering the shoulder peak of ammonia desorption at ~650 K for γ-Al2O3-nt. This results verifies the exposed surfaces of γ-Al2O3-nt, stronger acidic, agrees the property of {111} planes. It should be pointed out that the acid sites measured by NH3-TPD include the contribution of Lewis acid sites. By the OH removal at elevated temperatures, the left low coordinated aluminum ions on the surface turn to be Lewis acid sites, which are also very important for catalysis. In this work, the adsorption of P(CH3)3 (trimethylphosphine, TMP) is also used to detect the acid sites, including the Brönsted and Lewis acid sites, which are left by OH removal. The

31

P solid state NMR signals are sensitive to their

chemical environment. Figure 4c shows the 31P solid state NMR spectra recorded on the γ-alumina samples dehydrated at 673 K under vacuum. Generally, there are two groups of NMR signals observed on the samples of γ-Al2O3-nt and γ-Al2O3-c, i.e., one around -5 ppm and the other around -50 ppm. The peaks at ~-5 ppm correspond to the TMP adsorbed on Brönsted acid sites and the peaks at ~-50 ppm correspond to Lewis acid sites. Due to the spectra are recorded on dehydrated samples, the peaks at ~-50 ppm are much stronger than those at ~-5 ppm, indicating much more Lewis acid sites than Brönsted acid sites. And, the γ-Al2O3-nt sample shows larger density of acid than the γ-Al2O3-c, in agreement with NH3-TPD results. Noteworthy that the subtle 13


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Page 14 of 26

differences in the spectra of the two samples just reflect the property of their external surfaces, as the assignment of NMR signals of TMP adsorbed on different surfaces of zinc oxide by Peng et al. 56

. Referenced to the results, schematic illustrations of the TMP adsorption on the two surfaces of

current alumina are summarized in Figure 4d. It is noted that the {110} plane is nonpolar, while {111} plane is a polar surface, according to the literature,56 the interaction between TMP and polar surface is different from nonpolar ones, thus leading to differences in 31P chemical shifts.

Figure 4. Characterization of the samples. a, The HD evolution profile during the D2/OH exchange from the γ-alumina nanotubes (γ-Al2O3-nt), the traditional γ-alumina (γ-Al2O3-c) and the industry catalyst γ-alumina (γ-Al2O3-i). The values in the parenthesis are OH density of the samples (in unit of 1015 cm-2), derived from the HD evolution. b, The profiles of NH3 desorption during the NH3-TPD process. c,

31

P MAS NMR spectra of

γ-Al2O3-nt and γ-Al2O3-c adsorbed with trimethylphosphine (TMP). d, Schematic illustrations of molecular interaction between TMP and various surface features: (111) Al2O3-nt and (110) Al2O3-c . Yellow sphere: Al, Red sphere: O, White sphere: H, Gray sphere: C, Pink sphere: P.

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ACS Catalysis

Interaction between Pd and γ-alumina surfaces

Besides the acidity, the special property of the new-type γ-alumina must result in novel interaction between its surface and loaded metals such as palladium and ultimately affect catalytic properties of the metal. The calculation results on the palladium cluster (Pd4) interaction of the (110) or (111) surfaces of γ-alumina are summarized in Figure 5 and the numerical results are listed in Table 2. Regarding Pd4 cluster adsorption, the calculated adsorption energy clearly demonstrates that the interaction between Pd4 cluster and (111) surface is stronger than those of the (110) surface. Hydroxylation of the surface reduces the cluster-surface interaction greatly, as shown by the decreased adsoprtion energy, but the (111) surface of γ-alumina is still much more attractive to Pd4 cluster. Thus one can expect that the (111) surface favors the dispersion of Pd metal catalyst. The experimental results of Pd 3d binding energy is measured by XPS and the degree of dispersion of Pd is measured by H2-TPD, with different γ-alumina as supports, are also listed in Table 2. Indeed, the palladium shows more positive charge on γ-Al2O3-nt than on γ-Al2O3-c which indicated that the palladium interacts more strongly with γ-Al2O3-nt than γ-Al2O3-c. Very interestingly, treating the catalysts at higher temperature causes different results on dispersion of Pd, i.e, higher dispersion on γ-Al2O3-nt but less dispersion on γ-Al2O3-c. In situ IR spectra of CO chemisorption is an excellent method to probe the geometric and electronic structures of palladium on γ-Al2O3. Figure 6 compares the in-situ IR spectra of CO adsorbed on 0.3% Pd/γ-Al2O3-nt, 0.3% Pd/γ-Al2O3-c, 1% Pd/γ-Al2O3-nt and 1% Pd/γ-Al2O3-c catalysts. In comparison with the Pd on the surface of γ-Al2O3-c support, there is a significant blue shift (ca. 10 cm-1) for both linear (~2070 cm-1) and multi-coordinated (~1920 cm-1) CO adsorption modes of the Pd on the surface of γ-Al2O3-nt support, which is a result of electronic interaction between Pd and γ-Al2O3-nt support, and corresponds to a positively charged Pd particles due to stronger interaction with high energy surface of γ-Al2O3-nt, consistent with XPS characterization. The stronger interaction effectively prevents the metallic particles from diffusion and aggregation in process of drying, roasting and reduction, and suppresses the growth of metallic particles in the calcination process, and ultimately favors the palladium dispersion on γ-Al2O3-nt than on γ-Al2O3-c. Particle sizes of palladium in 0.5% Pd/γ-Al2O3-nt and 0.5% Pd/γ-Al2O3-c catalysts are 15


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subsequently observed by TEM and shown in Figure S2. The palladium particle size of 0.5% Pd/γ-Al2O3-nt catalyst is about 1.6 nm (Figure S2 a,b), which is much smaller than those on γ-Al2O3-c at the same palladium content (5.8 nm) as shown in Figure S2 c,d, indicating the higher dispersion of Pd on the surface of γ-Al2O3-nt support.

Figure 5. Optimized model of Pd4 molecular interaction with (110) or (111) surface of γ-Al2O3 by theoretical calculation and the density of states and d-band center (dε in eV) of Pd4 cluster on the hydrated γ-Al2O3 (110) and (111) surfaces. Typical calculation results of C2H4 and C2H2 molecular interaction with Pd4/γ-Al2O3 (110) or Pd4/γ-Al2O3 (111) and corresponding adsorption energy are also listed. Yellow sphere: Al, Red sphere: O, White sphere: H, dark blue sphere: Pd, gray sphere: C. The figures in blue means the adsorption energy.

16


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0.3% Pd/γ-Al2O3-nt 1,920 cm 2,063 cm

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

-1

-1

0.3% Pd/γ-Al2O3-c 1,908 cm 2,053 cm

-1

1,946 cm

1% Pd/γ-Al2O3-nt 2,074 cm

-1

-1

1% Pd/γ-Al2O3-c

1,936 cm

2,069 cm

2,100

-1

-1

-1

2,000

1,900 -1

1,800

Wavenumber (cm ) Figure 6. In-situ FT-IR spectra of CO adsorbed on the catalysts.

Catalytic performance The catalyst γ-Al2O3-i, produced by Sinopec, has been used as dehydration catalyst in industry for a long period of time. To elucidate the shape and surface-specific effects of the current γ-alumina, the three samples are tested for the dehydration reaction of ethanol under the same conditions and the results are shown in Figures 7a,b. The γ-Al2O3-nt catalyst has all-round advantage over the other two catalysts, considering the effects of reaction temperatures (Figure 7a) and space velocities (Figure 7b). The results show the γ-alumina nanotubes synthesized under the oleic acid protection are better catalyst for acid catalyzed reactions. The catalysts Pd/γ-Al2O3-nt and Pd/γ-Al2O3-c are tested for semi-hydrogenation of acetylene to ethylene and phenylacetylene to styrene, both of the two reactions are important in industry, and the results are depicted in Figures 7c-f. Figure 7c displays the selectivity to ethylene as a function of the acetylene conversion, values for the Pd/γ-Al2O3-c catalysts are mainly distributed in region I and III wherein acetylene conversion or ethylene selectivity is relatively poor. The Pd/γ-Al2O3-nt catalysts, by contrast, the majority of values fall into region II wherein high conversion and 17


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selectivity are achievable concurrently. Subsequently, the durability of the 0.01% Pd/γ-Al2O3 catalysts are examined as shown in Figure 7d, the 0.01% Pd/γ-Al2O3-nt catalyst showed excellent durability against deactivation during a total 76 h on-stream, in contrast, acetylene conversion fell by 43% over this period for the 0.01% Pd/γ-Al2O3-c catalyst. Furthermore, the 0.5% Pd/γ-Al2O3 catalysts are applied in the semi-hydrogenation of phenylacetylene reaction. Figure 7e show that the 0.5% Pd/γ-Al2O3-c catalyst have the higher selectivity of byproduct ethylbenzene than the 0.5% Pd/γ-Al2O3-nt catalysts at the same pretreatment temperature. The evolution of phenylacetylene conversion with the reaction time is shown in Figure 7f. The results show that the phenylacetylene conversion of the 0.5% Pd/γ-Al2O3-nt catalyst is better than that of the 0.5% Pd/γ-Al2O3-c catalyst no matter how the pretreatment temperature changes. Whatever essential factors about catalyst evaluation are concerned, i.e., activity, selectivity or stability, the catalyst of palladium loaded on γ-Al2O3-nt is predominantly better than the contrast catalyst Pd/γ-Al2O3-c, which possesses mainly {110} faces as external surfaces.

18


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a

b 100

Yield of ethene (%)

Yield of ethene (%)

100 80 60 40 γ-Al2O3-nt

20

560

600

640

d

Ⅰ

Ⅱ

Ⅳ

Ⅲ

20 0 0

20

40

60

4

80

0.01% Pd/γ-Al2O3-nt-Conv. 0.01% Pd/γ-Al2O3-nt-Sele. 0.01% Pd/γ-Al2O3-c-Sele.

0

20

60

80

100

Conversion of phenylacetylene (%)

0.0 40

40

60

80

Time on stream (h)

0.4

20

12

40

0

100

0.8

0

-2

0.01% Pd/γ-Al2O3-c-Conv.

f

1.2

10

-1

60

20

0.5% Pd/γ-Al2O3-nt 373K 0.5% Pd/γ-Al2O3-c 373K 0.5% Pd/γ-Al2O3-nt 573K 0.5% Pd/γ-Al2O3-c 573K 0.5% Pd/γ-Al2O3-nt 773K 0.5% Pd/γ-Al2O3-c 773K

1.6

8

-2

100

Conversion of acetylene (%)

e

6

80

0.01% Pd/γ-Al2O3-nt 0.01% Pd/γ-Al2O3-c 0.015% Pd/γ-Al2O3-nt 0.015% Pd/γ-Al2O3-c 0.02% Pd/γ-Al2O3-nt 0.02% Pd/γ-Al2O3-c

40

γ-Al2O3-i

2

Intensity (%)

Selectivity of ethylene (%)

60

γ-Al2O3-nt

60

Space velocity (10 g h m )

100 80

70

680

Temperature (K)

c

80

50

γ-Al2O3-i

520

90

γ-Al2O3-c

γ-Al2O3-c

0

Selectivity of ethylbenzene (%)

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ACS Catalysis

Conversion of phenylacetylene (%)

100 80 60 40 0.5% Pd/γ-Al2O3-nt 373K 0.5% Pd/γ-Al2O3-c 373K 0.5% Pd/γ-Al2O3-nt 573K 0.5% Pd/γ-Al2O3-c 573K 0.5% Pd/γ-Al2O3-nt 773K 0.5% Pd/γ-Al2O3-c 773K

20 0 0

2

4

6

8

10

12

Time of reaction (min)

Figure 7. Results of catalytic tests. a, The yield of ethylene as a function of the reaction temperature (space

velocity is 0.02 g· h-1· m-2) in dehydration of ethanol. b, The yield of ethylene as a function of the reaction space velocity (reaction temperature: 653K) in dehydration of ethanol. c, d, The catalytic performance of the catalysts for semi hydrogenation of acetylene to ethylene. e, f, The catalytic performance of the catalysts for semi hydrogenation of phenyl acetylene to styrene. The xxxK in the sample name means the catalyst is calcinated at the temperature of xxxK before catalytic test.

Further discussion on thermodynamic calculation The differences are further understood by theoretical calculation of thermodynamics. The 19


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theoretically optimized models of ethylene/acetylene adsorptions on Pd4 cluster adsorbed on the different surfaces of γ-Al2O3 are also summarized in Figure 4 and the numerical results in detail are listed in Table 2. Based on the data of calculation, the further discussions should be: (1) C2H2 adsorption: C2H2 prefers the di-σ adsorption mode. The surface hydroxylation of alumina tends to weaken the bonding of C2H2 with the cluster on the (110) surface. On the contrast, hydroxylation remarkably enhances the C2H2-cluster interaction on the (111) surface, as indicated by the adsorption energy changes, from -0.39 to -1.77eV for the π mode or from -1.30 to -2.16 eV for the di-σ mode. The adsorption energy changes are consistent with the upshift of the d-band center of the cluster (from -2.06 to -1.68 eV). (2) C2H4 adsorption: Similar to acetylene adsorption, the adsorption energy of C2H4 is also significantly increased upon surface hydroxylation of the (111) facet, while the calculated adsorption energy is almost unchanged from the dehydrated (110) to the hydrated one. (3) (110) vs. (111): Both C2H2 and C2H4 are more stable on the dehydrated (110) surface than on the dehydrated (111) surface. However, on the hydrated (110) surface the adsorption energy of C2H2 is very close to that of C2H4 for the di-σ mode, -1.33 eV vs. -1.36 eV. While on the hydrated (111) surface, the adsorption energy of C2H2 (-2.16 eV) is 0.38 eV higher than C2H4 (-1.78 eV) also for the di-σ mode. Generally, the adsorptions of acetylene and ethylene on Pd4 cluster loaded on (111) surface are weaker than on (110) surface of alumina. The ethylene adsorption on Pd4 cluster loaded on (111) surface are weaker more than acetylene adsorption and this would be the main difference in surface-specific of γ-Al2O3 effects on palladium loaded. The Pd/γ-Al2O3 (111) should give better selectivity for hydrogenation of acetylene to ethylene, in agreement with experiment. Although the adsorption of acetylene on Pd/γ-Al2O3 (111) would be too strong to influence the catalyst activity, the catalyst compensates activity with larger dispersion and better stability against sintering of palladium, which leads to more active and more selective catalyst for the reaction. More details about characterization and calculation are shown in Figure S3-S13.

20


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ACS Catalysis

Table 2. Experimental measurements on Pd electronic states (XPS) and dispersion on the alumina (H2 adsorption) and the results of theoretical calculation of interaction between Pd4 cluster and the alumina and the calculated adsorption energies (eV) of C2H4 or C2H2 on the Pd4 cluster supported on different γ-Al2O3 surfaces are also listed.* (110) or Al2O3-c *1

(111) or Al2O3-nt *1

335.4 (573K)

335.9 (573K)

335.4 (773K)

335.7 (773K)

31 (573K)

65 (573K)

22 (773K)

77 (773K)

Pd4 adsorption energy /eV

-3.20/-1.79(-1.82)

-8.27/-5.92

Pd4 d band center /eV

-2.06

-1.68

di-σ mode of C2H2

-1.97/-1.33

-1.30/-2.16

di-σ mode of C2H4

-1.32/-1.36(-0.99)

-0.68/-1.78

π mode of C2H2

-1.11/-1.22

-0.39/-1.77

π mode of C2H4

-1.06/-1.16(-1.09)

-0.74/-1.81

Surface or sample Pd 3d binding energy /eV *2 Experimental Pd dispersion (%) *3

Theoretical

*

The models and methods involved are fully described in experiment section. Data before and after the slash

denote the adsorption energy on the dehydrated and hydrated surfaces of alumina respectively. values in the parenthesis are the corresponding adsorption energy on the hydrated surface from reference 57.

*1

(110) and

(111) planes correspond to DFT calculations and γ-Al2O3-c and γ-Al2O3-nt for XPS and H2-TPD experimental measurements. *2 XPS of 0.5% Pd/γ-Al2O3 samples. *3 H2-TPD of 0.5% Pd/γ-Al2O3 samples.

CONCLUSIONS A new-type γ-alumina in nanotubular morphology with high energy {111} facets as main exposed surfaces has been obtained by the hydrothermal assembly of colloidal boehmite alumina in very tiny particles using oleic acid as anionic surfactant and surface stabilizer. The novel material revealed by current systematic investigation in unique surface structure with high energy, exhibiting excellent properties as solid acidic catalyst or catalyst support for noble metals is promising for future applications in catalysts design and progress for more important industrial reactions.

Supporting Information XRD results of boehmite colloidal particulates，HRTEM images of 0.5% Pd/γ-Al2O3-nt and 0.5% 21


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Pd/γ-Al2O3-c and detailed results of computation characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. References： (1) Hughes, M. D.; Xu, Y.-J.; Jenkins, P. Nature 2005, 437, 1132-1135. (2) Choi, M.; Na, K.; Kim, J.; Sakamoto, Y.; Terasaki, O.; Ryoo, R. Nature 2009, 461, 246-249. (3) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732-735. (4) Xie, X.-W.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W.-J. Nature 2009, 458, 746-749. (5) Yang, H.-G.; Sun, C.-H.; Qiao, S.-Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H.-M.; Lu, G.-Q. Nature 2008, 453, 638-641. (6) Ma, X.-C.; Jiang, P.; Qi, Y.; Jia, J.-F.; Yang, Y.; Duan, W.-H.; Li, W.-X.; Bao, X.-H.; Zhang, S.-B.; Xue, Q.-K. P. Natl. Acad. Sci. USA. 2007, 104, 9204-9208. (7) Zhou, K.-B.; Wang, X.; Sun, X.-M.; Peng, Q.; Li, Y.-D. J. Catal. 2005, 229, 206-212. (8) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.-D.; Somorjai, G. A. Nano Lett. 2007, 7, 3097-3101. (9) Pulido, A.; Concepcion, P.; Boronat, M.; Corma, A. J. Catal. 2012, 292, 138-147. (10) Narayanan, R.; Sayed, M. A. E. J. Phys. Chem. B 2005, 109, 12663-12676. (11) Wu, Z.-L.; Li, M.-J.; Overbury, S. H. J. Catal. 2012, 285, 61-73. (12) Si, R.; Maria, F. S. Angew. Chem. Int. Ed. 2008, 47, 2884 -2887. (13) Zhang, H.; Sun, J.-M.; Dagle, V. L.; Halevi, B.; Datye, A. K.; Wang, Y. ACS Catal. 2014, 4, 2379-2386. (14) Hu, L.-H.; Peng, Q.; Li, Y.-D. J. Am. Chem. Soc. 2008, 130, 16136-16137. (15) Yuan, Q.; Yin, A.-X.; Luo, C.; Sun, L.-D.; Zhang, Y.-W.; Duan, W. T.; Liu, H. C.; Yan, C. H. J. Am. Chem. Soc. 2008, 130, 3465-3472. (16) Kwak, J. H.; Hu, J.-Z.; Mei, D.-H.; Yi, C.-W.; Kim, D. H.; Peden, C. H. F.; Szanyi, L. F. J. Science 2009, 325, 1670-1673. (17) Starzyk, F. T.; Seguin, E.; Thomas, S.; Daturi, M.; Arnolds, H.; King, D. A. Science 2009, 324, 1048-1051. (18) Liu, Y.; Huang, F.-Y.; Li, J.-M.; Weng, W.-Z.; Luo, C.-R.; Wang, M.-L.; Xia, W.-S.; Huang, C. -J.; Wan, H.-L. J. Catal. 2008, 256, 192-203. (19) Peri, J. B. J. Phys. Chem. 1965, 69, 220-230. 22


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ACS Catalysis

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(46) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188-5192. (47) Raybaud, P.; Digne, M.; Iftimie, R.; Wellens, W.; Euzen, P.; Toulhoat, H. J. Catal. 2001, 201, 236-246. (48) Krokidis, X.; Raybaud, P.; Gobichon, A. E.; Rebours, B.; Euzen, P.; Toulhoat, H. J. Phys. Chem. B 2001, 105, 5121-5130. (49) Digne, M.; Raybaud, P.; Sautet, P.; Rebours, B.; Toulhoat, H. J. Phys. Chem. B 2006, 110, 20719-20720. (50) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L. J. Phys. Chem.1981, 85, 2238-2243. (51) Ding, W.-P.; Meitzner, G. D.; Iglesia, E. J. Catal. 2002, 206, 14-22. (52) Gu, W.-M.; Xie, Z.-K. J. Catal. 2007, 248, 20-28. (53) Ding, W.-P.; Li, S.-Z.; Meitzner, G. D.; Iglesia, E. J. Phys. Chem. B 2001, 105, 506-513. (54) Xue, N.-H.; Chen, X.-K.; Nie, L.; Guo, X.-F.; Ding, W.-P.; Chen, Y. J. Catal. 2007, 248, 20-28. (55) Gonzales, N. O.; Chakraborty, A. K.; Bell, A. T. Top. Catal. 1999, 9, 207-213. (56) Peng, Y.-K.; Ye, L.; Qu, J.; Zhang, L.; Fu, Y.-Y.; Teixeira, I. F.; Mcpherson, I. J.; He, H.-Y.; Tsang, S. C. E. J. Am. Chem. Soc. 2016, 138, 2225-2234. (57) Valero, M. C.; Raybaud, P.; Sautet, P. J. Catal. 2007, 247, 339-355.

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ACS Catalysis

Acknowledgements The authors thank the financial supports from the Ministry of Science and Technology of China (2009CB623504), the National Natural Science Foundation of China (20673054, 21273103, 21273107,91434101), and Sinopec Shanghai Research Institute of Petrochemical Technology.

Competing financial interests The authors declare no competing financial interests.

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Nanotubular Gamma Alumina with High-Energy ... - ACS Publications

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