Synthesis, Characterization, and Adsorption Studies of Nanocrystalline Aluminum Oxide and a Bimetallic, страница 5

8 aluminum tri-tert-butoxide toluene/tert-butanol 10 810

(100 mL/40 mL)

9 aluminum tri-tert-butoxide toluene/tert-butanol 20 735

(100 mL/40 mL)

10 aluminum tri-tert-butoxide toluene/tert-butanol 10 673

(50 mL/40 mL)

11 aluminum tri-tert-butoxide toluene/tert-butanol 10 779

(150 mL/40 mL)

Nanocrystals of Al2O3 and Al2O3/MgO Chem. Mater. C

as indicated by comparing the shape of the adsorptiondesorption

curve with standards.

Compressed pellets were prepared of the NC-Al2O3

at various compaction pressures and then studied by

BET methods to see how the pressure affected the

surface area, pore diameter, pore volume, and pore

shape. The pressures tested in pounds per square inch

(psi) were 2000, 5000, 10000, and 20000. Before being

pressed, the samples had about 800 m2/g surface area,

when pressed at 2000 psi the surface area fell to 752

m2/g, and the area fell to 486 m2/g at 20000 psi. The

average pore diameter also changed with increasing

pressure. Before pressing, the samples have 10.8 nm

pore openings and did not change much when the

samples are pressed at 2000 psi. However, pressing at

20000 psi caused a marked decrease in the size of the

pore openings down to 7 nm. Pore volume was also

changed with pressure, but not as drastically as the

average pore diameter. Before pressing the samples had

2.05 cm3/g volume, and after being pressed at 2000 psi,

the volume dropped slightly to 1.85 cm3/g, while after

pressing at 10000 psi the volume decreased to 1.22 cm3/g

where it generally remained even after being pressed

at 20000 psi. The pore shape of the NC-Al2O3 sample

changed with increasing pressure, according to De

Boer’s hysteresis. 37 Before any pressure was applied,

the sample had a pore structure consisting of cylindrical

pores open at both ends; when pressure was applied,

this pore structure remained, until 20000 psi, where it

was replaced with slit-shaped pores, the space between

parallel plates.

X-ray Diffraction (XRD). From XRD, we obtained

diffraction patterns that showed the NC-Al2O3 sample

to be less crystalline than the commercial Al2O3 samples.

The NC-Al2O3 had a completely amorphous pattern,

due to particle size broadening. 38 From the diffraction

patterns the crystallite size of NC-Al2O3 could not be

determined, even as the temperature was increased.

The results show that the NC-Al2O3 had a significantly

smaller crystallite size than the commercial Al2O3

samples; indeed, the average crystallite size for NCAl2O3

activated at 500 °C was very small, less than 2

nm, while the average crystallite size for CM-Al2O3 was

19 nm.

Infrared Spectroscopy (IR). The heat-treated samples

were ground with KBr, and pressed into pellets. IR

spectra taken after heat treatment at 25, 50, 100, 150,

200, 250, 300, 400, and 500 °C indicate a gradual loss

of water and a small amount of residual alkoxy groups.

After 500 °C heat treatment, some residual -OH groups


Thermogravimetric Analysis (TGA). Weight loss under

nitrogen flow was observed. Total weight loss was about

35% and was found to be the same when conducted in

air. Although a gradual loss was observed, most mass

loss occurred in the 75-175 and 350-460 °C ranges,

which corresponds mainly to surface and internal water

loss. These findings are important, since the conversion

of Al(OH)3 to Al2O3 should yield a 35% weight loss

(whereas conversion to AlOOH would be 23%)

Transmission Electron Microscope (TEM). Parts a and

b of Figure 1 show TEM photographs, the CM-Al2O3

sample exhibited a grainy material with crystallites

greater than 10 nm. The NC-Al2O3 (Figure 1b) sample

(37) Lowell, S. Introduction To Powder Surface Area; John Wiley

& Sons: New York, 1979.

(38) Azafoff; Buerger. The Powder Method in X-ray crystallography;