Synthesis, Characterization, and Adsorption Studies of Nanocrystalline Aluminum Oxide and a Bimetallic, страница 6

McGraw-Hill Book Co.: New York, 1958; Chapter 16.

Table 2. Surface Area, Pore Volume, and Diameter of

Nanocrystalline Samples Compared to Commercially

Available Samples



area (m2/g)

av pore

vol (cm3/g)

av pore

diam (nm)

CM-Al2O3 103 0.19 7.4

CM-MgOa 18.7 0.077 16

NC-Al2O3 805 2.1 11

NC-Al2O3/MgOb 793 1.9 11

NC-MgO 400 0.90 9.0

a Also referred to as AP-MgO for aerogel prepared. b One mole

Al2O3 to 1 mol MgO.

Figure 1. Transmission electron microscope pictures: (a)

CM-Al2O3; (b) NC-Al2O3.

D Chem. Mater. Carnes et al.

consisted of a weblike material, quite different in

texture. By compiling data from XRD, TEM, and BET

we have concluded that the NC-Al2O3 samples are

made up of <2 nm crystallites. HRTEM confirmed that

the average crystallite size was very small, and the

crystals were disordered (Figure 2).

Elemental Analysis. Elemental analysis results for

NC-Al2O3 preheat-treated to 500 °C under dynamic

vacuum gave Al ) 47.1% (52.9% calculated for Al2O3).

These results suggest the presence of some residual OH/

H2O as well as adsorbed CO2 which was indicated by

IR. If CO2 and surface OH are assumed to be the only

adsorbed species, the formula Al2O2.7 (OH)0.53(CO2)0.03

fits the data (oxygen by difference).

Preparation of the Aluminum/Magnesium Oxide.

Several experiments were conducted varying the

solvent, stirring time, and molar ratios, and all were

found to have an effect on the surface area of the resulting

sample. Some of the results are shown in Table 3

and demonstrate that the highest surface area was obtained

for sample number seven, where a 2:1 Al2O3 to

MgO ratio was used. For 1/1 Al2O3 to MgO samples,

time was found to be a factor in the surface area. The

amount of solvent (ethanol) used to dilute the water in

the hydrolysis step was also found to have an important

role in the surface area. In samples 1 and 2, decreasing

the amount of ethanol from 70 to 20 mL caused a significant

decrease in surface area, going from 762 to 559 m2/g.

Activation of the Aluminum/Magnesium Oxide

(Thermal Dehydration). Aluminum/magnesium oxide

was activated under both argon flow and dynamic

vacuum, and a small advantage was realized with the

vacuum treatment (800 m2/g using vacuum and 750

m2/g using argon for 500 °C treatment).

(1/1) Aluminum Oxide/Magnesium Oxide Characterization.

Brunauer-Emmet-Teller Method (BET).

The NC-(1/1)Al2O3/MgO samples typically possessed

surface areas within the range 770-810 m2/g after heat

treatment at 500 °C. Figure 3 shows the heat treatment

temperature dependence observed in NC-(1/1)Al2O3/

MgO compared to NC-MgO, NC-Al2O3, CM-Al2O3,

and CM-MgO. The pore volumes and pore size openings

remained very large, comparable to the NC-Al2O3

sample and quite different from the NC-MgO and CM

samples (Table 2).

Compressed pellets were also prepared of the NC(1/1)Al2O3/MgO heat treated at 500 °C. The pressures

employed in pounds/square inch (psi) were the same as

those for the Al2O3 experiments. Before being pressed

the samples had 772 m2/g surface area, when pressed

at 2000 psi the surface area fell to 547 m2/g, and it fell

to 502 m2/g at 20000 psi. The pore diameter was also

affected by pressure: when not pressed the samples had

10.8 nm pores which decreased to 7.8 nm at 2000 psi

and then dropped to 6 nm at 20000 psi. Pore volume

also changed with pressure, but not as drastically as

the diameter or the surface area. Before being pressed

the samples had 1.90 cm3/g volume, while after being

pressed at 2000 psi the volume dropped to 0.742 cm3/g,

and then after being pressed at 10000 psi the volume

decreased to 0.635 cm3/g where it generally remained

even after being pressed at 20000 psi. The pore shape

of the NC-Al2O3/MgO sample changed with increasing