McGraw-Hill Book Co.: New York, 1958; Chapter 16.
Table 2. Surface Area, Pore Volume, and Diameter of
Nanocrystalline Samples Compared to Commercially
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
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