Synthesis, Characterization, and Adsorption Studies of Nanocrystalline Aluminum Oxide and a Bimetallic, страница 7

pressure, according to De Boer’s hysteresis,: at first the

pore structure consisted of cylindrical pores open at both

ends until 10000 psi, where it is replaced with tapered

or wedged shaped pores with narrow necks, open at one

or both ends.37 Indeed, these pore structures are different

from the NC-Al2O3 samples (see earlier results) and

quite different from NC-MgO, where pore volumes and

pore openings are smaller, and bottleneck and cylindrical

pores were observed.39

(39) Richards, R.; Li, W.; Decker, S.; Davidson, C.; Koper, O.;

Zaikovski, V.; Volodin, A.; Rieker, T.; Klabunde, K. J. J. Am. Chem.

Soc. 2000, 122, 4921.

Figure 2. HRTEM of NC-Al2O3.

Table 3. Aluminum/Magnesium Oxide Preparation

Variables

sample

no.

molar ratio

Al2O3/MgO

amt of

solvent (mL)

stirring

time (h)

surface

area (m2/g)

1 1/1 20 1 559

2 1/1 70 1 762

3 1/1 120 1 743

4 1/1 70 10 815

5 1/1 70 20 796

6 1/2 70 10 775

7 2/1 70 10 834

Figure 3. Surface areas of NC-Al2O3, CM-Al2O3, NC-MgO,

CM-MgO, and NC-(1/1)Al2O3/MgO upon heat treatment.

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

X-ray Diffraction. Figure 4 compares the XRD pattern

for the Mg and Al samples. The Al2O3/MgO did not

sinter even upon heating to 700 °C.

Infrared Spectroscopy. IR spectra were taken after

heat treatment at 25, 100, 200, 300, 400, and 500 °C. A

gradual loss of water and solvent was observed from 25

to 500 °C. So when the sample is activated at 500 °C, it

can be inferred that there will be a small amount of

residual surface -OH present.

Thermogravimetric Analysis. Weight loss under nitrogen

flow was about 40% and was found to be the

same when conducted in air. There was a gradual

weight loss throughout the heating, due to the hydroxide

converting to oxide and the removal of the water. The

theoretical weight loss is 34%, so the observed 40%

indicated that some adsorbed water was also present.

Transmission Electron Microscopy. Figures 1a, 5a,

and 5b show TEM photographs, of CM-Al2O3, NCAl2O3/

MgO, and CM-MgO, respectively. The CM-MgO

sample consists of single crystals of varying fairly large

sizes. The NC-Al2O3/MgO sample however consists of

crystallites having an average crystallite size of about

5 nm or smaller, and of quite different morphology from

analogous NC-Al2O3 and NC-MgO samples.39

ElementalAnalysis.Elementalanalysis results of NC(1/1)Al2O3/MgO preheat-treated to 500 °C under dynamic

vacuum were Mg ) 13.3% (17% calculated), Al

) 30.9% (37.9% calculated), C ) 0.65% (0% calculated)

and H ) 1.2% (0% calculated). Obviously some adsorbed

H2O and CO2 affected these results (as indicated by IR).

The formula MgAl2O3.3(OH)1.4(CO2)0.088(H2O)0.35 fits the

data (oxygen by difference). It was apparent from TGA

that some weakly bound water was present.

Adsorption Studies. Reaction of Al2O3 and (1/1)

Al2O3/MgO with CCl4. The reaction of CCl4 with Al2O3

and Al2O3/MgO was carried out to understand the

destructive adsorption abilities of the metal oxides

toward a model chlorocarbon at elevated temperatures.

Thermodynamics predicts that the samples containing

MgO should be more reactive than the Al2O3 samples.40

However, it will be seen how surface area, crystallite

size, and morphology can play an active role.

These reactions were conducted via the pulse method,

and the products were identified by GC. The breakthrough

injection, saturation injection, and molar ratios

are reported in Table 4.

Sulfur Dioxide Adsorption on Alumina, and Aluminum/

Magnesium Oxide. Adsorption of SO2 was carried

out to learn if the adsorption properties are different

for nanocrystals, when compared to commercial microcrystals.

Using 19.2 Å2 as the area of an SO2 molecule,

it can be determined that 5.2 molecules of SO2/nm2

would form a monolayer. The experimental results