Synthesis, Characterization, and Adsorption Studies of Nanocrystalline Aluminum Oxide and a Bimetallic, страница 8

showed that at atmospheric pressure and room temperature,

SO2 adsorbed onto NC-Al2O3 up to 3.5

molecules of SO2/nm2, similarly on CM-Al2O3 there

were 3.5 molecules of SO2/nm2 adsorbed, and on NC(1/1)Al2O3/MgO there were 6.8 molecules of SO2/nm2,

whereas on CM-MgO there were only 0.68 molecules

of SO2/nm2 adsorbed (Table 5). These data indicate that

NC-Al2O3/MgO efficiently adsorb SO2 in slightly more

than one layer. After adsorption had ceased, the samples

were subjected to dynamic vacuum for 100 min to

remove the physisorbed species. After this vacuum

treatment, there remained 1.70 and 0.45 molecules of

SO2/nm2 present chemisorbed onto the NC-Al2O3, and

CM-Al2O3 samples, respectively. The vacuum treatment

removed most of the adsorbed SO2 from the CMMgO,

whereas the NC-Al2O3/MgO sample retained 3.9

molecules of SO2/nm2.

Next, an in situ IR study was performed to help

identify how the SO2 was binding.41 This setup allows

IR spectra to be obtained of the exact same spot before

SO2, with SO2, and after evacuation of SO2. The study

was conducted at room temperature with 20 Torr of SO2.

(40) Barin, I.; Knacke, O. Thermochemical Properties of Inorganic

Substances; Springer-Verlag: Berlin, 1973.

Table 4. Reaction of Pulses of CCl4 with Oxide Samples at 500 °C

sample breakthrough saturation molar ratio

NC-Al2O3 57 98 1.44 mol of CCl4:1 mol of Al2O3

CM-Al2O3 2 17 1 mol of CCl4:16 mol of Al2O3

NC-(1/1)Al2O3/MgO 25 88 1.8 mol of CCl4:1 mol of Al2O3/MgO

CM-MgO 1 4 1 mol of CCl4:32 mol of MgO

a Theoretical molar ratio: 1 mol of CCl4:2 mol of MgO, 1.5 mol of CCl4:1 mol of Al2O3, 2 mol of CCl4:1 mol of Al2O3/MgO. b NC-MgO

studied at 400 °C.51

Figure 4. X-ray diffraction patterns for CM-Al2O3, CMMgO,

NC-MgO, NC-Al2O3, and NC-(1/1)Al2O3/MgO.

Table 5. Adsorption of SO2 on Al2O3, MgO, and

Al2O3/MgO Samples at Room Temperature

sample

tot. molecules

of SO2/nm2

adsorbed

molecules of

SO2/nm2

physisorbed

molecules of

SO2/nm2

chemisorbed

NC-Al2O3 3.5 1.8 1.7

CM-Al2O3 3.5 3.0 0.45

NC-Al2O3/MgO 6.8 2.9 3.9

CM-MgO 0.68 0.51 0.17

NC-MgO35 6.0 1.5 4.5

2 Al2O3 + 3CCl4 f 3CO2+ 2Al2Cl6

2 MgO + CCl4 f CO2+ 2MgCl2

F Chem. Mater. Carnes et al.

Figure 6 shows the spectra after a 2 h evacuation for

all four samples. Both CM-MgO, and CM-Al2O3 showed

no adsorbed species. Both NC-Al2O3 and NC-Al2O3/

MgO show new peaks at 1135 and 1330 cm-1 that

correspond to chemisorbed monodentate SO2 adsorbed

species.35,42-46 These results indicate that the NC

samples have a high capacity for chemisorption of SO2

(Table 5) per unit surface area, indicating an intrinsically

higher activity.

Destructive Adsorption of Diethyl 4-Nitrophenyl

Phosphate (Paraoxon). The adsorption of Paraoxon

was carried out to compare the rates and capacities for

the metal oxide samples to dissociatively chemisorb a

polar organic, more specifically a toxic insecticide. By

monitoring of the disappearance of an UV band for

Paraoxon in pentane, the data shown in Table 6 were

obtained, and the results are striking. Neither of the

CM samples adsorbed much Paraoxon, while the NC

samples rapidly adsorbed the entire sample and developed

a bright yellow color, indicating the likely formation

of the p-nitrophenoxide anion on the surface.

Additional experiments with larger and larger amounts

of Paraoxon were studied, and it was found that about

5.5 ÌL of Paraoxon (2.55 _ 10-5 mol, 1.54 _ 1019

molecules) was adsorbed by 0.0300 g of NC-Al2O3, and

about 9.5 ÌL of liquid Paraoxon (4.40 _ 10-5 mol, 2.65

_ 1019 molecules) was taken up by 0.0350 g of NCAl2O3/

MgO. One molecule of Paraoxon dissociated and

with the p-nitrophenoxide lying flat would occupy about

1 nm2 of surface area. In a 0.0300 g sample of NCAl2O3

2.4 _ 1019 nm2 surface area is available, and so