we can estimate that about 0.77 monolayer is adsorbed
under these conditions. In a 0.0350 g sample of NCAl2O3/
MgO 2.8 _ 1019 nm2 surface is available, and so
we can estimate that about 1.04 monolayers are adsorbed
under these conditions (room temperature, 0.01
M concentration in pentane).
After the reaction was complete, the powders were
filtered, and IR studies were done on the solids. The
results from the IR show that the NC samples have
many new species adsorbed to the powder, whereas the
CM samples have little if any new species adsorbed.
Table 7 gives IR spectra assignments for free Paraoxon
and for adsorbed Paraoxon which are based on the
literature.33,41,42,47,48 According to IR spectra taken following
the reaction with Paraoxon for CM-Al2O3, NC(41) Koper, O. Properties of High Surface Area Calcium Oxide and
its Reactivity Towards Chlorocarbons. Ph.D. Thesis, Kansas State
(42) Stark, J. V.; Klabunde, K. J. Chem. Mater. 1996, 8, 1913-1918.
(43) Goodsel, A. J.; Low, M. J. D.; Takezawa, N. Environ. Sci.
Technol. 1972, 6, 3, 268.
(44) Dunn, J. P.; Koppula, P. R.; Stenger, H. G.; Wachs, I. E. Appl.
Catal., B 1998, 19, 103.
(45) Stark, J. Characterization and Studies of Reactivity of Magnesium
Oxide. M.S. Thesis, Kansas State University, 1995.
(46) Li, Y.; Schlup, J.; Klabunde, K. J. Langmuir 1991, 7, 1388.
Figure 5. Transmission electron microscope pictures: (a)
NC-Al2O3/MgO; (b) CM-MgO.
Figure 6. IR spectra after 2 h evacuation at room temperature.
Table 6. Destructive Adsorption of Paraoxon in Pentane
on the Oxide Powders
sample molar ratio
NC-Al2O3 1 mol of Paraoxon:11 mol of Al2O3
CM-Al2O3 1 mol of Paraoxon:63 mol of Al2O3
NC-(1/1) Al2O3/MgO 1 mol of Paraoxon:5.6 mol of Al2O3/MgO
CM-MgO 1 mol of Paraoxon:188 mol of MgO
NC-MgO 1 mol of Paraoxon:46 mol of MgO
Nanocrystals of Al2O3 and Al2O3/MgO Chem. Mater. G
Al2O3, CM-MgO, and NC-(1/1)Al2O3/MgO, there was
some change when Paraoxon was adsorbed. A band at
1296 cm-1 assigned to Ó(PdO) was broadened and
shifted to approximately 1313 cm-1. Also, the original
peak for free Paraoxon at 1232 cm-1 assigned to the
Ó(P-O-Ar) stretch had disappeared. The band due to
Ó(P-O-Et) at 1045 cm-1 did not change much (see
Table 7), suggesting that the EtO-P moieties were not
perturbed or destroyed. We tentatively conclude from
these data that PdO bond is strongly perturbed through
binding to Lewis acid sites on the Al2O3 and MgO
surfaces and that the P-OAr bond is broken.
The modified aerogel procedure employed is designed to yield high quality gels under apid gelation conditions. 49 Solvent-to-alkoxide ratios are important as well as the solvent choice. The addition of toluene has at least two effects; lowering viscosity and polarity. This in turn, causes a more rapid hydrolysis step and faster gelation, which results in a more “open” structure.49 After supercritical drying50 very high surface area Al(OH)3 or mixed MgAl(OH)4 were obtained. Subsequently, after dehydration of the hydroxides, high surface area oxides with large pore volumes were obtained. This synthetic approach is very different from earlier reports, where reactive sputtering,12 dc arc plasmas,13 crysol techniques,14 and related sol-gel methods were
used.16,17 For example Ramesh and co-workers17 used aluminum triisopropoxide as a starting material in a sol-gel synthesis and obtained amorphous Al2O3, surface
area 97-183 m2/g, much lower than the Al2O3 samples reported herein. Earlier work on mixed Al2O3/MgO systems dealt with impregnating Al2O3 with magnesium salts followed by heat treating; surface areas of 136-184 m2/g were reported. In addition, Chi and Chang report that Al2O3 impregnated with MgO could adsorb more NO at 523 K than did a series of other similar compositions. It should be emphasized that the modified aerogel procedure reported herein is intended to prepare MgOAl2O3
Чтобы распечатать файл, скачайте его (в формате Word).
Ссылка на скачивание - внизу страницы.