| dc.description.abstract | The distribution of metals in the shell of large silicon dioxide beads plays a crucial role
in many industry-oriented applications over the past decades and an understanding of
its preparation method is much helpful for the improvement of catalytic efficiency of
the catalyst. The main purpose of this study was to develop a method to prepare the
formation of alloys of palladium-copper nano-metal in the shell of silicon dioxide bead
which was used as the carrier. How to prepare bi-metal on bead catalyst with an eggshell
structure using in production of allyl acetate is very important to chemical industry
because PdCu/SiO2 is used as the catalyst for the reaction of the gas phase acetoxylation
of ethylene and propylene in industry. Since the reaction medium is acidic, Pd and Cu
would migrate and sinter as time went by, causing severe deactivation. It was found that
Cu would migrate to the inside core of SiO2 bead, causing deaction of the reaction. By
forming PdCu nano alloy, the migration of Cu may be limited. The composition of SiO2
carriers also has some effect on the catalytic properties of the catalyst. Two SiO2 beads
were tested in this study, natural and synthetic spherical silicon dioxide with 5 and 3
mm size, respectively. The natural SiO2 beads contain Na cations, which may influence
the deactivation rate. In preparation, Pd and Cu metal salts were added by impregnation
and potassium hydroxide solution was added to push the metal cations to shell side. The
preparation procedure, such as drying, calcination, concentration of KOH solution, and
the order of the above steps influence the thickness of metal shell on silica beads. The
reduction method is needed to ensure Pd and Cu to form alloy on the silica surface.
Eight preparation methods with various steps were carried out in this study. All the
samples were characterized by optical microscope, X-ray diffraction, transmission
electron microscopy, and high resolution-transmission electron microscopy.
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The synthetic silica does not have sodium cation, the pore structure was very stable
after preparation of catalyst. Instead, natural silica bead contains sodium cation, it was
not stable in acidic condition during preparation process, and resulted in loss of pore
structure. Using KOH solution before adding metal solution in preparation could form
egg-shell structure. All samples under KOH solution and reduction showed dark black
spots in shell side of SiO2 beads. PdCu had thinner distribution in the shell side in
synthetic silica than natural silica.
The catalyst prepared by the following method had the thinnest shell among all
catalysts: The silica particles were synthetic silica. AlCl3 in 10 ml DI water was added
into sample and then dried at 180 ℃ for 2 h. KOH in 10 ml DI water was added into
the sample and dried at 110 ℃ for 4 h. The sample was impregnated with metal salts
(Na2PdCl4 and CuCl2) solution and dried at 80 ℃ for 6 h. After that, the sample was
calcined at 250 ℃ for 2 h and was stirred in an ice bath and reduced with a NaBH4
solution drop by drop. Next, keeping the sample in an ice bath for 30 min and washed
5 times by DI water. The sample was vacuum dried at 25 ℃ for 12 h. | en_US |