Selective Deposition of Pd on Porous Alumina Support Using Supercritical CO 2
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AA12.1.1
Selective Deposition of Pd on Porous Alumina Support Using Supercritical CO 2 Masahiko Matsukata, Takashi Nishizuka, Yasushi Sekine and Eiichi Kikuchi Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ABSTRACT A Pd/α-Al 2 O 3 composite membrane was prepared on a porous α -Al 2 O 3 disk via the thermal decomposition of bis(hexafluoroacetylacetonato)Pd(II), Pd(hfac) 2 . Pd(hfac) 2 was dissolved in supercritical CO 2 at 303 K, supplied to the substrate, and thermally decomposed at 403 and 443 K. The decomposition of Pd(hfac) 2 at 443 K resulted in a sufficient pore filling of substrate with Pd as well as the formation of a thin Pd top layer. The hydrogen flux through this membrane was about 1.7 times than that of a electro-less plating Pd membrane with about 20 µ m thick. Hydrogen permeation was governed by the solution-diffusion mechanism which gave a high separation selectivity. INTRODUCTION Palladium-based membranes have been applied to hydrogen separation. The hydrogen permeation mechanism of Pd membrane is the solution-diffusion mechanism, which allows us to separate with a very high selectivity. The current requirements with Pd membrane are cost reduction and an increase in hydrogen flux. The most effective wa y to overcom e these problems is the reduction in membrane thickness. Much effort has been devoted to develop a variety of techniques [1-9] for preparing a thin Pd layer on a porous substrate without loosing separation selectivity and mechanical strength. Chemical vapor deposition (CVD) is one of the techniques to prepare a Pd thin layer inside the pores of porous substrate. In comparison with the case that a Pd layer is prepared on the surface of substrate, one can expect several advantages on the filling of the pores of substrate with Pd, namely, the resistance against the exfoliation of Pd layer from the substrate during heating and cooling cycles, the improvement of strength against hydrogen embrittelement, and a further decrease in membrane thickness [3]. Palladium particles are, however, easily generate in gaseous phase in parallel with the pore filling with Pd in the substrate, because the sublimation and decomposition temperatures of Pd precursors are close [6]. Using supercritical CO 2 has recently been proposed to carry Pd precursors at a relatively low temperature at which its decomposition does not occur. Blackburn et al. first successfully deposited Pd on a substrate at a low temperature via the hydrogenolysis of a Pd complex, Bis(hexafluoroacet ylacetonato)Pd(II) [C 1 0 H 2 F 1 2 O 4 Pd(II), Pd(hfac) 2 ] [10-12]. In this study, a Pd/ α -Al 2 O 3 composite membrane was prepared by the thermal decomposition of Pd(hfac) 2 in a supercritical CO 2 medium. The influence of the decomposition temperature of Pd(hfac) 2 on the pore filling of substrate with Pd was explored.
AA12.1.2
EXPERIMENTAL DETAILS A comm ercially available, porous α-Al 2 O 3 disk, manufactured by NGK Co., was used as the substrate. The porous α-Al 2 O 3 disk was 20×2
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