New hydrogenation systems of unsaturated organic compounds using noble metal-deposited palladium sheet electrodes with t
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New hydrogenation systems of unsaturated organic compounds using noble metal-deposited palladium sheet electrodes with three-dimensional structures Chiaki Iwakura Department of Applied Chemistry, College of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
Yasuki Yoshida and Setsuro Ogata Permelec Electrode Ltd., Fujisawa, Kanagawa 252-0816, Japan
Hiroshi Inoue Department of Applied Chemistry, College of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan (Received 21 August 1997; accepted 5 January 1998)
A successive hydrogenation system was constructed using a two-compartment cell separated by a Pd sheet. The hydrogenation rate changed greatly, depending on the kind of substrates used. For the purpose of improving the hydrogenation rate, the surface of a Pd sheet was modified with highly active catalysts such as Pd black, Pt, and Au by using active hydrogen passing through the Pd sheet as a reducing agent. As a result, the hydrogenation rate of unsaturated organic compounds such as 4-methylstyrene was markedly increased by the surface modification with these catalysts due to the increase in reaction zone and appearance of new active sites.
I. INTRODUCTION
The catalytic hydrogenation of unsaturated organic compounds using catalysts such as Pd, Pt, Ni, and so on is very important in organic syntheses.1–5 On these catalysts, the chemisorbed hydrogen atoms (active hydrogen) are easily formed, especially using an electrochemical method,6,7 leading to the bypass of the kinetic barrier for dissociation and mass transport of poorly soluble molecular hydrogen. Pd, one of the catalysts for hydrogenation, has unique properties different from Pt and Ni. It can absorb and desorb hydrogen reversibly,8,9 and therefore hydrogen-absorbed Pd could operate not only as a catalyst but also as a source of hydrogen for hydrogenation. Based on this idea, we have successfully constructed a new successive hydrogenation system of organic compounds such as styrene.10,11 The cell was made up of two compartments separated by the Pd sheet, where galvanostatic electrolysis of water was carried out to produce the active hydrogen at one side of a Pd sheet, and the substrate was hydrogenated at another side by using electrogenerated active hydrogen atoms passing through the Pd sheet. This system has an advantage of eliminating the need for supporting electrolytes at the substrate side. Moreover, it has a remarkable merit that the rate of hydrogenation can be controlled precisely by an applied current. However, in the above system, the hydrogenation rate of 4-methylstyrene at the bare Pd sheet electrode was J. Mater. Res., Vol. 13, No. 4, Apr 1998
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about 40 times slower than that of styrene,12 probably due to the difference in the strength of the interaction between the substrate molecules and Pd atoms based on the difference in electronic and steric factors of the substituent groups bound to a double bon
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