Toshikatsu Mori

Oxidation and reduction of substances in aqueous solution in presence of water-repel lent catalyst

The surface of a conventional solid catalyst composed mainly of metal oxides such as A1203, SiOz, TiOz, and Fez03 is hydrophilic. When such a catalyst is used in an aqueous solution, the surface is completely covered by water. It has been understood, therefore, that a conventional solid catalyst has little effect on the reaction of a substance in an aqueous solution with an insoluble gas, for example, oxidation of SHion by 02 and reduction of NO3ion by HZ. Since the surface of a hydrophilic catalyst is covered with a layer of water, 02 or H2 gas is not brought into contact with active sites of the catalyst surface. Under such circumstances the reaction rate may be controlled by the dissolution of O2 or H2 gas into the aqueous solution. In order to overcome the drawbacks of a hydrophilic catalyst an attempt has been made in the present study to prepare and utilize a water-repellent or hydrophobic catalyst for a reaction in an aqueous solution involving 02 and Hz gas. It has been discovered that several reactions are greatly accelerated in the presence of a water-repellent catalyst. The term “waterrepellent catalyst” instead of “hydrophobic catalyst” is used in the present paper, since it better represents the nature and function of the catalyst. A water-repellent catalyst is defined as a catalyst whose surface is at least partly water repellent and forms a gas-solid interface when it is in contact with a gas in an aqueous solution. 264 0021-9517/83/020264-07

Stability and activity of platinum/carbon catalysts.

03.00/O Cowi&t Q 1983 by Academic Press, Inc. Au rights of reproduction in any form reserved. Consequently a function of a water-repellent catalyst is to form a three-phase, i.e., gas-liquid-solid interface, which provides reaction sites for the gas and liquid components. A water-repellent catalyst has been used in the HT02 recombiner for a lead-acid battery (1, 2) and in the isotopic exchange between HDO(liquid) and Hz(gas) for the deuterium concentration (3-5). In these applications the catalyst is not used to cause a chemical reaction of a substance in the aqueous solution, but the water repellency is utilized merely to avoid surface wetting. Recently it has been reported in patents that a water-repellent catalyst with an electroconductive support (carbon) has been applied to various chemical reactions in aqueous solutions (6, 7). An electrode catalyst which is used in the HT02 fuel cells with KOH or H3P04 electrolyte can be said to be a water-repellent catalyst with electroconductivity (8, 9). We have prepared a number of waterrepellent catalysts from various support materials (electroconductive and nonconductive) and applied the catalysts to several reactions. The fundamental nature of the water-repellent catalyst is investigated and the preliminary experimental results are reported in this paper.

Catalytic process for reducing nitrogen oxides to nitrogen

リン酸型燃料電池に使用する触媒担体について, 3種の炭素粉末の酸素および水蒸気に対する反応性を調べ, 炭素粉末の安定性を評価した。また, 白金を担持した触媒の酸素還元反応に対する活性を, 190℃ の 98wt% リン酸中で測定した。黒鉛結晶子め層間隔が小さく, かつ結晶子径が大きい炭素粉末は, 酸素および水蒸気との反応における速度定数が小さく, 安定であることが明らかになった。アセチレンブラックであるデンカブラック (電気化学工業製) は, ファーネスブラックである Regal 660R および Vulcan XC-72R (いずれも Cabot Corp. 製) にくらべて, 黒鉛結晶子の層間隔が小さく, かつ結晶子径が大きかった。触媒活性においても, デンカブラックを担体とする触媒が高活性を示した。白金粒子径は変わらないが, 白金粒子の分散性が他の炭素粉末にくらべて高いことが一因と考えられた。