By ARTHUR JOHN BERRY. THE numerous investigations which have been carried out on the occlusion of hydrogen by palladium favour the view that the process consists of the solution of the gas in the metal rather than of chemical combination. Hoitsema (Zeitsch. physikal. Chem. , 1895, 17, l), in a very complete investigation of the vapour pressure of '' palladium-hydrogen," concluded that two immiscible solid solutions are formed. A t the same time, the precise nature of the product can scarcely be regarded as completely established.
The dissolution of powdered periclase, MgO, in near neutral to acidic solutions, proceeds via a brucite, Mg(OH)2, intermediate. The hydration reaction is apparent from the similarity of the dissolution rates (Vermilyea, 1969) and X-ray diffraction of the reacting powder. There is evidence from both natural samples (Gorshkov et al., 1992) and calculations (Refson et al., 1995) that a topotactic relationship exists between the (111) planes of periclase and the (0001) planes of brucite. Calculations indicate that the dissociation of water on the (001) surface is endothermic. Chemisorption on surfaces comprising atoms of lower coordination is more favourable, with the hydroxylated (111) surface being of lowest energy (Refson et al., 1995; de Leeuw et al., 1995). A proposed reaction mechanism involves the replacement of magnesium by hydrogen along (111) periclase planes. This was studied for the periclase (001) surface. Elastic recoil detection analysis (ERDA) results have been reported which suggested the formation of a 1000 ,~ layer with composition Mg(OH)2 for a sample reacted at pH 2 for 2 hours (Wogelius et al., 1995). In this paper we present ERDA and SIMS results which contradict this finding. Experimental Periclase cleavage plates were obtained commercially. These were either freshly cleaved or chemomechanically polished before use. Samples were reacted at constant pH using a Mettler DL-77 auto-titrator. Deuteration experiments used D20 (99.9%) acidified with HNO3 (10 M). ERDA used a 7.5 MeV J604+ beam. Recoiled protons were counted using a surface barrier detector covered with a kapton filter to reject scattered 160. The sample was tilted 15 ~ , and the detector 40 ~ , relative to the incident beam. The detector energy
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