Engineering Defect-Free Nanoporous Pd from Optimized Pd-Ni Precursor Alloy by Understanding Palladium-Hydrogen Interacti
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NANOPOROUS palladium (np-Pd) features vastly increased surface area vs bulk Pd,[1] on a per volume basis. Bulk Pd readily adsorbs hydrogen, and np-Pd’s large surface area strongly enhances this effect.[2] Nanoporous metals have potential applications as actuators, and their high surface-area-to-volume ratio makes them appealing candidates for catalytic structures.[3] The adsorption of hydrogen on palladium surfaces allows for consequent diffusion of hydrogen into the octahedral sites of the face-centered cubic Pd lattice.[4] At room temperature and relatively low hydrogen concentration (stoichiometric ratios of H/Pd less than 0.6), immiscible hydrides are formed.[5] Rather et al.[6] showed that Pd nanoparticles with size on the length scale of np-Pd ligaments (5 to 10 nm) are capable of hyperstoichiometric palladium hydride formation, up to PdH1:12 . Because of this high reactivity with hydrogen, palladium nanostructures have promising potential as hydrogen sensors.[7–9] In addition to hydrogen sensing, hydrogen storage presents a possible application for both np-Pd and Pd nanoparticles.[10,11] It is worth noting that the rapid ad- and desorption of H2 by Pd poses a potential difficulty for retaining H atoms within the Pd matrix, unless the diffusion of hydrogen near the Pd surface is limited by a third mechanism.[1,10]
JULIUS SCHOOP, Graduate Student, and T. JOHN BALK, Associate Professor, are with the Department of Chemical and Materials Engineering, University of Kentucky, 177 F. Paul Anderson Tower, Lexington, KY 40506-0046. Contact e-mail: [email protected] Manuscript submitted July 30, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
For this study, thin films of np-Pd were produced using the dealloying process, during which a Pd-Ni precursor alloy film was subjected to selective corrosion and removal of a sacrificial element (Ni). In order to obtain a specific nanoporous morphology, the first step is to select the proper precursor alloy.[12] An asdeposited Pd-Ni composition was identified that allows the formation of fine pores (average diameter of 7 nm) with completely interconnected ligaments. To achieve an open and stable nanoporous structure, proper dealloying conditions must be selected.[13] Moreover, the development of stress during dealloying should be well understood to reliably produce stable, crack-free porous structures. Erlebacher et al.[14] described the evolution of nanoporosity during dealloying via spinodal decomposition at the moving metal/solution interface, resulting in a sponge-like structure of the more noble metal. Previous work indicates that stress evolution during dealloying has a direct impact on the obtainable nanoporous morphology of dealloyed specimens.[15] Dealloying is an inherently electrochemical process that involves anodic dissolution of the less noble metal of the precursor alloy, as well as a cathodic reaction. In the Pd-Ni system, the cathodic reaction involves hydrogen reduction. This paper shows how the strong interaction between hydrogen and palladium affects stress evolution
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