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Posted: April 25, 2008
Caught in a trap
(Nanowerk News) The environmental impact of the use of hydrocarbons as fuels has led to a global search for cleaner energy sources. Hydrogen offers a greener alternative for transportation fuels, but a critical issue is the requirement of a safe and reliable hydrogen storage medium. Nanoparticles have advantages over bulk materials for hydrogen storage applications: they have a larger solid/gas interface area and shorter hydrogen diffusion paths, yielding potentially faster kinetics for gas absorption and desorption.
In two recent communications published in the Journal of the American Chemical Society, Masaki Takata from the SPring-8 Centre, Harima, and his colleagues, including Hiroshi Kitagawa from Kyushu University, explore the hydrogen absorption and desorption behavior of palladium nanoparticles and of palladium core–platinum shell nanoparticles.
In the first communication ("Hydrogen Absorption in the Core/Shell Interface of Pd/Pt Nanoparticles"), the researchers address whether core–shell nanoparticles made of two metals store hydrogen. The team prepared structures with crystalline palladium cores of 6 nm diameter and crystalline platinum shells of thickness around 2 nm, and then characterized them using a variety of techniques.
Pressure–composition isotherms showed that the core–shell nanoparticles absorbed the same amount of hydrogen as homogenous palladium nanoparticles. Takata, Kitagawa and colleagues then performed solid state nuclear magnetic resonance (NMR) measurements with deuterium, a hydrogen isotope, to identify the absorption site of hydrogen. Surprisingly, they have found that while deuterium was dispersed in both palladium and platinum lattices, it was concentrated in the boundary region between the core and the shell.
A schematic depiction of hydrogen storage of palladium (Pd) and platiunum (Pt) nanoparticles (green, hydrogen; red, Pd; blue Pt). (IMage: RIKEN)
Palladium nanoparticles do not demonstrate complete reversibility in their hydrogen uptake and release, in contrast to their bulk counterparts. Takata, Kitagawa and colleagues explored this hysteresis in their second communication ("On the Nature of Strong Hydrogen Atom Trapping Inside Pd Nanoparticles"). Using x-ray diffraction, they have found that the lattice constant of palladium nanoparticles of 6 nm diameter increases with exposure to increased hydrogen pressures. However, on evacuation of the hydrogen, the lattice does not return to its original value; it remains slightly larger.
Then, again using solid state NMR measurements with deuterium, the researchers have found that some deuterium atoms remained within the palladium lattice after evacuation of ‘free’ deuterium from the system. They suggest that hydrogen atoms are trapped firmly within the lattice, which expands the crystal lattice, and hence lattice constant, of palladium. This, they say, explains why hydrogen absorption in these materials is not completely reversible.
The researchers conclude that their work provides a new understanding of the interactions between hydrogen and ‘nano-structured’ solids, and could contribute to the development of practical hydrogen-storage materials.