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Acceleration of hydrogen absorption by palladium through surface alloying with gold

 The University of Tokyo announced on 10 July 2018, that the research group led by Prof. Katuyuki Fukutani at Institute of Industrial Science discovered that hydrogen absorption by palladium (Pd) can be accelerated over 40 times through surface alloying with gold (Au). Details were published in PNAS*.

 Hydrogen- storage is a key process in utilizing hydrogen as a carbon-free energy carrier, and the best known hydrogen absorber has been Pd. The absorption of hydrogen into Pd is composed of two sequential processes. In the first step, hydrogen molecule (H2) collides with Pd and chemically adsorbs to the surface. In the second step, adsorbed H atoms diffuse into the sub-surface and this step is the bottleneck to the overall process. 

 The group found that when the Pd surface is alloyed with Au, the absorption rate of hydrogen is accelerated over 40 times. The amount of Au is crucial and the hydrogen absorption is maximized when the Au/Pd ratio is around 0.4 in a single monolayer of Pd. Instrumental analyses showed that although the Au atoms are present only in the surface layer, hydrogen content profile is enhanced to a depth of about 10 nm. This means that Au accelerate the diffusion into the sub-surface, rather than increasing the solubility of hydrogen. Density functional theory (DFT) calculation has shown that the Au atoms increase the energy of the chemisorped hydrogen by destabilization and reduce the barrier for H atoms to diffuse into Pd.

*Kazuhiro Namba, Shohei Ogura, Satoshi Ohno, Wen Di, Koichi Kato, Markus Wilde, Ivo Pletikosić, Petar Pervan, Milorad Milun, and Katsuyuki Fukutani, "Acceleration of hydrogen absorption by palladium through surface alloying with gold", Proceedings of the National Academy of Sciences of the United States of America (PNAS), DOI:10.1073/pnas.1800412115