Hydrogen adsorption on PdGa(100), (111) and () surfaces: A DFT study

Hydrogen adsorption on PdGa intermetallic compound is analyzed using density functional theory (DFT) calculations. Changes in the electronic structure of PdGa(100), (111) and ( 1 ¯ 1 ¯ 1 ¯ ) surfaces and H bonding after adsorption are addressed. H interacts with Pd atoms with a tilted geometry on (100) and (111) surfaces. On the ( 1 ¯ 1 ¯ 1 ¯ ) surface two possible forms for H adsorption are detected: one is observed atop perpendicular to the surface and the other one is subsuperficial, both with similar adsorption energies. The Ga–H interaction is energetically less stable and is only present on the (100) plane. Pd–Pd bond strength decreases up to 53.8% as the new Pd–H bond is formed. The Pd–H bond length differs less than 1%, compared to the gas phase value for the (100), (111) and atop ( 1 ¯ 1 ¯ 1 ¯ ) . However in the ( 1 ¯ 1 ¯ 1 ¯ ) subsurface-H bond length is about 2.17 Å. The effect of H is limited to its first Pd neighbor. Analysis of orbital interaction reveals that the Pd–H bond mainly involves s–s and s–p orbitals with lower participation of Pd 4d orbitals. The computed H vibration frequencies after adsorption show values of 1786, 1289 and 633.5 cm−1 that correspond to top, bridge and hollow sites respectively.