Electrochemical adsorption of hydrogen on mixed Pd2Pt nanostructures

In the present contribution we have focused on the electrochemical adsorption of a proton from the solution-the Volmer reaction-on a variety of systems based on bimetallic nanostructures-clusters and wires-of Pd and Pt deposited on a surface of Au(111). We have calculated the free energy surface for the electron transfer step by a combination of DFT calculations, MD simulations and the theory of electrocatalysis. We analyze in detail the interaction of the metal d band with the valence orbital of the hydrogen and its effect on the catalytic activity as well as several aspects that influence the electrode reactivity such as spatial arrangements of the nanostructures, the solvation shell and chemical factors. We found that the mixed Pd₂Pt wire interacts strongly with hydrogen, and retains an almost complete solvation shell, which is reflected in a substantially reduced activation energy for the Volmer step. Thus, Pd₂Pt wires on Au(111) are predicted to be efficient electrocatalysts for the reaction.

Theoretical study of hydrogen storage in metal hydrides

Adsorption, absorption and desorption energies and other properties of hydrogen storage in palladium and in the metal hydrides AlH₃, MgH₂, Mg(BH₄)₂, Mg(BH₄)(NH₂) and LiNH₂ were analyzed. The DFT calculations on cluster models show that, at a low concentration, the hydrogen atom remains adsorbed in a stable state near the palladium surface. By increasing the hydrogen concentration, the tetrahedral and the octahedral sites are sequentially occupied. In the α phase the tetrahedral site releases hydrogen more easily than at the octahedral sites, but the opposite occurs in the β phase. Among the hydrides, Mg(BH₄)₂ shows the highest values for both absorption and desorption energies. The absorption energy of LiNH₂ is higher than that of the palladium, but its desorption energy is too high, a recurrent problem of the materials that have been considered for hydrogen storage. The release of hydrogen, however, can be favored by using transition metals in the material structure, as demonstrated here by doping MgH₂ with 3d and 4d-transition metals to reduce the hydrogen atomic charge and the desorption energy.

Optical readout of hydrogen storage in films of Au and Pd

For hydrogen sensor and storage applications, films of Au and Pd were (i) co-sputtered at different rates or (ii) deposited in a sequentially alternating fashion to create a layered structure on a cover glass. Peculiarities of hydrogen uptake and release were optically monitored using 1.3 μm wavelength light. Increase of optical transmission was observed for hydrogenated Pd-rich films of 10-30 nm thickness. Up to a three times slower hydrogen release took place as compared with the hydrogen uptake. Compositional ratio of Au:Pd and thermal treatment of films provided control over the optical extinction changes and hydrogen uptake/release time constants. Higher uptake and release rates were observed in the annealed Au:Pd films as compared to those deposited at room temperature and were faster for the Auricher films. Three main parameters relevant for sensors: sensitivity, selectivity, stability (reproducibility) are discussed together with the hydrogenation mechanism in Au:Pd alloys.

How the hydrogen sorption properties of palladium are modified through interaction with iridium

The instability of hydrogen sorption properties in Ir (positive sorption energies) leads to the destabilization of hydrogen sorption in a Pd layer interacting with Ir.