We have investigated the potential of metal-organic frameworks for immobilising single atoms of transition metals using a model system of Pd supported on NH₂-MIL-101(Cr). Our transmission electron microscopy and in situ Raman spectroscopy results give evidence for the first time that functionalised metal-organic frameworks may support, isolate and stabilise single atoms of palladium. Using thermal desorption spectroscopy we were able to evaluate the proportion of single Pd atoms. Furthermore, in a combined theoretical-experimental approach, we show that the H-H bonds in a H₂ molecule elongate by over 15% through the formation of a complex with single atoms of Pd. Such deformation would affect any hydrogenation reaction and thus the single atoms supported on metal-organic frameworks may become promising single atom catalysts in future.

Mg-Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH ₄)₂ is already formed from the elements at p(H2)=1 bar and T=100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg-Al with Ti has a negative influence on hydrogen absorption.

The catalytic activity of niobium oxide for hydrogen absorption is determined using an optical indicator technique to measure hydrogen sorption kinetics. The catalytic activity of NbO_x is studied as a function of the oxygen concentration determined from electron spectroscopy. STM measurements display a clustered NbO_x surface, disproving a diffusion-controlled kinetics. The catalytic effect results from dissociation of hydrogen by niobium oxide. The measured activation energy depends strongly on the oxygen concentration and reaches a minimum of 0.6 eV at the highest oxygen content (x=2.5, i.e., Nb₂O₅). Pure Nb does not display a catalytic effect. Oxide surfaces with a high concentration of artificial oxygen vacancies are less catalytically active.