  Catalyzing capping layers on top of metal hydrides are often employed to
enhance the hydrogenation kinetics of metal-hydride based systems such as
hydrogen sensors. Here, we experimentally study the hydrogenation kinetics of
capping layers composed of several alloys of Pd and Au as well as Pt, Ni and
Ru, all with and without an additional PTFE protection layer using a novel
method and under the same set of experimental conditions. Our results
demonstrate that doping Pd with Au results in significantly faster
hydrogenation kinetics, with response times up to five times shorter than Pd
through enhanced diffusion and a reduction of the activation energy. The
kinetics of non-Pd based materials turns out to be significantly slower and
mainly limited by the diffusion through the capping layer itself. Surprisingly,
the additional PTFE layer was only found to improve the kinetics of Pd-based
capping materials and has no significant effect on the kinetics of Pt, Ni and
Ru. Taken together, the experimental results aid in rationally choosing a
suitable capping material for the application of metal hydrides and other
materials in a green economy. In addition, the developed method can be used to
simultaneously study the hydrogenation kinetics and determine diffusion
constants in thin film materials for a wide set of experimental conditions.
