  The potential of graphene for hydrogen storage, coupled with the established
role of Platinum as a catalyst for the hydrogen evolution reaction and the
spillover effect, makes Pt-functionalized graphene a promising candidate for
near-ambient hydrogen storage. This paper focuses on examining the process of
Pt cluster formation on epitaxial graphene and assesses the suitability of the
system as hydrogen storage material. Scanning tunneling microscopy unveils two
primary pathways for Pt cluster growth. In the initial phase, up to ~1 ML of Pt
coverage, Pt tends to randomly disperse and cover the graphene surface, while
the cluster height remains essentially unchanged. Beyond a coverage of 3 ML,
the nucleation of new layers on existing clusters becomes predominant. Then,
the clusters mainly grow in height. Thermal desorption spectroscopy on
hydrogenated Pt-decorated graphene reveals the presence of multiple hydrogen
adsorption mechanisms, manifested as two Gaussian peaks superimposed on a
linearly increasing background. We attribute the first peak at 150{\deg}C to
hydrogen physisorbed on the surface of Pt clusters. The second peak at
430{\deg}C is attributed to chemisorption of hydrogen on the surface of the
clusters, while the linearly increasing background is assigned to hydrogen
bonded in the bulk of the Pt clusters. These measurements demonstrate the
ability of Pt-functionalized graphene to store molecular hydrogen at
temperatures that are high enough for stable hydrogen binding at room
temperature.
