  Hydrogen adsorption on graphene-supported metal clusters has brought much
controversy due to the complex nature of the bonding between hydrogen and metal
clusters. The bond types of hydrogen and graphene-supported Ti clusters are
experimentally and theoretically investigated. Transmission electron microscopy
shows that Ti clusters of nanometer-size are formed on graphene. Thermal
desorption spectroscopy captures three hydrogen desorption peaks from
hydrogenated graphene-supported Ti clusters. First principle calculations also
found three types of interaction: Two types of bonds with different partial
ionic character and physisorption. The physical origin for this rests on the
charge state of the Ti clusters: when Ti clusters are neutral, H2 is
dissociated, and H forms bonds with the Ti cluster. On the other hand, H2 is
adsorbed in molecular form on positively charged Ti clusters, resulting in
physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on
graphene-supported Ti clusters.
