  The d-band center descriptor based on the adsorption strength of adsorbate
has been widely used in understanding and predicting the catalytic activity in
various metal catalysts. However, its applicability is unsure for the
single-atom-anchored two-dimensional (2D) catalysts. Here, taking the hydrogen
(H) adsorption on the single-atom-anchored 2D basal plane as example, we
examine the influence of orbitals interaction on the bond strength of hydrogen
adsorption. We find that the adsorption of H is formed mainly via the
hybridization between the 1s orbital of H and the vertical dz2 orbital of
anchored atoms. The other four projected d orbitals (dxy/dx2-y2, dxz/dyz) have
no contribution to the H chemical bond. There is an explicit linear relation
between the dz2-band center and the H bond strength. The dz2-band center is
proposed as an activity descriptor for hydrogen evolution reaction (HER). We
demonstrate that the dz2-band center is valid for the single-atom active sites
on a single facet, such as the basal plane of 2D nanosheets. For the surface
with multiple facets, such as the surface of three-dimensional (3D) polyhedral
nanoparticles, the d-band center is more suitable.
