  Spin plays a key role in physical and chemical reactions, such as oxygen
evolution and hydrogen evolution reactions (OER/HER); but the spin-activity
correlation has remained unclear. Based on a transition metal (TM)-doped PtN2
monolayer model with a well-defined spin center as adsorption site, we here
reveal that only active spin state can enhance the strength of hydrogen
adsorption, while inert spin state offers very little influence. Specifically,
the unpaired electron along the out-of-plane direction such as in dZ2 orbital,
acting as an active spin state, will strongly hybridize with hydrogen,
resulting in enhanced hydrogen binding energy because dZ2 orbital is just
enough to accommodate two electrons to form a bonding orbital. While the
in-plane unpaired electron such as in dX2-Y2 orbital, plays a negligible role
in adsorbing hydrogen atom. This is verified by a series of single atom
catalysts comprising of PtN2 monolayer by replacing Pt atom with a TM (Fe, Co,
Ni, Ru, Rh, Pd, Os, or Ir) atom, or subsequent adsorbing a Cl atom. One of the
most promising materials is Pd@PtN2-Cl that offers superior HER activity, even
better than pure Pt. This work uncovers the nature of spin-activity
correlation, thus paving the way for the design of high-performance catalysts
through spin-engineering.
