  Photocatalytic nitrogen reduction is the promising way for ammonia
production, the question now is that the search of highly active and low active
catalysts. Based on the first-principles calculation, single boron atom is
anchored on the g-C$_2$N to form B/g-C$_2$N, the results show that B/g-C$_2$N
can serve as a potential photocatalyst for N$_2$ fixation. The introduction of
B atom to g-C$_2$N, the energy gap will reduce from 2.45 eV to 1.21 eV, and
also show strong absorption in the visible light region. In addition, N$_2$ can
be efficiently reduced on B/g-C$_2$N through the enzymatic mechanism with low
onset potential of 0.07 V and rate-determing barrier of 0.50 eV. The
"acceptance-donation" interaction between B/g-C$_2$N and N$_2$ plays a key role
to active N$_2$, the BN$_2$ moiety of B/g-C$_2$N acts as active and
transportation center. And the activity originates from the strong interaction
between 1$\pi$1$\pi$* orbitals of N$_2$ and molecular orbitals of B/g-C$_2$N,
the ionization of 1$\pi$ orbital and the filling of 1$\pi$* orbital can
increase the N$\equiv$N bond length greatly, making the activation of N$_2$.
Overall, this work demonstrates B/g-C$_2$N is a promising photocatalyst for
N$_2$ fixation.
