In this work, we demonstrate the finite-size effect on MCE over nickel and cobalt NPs by performing micro-magnetic simulations based on Heisenberg's classical spin Hamiltonian. Within the fine NPs below a specific size threshold, the 𝑇 ! of the catalyst is directly decreased from the bulk value, while an extra demagnetization in the outer-most shells can further reduce the working temperature to induce MCE. Combined with the temperature-dependent N-binding energy calculated from ab initio spin dynamics, we propose that the optimal reaction temperature of Cobalt NPs catalyzed ammonia synthesis can be decreased by 300 K. On the other hand, this is the in-situ reaction situation where catalysis occurs, rather than the oversimplified models in our previous simulations. We believe this work indeed bridges the gap between the bulk phase transition and interfacial chemical reaction and further provides a feasible possibility for practical applications of MCE in heterogeneous catalysis.