The MCE describes a distinct chemical reactivity of a ferromagnet between its FM ground state and paramagnetic (PM) state. In 1934, this interesting phenomenon was first observed by Hedvall and coworkers in the N2O decomposition on Ni , while a similar effect was also reported for CO decomposition on Ni . The reactivities of these processes have been significantly enhanced around the 𝑇 ! of Ni. In the early 1980s, Hamilton and coworkers observed a reversible atomic C segregation over Ni near its 𝑇 ! , where the measured C coverages indicate a variation in segregation heat over 0.2 eV . The detailed energetics of the process have been further investigated by density functional theory (DFT) calculations and classical spin dynamics, and the results show a stronger C-binding strength in the PM Ni . Using ammonia synthesis as a model reaction, we have recently revisited the MCE over metallic ferromagnets . According to the static DFT results, the N-binding strength undergoes a notable enhancement in the PM phase, leading to 2-4 orders of magnitude higher NH3 production rate for Ni and Co. The case of Co has been further investigated by ab initio spin dynamics, and the 55% thermally demagnetized Co is predicted to be the optimal transition metal catalyst for ammonia synthesis . However, the magnetic status mentioned above refers to a reaction temperature over 1300 K, which severely limits the application of this exciting discovery.