Understanding the magnetocatalytic effect: Magnetism as a driving force for surface segregation

The magnetocatalytic (or Hedvall) effect refers to a change in the rate of a chemical reaction on a magnetic surface at the Curie point T-C. For Ni catalysts, experiments suggest the effect is related to a sudden increase in segregated surface C, a strong catalytic poison, at temperatures below T-C. However, the connection between magnetism and surface segregation is not understood. Using density functional theory and spin-dynamics simulations, we show that the solubility of C in Ni is significantly reduced in the ferromagnetic state, because C suppresses Ni magnetism and thereby increases the heat of solution. This explains the observed increase in C segregation and the reduced catalytic activity below T-C.