  The d-band center model of Hammer and N{\o}rskov is widely used in
understanding and predicting catalytic activity on transition metal (TM)
surfaces. Here, we demonstrate that this model is inadequate for capturing the
complete catalytic activity of the magnetically polarized TM surfaces and
propose its generalization. We validate the generalized model through
comparison of adsorption energies of the NH$_3$ molecule on the surfaces of 3d
TMs (V, Cr, Mn, Fe, Co, Ni, Cu and Zn) determined with spin-polarized density
functional theory (DFT)-based methods with the predictions of our model.
Compared to the conventional d-band model, where the nature of the
metal-adsorbate interaction is entirely determined through the energy and the
occupation of the d-band center, we emphasize that for the surfaces with high
spin polarization, the metal-adsorbate system can be stabilized through a
competition of the spin-dependent metal-adsorbate interactions.
