483504.fig.009
Figure 9: A lattice model with a few Ni atoms for the circle reaction processes of hydrogen. , and are the abbreviations for a hcp hollow site, a fcc hollow site, a subsurface octahedral vacancy, and a subsurface tetrahedral vacancy, respectively. The black lines stand for the directions of the reaction paths. The atoms of , and Ni3 colored in orange are specially used to determine the reaction coordinate of the process .