Is the preadsorbed sulfur atom always acting as a poison for the surface reaction?

We have systematically studied the water dissociation on clean, oxygen preadsorbed and sulfur preadsorbed Mo(100), Cu(111), and Au(111) by the DFT-GGA method. The total energy change and the activation energy have been calculated for the direct, the oxygen-assisted, and the sulfur-assisted cleavage of the O-H bonds of water, respectively. The results show that the activation energy on the oxygen preadsorbed Mo(100) is larger than that on the clean Mo(100), and on the sulfur preadsorbed Mo(100) the activation energy is the largest one, which indicates that these preadsorbed atoms are inhibiting the water dissociation on the active metal surface. However, the presence of the preadsorbed oxygen dramatically decreases the value of activation energy on Cu(Ill), while the presence of the preadsorbed sulfur increases the value of activation energy. More interestingly, the activation energy of the water dissociation on both of the oxygen preadsorbed and the sulfur preadsorbed Au(111) decreases sharply, which indicates that the preadsorbed sulfur is not always acting as a poison, but as an activator at some cases, especially at An, the noblest metal surface.