(376bs) Computational Studies of the Oxidation and Reduction of Pd-Ag Alloy

The Pd-Ag alloy is synthesized by deposition of Pd onto Ag(111) surface at room temperature, both Pd and Ag can be oxidized by O₂ in annealing conditions. We built models of single and double PdO(101) layers on Ag(111) as well as the AgO-p(4x4), and calculated CO adsorption and H₂ oxidation on the surface by using density functional theory (DFT). The IRRAS experiments at 150 K indicated that CO adsorbs on Pd top sites rather than Pd bridge sites, CO can also adsorb on Ag oxides at higher pressure. In our calculations, we found that CO adsorbs on Ag top sites, with a much weaker binding energy than PdO(101). These results are consistent with experiments. We also found that CO adsorption on single layer PdO(101) is weaker than on double and multiple layers of PdO, which agrees with previous studies of PdO(101) on Pd(100). The AP-XPS experiments showed that pure AgO cannot be reduced by H₂ at room temperature. However AgO in the mixed oxide can be quickly reduced in the same conditions. We calculated the reduction of the oxides by H₂. We found that H₂ activation on AgO has a high barrier, which makes the reduction reaction difficult at room temperature. On the other hand, H₂ dissociates on PdO with a lower energy barrier than AgO, but the further reduction of PdO, which leads to the formation of water, has a relatively high barrier. With these calculations of reduction mechanisms, we can explain why AgO reduction can be facilitated by PdO found in experiments. H₂ can be easily dissociated on PdO, and then migrate to AgO surface. Since the water formation barrier on AgO is lower than PdO, AgO will be reduced easily.