کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
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5425585 | 1395861 | 2006 | 10 صفحه PDF | دانلود رایگان |
The oxidation of the Pd(1Â 1Â 1) surface was studied by in situ XPS during heating and cooling in 0.4Â mbar O2. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430Â K, the Pd(1Â 1Â 1) surface was covered by a 2D oxide and by a supersaturated Oads layer. The supersaturated Oads layer transforms into the Pd5O4 phase upon heating and disappears completely at approximately 470Â K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655Â K, the bulk PdO phase appears and this phase decomposes completely at 815Â K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4Â mbar O2, but the Pd5O4 phase appears below 745Â K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.
Journal: Surface Science - Volume 600, Issue 15, 1 August 2006, Pages 2980-2989