Reduction behavior of oxidized Pd(100) and Pd_(75)Ag_(25)(100) surfaces using CO

摘要

The reduction of the (5_(1/2)× 5_(1/2)) R27° surface oxide on Pd(100) and Pd_(75)Ag_(25)( 100) surfaces by CO has been investigated by high-resolution X-ray photoelectron spectroscopy in combination with kinetic analysis using the Avrami-Erofeev theory. For both surfaces we observed faster reduction with increasing temperature. Kinetic analysis indicates that the reduction process is phase boundary controlled for Pd(100) in the temperature range investigated, from 30 ℃ to 120 ℃. On Pd_(75)Ag_(25)(100) the surface oxide reduction is significantly slower compared to Pd(100). Also in this case, a phase boundary controlled reduction of the surface oxide is observed at temperatures of 120 ℃ and above, while at T ≤ 70 ℃ the reduction is found to be diffusion limited. Density functional theory calculations show that the presence of silver in the outermost surface layer significantly increases the CO diffusion barriers on the reduced areas, supporting a diffusion limited reduction process for Pd_(75)Ag_(25)(100) at lower temperatures.