Y. Yun et al. / Journal of Catalysis 253 (2008) 295–302
301
ing C–C bonds on the metallic surface. Because oxygen tends
to island at low coverages on Pd surfaces, and because it can be
compressed from a low coverage p(2 × 2) phase to higher cov-
erage c(2 × 2) and reconstructed phases on Pd(100) [13,15,16],
after the water desorbs from the chemisorbed oxygen-covered
surface there is space available for the remaining C atoms to
bond to the metal surface. Carbon interacts strongly with Pd
and thus the higher activation energy for CO2 formation on the
chemisorbed oxygen-covered surface can reflect deeper adsorp-
tion wells for both C and O.
than the surface oxide. As a result of the high sticking coeffi-
cient, CO2 production peaked immediately upon exposing the
oxide clusters to propene in oxygen titration experiments, as
opposed to the surface oxide which requires reduction before
substantial reactivity is seen. Thus, for higher temperatures,
above roughly the 490 K CO2 and water desorption peaks,
the oxide clusters are more reactive for hydrocarbon oxidation
than the surface oxide because of the higher sticking coeffi-
cient and more reactive than chemisorbed oxygen on metallic
Pd because of the lower activation energy of the direct oxida-
tion pathway.
The much higher sticking coefficient and somewhat higher
reaction temperature for the bulk oxide compared to the sur-
face oxide cannot be readily explained with the data at hand.
As noted in Section 1, low energy ion scattering indicates that
the surface and bulk oxides expose similar surface densities
of Pd atoms [14], and so the lower sticking coefficient on the
surface oxide cannot be due to an inaccessibility of Pd sites
to propene on the surface oxide. As described above, oxygen
bonding strengths appear to be similar on the bulk and surface
oxides and so this cannot explain the higher reaction temper-
ature on the bulk oxide. Instead, structural effects may play
an important role in determining the reactivity. Although hy-
drocarbon oxidation over Pd catalysts is considered structure
insensitive, this reflects the fact that the structure of the oxide
formed when metallic Pd is oxidized is insensitive to the struc-
ture of the starting surface [18]. In contrast, the surface oxide on
Pd(100) has been assigned to a rumpled PdO(101) plane while
the bulk oxide is in the form of poorly ordered clusters pre-
dominantly in an (001) orientation [17,34]. For PdO, the (101)
orientation is non-polar and so this surface may b expected to
be unreactive compared to polar (001) surfaces with a high den-
sity of defect sites. We are currently investigating the structure
sensitivity of PdO-catalyzed reactions using ordered, epitaxial
PdO thin films.
Acknowledgments
The authors thank Dr. Min Li for his help in carrying out this
work. This project was supported by the Petroleum Research
Fund of the American Chemical Society through Grant num-
ber 42178-AC5. The authors also acknowledge the use of Yale
Materials Research Science and Engineering Center facilities
through NSF Grant No. DMR-0520495.
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