Qualitative evidence for adsorbate-induced edge metal atom localization:: The characterization of lifted backbond degeneracy in adsorbed CO through the model adsorption of methyl isocyanide on Pd/Al2O3

Qualitative synthetic chemistry strategies are used to show that electron-deficient (i.e., unstable) edge Pd atoms can find stability through separation from the surface band structure and strong bonding to a CO adsorbate state that displays lifted backbond degeneracy. This conclusion is drawn from the transmission infrared observation of a thermally activated bonding mode of CO on Pd/Al2O3 that has characteristics inconsistent with conventional atop- or bridge-bonded CO but similarities with an "atop bent" methylisocyanide adsorbate mode, characterized on the same surface, which is known to have lifted backbond degeneracy. Analysis of qualitative observations in both the data and the literature indicates the thermally stable CO state to be atop bound to an edge atom. Since this state is populated over a wide temperature range, the Delta H for its formation must arise from the attached surface metal atom gaining stability in the process even at the expense of CO stability. Published work [J. Phys. Chem. 93, 4890 (1989); Phys. Rev. B 24, 754 (1981)] indicates that unstable edge atoms are the likely candidates for that behavior. A TLEED study [Chem. Phys. Lett, 201, 393 (1993)] that reports the identical CO adsorbate state on Pd(110) and a study reporting the ability of adsorbed CO to reconstruct the Pd(110) surface [Chem. Phys. Lett. 167, 391 (1990); Surf. Sci. 249, 1 (1991)1 are published observations that are consistent with this conclusion. (C) 2000 Academic Press.