CO Adsorption and Disproportionation on Smooth and Defect-RichIr(111)

CO adsorption and dissociation on"perfect"and"defect-rich"Ir(111)surfaces were studied by a combination of surface-analytical techniques, includingpolarization-dependent (PPP and SSP) sum frequency generation (SFG) vibrationalspectroscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy,X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT)calculations. CO was found to be ordered and tilted from the surface normal at highcoverage on the"perfect"surface (e.g.,theta=30 degrees at 0.70 ML), whereas it was lessordered and preferentially upright (theta=4-10 degrees) on the"defect-rich"surface forcoverages of 0.55-0.70 ML. SFG, LEED, and XPS revealed that CO adsorption atlow pressure/high temperature and high pressure/low temperature was reversible. In contrast, upon heating to similar to 600 K in near mbarCO pressure,"perfect"and even more"defect-rich"Ir(111) surfaces were irreversibly modified by carbon deposits, which, accordingto DFT, result from CO disproportionation.