The chemisorption of carbon monoxide on the iridium (III) surface

Abstract

The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.