INSITU FT-IRAS STUDY OF THE CO OXIDATION REACTION OVER RU(001) .2. COADSORPTION OF CARBON-MONOXIDE AND OXYGEN

Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), Thermal Desorption Mass Spectrometry (TDMS), and Low-Energy Electron Diffraction (LEED), we have investigated the coadsorption of CO and O on a Ru(001) surface under UHV conditions. Preadsorption of oxygen is found to weaken the adsorption energy of CO from 38 kcal/mol (clean Ru) to < 11 kcal/mol (theta-O --> 1). Co adsorption on the O-(1 x 1) monolayer is suppressed completely even at 85 K. Vibrational data show in agreement with earlier work that this reduction in the adsorption energy results from a reduction of back-donation from the metal substrate into the 2-pi-*-orbital of CO. Vibrational and thermal desorption data show the existence of distinctly different CO adsorption states on the O-(2 x 2) and O-(2 x 2) surfaces, where CO is found to order in linear on-top sites with identical (2 x 2) structures, but with different numbers of oxygen neighbors surrounding each CO molecule. Infrared lineshapes reveal a high degree of CO ordering on the O-(2 x 2) surface in contrast to the O-(2 x 1) surface which appears to be less well ordered. At higher CO coverage in both cases a compressed CO layer is observed with drastically reduced adsorption energy and the additional occupation of bridging adsorption sites. At intermediate oxygen coverages mixed phases of clean Ru, O-(2 x 2) and O-(2 x 1) are observed. Vibrational coupling observed at higher CO coverage, however, suggests rather small domain sizes.