Electronic and chemical interactions between boron and carbon monoxide on Ru(0001)

The interaction between B and CO on Ru(0001) has been studied by means of thermal desorption mass spectroscopy, Fourier transform infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. Boron adatoms poison CO chemisorption approximately on a one-to-one basis. No reaction or direct bonding between B and CO was observed. The B⋅⋅⋅CO interaction is repulsive due to the electron-acceptor nature of both adsorbates. Boron adatoms modify the electronic and chemical properties of first and second nearest-neighbor metal atoms. In the presence of B, the CO desorption temperature decreases, whereas the O(1s) binding energy and CO stretching frequency increase. These trends are a consequence of (1) a reduction in 2π* back donation caused by competition for metal electrons between CO and B and (2) repulsive electrostatic interactions between the negative charges on CO and B. The infrared results indicate that metal atoms strongly affected by B are only occupied when no more unperturbed Ru sites are available on the surface, or when the high temperature of the system favors CO migration (due to entropic effects) onto these energetically less favorable sites.