An HREELS study of CO adsorption on Pd(311)

CO adsorption on Pd(311) at 100 K has been investigated by means of HREELS. The results resemble the adsorption behaviour of CO on Ni(311), suggesting that a decisive role is played by the substrate geometry. An analysis of the observed CO stretch vibrations as a function of coverage reveals that in the saturated phase, mainly twofold bridge sites are occupied. These results clearly demonstrate that the so called “frequency rule” for CO adsorption in ontop and bridge sites is still a reliable guide in cases where the vibrational frequencies lie in the “grey area” between 235 and 248 meV.