CO ADSORPTION ON RH(110) COVERED WITH ORDERED (3X1) AND (2X1) NITROGEN LAYERS

Room temperature CO adsorption on Rh(110) modified with ordered (3 x 1)N and (2 x 1)N overlayers has been studied by means of XPS, TPD and LEED. The CO and N2 TPD spectra and the O 1s and N 1s XPS spectra were used for measuring the coverage and determining the coadsorption effect on the CO and N adsorption states. It has been found that CO coadsorbed with N is characterised by a O 1s binding energy at approximately 532.1 eV which is slightly higher than the value of 531.9 eV measured for on-top CO on a N-free surface. Both the adsorption rate and the adsorptive capacity of the surface towards CO are reduced by the presence of ordered N layers. The CO and N2 TPD spectra from mixed CO + N layers show an enhanced desorption rate and exhibit new low-temperature features not present in the TPD spectra when each species is adsorbed alone. LEED data have revealed that CO coadsorption at 305 K leads to substantial changes of the initial N-related (3 x 1) and (2 x 1) patterns. When CO is coadsorbed on a surface precovered with a (3 x 1) N layer the CO-induced changes involve streaking of the 1/3 N-related fractional order spots and an appearance of weak extra spots indicating formation of poorly ordered c(2 x 2) and (2 x 1)p2mg CO domains. CO coadsorption on a surface with a (2 x 1) N layer causes streaking in the [110BAR] direction with an enhanced intensity at 1/4 order positions. Stepwise annealing and partial desorption of CO from dense CO + N layers lead to structural rearrangements and formation of new composite LEED patterns due to formation of separate ordered CO and N islands. The results are interpreted considering the possible interactions between CO and N in the coadsorbed layer.