The effect of atomic steps on adsorption and desorption of CO on Ru(1 0 9)

The chemisorption and dissociation of CO has been studied on an atomically-stepped single crystal Ru(1 0 9) surface using several methods. CO chemisorbs on Ru(1 0 9) by means of a mobile precursor mechanism with an initial sticking coefficient of ∼0.85 at 88 K. The only molecular adsorbate observed is CO in an on-top configuration, which exhibits a single IR absorption band shifting from 1973 cm−1 at the lowest coverages to 2063 cm−1 at the saturation coverage. The individual IR features from step- and terrace-bound CO have not been resolved. The formation of the √3×√3 R30° CO superstructure has been detected at intermediate coverages. CO has been found to dissociate at the step Ru sites, and then to recombine near 530 K, to produce desorbing CO. The involvement of the step sites was demonstrated by poisoning them with carbon. These processes are not observed on the atomically-smooth Ru(0 0 1) surface. The combination of vibrational spectroscopic and thermal desorption measurements indicates that in the range 450–500 K, active carbon species may be produced from the dissociation of CO if step defect sites are present. The step sites on Ru(1 0 9) have been found to be two atoms in height as a result of a surface reconstruction producing step height doubling (Ru[9(0 0 1)×2(1 0 1)] surface structure) which occurs upon annealing the clean crystal in ultrahigh vacuum.