Subsurface species and compound formation

With hydrogen, penetration of the adsorbed atoms below the surface is particularly pronounced in the case of Pd because of the possibility for the formation of bulk hydride phases [39] however, it may also be relevant for hydrogen on Ni or oxygen on Pt [40]. 

The formation of bulk phases is most common in the case of oxygen. Transformation of the chemisorbed phase into oxide proceeds generally through a nucleation and growth mechanism. As an example. Fig. 2.26a shows an STM image from a Ru(0 001) surface that had been exposed to O2 at elevated temperature [41 ]. The right part is still the Ru(0 001) surface covered by a 1 x 1 O adlayer, while the left part had been transformed into a thin 

FIGURE 2.26. Structural transformation of the Ru(OOOl) surface into a Ru02(1 1 0) overlayer under the influence of oxygen (a) STM image exhibiting both phases (b) ball model of the Ru02(l 10) surface. (See color insert.) 

At elevated temperatures ( 530K), the chemisorbed O atoms are more mobile and form larger 1x1 Oad islands, but nucleation and formation of the oxide proceed along with similar mechanisms [43]. 

---