The flexible surface or the rigid surface?

In this paper we discuss aspects of the concept described by Somorjai as th e "flexible surface", and whether some surfaces can be considered to be inf lexible, or rigid. We present STM results which appear to manifest both typ es of behaviour for surfaces. depending on their oxidation state. Copper me tal surfaces can be classed as flexible, showing facile reconstruction in t he presence of oxygen, whereas an oxidised Pd(110) surface shows no apparen t diffusivity, even at 500 K. We go on to show data for a bulk oxide which indicates that sub-stoichiometry in the sample induces an element of flexib ility in the surface. especially during reaction with oxygen. Finally, this is related to the direct observation of spillover on model catalysts of Pd nanoparticles supported on TiO2. It must be recognised that flexibility re lates to surface diffusivity and hence length- and time-scales. Surfaces wh ich appear inflexible at short times may be flexible at long times. In rela tion to catalysis then. surface flexibility depends on the relationship bet ween the time-scale of diffusive events on the surface and the catalytic tu rnover number.