THE EXPERIMENTAL-EVIDENCE OF THE ROLE OF SURFACE RESTRUCTURING DURING CATALYTIC REACTIONS

Changes of surface structure of transition metals, and oxides were observed to occur on three different time scales. Chemisorption induced restructuring is localized to the proximity of the adsorption site and is likely to take place within less than 10(-6) sec. In this regime the coverage dependence of restructuring can also be detected. Surface reconstruction has also been found to occur on the catalytic turnover time scale (10(-2)-10(2) sec) and is detectable readily during oscillatory and partial oxidation reactions. Structural changes also occur at oxide-metal interfaces in this time scale under reaction conditions. Surface atom transport controlled restructuring takes place during longer time-periods, 10(2)-10(4) sec. These diffusion controlled processes can be correlated with catalyst activation and deactivation that are detectable on a similar time scale. Oxidation and reduction of catalyst particles, sulfur or alkali metal adsorption can all lead to restructuring of this type. The results of experiments are reviewed and the need for developing time resolved techniques to detect surface structural changes at high pressures and temperatures is discussed.