Velocity components in the desorbing product CO2 in CO oxidation over Pd(100) under steady-state conditions

The velocity distributions of desorbing product CO2 were studied in the ste ady-state CO oxidation on Pd(100) by means of cross-correlation time-of-fli ght techniques. CO2 desorption is sharply collimated along the surface-norm al throughout the switching region of the rate-determining step. At O-2 pre ssures above 1x10(-3) Torr and surface temperatures below ca. 520 K, the ve locity distribution curve showed two components in the inhibited region whe re the overall reaction rate was controlled by O-2 dissociation. One compon ent showed a translational temperature of 1500-2000 K and the other equaled the surface temperature. On the other hand, only the fast component was fo und in the velocity distribution curves in the active region where the reac tion rate was controlled by CO adsorption, or in both regions below 1x10(-3 ) Torr O-2 and above 520 K. The fast component showed a remarkable surface- structure dependence of the translational energy.