Two desorption components of product CO2 in steady-state CO oxidation on Pd(110)

The velocity and angular distributions of desorbing product CO2 were studied in steady-state CO oxidation on Pd(110) by means of cross-correlation time-of-flight techniques. CO2 desorption was always collimated along the surface-normal direction. The velocity distribution curves involved two desorption components, a fast one and a slow one. The former showed translational temperatures above 2300 K and was suddenly suppressed above a critical CO pressure at which CO retarded the reaction. On the other hand, the latter showed a Maxwellian distribution at the surface temperature and was not suddenly suppressed around the critical CO pressure, decreasing slowly at higher CO pressures. Below the critical CO pressure, the fraction of this component was around 0.3, and above it, the value abruptly increased to approximately 0.9. The slow component appears to be formed on sites which are suitable for oxygen dissociation, such as structural defects.