INFLUENCE OF PREADSORBED OXYGEN ON ACTIVATED CHEMISORPTION OF METHANEON PD(110)

Dissociative chemisorption of methane on clean and oxygen modified Pd( 110) has been studied by using molecular beam surface scattering. The absolute dissociation probability of CH4 (S-tot) is found to increase exponentially with the incident normal energy (E(n)) of CH4 and with s urface temperature (T-s) on clean Pd(110). The kinetic isotope effect is also found; namely, S-tot of CD4 is 4 to 5 times smaller than S-tot of CH4 throughout the entire range of E(n) studied. These results are consistent with a direct dissociation mechanism. Measurements on prea dsorbed oxygen on Pd(110) show that S-tot of CH4 decreases linearly, a s oxygen coverage is increased from 0 to 0.4 ML in good agreement with the first-order Langmuir kinetics when approximately two active sites are blocked by one oxygen atom. No influence of the oxygen induced su rface reconstructions on the dissociative adsorption kinetics of CH4 i s observed.