Hydrogen adsorption and absorption on ultrathin Pd films on Ta(110)

The H₂ adsorption and absorption properties of monolayer and ultrathin Pd films deposited on Ta(110) were studied with AES, LEED, and temperature programmed desorption (TPD). The interaction of H₂ with the fcc (111) Pd monolayer (θ_Pd = 1) at 100 K is characterized by an initially high value of the H₂ dissociative sticking coefficient, S ~ 0.6, that decreases rapidly to ~ 0.04 with increasing H₂ uptake. Only a very small amount (< 1% of a monolayer) of hydrogen desorbs from chemisorption sites on the θ_Pd = 1 film although there is substantial desorption of hydrogen from bulk absorption sites. We also observed desorption at 150 K from a near-surface hydride or interface state from the fcc (111) Pd monolayer. The absence of any appreciable amount of desorption from a Pd-H chemisorption state on the Pd monolayer is due to destabilization of the Pd-H bond and diffusion of hydrogen into Ta to populate energetically more favorable sites. Increasing the temperature of the Pd monolayer to 500 K caused S to increase to only 0.1 for large H₂ exposures. We attribute this relatively small value of S to the population of hydrogen absorption sites just below the Ta surface which, in effect, create a barrier to further H₂ uptake. LEED observations following H₂ exposures on the θ_Pd = 1 film showed that hydrogen, unlike CO, had no effect on the observed LEED pattern, i.e., no large change in the geometric structure of the Pd film occurred. The H₂ chemisorption properties of the pseudomorphic bcc (110) Pd monolayer (θ_pd = 0.7-0.8) were almost identical to the fcc (111) Pd monolayer with only a slightly smaller value of S and no desorption peak at ~ 150 K. As the Pd film thickness was increased beyond one monolayer, the H₂ sticking coefficient and desorption from the H-Pd chemisorption state increased. If adsorption experiments were carried out on these thicker Pd films at 100 K, S reached a value of only 0.23 for large H₂ exposures even for films as thick as θ_Pd ≈ 50. We propose that slow diffusion into the bulk at this low temperature limits the uptake rate. Increasing the temperature of thick Pd films to 500 K caused a large increase in the H₂ dissociative sticking coefficient to S ≈ 0.4 for θ_Pd = 10 and large H₂ exposures.