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 (θPd = 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 θPd = 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 θPd = 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 (θpd = 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 θPd ≈ 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 θPd = 10 and large H2 exposures.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry