HYDROGEN ADSORPTION AND ABSORPTION ON ULTRATHIN PD FILMS ON TA(110)

The H-2 adsorption and absorption properties of monolayer and ultrathi n Pd films deposited on Ta(110) were studied with AES, LEED, and tempe rature programmed desorption (TPD). The interaction of H-2 with the fc c (111) Pd monolayer (theta(Pd) = 1) at 100 K is characterized by an i nitially high value of the H-2 dissociative sticking coefficient, S al most-equal-to 0.6, that decreases rapidly to approximately 0.04 with i ncreasing H-2 uptake. Only a very small amount (< 1% of a monolayer) o f hydrogen desorbs from chemisorption sites on the theta(Pd) = 1 film although there is substantial desorption of hydrogen from bulk absorpt ion sites. We also observed desorption at 150 K from a near-surface hy dride or interface state from the fcc (111) Pd monolayer. The absence of any appreciable amount of desorption from a Pd-H chemisorption stat e on the Pd monolayer is due to destabilization of the Pd-H bond and d iffusion 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 H-2 exposures. We attribute this r elatively small value of S to the population of hydrogen absorption si tes just below the Ta surface which, in effect, create a barrier to fu rther H-2 uptake. LEED observations following H-2 exposures on the the ta(Pd) = 1 film showed that hydrogen, unlike CO, had no effect on the observed LEED pattern, i.e., no large change in the geometric structur e of the Pd film occurred. The H-2 chemisorption properties of the pse udomorphic bcc (110) Pd monolayer (theta(Pd) = 0.7-0.8) were almost id entical to the fcc (111) Pd monolayer with only a slightly smaller val ue of S and no desorption peak at approximately 150 K. As the Pd film thickness was increased beyond one monolayer, the H-2 sticking coeffic ient and desorption from the H-Pd chemisorption state increased. If ad sorption experiments were carried out on these thicker Pd films at 100 K, S reached a value of only 0.23 for large H-2 exposures even for fi lms as thick as theta(Pd) almost-equal-to 50. We propose that slow dif fusion into the bulk at this low temperature limits the uptake rate. I ncreasing the temperature of thick Pd films to 500 K caused a large in crease in the H-2 dissociative sticking coefficient to S almost-equal- to 0.4 for theta(Pd) = 10 and large H-2 exposures.