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

Citation
Jm. Heitzinger et al., HYDROGEN ADSORPTION AND ABSORPTION ON ULTRATHIN PD FILMS ON TA(110), Surface science, 294(3), 1993, pp. 251-264
Citations number
38
Categorie Soggetti
Chemistry Physical
Journal title
ISSN journal
00396028
Volume
294
Issue
3
Year of publication
1993
Pages
251 - 264
Database
ISI
SICI code
0039-6028(1993)294:3<251:HAAAOU>2.0.ZU;2-G
Abstract
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.