Vibrational states of a H monolayer on the Pt(111) surface

Abstract

We present high-resolution electron energy-loss data and theoretical modeling for the vibrational properties of an atomic monolayer of H ͑D͒ on the Pt͑111͒ surface. Experimentally we find three loss peaks, in contrast with two peaks visible in the low-coverage case. A three-dimensional adiabatic potential-energy surface at full coverage of hydrogen is obtained through first-principles calculations. When the zero-point energy effects are included, the minimum energy adsorption site is found to be the fcc site just as in the low-coverage case. Vibrational band states for motion in this potential-energy surface are computed and the excited states associated with the observed loss peaks identified.

Revisiting H/Pt(111) by a combined experimental study of the H-D exchange reaction and first-principles calculations

“…These theoretical results are consistent with previous experimental results [2,3,9,11,13,16,19] which indicated that only one type of H a (H a on fcc sites) exists on the Pt(111) surface. The adsorption energy of H a on the fcc sites of Pt(111) decreases by 0.14 eV/atom when the surface coverage is increased from 0.25 ML to 1.0 ML, which clearly shows that strong repulsive interactions exist within H a layers on Pt(111) at high coverages.…”

“…Two similar desorption peaks have been also observed in previous investigations [2,17,22], and attributed to repulsive H···H interactions between the dissociatively adsorbed H atoms on Pt(111) at 110 K. The slight differences in desorption temperature and relative peak intensity between our results and those in the literature may be due to different adsorption temperatures and heating rates. Considering that the adsorption temperature is around 110 K in our TDS measurements, we assume [2] that the saturation coverage of H a on Pt(111) is 1 ML. The desorption activation energy of the  2 peak was determined by employing the leading edge analysis method [17].…”

“…This interpretation has been partly supported by other experimental results [2,3,9,11,13,16,19], which also indicate that the dissociative adsorption of H 2 on Pt(111) gives rise to only one type of H a . In order to explore the possibility of other types of H a , we exposed Pt(111) to H(g) at 110 K, and the resulting TDS spectra are shown in Figure 1(b).…”

“…By calibrating the TDS data with those reported by Norton et al [18], Bădescu et al [2] concluded that the saturation coverage of H a on Pt(111) below 100 K could reach 1 ML. Interestingly, Greenlief et al [17] proposed the preferential accommodation of H a into the near-surface region of Pt(111) during the course of the H 2 (g) + D a reaction at temperatures between 120 and 250 K. The first high-resolution electron energy loss spectroscopy (HREELS) study of H/Pt(111) showed that H is adsorbed at the three-fold hollow sites [19], and this conclusion has been supported by other experimental results [2,3,9,11,13,16,29]. Furthermore, by using low-energy recoil scattering (LERS), Koeleman et al showed unambiguously that H occupies face-centered cubic (fcc) type adsorption sites on Pt(111) terraces [16,29]; this was later confirmed by Umezawa et al [11].…”

“…More recent theoretical calculations [4][5][6][7][8] found that adsorption of H on an fcc site was only slightly more favorable than adsorption at other sites at low coverage ( (H) = 0.25 ML). Bădescu et al [2,3] showed that protonic band structure calculations derived from a full 3-D adiabatic potential energy surface (APES) could explain the observed loss peaks in HREELS of H/Pt(111) at low coverages (  (H)  0.75 ML).…”