Anisotropy in the Adsorption ofH2O at Low Coordination Sites on Pt(111)

Abstract

The H 2 O adsorption on Pt(111) at 140 K is investigated by temperature-variable scanning tunneling microscopy. H 2 O adsorbs preferentially at the upper side of step edges. At these low coordination sites the adsorbates are bound stable as quasi-one-dimensional chains up to 160 K. In contrast, the desorption from the two-dimensional H 2 O islands on the terrace has already started at 145 K. The occupancy of the sites at the upper side of step edges is different at the two types of dense packed steps. This adsorption anisotropy is correlated with the different dipole moment of the two types of steps. [S0031-9007 (96)00492-9] PACS numbers: 68.45.Da, 81.10.Aj, 82.65.My The adsorption of H 2 O on metal surfaces was intensively studied in the past [1]. Different investigations of the system H 2 O͞Pt(111) [2][3][4][5][6] show that water adsorbs via island formation [3,5]. The desorption peak temperatures in thermal desorption spectroscopy (TDS) experiments were determined to be 165 and 175 K for the multilayer and the so-called bilayer, respectively [3,4]. First principle calculations of the interaction of single H 2 O molecules with a Pt 10 cluster have shown that the binding between Pt and H 2 O is mainly caused by the mixing of unoccupied Pt 5d states with the occupied lone pair orbitals of the H 2 O molecule. This results in an energy gain of about 0.5 eV. The adsorption site was found to be on top. The reason is that the Pt charge density on top is lower, which reduces the repulsive part of the interaction [7].In this work we present the first scanning tunneling microscopy (STM) study of water adsorption on a metal surface, which shows that H 2 O adsorbs preferentially at the low coordination sites at the upper side of step edges. This appears to be the adsorption site of highest thermal stability. Surprisingly, the H 2 O occupancy of these sites is different for the two types of dense packed steps.The experiments were performed in an UHV-STM apparatus described elsewhere [8]. The background pressure was below 1 3 10 210 mbar. The sample was prepared by repeated cycles of Ar 1 bombardment, oxygen exposure, and subsequent flashing to 1000 ± C. This procedure results in a clean, well-ordered surface as checked by low energy electron diffraction Auger electron spectroscopy and the STM. Water of milli-Q quality with an electrical resistivity of 10 7 Vcm was used for the exposure. Just before water exposure the sample was flashed to 500 ± C to desorb all adsorbates from the surface. The water adsorption has been performed by filling the chamber with a H 2 O pressure of 5 3 10 29 1.1 3 10 28 mbar and keeping the sample temperature at 140 K. The gas inlet was routinely checked by a mass spectrometer with automatic background subtraction. The only peak not related to H 2 O, which has an intensity of more than 0.1% of the H 2 O peak, was M 28 with an intensity corresponding to somewhat less than 1% of H 2 O. We assume that this is due to residual N 2 solved in the water reservoir. However, N 2 does not adsorb on Pt(111) d...

Water Molecular Adsorption on the Low-Index Pt Surface: A Density Functional Study

“…Using a temperature programmed desorption (TPD) and spectroscopic methods (XPS, UPS), Sexton and Hughes, obtained the adsorption energy of water on Pt(111) is 0.412 eV [48]. However, for the case of (111) surface index, our result of the atop position as the most preferential adsorption site is in agreement with STM results by Morgenstern et al [23,49]. The weakest interaction strength of H2O/Pt(111) compared to Pt(100) and Pt(110) indicates the most unflavored water dissociation at the Pt(111) surface.…”

“…While the mechanism of adsorption of methanol and its derived fragments on Pt and Pt alloy surfaces was explained in our earlier work [17][18][19][20], we now come to H2O-Pt systems. In this case, the existence of H2O molecules on Pt surfaces was confirmed by several experiments using different techniques, such as atom superposition and electron delocalization (ASED) [21], low energy electron diffraction (LEED) [22], or scanning tunneling microscopy (STM) [23]. Theoretical studies have also extended the investigation of water at various Pt-surface models in order to obtain the fundamental correlation between surface geometry and electronic structure and reactivity [24][25][26][27].…”

Disentangling Catalytic Activity at Terrace and Step Sites on Selectively Ru-Modified Well-Ordered Pt Surfaces Probed by CO Electro-oxidation

“…Our data neither rule out nor corroborate the hypothesis that PtRu interfaces are the most catalytic ones, but they convincingly show that PtRusteps and remaining Ptsteps are little catalytic active for CO oxidation ( Figures 5 and 7 Figures 6B and 7B). Indeed, from solid/gas interface studies, [72][73][74] it is well known that water adsorption and its dissociation preferentially occur in the upper parts of step edges compared to closepacked domains. In this respect, once the adsorbates strongly adsorb at the steps, this could at least partially explain the higher over-potentials required for the reaction Ptκ Figure 4 evidence that after partial oxidation of a CO adlayer, on a Ru partially modified Pt step, the sites released in the first voltammetric cycles apparently were not reoccupied by the remaining CO molecules on surface.…”

Desorption of H2O from Flat and Stepped Pt(111)

“…11 There is evidence that water molecules preferentially bind to steps on Pt(111). On one hand, STM measurements 20 show that annealing (2 min at 160 K) of a monolayer desorbs all water except chains along both types of densely packed steps (steps in the 〈11 j 0〉 direction with either (100) microfacets (also called A-steps) or (111) microfacets (B-steps), see Figure 1). Density functional theory (DFT) calculations indicate that water molecules form one-dimensional, hydrogen bonded chains along step edges.…”

“…23 A few earlier studies reported additional signals in the TDS for higher temperatures; namely, a peak/shoulder at T ≈ 198 K, 24,25 which has been tentatively assigned to desorption from step edges. 20 This issue was clarified by a study on a vicinal Pt(335) surface, which clearly revealed a peak at T ) 198 K due to molecules bound to step edges in addition to the multilayer (T ) 160 K) and monolayer peak (T ) 185 K). 21 A further source for additional high temperature desorption peaks is the presence of OH, which tends to stabilized, mixed OH-H 2 O overlayer on Pt(111).…”