H- and O-induced compressive surface stress on Pt(111): Experiments and density functional theory calculations (original) (raw)
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Surface Science, 1990
The hydrogen-oxygen reaction on the Pt(ll1) surface was investigated with a slowly modulated (v I 0.5 Hz) molecular beam: oxygen atoms, adsorbed on the surface from the ambient gas, are periodically titrated by a pure D, or a mixed HZ/D, beam. The reaction is characterized by monitoring the desorption of D,O and HD. The adsorbate coverage is also continuously measured by monitoring the specular H, intensity. LEED and AES were used to characterize the clean and oxygen covered Pt(ll1) surface. LEED measurements show that the (2 X 2)0 structure observed at 400 K becomes disordered above 500 K. In the same temperature range a change in the dependence of the hydrogen sticking probability SH, on the oxygen coverage So is found: at 400 K SHI(Bo) is independent of oxygen coverage, whereas at 550 K S,*(tio) decreases with 19,. We propose that the decrease of SH2 is due to disorder in the oxygen adlayer. With this model, one can also explain the extreme sensitivity of the reaction to contamination, which we observe, and the discrepancies with respect to previously reported results.
Identification of an Adsorbed Hydroxyl Species on the Pt(111) Surface
Physical Review Letters, 1980
The electronic structure and vibrations of an adsorbed hydroxyl (OH) species are identified and characterized for the first time on a transition-metal surface. In this study of water's interaction with clean Pt(ill) in ultrahigh vacuum, water adsorbed at 100 K desorbs at 180 K without appreciable dissociation. However, in the presence of adsorbed atomic oxygen, water dissociates above 150 K to form adsorbed hydroxyl species. The O-H axis appears to be bent relative to the surface normal.
Hydrogen-induced low temperature CO displacement from the Pt(111) surface
Surface Science, 1990
A new low temperature displacement mechanism for CO on the Pt(ll1) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr Hz). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K (E,, = 10.6 kcal/mol) instead of near the desorption temperature of 400 K (Ed = 26 kcal/mol).
The Journal of Physical Chemistry B, 1999
The variation of the adsorption pseudocapacitance with temperature is used to obtain the enthalpy, entropy, and free energies of adsorption of H upd and OH ad on Pt(111) as a function of pH and nature of the anion of the supporting electrolyte. It is shown that the heat (enthalpy) of adsorption of hydrogen on Pt(111) at the electrochemical interface is essentially independent of either the pH of the electrolyte or the nature of the supporting anion. The heat of adsorption has a linear decrease with Θ Hupd , from ∼42 kJ/mol at Θ Hupd ) 0 ML to ∼24 kJ/mol at Θ Hupd ) 0.66 ML. The heat of adsorption of OH ad is more sensitive to the nature of the anion in the supporting electrolyte. This is presumably due to coadsorption of the anion and OH ad in electrolytes other than the simple alkali bases. From the isosteric heat of adsorption of OH ad in alkaline solution (ca. ∼200 kJ/mol) and the enthalpy of formation of OH • we estimated the Pt(111)-OH ad bond energy of 136 kJ/mol. This value is much smaller than the Pt-O ad bond energy at a gas-solid interface (∼350 kJ/mol). In basic solution the electrooxidation of CO proceeds at low overpotentials (<0.2 V) between the adsorbed states of CO ad and OH ad , the latter forming at low overpotentials selectively at defect sites. In acid solution, however, these sites are not active because they are blocked by specific adsorption of anions of the supporting electrolyte.
Electronic changes at the Pt(111) interface induced by the adsorption of OH species
Catalysis Today, 2013
We perform a detailed analysis of the modifications in the electronic properties when an OH radical adsorbs on Pt(1 1 1). On the basis of first principle calculations, we provide an overview of the interface at the atomic level at low coverages (1/9 of a monolayer). The electronic factors that govern the adsorption phenomenon are discussed. The interaction of the electronic states involved in the bond formation is investigated. In this context, we examine the charge redistribution and the projected density of states onto the different participating atoms and orbitals. We establish a comprehensive picture which provides a valuable guideline to understand the complicated interplay of the electronic states in the formation of bonds.
Molecular structure of the H_{2}O wetting layer on Pt(111)
Physical Review B, 2010
The molecular structure of the wetting layer of ice on Pt(111) is resolved using scanning tunneling microscopy (STM). Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140 K only a √ 37 × √ 37 R25.3 • superstructure can be established, while at 130 K also a √ 39 × √ 39 R16.1 • superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries.
The Energy of Adsorbed Hydroxyl on Pt(111) by Microcalorimetry
The Journal of Physical Chemistry C, 2011
Surface hydroxyl groups are reaction intermediates in a large number of important catalytic and electrocatalytic reactions on Pt and other late transition metal surfaces. These reactions include the combustion and partial oxidation reactions of practically any organic molecule, the water-gas shift reaction, steam reforming of oxygenates or other organic molecules, the three-way catalyst for automotive exhaust cleanup, fuel cells for generating electricity from H 2 or any other hydrogen-containing fuel, and water splitting. Platinum has been identified as the most active catalyst for many of these reactions, 1 and therefore, the enthalpy of formation of adsorbed OH on Pt surfaces is particularly important. Because of its importance, much work has been devoted to understanding the interaction between water on clean and oxygen precovered transition metal surfaces, as has been summarized recently in a review. 2 Water adsorbs intact on clean Pt(111), and desorption from the multilayer and monolayer happens at ∼150 and 165 K, respectively. In the temperature range from 130 to 185 K, water is known to react with preadsorbed oxygen adatoms on Pt(111) to produce adsorbed OH groups in a coadsorbed, hydrogen-bonded H 2 O 3 3 3 OH complex. 3À6 This complex can be stable up to 205 K, with its stability dependent on the H 2 O:OH surface stoichiometry. At ∼205 K, water desorbs from the surface, restoring the initial oxygen-covered surface. Careful measurements by Clay et al. 5 of the amount of water adsorbed per oxygen adatom over a wide range of oxygen precoverages at 163 K (where water on oxygen-free Pt(111) is no longer stable) indicate that the complex formation can be described by the following reaction: 3H 2 O g þ O ad f 2ðH 2 O 3 3 3 OHÞ ad ð1Þ
Structure and bonding of water on Pt (111)
Physical review letters, 2002
We address the adsorption of water on Pt(111) using x-ray absorption, x-ray emission, and x-ray photoelectron spectroscopy along with calculations in the framework of density functional theory. Using the direct relationship between the electronic structure and adsorbate geometry, we show that in the first layer all the molecules bind directly to the surface and to each other through the in-layer H bonds without dissociation, creating a nearly flat overlayer. The water molecules are adsorbed through alternating metal-oxygen (M-O) and metal-hydrogen (M-HO) bonds.
Adsorption of hydrogen on Pt(111) and Pt(100) surfaces and its role in the HOR
Electrochemistry Communications, 2008
Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO 4 . We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H ad ). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at À0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H ad is in fact a spectator in the HOR.
Atomistic Mechanism of Pt Extraction at Oxidized Surfaces: Insights from DFT
Electrocatalysis, 2016
In this article, we propose a novel mechanism for the atomic-level processes that lead to oxide formation and eventually Pt dissolution at an oxidized Pt(111) surface. The mechanism involves a Pt extraction step followed by the substitution of chemisorbed oxygen to the subsurface. The energy diagrams of these processes have been generated using density functional theory and were analyzed to determine the critical coverages of chemisorbed oxygen for the Pt extraction and O ads substitution steps. The Pt extraction process depends on two essential conditions: (1) the local coordination of a Pt surface atom by three chemisorbed oxygen atoms at nearestneighboring fcc adsorption sites; (2) the interaction of the buckled Pt atom with surface water molecules. Results are discussed in terms of surface charging effects caused by oxygen coverage, surface strain effects, as well the contribution from electronic interaction effects. The utility of the proposed mechanism for the understanding of Pt stability at bimetallic surfaces will be demonstrated by evaluating the energy diagram of a Cu ML /Pt(111) near-surface alloy.