Coadsorption of Oxygen and Water on Ru (001): Vibrational and Structural Properties

Abstract

A thermally induced change in the vibrational properties of a coadsorbed oxygen-water overlayer on Ru(OOl) is attributed to the formation of a local 0-H 2 0 complex. An O-H stretching mode of this complex is observed in electron-energy-loss data, but not in infrared reflection-absorption data available from another laboratory. This provides the first direct experimental evidence of a case in which the surface dipolar selection rule applies in an infrared but not in an energy-loss measurement.PACS numbers: 82.65.Jv, 68.20.+t, 68.90.+g, 82.45,+z Recent studies of the coadsorption of oxygen and H 2 0 on hexagonally close-packed surfaces of Ni, Ru, and Pt have provided several very intriguing results. On Ni, electron-stimulateddesorption ion angular distribution (ESDIAD) measurements 1 indicate that adsorbed molecular water forms structures at 80 K which are azimuthally oriented with respect to the substrate atoms only if oxygen is coadsorbed. In the case of Ru, infrared reflection-absorption spectroscopy (IRAS) showed complete suppression of the infrared activity of the O-H stretching mode of adsorbed water in the presence of a critical coverage of coadsorbed oxygen. 2 On Pt, vibrational and photoelectron spectra were interpreted as evidence for H 2 0 dissociation and the formation of hydroxyl species only if oxygen is preadsorbed.' In all cases, therefore, the presence of adsorbed oxygen exerts a strong effect on the properties of water which is coadsorbed. This effect is evident in the present study also.We have used electron energy-loss spectroscopy (EELS) to study the vibrational properties of a mixed 0-H 2 0 adlayer on Ru(001). We observe that a critical coverage of oxygen can affect the intermolecular structure of the adsorbed water. The O-H stretching vibration which we observe in EELS for this structure has been shown to be IRAS inactive. 2 Therefore, different vibrational "selection rules" must be operative in the IRAS and EELS measurements in this case.The EEL spectra of water adsorbed on clean Ru(001) have been studied extensively in this laboratory, and experimental details have been reported previously. 4 Specific vibrational features have been shown to reflect the intermolecular and intramolecular structure of the adsorbed H 2 0 overlayer. The frequencies of the losses due to frustrated rotations of molecular H 2 0 depend upon the degree of local order in the adlayer. 4 Two of the other fundamental vibrations of H 2 0, the scissoring mode (v s ) and the O-H stretching mode (^OH)> are not sensitive to this type of order. However, v s can be used as evidence of nondissociative adsorption since it involves the motion of all three atoms in the H 2 0 molecule. Significant dissociation should affect strongly the intensity of this vibrational feature. The frequency of ^OH is sensitive to intermolecular hydrogen bond formation, occurring at 3630-3750 cm" 1 for isolated H 2 0 molecules, and 3000-3600 cm" 1 in ice. Figure 1 shows a series of EEL spectra following sequential adsorption of oxygen and H 2 0 on R...

Modifying the adsorption characteristics of water on Ru{0001} with preadsorbed oxygen

“…After desorption, the exact same amount of O at is present at the surface as before water was adsorbed. In agreement with earlier studies, [2][3][4]12 we find that about five water molecules are affected per oxygen atom.…”

“…5,8 In view of the debate over partial dissociation of water on Ru͕0001͖, it is imperative to reevaluate the assumptions made by previous authors who generally assumed that water stays intact when interacting with oxygen. [2][3][4] Only in a recent infrared spectroscopy study by Clay et al 12 was the possibility of water dissociation pointed out when coadsorbed with low coverages of oxygen. Using temperature-programed desorption ͑TPD͒, Doering and Madey found a significant change in the overall shape of the desorption spectra between low and high oxygen coverages with a sharp transition between 0.20 and 0.25 ML O, indicating a change in the interaction of H 2 O and O adatoms on the surface.…”

“…After more than 25 years of studying water adsorption on Ru͕0001͖, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] there is still an ongoing debate about the exact nature of the system. Feibelman's recent suggestion that the most stable adsorption structure is in fact a partially dissociated hydrogen-bonded network of H 2 O and OH ͑Ref.…”

Water-induced surface reconstruction of oxygen(2×1)covered Ru(0001)

“…Dissociation is observed at low oxygen coverage ͑ Ͻ 0.2 ML͒ while it is inhibited at larger O coverage ͑ = 0.25-0.5 ML͒ contrary to studies that assume that water remains intact when interacting with oxygen. [6][7][8][9] Preadsorbed oxygen on the ruthenium surface does not only influence the dissociation characteristics of water but also its structure. On the p͑2 ϫ 2͒ oxygen terminated surface, water adsorbs in a p͑2 ϫ 2͒ symmetry, 4,10 compared to a hexagonal arrangement ͑ͱ3 ϫ ͱ 3͒R30°observed on clean hexagonalclose-packed metal surfaces.…”

Hydrogen Transfer on Metal Surfaces

“…Very extensive and revealing studies of water interaction with the Ru(0001) surface were performed by Thiel et al [130,131] using a variety of techniques (TDS, AES, HREELS, UPS, and ESDIAD = electron-stimulated desorption ion angular distribution). The authors observed several binding states in their TD spectra which develop simultaneously with H 2 O exposure, again indicating that a first and a second water layer grow at the same time, whereby the second layer is H-bonded to the first layer.…”