Identification of Hydroxyl on Ni(111)

Abstract

Hydroxyl (OH) is identified and characterized on the Ni(111) surface by high-resolution electron energy loss spectroscopy. We find clear evidence of stretching, bending, and translational modes that differ significantly from modes observed for H(2)O and O on Ni(111). Hydroxyl may be produced from water by two different methods. Annealing of water co-adsorbed with atomic oxygen at 85 K to above 170 K leads to the formation of OH with simultaneous desorption of excess water. Pure water layers treated in the same fashion show no dissociation. However, the exposure of pure water to 20 eV electrons at temperatures below 120 K produces OH in the presence of adsorbed H(2)O. In combination with temperature-programmed desorption studies, we show that the OH groups recombine between 180 and 240 K to form O and immediately desorbing H(2)O. The lack of influence of co-adsorbed H(2)O at 85 K on the O-H stretching mode indicates that OH does not participate in a hydrogen-bonding network.

Adsorbed Hydroxyl and Water on Ni(111): Heats of Formation by Calorimetry

“…As the literature has shown, D 2 O molecularly adsorbs on Oprecovered Ni(111) at 100 K. 7,18,25 Figure 2 shows that the integral heat at 100 K is 56.1 kJ/mol at 0.5 ML of adsorbed D 2 O for the Ni(111) surface precovered with 0.25 ML O, which is 2.5 kJ/mol higher than that of D 2 O on clean Ni(111). This is in line with the literature; that is, the attractive force between adsorbed D 2 O and O makes the coadsorbed molecules more stable on O-precovered Ni(111).…”

“…19 For O-precovered Ni(111), water molecularly adsorbs on the surface below ∼150 K but desorbs at higher temperature compared to that for clean Ni(111). 7 Figure 2 an attractive force between adsorbed water molecules at low coverage. From 0.3 to 0.5 ML, the heat slightly decreases again but remains almost constant from 0 to ∼0.5 ML.…”

“…20 We can compare the measured monolayer and multilayer heats of D 2 O on Ni(111) to the previous TPD studies (the maximum desorption peaks for the monolayer and multilayer are at 160 and 170 K, respectively). 7,8 To do this, the prefactors for D 2 O desorption are estimated to be 3.05 × 10 14 s −1 at 170 K (monolayer desorption) and 2.87 × 10 14 s −1 at 160 K (multilayer desorption), respectively, following the entropy method developed by Campbell and Sellers 21,22 using the standard gas-phase entropy of D 2 O of 181.26 J/(mol K) at 170 K and 178.89 J/(mol K) at 160 K. 23,24 The activation energies are estimated to be 46.4 kJ/mol for the multilayer desorbed at 160 K and 49.4 kJ/mol for the monolayer desorbed at 170 K using the simple first-order Redhead analysis. 21 In comparison to the calorimetric heats, 1 / 2 RT has to be added to the TPD values, giving 47.1 kJ/mol as the multilayer heat and 50.1 kJ/ mol as the monolayer heat, showing a quite good agreement with our calorimetric heat values (46.9 and 53.6 kJ/mol, respectively).…”

“…The adsorption of D 2 O on clean Ni(111) and oxygenprecovered Ni(111) were previously studied by temperatureprogrammed desorption (TPD) and high-resolution electron energy loss (HREEL) spectroscopy. 7,8 Adsorbed hydroxyl was identified as the product of heating D 2 O adlayers on oxygenprecovered Ni(111) to 170 K. 7 In our experiments, a molecular beam of D 2 O was introduced onto a clean Ni(111) surface and an O-precovered Ni(111) surface at different temperatures. Single crystal adsorption calorimetry (SCAC) was employed to directly measure the heats of D 2 O adsorbed onto the surfaces, molecularly and dissociatively, allowing us for the first time to extract the enthalpy of formation and the bond enthalpy of adsorbed hydroxyl on Ni(111).…”

Effects of Residual Gases on Tribo-Chemical Reaction of Nickel in Hydrogen Gas Atmosphere

“…Keiser et al [20] have studied the reactions of H 2 and O 2 mixtures and the dissociation of H 2 O on polycrystalline nickel. The formation of OH fragments has also been observed from water decomposition on Ni(1 1 0) [21] and Ni(1 1 1) [22] by high-resolution electron energy loss spectroscopy (HREELS) at around 200 K. Among these experimental results, we paid attention to the presence of the O-H species on nickel surface. Nickel hydroxide has a hexagonal structure.…”