Patricia A. Thiel

The interaction of water with solid surfaces: Fundamental aspects

Abstract The purpose of this review is to compare and discuss recent experimental and theoretical results in the field of H2O-solid interactions. We emphasize studies of low (submonolayer) coverages of water on well-characterized, single-crystal surfaces of metals, semiconductors and oxides. We discuss the factors which influence dissociative versus associative adsorption pathways. When H2O adsorbs molecularly, it tends to form three-dimensional hydrogen-bonded clusters, even at fractional monolayer coverages, because the strength of the attractive interaction between two molecules is comparable to that of the substrate-H2O bond. The template effect of the substrate is important in determining both the local orientation and long-range order of H2O molecules in these clusters. The influence of surface additive atoms (e.g., O, Br, Na, K) and also surface imperfections (e.g. steps and defects) on the surface structure and chemistry of H2O is examined in detail. Some results on single-crystal substrates are compared with earlier measurements of H2O adsorption on high-area materials.

Oxygen on Pd(100): Order, reconstruction, and desorption

We have investigated the temperature and coverage‐dependent transformations of the ordered structures of oxygen on Pd(100). The four ordered structures are the chemisorbed c(2×2) and p(2×2) lattices, and the reconstructed p(5×5) and ((5)1/2×(5)1/2)R27° lattices. We present evidence that the p(5×5) reconstruction forms in an activated stepfrom c(2×2) regions. The onset of p(5×5) formation is associated with an increase in oxygen sticking coefficient. In thermal desorption, there are three states which can be correlated directly with the structure of the adsorbed phase during desorption: At lowest coverage, the α state shows the traits of second‐order kinetics and is due to desorption from a disordered adlayer. At higher coverage, the β state appears and is due to desorption from a layer with c(2×2) order. There is a lower barrier to desorption in the β state than in the α state because of the repulsive second‐nearest‐neighbor interactions in the c(2×2). At highest coverage, the sharp and narrow γ state eme...

The interaction of formic acid with transition metal surfaces, studied in ultrahigh vacuum

Abstract In this report, we review the interaction of formic acid (HCOOH) with transition metal surfaces, as studied with the tools of modern surface science in an ultrahigh vacuum environment. Our goal is to provide a broad overview of the fundamental adsorption and decomposition processes which HCOOH undergoes at the surfaces of these metals. The metal substrates include Cu, Fe, W, Ni, Ru, Ag, Pt, Au, Rh and Pd.

The vibrational spectra of chemisorbed molecular clusters: H2O on Ru(001)

Electron energy loss spectroscopy and thermal desorption mass spectroscopy have been used to correlate vibrational spectra of H2O on Ru(001) with ESDIAD patterns reported recently. We find that the ‘‘extended bilayer’’ is characterized by a sharp fundamental O–H vibration at 3500–3565 cm−1 which is assigned to nonhydrogen‐bonded OH bonds of molecularly adsorbed water. Hydrogen bonds within the well‐ordered bilayer give rise to features at ∼3290–3450 cm−1 which are of relatively weak intensity in electron scattering due to the orientations of these bonds. In addition, very small clusters exhibit a weak vibrational mode at ∼2935 cm−1 which possibly represents H2O molecules with one O–H bond pointing into the surface. Isolated H2O molecules at low temperature and low coverage exhibit a fundamental O–H vibration at 3600 cm−1. A reinterpretation of the thermal desorption spectra is suggested.

Surface oxidation of an AlPdMn quasicrystal, characterized by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) is used to determine the extent of oxidation of each of the three metals which comprise a quasicrystalline alloy. A single-grain sample, oriented with the fivefold axis perpendicular to the surface plane, and with nominal bulk composition Al 70 Pd 21 Mn 9 is used. The oxide which results from exposure to ambient gas at room temperature is compared with that which results from exposure to pure oxygen in ultrahigh vacuum at temperatures up to 870 K. XPS probes the near-surface region (ca. top 100 A), and shows that only the Al can be oxidized. The depth of the oxide layer depends systematically upon the conditions of treatment, but is always very thin — in the range of about 5–30 A. Taken together, the data suggest that the surface forms a thin, passivating, surface layer of aluminum oxide.

Oxygen adsorption on a single-grain, quasicrystal surface

Abstract Oxygen adsorbs on the five-fold surface of Al 70 Pd 21 Mn 9 . A chemisorbed phase, possibly in or below the surface plane, serves as a precursor to oxidation of Al. This chemisorbed phase destroys the quasiperiodicity of the surface. The major features of oxygen adsorption and oxidation are similar in the temperature range 105–500 K, but are different at 870 K, where Al segregates strongly to the surface. We postulate that Al segregation is driven by the exothermicity of its oxide, which is higher than that of the other constituents of this alloy. At all temperatures, the oxide layer is passivating (under the conditions of these experiments), is quite thin ( ⪅ 10 A ) and is thermally stable (at least upon heating to 870 K). The oxidation characteristics of this quasicrystal are broadly similar to those of its major constituent, Al, with the possible exception of the oxygen sticking coefficient.

The formation and decomposition of KOH on Ru(001)

Abstract Potassium hydroxide is formed when water is adsorbed at 80 K on a Ru(001) surface precovered with potassium. The vibrational spectrum of KOH obtained with EELS is characterized by an OH stretch at 3570–3620 cm −1 ; losses at 1340–1500 cm −1 and 200–800 cm −1 are tentatively assigned to KOH bending and KO stretching vibrations respectively. Heating the surface to 500–600 K results in decomposition of KOH and desorption of the dissociation products. The decomposition pathway is found to depend strongly on the potassium precoverage.

Structural steps to oxidation of Ni(100)

In this paper, we emphasize the temperature‐ and exposure‐dependent development of low‐energy electron diffraction patterns,measured quantitatively during oxidation of Ni(100) at 80 to 400 K. We find a strong temperature dependence in the development of LEED patterns associated with NiO. NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7×7)‐like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a ‘‘crossover’’ point between these two forms of the oxide. The final oxide depth is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. When the sample is heated in vacuum after adsorption, massive rearrangements take place above 500 K. Some of the nickel reverts to metallic nickel covered by a c(2×2) oxygen overlayer, and some forms NiO crystallites which are probably deeper than the initial oxide skin. Effectively, the parent oxide disproportionates into a less‐oxygen‐rich phase and a more‐oxygen‐rich phase...

Surface properties of quasicrystals

There is currently great interest in the surface reactivity of quasicrystalline materials, generated largely by a model, proposed by Janot, for the bulk atomic and electronic structure. This {open_quotes}hierarchical cluster{close_quotes} model predicts that quasicrystal surfaces should be intrinsically inert and rough, and is being used to rationalize their practical properties such as low friction coefficients and oxidation resistances Surface structure and surface preparation may play a role in the applicability of this model. In this talk, we examine these factors and present experimental measurements of the surface reactivity of Al-based icosahedral alloys. We make some comparisons with surface reactivity of pure, crystalline aluminum, and with that of crystalline alloys which are similar in composition to the quasicrystal.

Isotope effect in water desorption from Ru(001)

It is well-established that three features are resolved in the thermal desorption spectra of H 2 O from clean Ru(001): one at low temperature due to desorption from ice multilayers, and two at higher temperatures due to desorption from a chemisorbed bilayer. However, we find that for D 2 O the highest-temperature state is strongly suppressed. By varying the heating rate during H 2 O desorption, we obtain evidence that the highest-temperature state can be formed by conversion from the intermediate state. Conversion is much slower for D 2 O than for H 2 O, so that direct desorption is kinetically favored over conversion and the high-temperature state is suppressed for D 2 O. The rate of conversion is about 3 to 8 times slower for D 2 O than for H 2 O, which is consistent with a model in which the rate-determining step for conversion is rotation of one or more water molecules within a hydrogen-bonded cluster. This work provides the first evidence for an isotope effect in thermal desorption of water. An important ramification is that adsorbed D 2 O is not always interchangeable with H 2 O for chemical studies of isotopic mixing or for spectroscopic studies of vibrational frequency shifts, as is commonly assumed.