Water adsorbed on Ni(111) forms an ordered, hydrogen bonded ice structure with a (2√7×2√7)R19° unit cell. The 2√7 wetting structure forms as islands and persists up to saturation of the first layer. Adsorption of a fraction of a monolayer more water into a second layer destroys the 2√7 registry and creates a disordered ice film. Gas adsorption measurements indicate that the wetting layer is completely covered by a second layer of water before thicker multilayer ice forms. As the second layer is completed the film orders to form an incommensurate crystalline ice film with a hexagonal LEED pattern, oriented to the Ni close packed rows. This ordered, incommensurate structure persists as the ice multilayer grows thicker.

A combination of in situ surface X-ray scattering (SXS) and cyclic voltammetry (CV) measurements have been performed to determine the effect that the surface atomic structure of the low-index faces of Au single crystals has on carbon monoxide (CO) oxidation in alkaline solution. For the (1 1 1), (1 0 0) and (1 1 0) surface orientations, potentiodynamic measurements of the scattered X-ray intensity at key reciprocal lattice points have been employed to determine the potential window of stability for the reconstructed surfaces. By saturation of the electrolyte with CO, the effect that adsorbed CO has on the stability of the reconstructed surfaces has been investigated. The presence of CO acts to stabilize the surface reconstructions over a wider potential range, causing a positive shift in the potential for the lifting of the Au(1 1 1) surface reconstruction of ∼200 mV. In the case of the (1 0 0) and (1 1 0) orientations, the adsorption of CO stabilizes the reconstruction over the entire potential range studied. The oxidation of dissolved CO was found to be structure-sensitive, with the activity decreasing in the order Au(1 1 0)-(1 × 2) > Au(1 0 0)-“hex” > Au(1 1 1)-(23 × √3). The structure-sensitivity is correlated with the surface density of low coordinated sites.

The surface reconstruction of Au(100) and the formation of an ordered commensurate c(root2 x 22root2)R45degrees adlayer of Br (Br-ad) have been studied by a combination of electrochemical (EC) and surface X-ray scattering (SXS) measurements. Emphasis is placed on linking the microscopic structural information concerning the Br-ad adlayer to the voltammetric and other macroscopic electrochemical responses, including using the rotating ring disk electrode (RRDE) measurements for determining the surface coverage by Br-ad. It is found that the potential-induced hexagonal ("hex") to (1 x 1) transition of the An surface coincides with Br- adsorption and occurs faster in solutions containing Br- anions than in Br- free Solutions. In agreement with previous SXS results, in acid solutions, Br-ad forms a c(root2 x 2root2)R45degrees structure at around 0.15 V. However, no ordered structures of Br-ad are observed in alkaline solution, although the cyclic voltammetry indicates that the order structure should be present at 0.12 V. Absence of an ordered Br-ad adlayer in alkaline solution is attributed to competitive adsorption between Br-ad and OHad. To probe the role of OHad on the ordering of the bromide adlayer, coadsorbed OHad is consumed in an electrochemical reaction in which strongly adsorbed OHad is removed from the surface by a relatively weakly adsorbed reactant, viz. COad. Under such experimental conditions, we found that (i) in acid solution the c(root2 x 2root2)R45degrees structure develops/disappears more rapidly than in CO-free solution and (ii) in alkaline solution the c(root2 x 2root/2)R45degrees structure is formed in exactly the same potential region as in acid solution. We propose that the continuous removal of OHad in the Lanmmuir-Hinshelwood reaction (COad + OHad = CO2 + H+ + e(-)) may stabilize the c(root2 x 2root2)R45degrees structure in both acid as well as alkaline media.

The effects of pH on the surface reconstruction of Au(100), on CO oxidation, and on the oxygen reduction reaction (ORR) have been studied by a combination of surface X-ray scattering (SXS), Fourier transform infrared (FTIR) spectroscopy, and rotating ring-disk electrode (RRDE) measurements. In harmony with previous SXS and scanning tunneling microscopy (STM) results, the potential-induced hexagonal ("hex") to (I x 1) transition occurs faster in an alkaline electrolyte than in acidic media. In alkaline solution, CO adsorption facilitates the formation of a "hex" phase; in acid solution, however, CO has negligible effect on the potential range of thermodynamic stability of the "hex" <----> (1 x 1) transition. We propose that in KOH the continuous removal of OHad in the Langmuir-Hinshelwood reaction (CO + OH = CO2 + H+ + e(-)) may stabilize the "hex" phase over a much wider potential range than in CO-free solution. In acid solution, where specifically adsorbing anions cannot be displaced by CO from the Au(100) surface, CO has negligible effect on the equilibrium potential for the "hex" <----> (1 x 1) transition. Such a mechanism is in agreement with the pH-dependent oxidation of CO. The ORR is also affected by the pH of solution. It is proposed that the pH-dependent kinetics of the ORR on Au(100) can be unraveled by finding the relationship between kinetic rates and two terms: (i) the energetic term of the Au(100)-O-2(-) interaction determines the potential regions where the rate-determining step O-2 + e = O-2(-) occurs, and (ii) the preexponential term determines the availability of active sites for the adsorption of O-2(-).

In situ surface X-ray scattering (SXS) measurements have been performed to determine the surface structure of Pt3Sn(1 1 1) in sulfuric acid electrolyte. Potentiodynamic measurements indicate that the ultra high vacuum (UHV) prepared p(2 × 2) alloy surface structure is stable upon transfer to electrolyte and remains stable during subsequent cycling of the applied potential. A detailed structural study by crystal truncation rod (CTR) analysis shows that the surface layer of Pt and Sn atoms undergoes an expansion of ∼2% of the (1 1 1) layer spacing at low potential (0.05 V vs. reversible hydrogen electrode) in CO-free electrolyte. At 0.55 V the expansion of the Pt atoms is reduced to ∼0.6%, whereas the Sn atoms are expanded by ∼6% of the layer spacing. The potential-induced buckling of the surface layer is also observed in CO-saturated electrolyte and is a precursor to Sn dissolution which occurs at ∼1.0 V, causing irreversible roughening of the surface.

In situ surface X-ray scattering (SXS) measurements have been performed to determine the surface structure of Pt3Sn(111) in sulfuric acid electrolyte. Potentiodynamic measurements indicate that the ultra high vacuum (UHV) prepared p(2×2) alloy surface structure is stable upon transfer to electrolyte and remains stable during subsequent cycling of the applied potential. A detailed structural study by crystal truncation rod (CTR) analysis shows that the surface layer of Pt and Sn atoms undergoes an expansion of ∼2% of the (111) layer spacing at low potential (0.05 V vs. reversible hydrogen electrode) in CO-free electrolyte. At 0.55 V the expansion of the Pt atoms is reduced to ∼0.6%, whereas the Sn atoms are expanded by ∼6% of the layer spacing. The potential-induced buckling of the surface layer is also observed in CO-saturated electrolyte and is a precursor to Sn dissolution which occurs at ∼1.0 V, causing irreversible roughening of the surface.