The adsorption of carbon dioxide on potassium-dosed Ag(111) has been investigated with temperature-programmed desorption (TPD), work function measurements, and Auger electron (AES), X-ray photoelectron (XPS), and high-resolution electron energy loss (HREELS) spectroscopies. Unlike the behavior observed for other K-modified single-crystal metal surfaces, the TPD spectra of near-saturation coverages of CO2 on K/Ag(111) for K coverages in the range 0.13 < theta(K) < 0.47, where the close-packed monolayer corresponds to theta(K) = 1/3, exhibit a sharply defined m/e = 44 peak at 796 +/- 6 K with no evidence for the desorption of CO at any temperature. Similar TPD experiments involving mixtures of natural and O-18-labeled CO2 indicate that the oxygen atoms undergo partial scrambling, suggesting that the overall process cannot be represented in terms of a simple adsorption/desorption of CO2. The HREELS spectra of CO2-saturated K/Ag(111) show, in addition to very minor features, a sharp peak at approximately 1480 cm-1, and XPS spectra of the same interface display a C(1s) peak with a binding energy characteristic of an electron-rich carbon species. This information is consistent with the presence of a carbon-bound CO species on the surface. Evidence against the complete dissociation of CO2 was obtained from TPD, which failed to reveal features associated with carbonate (decomposition) expected to be formed via the reaction of CO2 and adsorbed O. On the basis of these results, it is proposed that CO2 on K/Ag(111) binds through the carbon to the surface, leading to the "partial" dissociation (or activation) of each CO2 molecule into adsorbed CO and O. Within this model, such adsorbed O would serve as a bridge between the carbon atoms of neighboring "activated" CO2 molecules and therefore undergo exchange prior or during thermal desorption. Adventitious water or oxygen in the system and/or defect sites on the surface give rise to an additional m/e = 44 TPD peak at a much higher temperature. The height of this new feature is increased significantly by predosing the K/Ag(111) surface with O2 or H2O at coverages as low as 0.05 L. The XPS spectra for these purposely contaminated surfaces reveal features very different from those observed in the absence of such impurities, but consistent with the presence of an ordinary form of carbonate. Ag(111) surfaces which had been damaged prior to K deposition and subsequent CO2 adsorption were found to yield significant amounts of CO in the TPD spectra at lower temperatures.

AbstractThe passive films grown either electrochemically (10 mM HClO4) or by dosing with O2 in ultrahigh vacuum share a common NiO component, but the electrochemically grown films also contain other forms of oxygen (apparently some higher oxides and a superficial hydroxide).

The thermal power output of a galvanostatic D2O electrolysis cell was determined simultaneously by two methods. The first, water-flow calorimetry, gave results rigorously independent of the nature of the heat source. The second, an isoperibolic method involving measurement of temperatures within the electrolysis cells, gave results in agreement with the first when all factors were accounted for. However, neglect of a drop in effective cell impedance accompanying operation of an in-cell calibration heater, or neglect of effects of the dropping electrolyte level, can produce spurious indications of excess heat. This work demonstrates the need for extreme care in application of isoperibolic methods to electrolytic cells.

Aqueous chloride ions accelerate the corrosion of all metals, and chloride chemistry is essential to the preparation of noble metal catalysts. To improve the understanding of the interactions of chloride species with metals, the adsorption of anhydrous HCl on Pt(111) at 90 K and its coadsorption with hydrogen and water were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Low coverages of HCl fully dissociate to form a disordered mixture of adsorbed H and adsorbed Cl. Higher exposures produce first a well-ordered 3 × 3 phase and then an increasingly disordered form which saturates just above the density of one layer of close-packed Cl; no multilayer of HCl ice can be grown. Coadsorption of HCl and water produces adsorbed H3O+. The thermal desorption of water indicates two types of water stabilization by HCl, but there is no evidence for water molecules bound directly to adsorbed Cl. The chemistry of HCl+H2O coadsorption, which is dominated by strong PtCl interactions, is contrasted with that of the previously-studied HF+H2O system, which is dominated by hydrogen bonding effects.

Carbon monoxide is a major poison to anodic reactions in aqueous fuel cells. To ascertain the effects of a controlled aqueous environment on the adsorption of CO on electrocatalytically active metals, the coadsorption of CO and water on Rh(111) and Pt(111) surfaces at 100 K was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). On Rh(111) low coverages of CO shift monolayer water desorption from 182 to 207 K, indicating net attractive COH2O interactions. Water shifts the CO stretching frequency from 2020 to 1620 cm−1, suggesting a displacement of CO from atop to three-fold hollow sites. A site shift is corroborated by XPS data. Concurrent changes in water vibrational features suggest formation of a mixed phase in which CO and water occupy adjacent sites. It has been hypothesized that such an adsorption geometry would facilitate the normally slow electrooxidation of CO and of fuels such as methanol.

The cyclic voltammetry of Pt (111) in three different structural states (well-ordered, restructured by electrochemical redox cycling, and restructured by Ar+-sputtering) was studied in aqueous acidic and basic electrolytes using a UHV-electrochemical transfer system. The well-ordered surface gave distinctive voltammograms with anomalous features identical to those first reported by Clavilier using flame-an-nealed (111) surfaces transferred through the air while hot. Electrochemical cycling above the sharp oxidation peak removed the anomalous features and introduced randomly spaced monatomic steps to the surface, as shown by low energy electron diffraction (LEED). Ar+-sputtered surfaces, with a similar randomly stepped structure, yielded on the first cycle the same voltammetry as the redox-cycled surfaces, demonstrating clearly that the loss of the anomalous features upon cycling is due to a disruption of the long-range (> 5 atomic spacings) order of the surface, rather than to a simple irreversible oxidation of adventitious impurities. Cycling in basic electrolytes produced voltammetric and structural changes similar to those seen in acid. Possible origins of the anomalous features are discussed in the light of the collected electrochemical and surface science data. The extreme sensitivity of these systems to disruption of long-range surface order is ascribed to the ability of hydrogen-bonded aqueous networks to transmit structural phase information over greater-than-molecular distances.

The adsorption of HF and its coadsorption with water were studied on Pt(111) by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) as a step in the UHV modeling of the acidic aqueous electrolyte/electrode interface. Anhydrous HF adsorbs without dissociation. HF coadsorbed with water reacts to form several phases distinguishable by TPD. HREELS spectra show that the reaction forms the H3O+ ion. The stoichiometries of thermal desorption identify an acid monohydrate phase ([H3O+][F−]) and fully hydrated phases with stoichiometries of HF · 5H2O in the monolayer and HF · 8H2O in the multilayer. To the as-yet-unknown extent that low-temperature measurements are relevant to normal aqueous electrochemistry, these results indicate that even such classically “non-specifically” adsorbed ions as H+ and F− interact sufficiently strongly with Pt surfaces to displace some water from their inner solvation shells. These data also show that Brönsted acid-base chemistry can be carried out and spectroscopically observed in low temperature monolayers in UHV, and point the way towards UHV studies of such pH-dependent phenomena as corrosion and electrocatalysis.

The adsorption of water and its interactions with oxygen on Rh(111) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for Pt(111). On Rh(111) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on Rh(111) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(111), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H2O/Rh(111), more closely resemble those for other noble metal surfaces than the sharp, single feature observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm−1 seen for H2O/Pt(111) were absent on Rh(111). The middle (3a1) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on Pt(111) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O-O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H2O. On Rh (111) this phase contains both OH and H2O in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.

Anodic films formed on Pt(100) in 0.3M HF using a quasi thin-layer electrochemical cell within a vacuum envelope were transferred to ultra-high vacuum for study by AES and TDS. Films generated at potentials above 1.1 V (RHE) survived emersion and pumpdown in a hydrated state. As the emersion potential increased, the integrated H2O and O2 thermal desorption signals increased in parallel, indicating a constant stoichiometry consistent with the formation of a platinum hydroxide layer. The oxygen TDS and AES signals after holding the electrode at constant potentials above 1.9 V (RHE) for several minutes saturated with formation of a surface phase containing 2.3 O/Pt (desorbing as O2) and 2 H2O/Pt. Much thicker films could be grown by AC polarization. XPS analysis combined with TDS indicated the most likely chemical state of the saturation layer to be Pt(OH)4. Water evolved from all films at 400 K and higher, temperatures much higher than that reported for surface adsorbed hydroxyl groups produced by low-temperature gas-phase coadsorption of O2 and h2O [G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683]. The higher temperature desorption is ascribed to the incorporation of hydroxyls into a surface phase involving place-exchange between Pt and OH.

The structures formed by oxidation-reduction cycling of ordered Pt(100) and Pt(111) surfaces in aqueous electrolytes were studied using LEED spot profile analysis. Surfaces cycled to anodic potential limits above 1.0 V (RHE) and emersed at 0.5 V gave LEED spots which varied in width periodically with beam energy. The varying spot widths indicated a type of randomly stepped surface, and mean terrace widths were estimated by comparison with the previously published scattering calculations of Henzler and Lu and Lagally. LEED spots from surfaces cycled to potentials below 1.0 V (RHE) remained sharp at all beam energies. The restructuring resulted from the anodic formation of an amorphous surface oxide phaes involving place exchange of platinum and oxygen followed by a reduction process in which the platinum atoms do not all return to their original positions in the surface lattice. The observed spot profiles changed both with the the number of cycles and with the upper potential limit. Cycling to 1.28–1.58 V produced characteristically different structures than cycling to limits of 1.08–1.28 V. At the higher potentials, a randomly-stepped surface was formed whose mean terrace width and extent of vertical relief were functions of the total anodic charge. Cycling to the lower potential limits produced a type of structure which is intermediate between the classical two-level island and the multi-level randomly-stepped structure. A three-level structure with a high degree of correlation between the first (island-like) and third (hole-like) levels is proposed for the latter.

An ultra-high vacuum Low Energy Electron Diffraction apparatus directly coupled to a three-electrode electrochemical cell was used to identify the atomic-scale structural changes induced by potentiodynamic cycling of a well-characterized Pt(100) surface in 0.3 M aqueous HF. Cycling between the hydrogen electrosorption region and the edge of the oxygen electrosorption region (0–0.82 V RHE) caused no discernable change in the Pt(100)−1×1 LEED pattern. However, cycling well into the oxygen electrosorption region (to 1.58 V RHE) produced LEED patterns characterized by alternate broadened and sharp spots. The breadth of a given spot fluctuated with the energy of the incident electron beam in a manner consistent with the formation of random monoatomic-height steps on the surface. From the maximum spot width a mean terrace width of 5–7 atoms was estimated. Cyclic voltammetry (0–0.8 V) on vacuum-prepared Pt(100) and (111) surfaces showed features not seen on single-crystal surfaces cleaned by potentiodynamic cycling into the oxygen electrosorption region. Pt(100) showed excess cathodic charge in the hydrogen electrosorption region, a cathodic spike during the positive sweep just above the hydrogen region, and an associated, highly irreversible, couple in the region 0.4–0.8 V RHE. Intentional adsorption of CO accentuated these features. Pt(111) showed no cathodic spike and a highly reversible couple at 0.76 V. Sweeping up to oxygen evolution potentials removed the “anomalous” peaks on both surfaces, yielding single hydrogen electrosorption peaks in agreement with previous work. The present work suggests that the anomalous peaks are due to structure-sensitive reactions with impurities which are irreversibly oxidized during potential excursions into the oxygen electrosorption region.