Eric M. Stuve

The adsorption of water and oxygen on Ag(110): A study of the interactions among water molecules, hydroxyl groups, and oxygen atoms

The adsorption of water on clean and oxygen covered Ag(110) in ultrahigh vacuum was studied with thermal desorption spectroscopy (TDS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED), work function measurements (Δ), and Auger electron spectroscopy (AES). The ranges of experimental conditions were θ(O) between 0 and 0.5, θ(H 2 O) between 0 and 2, and temperature between 80 and 400 K. Water adsorbs on the clean surface at 80 K with no long-range order and little, if any, short-range order as determined by LEED and ESDIAD, respectively. Electron bombardment of water adlayers on clean Ag(110) results in dissociation to form OH groups at 80 K. Water also reacts with preadsorbed oxygen, either in atomic or molecular form, to form OH groups at 80 K. The O-H bond axis of the OH groups is tilted with respect to the surface normal and its thermodynamically stable, azimuthal orientation is along the [001] direction. In the absenoe of excess, molecular H 2 O, the OH groups exhibit no hydrogen bonding. The OH groups form (1× m ) ( m =2,3) LEED compression patterns upon annealing to 200 K. The γ adsorption state, which evolves water in a desorption peak at 225 K, was carefully studied. This state consists of coadsorbed H 2 O and OH in a 1:2 ratio. The results of this work are combined with earlier TDS and vibrational measurements for this system to propose an adsorption model for the γ state, in which an H 2 O molecule is hydrogen bonded to two OH groups to form an (HO) 2 -H 2 O surface complex.

Promotion and poisoning of the reaction of methanol on clean and modified platinum (100)

The reactivity of Pt(100) towards methanol depends on surface structure and the nature and amount of coadsorbed reaction modifiers. We studied the adsorption and reaction of CH3OH in ultrahigh vacuum on the clean hex and (1 × 1) surfaces of Pt(100) and with coadsorbed O, Bi, and CO with thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. Methanol adsorbs molecularly on Pt(100) at 100 K and decomposes to adsorbed H and CO in the temperature range of 170–220 K. The vibrational spectra show evidence for surface interactions involving both the hydroxyl (O-H … Pt) and methyl (C-H … Pt) hydrogens of molecular methanol. Under static conditions (no desorption of reaction products) as much as 0.085 ± 0.01 monolayers of methanol react on the clean hex surface. Chemical saturation of the surface, which occurs when the total coverage of surface reaction products reaches about one-half of a monolayer, limits the maximum extent of reaction on the hex, (1 × 1), and O-covered surfaces. The (1 × 1) surface enhances methanol reaction by factors of 1–3.4 compared to reaction following the same exposure on the hex surface. Reaction with preadsorbed oxygen leads to a methoxy intermediate (CH3O) and enhancement factors of 1–2.5 for oxygen coverages below 0.15 and above 0.3 monolayers. Coadsorbed oxygen enhances methanol reaction stoichiometrically, by scavenging H atoms to form OH and H2O, and nonstoichiometrically, most likely by local lifting of the hex reconstruction. Oxygen coverages between 0.15 and 0.3 monolayers inhibit the reaction by factors of 1—0.85 through blocking of reactive sites. Both Bi and CO inhibit the reaction by factors of 1—0.05 through a combination of ensemble and site blocking effects. Methanol decomposition requires an ensemble of 5 adsorption sites, as determined by Bi coadsorption experiments. The adsorbate/substrate combinations in this study represent model electrochemical situations, and we briefly discuss the implications of these findings toward methanol electrocatalysis.

Reaction mechanism and dynamics of methanol electrooxidation on platinum(111)

Surface reactions in methanol electrooxidation to carbon dioxide are influenced by the accumulation of adsorbed partial decomposition products. We developed a Langmuir–Hinshelwood kinetic model for the surface chemistry based on current understanding of methanol electrooxidation. The model, which contains four kinetic and four mechanistic parameters, accounts for CO2 production by both CO oxidation (serial pathway) and oxidation of a non-CO reactive intermediate (parallel pathway). Model parameters were estimated by fitting time-dependent kinetic measurements of the charge passed following a potentiostatic step to 0.6 V on Pt(111) in 0.1 M methanol bearing electrolyte and the charge passed during voltammetric stripping of accumulated residue in blank electrolyte. The model gave an excellent fit to the experimental data, which spanned four orders of magnitude in time (0.03–300 s). Results from this exercise show that the non-CO reactive intermediate has the stoichiometry H:C:O prior to reacting to either CO or CO2; oxidation of H:C:O accounts for most of the CO2 produced. The most abundant adsorbate varies from H:C:O at short times, when the total adsorbate coverage is low, to CO at long times, when the total adsorbate coverage is high. Accumulation of H:C:O and CO not only poisons the surface, but also transfers kinetic control between different surface reactions. The predictive capability of the model was tested by comparing model predictions with experimental data obtained in 0.5 M methanol bearing electrolyte. We discuss the influence of residue accumulation on the rates of individual surface reactions, the significance of adsorbate interactions, and limitations of the model.

Coadsorption of water and hydrogen on Pt(100): formation of adsorbed hydronium ions

Abstract Coadsorption of water and hydrogen on Pt(100) was studied under conditions of ultrahigh vacuum with thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. The experiments were conducted over a temperature range of 100–700 K and coverages ranging from submonolayer to several multilayers. The surface reaction H 2 O + H → H 3 O + + e − occurs in water/hydrogen coadsorbate; H 2 O + was identified by a new vibrational peak at 1150 cm −1 , assigned to the umbrella mode, and isotope exchange between coadsorbed H 2 O and D. Isotope exchange in the coadsorbate involves three reaction equilibria: intralayer exchange in the aqueous layer (primarily water), intralayer exchange in the chemisorbed deuterium layer, and interlayer exchange between the aqueous and chemisorbed deuterium layers. Both intralayer exchanges are essentially completely equilibrated for the conditions of the experiment, whereas interlayer exchange is kinetically limited, ranging from zero to 55% approach to equilibrium depending on the coverages of both species. Irreversible hydration of chemisorbed hydrogen, as probed by thermal desorption, is essentially negligible. This behavior is consistent with the production of hydronium ions, since the above surface reaction is approximately energetically neutral. These results are also consistent with recent attempts to unify the electrochemical and vacuum potential scales, which predict hydronium ion formation on high work function surfaces, like platinum, but not on low work function surfaces, like copper.

An ex situ study of electrodeposited lead on platinum (111): I. Examination of the surface redox behavior of lead and dynamic emersion

Electrochemical deposition of lead onto a Pt(111) electrode was studied ex situ by means of a combined electrochemical and ultrahigh vacuum system. The Pb/Pt(111) system was examined with cyclic voltammetry in the electrochemical cell and Auger spectroscopy, high energy resolution X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, thermal desorp- tion spectroscopy, and low energy electron diffraction in the vacuum system. Electrodeposition of Pb from 0.1 M HClO4 involves both classical electrodeposition (i.e., underpotential deposition) and a surface redox reaction. X-ray photoelectron spectra show two distinct types of adsorbed lead, Pb0 and Pb2+, in amounts that vary with emersion potential. The standard reduction potential for the reaction Pb2+ab + 2 e− Pb0ab, as determined by XPS, is E0 = 0.65 ± 0.05 VRHE, in excellent agreement with reversible peaks at 0.63 V in the cyclic voltammogram. A consequence of the surface redox reaction is that emersion is a dynamic process; discharge of the double layer after emersion results in a final emersed potential close to E0, regardless of the emersion potential. XPS measurements of the shifts of adsorbate binding energies as a function of emersion potential show that the double layer fully collapses upon emersion at potentials of −0.3 to 0.55 VRHE whereas a partially intact double layer remains after emersion at other potentials. We propose a model of the Pb surface redox process in which the oxidized species Pb2+ab must be accompanied by some form of oxygen-containing species, such as Oad or OHad.

Relating the in-situ, ex-situ, and non-situ environments in surface electrochemistry

The relationship between ultrahigh vacuum surface science and electrochemistry is examined by comparison of so-called non-situ and ex-situ experiments performed in the vacuum environment with in-situ electrochemical experiments. Preadsorbed ClO4 on Ag(110) may be hydrated by post-adsorbed water and lifted off the surface as a hydrated complex at 170 K. This result directly illustrates the electrochemical concepts of anion desorption and nonspecific adsorption and can be explained by analogy to electrochemistry. For ex-situ studies three models exist to describe the key step of removing the electrode from the electrolyte (emersion): ideal, superequivalent, and dynamic. Ideal emersion obtains upon satisfying the criteria of (1) a 1 : 1 relationship of emersed work function with emersion potential and (2) zero charge transfer upon emersion. These criteria can be tested by Kelvin probe measurements of the work function in vacuum and re-immersion charge transient measurements in the electrochemical cell, respectively. Emersion of Pt(111) from 0.1M HClO4 + 1mM Cu2+ exhibits ideal emersion at potentials greater than 0.7VRHE and superequivalent emersion, so called because superequivalent adsorption of ClO−4 and Cl− establishes a constant work function, at emersion potentials less than 0.6VRHE. The Pb/Pt(111) system exhibits dynamic emersion behavior, characterized by a surface redox reaction between Pb0 and Pb2+ that discharges the double layer after emersion. Theoretical relationships among the non-situ, ex-situ, and in-situ methodologies are also briefly reviewed.

Development of an Electrochemical Flow-Cell Technique for Studying Methanol Electro-oxidation at Elevated Temperatures

Development of fuel-cell catalysts for elevated temperature applications requires electrochemical techniques that simulate these conditions. A pulsed reactant electrochemical flow cell (PREFC) technique has been developed, capable of independent half-cell measurements of methanol electro-oxidation current and poisoning adsorbate charge on Pt/C-Nafion catalyst layers over a wide range of temperatures (25-100°C) and pressures while maintaining potential control at all times. The cell was fabricated from Pyrex and silicon using standard microfabrication techniques and exhibits adequate corrosion resistance to the sulfuric acid electrolyte for accurate electrochemical measurements up to 100°C. Thin Pt/C-Nafion catalyst layers of 50-100 μg Pt/cm 2 were prepared at roughly 100% catalyst utilization, as determined independently by in situ hydrogen voltammetry and ex situ transmission electron microscopy analysis. Increasing the temperature from 50 to 100°C at 0.35 V PdH resulted in a substantial increase in methanol electro-oxidation rates with an activation energy of 70 kJ/mol. This kinetic enhancement is not due to thermal desorption of the poisoning adlayer, as determined by experiments showing the adlayer to be stable in flowing electrolyte at 95°C for durations up to 15 min. These results also show that the technique is able to perform the methanol oxidation and poisoning adlayer determinations without any significant effects of oxygen or other homogeneous contamination.

Metal/Electrolyte surface chemistry: The adsorption of nitric acid and water on Ag(110)

Abstract The adsorption of HNO 3 /H 2 O mixtures on Ag(110) was investigated to learn more about the chemistry of the metal/electrolyte interface. The experiments were performed in ultrahigh vacuum (UHV) using thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and electron stimulated desorption ion angular distribution (ESDIAD) over temperatures of 80–650 K and coverages of 0–10 monolayers (ML). As this is the first known study of HNO 3 in UHV, the mass spectrometer cracking pattern for HNO 3 is here reported. HNO 3 adsorbs irreversibly on the clean surface at 80 K and loses its acidic proton to form an adsorbed surface nitrate (NO 3 ) below 150 K. The saturation amount of adsorbed NO 3 is 0.4 ± 0.1 ML for which adsorption occurs in either a normal or split c(2 × 2) structure. N03 is stable on the surface up to 450 K beyond which it decomposes directly to gaseous NO 2 and NO and adsorbed atomic oxygen. NO 3 decomposition is first order with an activation energy E a = 151±4 kJ mol −1 and a pre-exponential factor of A = 10 15.4±0.4 s −1 . NO 3 stabilizes adsorbed H 2 O by about 8 kJ mol −1 and is hydrated by as many as three H 2 O molecules. Multilayers of HNO 3 /H 2 O desorb at 150–220 K and show evidence of extensive hydrogen bonding and hydration interactions. No evidence for HNO 3 -induced corrosion or other surface damage was detected in any of these experiments.

Adsorption of Water Dimer on Platinum(111): Identification of the −OH···Pt Hydrogen Bond

The fundamental structure of an isolated water dimer on Pt(111) was determined by means of a spectroscopic method using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Two water molecules on adjacent atop sites form a dimer through a hydrogen bond, and they rotate even at a substrate temperature of 5 K. Action spectroscopy using STM (STM-AS) for water dimer hopping allows us to obtain the vibrational spectrum of a single water dimer on Pt(111). Comparisons between the experiments and theory show that one of the OH groups of the acceptor water molecule points toward the surface to form an -OH···Pt hydrogen bond.

II. Mechanisms of Elevated Temperature Methanol Electro-oxidation and Poisoning on Pt/C-Nafion Catalyst Layers

Half-cell measurements using fuel cell catalysts at elevated temperatures were made in a pulsed reactant electrochemical flow cell. Kinetics of methanol and the poisoning adlayer electro-oxidation were investigated on 20%Pt/Vulcan C-Nafion catalyst layers at 95°C. Transient measurement of the adlayer coverage during methanol electro-oxidation at 0.30 V r h e and 95°C showed that the surface reached a steady-state adlayer coverage after 30 s. Almost no change in adlayer coverage or current occurred from 30 to 300 s, indicating that steady-state conditions were maintained at 95°C. With increasing potential from 0.30-0.40 Vine in 0.015 M methanol, the steady-state CO coverage decreased from 0.44 to 0.28 monolayers, while the steady-state current increased from 3.3 to 29 mA/mg Pt. The kinetics of CO adlayer oxidation were determined independently over this potential range for adlayers prepared by repeated, 60 s periods of methanol oxidation. The initial rates of adlayer electro-oxidation were 80-90% of the steady-state methanol electro-oxidation rates throughout the potential range. This indicates high selectivity through the CO adlayer at 95°C. Poisoning adlayer electro-oxidation occurred with faster kinetics on Pt(110) facets relative to Pt(100) facets. Dissociative adsorption of methanol at 0.05 V r h e and 95°C was also studied. The adlayer coverage reached 0.36 monolayers after a 60 s exposure to 0.015 M methanol, indicating that the Pt/C electrocatalyst reacts with methanol under these conditions.