A unified theory of homogeneous and electrochemical electron‐transfer rates is developed using statistical mechanics. The treatment is a generalization of earlier papers of this series and is concerned with seeking a fairly broad basis for the quantitative correlations among chemical and electrochemical rate constants predicted in these earlier papers. The atomic motions inside the inner coordination shell of each reactant are treated as vibrations. The motions outside are treated by the "particle description," which emphasizes the functional dependence of potential energy and free energy on molecular properties and which avoids, thereby, some unnecessary assumptions about the molecular interactions.

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On the Theory of Electron-Transfer Reactions. VI. Unified Treatment for Homogeneous and Electrode Reactions

Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

Spectra of porphyrins

Chemistry has its origins as a quantitative science in the careful weighing of products and reactants by Lavoisier and his followers beginning some 200 years ago. Ever since then, the constantly evolving gravimetric balance has been a faithful servant of the laboratory chemist and has played a major role in developing the analytical methods that are the foundation of contemporary chemical science. Perhaps the ultimate stage in the evolution of that balance is represented by the modern mass spectrometer. It is able to determine with high precision the masses of individual atoms and molecules by transforming them into ions and measuring the response of their trajectories in vacuo to various combinations of electric and magnetic fields. Clearly, the sine qua non of such mass determination is the transformation of analyte atoms and molecules from their initial state in a sample to ions in vacuo ready for ”weighing.” Over the years, ingenious investigators have produced a variety of methods for achieving this transformation. One of them, electrospray ( E S ) ionization, has recently shown itself capable of producing intact ions, with multiple charges, from remarkably large, complex, and fragile parent species. Our assignment here is to review what has thus far been learned about this still uncommon technique and what it seems able to offer practitioners of mass spectrometric analysis. Our approach will be to set forth the present state of the ES ionization art in terms of a sort of menu of its procedures, processes, performance, and promise. Until very recently we have been almost the only group that has worked with ES ionization since the pioneering efforts of Malcolm Dole (1) some 20 years ago. Consequently, this review is more tutorial than most. Moreover, it may seem like a cook book that is overly preoccupied with the authors’ own culinary adventures. The reasoil is that many of the dishes to be described were first tried out in our own kitchen. Therefore, we earnestly urge the reader to remember what every gourmet knows: the piquancy of any dish on a bill of fare is due much less to its ingredients than to the skill of the chef whc. prepares it.

Electrospray ionization–principles and practice

Work function, electronegativity, and electrochemical behaviour of metals: III. Electrolytic hydrogen evolution in acid solutions

The effect of low‐energy (15–200‐eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field‐emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre‐exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σvβ≥10—19 cm2. These results are interpreted in terms of transitions from the adsorbed ground state to repulsive portions of excited states, followed by de‐exciting transitions which prevent desorption. Arguments are made to show that the excitation cross sections should be essentially ``normal,'' i.e., ∼10—16 to 10—17 cm2, and that the much smaller over‐all cross sections observed are due to high transition probabilities to the ground state, estimated as 1014 to 1015 sec—1. A detailed calculation for the case of exponentially varying transition probabilities and repulsive upper states is presented and discussed, and the variations in cross section with binding mode made plausible. It is shown that low‐energy electron impact constitutes a sensitive tool for studying chemisorption.

Desorption from Metal Surfaces by Low‐Energy Electrons

The theory of field desorption proposed previously has been extended by a more detailed quantum‐mechanical calculation of the rate constants of desorption for various cases, including an attempt to evaluate matrix elements for the electronic transition when the latter is slow. A simple, generalized model of chemisorption on metals is presented and used in these calculations. It is found that two‐electron transitions lead to somewhat smaller rate constants than one‐electron transitions, but that the effect is not very large. In general, a one‐dimensional theory leads to rate constants with essentially normal pre‐exponential terms, except under conditions where adsorbate tunneling becomes important. A tentative explanation for the compensation effect, i.e., decreases in pre‐exponential term with decreasing activation energy observed experimentally in a number of different cases, is given in terms of changes of reaction path with temperature.

Theory of Field Desorption

The adsorption of potassium on thermally annealed and field‐evaporated tungsten surfaces was studied by field emission. The φ‐ vs‐θ curve has a minimum of 1.78 eV at θ=0.83, n=3.2×1014 atom/cm2 (which coincides with the minimum of φ110), and approaches φK at θ=2–3. Distinct minima also occur for other close‐packed planes, like 211. At θ=1, n=3.9×1014 atom/cm2, φ=1.95 eV the emission pattern is indistinguishable from that of clean tungsten. The average charge per K atom, q, varies from 0.27 at low θ to 0.2 electron charges at θ=0.8. The value of q on the close‐packed W planes at low coverage may be considerably higher.The activation energy of neutral desorption varied from 2.67 to 2.4 eV over the interval 0<θ<0.4 and fell rapidly at higher coverage, approaching the heat of sublimation of K, 1.0 eV, at θ=1.0. The activation energy of ionic desorption was measured over the interval 0<θ≤0.07, and indicates that ionic desorption occurs on regions of φ=5.3 eV. dφ/dθ on these regions is very high, ...

Adsorption of Potassium on Tungsten

Current fluctuations from small regions of adsorbate covered field emitters: A method for determining diffusion coefficients on single crystal planes

Field emission from tungsten emitters into liquid H2, D2, He, Ar, N2, O2, and C6H6 has been carried out in modified field emission tubes, which permitted switching from ultrahigh vacuum to very pure liquids and back to vacuum. In all cases space‐charge‐limited behavior was found at >10−9 A, from which the following mobilities (in centimeter2 volt−1·second−1/units) could be estimated: 3 × 10−2 cm2 V−1·sec−1 for H2 at 21°K, 0.9∼10−2 for D2 at 21°K, 4 × 10−2 for He at 4.2°K, 0.36 for He at 1.8°K, 8 × 10−3 for O2, 8 × 10−3 for N2, and 4 × 10−3 for C6H6. In the case of Ar a limiting velocity of 5 × 106 cm/sec rather than a field‐dependent mobility was observed. For He and Ar onset of high emission (10−9 A) is abrupt, indicating electron avalanching with concomitant gas‐bubble formation at the tip which leads to sputtering. In the case of O2 electrochemical etching occurred even at low currents. For H2 and D2 a true Fowler–Nordheim field emission regime could be observed for i≤10−10 A, from which an energy barr...

Robert Gomer

Papers

Desorption from Metal Surfaces by Low‐Energy Electrons

Theory of Field Desorption

Adsorption of Potassium on Tungsten

Current fluctuations from small regions of adsorbate covered field emitters: A method for determining diffusion coefficients on single crystal planes

Field Emission in Liquids