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The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding

Chemical Cogeneration in Solid Electrolyte Cells The Oxidation of H2S to S02

ABSTRACT The anodic oxidation of H2S was investigated in the solid electrolyte fuel cell H2S, Sx, SO2, Pt/ZrO2(8% Y~O3)/Pt, air operating at atmospheric pressure and temperatures 650 ~ to 800~ It was found that the fuel cell product selectivity crucially depends on the ratio M of the fluxes of oxygen anions 02_ and H2S reaching the porous Pt anode. When M < 0.33, elemental sulfur is the major product, and the anode is severely polarized. For higher M values, the product selectivity to SO2 exceeds 99% at H2S conversions as high as 99%. The cell appears to be a promising candidate for the cogeneration of electric energy and sulfur dioxide. The oxidation of hydrogen sulfide to sulfur dioxide is one of the basic steps of the Claus process and, ultimately, of the industrial manufacture of sulfuric acid, which ranks first in volume among all chemicals produced with an annual worldwide production exceeding 3 -108 tons. The conversion of H2S to SO2 is a highly exothermic reaction with AG ~ = -103,6 kcal/mole SO2 at 800~ Due to the high exothermicity of the reaction, large amounts of thermal energy are generated. It has been a long-sought goal to obtain this energy as electric rather than thermal energy by oxidizing H2S to SO2 in a fuel cell (1, 2). Low-temperature fuel cells are severely polarized by H2S and: sulfur. High-temperature solid electrolyte cells have been tested for years as fuel cells with H2, CO, or CH4 as the fuel (3-8). The same:type of cells can be used to study the mechanism of catalytic reactions on metals (9-12) and also to influence the activity and selectivity of metal catalysts by electrochemically pumping oxygen anions O 2-onto catalyst surfaces (13-17). Progress in this area has been reviewed recently (18). In some very recent studies (17,(19)(20)(21) it has been found that the increase in catalytic reaction rate can exceed the rate of 02 pumping to the catalyst by as much as a factor of l0 s with a concomitant 40-fold increase in catalytic reaction rate over its open-circuit value (19)(20). The acronym NEMCA (non-faradaic electrochemical modification of catalytic activity) has been used to describe this new phenomenon which has been attributed to changes induced to the average catalyst work function due to the interaction of the catalyst surface with excess 02 (17,(19)(20)(21). One of the emerging uses of solid electrolyte cells is chemical cogeneration, i.e., the simultaneous production of electrical power and useful chemicals. This mode of operation combines the concepts of a fuel cell and of a chemical reactor. Its feasibility was first demonstrated in 1980 when it was shown that solid oxide fuel cells with Pt-based electrodes can quantitatively convert NH3 to NO with simultaneous generation of electrical power (22)(23)(24). Subsequent work has shown that four other exothermic reactions of industrial importance can also be carried out successfully in solid oxide fuel cell reactors with appropriate electrocatalytic anodes. These are the oxidative dehydrogenation of ethylbenzene to styrene (25, 26) and l-butene to butadiene (27), the Adrussov process, i.e., the ammoxidation of methane to form HCN (28) and, more recently, the partial oxidation of methanol to formaldehyde (18, There have been two very recent studies of the anodic oxidation of H2S in high-temperature yttria-stabilized zirconia cells In this work, H2S was used as the fuel in a high-temperature solid electrolyte fuel cell with porous Pt electrodes in order to study the electrochemical characteristics and product distribution of the cell and explore the possibility of simultaneous generation of SO2 and electrical power. Experimental Apparatus A schematic diagram of the experimental apparatus, which has been described in previous communications (9-12, 17, 24), is shown i