Environmental Engineering

A kinetic model was prepared for the simulation of the production of HCN in a yttria-stabilizedzirconia cell that uses CH4-NH3 mixtures as fuel. The model considers 14 surface reactions occuring simultaneously at the anode. The effects of temperature, 0% flux, inlet gas composition, and inlet gas velocity on the HCN yield and on the overall exothermicity of the system were examined. It was found that the cell can operate without external heat supply if the CHJNH3 feed ratio is higher than 1.25. Two different cell designs were examined, one in which the anode is inside the zirconia tube, and one in which air flows inside the tube and the fuel flows outside perpendicular to the direction of air flow. The latter design exhibits certain additional advantages over the traditional tubular cell design. Cowans, B. A.; Jurman, K. A.; Delgass, W. N.; Li, Y. Z.; Reifenberger, R.; Koch, T. A. Scanning Tunneling Microscopy of Platinum-Rhodium Gauze HCN Catalysts. J. Catd. 1990,125, 513. Debenedetti, P. G.; Vayenas, C. G.; Yentekakis, I.; Hegedus, L. L. Mathematic Modeling of Cross-Flow, Solid State Electrochemical Reactors. ACS Symp. Ser. 1984,237, 181. Grosz, F.; Zegers, P.; Singhal, S. C.; Yamamoto, 0. Proceedings of the Second International Symposium on Solid Oxide Fuel Cells, Athens; Commission of the European Communities: Luxembourg, 1991. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from CHI and NH3 on Clean Rh. J. Catal. 1985, 91, 116. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from C& and NH3 on Clean Pt. J. CUtUl. 1986,97,166. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from C&, NHa and 0 2 on Clean Pt. J. C4td 1987, 104,441. Kiratzis, N.; The Synthesis of Hydrogen Cyanide ina Solid Electrolyte Fuel Cell. Ph.D. Thesis 1991, Tufta University. Kiratzis, N.; Stoukides, M. The Synthesis of Hydrogen Cyanide in a Solid Electrolyte Fuel Cell. J. Electrochem. SOC. 1987, 134, 1925. Kiratzis, N.; Stoukides, M. Kinetics of the HCN Synthesis in a Solid Electrolyte Cell. J. C4td 1991,132, 257. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; Wiley: New York, 1978. Koberstein, E. Model Reactor Studies of the Hydrogen Cyanide Synthesis from Methane and Ammonia. Znd. Eng. Chem. Process Des. Deu. 1973,12,444. McKenna, L. A. Modeling of HCN Synthesis in a Solid Electrolyte Fuel Cell. M.S. Thesis 1992, Tufts University.

Limited corrosion tests were also carried out with an experimental Fe-30Ni-15Mo alloy. The objective of these tests was to determine whether molybdenum might provide better corrosion protection from Na2S3 in an ironbased alloy. However, 100h exposure tests showed metal losses greater than those observed with Hastelloy B.

The technique of air-pressurized solution spray pyrolysis was used to fabricate thin films of La 0.75 Sr 0.25 MnO 3 (LSM) and CuO-CeO 2 on dense YSZ substrates. These films are widely used in several applications such as high temperature ceramic electrochemical cells, membranes and sensors. The technique was tailored to produce films with differing morphologies and thicknesses in terms of applied substrate deposition temperatures. The droplet drying and decomposition kinetics is discussed as a function of substrate temperature, given the other processing parameters (i.e. nozzle to substrate distance, solution flow rate, air pressure/air flow rate and deposition time) constant. A graph-map is drawn containing morphology and quality of fabricated films in terms of substrate temperatures and concentration levels for this particular set of precursor salts in aqueous solutions. Within the deposition temperature range examined, film thicknesses were found to decrease with increasing substrate temperature along with a concomitant change in quality from irregular texture to smoother and of more uniform thickness. A maximum deposition temperature of about 280 • C was identified for 0.025M concentration for both types of films which correlates quite well with the Leidenfrost point measured in the literature for solvents with boiling point less than 150 • C.

Pure diamondlike carbon thin films largely bonded by four-fold coordination suffer from a large internal compressive stress that gives rise to a serious adhesion problem. In this work, functionally gradient (FG) diamondlike carbon thin coatings were prepared by pulsed laser deposition in a high vacuum chamber as an alternative approach to address the adhesion problem of diamondlike films. Copper, silver and titanium were incorporated into the growing films with their concentration as a function of the distance from the substrate surface. The top of the thin coating is pure DLC of about 400 nm in thickness. The total thickness of the functionally graded superhard DLC coatings can exceed 1.0 pm without buckling. Visible micro-Raman spectroscopy was used to characterize the bonding structure of the layers which contain alloy atoms. High resolution transmission electron microscopy was employed to study the microstructure of the coatings. Nanoscale mechanical characterizations using Nanoindenter XPTM were carried out to study the mechanical behavior of the functionally gradient DLC films.

Results are presented of the application and process parameter optimization of the technique of solution spray pyrolysis for the fabrication of functional films of the type Cu-CeO 2 και La 0.75 Sr 0.25 MnO 3 (LSM) on dense pellets of yttria-stabilized zirconia (YSZ). The first type of deposited film constitutes a cermet (ceramic-metal composite) that combines the properties of an inert electronic conductor (Cu) and a catalyst while the second is a typical perovskite that exhibits mixed (i.e. ionic and electronic) conductivity. Both types of electrodes find wide use in ceramic fuel cells, gas sensors and catalytic processes.

Manmade climate change caused by the burning of fossil fuels has created the need for an energy transition to renewable energy sources. Reversible solid oxide  cell (rSOC) technologies, can play an important role in the energy transition as they offer the potential to produce and store energy on demand. Current challenges are associated with cost, durability, material stability, and low round-trip efficiency. These challenges can be addressed through the prudent selection and fabrication of ceramic cell materials. After decades of research, a lot of achievements and progress have been made. According to the composition and requirements of each component of a reversible solid oxide cell, this review summarizes the research progress based on ceramic materials and discusses the merits and demerits of current cell materials in electrochemical performance.