Gopala N. Krishnan

Electrocatalytic reduction of NOx on La1−xAxB1−yB′yO3−δ: evidence of electrically enhanced activity

Abstract Solid-state electrochemical cells can be used to sense and reduce NO x from combustion exhaust gases. In the reduction process the products are N 2 and O 2 . For these cells to be effective in fuel-lean combustion exhaust, cathode materials with high selectivity for NO vs. O 2 are necessary. Numerous compositions of La 1− x A x B 1− y B′ y O 3− δ (where A is Sr or Ba; and B and B′ are transition metals) were investigated for heterogeneous catalytic activity and selectivity for NO reduction using temperature programmed reaction (TPR). Ceramic cells using these materials as cathodes were fabricated and used to electrocatalytically reduce NO in NO/He and simulated exhaust atmospheres. An enhanced electrocatalytic three-way activity for NO x reduction was demonstrated that increases the window of operation into fuel-lean conditions.

Elucidation of behavior of sulfur on nickel-based hot gas cleaning catalysts

Abstract A closed-loop gas-recirculation system was used to measure the isosteric heat of sulfur chemisorption on supported nickel catalysts in hot gas cleaning conditions of gasification gas. During sulfur adsorption, reconstruction of the catalysts occurred. In addition, probably the enormous increase in surface diffusion due to sulfur adsorption on some nickel catalysts with high flow rates resulted in melt formation of adsorbed species on the surfaces of catalyst particles. Heat of sulfur adsorption on nickel decreased when sulfur coverage was increased. However, the enthalpy of adsorption decreased even below the heat of formation of bulk Ni3S2, indicating most likely multi-layer or subsurface sulfur formation on catalyst surfaces. The structural properties of the catalysts had a great influence on sulfur adsorption behavior. The effect of sulfur on ammonia decomposition in synthetic gasification gas tests was explained by the change of heat of sulfur chemisorption on nickel.

Microwave-assisted NO reduction by methane over Co-ZSM-5 zeolites

The microwave-assisted reduction of NO by methane in the presence of oxygen over Co-NaZSM-5, and H-ZSM-5 catalysts was studied. We demonstrated that microwave radiation at 2.45 GHz significantly enhances the catalytic conversion of NO to N2. High conversions (>70%) of NO were achieved over both Co-NaZSM-5 and Co-HZSM-5 zeolite catalysts at temperatures 250–400 ºC. Under similar conditions, thermal runs failed to show significant conversion of either NO or methane.

Interaction of methane and carbon monoxide with oxygen adspecies on Ni(111)

Abstract The elementary step in the surface-catalyzed oxidation of methane was investigated by studying the reaction kinetics of chemisorbed oxygen on Ni(111) with gaseous methane over a temperature range from 423 to 573 K. The rates of disappearance of oxygen aadatoms at specified initial surface densities was monitored by means of Auger electron spectroscopy. At oxygen coverages of less than 1 4 of a monolayer, the rate of surface oxygen removal was of first order. The reaction was more rapid with methane than with carbon monoxide under the same conditions of temperature and reactant concentration. The activation energies were found to be 25±2 kJ mol−1 for methane, and 30±2 kJ mol−1 for carbon monoxide. At oxygen coverages greater than 1 2 of a monolayer, the rates of oxygen removal were reduced significantly. This decline in reaction rate reflects the change in surface coordination of the oxygen adspecies. In comparison with an oxygen-free Ni(111) surface, the presence of oxygen adatoms is highly effective in activating methane.

WSi, FeSi, and CuSi interfaces

The formation and properties of metal-silicon interfaces are of great technological interest in semiconductor, high temperature material, and corrosion applications. Si can be deposited and diffused on metal surfaces to increase corrosion resistance in aqueous acidic environments, oxidation resistance at high temperatures, and erosion resistance. Siliconization of steel to increase its oxidation resistance is performed above 900 C to permit diffusion of Si into the metal. However, the deposition of Si on metals has not been studied in depth. Recently, the authors developed a new coating technique based on chemical vapor deposition (CVD) that allows deposition of Si at much lower temperatures than conventional CVD or pack siliconization techniques. The authors have been able to deposit silicon on W, Fe, and Cu from 350 to 850 C. This paper describes some of the preliminary findings on the metal-Si interfaces formed at these low temperatures.

Impedance Study of the Synergistic Effects of Coal Contaminants: Is Cl a Contaminant or a Performance Stabilizer?

Impedance spectroscopy and current-voltage measurements were used to study the synergistic effects of the coal gas contaminants As, P, S, and Cl at the parts per million level on the performance of the anode-supported Ni-yttria-stabilized zirconia (YSZ)/YSZ/ lanthanum strontium manganese solid oxide fuel cells (SOFCs) at 750°C. Three semicircles were differentiated in cell impedance spectra. The results indicate that the effects of the contaminants are synergistic, can be destructive or constructive, and are not a simple addition of individual contaminants. Adding H 2 S to As and P accelerated the performance degradation. The addition of H 2 S also increased the charge-transfer resistance, whereas the removal of H 2 S resulted in a substantial ohmic resistance increase. The presence of Cl could mitigate or prevent performance loss from an attack by As and P. Impedance spectra show that the presence of Cl species protected the electronic percolation and slowed the rate of the charge-transfer resistance increase. Therefore, the tolerance limit of SOFCs for each contaminant could be significantly increased with Cl species in the gas stream. It is speculated that Cl species could react with nickel phosphides and Ni-P alloy to form NiCl(g), resulting in the restructuring of the catalyst surface.

Production of low-cost solar-grade silicon by reduction of SiF 4 gas with sodium: Technical and industrial developmental status

To meet increasing demand for electrical power using solar photovoltaics, millions of tons of solar-grade silicon costing <

Catalytic Ammonia Decomposition for Coal-Derived Fuel Gases

20/kg will be needed. Low-cost solar-grade silicon can be mass produced by burning Na metal in an atmosphere of pure SiF4 gas—an exothermic, fast, and complete reaction. SRI International routinely uses reactors that can produce at rates >20 metric tons per year (mty), and has licensed the technology to several companies. The Si product is completely separated from the by-product NaF by leaching or melt separation. With the direct use of industrially available Na, B and P levels in the Si product are <1 ppm. Using purified Na results in Si with dopant levels as low as 20 ppb. Experimental ingots grown have resistivity of 1 to 30 Ω·cm, with >100 Ω·cm when purified reactants were used. The purity, electronic parameters, and solar cell efficiency of 15% indicate that this silicon is of solar grade and can be a key contributor in developing solar power markets.

Diffusion Coatings for Corrosion Resistant Components in Coal Gasification Systems

The objective of this study is to develop and demonstrate catalytic approaches for decomposing a significant percentage (up to 90 percent) of the NH{sub 3} present in fuel gas to N{sub 2} and H{sub 2} at elevated temperatures (550 to 900{degrees}C). The NH{sub 3} concentration considered in this study was {similar_to}1,800 to 2,000 ppmv, which is typical of oxygen-blown, entrained-flow gasifiers such as the Texaco coal gasifier being employed at the TECO Clean Coal Technology Demonstration plant. Catalysts containing Ni, Co, Mo, and W were candidates for the study. Before undertaking any experiments, a detailed thermodynamic evaluation was conducted to determine the concentration of NH{sub 3} in equilibrium with the Texaco gasifier coal gas. Thermodynamic evaluations were also performed to evaluate the stability of the catalytic phases (for the various catalysts under consideration) under NH3 decomposition conditions to be used in this study. Two catalytic approaches for decomposing NH{sub 3} have been experimentally evaluated. The first approach evaluated during the early phases of this project involved the screening of catalysts that could be combined with the hot-gas desulfurization sorbents (e.g., zinc titanate) for simultaneous NH{sub 3} and H{sub 2}S removal. In a commercial system, this approach would reduce capital costs by eliminating a process step. The second approach evaluated was high-temperature catalytic decomposition at 800 to 900{degrees} C. In a commercial hot-gas cleanup system this could be carried out after radiative cooling of the gas to 800 to 900{degrees}C but up stream of the convective cooler, the hot particulate filter, and the hot-gas desulfurization reactor. Both approaches were tested in the presence of up to 7,500 ppmv H{sub 2}S in simulated fuel gas or actual fuel gas from a coal gasifier.

A New Activated Carbon for CO2 Capture from Coal-Fired Boiler Flue Gas

Heat-exchangers, particle filters, turbines, and other components in integrated coal gasification combined cycle system must withstand the highly sulfiding conditions of the high temperature coal gas over an extended period of time. The performance of components degrades significantly with time unless expensive high alloy materials are used. Deposition of a suitable coating on a low-cost alloy may improve its resistance to such sulfidation attack, and decrease capital and operating costs. The alloys used in the gasifier service include austenitic and ferritic stainless steels, nickel-chromium-iron alloys, and expensive nickel-cobalt alloys. The primary activity this period was preparation and presentation of the findings on this project at the Twenty-Third annual Pittsburgh Coal Conference. Dr. Malhotra attended this conference and presented a paper. A copy of his presentation constitutes this quarterly report.