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NIRE Annual Report
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1997

Development of Flue Gas DeNOx Catalyst

Combustion Engineering Division
Thermal Energy and Combustion Engineering Department
Objectives
Nitrogen oxides in exhaust gases from automobiles and stationary combustion facilities have been causing serious air pollution in urban areas. In this study, an attempt was made to develop new catalysts which can be used for the selective reduction of NOx with hydrocarbons and alcohols.
Results
NOx reduction activities of various alumina-supported catalysts was investigated by using a fixed bed flow reactor. Alumina was selected as a support because it has higher thermal stability than zeolites. Among the tested catalysts, Ag/ Al2O3 had the highest activity in the reduction of lean NOx with hydrocarbons in the presence of water vapor and SO2. It was found that Ag/Al2O3 was quite effective in reducing NOx by oxygen-containing organic compounds such as ethanol and 2-propanol even in the presence of water vapor and excess oxygen. There was a substantial amount of nitrogen-containing by-products such as N2, NH3, CH3CN, and HCN besides the main product, N2, when a gas mixture of NO + ethanol + O2 /He is allowed to react over Ag/Al2O3. The effect of SO2 on the reduction of NOx with ethanol over Ag/Al2O3 was also investigated. The efficiency of NOx reduction at low temperatures was extremely decreased in the presence of SO2. It was found that the reaction between ethanol and NOx was suppressed by SO2 and that the dehydration of ethanol was occurred. This dehydration causes the change of ethanol to ethylene which is less efficient reductant of NOx.
Surface species adsorbed on the catalyst in the catalytic reduction of NOx with ethanol were observed by means of infrared spectroscopy. Two intense IR absorption bands due to isocyanate (-NCO) species appeared at 2262 cm-1 and 2232 cm-1 when Ag/Al2O3 was exposed to a mixture of NO, O2 and ethanol at 150°C and subsequently heated to >300°C in vacuum. It has been already revealed that surface NCO plays an important role as an intermediate in the NO/O2/hydrocarbon system. When hydrocarbons were used as reductant both the NOx reduction efficiency and the formation of NCO were extremely decreased by water vapor. On the other hand, the intensity of NCO was hardly affected by water in the case where ethanol was used as reductant. Since the ethanol-containing system was effective in reducing NOx even in the presence of water, the NCO species seems to be a reaction intermediate of the NOx reduction.
Selected Publications
1) Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene and oxygen-containing organic compounds, Appl. Catal. B 2(1993) 225.
2) Alumina-supported Catalysts for the Selective Reduction of Nitric Oxide by Propene, Chemistry Letters, (1993) 1483.
3) Selective Reduction of NOx by Hydrocarbons over Supported Silver Catalysts, J. of the Japan Institute of Energy, 73(1994) 987.
4) Infrared Study of Catalytic Reduction of Lean NOx with Alcohols over Alumina-supported Silver Catalyst, Catal. Lett., 39(1996)265.

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