Characterization of Porous ß-Alumina and Perovskite based Materials for their Selective Catalytic NOx-Reduction Activity at Temperatures up to 700°C

Abstract

High temperature lean-burn combustion is one of the most effective approaches for the improvement of engine efficiency in both gas turbines and diesel engines. Stringent legislation to be released beyond 2008 concerning emissions for turbines and vehicles is already underway. NOx-emission reduction can be dealt effectively by catalytic combustion as it offers the potential of ultra-low NOx emissions (< 3 ppm), improved flame stability and fewer pulsations. Drawbacks are extremely high gas temperatures, restrictions in provided space in combustor and problems related to pre-mixture conditions. Exhaust gas after treatment is common application for fuel engines and can be adapted into the diesel engines and stationary gas turbines. Possible positions for a NOx-catalytic converter are hot-gas outlets behind the combustor and turbo-engines which require high-temperature stability of materials. ß-alumina and perovskite based ceramics doped with redox-active elements (Mn, Co) are good candidates for such applications through their high oxygen vacancies. Such ceramics can be produced as porous layers by chemical processing or electron-beam vapor deposition on porous-matrix fiber reinforced oxide composites. Sol-Gel-processed powders with surface areas from 9 to 13 m2/g are tested for their catalytic activity in a rig where a pre-heated carrier gas having 100-500 ppm NO and 4-8% O2 with a space velocity of 75 sccm/h.kg is sent over the catalytic material. Outlet gas is analyzed by means of a mass spectroscopy to determine the quantity and type of produced gas after conversion on the catalytic material. The properties of the 5 different catalytic materials are tested for direct NO-decomposition capacity and for selective catalytic reduction with addition of methane or propane. As the FT-IR analysis indicated, Mn-doped ß-alumina materials are restricted in terms of methane reduction since the adsorbed NO-species are non-reactive with methane. The experiments are conducted to overcome this behavior by using high chained hydrocarbons for reduction and/or by altering the composition with incorporation of Sr.