Continuous reduction of NO with octane over a silver/alumina catalyst in oxygen-rich exhaust gases:combined heterogeneous and surface-mediated homogeneous reactions

摘要

An Ag/alumina catalyst was prepared and tested for its HC(octane)-SCR activity using a gas mixture simulation exhaust gas from diesel and lean-burn vehicles The catalyst showed excellent NO to N_2 activity having a maximum conversion of approcimately 90% at 450 deg C and an average conversio of 66% in the temperature window 300-600 deg C During the high conversion of NO formation of CO was recorded A commercial oxidatin ("cheanup") catalyst was placed after the Ag/alumina in order to remove the formed CO as well as unburnt hydrocarbons This resulted however in a drastic drop of NO conversion which was more pronounced when the distance was short between the tow catalysts When teh distance between the two catalysts was increased to 33 mm (maximum value) the conversion was almost unaffected This raised the question how nitrogen once formed from nitrogen oxides can seemingly disappear over the oxidizing catalyst at temperatures far too low to make this phenomenon thermodynamically feasible In this study we propose that the HC-SCR reaction mechanism over Ag/alumina involves a gas-phase reaction bwtween species created over the catalyst surface and that this reaction is dependent on the residence time behind the catalyst.