Selective catalytic reduction of NOx with methanol over supported silver catalysts

Abstract

Methanol is a potential renewable fuel for the transport sector and is thus interesting to study as reducing agent for NOx in hydrocarbon assisted selective catalytic reduction (HC-SCR). The effect of catalyst composition on the lean NOx reduction during methanol-SCR conditions was investigated. In particular silver supported on alumina and ZSM-5 were in focus and parameters that can improve the NOx reduction performance, i.e. the C/N molar ratio and the addition of hydrogen, were specifically studied. Five catalysts were prepared (H-ZSM-5, Ag/H-ZSM-5, Pd/Ag/H-ZSM-5, γ-Al2O3 and Ag-Al2O3) and compared in flow reactor experiments. The Ag-Al2O3 (2wt% Ag, sol–gel) sample was found to give higher NOx reduction compared to the other tested samples, where important factors are suggested to be the support material, the preparation method and the Ag loading. The NOx reduction over Ag-Al2O3 with methanol is proposed to proceed by adsorbed nitrates or nitrites (or gas phase NO2) reacting with adsorbed acetate or formaldehyde like species forming adsorbed RNO2 or RONO, with subsequent conversion into NCO, CN, RNH2 or NH3, with final desorption of N2 and COx (or HCHO). The addition of hydrogen and an increase of the supplied methanol concentration resulted in an increased NOx reduction mainly over the Ag-Al2O3 sample, where the addition of hydrogen also extended the active temperature interval towards lower temperatures. Another effect of the addition of hydrogen was formation of more oxidized carbon containing reaction products. The Ag-Al2O3 catalyst prepared by the sol–gel method, including freeze-drying of the formed gel, is concluded to be the most promising candidate of the tested catalysts for methanol-SCR.