Selective catalytic reduction of NOx with NH3 and CH4 over zeolite supported indium-cerium bimetallic catalysts for lean-burn natural gas engines

Abstract

Selective catalytic reduction (SCR) is recognized as an effective technology to control NOx emissions from lean-burn natural gas engines, but combining ammonia and methane as reductants for SCR of NOx has not been studied systematically. Herein, we investigated the SCR of NOx with NH3 and CH4 over InCeOx/HBEA catalysts. Different characterization methods were applied, including XPS, XRD, TPR, EPR, NMR, TEM, SEM, TPD and in-situ DRIFTS. The results reveal that the In8CeOx/HBEA catalyst presents high activity in a wide temperature range. Its NOx conversion, N2 selectivity and NH3 consumption are nearly 100% from 250 to 600 °C during the NH3/CH4-SCR process. Meanwhile, the In8CeOx/HBEA catalyst shows good tolerance to CO2, H2O and SO2. In addition, it was also found that NOx were mainly adsorbed on cerium species, and then oxidized to nitrite and nitrate species. NH3 is mainly adsorbed on the Bro¨nsted and lewis acid sites to form NH4+ and coordinated NH3 species. The ammonia nitrite and nitrate species are the main active intermediates for NH3-SCR. Meanwhile, methane is activated on InO+Z– sites, and then reacts with surface nitrate/nitrite species to form a short-lived intermediate nitromethane (CH3NO2) for CH4-SCR. Furthermore, a series of elementary steps are proposed to elucidate the SCR reaction mechanism. The fast NH3-SCR process is mainly responsible for NOx reduction in the low temperature range, and the standard NH3-SCR and CH4-SCR process are mainly responsible for NOx reduction in the high temperature range. These results show that In8CeOx/HBEA is a promising catalyst for the removal of NOx from natural gas fueled engines.