Activation by pretreatment of Ag–Au/Al2O3 bimetallic catalyst to improve low temperature HC-SCR of NOx for lean burn engine exhaust

Abstract

Bimetallic Ag–Au/Al2O3 catalyst was synthesised by successive impregnation of 1% Au and 1% Ag on in-house prepared high surface area alumina (450m2/g). The corresponding monometallic catalysts were also prepared by loading 1% Ag or 1% Au on the same high surface area alumina for comparison. The catalysts were characterised by various physico-chemical techniques and tested for SCR activity under lean burn engine exhaust conditions. Ag–Au/Al2O3 catalyst prepared by successive impregnation method showed considerably higher NO reduction (100%) to N2 compared to 1% Au/Al2O3 (70%) whereas the activity was comparable with that of 1% Ag/Al2O3 (96%). The effect of various pretreatments on SCR activity of Ag–Au/Al2O3 was studied and pretreatment at 250°C in flow of hydrogen was found to give the best results with 100% NO conversion to N2 at 353°C. Further ageing of the catalyst under reaction feed at 500°C resulted in considerable increase in low temperature activity of bimetallic catalyst with ∼40% NO conversion at 222°C. Even though the SCR activity of pretreated Ag–Au/Al2O3 and Ag/Al2O3 were comparable, after ageing the Ag–Au/Al2O3 showed significantly higher NO conversion (95%) compared to Ag/Al2O3 (83%) and Au/Al2O3 (70%). The formation of H2 and CO due to steam reforming of higher hydrocarbon (decane) was evidenced at the temperature of highest deNOx activity. Detailed investigation of the textural properties of the pretreated and aged catalysts showed presence of well dispersed metallic Au and Agnδ+ clusters after pretreatment in hydrogen at 250°C.