Silver/alumina for methanol-assisted lean NOx reduction—On the influence of silver species and hydrogen formation

Abstract

High low-temperature activity for lean NOx reduction can be achieved for silver/alumina by using an oxygenated reducing agent. In this system the catalytic reactions, including the H2 formation previously observed during methanol-SCR conditions, are strongly dependent on the composition of surface silver species. With the aim to increase the understanding of the role of supported silver species in combination with the methanol-SCR reactions, catalysts with the same silver loading but different composition of silver species are prepared by utilizing different preparation methods. The supported silver species are characterized by UV–vis spectroscopy and transmission electron microscopy, while the catalytic performance for lean NOx reduction with methanol is investigated in flow reactor experiments and surface species studied by DRIFT spectroscopy. The results indicate that hydrogen atoms are abstracted mainly from the methyl group during the conversion of surface species formed from methanol. The hydrogen atoms could contribute to reduction of the catalyst or affect the catalytic reactions in other ways, before they react to form H2 or H2O. Here, more H2O is formed over the samples containing more silver nanoparticles. The released hydrogen atoms are suggested to explain the high NOx reduction at low temperature associated with oxygenated reducing agents, rather than the subsequently formed gaseous H2. Furthermore, the results show that lean NOx reduction with methanol is determined by silver species in the size range from small silver clusters to small silver nanoparticles (<ca20nm). Small silver nanoparticles (<ca20nm) are concluded to catalyse formation of N2O. The role of silver ions and small clusters, on the other hand, is suggested to be promotion of N2 formation. The largest silver nanoparticles observed in this study (>ca20nm), however, do not have a significant impact on the catalytic reactions. Finally, in order to achieve both high low-temperature activity, as well as high selectivity to N2 a combination of small and somewhat larger silver species are needed.