High-surface-area zinc aluminate supported silver catalysts for low-temperature SCR of NO with ethanol

Abstract

The NOx selective catalytic reduction of ethanol (EtOH-SCR) was studied using a complex gas mixture representative of a diesel exhaust, over zinc alumina mixed oxide supported silver catalysts (2wt.% Ag/Znx-Al2O3, with x=10–20–33at.%). The supports were obtained using a template assisted sol–gel route in order to achieve high surface areas. For the higher Zn loading (33%), the calcination temperature has been raised from 600°C to 800°C then 1000°C in order to evaluate thermal stability of these materials. Addition of zinc in alumina network leads to the formation of the spinel-type zinc aluminate structure. Chemical and physical characterizations of the catalysts have been confronted with the EtOH-SCR results, in order to understand the respective influence of the metal and support in nitrogen formation. This study shows that addition of zinc and modification of the support calcination temperature both play upon the Lewis acidic sites (LAS) concentration and density, determined by pyridine adsorption monitored by FTIR spectroscopy. This parameter is shown to be related with ammonia emission at T≥350°C. Besides, formation of nitrogen at T≤350°C is shown to be dependent on (i) the rate of acetaldehyde formation and (ii) the reactivity of acetaldehyde in SCR of NO reaction. Modification of the alumina support directly impacts these parameters. Finally, it is demonstrated that Zn is a hopeful candidate to increase the EtOH-SCR activity at low temperature (T≤300°C).