On the Characterisation of Silver Species for SCR of NO x with Ethanol

Reducing of nitrogen oxides (NO (x) ) in a lean exhaust gases has become one of the most important environmental concerns. Among the different active phases studied for NO (x) reduction reaction, silver-based catalysts supported over alumina show good performances using, as reducing agents, either hydrocarbons or oxygenated compounds. Nevertheless, a good understanding of the mechanism reaction has not been reached yet. This comprehension requires a better characterisation of the silver-based catalysts system. In our study, Ag/Al2O3 catalysts showed high efficiency in NO (x) reduction using ethanol as reducing agent. The conversion plots, in steady state conditions for the different samples Ag/Al2O3 (0.8-3.5% Ag wt), show a great dependance of the activity with the metal loading. The optimal silver loading has been established around 2 wt.% Increasing the silver loading, the temperature of maximal NO (x) conversion shifted toward the lower temperatures. According to the literature, a reduced and an oxide phase of silver have been observed by UV-Vis spectroscopy. The ratio between the two phases is changing with the silver loading. However, temperature programmed reduction (TPR) measurements reveal the presence of two types of oxide phases. TPR reveal the coexistence of a silver oxide phase (Ag2O), according to a production of water in the course of the reaction, and a non-oxygenated phase attributed to isolated Ag+ cation. Thus, an original way using TPR measurements has been developed to differentiate the various oxidized phases. The aim of this characterisation is to correlate the catalyst's activity with the observed silver phases, in order to understand the nature of phase active for NO (x) reduction at low temperatures.