AgNaMordenite catalysts for hydrocarbon adsorption and deNO x processes

Abstract

Silver catalysts were prepared by ion exchange of NaMordenite with 5, 10 and 15 wt% Ag. Various characterization techniques such as TPR, UV–vis and XPS indicated the presence of small particles of highly dispersed Ag 2O together with isolated Ag + cations located in α, β and γ exchange sites of NaMOR. The formation of clusters of cationic silver (Ag n m+ ) was also considered. The prepared samples were active in the Selective Catalytic Reduction of NO x in the presence of toluene or butane as reducing agents, excess oxygen and 2% H 2O. The solid with 15 wt% Ag was the most active one in the presence of water, reaching a maximum conversion of NO x to N 2 of 47.5% or 51.2% when butane or toluene were respectively used. Under dry conditions, the maximum conversion of NO had an optimum between 5 and 10 wt% Ag for both hydrocarbons. The NaMOR support showed a higher adsorption capacity than the exchanged samples with both hydrocarbons. For the silver loaded solids, the toluene adsorption capacity at 100 °C increased with the increase of the metal content. In contrast, the amount of butane adsorbed was similar for the different contents of Ag. Consequently, silver has two opposite effects: one is the partial obstruction of the mordenite channels, as seen by the loss of crystallinity and the decrease of surface area and pore volume; and the other effect is the chemical interaction that depends on the nature of the adsorbed hydrocarbon. The interaction between toluene and Ag + ions is stronger with the π-electrons of the aromatic ring of the toluene molecule than with the σ-electrons of the linear chain of butane. For this reason, toluene is retained at higher temperatures than butane. In addition, between 300 and 500 °C, the appearance of signals corresponding to H 2, CO 2 and H 2O is observed during the TPD of toluene. This indicates that the toluene decomposition occurs, producing coke and hydrogen. Most probably, the generation of CO 2 and H 2O is a consequence of the reduction of Ag 2O particles with toluene.