The role of AgOAl species in silver–alumina catalysts for the selective catalytic reduction of NOx with methane

We examined the role of silver and alumina in Ag–alumina catalysts for the selective catalytic reduction (SCR) of NOx by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag–alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 °C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [AgOAl] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV–vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [AgOAl] species after leaching. NO–O2-TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NOx adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NOx species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH4 to produce N2 in the presence of oxygen, beginning at ∼300 °C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag–alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH4-SCR of NOx. These catalyze the direct oxidation of methane at temperatures as low as 300 °C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples.