Activity and stability of silver-alumina catalysts for the selective catalytic reduction of nitric oxides with methane.

摘要

Nitric oxides (NOx) emitted from automobiles and stationary sources such as power plants, are major air pollutants which cause acid rain and smog. Two commercialized technologies for NOx abatement from gasoline-fueled vehicles and stationary large-scale boilers, respectively, are the three-way catalyst (TWC) and the selective catalytic reduction (SCR) of NOx by ammonia. However, these technologies have well-known drawbacks.; Lean NOx-SCR with hydrocarbons is a promising technology currently under development for application to fuel-efficient vehicles. The main barrier to development of HC-SCR technology is the present low activity of catalysts in realistic gas mixtures, especially if they contain SO2. Silver-alumina materials are promising catalysts for NOx-SCR with hydrocarbons and oxygenates used as reductants. These catalysts have good activity and selectivity to dinitrogen and moderate resistance to water and SO2. In this thesis work, a cogelation method was used to prepare Ag-alumina catalysts with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650°C. These are highly active catalysts for CH 4-SCR at 450-650°C. Typically, a part of silver is present as fine nanoparticles on the alumina, while another part is ionic, bound with the alumina as [Ag-O-Al] species. We further examined the roles of silver and alumina in Ag-alumina catalysts for the selective catalytic reduction (SCR) of NOx by methane in gas streams containing excess oxygen. 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 content of silver, the higher the amount of the residual [Ag-O-Al] 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 CH 4 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. (Abstract shortened by UMI.)