Selective Reduction of No By C3 and C8 Alkanes Over Silver Catalysts on Structured Al2O3/Cordierite Supports

Abstract

O. Soloviev, and S. M. OrlykAn extremal dependence of the catalytic activity of Ag/Al 2 O 3 /cordierite on the silver content in the selective catalytic reduction of NO with propane and octane was discovered. The optimum content of Ag was higher during reduction with octane (0.5 wt.%) than with propane (0.3 wt.%). The role of the silver involves formation not only of oxidation-reduction centers but also of additional Lewis acid centers at which activation of the reagents (NO and C n H m ) occurs.Neutralization of nitrogen oxides in the exhaust gases of diesel and lean-burn gasoline engines with a high oxygen content is an important problem for environmental catalysis [1,2]. Silver-alumina catalysts have been widely investigated for the selective catalytic reduction (SCR) of nitrogen oxides by the C 3 -C 10 hydrocarbons that are products from incomplete combustion of the fuel [3-6]. The effectiveness of block catalysts with honeycomb structure Ag/Al 2 O 3 /cordierite in the SCR of NO by ethanol and butanol, which can be components of motor fuel, was demonstrated in [7-9]; the effect of the composition of the catalysts on their activity was determined.In the present work the role of the silver in structured Ag/Al 2 O 3 /cordierite catalysts in SCR of NO by propane and octane, i.e., in reaction mixtures close in composition to exhaust gases, was investigated.Samples of silver-alumina catalysts deposited on a structured support from cordierite were prepared by the previously developed method [7,8], in which the optimum concentration of Al 2 O 3 amounts to 45-50 wt.%. The prepared Ag/Al 2 O 3 /cordierite catalysts contained 0.1, 0.3, 0.5, and 1.0 wt.% of silver and 46 wt.% of alumina.The catalytic activity of the samples in the selective reduction of NO with propane and octane were characterized by the conversion of NO to N 2 /N 2 O. A fragment of the block catalyst (diameter 8 mm, height 5 mm) was loaded into a flow-type quartz reactor. The investigations were carried out with a gas mixture with the composition (vol.%) 0.05NO + 0.2HC 1 (C 3 H 8 or C 8 H 18 ) + 10.0O 2 + He with a volume flow rate of 48000 h -1 in the presence of water vapor (2.0 vol.%). The NO was analyzed by means of a gas analyzer with a chemiluminescence detector, and the other reactants were analyzed by chromatography (thermal conductivity detector).In order to study the acidic characteristics of the active phase of the catalysts by IR spectroscopy Ag/Al 2 O 3 samples were prepared by analogy with the method in [7,8]; they contained 0.2, 0.6, and 1.2 wt.% of Ag, which is close in the ratio of the components to the active phase of the investigated structured catalysts. The samples in the form of tablets were submitted to preliminary heat treatment under vacuum at 400°C for 1 h, after which the background IR spectrum at room temperature was 122 0040-5760/15/5102-0122