The advent of highly energy-efficient automotive engines such as diesel and lean-burn engines for reducing fuel consumption and CO2 emissions has not resolved the emission of NOx due to the requirement of the oxygen-rich feed gas stream to the engines [1-2]. The selective catalytic reduction by hydrocarbons (HC-SCR) might be a promising optional technology to meet the ever stringent air pollution regulations including SULEV and EURO V. Among a number of catalysts suggested to be effective for the SCR reaction, Cu-ZSM-5 and Ag/Al2O3 have been commonly recognized as a good catalyst [3-7]. Iwamoto et al. [3] proposed Cu/ZSM-5 catalyst as a promising HC-SCR catalyst under oxygen rich condition, though its water tolerance has been improved yet [4]. Ag/Al2O3 catalyst has been suggested to be active for HC-SCR by long-chain hydrocarbons such as octane, decane, dodecane, etc [5-6]. The effect and role of the preparation methods including Ag content, the state of Ag, alumina support, etc., and the hydrocarbons as a reducing agent on the high de-NOx performance of the HC-SCR have been extensively investigated [6-7]. However, the effect of thermal aging of Ag/Al2O3 catalyst on the NO removal activity by dodecane-SCR has not been clearly identified yet. The purpose of the resent study is to examine the alteration of the deNOx activity over Ag/alumina after thermal aging at 550, 700 and 800oC by dodecane-SCR. The NO conversion activity to N2 generally shifts to the higher reaction temperatures as the aging temperature decreases and C1/NO feed ratio to the reactor increases. This trend by the alteration of the reaction temperature and C1/NO ratio achieving the maximum NO conversion may offer a significant basis for synthesizing an effective HC-SCR process by combining the catalytic systems. Ag/γ-Al2O3 catalysts aged at the temperature higher than 600oC revealed the high performance of NO conversion into N2 within the reaction temperature range less than 300oC. In the reaction temperature region greater than 350oC, the conversion exponentially decreased with respect to the catalyst aging temperatures. In particular, Ag/γ-Al2O3 catalyst aged at 800oC showed a considerably low NO conversion to N2 within the range of the reaction temperatures higher than 400oC when C1/NO ≈ 6, but revealed a considerably wide operating temperature window of deNOx activity at C1/NO ≈ 9. It may be simply due to the formation of Ag metal clusters upon the catalyst sintering on the catalyst surface by UV-Vis spectroscopy. Three distinguishable spectroscopic bands at ~220, ~290 and ~350 nm over Ag/γ-Al2O3 catalysts were confirmed as shown in Figure 1. They are mainly attributed to Ag+, Agn1 small metallic clusters and Agn2 large metallic clusters, respectively [7]. The number of large Ag metal clusters on the catalyst surface increases as the aging temperature increases. Although Ag+ is commonly recognized as a major active reaction site for dodecane-SCR process in the range of the C1/NO feed ratio less than 6, Ag0 species plays an important role for the high deNOx catalytic activity when the C1/NO ratio is higher than 6. It is probably due to the stronger activation of the reductant, dodecane to the active NO reducing intermediate such as CHN compound on Ag metal clusters than that of Ag+. It can be concluded that the appropriate combination of Ag+ and Ag0 species on the catalyst surface might be critical for the high deNOx performance of Ag/γ-Al2O3 catalyst by dodecane-SCR technology in the range of the high C1/NO feed ratio greater than 6. In addition, by in-situ FT-IR measurements under the feed gas stream containing NO+O2+H2O+n-dodecane, NO+O2, etc., the surface oxygenated hydrocarbons such as acetate-like species has been confirmed as a hydrocarbon-derived active reaction intermediate for the reduction of NO to N2 [7].
References [1] W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, E.P. James, Catal. Rev. 46 (2004) 163. [2] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology, Van Nostrand Reinhold, New York, 1995, p. 94. [3] M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u, N. Mizuno, Appl. Catal. 70 (1991) L15. [4] P. Konova, K. Arve, F. Klingstedt, P. Nikolov, A. Naydenov, N. Kumar, D. Yu. Murzin, Appl. Catal. B 70 (2007) 138. [5] L.-E. Lindfors, K. Eränen, F. Klingstedt, D. Murzin, Top. Catal. 28 (2004) 185. [6] K. Arve, L. Čapek, F. Klingstedt, K. Eränen, L.-E. Lindfors, D. Murzin, J. Dědeček, Z. Sobalik, B. Wichterlová, Top. Catal. 30/31 (2004) 91. [7] K. Shimizu, A. Satsuma, Phys. Chem. Chem. Phys., 8 (2006) 2677.
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