Rate-determining Step of Selective Catalytic Reduction of NO by Acetylene Over HZSM-5
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Rate-determining Step of Selective Catalytic Reduction of NO by Acetylene Over HZSM-5 Xinping Wang Æ Qing Yu Æ Guangfeng Li Æ Zhiguang Liu
Received: 18 January 2008 / Accepted: 24 January 2008 / Published online: 12 February 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Nitrate species is crucial intermediate of selective catalytic reduction of NO by acetylene (C2H2SCR) over HZSM-5. It is proposed that formation of nitrate species on the zeolite is rate-determining step of C2H2SCR. The proposition is supported by the experimental results: (1) No nitrate species could be detected by in situ FTIR in steady C2H2-SCR over HZSM-5 at 350 °C, while the bands due to nitrate species were clearly observed on the zeolite both in situ in NO + O2 and after a brief evacuation at the temperature; (2) Yttrium incorporation into HZSM-5 zeolite considerably enhanced nitrate species formation on the catalyst and correspondingly the activity of the catalyst for C2H2-SCR was significantly increased. Both the promotional effect of yttrium on C2H2-SCR and the rate-determining step of C2H2-SCR being nitrate species formation over the catalyst were first reported herein. Keywords Nitric oxide Nitrate Rate-determining step Promotional effect Selective catalytic reduction of NO
1 Introduction Since the first reports on selective catalytic reduction of NO by hydrocarbons (HC-SCR) [1, 2], extensive research into the HC-SCR applications for the control of NOx in exhaust gases of diesel and lean-burn engines has been carried out [3–9]. For the practical application of the HCSCR, an active catalyst capable of reducing NO in large space velocity at ca. 300 °C is required, in addition to high X. Wang (&) Q. Yu G. Li Z. Liu State Key Laboratory of Fine Chemicals, Dalian University of Technology, 288# Linggong Road 2, Dalian 116024, China e-mail: [email protected]
selectivity of the reductant towards the target reaction. For this purpose, having an insight into the reaction mechanism of the HC-SCR is significantly of importance to the researchers investigating in this field. In the previous investigation, we reported that acetylene is highly active and selective towards NO reduction under lean-burn conditions over Ce-HZSM-5 [10] and Mo/HZSM-5 [11] in the temperature range 250–450 °C. In this paper, we report that yttrium incorporation into HZSM-5 can also significantly increase the activity of the zeolite for selective catalytic reduction of NO by acetylene (C2H2-SCR). NO removal with about same level as those over Ce-HZSM-5 and Mo/HZSM-5 could be obtained under same reaction conditions over 3% Y/HZSM-5 catalyst. As it is the first time to report the effect of yttrium on selective catalytic reduction of NO by hydrocarbons, the mechanism concerning the promotional effect of yttrium on C2H2-SCR was investigated. We found that nitrate species is a crucial intermediate of C2H2-SCR over HZSM-5 and that the nitrate species formation on the zeolite is rate-determining step of the C2H2-SCR. Yttrium incorporation into HZSM-5 prom
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