Applied Catalysis A: General 393 (2011) 323–330 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Synthesis, characterization and catalytic activities of vanadium–cryptomelane manganese oxides in low-temperature NO reduction with NH 3 Liang Sun a,c , Qingqing Cao a , Bingqing Hu a , Junhua Li b , Jiming Hao b , Guohua Jing c , Xingfu Tang a,∗ a Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China b Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China c College of Chemical Engineering, Huaqiao University, Xiamen 361021, China article info Article history: Received 26 September 2010 Received in revised form 30 November 2010 Accepted 7 December 2010 Available online 13 December 2010 Keywords: NH3-SCR Cryptomelane-type manganese oxides Vanadium doping Lewis acid Surface redox abilities abstract Vanadium doped cryptomelane-type manganese oxides (V-OMS-2) with V/Mn molar ratios of 0–10% were synthesized and investigated for low-temperature selective catalytic reduction of NO by NH 3 (NH 3 -SCR). The characterization results of XRD patterns and Raman spectra demonstrated that V 5+ iso- morphously substituted for framework Mn 4+ of the V-OMS-2. The adsorption and desorption of NH 3 and H 2 -TPR results revealed that both surface defect sites (Lewis acid) and redox abilities were efficiently controlled by amounts of vanadium doping, and more vanadium dopings resulted in more Lewis acid sites and weaker bulk redox abilities. The results of catalytic tests revealed that the 2%V-OMS-2 catalyst showed the highest catalytic activities among the V-OMS-2 catalysts. In fact, the desired doping amount of V 5+ can not only provide more Lewis acid sites for adsorption of NH 3 but also maintain excellent surface redox abilities for activation of NH 3 , which resulted in higher catalytic activities in the low-temperature NH 3 -SCR reactions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Nitrogen oxides (NO x , x = 1 or 2) as air pollutants can cause adverse effects on the environment such as urban smog, acid rain and ozone depletion [1,2], and nearly 45% NO x emission origi- nates from the stationary source [3]. Low-temperature selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) is more promising to treat stack gases from stationary source than the conventional high-temperature NH 3 -SCR (300–400 ◦ C) from the economic view- point. Some transitional metal oxide catalysts have been proven to be active for low-temperature NH 3 -SCR, and among them man- ganese oxides showed the best catalytic activities [4–6]. Specially, cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) showed strong adsorption and activation abilities to NH 3 and excellent low-temperature NH 3 -SCR activities [7,8]. The OMS-2 have MnO 6 octahedra as basic building units, which share two opposite edges to form a single octahedral chain, and two single chains further share two other neighboring edges from each octahedron to build a double chain. Four double chains construct corner-linking one-dimensional tunnel [9]. The cryp- tomelane structure consists of a hexagonal close packed anionic ∗ Corresponding author at: Department of Environmental Science and Engineer- ing, Fudan University, No. 220, Handan Road, Shanghai 200433, China. Tel.: +86 21 55664880; fax: +86 21 65643597. E-mail address: tangxf@fudan.edu.cn (X. Tang). lattice and contains two different kinds of oxygen atoms, one at the center of an almost equilateral triangle of Mn 4+ cations, adopting an sp 2 hybridization with average Mn–O sp2 bond length of 0.188 nm, and the other at the apex of a trigonal pyramid of Mn 4+ cations, adopting an sp 3 hybridization with average Mn–O sp3 bond length of 0.192 nm [10]. Obviously, the Mn–O sp3 bonds with lower bond energies show stronger adsorption and activation abilities to NH 3 than the Mn–O sp2 ones. For instance, Wang et al. reported that the OMS-2 had strong adsorption ability to NH 3 [7], and that the pres- ence of NO accelerated the oxidation of NH 3 to produce N 2 at low temperatures [8]. For low-temperature NH 3 -SCR, the adsorption and activation of NH 3 have been proven to be crucial for governing catalytic activities, whereas adsorption and activation of NO appear to be less crucial for these reactions [11,12]. Correspondingly, Lewis acid and redox ability of the catalysts are intimately associated with the adsorption and activation of NH 3 in the NH 3 -SCR reac- tions. As a matter of fact, the increase of Lewis acid frequently enhances adsorption capability to NH 3 because the existence of Lewis acid appears to be a requisite for low-temperature NH 3 adsorption [13–15], and the enhancement of redox property favors the activation of NH 3 . Lietti and Forzatti reported that both sur- face acidity and redox property of the catalyst had important roles in determining its NH 3 -SCR activities under the certain reaction conditions [16]. Kapteijn et al. found that manganese oxide with lower temperature of the onset of reduction in the H 2 -TPR profiles showed higher activities in the NH 3 -SCR reactions, which implied 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.12.012