Journal of Catalysis 237 (2006) 426–430 www.elsevier.com/locate/jcat Research Note Surface structure effects in nanocrystal MnO 2 and Ag/MnO 2 catalytic oxidation of CO Run Xu, Xun Wang, Dingsheng Wang, Kebin Zhou, Yadong Li ∗ Department of Chemistry and the Key Laboratory of Atomic & Molecular Nanosciences (Ministry of Education, China), Tsinghua University, Beijing 100084, PR China Received 4 July 2005; revised 22 October 2005; accepted 25 October 2005 Abstract Pure-phase α-MnO 2 and β -MnO 2 nanowires/nanorods were synthesized through an easy solution-based hydrothermal method, and the effect of the manganese dioxide phase on the activity of MnO 2 and Ag/MnO 2 for the oxidation of CO was investigated. MnO 2 is an effective catalyst in CO oxidation, and its activity depends on the crystal phase of MnO 2 . α-MnO 2 exhibits a higher activity than β -MnO 2 , because the α-MnO 2 nanowires can be reduced more easily than the β -MnO 2 nanorods. Moreover, when Ag was introduced to MnO 2 , a strong interaction occurred between Ag and MnO 2 . The catalytic activity clearly correlates with this interaction, which is determined by crystal phase and surface structure.  2005 Elsevier Inc. All rights reserved. Keywords: MnO 2 nanocrystal; Controlled-synthesis; Crystal phase; CO oxidation 1. Introduction The performance of catalysts depends strongly on their sur- face structure and surface active sites, which have a direct relationship with the crystal planes, crystal phases, and stere- ochemistry of catalysts [1,2]. A typical case is iron catalysts used for the synthesis of ammonia, for which the reaction rate on the {111} planes is more than 400 times higher than that on the {110} planes and roughly 15 times higher than that on the {100} planes [1,3]. Bell et al. [4–7] reported that differ- ent crystal phases of zirconia-supported copper catalysts exhibit different activity for the synthesis of methanol, and the cat- alyst prepared on monoclinic-ZrO 2 is 7.5 times more active than that prepared on tetragonal-ZrO 2 . Consequently, a feasible approach to designing and constructing catalysts with high ac- tivity and selectivity may be through the controllable synthesis of catalyst materials with well-defined crystal planes or crystal phases. We recently reported that using an easy solution-based hydrothermal method, CeO 2 nanorods exposing mostly {001} and {110} planes can be synthesized; these CeO 2 nanorods * Corresponding author. Fax: +86 10 62788765. E-mail address: ydli@mail.tsinghua.edu.cn (Y. Li). show higher catalytic activity for CO oxidation than CeO 2 nanoparticles [8,9]. Morphology-controlled synthesis of nano- materials may present an opportunity for the synthesis of cat- alytic materials with desired properties, because these novel materials nucleate and grow in an epitaxial manner [10,11]. CO oxidation is an important process in three-way cataly- sis for the treatment of exhaust gas from automobiles and in selective oxidation of CO in reformer gas for fuel cell applica- tions [12]. Precious metals, such as Au, Pt, and Pd, have high activity and stability for the oxidation of CO [13,14]. Recently, considerable research has focused on base metal catalysts for CO oxidation, because of the limited availability of precious metals. Manganese dioxides and derivative compounds have at- tracted special attention and have been widely used as catalysts and catalyst supports due to their redox capabilities [15,16]. These redox capabilities are strongly enhanced when other el- ements are combined [17]. Imamura et al. [15] reported that Ag–Mn composite oxides exhibited high activity for CO oxi- dation. A combination of Ag and Mn seems to be an important contributing factor to catalytic activity [18,19], but details of the action of Ag and Mn in these catalysts are not yet avail- able. The main reason for this may be that the performance of the manganese oxides for those applications is critically controlled by their phase composition and textural properties, 0021-9517/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2005.10.026