Applied Surface Science 257 (2011) 8647–8652 Contents lists available at ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Density functional theory study on activity of -Fe 2 O 3 in chemical-looping combustion system Changqing Dong ∗ , Shuhui Sheng, Wu Qin, Qiang Lu, Ying Zhao, Xiaoqiang Wang, Junjiao Zhang National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Engineering, North China Electric Power University, Beijing 102206, China a r t i c l e i n f o Article history: Received 4 January 2011 Received in revised form 28 March 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: CO -Fe2O3 Chemical-looping combustion Density functional theory a b s t r a c t The dominant growth planes (0 0 0 1) and (1 1 0 2) have been used to investigate the activity of the natural -Fe 2 O 3 in chemical-looping combustion system based on density functional theory (DFT) calculations. In the chemical-looping combustion system, CO is selected as the probe fuel gas to detect the activities of the different surfaces. CO interacts stronger to Fe 2 O 3 (1 1 0 2) than Fe 2 O 3 (0 0 0 1). CO can be oxidized into CO 2 species directly on Fe 2 O 3 (1 1 0 2) rather than Fe 2 O 3 (0 0 0 1). The formation of CO 2 accompanying with a transformation from hematite to magnetite acted as the key step for the reduction process of hematite. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The chemical-looping combustion (CLC) has received increas- ing interest as a promising technology for the CO 2 capture [1–7]. This technique not only increases the thermal efficiency in power generation stations, but also exhibits inherent advantages for CO 2 separation. Thus it is attractive for minimizing emissions of green- house gases. To date, many research efforts have been taken in the design of reactor [1], the selection and synthesis of oxygen car- rier [2–5], the chemistry and thermodynamics of CLC [6], and the reaction mechanism and kinetics parameters of the reaction system [6,7]. Hematite has been discovered to be one of the most important oxygen carriers, and several investigations have been performed to detect the surface structures and adsorption properties of hematite. For example, (0 0 0 1) surface structure was characterized under ultrahigh vacuum and clean conditions [8]; the pure and hydrated (0 0 0 1) and (1 ¯ 1 02 ) surfaces were crystal truncation rod diffraction [9,10]; the relaxation of the (0 0 0 1) surface was detected [11,12]; the binding energies, magnetic, electronic properties, and surface stability of (1 ¯ 1 02 ) surface were discussed [13]; the adsorption of methyl on (0 0 0 1) surface was carried out by TPD, AES, QMS, and LEED [14]; the active site and geometric structure of the adsorption of arsenate on (1 ¯ 1 02 ) surface were investigated by surface X-ray standing wave (XSW) measurements [15]; the adsorption of H 2 O, methyl, and other sorbates on the ideal and hydrated (0 0 0 1) and ∗ Corresponding author. Fax: +86 10 61772031. E-mail address: cqdong1@163.com (C. Dong). (1 ¯ 1 02 ) surfaces was investigated, respectively by density func- tional theory (DFT) calculations [16–19]. Besides, the preparation of Fe 2 O 3 nano-particles aimed to oxi- dize CO into CO 2 in the absence of O 2 [20,21]. Theoretical studies have also been done to investigate the reaction between CO and iron oxide clusters. Reddy Group found Fe 2 O 3 clusters can oxidize two CO molecules in absence and presence of O 2 without energy barrier [22–24]. Reilly Group found that among anionic iron oxide clusters, species with one more oxygen atom than iron atom (FeO 2 - and Fe 2 O 3 - ) were the most reactive for CO oxidation [25]. Xue com- bined single-photon ionization and DFT calculations to study the reactivity of small neutral iron oxide clusters for CO oxidation [26]. However, the detailed understanding of microscopic structures and physicochemical properties of CO interaction to different periodic crystal surfaces of Fe 2 O 3 have not yet been fully understood, by either experiments or theoretical simulations. Here we performed DFT calculations based on the peri- odic surface model to investigate the activity of -Fe 2 O 3 in chemical-looping combustion system related to the oxidation of CO molecules on its dominant growth faces (0 0 0 1) and (1 ¯ 1 02 ). The questions we addressed were as follows: the detail interac- tion between CO and different active sites of Fe 2 O 3 (1 ¯ 1 02 ) and Fe 2 O 3 (0 0 0 1); the oxidation of CO in the absence of oxygen. Results will promote the fundamental understanding and applications of hematite. 2. Computational methods Our calculations were performed with a spin-polarized first- principles pseudopotential plane-wave approach based on DFT 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.042