32494 | Phys. Chem. Chem. Phys., 2016, 18, 32494--32502 This journal is © the Owner Societies 2016 Cite this: Phys. Chem. Chem. Phys., 2016, 18, 32494 Effects of strong interactions between Ti and ceria on the structures of Ti/CeO 2 † Xiao-Dan Yao,‡ a Kong-Jie Zhu,‡ a Bo-Tao Teng,* ab Cao-Ming Yu, a Yun-Lei Zhang, a Ya Liu, a Maohong Fan* b and Xiao-Dong Wen* c The effects of strong interactions between Ti and ceria on the structures of Ti/CeO 2 (111) are systematically investigated by density functional theory calculation. To our best knowledge, the adsorption energy of a Ti atom at the hollow site of CeO 2 is the highest value (7.99 eV) reported in the literature compared with those of Au (0.88–1.26 eV), Ag (1.42 eV), Cu (2.69 eV), Pd (1.75 eV), Pt (2.62 eV) and Sn (3.68 eV). It is very interesting to find that Ti adatoms disperse at the hollow site of CeO 2 (111) to form surface TiO x species, instead of aggregating to form Ti metal clusters for the Ti–CeO 2 interactions that are much stronger than those of Ti–Ti ones. Ti adatoms are completely oxidized to Ti 4+ ions if they are monatomically dispersed on the next near hollow sites of CeO 2 (111) (xTi-NN- hollow); while Ti 3+ ions are observed when they locate at the near hollow sites (xTi-N-hollow). Due to the electronic repulsive effects among Ti 3+ ions, the adsorption energies of xTi-N-hollow are slightly weaker than those of xTi-NN-hollow. Simultaneously, the existence of unstable Ti 3+ ions on xTi-N-hollow also leads to the restructuring of xTi-N-hollow by surface O atoms of ceria transferring to the top of Ti 3+ ions, or oxidation by O 2 adsorption and dissociation. Both processes improve the stability of the xTi/CeO 2 system by Ti 3+ oxidation. Correspondingly, surface TiO 2 -like species form. This work sheds light into the structures of metal/CeO 2 catalysts with strong interactions between the metal and the ceria support. 1. Introduction Due to the reversible transformation between Ce 4+ and Ce 3+ ions, ceria has a relatively high oxygen storage capacity (OSC) and has been widely used in heterogeneous catalysis. To improve the OSC capacity and catalytic performance, metal nanoparticles are generally supported on ceria (M/CeO 2 ) catalysts. 1,2 Extensive experimental results indicate that the performance of M/CeO 2 catalysts is highly related to the structures of metal clusters on the ceria support. 3 Therefore, great efforts in experiment and theory have been devoted to explore the structures of metal clusters on the ceria support and their interactions. 4–6 Using STM and XPS in an ultrahigh vacuum chamber, Mullins and Zhou’s group systematically investigated the possible structures of Rh, 7 Pd, 8 Ni, 9 Pt 10 and Mn 11 nanoparticles on CeO 2 (111). Farmer and Campbell 12 found that Ag nanoparticles are more stable on the reduced CeO 2 (111). Similar conclusion was obtained by Kong et al. 13 Akita et al. 14–16 studied the structures of Au nanoparticles and their restructuring on the ceria support by TEM. They observed that the three dimensional (3D) structures of Au nanoparticles shrank layer by layer under electron beam irradiation. In the aspect of theory, Zhang et al. 17,18 systematically calculated the structures of Au clusters on the ceria surface using the density functional theory (DFT) method and proposed a 3D fcc or hcp structure of Au clusters on CeO 2 (111). It was reported by Teng et al. 2,19 that the stable structures of Au/CeO 2 catalysts are highly related to the structure of the ceria surface, as well as the size and arrangement of Au clusters. Zhao et al. 20 reported that the stable Sn x (x = 1–4) configurations varied with the increase of Sn coverage on CeO 2 (111). Yang’s work indicated that copper adatoms tend to form 3D clusters on CeO 2 (111). Similar conclusions are also obtained for Pd and Pt clusters on the ceria surface. 21 According to the experimental and theoretical results in the literature, most of the metals on the ceria surface incline to form 3D stable clusters. This might be attributed to the relatively strong M–M interactions compared with M–CeO 2 , or the interactions between M–M and M–CeO 2 are comparable. What are the possible stable structures if very strong inter- action actions occur between metal and ceria? This will provide valuable information for the further understanding of the a School of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China. E-mail: tbt@zjnu.cn b Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA. E-mail: mfan@uwyo.edu c Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China. E-mail: wxd@sxicc.ac.cn † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp05406d ‡ Xiao-Dan Yao and Kong-Jie Zhu are the co-first authors. Received 4th August 2016, Accepted 7th November 2016 DOI: 10.1039/c6cp05406d www.rsc.org/pccp PCCP PAPER Published on 07 November 2016. Downloaded by University of Wyoming on 09/01/2017 16:01:53. View Article Online View Journal | View Issue