Theoretical Confirmation of the Enhanced Facility to Increase Oxygen Vacancy Concentration in TiO 2 by Iron Doping Alberto Rolda ´n, †,‡ Merce ´ Boronat, § Avelino Corma,* ,§ and Francesc Illas † Departament de Quı ´mica Fı ´sica & Institut de Quı ´mica Teo `rica i Computacional (IQTCUB), UniVersitat de Barcelona, C/ Martı ´ i Franque `s 1, 08028 Barcelona, Spain, Departament de Quı ´mica Fı ´sica i Inorga `nica, UniVersitat RoVira i Virgili, C/ Marcel · lı ´ Domingo s/n, 43007 Tarragona, Spain, and Instituto de Tecnologı ´a Quı ´mica, UPV-CSIC, AV. los Naranjos, s/n, Valencia, Spain ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: February 24, 2010 The effect of Fe-doping on the oxygen vacancy energy formation and on the electronic structure of stoichiometric bulk and (001) surface of TiO 2 anatase has been studied by means of periodic density functional calculations within the GGA+U approach. The vacancy energy formation is always lower for the surface than for the bulk. The presence of Fe causes only a minor perturbation of the atomic structure of anatase but strongly reduces the oxygen vacancy energy formation, especially at the surface. The present results provide additional and independent support to the claim that the enhanced catalytic activity of Au nanoparticles supported on Fe-doped TiO 2 toward CO oxidation has its origin in the ease to create oxygen vacancies in the support. 1. Introduction Titanium dioxide is widely used in several fields related to energy conversion, heterogeneous catalysis, and photocatalysis. 1-4 The discovery of the high catalytic activity of gold nanoparticles supported on transition metal oxides 5-11 and, specially, the remarkable activity and selectivity for CO and alcohol oxidation 12,13 increased the interest in TiO 2 as a support for heterogeneous oxidation catalysts. Since then, numerous experimental 14-18 and theoretical 19-25 studies have been pub- lished about the mechanism of CO oxidation by supported gold. Although the way in which oxygen is activated is not completely clear, the positive role of oxygen vacancy defects on reduced oxide surfaces has been stressed. 26-29 In this sense, the high catalytic activity of gold supported on nanostructured ceria has been related to the ability of Ce to change oxidation state from Ce 4+ to Ce 3+ thus facilitating the formation of surface oxygen vacancies. 30-33 It has also been demonstrated that doping of CeO 2 with other elements such as Zr, Ti, Hf, Ni, or Pd results in an increased concentration of oxygen vacancies, presumably because doping with these elements reduces the oxygen vacancy formation energy, and a concomitant enhancement of the corresponding surface reactivity. 34-38 In a similar way, it has been reported that doping of TiO 2 with Fe increases the oxidation activity of Au/TiO 2 catalyst and this has been related to a higher density of oxygen vacancies which contribute to activate O 2 . 39,40 Again, this catalytic enhancement has been attributed to the increased presence of oxygen vacancies whose formation would be facilitated by the presence of Fe. In the case of CO oxidation by Au clusters supported on Fe- doped TiO 2 , the experiments provide clear indication that the main effect due to the presence of Fe is likely to facilitate O vacancy formation 28 although one must realize that this is rather indirect evidence. Unfortunately, the direct measurement of the oxygen vacancy formation energy at the surface of the support and the effect of Fe on this quantity is very hard if not impossible. Theoretical models offer, no doubt, a good alterna- tive since the calculations based on density functional theory have proven to be very robust, predictive, and useful although they are not exempt of problems. Obtaining a direct estimate of the effect of the presence of Fe on the energy formation of oxygen vacancies is precisely the aim of the present work. The structural and electronic properties of the most common phases of TiO 2 , rutile and anatase, have been extensively investigated from a theoretical point of view. 41-46 These studies have concluded that standard DFT methods in the GGA approximation fail to describe the nature of oxygen vacancy defects present in reduced TiO 2 . This is due to the well-known shortcomings of standard LDA and GGA to describe the electronic structure of simple and transition metal oxides. 47 In fact, these methods usually lead to too small values of the calculated band gap, which impedes the study of changes in the electronic structure induced by the presence of these point defects and predict these materials to be metallic even when many of them are commonly antiferromagnetic insulators. Therefore, a correct description of these systems requires methods that go beyond the standard LDA and GGA imple- mentations of density functional theory such as the use of hybrid functionals or the inclusion an on-site correlation Hubbard term resulting in the so-called LDA+U or GGA+U methods. 48 In the present work, the formation energy of an isolated oxygen vacancy in the bulk of anatase and on different surface and subsurface positions of its (001) crystal face have been calculated using standard GGA and also GGA+U density functional theory based methods. The choice for the anatase polymorph of TiO 2 comes form the fact that this is the most abundant phase of the TiO 2 catalyst. Likewise, the choice of the (001) surface is justified because this is the most reactive and hence the one which, in principle, is active in the catalyst and has been the object of special methods of preparation to increase its surface in TiO 2 nanoparticles. 49 The influence of the methodology on the computed oxygen vacancy formation energies, geometrical distortions, degree of localization of the † Universitat de Barcelona. ‡ Universitat Rovira i Virgili. § UPV-CSIC. J. Phys. Chem. C 2010, 114, 6511–6517 6511 10.1021/jp911851h  2010 American Chemical Society Published on Web 03/12/2010