Published: December 06, 2011 r2011 American Chemical Society 994 dx.doi.org/10.1021/jp209680r | J. Phys. Chem. C 2012, 116, 994–1001 ARTICLE pubs.acs.org/JPCC Chromium Oxide Species Supported on Silica: A Representative Periodic DFT Model Hazar Guesmi and Frederik Tielens* UPMC, Univ Paris 06, UMR 7197, Laboratoire de R eactivit  e de Surface, Site d’IvryÀLe Rafa€ el 3, rue Galil  ee, 94200 Ivry-Sur-Seine, France and CNRS, UMR 7197, Laboratoire de R eactivit  e de Surface, Site d’IvryÀLe Rafa€ el, 3 rue Galil  ee, 94200 Ivry-Sur-Seine, France 1. INTRODUCTION Chromium oxides supported on inorganic oxides, such as SiO 2 , Al 2 O 3 , MCM-41, etc., are typically used as catalysts. Chromium oxide on silica (Cr/SiO 2 ) is the famous Philips catalyst for the polymerization of ethylene at relatively low pressures. 1À3 This catalyst is the basis for particle form process in the production of high-density polyethylene (HDPE), one of the most extensively used polymers. Other important catalytic activities are hydrogenationÀdehydrogenation, oxidation, iso- merization, aromatization, DeNOx reactions, and complete combustion. 4À6 The basis for the activity of Cr in such a wide spectrum of reactions lies in the variability of oxidation states, of coordination environments, and of degree of polymerization of Cr oxide species. This variability is especially pronounced on the surface. Thus, knowledge about the surface chemistry of sup- ported chromium oxide species is of key importance in environ- mental sciences and heterogeneous catalysis. Mainly due to the importance of this system, it has been studied in detail for the last 50 years, experimentally and more recently theoretically. The still growing computational power enables to study models with increasing complexity and reli- ability. Until now ab initio quantum chemical calculations have been performed on cluster models involving a dozen of atoms. 7À9 One of the difficulties to model Cr/SiO 2 catalyst, among other points, is the representation of the silica surface, due to its amorphous nature. The objective of this study is to investigate possible atomic models of the silica supported chromium oxide catalysts in hydrated and dehydrated forms, in a similar way to our study on the characterization of vanadium containing silica. 10 To this end, density functional theory (DFT) is used to calculate the structures, vibrational frequencies, and relative stabilities of isolated monomeric chromium species grafted to the surface of amorphous silica. The effect of siloxane ring size was also investigated. The inclusion of small amounts of titanium has a promotional effect on both the polymerization activity and the termination rate of the catalysts. 11,12 A further objective of the work is to determine the surface doping effect by Ti on the structure and stability of chromium species. In particular, the effect of Ti content on the charge transfer between Ti and Cr has been investigated together with vibrational frequency analysis. The structures calculated in the present study are compared with experimental literature data obtained by EXAFS and Raman spectra reported for highly dispersed CrO x on silica. 7,13À16 2. METHODOLOGY 2.1. Computational Details. All calculations are performed using ab initio plane-wave pseudopotential approach as imple- mented in VASP. 17,18 The PerdewÀBurkeÀErnzerhof (PBE) functional 19,20 has been chosen to perform the periodic DFT calculations with an accuracy on the overall convergence tested elsewhere. 21À24 The valence electrons are treated explicitly, and their interactions with the ionic cores are described by the projector augmented-wave method (PAW), 25,26 which allows the use of a low energy cut off equal to 400 eV for the plane-wave basis. The Gamma point is used in the Brillouin-zone integration. Received: October 8, 2011 Revised: November 26, 2011 ABSTRACT: The Cr/SiO 2 system is investigated using peri- odic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics model confirms the higher stability of mono-oxo and dioxo chromium, in comparison with species containing CrÀOH groups. In addition, the effect of the siloxane ring size on the spectroscopic signature of chromium is analyzed. A preliminary study is presented on the surface doping effect by Ti on the structure and stability of chromium species. The results reveal that the charge transfer process between Ti and Cr can explain the observed change in the reactivity of chromium species.