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53 PBE0/GTO 386 604 540 353 71 B3LYP/GTO 389 617 547 340 71 PBE+U(4.5)/pw 392 618 −1435 e 549 −834 e 221 −20 53 PBE/pw f 383 608 −1573 547 −988 403 162 53 PBE/pw g 385 604 −1589 547 −999 409 168 53 a Reference 81. b Zero-point vibrational energy and thermal contributions to the heats of formation are very small (2.1 and 1.4 kJ/mol for CeO 2 and Ce 2 O 3 , respectively) and even smaller for reaction 2.1 (0.8 kJ/mol). Therefore, we use ΔE f,r ≈ ΔH f,r 0 (values from Table 2). c Experimental formation enthalpy of water at 0 K (−239 kJ/mol) has been taken from the literature. 82 d Reference 63. e Reference 80. Since GGA+U yields only the less stable γ phase of cerium, the energy difference between α and γ phase (−3 kJ/mol) has been added to the heats of formation. f PAW ansatz to describe the electron−ion interaction using plane waves. g LAPW+LO ansatz to describe the electron−ion interaction using plane waves, which can be considered as the benchmark method in the solid-state community. Both, PAW and LAPW are so-called full-potential methods.

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The methylation of ethene, propene, and t-2-butene by methanol over the acidic microporous H-ZSM-5 catalyst has been investigated by a range of computational methods. Density functional theory (DFT) with periodic boundary conditions (PBE functional) fails to describe the experimentally determined decrease of apparent energy barriers with the alkene size due to inadequate description of dispersion forces. Adding a damped dispersion term expressed as a parametrized sum over atom pair C(6) contributions leads to uniformly underestimated barriers due to self-interaction errors. A hybrid MP2:DFT scheme is presented that combines MP2 energy calculations on a series of cluster models of increasing size with periodic DFT calculations, which allows extrapolation to the periodic MP2 limit. Additionally, errors caused by the use of finite basis sets, contributions of higher order correlation effects, zero-point vibrational energy, and thermal contributions to the enthalpy were evaluated and added to the "periodic" MP2 estimate. This multistep approach leads to enthalpy barriers at 623 K of 104, 77, and 48 kJ/mol for ethene, propene, and t-2-butene, respectively, which deviate from the experimentally measured values by 0, +13, and +8 kJ/mol. Hence, enthalpy barriers can be calculated with near chemical accuracy, which constitutes significant progress in the quantum chemical modeling of reactions in heterogeneous catalysis in general and microporous zeolites in particular.

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One of the most topical issues surrounding oxygen vacancies on CeO2(111) is the relative stability of surface and subsurface defects. Using density-functional theory (DFT) with the HSE06 (Heyd-Scuseria-Ernzerhof) hybrid functional as well as the DFT+U approach (where U is a Hubbard-like term describing the on-site Coulomb interactions), we find subsurface vacancies with (2x2) periodicity to be energetically more favorable by 0.45 (HSE06), 0.47 [PBE+U (Perdew-Burke-Ernzerhof functional)], and 0.22 eV [LDA+U (local density approximation)]. The excess electrons localize not on Ce ions which are the nearest neighbor to the defect as priorly suggested, but instead on those that are next-nearest neighbors. The excess-electron distribution and the preference for subsurface vacancies are explained in terms of defect-induced lattice relaxation effects.

Thin SiO 2 films were grown on a Ru(0001) single crystal and studied by photoelectron spectroscopy, infrared spectroscopy and scanning probe microscopy. The experimental results in combination with density functional theory calculations provide compelling evidence for the formation of crystalline, double-layer sheet silica weakly bound to a metal substrate. DOI: 10.1103/PhysRevLett.105.146104 PACS numbers: 68.35.Àp, 68.47.Gh, 68.55.Àa Silicon dioxide (SiO 2 ) plays a key role in many modern technologies and applications that range from insulating layers in integrated circuits to supports for metal and oxide clusters in catalysts. For better understanding of structureproperty relationships on silica-based materials, particularly of reduced dimensions, thin silica films grown on metal single crystal substrates are suggested as suitable model systems that allow the facile application of many ''surface science'' techniques. It has recently been shown that crystalline silica films and nanowires can be grown on Mo(112) [1][2][3][4][5]. The ultrathin film consists of a monolayer honeycomblike network of corner-sharing [SiO 4 ] tetrahedra, thus resulting in a SiO 2:5 stoichiometry of the film. The Si atoms in these films can be partly substituted by Al in the course of preparing metal supported aluminosilicate films [6], which is the first step towards experimental modeling of catalytic centers in zeolitelike materials. However, attempts to grow thicker silica films on the Mo substrates resulted in amorphous structures [7][8][9], most likely due to the formation of strong Si-O-Mo bonds at the interface that govern the growth mode [9]. Recently, the preparation of crystalline silica films on other supports such as Pd(100) [10] and Ni(111) [11] has been reported. However, the atomic structure of the films, film surface termination, and the nature of the silica-metal interface were not determined.In this Letter, we report on the preparation and the atomic structure of well-defined silica films on Ru(0001). The experimental results, obtained by photoelectron and vibrational spectroscopies and high-resolution scanning probe microscopy, are complemented by density functional theory calculations which together provide compelling evidence for the formation of a double-layer sheet silicate, with a SiO 2 stoichiometric composition, weakly bound to a metal support. The results open new perspectives for employing a ''surface science'' approach to understand the reactivity of silicate surfaces consisting of hydrophobic Si-O-Si bonds, such as those of microporous all-silica zeolites [12]. Also, these films can be used as model supports for catalytically active metal and oxide clusters [4,13].The experiments were performed in an ultrahigh vacuum chamber equipped with low energy electron diffraction (LEED) and Auger electron spectroscopy, x-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and scanning tunneling microscopy (STM). Atomically resolved atomic force microscopy (AFM) and STM image...

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