Sergio Tosoni

Quantum mechanical calculation of the OH vibrational frequency in crystalline solids

The OH vibrational frequency of four crystalline compounds ranging from ionic (brucite, Mg(OH)2, and portlandite, Ca(OH)2) to semi-covalent (edingtonite, as representative of free surface OH groups in silica, and acid chabazite, as representative of acid zeolites) has been investigated at quantum mechanical level with the CRYSTAL program using the B3LYP hybrid functional. The OH vibration is calculated in two ways: (i) in the harmonic approximation, by diagonalizing the fully coupled dynamical matrix to yield the harmonic frequency ω h . (ii) at the anharmonic level, by decoupling the OH stretching mode from the bulk phonons and by numerically solving the one-dimensional Schrödinger equation associated with the OH potential energy to yield the fundamental ω01 and the first overtone ω02 frequencies. The harmonic and anharmonic frequencies differ by more than 150 cm−1. In the cases where direct comparison is possible (brucite, portlandite and edingtonite), the experimental and calculated frequencies differ by less than 10 cm−1; the calculated anharmonicity constant, ω e x e  = (2ω01 − ω02)/2, is systematically smaller than the experimental value by about 10 cm−1. The effect of the computational parameters on the computed frequencies is explored, with particular attention to the grid used for the construction of the DFT exchange and correlation contribution to the Hamiltonian and the accuracy in the geometry optimisation.

Role of dispersive interactions in layered materials: a periodic B3LYP and B3LYP-D* study of Mg(OH)2, Ca(OH)2 and kaolinite

The role of dispersive interactions on the structure, energetic and vibrational features of brucite [Mg(OH)2], portlandite [Ca(OH)2] and kaolinite [Al2Si2O5(OH)4] layered materials has been addressed for the first time. Dispersion contribution is included with a −C6/R6 empirical correction to the B3LYP functional (B3LYP-D* recipe) which has recently been employed to study molecular crystals. To decrease the spurious effect of the basis set superposition error, Gaussian basis sets of triple-zeta plus polarization functions were adopted. Comparing B3LYP and B3LYP-D* results shows the latter to provide significant improvement as far as structure and energetic data are concerned. For the treated systems the cell parameter controlling the inter-layer distance, usually overestimated by the pure B3LYP, is in good agreement with experiment. The inter-layer interaction energy is dramatically increased by the dispersion contribution which, for kaolinite, fully justifies the request of strong Lewis basic molecules needed to swell the material. In general, B3LYP harmonic frequencies are scarcely changed by the dispersive correction albeit some modes sensitive to the inter-layer separation may be significantly perturbed at B3LYP-D*. The present results are encouraging although fine tuning of the proposed empirical correction for inorganic systems might be needed.

A comparison between plane wave and Gaussian-type orbital basis sets for hydrogen bonded systems: formic acid as a test case.

The formic acid molecule, its dimers, and its molecular crystal are adopted as test systems to compare results obtained with plane wave (PW) basis sets and norm-conserving pseudopotentials to all-electron Gaussian-type orbital (GTO) calculations. The CPMD and CRYSTAL06 codes, respectively, are applied with the PBE, PW91, and BLYP density functionals. Hydrogen bonding is the leading interaction in the dimers and the crystal. In the latter, dispersive and weak C-H...O interactions are also relevant. Irrespective of the adopted functional, for all considered structures PW and GTO results converge smoothly as a function of the quality of the adopted basis sets to the same values for structures, energies of interaction, and harmonic vibrational features. To achieve a high level of mutual agreement the use of GTO basis sets of at least of triple-zeta quality including one set of polarization functions and PW basis sets with a kinetic energy cutoff higher than 110 Ry is recommended. Pros and cons of both approaches for studying molecular crystals are also discussed.

Comparison between Gaussian-type orbitals and plane wave ab initio density functional theory modeling of layer silicates: talc [Mg3Si4O10(OH)2] as model system.

The quantum chemical characterization of solid state systems is conducted with many different approaches, among which the adoption of periodic boundary conditions to deal with three-dimensional infinite condensed systems. This method, coupled to the Density Functional Theory (DFT), has been proved successful in simulating a huge variety of solids. Only in relatively recent years this ab initio quantum-mechanic approach has been used for the investigation of layer silicate structures and minerals. In the present work, a systematic comparison of different DFT functionals (GGA-PBEsol and hybrid B3LYP) and basis sets (plane waves and all-electron Gaussian-type orbitals) on the geometry, energy, and phonon properties of a model layer silicate, talc [Mg3Si4O10(OH)2], is presented. Long range dispersion is taken into account by DFT+D method. Results are in agreement with experimental data reported in literature, with minimal deviation given by the GTO∕B3LYP-D* method regarding both axial lattice parameters and interaction energy and by PW/PBE-D for the unit-cell volume and angular values. All the considered methods adequately describe the experimental talc infrared spectrum.

Origin of optical excitations in fluorine-doped titania from response function theory: Relevance to photocatalysis

We investigate the effect of fluorine doping on the optical spectra of stoichiometric and reduced TiO2 anatase, brookite, and rutile using density functional methods. The present approach is able to reproduce the main features of experiments and high-level quasiparticle calculations for undoped titania but at a much lower computational cost, thus allowing the study of doped titania, which requires large supercells. Whereas the simulated spectra of F-substituted brookite and rutile do not show any significant new feature, a relatively intense new band near the visible region is predicted for F-substituted anatase. This allows one to suggest assigning the spectral features near the visible region, observed on multiphase F-doped titania samples, to the presence of anatase. The physical origin of the new absorption band in F-doped anatase is unambiguously attributed to the presence of Ti(3+) centers.

Hydroxylated crystalline edingtonite silica faces as models for the amorphous silica surface

Fully hydroxylated surfaces derived from crystalline edingtonite were adopted to model the variety of sites known to exist at the amorphous silica surface, namely isolated, geminal and interacting silanols. Structures, energetics and vibrational features of the surfaces either bare or in contact with water were modelled at DFT level using the B3LYP functional with a GTO basis set of double-zeta polarized quality using the periodic ab-initio CRYSTAL06 code. Simulated infrared spectra of both dry and water wet edingtonite surfaces were in excellent agreement with the experimental ones recorded on amorphous silica. Water interaction energies were compared with microcalorimetric differential heats of adsorption data showing good agreement, albeit computed ones being slightly underestimated due to the lack of dispersive forces in the B3LYP functional.

A DFT Study of the Reactivity of Anatase TiO2 and Tetragonal ZrO2 Stepped Surfaces Compared to the Regular (101) Terraces

It is generally assumed that low-coordinated sites at extended defects of oxide surfaces like steps or edges are more reactive than the regular, fully coordinated sites at the flat terraces. In this work we have considered the properties of stepped surfaces of anatase TiO2 and tetragonal ZrO2 by means of periodic DFT+U calculations. For both oxides, the stability of oxygen vacancies located near the step edges is compared to that of the same defects at the regular terraces. The capability of the steps to induce nucleation of metal nanoparticles on the surface has been evaluated by simulating the adsorption of a single ruthenium adatom. We conclude that, for anatase, step edges have no particular role in favouring the reduction of the oxide by reducing the cost for oxygen abstraction; in the same way, there is no special role of the stepped anatase surface in stabilizing adsorbed Ru atoms. On the contrary, step edges on zirconia display some capability to stabilise oxygen vacancies and ruthenium adatoms.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

TiO2 and ZrO2 in biomass conversion: why catalyst reduction helps

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase TiO2, a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm. This article is part of a discussion meeting issue ‘Providing sustainable catalytic solutions for a rapidly changing world’.

Nature of Sintering-Resistant, Single-Atom Ru Species Dispersed on Zirconia-Based Catalysts: A DFT and FTIR Study of CO Adsorption

Recent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.