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The most widely used approach to predict catalytic activity is density functional theory, whose results however depend on the adopted exchange‐correlation functional. In this work, the role played by the functional in predicting the activity of single atom catalysts (SAC) in the hydrogen and oxygen evolution reactions (HER and OER) is studied. 16 transition metal (TM) atoms embedded in N‐doped graphene are simulated and the performance of the widely adopted Perdew–Burke–Ernzerhof (PBE) functional against the hybrid PBE0 functional is assessed. The PBE+U approach, a computationally less complex way to correct for the self‐interaction error in density functional theory, is also considered. The predictions obtained with PBE have a substantial deviation from PBE0 for first row TMs, i.e., 3d systems, while smaller deviations are found for the 4d and 5d series. The PBE+U results represent an improvement with respect to PBE, although some differences from PBE0 remain. This study underlines the importance of the choice of the DFT functional in screening new catalysts and in predicting catalytic activities. The use of PBE appears acceptable for 4d and 5d metals, while in the case of 3d systems PBE+U or PBE0 approaches are recommended, in particular for magnetic ground states. [ABSTRACT FROM AUTHOR]

In this work we present a computational study of the nature of a Single Atom Catalyst (SAC) consisting of a Pt1 atom anchored on a C3N4 support, and of its reactivity in the water splitting semi-reactions, the Hydrogen Evolution (HER) and Oxygen Evolution (OER) Reactions. The work is motivated by the intense research in designing catalytic materials for water splitting characterized by a low amount of noble metal species, maximization of active phase, and stability of the catalyst. C3N4-based SACs are promising candidates. The results indicate that the chemistry of a single atom is complex, as it can be anchored to the support in different ways resulting in a different stability. The reactivity of the most stable structure in HER and OER has been considered, finding that Pt1@C3N4 is more reactive than metallic platinum. Furthermore, unconventional but stable intermediates can form that differ from the intermediates usually found on extended catalytic surfaces. The work highlights the importance of considering the complex chemistry of SACs in view of the analogies existing with coordination chemistry compounds. [ABSTRACT FROM AUTHOR]

The role of the support in tuning the properties of transition metal (TM) atoms is studied by means of density functional theory calculations. We have considered the adsorption of Cu, Ag, Au, Ni, Pd, and Pt atoms on crystalline silica bilayers, either free-standing or supported on Ru(0001) and Pt(111) metal surfaces. These systems have been compared with an hydroxylated SiO2/Si(100) film simulating the native oxide formed on a silicon wafer. The properties of the TM atoms change significantly on the various supports. While the unsupported silica bilayer weakly binds some of the TM atoms studied, the SiO2/Ru(0001) or SiO2/Pt(111) supports exhibit enhanced reactivity, sometimes resulting in a net electron transfer with the formation of charged species. Differences in the behavior of SiO2/Ru(0001) and SiO2/Pt(111) are rationalized in terms of different work functions and metal/oxide interfacial distances. No electron transfer is observed on the SiO2/Si(100) films. Here, the presence of hydroxyl groups on the surface provides relatively strong binding sites for the TM atoms that can be stabilized by the interaction with one or two OH groups. The final aspect that has been investigated is the porosity of the silica bilayer, at variance with the dense SiO2/Si(100) film. Depending on the atomic size, some TM atoms can penetrate spontaneously through the six-membered silica rings and become stabilized in the pores of the bilayer or at the SiO2/metal interface. This study shows how very different chemical properties can be obtained by depositing the same TM atom on different silica supports. [ABSTRACT FROM AUTHOR]

In this work, we investigate the structural and electronic properties of the SrTiO3/TiO2 (anatase) heterostructure by means of hybrid density functional theory calculations. The work is motivated by several experiments that pointed to SrTiO3/TiO2 as a good system for photocatalytic applications, due to the small lattice mismatch between these two oxides and their favorable band alignment, leading to a type-II heterojunction, favoring the charge-carrier separation. The present results provide insights into the nature of the contact region and an estimation of the band offsets in the composite system. Our results are also compared with the available experimental values and with previous theoretical reports. The calculated offsets quantitatively agree with experimental measurements. In addition, we found significant interfacial effects that make the band offsets slightly increase with respect to those of the separated components. Last, we also discuss the role of point defects such as oxygen vacancies, finding that they do not remarkably affect the band alignment. [ABSTRACT FROM AUTHOR]

Calcium-molybdate ultrathin films were prepared on a Mo(001) crystal and characterized by means of scanning tunneling microscopy (STM), electron diffraction, photoelectron spectroscopy, and density functional theory (DFT). The films were grown via reactive Ca deposition, followed by a vacuum annealing step to trigger Mo diffusion from the support into the Ca—O ad-layer. A series of crystalline oxide configurations was revealed that evolves from a (3 × 3) to a (4 × 4) and (6 × 6) superstructure with increasing annealing temperature and finally decays to a binary MoOx phase. The stoichiometry of the initial (3 × 3) phase was estimated to CaMo3O6, yet with decreasing Ca concentration at higher temperature. In the search for a suitable structure model for DFT calculations, we have started with a bulk CaMo5O8 configuration that was iteratively modified to match the experimental data. The optimized structure is made of regular sequences of flat-lying and upright standing Mo octahedrons, being separated from each other by Ca2+ ion rows. With decreasing Ca content, the central Mo units grow in size, which explains the observed transition from (3 × 3) to (6 × 6) superstructures upon annealing. The proposed structure model rationalizes the periodicity and corrugation of the regular oxide surface as well as the characteristic domain patterns in the film. Its electronic properties, as deduced from STM conductance spectroscopy, can be correlated with an increasing metallicity of the ad-layer upon annealing. Our work presents a facile pathway to produce high-quality ternary oxide films via interdiffusion of atoms from a suitable metal support into a binary oxide layer. [ABSTRACT FROM AUTHOR]

The surface basicity of the alkaline-earth metal oxides has been investigated by studying the properties of 17O nuclear magnetic resonance (NMR). To this end, we performed density functional theory calculations and determined the 17O chemical shift and the quadrupolar coupling constants of the regular and stepped surfaces of MO (M = Mg, Ca, Sr, and Ba) oxides. The computed average chemical shift (δisoav) for 17O NMR of bulk MgO, CaO, SrO, and BaO is 46, 301, 394, and 636 ppm, respectively, in excellent agreement with the experiment. The 17O NMR chemical shifts correlate linearly with the Madelung potential in the four oxides. Next, we considered the changes in the 17O chemical shift due to the adsorption of BR3 (R = F and OCH3) and pyrrole as probe molecules. We found that the 17O NMR signal of the O ion directly bound to the probe molecule shifts considerably compared to the clean surface. This is due to a change in the polarization of the O charge distribution due to the molecular adsorption. This change is the largest for BaO, with the strongest bond and the shortest surface-adsorbate distance, and the smallest for MgO, thus showing a direct correlation between 17O NMR and surface basicity. The 17O chemical shift of the basic site correlates linearly also with several properties of the adsorbed molecules, providing a direct measure of the surface basicity. [ABSTRACT FROM AUTHOR]

CO2 adsorption and activation on Cu single atom catalysts and Cu nanoclusters supported on the (110) surface of rutile and on the (101) surface of anatase TiO2 have been investigated by means of first principles electronic structure calculations. The role of oxide reduction associated to the presence of oxygen vacancies has been considered. Five main messages emerge from this study. (1) CO2 activation on Cu/TiO2 nanostructures is surface sensitive, as the rutile and anatase surfaces can exhibit different behaviors; (2) the surface morphology is essential since CO2 is activated only when the molecule can simultaneously bind to at least two active sites, such as a Cu atom on one side and an oxide ion on the other site; (3) Cu atoms on TiO2 are in the +I oxidation state and can bind and activate CO2 via charge transfer from the oxide; (4) on supported Cu clusters CO2 activation occurs mostly at the metal/oxide interface; (5) the presence of O vacancy sites facilitates the spontaneous dissociation of CO2 to CO, or increases the electron density of the metal catalyst, two effects that can influence the mechanism of CO2 reduction to methanol or other chemicals. [ABSTRACT FROM AUTHOR]