Polina Tereshchuk

Comparative study of van der Waals corrections to the bulk properties of graphite.

Graphite is a stack of honeycomb (graphene) layers bound together by nonlocal, long-range van der Waals (vdW) forces, which are poorly described by density functional theory (DFT) within local or semilocal exchange-correlation functionals. Several approximations have been proposed to add a vdW correction to the DFT total energies (Stefan Grimme (D2 and D3) with different damping functions (D3-BJ), Tkatchenko-Scheffler (TS) without and with self-consistent screening (TS  +  SCS) effects). Those corrections have remarkly improved the agreement between our results and experiment for the interlayer distance (from 3.9 to 0.6%) [corrected] and high-level random-phase approximation (RPA) calculations for interlayer binding energy (from 69.5 to 1.5%). [corrected]. We report a systematic investigation of various structural, energetic and electron properties with the aforementioned vdW corrections followed by comparison with experimental and theoretical RPA data. Comparison between the resulting relative errors shows that the TS  +  SCS correction provides the best results; the other corrections yield significantly larger errors for at least one of the studied properties. If considerations of computational costs or convergence problems rule out the TS  +  SCS approach, we recommend the D3-BJ correction. Comparison between the computed π(z)Γ-splitting and experimental results shows disagreements of 10% or more with all vdW corrections. Even the computationally more expensive hybrid PBE0 has proved unable to improve the agreement with the measured splitting. Our results indicate that improvements of the exchange-correlation functionals beyond the vdW corrections are necessary to accurately describe the band structure of graphite.

The role of the CO adsorption on Pt monolayers supported on flat and stepped Au surfaces: a density functional investigation

Ultrathin metal films supported on transition-metal (TM) surfaces have been considered as promising catalysts due to the possibility to tune their chemical activity by controlling substrate strain, composition, and ligand effects, however, our atomistic understanding of the atomic structure of those systems is far from satisfactory due to the complex role of strain effects and adsorbed species. In this work, we will report a density functional theory investigation of the atomic structure of Pt overlayer, skin of PtAu alloy, and Pt submonolayer supported on the Au(111) and Au(332) surfaces, as well as the effects induced on the atomic structure by CO adsorption. For uncovered CO surfaces, we found the same trend for both Au substrates, i.e., there is a strong Pt preference for submonolayer sites, which is consistent with experimental findings and segregation energy calculations, however, the adsorption of CO molecules on the surfaces favors the location of the Pt atoms on the topmost surface layer due to the strong binding of CO molecules to the Pt atoms. For Pt/Au/Au(332), which is a high energy configuration, we found that the Pt overlayer adopts a Pt(111)-like structure instead of the expected Pt(332) overlayer following the Au(332) stacking, however, the steps are preserved once part of the Pt atoms are exchanged by substrate Au atoms or once CO molecules are adsorbed on the Pt overlayer, i.e., CO/Pt/Au/Au(332). Therefore, the atomic structure of Pt overlayer on flat and stepped Au surfaces under a CO atmosphere is complex due to the competing effects that favors different locations for the Pt atoms, i.e., segregation energy favors subsurface sites, while strong CO–Pt binding energy favors the formation of overlayers on the topmost surface layers.

Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections

Transition-metal (TM) nanoparticles supported on oxides or carbon black have attracted much attention as potential catalysts for ethanol steam reforming reactions for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory with van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters, where TM = Ni, Cu, Pd, Ag, Pt, and Au. We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at higher-energy structures, ethanol binds also via the H atom from the CH2 group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol; however, for few systems, the compact higher-energy icosahedron structure changes its configuration upon ethanol or water adsorption. That is, those configurations are only shallow local minimums in the phase space. Except few deviations, we found similar trends for the magnitude of the adsorption energies of water and ethanol, that is, Ni13 > Pt13 > Pd13 and Cu13 > Au13 > Ag13, which is enhanced by the addition of the vdW correction (i.e., from 4% to 62%); however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the d-states toward the highest occupied molecular orbital. On the basis of the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.01 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the red shifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of the O-H bonding.

The role of the cationic Pt sites in the adsorption properties of water and ethanol on the Pt4/Pt(111) and Pt4/CeO2(111) substrates: A density functional theory investigation

Finite site platinum particles, Ptn, supported on reduced or unreduced cerium oxide surfaces, i.e., CeO2-x(111) (0<x<12), have been employed and studied as catalysts for a wide range of applications, which includes hydrogen production using the ethanol steam reforming processes. Our atomic-level understanding of the interaction of Pt with CeO2-x has been improved in the last years; however, the identification of the active sites on the Ptn/CeO2-x(111) substrates is still far from complete. In this work, we applied density functional theory based calculations with the addition of the on-site Coulomb interactions (DFT+U) for the investigation of the active sites and the role of the Pt oxidation state on the adsorption properties of water and ethanol (probe molecules) on four selected substrates, namely, Pt(111), Pt4/Pt(111), CeO2(111), and Pt4/CeO2(111). Our results show that water and ethanol preferentially bind in the cationic sites of the base of the tetrahedron Pt4 cluster instead of the anionic lower-coordinated Pt atoms located on the cluster-top or in the surface Ce (cationic) and O (anionic) sites. The presence of the Pt4 cluster contributes to increase the adsorption energy of both molecules on Pt(111) and CeO2(111) surfaces; however, its magnitude increases less for the case of Pt4/CeO2(111). Thus, the cationic Pt sites play a crucial role in the adsorption properties of water and ethanol. Both water and ethanol bind to on-top sites via the O atom and adopt parallel and perpendicular configurations on the Pt(111) and CeO2(111) substrates, respectively, while their orientation is changed once the Pt4 cluster is involved, favoring H binding with the surface sites.

Atomic structure of the La/Pt(111) and Ce/Pt(111) surfaces revealed by DFT+U calculations

Platinum–lanthanide surface alloys have attracted great interest as promising catalysts in oxygen reduction reactions, however, our atomistic understanding of the surface structure of these systems is far from satisfactory. In this work, we investigated LnPt5/Pt(111) systems (Ln = La and Ce) employing ab initio molecular dynamics based on density functional theory with Hubbard model corrections. In the lowest energy structure, the surface layers are stacked as Pt4/LnPt2/Pt3/Pt(111) in the 2 × 2 surface unit cell, which implies a strong preference of the La and Ce atoms as subsurface layers, and hence, a full Pt monolayer is exposed to the vacuum region. Thus, the work function and d-band center of the occupied states of the topmost layer are only slightly affected by the presence of the La and Ce atoms in the subsurface layer compared with the clean Pt(111) results. In contrast, in the highest energy surface structures, LnPt2/Pt4/Pt3/Pt(111), the La and Ce atoms are exposed to the vacuum region, and hence, the work function is about 1.0 eV lower than in the clean Pt(111) surface, and the d-band center is shifted towards the Fermi level by about 0.40 eV. Although one of the Ce 4f-states is occupied and shows a localized nature, the surface structures, geometric parameters, and electronic properties are very similar for both LaPt5/Pt(111) and CePt5/Pt(111) systems.

Glycerol adsorption on a defected Pt6/Pt(100) substrate: a density functional theory investigation within the D3 van der Waals correction

We report an ab initio investigation based on density functional theory calculations within the van der Waals (vdW) correction to obtain an improved atomistic understanding of the adsorption properties of glycerol on a defected Pt6/Pt(100) substrate, which includes low-coordinated Pt sites and well defined Pt(100) terraces. We found that in the lowest energy structure glycerol weakly adsorbs on a low-coordinated cationic Pt site via one of the anionic O atoms with the central carbonate chain orientated nearly parallel to the surface plane. As expected, the vdW correction enhances the adsorption energy, however, while it does not change the adsorption site preference, it affects the orientation of the CCC frame with respect to the substrate. Our results of the work function and Bader charges suggest a negligible charge transfer between glycerol and the Pt6/Pt(100) substrate, which can be attributed mostly to polarizations between the atoms of the molecule and of the surface.

Adsorption of Li2O2, Na2O2, and NaO2 on TiC(111) Surface for Metal–Air Rechargeable Batteries: A Theoretical Study

We analyze, with Density Functional Theory (DFT) calculations, the adsorption energies of Li2O2, Na2O2 and NaO2 on clean and oxygen passivated TiC (111) surfaces. We show, that after deposition of two molecular layers of alkali metal oxides, the initial state of the TiC surface becomes unimportant for the adsorption energy and that all adsorption energies approach their native crystal values. The structure of the adsorbed molecular layers is analyzed and compared to their native oxide crystal structure. Finally, we discuss the possible implications for electrode optimization for Li-air and Na-air batteries.