arXiv subjects

Anastassia N. Alexandrova

Publications and source records attributed to Anastassia N. Alexandrova.

24 records · Page 2Linked to original sources

arXiv2020

Sn-modification of Pt7/alumina model catalysts: Suppression of carbon deposition and enhanced thermal stability

An atomic layer deposition process is used to modify size-selected Pt7/alumina model catalysts by Sn addition, both before and after Pt7 cluster deposition. Surface science methods are used to probe the effects of Sn-modification on the electronic properties, reactivity, and morphology of the clusters. Sn addition, either before or after cluster deposition, is found to strongly affect the binding properties of a model alkene, ethylene, changing the number and type of binding sites, and suppressing decomposition leading to carbon deposition and poisoning of the catalyst. Density functional theory on a model system, Pt4Sn3/alumina, shows that the Sn and Pt atoms are mixed, forming alloy clusters with substantial electron transfer from Sn to Pt. The presence of Sn also makes all the thermally accessible structures closed shell, such that ethylene binds only by π-bonding to a single Pt atom. The Sn-modified catalysts are quite stable in repeated ethylene temperature programmed reaction experiments, suggesting that the presence of Sn also reduces the tendency of the sub-nano-clusters to undergo thermal sintering.

physics.chem-ph↗

arXiv2018

Assembling Di- and Multiatomic Si Clusters in Graphene via Electron Beam Manipulation

We demonstrate assembly of di-, tri- and tetrameric Si clusters on the graphene surface using sub-atomically focused electron beam of a scanning transmission electron microscope. Here, an electron beam is used to introduce Si substitutional defects and defect clusters in graphene with spatial control of a few nanometers, and enable controlled motion of Si atoms. The Si substitutional defects are then further manipulated to form dimers, trimers and more complex structures. The dynamics of a beam induced atomic scale chemical process is captured in a time-series of images at atomic resolution. These studies suggest that control of the e-beam induced local processes offers the next step toward atom-by-atom nanofabrication and provides an enabling tool for study of atomic scale chemistry in 2D materials.

cond-mat.mtrl-sci↗

arXiv2012

Photo-driven Molecular Wankel Engine B$_{13}^+$

We report a molecular Wankel motor, the dual-ring structure B13+, driven by circularly-polarized infrared electromagnetic radiation, under which a guided uni-directional rotation of the outer ring is achieved with rotational frequency of the order of 300 MHz.

physics.chem-ph↗

arXiv2012

Structures of Li-Doped Alkali Clusters are Dictated by AO-Hybridization

Hybridization of atomic orbitals is a widely appreciated phenomenon whose impact on the structure and properties of, for example, organic molecules is well-established. Here, we demonstrate that hybridization also dramatically impacts the shapes of small alkali metal clusters. The seemingly similar and valence-iso-electronic LiNa4- and LiK4- clusters adopt very different global minimum structures: LiNa4- is a planar C2v (1A1) species distorted from the perfect pentagon due to the pseudo Jahn-Teller effect, and LiK4- is a planar square D4h (1A1g) species with Li being in the centre. This effect is rooted in the different degrees of the 2s-2p hybridization in Li in response to binding to Na versus K. Li inside the Na cluster exhibits a strong 2s-2p mixing, to achieve stronger covalent bonding. In contrast, Li inside of the K cluster does not show any hybridization, and the LiK4- cluster is reminiscent of an ionic salt. These differences are tied to the relative electronegativities of Li, Na, and K, and overlap of the valence atomic orbitals of Li with those of Na versus those of K. Atomic orbital hybridization is thus a pronounced effect in clusters, the understanding of which is important for the use of clusters in material science or catalysis, and designing clusters of desired shapes.

physics.chem-ph↗

arXiv2011

Aromaticity in a Surface Deposited Cluster: Pd$_4$ on TiO$_2$ (110)

We report the presence of σ-aromaticity in a surface deposited cluster, Pd$_4$ on TiO$_2$ (110). In the gas phase, Pd$_4$ adopts a tetrahedral structure. However, surface binding promotes a flat, σ-aromatic cluster. This is the first time aromaticity is found in surface deposited clusters. Systems of this type emerge as a promising class of catalyst, and so realization of aromaticity in them may help to rationalize their reactivity and catalytic properties, as a function of cluster size and composition.

cond-mat.mtrl-sci↗

arXiv2011

Structure, stability, and mobility of small Pd clusters on the stoichiometric and defective TiO$_2$ (110) surfaces

We report on the structure and adsorption properties of Pd$_n$ ($n=1-4$) clusters supported on the rutile TiO$_2$ (110) surfaces with the possible presence of a surface oxygen vacancy or a subsurface Ti-interstitial atom. As predicted by the density functional theory, small Pd clusters prefer to bind to the stoichiometric titania surface or at sites near subsurface Ti-interstitial atoms. The adsorption of Pd clusters changes the electronic structure of the underlying surface. For the surface with an oxygen vacancy, the charge localization and ferromagnetic spin states are found to be largely attenuated owing to the adsorption of Pd clusters. The potential energy surfaces of the Pd monomer on different types of surfaces are also reported. The process of sintering is then simulated via the Metropolis Monte Carlo method. The presence of oxygen vacancy likely leads to the dissociation of Pd clusters. On the stoichiometric surface or surface with Ti-interstitial atom, the Pd monomers tend to sinter into larger clusters, whereas the Pd dimer, trimer and tetramer appear to be relatively stable below 600 K. This result agrees with the standard sintering model of transition metal clusters and experimental observations.

cond-mat.mtrl-sci↗