Computational Modelling of Yttrium Stabilised Zirconia in Catalysis

<p>Abstract for a talk given at CATSA 2014 describing modelling of various properties of yttrium stabilised zirconia, particularly oxidative methane activation.</p> <p>Closely related work is published in Chem. Comm. here:</p> <p>http://pubs.rsc.org/en/Content/ArticleLanding/2015/CC/c4cc09010a</p> <p> </p> <p> </p

Simulation of the Embryonic Stage of ZnS Formation from Aqueous Solution

We investigate the processes of cluster formation and growth of ZnS from aqueous solution using molecular dynamics simulation techniques. The influence of both temperature and concentration is studied. We show that, at lower temperatures, the crucial process is the transformation of an outer-sphere Zn/S complex to an inner-sphere ion pair. Further growth of the latter is fast to generate negatively charged planar clusters. These clusters interact to form more stable, closed structures, which are found to be the global minima configurations in vacuo. At higher temperatures, no outer-sphere ion pairs are formed, and the larger cluster configurations form much more quickly

Computational Study of the Structural and Electronic Properties of Dopant Ions in Microporous AlPOs. 1. Acid Catalytic Activity of Divalent Metal Ions

Periodic ab initio QM calculations are applied to study the structure and acidity of Mg, Ca, Cr, Mn, Fe, Co, Ni, Zn, and Sr divalent metal ions in the AlPO-34 framework, charge compensated by an acid proton on a neighboring oxygen. Our results show that the local environment of the divalent dopants is a distorted tetrahedron, in which the Me−OH bond of the dopant to the protonated oxygen is ∼0.15 Å longer than the other three Me−O bonds. The nature of bonding between the Me2+ dopants and the neighboring oxygens is ionic in nature, explaining the Lewis acidity of the Me2+ ions. However, for Ni2+, the Lewis active orbitals are oriented within the framework, where they are screened from an effective Lewis- type interaction with adsorbed molecules. The attack of Lewis bases is favored from the side of the framework opposite to the Brønsted acid proton. The replacement energy of a framework Al3+ with a Me2+ ion increases linearly as a function of the ionic size of Me2+. Finally, we show that the acid strength is attributable to a complex combination of the structural and electronic features of the dopant ion and does not show appreciable correlation with the local environment or electronic distribution of the Me2+ dopant in the framework

A Computational Modelling Study of Methane Activation Over YSZ

<p>Poster presented at ICTAC 2014.</p> <p> </p> <p>Some images used from the literature or other sources are excluded since I do not own the copyright.</p> <p> </p> <p>Related methane activation work is described in more detail in the linked publication.</p> <p> </p

True Structure of Trigonal Bipyramidal SiO₄F^- Species in Siliceous Zeolites

This paper describes the true geometry of the pentacoordinated SiO4F- units found in several all-silica zeolite systems, as determined by ab initio density functional calculations. The SiO4F- units in the two zeolites (sodalite and ferrierite) studied contain Si−F bonds of 1.71 and 1.76 Å in length, and all the Si−O bonds in the units are elongated with respect to the Si−O bonds in the tetrahedral SiO4 unit typically found in a zeolite. All the bond angles in the SiO4F- unit agree with those expected for a regular trigonal bipyramidal XY5 unit. The calculated structure is shown to reproduce accurately an averaged structural model determined by standard crystallographic methods. The approach we have developed in this work has general implications for the solution of complex local structure problems in solids

A Computer Modeling Study of the Adhesion of Apatite Thin Films on Silicate Surfaces

Computer simulations of apatite thin films at a range of quartz surfaces have shown how the strength of adhesion between thin films of apatite material and ceramic silica surfaces is crucially dependent upon both the orientation of the film relative to the substrate and the nature of the silica surfacea finding that is important in a wide number of applications, from basic geological research on intergrowth of phosphate and silicate rock minerals to the search for more effective surgical implant materials

Nitrogen Activation in a Mars–van Krevelen Mechanism for Ammonia Synthesis on Co₃Mo₃N

Co₃Mo₃N is one of the most active catalysts for ammonia synthesis; however, little is known about the atomistic details of N₂ adsorption and activation. Here we examine whether N₂ can adsorb and activate at nitrogen surface vacancies. We have identified the most favorable sites for surface nitrogen vacancy formation and have calculated vacancy formation free energies (and concentrations) taking into account vacancy configurational entropy and the entropy of N₂ at temperature and pressure conditions relevant to ammonia synthesis (380–550 °C, 100 atm) via a semiempirical approach. We show that 3-fold hollow bound nitrogen-containing (111)-surfaces have surprisingly high concentrations (1.6 × 10¹⁶ to 3.7 × 10¹⁶ cm^–2) of nitrogen vacancies in the temperature range for ammonia synthesis. It is shown that these vacancy sites can adsorb and activate N₂ demonstrating the potential of a Mars–van Krevelen type mechanism on Co₃Mo₃N. The catalytically active surface is one where 3f-hollow-nitrogens are bound to the molybdenum framework with a hexagonal array of embedded Co₈ cobalt nanoclusters. We find that the vacancy-formation energy (VFE) combined with the adsorption energy can be used as a descriptor in the screening of materials that activate doubly and triply bonded molecules that are bound end-on at surface vacancies

Unique Organic−Inorganic Interactions Leading to a Structure-Directed Microporous Aluminophosphate Crystallization as Observed with in situ Raman Spectroscopy

We report on the direct observation of key organic template−framework interactions leading to the formation of specific aluminophosphate structures. In particular, we show how MeAPO-34 formation was governed by an interaction between the divalent framework substituted metal ion and the template conformation, while for AlPO-5 the structure formation was determined by the template conformation alone. Understanding such interactions therefore appears to be important for the rationalization of microporous material formation

Compressive Straining of Bilayer Phosphorene Leads to Extraordinary Electron Mobility at a New Conduction Band Edge

By means of hybrid DFT calculations and the deformation potential approximation, we show that bilayer phosphorene under slight compression perpendicular to its surface exhibits extraordinary room temperature electron mobility of order 7 × 10⁴ cm² V^–1 s^–1. This is approximately 2 orders of magnitude higher than is widely reported for ground state phosphorenes and is the result of the emergence of a new conduction band minimum that is decoupled from the in-plane acoustic phonons that dominate carrier scattering

Surface Energies Control the Self-Organization of Oriented In₂O₃ Nanostructures on Cubic Zirconia

Highly aligned one-dimensional (1D) nanorods of the transparent conducting oxide In2O3 have been grown on (110)-oriented Y-stabilized ZrO2 substrates, whereas growth on (100) and (111) substrates leads respectively to blocklike 3D islands and continuous films. It is shown that the striking influence of substrate orientation on the growth morphology is controlled by differences in energies between the low index surfaces of In2O3 and that spontaneous self-organization is driven by minimization of surface energies