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

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

Screening Divalent Metals for A- and B‑Site Dopants in LaFeO₃

Doping LaFeO₃, a mixed ionic electronic conductor, can serve to increase its ionic and electronic conductivity, as observed in La_1–<i>x</i>Sr_<i>x</i>Co_1–<i>y</i>Fe_<i>y</i>O_3−δ (LSCF), a promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material. In this study, <i>ab initio</i> methods have been employed to assess the viability of a range of divalent A- and B-site dopants for promoting ionic and electronic conductivity, through calculating solution energies and binding energies to charge compensating species. For the A-site, we find that all alkali earth metals considered promote increased conductivity properties, but strontium and calcium have the lowest solution energies and therefore will be suitable dopants, in full agreement with experiment. Surprisingly, we find manganese, which has typically been assumed to dope exclusively on the B-site, to have significant probability, on the basis of energetic considerations, to occupy the A-site and be equally as energetically favorable as the traditional strontium dopant under certain conditions. For the B-site, cobalt and nickel were found to be suitable dopants, promoting ionic and electronic conductivity, due to the variable oxidation state of transition metals. Magnesium also increases conductivity as a B-site dopant in contrast with the other alkali earth dopants studied, which favor the A-site. By considering two compensation mechanisms, O^2– vacancy and hole compensation, we show both oxygen vacancies and holes will be promoted in the doped system, in agreement with the experimentally observed mixed ionic electronic conducting properties of doped systems, including LSCF

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

Low‑T Mechanisms of Ammonia Synthesis on Co₃Mo₃N

Dispersion-corrected periodic DFT calculations have been applied to elucidate the Langmuir–Hinshelwood (dissociative) and an Eley–Rideal/Mars–van Krevelen (associative) mechanism for ammonia synthesis over Co₃Mo₃N surfaces, in the presence of surface defects. Comparison of the two distinct mechanisms clearly suggests that apart from the conventional dissociative mechanism, there is another mechanism that proceeds via hydrazine and diazane intermediates that are formed by Eley–Rideal type chemistry, where hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions. This result clearly suggests that via surface defects ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis

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

Why Are Polar Surfaces of ZnO Stable?

We probe and rationalize the complex surface chemistry of wurtzite ZnO by employing interatomic potential calculations coupled with a Monte Carlo procedure that sampled over 0.5 million local minima. We analyze the structure and stability of the (0001) and (0001̅) ZnO surfaces, rationalizing previous patterns found in STM images and explaining the (1 × 1) periodicity reported by LEED analysis. The full range of Zn/O surface occupancies was covered for a (5 × 5) supercell, keeping |<i>m</i>_Zn – <i>m</i>_O|/<i>N</i> ≈ 0.24 where <i>m</i> and <i>N</i> are the numbers of occupied surface sites and total surface sites, respectively. Our calculations explain why the (5 × 5) reconstructions seen in some experiments and highlight the importance of completely canceling the inherent dipole of the unreconstructed polar surfaces. The experimentally observed rich reconstruction patterns can be traced from the lowest occupancy, showing the thermodynamically most stable configurations of both polar surfaces. Triangular and striped reconstructions are seen, <i>inter alia</i>, on both polar surfaces, and hexagonal patterns also appear on the O terminated surface. Our results explain the main experimental structures observed on these complex surfaces. Moreover, grand canonical simulations of ZnO polar surfaces reveal that disorder is favored and, thus, configurational entropic factors is the the cause of their stability

Selection of pupils into classes and schools applying the pedagogy of Maria Montessori

This diploma thesis examines the pupil selection to Marie Montessori pedagogical classes and schools. The aim of the thesis was to find out how the individual schools choose pupils in Marie Montessori classes and schools. Furthermore, it was examined what reasons parents have to choose this pedagogy for their child and what conditions parents and pupils have to fulfill for admission to this education. Last but not least, I analyzed how the admission procedure is taking place. The basic research design was a multi-case study. I chose four primary schools with this pedagogy in the big city as well as in smaller towns. I divided the research into two parts. In the first part I conducted semi-structured interviews with employees of selected schools and with parents of children from Montessori classes. In the second part I observed the first class admission interviews. I have found out that each school has defined pupil selection requirements and that these requirements vary considerably between schools. Parents opt for the Montessori School because of its individual approach to individuals. Teachers believe that it is appropriate that parents apply the same educational methods at home and that they would like to apply this as an admission criterion. The whole admission interviews were the same as in..

Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles

Although [Ni­(S₂CNBuⁱ₂)₂] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 °C in oleylamine to give pure NiS nanoparticles, while in <i>n</i>-hexylamine at 120 °C a mixture of Ni₃S₄ (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral <i>trans-</i>[Ni­(S₂CNBuⁱ₂)₂(RNH₂)₂] result as shown by <i>in situ</i> XANES and EXAFS and confirmed by DFT calculations. Heating to 90–100 °C leads to changes consistent with the formation of amide-exchange products, [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­R}] and/or [Ni­{S₂CN­(H)­R}₂]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni­{S₂CN­(H)­Hex}₂] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 °C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni­{S₂CN­(H)­Hex}₂]. DFT modeling of [Ni­{S₂CN­(H)­R}₂] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S₂CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni­{S₂CN­(H)­R}­(μ-SH)]₂. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides

Designer Titania-Supported Au–Pd Nanoparticles for Efficient Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au–Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd_shell–Au_core nanoparticles immobilized on TiO₂ exhibit extremely high quantum efficiencies for H₂ production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO₂ is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis