Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase transition of point defects

A uniform distribution of point defects in an otherwise perfect crystallographic structure usually describes a unique pseudo phase of that state of a non-stoichiometric material. With off-stoichiometric uranium dioxide as a prototype, we show that analogous to a conventional phase transition, these pseudo phases also will transform from one state into another via changing the predominant defect species when external conditions of pressure, temperature, or chemical composition are varied. This exotic transition is numerically observed along shock Hugoniots and isothermal compression curves in UO2 with first-principles calculations. At low temperatures, it leads to anomalies (or quasi-discontinuities) in thermodynamic properties and electronic structures. In particular, the anomaly is pronounced in both shock temperature and the specific heat at constant pressure. With increasing of the temperature, however, it transforms gradually to a smooth cross-over, and becomes less discernible. The underlying physical mechanism and characteristics of this type of transition are encoded in the Gibbs free energy, and are elucidated clearly by analyzing the correlation with the variation of defect populations as a function of pressure and temperature. The opportunities and challenges for a possible experimental observation of this phase change are also discussed.Comment: 11 pages, 5 figure

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co₃Mo₃N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases

Silica grain catalysis of methanol formation

The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + COads and H + H2C=O-ads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH3OH to form efficiently at 10-20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H2C=O on the negatively charged defect are substantial, CH3OH may form efficiently during the warm-up period in star-forming regions

Embedded-Cluster Calculations in a Numeric Atomic Orbital Density-Functional Theory Framework

We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level DFT calculations of solid systems. We illustrate this capability by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO2(110).Comment: 12 pages, 4 figure

Modelling metal centres, acid sites and reaction mechanisms in microporous catalysts

We discuss the role of QM/MM (embedded cluster) computational techniques in catalytic science, in particular their application to microporous catalysis. We describe the methodologies employed and illustrate their utility by briefly summarising work on metal centres in zeolites. We then report a detailed investigation into the behaviour of methanol at acidic sites in zeolites H-ZSM-5 and H-Y in the context of the methanol-to-hydrocarbons/olefins process. Studying key initial steps of the reaction (the adsorption and subsequent methoxylation), we probe the effect of framework topology and Brønsted acid site location on the energetics of these initial processes. We find that although methoxylation is endothermic with respect to the adsorbed system (by 17–56 kJ mol−1 depending on the location), there are intriguing correlations between the adsorption/reaction energies and the geometries of the adsorbed species, of particular significance being the coordination of methyl hydrogens. These observations emphasise the importance of adsorbate coordination with the framework in zeolite catalysed conversions, and how this may vary with framework topology and site location, particularly suited to investigation by QM/MM technique

Building zeolites from precrystallized units: nanoscale architecture

This is the peer reviewed version of the following article: Angew. Chem. Int. Ed. 2018, 57, 15330 15353, which has been published in final form at https://doi.org/10.1002/anie.201711422. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.[EN] Since the early reports by Barrer in the 1940s on converting natural minerals into synthetic zeolites, the use of precrystallized zeolites as crucial inorganic directing agents to synthesize other crystalline zeolites with improved physicochemical properties has become a very important research field, allowing the design, particularly in recent years, of new industrial catalysts. This Review highlights how the presence of some crystalline fragments in the synthesis media, such as small secondary building units (SBUs) or layered substructures, not only favors the crystallization of other zeolites with similar SBUs or layers, but also permits control over important parameters affecting their catalytic activity (chemical composition, crystal size, or porosity, etc.). Recent advances in the preparation of 3D and 2D zeolites through seeding and zeolite-to-zeolite transformation processes will be discussed extensively in this Review, including their preparation in the presence or absence of organic structure-directing agents (OSDAs). The aim is to introduce general guidelines for more efficient approaches for target zeolites.This work has been supported by the Spanish Government (MINECO through "Severo Ochoa" (SEV-2016-0683) and MAT2015-71261-R), by the European Union through ERC-AdG-2014-671093 (SynCatMatch), and by the Fundacion Ramon Areces (through the "Life and Materials Science" program).Li, C.; Moliner Marin, M.; Corma Canós, A. (2018). Building zeolites from precrystallized units: nanoscale architecture. Angewandte Chemie International Edition. 57(47):15330-15353. https://doi.org/10.1002/anie.201711422S15330153535747Cundy, C. S., & Cox, P. A. (2005). The hydrothermal synthesis of zeolites: Precursors, intermediates and reaction mechanism. Microporous and Mesoporous Materials, 82(1-2), 1-78. doi:10.1016/j.micromeso.2005.02.016Martínez, C., & Corma, A. (2011). Inorganic molecular sieves: Preparation, modification and industrial application in catalytic processes. 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Efficient and accurate approach to modeling the microstructure and defect properties of LaCoO3

Complex perovskite oxides are promising materials for cathode layers in solid oxide fuel cells. Such materials have intricate electronic, magnetic, and crystalline structures that prove challenging to model accurately. We analyze a wide range of standard density functional theory approaches to modeling a highly promising system, the perovskite LaCoO3, focusing on optimizing the Hubbard U parameter to treat the self-interaction of the B-site cation's d states, in order to determine the most appropriate method to study defect formation and the effect of spin on local structure. By calculating structural and electronic properties for different magnetic states we determine that U=4 eV for Co in LaCoO3 agrees best with available experiments. We demonstrate that the generalized gradient approximation (PBEsol+U) is most appropriate for studying structure versus spin state, while the local density approximation (LDA+U) is most appropriate for determining accurate energetics for defect properties

Bulk and surface simulation studies of La1-xCax MnO3

The structural properties of La1-xCaxMnO3 are investigated by employing bulk and surface simulation\ud techniques. The potential parameters reproduce the crystal structures of both end compositions, giving\ud good agreement with experimental data. The calculated variation of the lattice parameters with Ca\ud concentration also agrees with experimental values. Our calculations predict the formation of a solid\ud solution and that, at very low temperatures and near x ) 1/2, the cations preferentially order on both A\ud and B sites. The driving mechanism for cation ordering arises from both the coulomb and ion-size terms,\ud with the relative proportions depending on composition. However, at temperatures where high-temperature\ud synthesis normally occurs, the ordering of the cations is random. Surface energies are calculated for\ud (010) and (110) surfaces for the end compositions: in LaMnO3, Mn-terminated layers are more stable,\ud whereas Ca-terminated layers are found to be more stable for the CaMnO3 system. It is proposed that the\ud surface structure of intermediate compositions is controlled by a subtle interplay between the different\ud cation ionic strengths and their respective concentrations