Composition-dependent morphologies of CeO2 nanoparticles in the presence of Co-adsorbed H2O and CO2: a density functional theory study.

Catalytic activity is affected by surface morphology, and specific surfaces display greater activity than others. A key challenge is to define synthetic strategies to enhance the expression of more active surfaces and to maintain their stability during the lifespan of the catalyst. In this work, we outline an ab initio approach, based on density functional theory, to predict surface composition and particle morphology as a function of environmental conditions, and we apply this to CeO2 nanoparticles in the presence of co-adsorbed H2O and CO2 as an industrially relevant test case. We find that dissociative adsorption of both molecules is generally the most favourable, and that the presence of H2O can stabilise co-adsorbed CO2. We show that changes in adsorption strength with temperature and adsorbate partial pressure lead to significant changes in surface stability, and in particular that co-adsorption of H2O and CO2 stabilizes the {100} and {110} surfaces over the {111} surface. Based on the changes in surface free energy induced by the adsorbed species, we predict that cuboidal nanoparticles are favoured in the presence of co-adsorbed H2O and CO2, suggesting that cuboidal particles should experience a lower thermodynamic driving force to reconstruct and thus be more stable as catalysts for processes involving these species.

The energetics of carbonated PuO2 surfaces affects nanoparticle morphology: a DFT+U study.

Radiolytic corrosion of actinide materials represent an issue for the long term storage and disposal of nuclear materials. Molecular species adsorbed at the surface of the actinides may impact the rate of radiolysis, and as the surfaces corrode, the soluble toxic and radioactive species leach into groundwater. It is therefore critical to characterise the surface composition of actinides. Here, we employ ab initio modelling to determine the surface composition of PuO2 with respect to adsorbed CO2. We found that CO2 interacts strongly with the surface forming carbonate species. By mapping the energetics of this interaction, we then calculate the temperature of desorption, finding that surface morphology has a strong impact on the adsorption of CO2, with the {100} being the most and the {111} the least affected by carbonation. Finally, we predict the effect of carbonation on the morphology of PuO2 nanoparticles as a function of temperature and pressure, finding that truncated octahedral is the preferred morphology. This modelling strategy helps characterise surface compensition and nanoparticle morphology, and we discuss the implication for radiolytically driven dispersal of material into the environment.

An introduction to classical molecular dynamics simulation for experimental scattering users.

Classical molecular dynamics simulations are a common component of multi-modal analyses of scattering measurements, such as small-angle scattering and diffraction. Users of these experimental techniques often have no formal training in the theory and practice of molecular dynamics simulation, leading to the possibility of these simulations being treated as a 'black box' analysis technique. This article describes an open educational resource (OER) designed to introduce classical molecular dynamics to users of scattering methods. This resource is available as a series of interactive web pages, which can be easily accessed by students, and as an open-source software repository, which can be freely copied, modified and redistributed by educators. The topics covered in this OER include classical atomistic modelling, parameterizing interatomic potentials, molecular dynamics simulations, typical sources of error and some of the approaches to using simulations in the analysis of scattering data.

Impact of Hydrogen on the Intermediate Oxygen Clusters and Diffusion in Fluorite Structured UO2+ x.

Uranium dioxide is the most prevalent nuclear fuel. Defect clusters are known to be present in significant concentrations in hyperstoichoimetric uranium oxide, UO2+ x, and have a significant impact on the corrosion of the material. A detailed understanding of the defect clusters that form is required for accurate diffusion models in UO2+ x. Using ab initio calculations, we show that at low excess oxygen concentration, where defects are mostly isolated oxygen interstitials, hydrogen stabilizes the initial clustering. The simplest cluster at this low excess oxygen stoichiometry consists of a pair of oxygen ions bound to an oxygen vacancy, namely the split mono-interstital, which resembles larger split interstitials clusters in UO2+ x. Our data shows that, depending on local hydrogen concertation, the presence of hydrogen stabilizes this cluster over isolated oxygen interstitials.

Computer-Aided Design of Nanoceria Structures as Enzyme Mimetic Agents: The Role of Bodily Electrolytes on Maximizing Their Activity.

Nanoceria, typically used for "clean-air" catalytic converter technologies because of its ability to capture, store, and release oxygen, is the same material that has the potential to be used in nanomedicine. Specifically, nanoceria can be used to control oxygen content in cellular environments; as a "nanozyme", nanoceria mimics enzymes by acting as an antioxidant agent. The computational design procedures for predicting active materials for catalytic converters can therefore be used to design active ceria nanozymes. Crucially, the ceria nanomedicine is not a molecule; rather, it is a crystal and exploits its unique crystal properties. Here, we use ab initio and classical computer modeling, together with the experiment, to design structures for nanoceria that maximize its nanozymetic activity. We predict that the optimum nanoparticle shape is either a (truncated) polyhedral or a nanocube to expose (active) CeO2{100} surfaces. It should also contain oxygen vacancies and surface hydroxyl species. We also show that the surface structures strongly affect the biological activity of nanoceria. Analogous to catalyst poisoning, phosphorus "poisoning", the interaction of nanoceria with phosphate, a common bodily electrolyte, emanates from phosphate ions binding strongly to CeO2{100} surfaces, inhibiting oxygen capture and release and hence its ability to act as a nanozyme. Conversely, the phosphate interaction with {111} surfaces is weak, and therefore, these surfaces protect the nanozyme against poisoning. The atom-level understanding presented here also illuminates catalytic processes and poisoning in "clean-air" or fuel-cell technologies because the mechanism underpinning and exploited in each technology, oxygen capture, storage, and release, is identical.