Dissociative adsorption of methane on the Cu and Zn doped (111) surface of CeO2

The development of economical heterogeneous catalysts for the activation of methane is a major challenge for the chemical industry. Screening potential candidates becomes more feasible using rational catalyst design to understand the activity of potential catalysts for CH4 activation. The focus of the present paper is the use of density functional theory to examine and elucidate the properties of doped CeO2. We dope with Cu and Zn transition metals having variable oxidation state (Cu), and a single oxidation state (Zn), and study the activation of methane. Zn is a divalent dopant and Cu can have a +1 or +2 oxidation state. Both Cu and Zn dopants have an oxidation state of +2 after incorporation into the CeO2 (111) surface; however a Hubbard +U correction (+U = 7) on the Cu 3d states is required to maintain this oxidation state when the surface interacts with adsorbed species. Dissociation of methane is found to occur locally at the dopant cations, and is thermodynamically and kinetically more favorable on Zn-doped CeO2 than Cu-doped CeO2. The origins of this lie with the Zn(II) dopant moving towards a square pyramidal geometry in the sub surface layer which facilitates the formation of two-coordinated surface oxygen atoms, that are more beneficial for methane activation on a reducible oxide surface. These findings can aid in rational experimental catalyst design for further exploration in methane activation processes

Enhancing the oxygen vacancy formation and migration in bulk chromium(iii) oxide by alkali metal doping: a change from isotropic to anisotropic oxygen diffusion

Oxygen vacancy formation and migration are vital properties for reducible oxides such as TiO2, CeO2 and Cr2O3 as the oxygen storage capacity (OSC) of these materials are important for a wide range of applications in photovoltaics, oxidative catalysis and solid oxide fuel cells. Substitutional doping these transition metal oxides enhances their OSC potential, in particular for oxygenation and surface reaction chemistry. This study uses density functional theory with on-site Coulomb interactions (PBE+U) for Cr 3d states (+U = 5 eV) and O 2p states (+U = 5.5 eV) to calculate the oxygen vacancy formation energy and oxygen diffusion pathways for alkali metal (Li, K, Na, Rb) doping of bulk chromium(III) oxide (α-Cr2O3). Substitutional doping of the lattice Cr3+ cations with alkali metals that have a +1 oxidation state, creates two hole states on the neighbouring lattice O atoms, and removal of a lattice oxygen charge compensates the dopants by filling the holes. The removal of the next oxygen describes the reducibility of doped Cr2O3. The oxygen vacancy formation energy is greatly promoted by the alkali dopants with a correlation between the ionic radius of the dopant cation and vacancy formation energy; larger dopants (K, Rb) improve the reducibility more than the smaller dopants (Li, Na). The activation barriers for oxygen migration along different directions in the alkali metal doped Cr2O3 bulk were also calculated to examine the effect of doping on the oxygen migration. The calculated activation energies for the undoped chromia are symmetric in three dimensions (isotropic) and the presence of the dopants break this isotropy. Alkali dopants promote oxygen migration in the oxygen intra-layers while suppressing oxygen migration across the Cr cation layers. The smaller dopants (Li, Na) facilitate easier migration in the oxygen intra-layers to a greater extent than the larger dopants (K, Rb). The Na–Cr2O3 bulk promotes both oxygen vacancy formation and migration which makes it a novel candidate for anode materials in medium temperature SOFCs and battery applications

Low valence cation doping of bulk Cr2O3: Charge compensation and oxygen vacancy formation

The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO2, and CeO2, the effect of substitutional metal dopants in bulk Cr2O3 and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard + U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal-doped bulk Cr2O3 to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr2O3 is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and, dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed "the active" oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr2O3 by lowering the energy cost

Comparison of Algorithms and Parameterisations for Infiltration into Organic-Covered Permafrost Soils

Infiltration into frozen and unfrozen soils is critical in hydrology, controlling active layer soil water dynamics and influencing runoff. Few Land Surface Models (LSMs) and Hydrological Models (HMs) have been developed, adapted or tested for frozen conditions and permafrost soils. Considering the vast geographical area influenced by freeze/thaw processes and permafrost, and the rapid environmental change observed worldwide in these regions, a need exists to improve models to better represent their hydrology. In this study, various infiltration algorithms and parameterisation methods, which are commonly employed in current LSMs and HMs were tested against detailed measurements at three sites in Canada’s discontinuous permafrost region with organic soil depths ranging from 0.02 to 3 m. Field data from two consecutive years were used to calibrate and evaluate the infiltration algorithms and parameterisations. Important conclusions include: (1) the single most important factor that controls the infiltration at permafrost sites is ground thaw depth, (2) differences among the simulated infiltration by different algorithms and parameterisations were only found when the ground was frozen or during the initial fast thawing stages, but not after ground thaw reaches a critical depth of 15 to 30 cm, (3) despite similarities in simulated total infiltration after ground thaw reaches the critical depth, the choice of algorithm influenced the distribution of water among the soil layers, and (4) the ice impedance factor for hydraulic conductivity, which is commonly used in LSMs and HMs, may not be necessary once the water potential driven frozen soil parameterisation is employed. Results from this work provide guidelines that can be directly implemented in LSMs and HMs to improve their application in organic covered permafrost soils

Improving our understanding of the Spitzer Space Telescope's pointing drifts

Spitzer observations of exoplanets routinely yield photometric accuracies of better than one part in 10,000. However, the attainable precision is limited in part by pointing drifts, which have the effect of moving the target to less stable or less-well characterized regions of Spitzer’s IRAC detector arrays. Here we examine a large sample of observing sequences in an effort to identify the causes of these pointing drifts. We find that short term and higher order drifts are correlated on various time scales to the temperatures of components in and around the spacecraft bus, and are most likely due to very slight angular displacements of the star trackers. Despite the constraints imposed by a limited pool of targets, such pointing drifts are best mitigated by optimal scheduling, minimizing large and/or lengthy excursions in telescope pitch angle within 24 hours of a high-precision photometry sequence. Such an effort is currently being initiated by the Spitzer Science Center

Using drift scans to improve astrometry with Spitzer

The Spitzer Space Telescope Infrared Array Camera (IRAC) is the only space-based instrument currently capable of continuous long duration monitoring of brown dwarfs to detect variability and characterize their atmospheres. Any such studies are limited, however, by the accuracy to which we know the positions and distances to these targets (most of which are newly discovered and therefore do not yet have multiple epochs of astrometric data). To that end, we have begun a new initiative to adapt the astrometric drift scanning technique employed by the Hubble Space Telescope to enhance Spitzer measurements of parallaxes and proper motions of brown dwarfs and other targets. A suite of images are taken with a set of sources scanned across the array. This technique reduces random noise by coaddition, and because each target covers multiple pixels we are able to average over residual instrumental distortion and intra-pixel variations. Although these benefits can be realized with appropriate dithering, scanning is much more effcient because we can take data concurrently with the spacecraft motion, covering many pixels without waiting to reposition and settle. In this contribution we demonstrate that the observing mode works and describe our software for analyzing the observations. We outline ongoing efforts towards simultaneously solving for source position and residual distortion. Initial testing shows a factor of more than 2 improvement in the astrometric precision can be obtained with Spitzer. We anticipate being able to measure parallaxes for sources out to about 50 pc, increasing the volume surveyed by a factor of 100 and enabling luminosity measurements of the young population of brown dwarfs in the β Pictoris moving group. This observing mode will be ready for public use around Winter of 2015

Influence of trivalent doping on point and Frenkel defect formation in bulk chromium (III) oxide

Substitutional doping in metal oxides is a well-known approach to modifying properties such as ionic conductivity or activity in catalysis. Chromium (III) oxide is an attractive reducible material that has potential use as an oxygenation catalyst, and the fact that it maintains its integrity at high operating temperatures makes it useful for high temperature methanol synthesis and solid oxide fuel cells. Understanding the defect chemistry of Cr2O3 is important for rational catalyst design, in particular when the material is modified by isovalent doping. Density functional theory calculations with a Hubbard +U correction applied to the Cr 3d and O 2p states are used to investigate isovalent doping with Al3+, Fe3+ and La3+ cations. Point defects including Cr and O vacancies, and Frenkel defects are investigated and the effect of cation doping on the defect formation energies in Cr2O3 is explored. Our calculations show that Cr Frenkel and peroxide point defects are the most favourable defects in undoped Cr2O3. Al3+ doping in Cr2O3 does not change the defect formation energies over undoped Cr2O3, and Fe3+ doping improves oxygen vacancy formation while greatly increasing the formation energies of other defects that are potential competing processes that may kill ionic conductivity. La3+ doping in Cr2O3 is found to also improve oxygen vacancy formation and induces a considerable decrease in the cost to form Cr vacancies and Frenkel defects. The modifications to the defect formation energies induced by these dopants in Cr2O3 can be used to impede formation of undesirable defects, thus enhancing ionic conductivity for oxygenation catalysis