107 research outputs found
Point Group Symmetry Analysis of the Electronic Structure of Bare and Protected Metal Nanocrystals
The electronic structures of a variety of experimentally identified gold and
silver nanoclusters from 20 to 246 atoms, either unprotected or protected by
several types of ligands, are characterized by using point group specific
symmetry analysis. The delocalized electron states around the HOMO-LUMO energy
gap, originating from the metal s-electrons in the cluster core, show symmetry
characteristics according to the point group that describes best the atomic
arrangement of the core. This indicates strong effects of the lattice structure
and overall shape of the metal core to the electronic structure, which cannot
be captured by the conventional analysis based on identification of spherical
angular momentum shells in the superatom model. The symmetry analysis discussed
in this paper is free from any restrictions regarding shape or structure of the
metal core, and is shown to be superior to the conventional spherical harmonics
analysis for any symmetry that is lower than Ih. As an immediate application,
we also demonstrate that it is possible to reach considerable savings in
computational time by using the symmetry information inside a conventional
linear-response calculation for the optical absorption spectrum of the Ag55
cluster anion, without any loss in accuracy of the computed spectrum. Our work
demonstrates an efficient way to analyze the electronic structure of
non-spherical, but atomically ordered nanocrystals and ligand-protected
clusters with nanocrystal metal cores and it can be viewed as the
generalization of the superatom model demonstrated for spherical shapes ten
years ago (Walter et al., PNAS 2008, 105, 9157)
Screening and Treatment for Subclinical Hypertensive Heart Disease in Emergency Department Patients With Uncontrolled Blood Pressure: A Costâeffectiveness Analysis
ObjectivesPoorly controlled hypertension (HTN) is extremely prevalent and, if left unchecked, subclinical hypertensive heart disease (SHHD) may ensue leading to conditions such as heart failure. To address this, we designed a multidisciplinary program to detect and treat SHHD in a highârisk, predominantly African American community. The primary objective of this study was to determine the costâeffectiveness of our program.MethodsStudy costs associated with identifying and treating patients with SHHD were calculated and a sensitivity analysis was performed comparing the effect of four parameters on cost estimates. These included prevalence of disease, effectiveness of treatment (regression of SHHD, reversal of left ventricular hypertrophy [LVH], or blood pressure [BP] control as separate measures), echocardiogram costs, and participant time/travel costs. The parent study for this analysis was a singleâcenter, randomized controlled trial comparing cardiac effects of standard and intense (<120/80 mm Hg) BP goals at 1 year in patients with uncontrolled HTN and SHHD. A total of 149 patients (94% African American) were enrolled, 133 (89%) had SHHD, 123 (93%) of whom were randomized, with 88 (72%) completing the study. Patients were clinically evaluated and medically managed over the course of 1 year with repeated echocardiograms. Costs of these interventions were analyzed and, following standard practices, a cost per qualityâadjusted lifeâyear (QALY) less than 117,044 to 50,000) was achieved when SHHD prevalence exceeded 11.1% for regression of SHHD, 4.7% for reversal of LVH, and 2.9% for achievement of BP control.ConclusionsIn this cohort of predominantly African American patients with uncontrolled HTN, SHHD prevalence was high and screening with treatment was costâeffective across a range of assumptions. These data suggest that multidisciplinary programs such as this can be a costâeffective mechanism to mitigate the cardiovascular consequences of HTN in emergency department patients with uncontrolled BP.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136283/1/acem13122.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136283/2/acem13122_am.pd
Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters : a computational study
Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ molâ1. At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ molâ1 relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and goldâpalladium alloys
Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters : a computational study
Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ molâ1. At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ molâ1 relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and goldâpalladium alloys
Au-ZSM-5 catalyses the selective oxidation of CH4 to CH3OH and CH3COOH using O2
The oxidation of methane, the main component of natural gas, to selectively form oxygenated chemical feedstocks using molecular oxygen has been a long-standing grand challenge in catalysis. Here, using gold nanoparticles supported on the zeolite ZSM-5, we introduce a method to oxidize methane to methanol and acetic acid in water at temperatures between 120 and 240 °C using molecular oxygen in the absence of any added coreductant. Electron microscopy reveals that the catalyst does not contain gold atoms or clusters, but rather gold nanoparticles are the active component, while a mechanism involving surface adsorbed species is proposed in which methanol and acetic acid are formed via parallel pathways. [Figure not available: see fulltext.]
Using the theory of planned behaviour to help predict consumer value of eco-labelling formats: a focus on fresh meat
Initiator-free hydrocarbon oxidation using supported gold nanoparticlec
In the catalytic oxidation of cycloalkenes and linear internal alkenes using oxygen radical initiators are not required if the stabilisers are removed prior to reaction.</p
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