Improved thermoelectric properties in Zn-doped Ca_(5)Ga_(2)Sb_6

Zintl compounds with the chemical formula Ca_(5)M_(2)Sb_6 have attracted attention as candidates for use in thermoelectric applications due to their low thermal conductivity and promising high temperature performance (i.e., zT = 0.6 at 1000 K in Ca_(5)Al_(2−x)Na_(x)Sb_6). We have shown previously that, relative to Ca_(5)Al_(2)Sb_6, both Ca_(5)Ga_(2)Sb_6 and Ca_(5)In_(2)Sb_6 have reduced phonon velocities and improved carrier mobility, suggesting that improved zT can be achieved in these materials. Here we further investigate Ca_(5)Ga_(2)Sb_6, which is an intrinsic semiconductor with a small concentration of p-type carriers. By substituting Zn^2+ on the Ga^3+ site, we show that it is possible to increase and control the carrier concentration in Ca_(5)Ga_(2−x)Zn_(x)Sb_6 and thus optimize its thermoelectric behavior. A single parabolic band model was used to estimate an effective mass of m* = 1.6me, which is slightly lower than Al-based compounds. Though the reduced m* leads to a lower Seebeck coefficient, it also leads to a much higher electronic mobility. The high mobility leads to increased thermoelectric figure of merit (zT) at low and intermediate temperatures relative to Zn-doped Ca5Al2Sb6. However, due to the decreased band gap in Ca_(5)Ga_(2)Sb_6 relative to Ca5Al2Sb6, the maximum zT in optimally doped Ca_(5)Ga_(2)Sb_6 is reduced (peak zT ~ 0.35 at T = 775 K)

Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both Ir^(III)(H)_2 and Ir^I–H^– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO_2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from Ir^I and Ir^(III) intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H_2/H^+ and HCOO^–/CO_2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO_2. The effect of solvation on calculated driving forces for hydride transfer is also discussed

Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both Ir^(III)(H)_2 and Ir^I–H^– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO_2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from Ir^I and Ir^(III) intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H_2/H^+ and HCOO^–/CO_2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO_2. The effect of solvation on calculated driving forces for hydride transfer is also discussed

Activation and Oxidation of Mesitylene C−H Bonds by (Phebox)Iridium(III) Complexes

A pincer iridium(III) complex, (Phebox)Ir(OAc)_2OH_2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C–H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF_3)_2OH_2 (2), was synthesized to compare with complex 1 for C–H activation, and (Phebox)Ir(mesityl)(OCOCF_3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)_2OH_2 catalyzed the benzylic C–H oxidation of mesitylene using Ag_2O as a terminal oxidant at 130 °C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C–H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C–H hydroxylation. Results of calculations of the C–H activation pathway appear to be the more consistent with the experimental observations

Design of robust 2,2′-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces

The attachment of the 2,2′-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker. Free energy calculations for the Cl-atom release process with model silyl-linker constructs revealed a modest barrier (14.9 kcal mol⁻¹) that decreased as the electrode potential became more negative. A small library of new bpy-derived structures has additionally been explored computationally to identify strategies that could minimize chlorine-induced linker instability. Structures with fluorine substituents are predicted to be more stable than their chlorine analogues, whereas fully non-halogenated structures are predicted to exhibit the highest stability. The behavior of a hydrogen-evolving molecular catalyst Cp*Rh(bpy) (Cp* = pentamethylcyclopentadienyl) immobilized on a silicon(111) cluster was explored theoretically to evaluate differences between the homogeneous and surface-attached behavior of this species in a tautomerization reaction observed under reductive conditions for catalytic H₂ evolution. The calculated free energy difference between the tautomers is small, hence the results suggest that use of reductively stable linkers can enable robust attachment of catalysts while maintaining chemical behavior on the electrode similar to that exhibited in homogeneous solution

Comportamento dos consumidores de carnes em Cabo Verde: sua percepção de qualidade.

TCC (graduação) - Universidade Federal de Santa Catarina. Centro de Ciências Agrárias. Curso de Zootecnia.Objetivou-se conhecer o perfil do consumidor de carne em Cabo Verde. Foram entrevistadas 168 pessoas por meio de questionário estruturado apenas com questões fechadas, enviadas ao acaso via internet. Os resultados demonstraram que a frequência de consumo de carnes é baixa, sendo que dos 168 entrevistados, apenas 70 consomem carne de 1 a 3 vezes por semana. As carnes de maior consumo são frango seguido por peixe. As variáveis, sexo, idade e renda influenciaram na decisão, sendo que o nível de renda foi estatisticamente significativo (P<0,05) para determinar o consumo do tipo de carne. No momento de avaliar as alternativas da compra, destacou-se o aspecto visual em geral seguido pelo local de conservação. Os consumidores mais jovens e de maior renda têm maior possibilidade de considerar essa característica para a compra da carne. O supermercado é o local com maior frequência de compra. A embalagem destacou-se como o principal agente fornecedor de informações, tendo sido citado por 44 dos entrevistados. As mulheres possuem maior possibilidade de considerar a etiqueta como um indicador de informação. A informação mais verificada na embalagem é a data de validade, seguido da integridade da embalagem. Indagados sobre o sistema de produção dos animais, 146 entrevistados mostraram-se interessados em saber como a carne foi produzida e os consumidores com maior idade, e os com nível superior completo e incompleto, possuem maior interesse em saber essa questão. Quando perguntado se verificam se a carne que compram passou pela inspeção de um técnico responsável, 87 (52%) dos entrevistados possuem essa preocupação enquanto que 81(48%) não verificam essa informação

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

Recent work has shown that Cp*Rh(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2′- bipyridine] undergoes endo protonation at the [Cp*] ligand in the presence of weak acid (Et_3NH^+; pK_a = 18.8 in MeCN). Upon exposure to stronger acid (e.g., DMFH+; pK_a = 6.1), hydrogen is evolved with unity yield. Here, we study the mechanisms by which this catalyst evolves dihydrogen using density functional theory (M06) with polarizable continuum solvation. The calculations show that the complex can be protonated by weak acid first at the metal center with a barrier of 3.2 kcal/mol; this proton then migrates to the ring to form the detected intermediate, a rhodium(I) compound bearing endo η^4-Cp*H. Stronger acid is required to evolve hydrogen, which calculations show happens via a concerted mechanism. The acid approaches and protonates the metal, while the second proton simultaneously migrates from the ring with a barrier of ∼12 kcal/mol. Under strongly acidic conditions, we find that hydrogen evolution can proceed through a traditional metal–hydride species; protonation of the initial hydride to form an H–H bond occurs before migration of the hydride (in the form of a proton) to the [Cp*] ring (i.e., H–H bond formation is faster than hydride–proton tautomerization). This work demonstrates the role of acid strength in accessing different mechanisms of hydrogen evolution. Calculations also predict that modification of the bpy ligand by a variety of functional groups does not affect the preference for [Cp*] protonation, although the driving force for protonation changes. However, we predict that exchange of bpy for a bidentate phosphine ligand will stabilize a rhodium(III) hydride, reversing the preference for bound [Cp*H] found in all computed bpy derivatives and offering an appealing alternative ligand platform for future experimental and computational mechanistic studies of H_2 evolution

Transition-Metal-Mediated Nucleophilic Aromatic Substitution with Acids

Transition-metal-mediated nucleophilic aromatic substitution (S_NAr) reactions prefer that a suitably strong nucleophile be in an aprotic medium. Usually, using protic nucleophile/medium requires high reaction temperatures (>180 °C) to overcome the attenuated nucleophilicity for attack on the arene π system. Surprisingly, we demonstrate herein a Rh^(III)-mediated S_NAr reaction of a fluoroarene moiety with RCO_2H (R = CH_3, CF_3) in acid media that proceeds at moderate temperatures (<100 °C). We show both by experimental and with DFT calculations that the mechanism proceeds through an internal nucleophilic aromatic substitution (I-S_NAr), where the nucleophile coordinates to the metal ion prior to substitution, thereby mitigating the acid influence

Proton–hydride tautomerism in hydrogen evolution catalysis

Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H_2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e^– + 2H^+ → H_2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, η^5-pentamethylcyclopentadienyl; bpy, κ^2-2,2′-bipyridyl) with acid, is not a hydride species but rather, bears [η^4-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H_2 and reformation of η^5-Cp* bound to rhodium(III). With suitable choices of acids and bases, the Cp*Rh(bpy) complex catalyzes facile and reversible interconversion of H^+ and H_2

Common core assessments in follow-up studies of adults born preterm-Recommendation of the Adults Born Preterm International Collaboration

Of all newborns, 1%-2% are born very preterm (VP; <32 weeks) or with very low birthweight (VLBW; ≤1500 g). Advances in prenatal and neonatal care have substantially improved their survival, and the first generations who have benefited from these advances are now entering middle age. While most lead healthy lives, on average these adults are characterised by a number of adversities. These include cardiometabolic risk factors, airway obstruction, less physical activity, poorer visual function, lower cognitive performance, and a behavioural phenotype that includes inattention and internalising and socially withdrawn behaviour that may affect life chances and quality of life. Outcomes in later adulthood are largely unknown, and identifying trajectories of risk or resilience is essential in developing targeted interventions. Joint analyses of data and maintenance of follow-up of cohorts entering adulthood are essential. Such analyses are ongoing within the Adults Born Preterm International Collaboration (APIC; www.apic-preterm.org). Joint analyses require data harmonisation, highlighting the importance of consistent assessment methodologies. To present an expert recommendation on Common Core Assessments to be used in follow-up assessments of adults born preterm. Principles of Common Core Assessments were discussed at APIC meetings. Experts for each specific outcome domain wrote the first draft on assessments pertaining to that outcome. These drafts were combined and reviewed by all authors. Consensus was reached by discussion at APIC meetings. We present a recommendation by APIC experts on consistent measures to be used in adult follow-up assessments. The recommendation encompasses both "core" measures which we recommend to use in all assessments of adults born preterm that include the particular outcome. This will allow comparability between time and location. The recommendation also lists optional measures, focusing on current gaps in knowledge. It includes sections on study design, cardiometabolic and related biomarkers, biological samples, life style, respiratory, ophthalmic, cognitive, mental health, personality, quality of life, sociodemographics, social relationships, and reproduction. [Abstract copyright: © 2020 The Authors. Paediatric and Perinatal Epidemiology published by John Wiley & Sons Ltd.