Homogeneous ligand-centered hydrogen evolution and hydrogen oxidation : exploiting redox non-innocence to drive catalysis.

Hydrogen is a promising carbon-free fuel / energy carrier and is an essential building block for many industrial and agricultural processes. Rising energy demands have ignited interest in the development of carbon-free and carbon neutral energy sources. In this context, hydrogen is an attractive candidate—being energy-dense, carbon-free—and easily accessible through a two-electron reduction of water. Accordingly, many electrochemical homogeneous catalyst systems have been studied, with a focus on understanding the mechanism of hydrogen evolution proceeding through metal-hydride intermediates. However, there has been a renaissance in hydrogen evolution reaction (HER) catalyst design, with many groups implicating ligand redox non-innocence as a crucial driving force for catalysis rather than metal-hydride formation. In this dissertation, using characterization techniques including, cyclic voltammetry, controlled potential coulometry, UV-visible spectroscopy, 1H NMR, cyclic voltammetry modeling, x-ray crystallography, kinetic isotope effect studies, and density functional theory, we investigate ligand-centered electrocatalysts, which function without the generation of metal-hydride intermediates, for the production and oxidation of dihydrogen. Chapter three expands upon the previous work in the Grapperhaus Lab, and focus on ReL3 (L = diphenylphosphinobenzenethiolate). ReL3 reduces acids to H2 in dichloromethane with an overpotential of 0.708 V and a turnover frequency (TOF) of 32 s-1, and also oxidizes H2 in the presence of base with an overpotential of 0.970 V and a TOF of 4 s-1. The mechanism is supported by kinetic isotope effect (KIE) studies and density functional theory calculations (DFT). Chapters four and five will build on Chapter three, aiming to develop sustainable approaches for ligand-centered catalysis. The non-transition metal complex, ZnL1, the metal-free complex, H2L1, and the transition metal complex, CuL1 (L1 = diacetyl-bis(N4-methyl-3-thiosemicarbazonato)), function as electrocatalysts for hydrogen evolution (ZnL1, H2L1 and CuL1) and hydrogen oxidation (ZnL1 and H2L1). H2L1and ZnL1 display TOF’s of 1,320 s-1 and 1,170 s-1 at overpotentials of 1.43 and 0.756 V, respectively, while the CuL1 complex demonstrates a TOF of 10,000 s-1. H2L1 and ZnL1 also display TOF values for H2 oxidation of 32 s-1 and 72 s-1 at overpotentials of 0.328 and 0.315 V, respectively. Mechanisms for the HER were modeled using digital simulations and are further supported by DFT calculations. ReL3, ZnL1, H2L1, and CuL1 represent a fundamentally new class of electrocatalysts. Contrary to traditional molecular electrocatalysts that employ a metal-hydride as the key mechanistic intermediate, this approach facilitates H2 evolution through ligand-centered proton and electron-transfer events resulting in the evolution of H2 through either ligand-centered H● radical coupling or ligand-centered hydride proton coupling

Differential tunnel transparency of a rectangular heterostructural barrier for the terahertz frequency range

Electron wave tunneling through a rectangular heterostructural emitter barrier is considered in the case of a homogeneous high-frequency (hf) alternating electric field directed normal to the barrier interfaces. This hf field leads not only to the well-known increase in a stationary tunnel current through the emitter barrier, which is proportional to EB2EB2 (where EBEB is the electric-field amplitude) but also to a linear ( ∼ EB)(∼EB) increase in an alternating current (ac) through this barrier with the same frequency ωω as the electric-field frequency. The ac is a sharp function of ωω, which grows significantly with an increase in ωω (typically in the terahertz range). In a certain intermediate current and frequency region, the above-mentioned increase in the ac is the dominating effect of the alternating field. Such an effect can be used to optimize tunnel barrier emitters for ballistic transit-time terahertz-range oscillators.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87677/2/093705_1.pd

A new methodology called dice game optimizer for capacitor placement in distribution systems

Purpose. Shunt capacitors are installed in power system for compensating reactive power. Therefore, feeder capacity releases, voltage profile improves and power loss reduces. However, determination optimal location and size of capacitors in distributionsystems is a complex optimization problem. In order to determine the optimum size and location of the capacitor, an objective function which is generally defined based on capacitor installation costs and power losses should be minimized According to operational limitations. This paper offers a newly developed metaheuristic technique, named dice game optimizerto determine optimal size and location of capacitors in a distribution network. Dice game optimizer is a game based optimization technique that is based on the rules of the dice game.Цель. Шунтирующие конденсаторы в энергосистеме устанавливаются для компенсации реактивной мощности. Следовательно, снижается емкость фидера, улучшается профиль напряжения и снижаются потери мощности. Однако определение оптимального местоположения и размера конденсаторов в системах распределения является сложной задачей оптимизации. Чтобы определить оптимальный размер и расположение конденсатора, целевую функцию, которая обычно определяется на основе затрат на установку конденсатора и потерь мощности, следует минимизировать в соответствии с эксплуатационными ограничениями. Данная статья предлагает недавно разработанный метаэвристический метод, называемый оптимизатором игры в кости, для определения оптимального размера и расположения конденсаторов в распределительной сети. Оптимизатор игры в кости – это игровой метод оптимизации, основанный на правилах игры в кости

Quantum wires and dots induced in a semiconductor by charged metallic filaments separated by an isolating barrier

A very thin positively charged metallic filament separated from a surface of a semiconductor (S)(S) by a thin nontunneling potential barrier (B)(B) induces a quantum wire (QWr) in the semiconductor at the B/SB∕S interface. Single-electron quantum states of this QWr are controlled by a potential (and a charge) of the metallic filament. Two close parallel metallic filaments placed over such a B/SB∕S interface form a double-quantum wire with the ground and the first excited electron states, which appear as a result of a symmetric–antisymmetric splitting of the ground electron state in the single QWr. Two crossed metallic filaments, which are parallel to the B/SB∕S interface, form a quantum dot with completely localized electron states under the crossing point of the metallic filaments. The analogous crossing of a metallic filament by a pair of close metallic filaments forms a double-quantum dot (DQD). The latter can serve as a two-level qubit cell. Such qubits can be controlled by potentials of three independent metallic filaments inducing the above-mentioned DQD. Besides this “outside” metallic wire control, the DQDs can be connected to each other across the “inside” quantum wires, which have formed these DQDs by crossing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87666/2/033516_1.pd

Split donor centers and split excitons in a semiconductor heterostructure

The first subject considered in the article is a donor center embedded in a thin heterostructural barrier separating a semiconductor medium into two halves. As a result of the small thickness of this barrier, the wave function of an electron bound by the donor center shifts almost completely into both halves of the surrounding semiconductor medium. The ground and first excited electron states of such a donor center are separated from each other by a narrow energy gap determined by the symmetric-antisymmetric tunnel splitting. Such structures can be implemented in both GaAs/AlXGa1−XAsGaAs∕AlXGa1−XAs and Si/GeXSi1−XSi∕GeXSi1−X material systems. The second considered subject is an exciton formed in analogous heterostructures when the staggered band alignment takes place between the heterobarrier and semiconductor medium. As a result of such band alignment, the hole participating in the exciton creation is located in the formed quantum well and the electron, which is the hole’s opponent, is separated into halves (on different sides of the quantum well) as before. Unlike the donor center, the exciton can be shifted and localized in arbitrary positions along the staggered “barrier-well” boundary by inhomogeneous electric fields of external controlling gates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87366/2/073711_1.pd

Time-dependent electron tunneling through time-dependent tunnel barriers

A plane electron wave incident on a tunnel-transparent potential barrier formed by the potential V(x,t)=V0(x)+V1(x)cos ωtV(x,t)=V0(x)+V1(x)cos ωt generates, in addition to the usual stationary transmitted and reflected stationary waves, also “transmitted” and “reflected” electron waves oscillating with the same frequency ωω. The transmitted oscillating wave can serve as the basis for transit-time microwave generators oscillating in the terahertz range. (Such oscillators are ballistic analogs of the tunnel-emission transit-time diode oscillators suggested almost half a century ago.) In the special case of a rectangular potential barrier, we describe the dependence of a small transmitted oscillating wave amplitude on the frequency ωω and the value of V1(x)V1(x). We consider two forms of V1(x)V1(x): (1) homogeneous oscillation of the height of the rectangular barrier and (2) V1(x)=aδ(x−x1)V1(x)=aδ(x−x1) [where δ(x)δ(x) is the Dirac delta function and 0<x1<w0<x1<w; ww is the barrier thickness]. For sufficiently high frequencies ωω determined by the time for tunneling, a much higher emission of the transmitted oscillating wave takes place in comparison with the results of quasistatic calculations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70729/2/JAPIAU-96-7-3831-1.pd

Ballistic and quasiballistic tunnel transit time oscillators for the terahertz range: Linear admittance

We have considered interactions between ballistic (or quasiballistic) electrons accelerated by a dc electric field in an undoped transit space (T space) and a small ultrahigh frequency ac electric field and have calculated the linear admittance of the T space. Electrons in the T space have a conventional, nonparabolic dispersion relation. After consideration of the simplest specific case when the current is limited by the space charge of the emitted electrons, we turned to an actual case when the current is limited by a heterostructural tunnel barrier (B barrier) separating the heavily doped cathode contact and the T space. We assumed that the B barrier is much thinner than the T space and both dc and ac voltages drop mainly across the T space. The emission tunnel current through the B barrier is determined by the electric field E(0)E(0) in the T space at the boundary B barrier/T space. The more substantial is, the tunnel current limitation the higher the electric field E(0)E(0) becomes. We have shown that for a space-charge limited current the change from parabolic dispersion to the nonparabolic branch induces narrowing and closing of the frequency windows of transit-time negative conductance starting with the lowest-frequency windows. These narrowing and closing frequency windows become effective only for very high voltages U across the T space: U≫mVS2/2e,U≫mVS2/2e, where m is the effective mass for the parabolic branch and VSVS is the saturated velocity for the nonparabolic branch. For moderate voltages U, the effects of nonparabolicity are not very substantial. The tunnel current limitation decreases the space-charge effects in the T space and diminishes the role of the detailed electron dispersion relation. As a result, restoration of the frequency windows of transit-time negative conductance and an increase in the value of this negative conductance occur. The implementation of the considered tunnel injection transit time oscillator diode promises to lead to efficient and powerful sources of terahertz range radiation. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70564/2/JAPIAU-93-9-5435-1.pd

Crop yields response to water pressures in the Ebro basin in Spain: risk and water policy implications

he increasing pressure on water systems in the Mediterranean enhances existing water conflicts and threatens water supply for agriculture. In this context, one of the main priorities for agricultural research and public policy is the adaptation of crop yields to water pressures. This paper focuses on the evaluation of hydrological risk and water policy implications for food production. Our methodological approach includes four steps. For the first step, we estimate the impacts of rainfall and irrigation water on crop yields. However, this study is not limited to general crop production functions since it also considers the linkages between those economic and biophysical aspects which may have an important effect on crop productivity. We use statistical models of yield response to address how hydrological variables affect the yield of the main Mediterranean crops in the Ebro river basin. In the second step, this study takes into consideration the effects of those interactions and analyzes gross value added sensitivity to crop production changes. We then use Montecarlo simulations to characterize crop yield risk to water variability. Finally we evaluate some policy scenarios with irrigated area adjustments that could cope in a context of increased water scarcity. A substantial decrease in irrigated land, of up to 30% of total, results in only moderate losses of crop productivity. The response is crop and region specific and may serve to prioritise adaptation strategies

Risk of water scarcity and water policy implications for crop production in the Ebro basin in Spain

The increasing pressure on water systems in the Mediterranean enhances existing water conflicts and threatens water supply for agriculture. In this context, one of the main priorities for agricultural research and public policy is the adaptation of crop yields to water pressures. This paper focuses on the evaluation of hydrological risk and water policy implications for food production. Our methodological approach includes four steps. For the first step, we estimate the impacts of rainfall and irrigation water on crop yields. However, this study is not limited to general crop production functions since it also considers the linkages between those economic and biophysical aspects which may have an important effect on crop productivity. We use statistical models of yield response to address how hydrological variables affect the yield of the main Mediterranean crops in the Ebro River Basin. In the second step, this study takes into consideration the effects of those interactions and analyzes gross value added sensitivity to crop production changes. We then use Montecarlo simulations to characterize crop yield risk to water variability. Finally we evaluate some policy scenarios with irrigated area adjustments that could cope in a context of increased water scarcity. A substantial decrease in irrigated land, of up to 30% of total, results in only moderate losses of crop productivity. The response is crop and region specific and may serve to prioritise adaptation strategies

Kyoto and Mañana: A CGE analysis of Spanish Greenhouse Gas targets to 2020

Employing a recursive dynamic computable general equilibrium (CGE) model of the Spanish economy, this study explicitly aims to characterise the potential impact of Kyoto and European Union environmental policy targets on the Spanish economy up to 2020, with a particular focus on the agricultural sector. The model code is modified to characterise the emissions trading scheme (ETS), emissions quotas and carbon taxes, whilst emissions reductions are applied to all six registered greenhouse gases (GHGs). As extensions to this work, the study attempts to integrate both the use of ‘Marginal Abatement Cost’ (MAC) curves for potential emissions reductions within the agricultural sector, and econometric estimates of the effects of global warming on land productivity in Spain. The study includes a no action baseline (with 2007 as the benchmark year), in which GHGs are not restricted in any sector of the economy. This is compared to an emissions stabilisation scenario, in which the European Union’s Emissions Trading Scheme (EU ETS) is implemented, and all of Spain’s commitments under Kyoto, and various pieces of EU climate change legislation, are met. Under this scenario, the policy-induced price rises of polluting inputs and processes determines the allocation of emissions reductions amongst the various industries in the economy. Given the agricultural focus of the study, the modelling of emissions response in this sector is further enhanced by the inclusion of MAC curves. These map out an endogenous technological response to price rises, and the extent to which the emissions coefficient (e.g. N2O per Kg of fertiliser applied, or CH4 per head of cattle) can be reduced, such that the same quantity of input emits a smaller amount of GHGs. A flexible functional form is used to calibrate the MAC curves to data from the IIASAs GAINS model , which includes potential emissions reductions, and associated costs, of all major technological advances in agriculture currently ...Publishe