Recursos de colección

Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

Group = Resnick Sustainability Institute

Mostrando recursos 1 - 20 de 76

  1. Anti-Markovnikov alkene oxidation by metal-oxo–mediated enzyme catalysis

    Hammer, Stephan C.; Kubik, Grzegorz; Watkins, Ella; Huang, Shan; Minges, Hannah; Arnold, Frances H.
    Catalytic anti-Markovnikov oxidation of alkene feedstocks could simplify synthetic routes to many important molecules and solve a long-standing challenge in chemistry. Here we report the engineering of a cytochrome P450 enzyme by directed evolution to catalyze metal-oxo–mediated anti-Markovnikov oxidation of styrenes with high efficiency. The enzyme uses dioxygen as the terminal oxidant and achieves selectivity for anti-Markovnikov oxidation over the kinetically favored alkene epoxidation by trapping high-energy intermediates and catalyzing an oxo transfer, including an enantioselective 1,2-hydride migration. The anti-Markovnikov oxygenase can be combined with other catalysts in synthetic metabolic pathways to access a variety of challenging anti-Markovnikov functionalization reactions.

  2. Nitrogen Fixation via a Terminal Fe(IV) Nitride

    Thompson, Niklas B.; Green, Michael T.; Peters, Jonas C.
    Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N_2. Here we show that a nitrogen-fixing Fe-N_2 catalyst can be protonated to form an Fe(NNH_2) hydrazido(2−) intermediate, which, upon further protonation, heterolytically cleaves the N–N bond to release [Fe^(IV)≡N]^+ and NH_3. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N_2-to-NH_3 conversion. The physical oxidation state range of the Fe complexes in this transformation is buffered...

  3. Crystalline nickel manganese antimonate as a stable water-oxidation catalyst in aqueous 1.0 M H_2SO_4

    Moreno-Hernandez, Ivan A.; MacFarland, Clara A.; Read, Carlos G.; Papadantonakis, Kimberly M.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Water oxidation is a required half-reaction for electrochemical water splitting. To date, the only well-established active oxygen-evolution catalysts stable under operating conditions and at rest in acidic aqueous media contain Ru or Ir, two of the scarcest non-radioactive elements on Earth. We report herein a nickel-manganese antimonate electrocatalyst with a rutile-type crystal structure that requires an initial voltammetric overpotential of 672 ± 9 mV to catalyze the oxidation of water to O_2(g) at a rate corresponding to 10 mA cm^(−2) of current density when operated in contact with 1.0 M sulfuric acid. Under galvanostatic control, the overpotential initially rose from...

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

    Johnson, Samantha I.; Gray, Harry B.; Blakemore, James D.; Goddard, William A., III
    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...

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

    Johnson, Samantha I.; Gray, Harry B.; Blakemore, James D.; Goddard, William A., III
    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...

  6. Adaptive charging network for electric vehicles

    Lee, George; Lee, Ted; Low, Zhi; Low, Steven H.; Ortega, Christine
    We describe a snapshot of an adaptive charging network (ACN) for electric vehicles at Caltech. We overview the system design, from the power distribution system to advanced charger design to control and communication system and the software system that integrates the overall network. We present a simple mathematical formulation of the charging problem. We have collected three months' of baseline charging data from the Caltech ACN. We demonstrate, by simulating a charging algorithm on the baseline data, the large potential benefit of ACN in saving infrastructure costs.

  7. Adaptive charging network for electric vehicles

    Lee, George; Lee, Ted; Low, Zhi; Low, Steven H.; Ortega, Christine
    We describe a snapshot of an adaptive charging network (ACN) for electric vehicles at Caltech. We overview the system design, from the power distribution system to advanced charger design to control and communication system and the software system that integrates the overall network. We present a simple mathematical formulation of the charging problem. We have collected three months' of baseline charging data from the Caltech ACN. We demonstrate, by simulating a charging algorithm on the baseline data, the large potential benefit of ACN in saving infrastructure costs.

  8. Optimal branch exchange for feeder reconfiguration in distribution networks

    Peng, Qiuyu; Low, Steven H.
    The feeder reconfiguration problem chooses the on/off status of the switches in a distribution network in order to minimize a certain cost such as power loss. It is a mixed integer nonlinear program and hence hard to solve. A popular heuristic search consists of repeated application of branch exchange, where some loads are transferred from one feeder to another feeder while maintaining the radial structure of the network, until no load transfer can further reduce the cost. Optimizing each branch exchange step is itself a mixed integer nonlinear program. In this paper we propose an efficient algorithm for optimizing a...

  9. Convex relaxations and linear approximation for optimal power flow in multiphase radial networks

    Gan, Lingwen; Low, Steven H.
    Distribution networks are usually multiphase and radial. To facilitate power flow computation and optimization, two semidefinite programming (SDP) relaxations of the optimal power flow problem and a linear approximation of the power flow are proposed. We prove that the first SDP relaxation is exact if and only if the second one is exact. Case studies show that the second SDP relaxation is numerically exact and that the linear approximation obtains voltages within 0.0016 per unit of their true values for the IEEE 13, 34, 37, 123-bus networks and a real-world 2065-bus network.

  10. Optimal power flow in tree networks

    Gan, Lingwen; Li, Na; Topcu, Ufuk; Low, Steven H.
    The optimal power flow (OPF) problem seeks to control power generation/demand to optimize certain objectives such as minimizing the generation cost or power loss. It is becoming increasingly important for tree distribution networks due to the emerging distributed generation and controllable loads. The OPF problem is nonconvex. We prove that after modifying the OPF problem, its global optimum can be recovered via a second-order cone programming (SOCP) relaxation for tree networks, under a condition that can be checked in advance. Empirical studies justify that the modification is “small”, and that the condition holds, for the IEEE 13-bus network and two...

  11. Equilibrium and dynamics of local voltage control in distribution systems

    Farivar, Masoud; Chen, Lijun; Low, Steven
    We consider a class of local volt/var control schemes where the control decision on the reactive power at a bus depends only on the local bus voltage. These local algorithms form a feedback dynamical system and collectively determine the bus voltages of a power network. We show that the dynamical system has a unique equilibrium by interpreting the dynamics as a distributed algorithm for solving a certain convex optimization problem whose unique optimal point is the system equilibrium. Moreover, the objective function serves as a Lyapunov function implying global asymptotic stability of the equilibrium. The optimization based model does not...

  12. The role of a market maker in networked Cournot competition

    Bose, Subhonmesh; Cai, Desmond W. H.; Low, Steven H.; Wierman, Adam
    We study the role of a market maker (or market operator) in a transmission constrained electricity market. We model the market as a one-shot networked Cournot competition where generators supply quantity bids and load serving entities provide downward sloping inverse demand functions. This mimics the operation of a spot market in a deregulated market structure. In this paper, we focus on possible mechanisms employed by the market maker to balance demand and supply. In particular, we consider three candidate objective functions that the market maker optimizes - social welfare, residual social welfare, and consumer surplus. We characterize the existence of...

  13. Quadratically Constrained Quadratic Programs on Acyclic Graphs With Application to Power Flow

    Bose, Subhonmesh; Gayme, Dennice F.; Chandy, K. Mani; Low, Steven H.
    This paper proves that nonconvex quadratically constrained quadratic programs can be solved in polynomial time when their underlying graph is acyclic, provided the constraints satisfy a certain technical condition. We demonstrate this theory on optimal power-flow problems over tree networks.

  14. Ultrafast Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

    Jhalani, Vatsal A.; Zhou, Jin-Jian; Bernardi, Marco
    GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron–phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5–1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot...

  15. Catalysis and chemical mechanisms of calcite dissolution in seawater

    Subhas, Adam V.; Adkins, Jess F.; Rollins, Nick E.; Naviaux, John; Erez, Jonathan; Berelson, William M.
    Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO_2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve ^(13)C-labeled calcites in natural seawater. We show that the time-evolving enrichment of δ^(13)C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the ^(13)C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium...

  16. Asymmetry in the Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

    Jhalani, Vatsal A.; Zhou, Jin-Jian; Bernardi, Marco
    GaN is a key material for lighting and power electronics. Yet, the carrier transport and ultrafast dynamics that are central in GaN devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons, and that for hot carriers with an initial 0.5−1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics...

  17. Toxic Byproduct Formation during Electrochemical Treatment of Latrine Wastewater

    Jasper, Justin T.; Yang, Yang; Hoffmann, Michael R.
    Electrochemical systems are an attractive option for onsite latrine wastewater treatment due to their high efficiency and small footprint. While concerns remain over formation of toxic byproducts during treatment, rigorous studies examining byproduct formation are lacking. Experiments treating authentic latrine wastewater over variable treatment times, current densities, chloride concentrations, and anode materials were conducted to characterize byproducts and identify conditions that minimize their formation. Production of inorganic byproducts (chlorate and perchlorate) and indicator organic byproducts (haloacetic acids and trihalomethanes) during electrolysis dramatically exceeded recommendations for drinking water after one treatment cycle (∼10–30 000 times), raising concerns for contamination of downstream water...

  18. A Spot Capacity Market to Increase Power Infrastructure Utilization in Multi-Tenant Data Centers

    Islam, Mohammad A.; Ren, Xiaoqi; Ren, Shaolei; Wierman, Adam
    Despite the common practice of oversubscription, power capacity is largely under-utilized in data centers. A significant factor driving this under-utilization is fluctuation of the aggregate power demand, resulting in unused "spot (power) capacity". In this paper, we tap into spot capacity for improving power infrastructure utilization in multi-tenant data centers, an important but under-explored type of data center where multiple tenants house their own physical servers. We propose a novel spot capacity market, called SpotDC, to allocate spot capacity to tenants on demand. Specifically, SpotDC extracts tenants' rack-level spot capacity demand through an elastic demand function, based on which the...

  19. Interpreting Latent Variables in Factor Models via Convex Optimization

    Taeb, Armeen; Chandrasekaran, Venkat
    Latent or unobserved phenomena pose a significant difficulty in data analysis as they induce complicated and confounding dependencies among a collection of observed variables. Factor analysis is a prominent multivariate statistical modeling approach that addresses this challenge by identifying the effects of (a small number of) latent variables on a set of observed variables. However, the latent variables in a factor model are purely mathematical objects that are derived from the observed phenomena, and they do not have any interpretation associated to them. A natural approach for attributing semantic information to the latent variables in a factor model is to...

  20. Cu metal embedded in oxidized matrix catalyst to promote CO_2 activation and CO dimerization for electrochemical reduction of CO_2

    Xiao, Hai; Goddard, William A.; Cheng, Tao; Liu, Yuanyue
    We propose and validate with quantum mechanics methods a unique catalyst for electrochemical reduction of CO_2 (CO_2RR) in which selectivity and activity of CO and C_2 products are both enhanced at the borders of oxidized and metallic surface regions. This Cu metal embedded in oxidized matrix (MEOM) catalyst is consistent with observations that Cu_2O-based electrodes improve performance. However, we show that a fully oxidized matrix (FOM) model would not explain the experimentally observed performance boost, and we show that the FOM is not stable under CO_2 reduction conditions. This electrostatic tension between the Cu^+ and Cu^0 surface sites responsible for...

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