Recursos de colección

Caltech Authors (170.931 recursos)

Repository of works by Caltech published authors.

Group = Resnick Sustainability Institute

Mostrando recursos 1 - 20 de 162

  1. Efficient solar hydrogen generation in microgravity environment

    Brinkert, Katharina; Richter, Matthias H.; Akay, Ömer; Liedtke, Janine; Giersig, Michael; Fountaine, Katherine T.; Lewerenz, Hans-Joachim
    Long-term space missions require extra-terrestrial production of storable, renewable energy. Hydrogen is ascribed a crucial role for transportation, electrical power and oxygen generation. We demonstrate in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel. The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen with current densities >15 mA/cm^2 in the absence of buoyancy. Conditions are described adverting the resulting formation of ion transport blocking froth layers on the photoelectrodes. The current limiting factors were overcome...

  2. Fusing triphenylphosphine with tetraphenylborate: introducing the 9-phosphatriptycene-10-phenylborate (PTB) anion

    Drover, Marcus W.; Nagata, Koichi; Peters, Jonas C.
    In a fusion of two ubiquitous organometallic reagents, triphenylphosphine (PPh_3) and tetraphenylborate (BPh_4−), the 9-phosphatriptycene-10-phenylborate (PTB) anion has been prepared for the first time. This borato species has been fully characterized by a suite of spectroscopic methods, and initial reactivity studies introduce it as a competent ligand for transition metals, including Co(II) and Fe(II).

  3. City-level climate change mitigation in China

    Shan, Yuli; Guan, Dabo; Hubacek, Klaus; Zheng, Bo; Davis, Steven J.; Jia, Lichao; Liu, Jianghua; Liu, Zhu; Fromer, Neil; Mi, Zhifu; Meng, Jing; Deng, Xiangzheng; Li, Yuan; Lin, Jintai; Schroeder, Heike; Weisz, Helga; Schelinhuber, Hans Joachim
    As national efforts to reduce CO_2 emissions intensify, policy-makers need increasingly specific, subnational information about the sources of CO_2 and the potential reductions and economic implications of different possible policies. This is particularly true in China, a large and economically diverse country that has rapidly industrialized and urbanized and that has pledged under the Paris Agreement that its emissions will peak by 2030. We present new, city-level estimates of CO_2 emissions for 182 Chinese cities, decomposed into 17 different fossil fuels, 46 socioeconomic sectors, and 7 industrial processes. We find that more affluent cities have systematically lower emissions per unit...

  4. Dynamic Roughness for Manipulation and Control of Turbulent Boundary Layers: An Overview

    McKeon, Beverley J.; Jacobi, Ian; Duvvuri, Subrahmanyam
    The use of dynamic roughness or small, oscillating, wall roughness elements to manipulate the structure of turbulent boundary layers is reviewed with a view to imposing true active control techniques. Linear and nonlinear responses to simple oscillations are detailed, such that the modifications of the overall structure of the turbulence and interactions between scales can be inferred. Although the work considered here uses mechanical actuation, the approach is considered sufficiently general to be implemented by a range of techniques, including engineered microelectromechanical system devices and metamaterial actuators.

  5. Emergent Failures and Cascades in Power Grids: A Statistical Physics Perspective

    Nesti, Tommaso; Zocca, Alessandro; Zwart, Bert
    We model power grids transporting electricity generated by intermittent renewable sources as complex networks, where line failures can emerge indirectly by noisy power input at the nodes. By combining concepts from statistical physics and the physics of power flows and taking weather correlations into account, we rank line failures according to their likelihood and establish the most likely way such failures occur and propagate. Our insights are mathematically rigorous in a small-noise limit and are validated with data from the German transmission grid.

  6. Reduction of aqueous CO_2 to 1-Propanol at MoS_2 electrodes

    Francis, Sonja A.; Velazquez, Jesus M.; Ferrer, Ivonne M.; Torelli, Daniel A.; Guevarra, Dan; McDowell, Matthew T.; Sun, Ke; Zhou, Xinghao; Saadi, Fadl H.; John, Jimmy; Richter, Matthias H.; Hyler, Forrest P.; Papadantonakis, Kimberly M.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS_2 or thin-film MoS_2 electrodes yields 1-propanol as the major CO_2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of -0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO_2 to 1-propanol were ~3.5% for MoS_2 single crystals and ~1% for thin films with low edge-site densities. Reduction of CO_2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e^- and 18...

  7. Globally Suppressed Dynamics in Ion-Doped Polymers

    Webb, Michael A.; Yamamoto, Umi; Savoie, Brett M.; Wang, Zhen-Gang; Miller, Thomas F., III
    We investigate how ion–polymer complexation suppresses molecular motion in conventional polymer electrolytes using molecular dynamics (MD) simulations of lithium hexafluorophosphate in poly(ethylene oxide) and a modified Rouse model. The employed model utilizes an inhomogeneous friction distribution to describe ion–polymer interactions and provides an effective way to examine how ion–polymer interactions affect polymer motion. By characterizing the subdiffusive Li^+ transport and polymer relaxation times at several salt concentrations, we observe that increases in local friction due to ion-polymer complexation are significantly smaller than previously assumed. We find that a Rouse-based model that only considers local increases in friction cannot simultaneously capture...

  8. The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO_2 Reduction Conditions: An Experimental and Theoretical Study

    Landers, Alan T.; Fields, Meredith; Torelli, Daniel A.; Xiao, Jianping; Hellstern, Thomas R.; Francis, Sonja A.; Tsai, Charlie; Kibsgaard, Jakob; Lewis, Nathan S.; Chan, Karen; Hahn, Christopher; Jaramillo, Thomas F.
    A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO_2 reduction (CO_2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO_2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO_2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO_2R. On Ni-MoS_2, a limitation is the kinetic barrier for the...

  9. Phase Directing Ability of an Ionic Liquid Solvent for the Synthesis of HER-Active Ni_2P Nanocrystals

    Roberts, Emily J.; Read, Carlos G.; Lewis, Nathan S.; Brutchey, Richard L.
    An ionic liquid (IL) solvent was used to synthesize small, phase-pure nickel phosphide (Ni_2P) nanocrystals. In contrast, under analogous reaction conditions, substitution of the IL for the common high-boiling organic solvent 1-octadecene (ODE) results in phase-impure nanocrystals. The 5 nm Ni_2P nanocrystals prepared in IL were electrocatalytically active toward the hydrogen evolution reaction. The synthesis in IL was also extended to alloyed Ni_(2–x)Co_xP nanocrystals, where 0.5 ≤ x ≤ 1.5.

  10. Iron Is the Active Site in Nickel/Iron Water Oxidation Electrocatalysts

    Hunter, Bryan M.; Winkler, Jay R.; Gray, Harry B.
    Efficient catalysis of the oxygen-evolution half-reaction (OER) is a pivotal requirement for the development of practical solar-driven water splitting devices. Heterogeneous OER electrocatalysts containing first-row transition metal oxides and hydroxides have attracted considerable recent interest, owing in part to the high abundance and low cost of starting materials. Among the best performing OER electrocatalysts are mixed Fe/Ni layered double hydroxides (LDH). A review of the available experimental data leads to the conclusion that iron is the active site for [NiFe]-LDH-catalyzed alkaline water oxidation.

  11. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis

    Chalkley, Matthew J.; Del Castillo, Trevor J.; Matson, Benjamin D.; Peters, Jonas C.
    Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N_2, CO_2, and O_2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P_3^BFe^+, for catalyzing the nitrogen reduction reaction (N_2RR, N_2-to-NH_3 conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pKa and N_2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of...

  12. Field-effect transistors made from solution-grown two-dimensional tellurene

    Wang, Yixiu; Qiu, Gang; Wang, Ruoxing; Huang, Shouyuan; Wang, Qingxiao; Liu, Yuanyue; Du, Yuchen; Goddard, William A., III; Kim, Moon J.; Xu, Xianfan; Ye, Peide D.; Wu, Wenzhuo
    The reliable production of two-dimensional (2D) crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with process-tunable thickness, from a monolayer to tens of nanometres, and with lateral sizes of up to 100 µm. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thickness-dependent shifts in Raman vibrational modes, which is...

  13. Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium–Aminophosphine Complexes

    Chu, Crystal K.; Lin, Tzu-Pin; Shao, Huiling; Liberman-Martin, Allegra L.; Liu, Peng; Grubbs, Robert H.
    Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation (k_1; initiation rate) and relative phosphine reassociation (k_(–1)) were determined for two series of catalysts bearing cyclohexyl(morpholino)phosphine and cyclohexyl(piperidino)phosphine ligands. In both cases, incorporating P–N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]–PCy_3 complex; however, this effect is muted for the tris(amino)phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT...

  14. Improved synthesis of 4-cyanotryptophan and other tryptophan analogs in aqueous solvent using variants of TrpB from Thermotoga maritima

    Boville, Christina E.; Romney, David K.; Almhjell, Patrick James; Sieben, Michaela; Arnold, Frances H.
    The use of enzymes has become increasingly widespread in synthesis as chemists strive to reduce their reliance on organic solvents in favor of more environmentally benign aqueous media. With this in mind, we previously endeavored to engineer the tryptophan synthase β-subunit (TrpB) for production of noncanonical amino acids that had previously been synthesized through multi-step routes involving water-sensitive reagents. This enzymatic platform proved effective for the synthesis of analogs of the amino acid tryptophan (Trp), which are frequently used in pharmaceutical synthesis as well as chemical biology. However, certain valuable compounds, such as the blue fluorescent amino acid 4-cyanotryptophan (4-CN-Trp),...

  15. Soliton Molecules and Multisoliton States in Ultrafast Fibre Lasers: Intrinsic Complexes in Dissipative Systems

    Gui, Lili; Wang, Pan; Ding, Yihang; Zhao, Kangjun; Bao, Chengying; Xiao, Xiaosheng; Yang, Changxi
    Benefiting from ultrafast temporal resolution, broadband spectral bandwidth, as well as high peak power, passively mode-locked fibre lasers have attracted growing interest and exhibited great potential from fundamental sciences to industrial and military applications. As a nonlinear system containing complex interactions from gain, loss, nonlinearity, dispersion, etc., ultrafast fibre lasers deliver not only conventional single soliton but also soliton bunching with different types. In analogy to molecules consisting of several atoms in chemistry, soliton molecules (in other words, bound solitons) in fibre lasers are of vital importance for in-depth understanding of the nonlinear interaction mechanism and further exploration for high-capacity...

  16. Datum: Managing Data Purchasing and Data Placement in a Geo-Distributed Data Market

    Ren, Xiaoqi; London, Palma; Ziani, Juba; Wierman, Adam
    This paper studies two design tasks faced by a geo-distributed cloud data market: which data to purchase (data purchasing) and where to place/replicate the data for delivery (data placement). We show that the joint problem of data purchasing and data placement within a cloud data market can be viewed as a facility location problem and is thus NP-hard. However, we give a provably optimal algorithm for the case of a data market made up of a single data center and then generalize the structure from the single data center setting in order to develop a near-optimal, polynomial-time algorithm for a...

  17. Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction

    Chapovetsky, Alon; Welborn, Matthew; Luna, M.; Haiges, Ralf; Miller, Thomas F., III; Marinescu, Smaranda C.
    The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional...

  18. Active Radiative Thermal Switching with Graphene Plasmon Resonators

    Ilic, Ognjen; Thomas, Nathan H.; Christensen, Thomas; Sherrott, Michelle C.; Soljačić, Marin; Minnich, Austin J.; Miller, Owen D.; Atwater, Harry A.
    We theoretically demonstrate a near-field radiative thermal switch based on thermally excited surface plasmons in graphene resonators. The high tunability of graphene enables substantial modulation of near-field radiative heat transfer, which, when combined with the use of resonant structures, overcomes the intrinsically broadband nature of thermal radiation. In canonical geometries, we use nonlinear optimization to show that stacked graphene sheets offer improved heat conductance contrast between “ON” and “OFF” switching states and that a >10× higher modulation is achieved between isolated graphene resonators than for parallel graphene sheets. In all cases, we find that carrier mobility is a crucial parameter...

  19. Stereoselective Enzymatic Synthesis of Heteroatom-Substituted Cyclopropanes

    Brandenberg, Oliver F.; Prier, Christopher K.; Chen, Kai; Knight, Anders M.; Wu, Zachary; Arnold, Frances H.
    The repurposing of hemoproteins for non-natural carbene transfer activities has generated enzymes for functions previously accessible only to chemical catalysts. With activities constrained to specific substrate classes, however, the synthetic utility of these new biocatalysts has been limited. To expand the capabilities of non-natural carbene transfer biocatalysis, we engineered variants of Cytochrome P450_(BM3) that catalyze the cyclopropanation of heteroatom-bearing alkenes, providing valuable nitrogen-, oxygen-, and sulfur-substituted cyclopropanes. Four or five active-site mutations converted a single parent enzyme into selective catalysts for the synthesis of both cis and trans heteroatom-substituted cyclopropanes, with high diastereoselectivities and enantioselectivities and up to 40 000 total...

  20. Monolayer atomic crystal molecular superlattices

    Wang, Chen; He, Qiyuan; Halim, Udayabagya; Liu, Yuanyue; Enbo, Zhu; Lin, Zhaoyang; Xiao, Hai; Duan, Xiangfeng; Feng, Ziying; Cheng, Rui; Weiss, Nathan O.; Ye, Guojun; Huang, Yun-Chiao; Wu, Hao; Cheng, Hung-Chieh; Shakir, Imran; Liao, Lei; Chen, Xianhui; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng
    Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach...

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