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Caltech Authors (119,780 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 549

1. Interfacial processes in energy storage and conversion devices - Tavassol, Hadi
My research has been focused on characterization and control of the interfacial processes in advanced electrochem. systems presented by solid acid fuel cells (SAFC) and Li ion batteries. The performance of these electrochem. systems is closely related to the processes occurring at the interface of electrodes and electrolytes. The electrocatalysis at the interface of the super-protonic (at 245 °C) CsH_2PO_4 solid acid and electrodes occurs at active sites which are also accessible to the humidified gases. Pt remains the best catalyst for both H_2 oxidn. and O_2 redn. reactions in the anodes and cathodes of the solid acid fuel cells....

2. Exploiting protein homeostasis for cancer therapy - Deshaies, Raymond; Li, Jing
Cancer cells often harbor numerous genomic insults and an altered metab. Together, these changes impede protein folding and assembly. As a result, cancer cells can have an elevated dependence on mechanisms that sustain protein assembly and quality control. To test whether cancer cells are hypersensitive to inhibition of protein homeostasis pathways, we seek to develop small mol. antagonists of ubiquitin system enzymes involved in protein quality control. In particular, we have been pursuing the proteasome, the COP9-Signalosome, and the ubiquitin-selective 'segregase' p97/VCP. Over the past 5 years we have carried out HTS against these targets through the NIH's Mol. Libraries...

3. Survey of nanostructured transition metal phosphide catalysts for carbon dioxide reduction - Francis, Sonja A.; Crompton, J. C.; Torelli, Daniel; Ferrer, Ivonne M.; Schaak, Raymond E.; Brunschwig, Bruce S.; Lewis, Nathan S.
Previous reports show a no. of nanostructured transition metal phosphides (e.g. Ni_2P, MoP, FeP, WP and CoP) as effective Earth-abundant alternatives to Pt for the hydrogen evolution reaction (HER) via electrochem. and/or photochem. redn. of water. We now present a comprehensive study of these and other new phosphides for the electrocatalytic carbon dioxide redn. (CO2R). We examine the effect of surface chem. compn. and electronic structure on the selectivity and activity of the catalysts towards CO2R, det. the HER behavior under CO2R conditions, and investigate the trends in product distribution and faradaic efficiencies as a function of compn., potential and supporting electrolyte.

4. CO2 reduction mechanisms by (PoCoP)Ir and (PeXeP)Co pincer catalysts for production of formate and CO - Johnson, Samantha I.; Nielsen, Robert J.; Shaffer, David W.; Yang, Jenny Y.; Goddard, William A.
Exptl., (PoCoP)Ir has been shown to reduce CO_2 (CO2RR) to formate at high Faradaic efficiencies both as a homogeneous catalyst and immobilized on an electrode. We use d. functional theory (B3LYP and M06 including solvation) to elucidate the mechanism for CO_2 redn. The derived mechanism agrees with the exptl. redn. potentials (irreversible redn. of (PoCoP)Ir(H)(NCMe)2+ ca. -1.4 V vs NHE; -1.37 V vs NHE calcd. for oneelectron redn. of IrIII(H)(NCMe)2+) and the surprising selectivity in this catalyst. We show that there are several thermodynamically favorable pathways for the hydrogen evolution reaction (HER); however, most are kinetically limited with calcd. barriers above 25 kcal/mol in neat water....

5. Soft skills of chemical research: Academic integrity and research ethics - Wrublewski, Donna; Leonard, Michelle; Buhler, Amy; Bharti, Neelam
The foundation of any scientist should be ethical behavior. Like many of these so-called "soft skills", talking about ethics is something that is assumed to be picked up on the fly, and instruction is minimal when present, and more often rolled into nebulous clauses such as "honor codes". Although not specific to chem., librarians teaching these skills is a growing phenomenon in academic libraries. This talk will look at academic integrity instruction as it underpins many of the skills and tools that will be discussed elsewhere in this session, and will describe specific outreach on academic integrity to chemists.

6. On-Demand Power Source for Medical Electronic Implants: Acousto-Mechanical Vibrations from Human Vocal Folds - Chen, S.; Rosenberg, J.; Balakrishna, A.; Ma, G.; Cho, H.; Lee, J. O.; Choo, H.
For use in vibration-driven power generation, we have quantitatively characterized the acousto-mechanical vibrations that propagate from the human vocal folds through the neck and head along the skeletal frames. We have used five MEMS accelerometers to characterize the acousto-mechanical vibrations present in various situations. The acousto-mechanical vibrations excite vibration-driven energy harvesters at their resonance frequencies between 90-300 Hz and generate up to 0.15 mW/cm^3 on demand.

7. Nanofabrication of 3 Dimensional Taper Structures for Nanofocusing Purposes - Kou, Junlong; Li, Zheng; Choo, Hyuck
We have demonstrated experimentally a highly efficient on-chip three-dimensional (3D) linearly tapered metal-insulator-metal (MIM) nanoplasmonic photon compressor (3D NPC) with a final aperture size of 14 x 80nm^2. An optimized and linearly tapered MIM gap plasmon waveguide could theoretically reduce the excessive losses that would occur during nanofocusing processes. This nanofocusing concept has existed for some time, yet researchers had difficulty in realizing structures based on the concept because precisely fabricating the nanoscale waveguides that taper in three dimensions had been very challenging. In simulation study, this approach could enable nanofocusing into a 2 X 5nm^2 area with the coupling loss...

8. Chemical threat responsive carbon nanotube membranes - Herbert, Myles B.; Belger, Christian; Weis, Jonathan G.; Swager, Timothy M.
Current research efforts are aimed at developing hybrid polymer-vertically aligned carbon nanotube (CNT) membranes that are breathable in their native state but become blocked upon exposure to low levels of chem. warfare agents (CWA). Our approach utilizes polymers formed by ring opening metathesis polymn. (ROMP) that have a highly non-planar structure in their neutral resting state to retain the high moisture vapor transport rate of the CNT membranes. Upon reacting with CWAs, these polymers can ionize and planarize, assuming a cationic, arom. structure. This π-stacking drives a gel collapse that will block the CNT pores. These types of polymers will be grafted from the membrane using...

9. Random walk through polymer information retrieval - Wrublewski, Donna; Roth, Dana L.
The history of scientific discovery has moved from observation and classification, through experimentation, and now to modeling and discovery. Models are built on knowledge, so how are knowledge databases constructed. It is worthwhile to understand how information is organized, so as to understand what can be discovered. Chem. information is unique in terms of representation, and polymers in particular have a somewhat sordid history in terms of naming and organization. This talk will take a look back at the history of polymer classification, how information has been organized, how it can be found, and what are some current initiatives underway to improve discoverability using improvements in...

10. Design of next-generation polymer electrolytes: A site network model for ion transport from mechanistic insight - Webb, Michael A.; Savoie, Brett M.; Miller, Thomas F.
We employ atomistic mol. dynamics (MD), in tandem with exptl. studies, to elucidate the mechanisms of lithiumion transport in new carbonyl-, allyl-, and ether-contg. polymer electrolytes. We find that that changes in the lithium-ion coordination environment play a crucial role in the lithium-ion hopping dynamics in all polymers, and this motivates a more general view of ion-transport in which the ion diffusion occurs via transitions between viable solvation sites in the polymer. This view is incorporated into the development of a general model for ion transport in polymer electrolyte solvents. In this model, ion diffusion depends on two essential characteristics: the distribution of sites and the...

11. Enabling small band-gap semiconductors for solar water oxidation using multifunctional NiOx coating - Sun, Ke; Saadi, Fadl H.; Kuang, Yanjin; McDowell, Matthew T.; Zhou, Xinghao; Verlage, Erik; Friedrich, Dennis; Brunschwig, Bruce S.; Tu, Charles W.; Lewis, Nathan S.
Technol. important small band gap (< 2 eV) semiconductors must be stabilized against corrosion or passivation in aq. electrolytes before they can be used as photoelectrodes that directly produce fuels from sunlight. In addn., incorporation of electrocatalysts on the surface of the photoelectrodes is required for efficient oxidn. of H_2O to O_2(g) and redn. of H_2O or H_2O and CO_2 to fuels. Stabilization of technol. important semiconductors against photocorrosion and photopassivation would have a significant impact on photoelectrochem. energy conversion, and could enable the development of a new generation of robust integrated devices for efficient solar-driven water splitting and solar-driven CO_2 redn. Previous efforts have been extensively...

12. Enhancements to electrocatalyic reduction of CO2 by cobalt phthalocyanine upon immobization in polyvinylpyridine membrane - Kramer, Wesley W.; Ferrer, Ivonne M.; McCrory, Charles C.
The factors governing the electrocatalytic CO_2 redn. reaction (CO_2RR) by cobalt phthalocyanine (CoPc) modified edge-plane graphite electrodes have been studied by a no. of electrochem. techniques. Electrodes modified with CoPc immobilized in poly-4-vinylpridine (PVP) show dramatic enhanced activity and selectivity compared to electrodes modified with CoPc alone. CoPc-PVP films reduce CO_2 to CO with Faradaic efficiencies of 98% while displaying large TON and TOF. Two properties of PVP contribute to enhancing the activity of CoPc; (1) the ability of individual pyridine residues to coordinate to CoPc and (2) the high concn. of uncoordinated pyridine residues throughout the film. Electrodes modified with polymer-free, five-coordinate CoPc(L) films (L =...

13. Band edge control of crystalline silicon by chemical functionalization of the surface - Plymale, Noah T.; Ramachandran, Anshul A.; Lim, Allison N.; Brunschwig, Bruce S.; Lewis, Nathan S.
Methyl-termination of the silicon (111) crystal plane has been shown to yield nearly complete termination of the silicon atop sites with Me groups to yield exceptional stability to oxidn. and low elec. defect densities at the surface. However, Me groups impart a -0.4 eV surface dipole that shifts the semiconductor band-edge positions of p-type silicon unfavorably for the prodn. of fuels, namely hydrogen, from sunlight. Incorporation of electroneg. elements, such as fluorine, into alkyl monolayers can effectively reverse the unfavorable shift on the band-edge positions and maximize the efficiency of solar-fuels devices. Thus, a mixed methyl/4- fluorobenzyl monolayer has been developed herein to shift the band-edge...

14. Redox-state effects on small molecule bin multimetallic iron complexes - De Ruiter, Graham; Thompson, Niklas B.; Agapie, Theodor
The biochem. processing of a variety of small mols. including H_2, N_2, CO_2, CO and NO_x requires complex machinery. Metalloenzymes are highly specialized in such transformations, and often employ multi-metal cooperativity. Controlling, rationally, the assembly of multiple metals into a single synthetic framework remains a challenge. Furthermore, understanding multi-metal cooperativity and esp. the role that different redox-states play in small mol. activation is of fundamental importance. As such, we have shown previously how redoxinactive metals influence the redn. potential in different hetero-metallic clusters. Here we present a new class of tetra-iron complexes that are site differentiated, with three iron centers that are six-coordinate and one unique four-coordinate...

15. Models of the oxygen-evolving complex of photosystem II - Kanady, Jacob; Agapie, Theodor
Structural and mechanistic understanding of the biol. site of water oxidn.-the oxygen-evolving complex (OEC) of photosystem II-has gradually developed over the last 30 years based on the highly collaborative work of biochemists, biophysicists, spectroscopists, inorg. chemists and computational chemists, culminating in the current proposal: a Mn_4CaO_5 heterometallic cluster, proposed to contain a Mn-_3CaO_4 cubane / open cubane unit with a fourth, dangling Mn bridged to the cubane through two oxido ligands. Our group joined this collaborative tradition in aims to synthesize accurate Mn_4Ca models. In past work we synthesized a Mn_3CaO_4 cubane supported by three acetates and a 1,3,5-triarylbenzene framework contg. six pyridines and three alkoxides....

16. Mechanistic insights into C-H activation using (phebox)Ir compounds - Johnson, Samantha I.; Nielsen, Robert J.; Zhou, Meng; Goldman, Alan S.; Goddard, William A.
Ph bisoxazolinyl (Phebox) iridium has been previously shown to catalyze C-H activation of benzene and hydrocarbon chains. The mechanisms for C-H oxidn. in (Phebox)Ir(X)_2 are explored using mechanistic templates previously developed for related NNC pincer compds. using d. functional theory (B3LYP and M06 with solvation). Pathways for both non-radical C-H activation and radical hydrogen atom abstraction are compared. These routes are compared to exptl. data for the activation of mesitylene, including oxidn. and H/D exchange reactions. In particular, we focus on the role of the monodentate ionic ligand (acetate and trifluoroacetic acid) in promoting C-H activation, showing that trifluoroacetic acid can lower non-radical C-H activation barriers. Using...

17. Molecular Fe-mediated nitrogen fixation catalysis: Improving turnover and mechanistic insights - Peters, Jonas C.; Creutz, Sidney E.; Del Castillo, Trevor J.; Rittle, Jonathan; Thompson, Niklas B.
Our lab. at Caltech has been exploring iron complexes for the redn. of nitrogen to ammonia in the presence of sources of electrons and protons at low temp. We have recently discovered several iron catalytsts that mediate this reaction, and are now focused on understanding the mechanism(s) by which these iron catalysts operate. We are particularly interested in establishing whether the distal (or Chatt), alternating, and/or a hyrbid mechanism is at work for these iron catalysts. We have also been exploring a no. of reaction parameters that substantially increase the TON of some of these iron systems. Finally, we have been exploring related potential catalysts that...

18. Mixed-metal nanosheet water oxidation catalysts made by pulsed-laser ablation in liquids - Part 1: Synthesis, characterization, and electrocatalysis - Hunter, Bryan M.; Blakemore, James D.; Gray, Harry B.; Winkler, Jay R.; Mueller, Astrid M.
Global sustainable energy solns. remain one of the greatest challenges of the 21st century. Health and climate risks assocd. with combustion of fossil fuels threaten global stability, development and national security. The sun is the most abundant and cleanest source of energy, but its intermittence on earth and unequal local energy needs require conversion into stored fuels. Chem. can meet this challenge by solar-driven water splitting with earth-abundant, efficient and robust materials. Water oxidn. is central to the prodn. of storable chem. fuels, since ample supply of cleanly, efficiently, and affordably generated protons and electrons is a prerequisite for all sustainable chem. transformations. Pulsed-laser ablation...

19. Mixed-metal nanosheet water oxidation catalysts made by pulsed-laser ablation in liquids - Part 2: Mechanistic insights gained by novel in-situ spectroscopies - Hunter, Bryan M.; Gray, Harry B.; Winkler, Jay R.; Mueller, Astrid M.
Sustainable energy solns. impact every aspect of human life. National security, health, access to clean water, the extent of climate change, and biodiversity all critically depend on the global availability of clean, affordable energy. The sun is our most abundant source of energy; more energy from sunlight strikes earth within a single hour than mankind consumes per yr. To sustainably power the planet, sunlight capture, charge transport, and catalysis are needed for fuel prodn. through water splitting. Water oxidn. provides reducing equiv. through a complex four-electron transfer process. Sufficiently active, robust, earth-abundant catalysts for this important reaction are much needed yet still elusive; they will...

20. Tandem catalytic processes for light alkane upgrading and ethylene polymerization - Bercaw, John E.; Leitch, David C.; Sattler, Aaron; Labinger, Jay A.
Progress towards a new tandem catalytic process for hydroalkylating and alkene with an alkane, using an alkene dimerization catalyst, Cp*TaCl_2(C_2H_4), and a hydrogen transfer catalyst, (PCP)Ir(alkene), will be described. Preliminary results for a tandem system that uses an ethylene trimerization catalyst and an ethylene/hexene copolymn. catalyst will also be described.

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