Search

The attachment of molecular catalysts to conductive supports for the preparation of solid-state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1-pyrenyl(2-pyridyl)ethanolate-κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2-pyridyl)methanolate-κO,κN)Cl (2), and [Cp*Ir(4-(1-pyrenyl)-2,2′-bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π-stacking to ordered mesoporous carbon (OMC). The resulting iridium-based OMC materials (Ir-1, Ir-2, and Ir-3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9–1.6 s−1 at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X-ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

The nature of intramolecular charge transfer (CT) of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting CT character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity (

Nanoscale spinel lithium manganese oxide is of interest as a high-rate cathode material for advanced battery technologies among other electrochemical applications. In this work, the synthesis of ultrathin films of spinel lithium manganese oxide (LiMn2O4) between 20 and 200 nm in thickness by room-temperature electrochemical conversion of MnO grown by atomic layer deposition (ALD) is demonstrated. The charge storage properties of LiMn2O4 thin films in electrolytes containing Li+, Na+, K+, and Mg2+ are investigated. A unified electrochemical band-diagram (UEB) analysis of LiMn2O4 informed by screened hybrid density functional theory calculations is also employed to expand on existing understanding of the underpinnings of charge storage and stability in LiMn2O4. It is shown that the incorporation of Li+ or other cations into the host manganese dioxide spinel structure (λ-MnO2) stabilizes electronic states from the conduction band which align with the known redox potentials of LiMn2O4. Furthermore, the cyclic voltammetry experiments demonstrate that up to 30% of the capacity of LiMn2O4 arises from bulk electronic charge-switching which does not require compensating cation mass transport. The hybrid ALD-electrochemical synthesis, UEB analysis, and unique charge storage mechanism described here provide a fundamental framework to guide the development of future nanoscale electrode materials for ion-incorporation charge storage.

Chemical looping is a promising approach for improving the energy efficiency of many industrial chemical processes. However, a major limitation of modern chemical looping technologies is the lack of suitable active materials to mediate the involved subreactions. Identification of suitable materials has been historically limited by the scarcity of high-temperature (> 600 °C) thermochemical data to evaluate candidate materials. An accurate thermodynamic approach is demonstrated here to rapidly identify active materials which is applicable to a wide variety of chemical looping chemistries. Application of this analysis to chemical looping combustion correctly classifies 17/17 experimentally studied redox materials by their viability and identifies over 1,300 promising yet previously unstudied active materials. This approach is further demonstrated by analyzing redox pairs for mediating a novel chemical looping process for producing pure SO2 from raw sulfur and air which could provide a more efficient and lower emission route to sulfuric acid. 12 promising redox materials for this process are identified, two of which are supported by previous experimental studies of their individual oxidation and reduction reactions. This approach provides the necessary foundation for connecting process design with high-throughput materials discovery to accelerate the innovation and development of a wide-range of chemical looping technologies.

Solar thermal water splitting (STWS) produces renewable hydrogen from water using concentrated sunlight. Because it utilizes energy from the entire solar spectrum to directly drive the redox reactions that split water, it can achieve high theoretical solar-to-hydrogen efficiencies. In two-step STWS, a metal oxide is first heated by concentrated sunlight to high temperatures to reduce it and produce O2. In the second step, the reduced material is exposed to H2O to reoxidize it to its original oxidation state and produce H2. Various aspects of this process are reviewed in this work, including the reduction and oxidation chemistries of the active redox materials, the effects of operating conditions, and the solar thermal reactors in which the STWS reactions occur, and a perspective is given on the future optimization of STWS. WIREs Energy Environ 2016, 5:261–287. doi: 10.1002/wene.174 This article is categorized under: Concentrating Solar Power > Science and Materials Energy and Climate > Science and Materials Energy Research & Innovation > Science and Materials

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H+/1e–/1H+/1e– reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP+/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e– reduction. A HT–PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H+ such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing di...

The temperature and pressure dependence of the reaction of OH + CO has been modeled using the (energy-resolved) master equation and RRKM theory. These calculations are based on the coupled-cluster potential energy surface of Yu and co-workers (Chem Phys Lett 349, 547–554, 2001). As is well known, this reaction shows a strong non-Arrhenius behavior at moderate and low temperatures because of the stabilization of the HOCO intermediate. Kinetic simulations are in excellent agreement with experiments at temperatures above 300 K, but the agreement is only modest at temperatures below 250 K. Our calculations indicate that the contribution of tunneling to the rate constant is marginal, given the small energy difference between the transition states corresponding to formation and decomposition of the HOCO intermediate. Parametric fits to the calculated rate constants are provided for modeling purposes. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 464–474, 2003

The shape of the light response curve of individual leaves has recently become important in models for light absorption by field crops. Studies of the light response of individual corn leaves under sunlight is discussed. A leaf chamber technique for the continuous measurement of net assimilation of CO/sub 2/ by a leaf or corn ear was developed. Uptake rates of corn leaves under optimum light were of the order of 45 mg. of CO/sub 2/ per square decimeter leaf area per hour. Corn ear temperature and percent dry matter were found to have a marked effect on the respiration rate. Corn ear husks photosynthesized approximately 30 mg. CO/sub 2/ per ear per hour under midday sun. 20 references, 5 figures, 3 tables.

A “quasi-diffusion resistance”, rq, is defined to accommodate the role which the thermochemical and photochemical phenomena of photosynthesis play in the control of CO2 fixation to the terminology and approach of the diffusion resistance analogue for the CO2 exchange of leaves. The relationship of rq to Rabinowitch's classical rectangular hyperbolic model of photosynthesis rate as a function of CO2 concen-tration at the carboxylating surface is discussed. Examination of Kmapp for phos-phopyruvate carboxylase (the predominant carboxylase in maize) suggests, as a reasonable hypothesis for light-saturated maize leaves, that rq may be essentially independent of ambient CO2-concentration up to at least 300 μ1/l. A corollary of the hypothesis is that an increase of diffusion resistance, rather than of rq, may account for the observed curvature of the response curves of light-saturated maize leaf photosynthesis to ambient CO2-concentration. An experiment carried out on fieldgrown maize plants, using a well controlled leaf chamber as a nitrous oxide diffusion porometer, gave evidence which strongly supported the hypothesis.

Maize (Zea mays L.) growing in the field was labeled with ¹⁴CO₂ at four leaf positions 15 to 18 days after flowering. On four occasions after labeling, entire plants were harvested and ¹⁴C in several plant fractions was analyzed by liquid scintillation spectroscopy to reveal patterns of translocation within the plants, losses of ¹⁴C by respiration, and patterns of mobilization and redistribution of ¹⁴C among and within organs. The first and third leaf blades above the ear (+1 and +3, respectively), and the first blade below the ear (∗1), exported ¹⁴C mainly kernels of the developing ear. Among these three leaf blades, the extent of export of ¹⁴C was more rapid from upper positions than from lower ones. Movement of ¹⁴C from the fifth leaf blade (∗5) below the ear was much slower than that from the upper three blades, but kernels of the developing ear ultimately became the principle sink for ¹⁴C exported by this blade. Compared with the upper three leaf blade positions, however, appreciable label from ∗5 accumulated in the lower stem and roots. Labeling plants bearing partly-fertilized ears caused a large accumulation of ¹⁴C in the stem, roots and to a lesser extent, husks, compared with completely-fertillzed controls. About 20% of the ¹⁴C assimilated by the +1, +3, and ∗1 blades was lost by respiration in the 5 weeks following labeling. During the same interval, nearly 70% of the ¹⁴C assimilated by the ∗5 blade was lost by respiration. Approximately half of the carbon assimilated by +3, +1, and ∗1 blades was translocated directly to kernels within 24 hr after assimilation. A similar quantity of label assimilated by +3, +1, and ∗1 blades was deposited in cobs and husks within 24 hr after assimilation. Estimates of redistribution of label within the plants suggest that 60 to 80% of the ¹⁴C accumulated in kernels between days 1 and 35 following labeling was derived from ¹⁴C originating in upper leaves and deposited in the cob and husk within 24 hr.