Electrolytic CO2 Reduction in a Flow Cell.

Electrocatalytic CO2 conversion at near ambient temperatures and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chemicals (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcohols). This process is particularly compelling when driven by excess renewable electricity because the consequent production of solar fuels would lead to a closing of the carbon cycle. However, such a technology is not currently commercially available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these experiments generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities ( J) by reducing the mass transport issues inherent to the H-cells. In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic reduction. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities ( J > 200 mA cm-2) for CO2 reduction. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liquid electrolyte. This physical facet also presents a number of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges associated with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.

Stabilizing Copper for CO2 Reduction in Low-Grade Electrolyte.

We demonstrate herein a CO2 reduction electrocatalyst regeneration strategy based on the manipulation of the Cu(0)/Cu2+ equilibrium with high concentrations of ethylenediaminetetraacetic acid (EDTA). This strategy enables the sustained performance of copper catalysts in distilled and tap water electrolytes for over 12 h. The deposition of common electrolyte impurities such as iron, nickel, and zinc is blocked because EDTA can effectively bind the metal ions and negatively shift the electrode potential of M/M n+. The Cu/Cu2+ redox couple is >600 mV more positive than the other metal ions and therefore participates in an equilibrium of dissolution and redeposition from and to the electrode in high concentrations of EDTA. These dynamic equilibria serve to further regenerate the surface copper catalyst to prevent the deactivation of catalytic sites. On the basis of this strategy, we show that >95% of initial hydrocarbon production activity can be maintained for 12 h in KHCO3 (99% purity) enriched distilled water and 6 h in KHCO3 (99% purity) enriched tap water.

Photodecomposition of Metal Nitrate and Chloride Compounds Yields Amorphous Metal Oxide Films.

UV light is found to trigger the decomposition of MClx or M(NO3)x (where M = Fe, Co, Ni, Cu, or Zn) to form uniform, amorphous films of metal oxides. This process does not elevate the temperature of the substrate and thus conformal films can be coated on a range of substrates, including rigid glass and flexible plastic. The formation of the oxide films were confirmed by a combination of powder X-ray diffraction, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy techniques. Amorphous oxide films of iron, nickel and a combination of iron and nickel demonstrated oxygen evolution reaction electrocatalytic activities commensurate with films of the same compositions prepared by widely used electrodeposition and sputtering methods. These results illuminate a potential route to amorphous oxides at scale using simple metal precursors without vacuum or heat.

Di- and Trivalent Metal-Ion Solution Studies with the Phosphinate-Containing Heterocycle DEDA-(PO).

A 7-membered triprotic heterocycle, DEDA-(PO), was synthesized, characterized, and tested for its solution properties with three trivalent lanthanides (La3+, Gd3+, and Lu3+) and three biologically relevant divalent metal ions (Ca2+, Zn2+, and Cu2+). The ligand synthesis has been reported once before; however, the characterization results were previously misinterpreted to correspond to a larger, 14-membered heterocycle, TETA-(PO)2. This manuscript serves to correct the original paper. Potentiometric titrations were carried out with each of the metal ions, and the thermodynamic stability values in terms of log ß and log KML were calculated showing a 1:1 metal-to-ligand ratio preference for the divalent metal ions and a 1:2 ratio for the lanthanides. The stability of the 1:2 complexes decreased across the lanthanide series, presumed to be a steric effect. Further resolution to the potentiometry results was given via pH-dependent NMR spectrometry (with La3+) and pH-dependent UV-vis spectroscopy (with Cu2+), and the pM values were calculated for all metal ions. The solid-state structure of the 1:1 Cu2+-DEDA-(PO) complex was further characterized by X-ray crystallography.

Dipicolinate Complexes of Gallium(III) and Lanthanum(III).

Three dipicolinic acid amine-derived compounds functionalized with a carboxylate (H3dpaa), phosphonate (H4dppa), and bisphosphonate (H7dpbpa), as well as their nonfunctionalized analogue (H2dpa), were successfully synthesized and characterized. The 1:1 lanthanum(III) complexes of H2dpa, H3dpaa, and H4dppa, the 1:2 lanthanum(III) complex of H2dpa, and the 1:1 gallium(III) complex of H3dpaa were characterized, including via X-ray crystallography for [La4(dppa)4(H2O)2] and [Ga(dpaa)(H2O)]. H2dpa, H3dpaa, and H4dppa were evaluated for their thermodynamic stability with lanthanum(III) via potentiometric and either UV-vis spectrophotometric (H3dpaa) or NMR spectrometric (H2dpa and H4dppa) titrations, which showed that the carboxylate (H3dpaa) and phosphonate (H4dppa) containing ligands enhanced the lanthanum(III) complex stability by 3-4 orders of magnitude relative to the unfunctionalized ligand (comparing log ßML and pM values) at physiological pH. In addition, potentiometric titrations with H3dpaa and gallium(III) were performed, which gave significantly (8 orders of magnitude) higher thermodynamic stability constants than with lanthanum(III). This was predicted to be a consequence of better size matching between the dipicolinate cavity and gallium(III), which was also evident in the aforementioned crystal structures. Because of a potential link between lanthanum(III) and osteoporosis, the ligands were tested for their bone-directing properties via a hydroxyapatite (HAP) binding assay, which showed that either a phosphonate or bisphosphonate moiety was necessary in order to elicit a chemical binding interaction with HAP. The oral activity of the ligands and their metal complexes was also assessed by experimentally measuring log Po/w values using the shake-flask method, and these were compared to a currently prescribed osteoporosis drug (alendronate). Because of the potential therapeutic applications of the radionuclides 67/68Ga, radiolabeling studies were performed with 67Ga and H3dpaa. Quantitative radiolabeling was achieved at pH 6.5 in 10 min at room temperature with concentrations as low as 10-5 M, and human serum stability studies were undertaken.

Evaluation of La(XT), a novel lanthanide compound, in an OVX rat model of osteoporosis.

PURPOSE: The purpose of this study was to evaluate the efficacy and toxicity of a novel lanthanum compound, La(XT), in an ovariectomized (OVX) rat model of osteoporosis. METHODS: Twenty-four ovariectomized female Sprague Dawley rats were divided into 3 groups receiving a research diet with/without treatment compounds (alendronate: 3 mg/kg; La(XT) 100 mg/kg) for three months. At the time of sacrifice, the kidney, liver, brain, lung and spleen were collected for histological examination. The trabecular bone structure of the tibiae was evaluated using micro-CT and a three-point metaphyseal mechanical test was used to evaluate bone failure load and stiffness. RESULTS: No significant differences were noted in plasma levels of calcium, phosphorus, creatinine, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) between the La(XT) treatment compared to the non-treated OVX group. Alendronate-treated animals (positive control) showed higher BV/TV, Tb.N and lower Tb.Th and Tb.Sp when compared to the non-treated OVX group. Mechanical analysis indicated that stiffness was higher in the alendronate (32.88%, p = 0.04) when compared to the non-treated OVX group. Failure load did not differ among the groups. CONCLUSIONS: No kidney or liver toxicities of La(XT) treatments were found during the three-month study. The absence of liver and kidney toxicity with drug treatment for 3 months, as well as the increased trabecular bone stiffness are encouraging for the pursuit of further studies with La(XT) for a longer duration of time.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode.

The photoelectrochemical decomposition of lignin model compounds at a BiVO4 photoanode is demonstrated with simulated sunlight and an applied bias of 2.0 V. These prototypical lignin model compounds are photoelectrochemically converted into the corresponding aryl aldehyde and phenol derivatives in a single step with conversion of up to ≈64 % over 20 h. Control experiments suggest that vanadium sites are electrocatalytically active, which precludes the need for a redox mediator in solution. This feature of the system is corroborated by a layer of V2 O5 deposited on BiVO4 serving to boost the conversion by 10 %. Our methodology capitalizes on the reactive power of sunlight to drive reactions that have only been studied previously by electrochemical or catalytic methods. The use of a BiVO4 photoanode to drive lignin model decomposition therefore provides a new platform to extract valuable aromatic chemical feedstocks using solar energy, electricity and biomass as the only inputs.

Solution-Deposited Solid-State Electrochromic Windows.

Commercially available electrochromic (EC) windows are based on solid-state devices in which WO3 and NiOx films commonly serve as the EC and counter electrode layers, respectively. These metal oxide layers are typically physically deposited under vacuum, a time- and capital-intensive process when using rigid substrates. Herein we report a facile solution deposition method for producing amorphous WO3 and NiOx layers that prove to be effective materials for a solid-state EC device. The full device containing these solution-processed layers demonstrates performance metrics that meet or exceed the benchmark set by devices containing physically deposited layers of the same compositions. The superior EC performance measured for our devices is attributed to the amorphous nature of the NiOx produced by the solution-based photodeposition method, which yields a more effective ion storage counter electrode relative to the crystalline NiOx layers that are more widely used. This versatile method yields a distinctive approach for constructing EC windows.

In vitro studies of lanthanide complexes for the treatment of osteoporosis.

Lanthanide ions, Ln(III), are of interest in the treatment of bone density disorders because they are found to accumulate preferentially in bone (in vivo), have a stimulatory effect on bone formation, and exhibit an inhibitory effect on bone degradation (in vitro), altering the homeostasis of the bone cycle. In an effort to develop an orally active lanthanide drug, a series of 3-hydroxy-4-pyridinone ligands were synthesized and eight of these ligands (H1 = 3-hydroxy-2-methyl-1-(2-hydroxyethyl)-4-pyridinone, H2 = 3-hydroxy-2-methyl-1-(3-hydroxypropyl)-4-pyridinone, H3 = 3-hydroxy-2-methyl-1-(4-hydroxybutyl)-4-pyridinone, H4 = 3-hydroxy-2-methyl-1-(2-hydroxypropyl)-4-pyridinone, H5 = 3-hydroxy-2-methyl-1-(1-hydroxy-3-methylbutan-2-yl)-4-pyridinone, H6 = 3-hydroxy-2-methyl-1-(1-hydroxybutan-2-yl)-4-pyridinone, H7 = 1-carboxymethyl-3-hydroxy-2-methyl-4-pyridinone, H8 = 1-carboxyethyl-3-hydroxy-2-methyl-4-pyridinone) were coordinated to Ln(3+) (Ln = La, Eu, Gd, Lu) forming stable tris-ligand complexes (LnL(3), L = 1(-), 2(-), 3(-), 4(-), 5(-), 6(-), 7(-) and 8(-)). The dissociation (pK(an)) and metal ligand stability constants (log ß(n)) of the 3-hydroxy-4-pyridinones with La(3+) and Gd(3+) were determined by potentiometric titrations, which demonstrated that the 3-hydroxy-4-pyridinones form stable tris-ligand complexes with the lanthanide ions. One phosphinate-EDTA derivative (H(5)XT = bis[[bis(carboxymethyl)amino]methyl]phosphinate) was also synthesized and coordinated to Ln(3+) (Ln = La, Eu, Lu), forming the potassium salt of [Ln(XT)](2-). Cytotoxicity assays were carried out in MG-63 cells; all the ligands and metal complexes tested were observed to be non-toxic to this cell line. Studies to investigate the toxicity, cellular uptake and apparent permeability (P(app)) of the lanthanide ions were conducted in Caco-2 cells where it was observed that [La(XT)](2-) had the greatest cell uptake. Binding affinities of free lanthanide ions (Ln = La, Gd and Lu), metal complexes and free 3-hydroxy-4-pyridinones with the bone mineral hydroxyapatite (HAP) are high, as well as moderate to strong for the free ligand with the bone mineral depending on the functional group.