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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Hydrogenation of Covalent Organic Framework Induces Conjugated π Bonds and Electronic Topological Transition to Enhance Hydrogen Evolution Catalysis

Recently, many topological materials have been discovered as promising electrocatalysts in chemical conversion processes and energy storage. However, it remains unclear how the topological electronic states specifically modulate the catalytic reaction. Here, the two-dimensional metal phthalocyanine-based covalent organic framework (MP c -COF) is studied by ab initio thermodynamic calculations to clearly reveal the promotional effect on the electrochemical hydrogen evolution reaction (HER) induced by topological gapless bands (TGBs). We find that the prehydrogenated (and fluorinated) H 4 CdPc-COF(F) shows the best HER performance, with 0.016 V (near zero) overpotential. By tracking changes to the electronic structure and free energy as the prehydrogenation and HER processes occur, we are able to separately attribute the high HER efficiency in part due to the increase of the electron bath by donating electrons to the conjugated π bonds and also to the existence of TGBs. Specifically, the significant catalytic promotion by TGBs is proven to decrease the free energy by 0.218 eV to near zero. When the TGBs are destroyed, e.g., by replacing N with P and opening a band gap, the HER efficiency is reduced. As a result, this study opens avenues for deterministically harnessing topological band features to improve electrocatalysis.

25 ENERGY STORAGE↗
DOE OSTI2018

Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru(II) complexes that use a planar “bda” ligand (bda is 2,2'-bipyridine-6,6'-dicarboxylate). These Ru(II)(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru(II)(bda) complexes remains challenging. We discuss here a new “bottom-up” synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm 2 at pH 7 in 0.1 M [PO 4 H 2 ]-/[PO 4 H] 2- solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Surface functionalization of metal-organic polyhedron for homogeneous cyclopropanation catalysis

A super-paddlewheel (comprised of two paddlewheels) metal-organic polyhedron (MOP) containing surface hydroxyl groups was synthesized and characterized. Condensation reactions with linear alkyl anhydrides lead to new MOPs with enhanced solubility. As a result, the surface-modified MOP 4 was demonstrated as a homogeneous Lewis-acid catalyst.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tandem Lewis/Brønsted homogeneous acid catalysis: conversion of glucose to 5-hydoxymethylfurfural in an aqueous chromium( iii ) chloride and hydrochloric acid solution

Here, a kinetic model for the tandem conversion of glucose to 5-hydroxymethylfurfural (HMF) through fructose in aqueous CrCl 3 –HCl solution was developed by analyzing experimental data. We show that the coupling of Lewis and Brønsted acids in a single pot overcomes equilibrium limitations of the glucose–fructose isomerization leading to high glucose conversions and identify conditions that maximize HMF yield. Adjusting the HCl/CrCl 3 concentration has a more pronounced effect on HMF yield at constant glucose conversion than that of temperature or CrCl 3 concentration. This is attributed to the interactions between HCl and CrCl 3 speciation in solution that leads to HMF yield being maximized at moderate HCl concentrations for each CrCl 3 concentration. This volcano-like behavior is accompanied with a change in the rate-limiting step from fructose dehydration to glucose isomerization as the concentration of the Brønsted acid increases. The maximum HMF yield in a single aqueous phase is only modest and appears independent of catalysts’ concentrations as long as they are appropriately balanced. However, it can be further maximized in a biphasic system. Our findings are consistent with recent studies in other tandem reactions catalyzed by different catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tandem catalysis for the production of alkyl lactates from ketohexoses at moderate temperatures

Significance Retro-aldol reactions of biomass-derived monosaccharides enable the production of a number of industrially relevant, renewable platform chemicals (e.g., C 2 –C 4 α-hydroxy carboxylic acids, acrylic acid, and esters thereof; and ethylene and propylene glycol). Here, we show a set of retro-aldol catalysts that can perform these reactions at significantly lower temperatures (∼100 °C) than previously reported, and demonstrate that these catalysts enable the key retro-aldol step when used in combination with a 1,2-hydride shift catalyst to create a reaction cascade that converts ketohexoses to lactic acid. The use of moderate temperatures enables numerous desirable features such as lower pressure and significantly less catalyst deactivation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗