Comparison of Material Activity and Selectivity in the Electrocatalytic Oxidation of Dibenzothiophene

There is an increasing demand for efficient methods to remove sulfur from oil products, such as oxidative desulfurization. In this work, a set of five materials (gold, glassy carbon, nickel, palladium and platinum) were evaluated as electrochemical catalysts for the oxidation of dibenzothiophene (DBT). Bulk electrolysis performed without water present, produced DBT dimer, while the addition of 2 M water, produced dibenzothiophene sulfoxide (DBTO), both more polar than DBT. LC-MS and NMR were used to characterize the oxidation products. Faradaic efficiencies ranged from 18.4–56.5% for DBT consumption without water present, and there was a correlation between higher rate constants, lower activation energies and more efficient DBT oxidation. With water present, selectivity for DBTO formation was highest using gold, with a Faradaic efficiency of 87.9%. Group ten metals demonstrated low Faradaic efficiencies due to competitive water oxidation. Though there were differences in the observed selectivity for DBT oxidation, all catalysts reduced the concentration of DBT in solution by similar amounts. Our findings indicate that the overall percent conversion does not give a complete picture of catalytic activity. Of the materials tested, gold was the most selective for oxidation to DBTO, with the presence of water improving the overall reaction activity.

On the Mechanism of Electrochemical Generation and Decomposition of Phthalimide N-oxyl (PINO)

Phthalimide <i>N</i>-oxyl (PINO) is a potent hydrogen atom transfer (HAT) catalyst that can be generated electrochemically from <i>N</i>-hydroxyphthalimide (NHPI). However, catalyst decomposition has limited its application. This paper details mechanistic studies of the generation and decomposition of PINO under electrochemical conditions. Voltammetric data, observations from bulk electrolysis, and <a>computational</a> studies suggest two primary aspects. First, base-promoted formation of PINO from NHPI occurs via multiple-site concerted proton-electron transfer (MS-CPET). Second, PINO decomposition occurs by at least two second-order paths, one of which is greatly enhanced by base. Optimal catalytic efficiency in PINO-catalyzed oxidations occurs in the presence of bases whose corresponding conjugate acids have <a>p<i>K</i>_a</a>s in the range of 12-15, which strike a balance between promoting PINO formation and minimizing its decay.

On the Mechanism of Electrochemical Generation and Decomposition of Phthalimide N-oxyl (PINO)

Phthalimide <i>N</i>-oxyl (PINO) is a potent hydrogen atom transfer (HAT) catalyst that can be generated electrochemically from <i>N</i>-hydroxyphthalimide (NHPI). However, catalyst decomposition has limited its application. This paper details mechanistic studies of the generation and decomposition of PINO under electrochemical conditions. Voltammetric data, observations from bulk electrolysis, and <a>computational</a> studies suggest two primary aspects. First, base-promoted formation of PINO from NHPI occurs via multiple-site concerted proton-electron transfer (MS-CPET). Second, PINO decomposition occurs by at least two second-order paths, one of which is greatly enhanced by base. Optimal catalytic efficiency in PINO-catalyzed oxidations occurs in the presence of bases whose corresponding conjugate acids have <a>p<i>K</i>_a</a>s in the range of 12-15, which strike a balance between promoting PINO formation and minimizing its decay.

Reproducible Electrodeposition of Hydrogen Molybdenum Bronze Films and Electrochemical Reduction of Carbon Dioxide

Electrochemical synthesis of hydrogen bronze films including molybdenum, tungsten and vanadium are useful electrocatalytic films. This paper describes reproducible hydrogen molybdenum bronze film formation on indium tin oxide and carbon paper substrates by electrodeposition. Film formation is a kinetic process dependent on concentration, time and potential. Bulk electrolysis over time determined the dependence of film thickness on time of deposition. Once the films were prepared, the films were characterized by thickness, conductivity, XPS and X-Ray Diffraction. Cyclic voltammetry in dilute sulfuric acid confirmed that these films are not electrochromic. Hydrogen bronze films on conductive carbon paper were also prepared. Carbon dioxide bubbled into 0.5 M NaHCO3 using a hydrogen bronze film as the working electrode resulted in formate quantified by ion chromatography. Cyclic voltammetry and Tafel plots using the as deposited films in 0.5 M NaHCO3 saturated with CO2 showed catalytic activity toward reduction of carbon dioxide. A Farradaic efficiency of 8% was obtained with an applied potential of -0.4 V.

Evaluating aqueous flow battery electrolytes: a coordinated approach

Here, we outline some basic pitfalls in the electrochemical investigation of aqueous metal complexes, advocate for the use of bulk electrolysis in redox flow cells for electrolyte analysis, and demonstrate methods of operation and performance of a lab scale redox flow battery.

Iron complex of a quadruply fused porphyrin: Synthesis, structure and redox properties

An iron(II) complex of a quadruply ring-fused porphyrin (QFP), Fe-1, in which four mesityl groups were introduced at the periphery to improve the solubility in organic solvents, has been newly synthesized and characterized. In the pyridine solution, two pyridine molecules bind to the low-spin Fe[Formula: see text] center of Fe-1 as axial ligands to make the complex stable even under air. Characterization of Fe-1 was performed using 1H NMR spectroscopy, MALDI-TOF-MS spectrometry and single-crystal X-ray diffraction analysis. The 1H NMR signals of Fe-1 were observed in the diamagnetic region, reflecting the low-spin state of the FeII center. In the differential pulse voltammogram of Fe-1, three oxidation waves and four reduction waves were observed in pyridine; the first oxidation wave at -0.08 V vs. Fc/Fc[Formula: see text] can be ascribed to the oxidation process of the FeII center, FeII/FeIII, and other six waves can be assigned to the redox processes of the QFP ligand. Furthermore, the ESR measurement of 1e[Formula: see text]-reduced Fe-1 upon controlled-potential bulk electrolysis in pyridine exhibited a signal at [Formula: see text] = 2.003 with a well-resolved 45-line hyperfine splitting at room temperature, due to the coupling with four nitrogen nuclei and twelve hydrogen ones of the QFP ligand. This indicates that the ligand radical anion of Fe-1 is stabilized by delocalization of the spin density owing to the peripheral ring fusion and resultant expansion of the [Formula: see text]-conjugation.

Electrochemical reduction of artemisinin: Chromatographic identification of bulk electrolysis products

Artemisinin is a naturally occurring sesquiterpene lactone with an endo-peroxide bond. This drug is used for treatment of many diseases including malaria. The reduction of this molecule on an electrode surface was carried out by cyclic voltammetry as well as amperometry. Cyclic voltammetry of artemisinin generated one prominent peak wave at -1.0 V and another, smaller one at -0.3 V vs Ag/AgCl reference electrode. The bulk electrolysis of artemisinin on a carbon electrode generated two other irreversible peak waves at around -0.7 and -0.1 V. The concentration of the products was dependent on the time of electrolysis. LC-MS was used to determine the bulk electrolysis products of artemisinin. Initially dihydroartemisinin was generated as the main reduction product. Other reduction products were formed after further reduction of dyhidroartemisinin.