Electrochemical reduction of halogenated organic compounds is gaining increasing attention as a strategy for the remediation of environmental pollutants. We begin this review by discussing key components (cells, electrodes, solvents, and electrolytes) in the design of a procedure for degrading a targeted pollutant, and we describe and contrast some experimental techniques used to explore and characterize the electrochemical behavior of that pollutant. Then, we describe how to probe various mechanistic features of the pertinent electrochemistry (including stepwise versus concerted carbon-halogen bond cleavage, identification of reaction intermediates, and elucidation of mechanisms). Knowing this information is vital to the successful development of a remediation procedure. Next, we outline techniques, instrumentation, and cell designs involved in scaling up a benchtop experiment to an industrial-scale system. Finally, the last and major part of this review is directed toward surveying electrochemical studies of various categories of halogenated pollutants (chlorofluorocarbons; disinfection byproducts; pesticides, fungicides, and bactericides; and flame retardants) and looking forward to future developments.

Environmental impacts of continued CO2 production have led to an increased need for new methods of CO2 removal and energy development. Nanomaterials are of special interest for these applications, because of their unique chemical and physical properties that allow for highly active surfaces. Here, we successfully synthesize AgPd nanodendrite-modified Au nanoprisms in various shapes (nanoprisms, hexagonal nanoplates, and octahedral nanoparticles) by selective metal deposition. This strategy involves coupling galvanic replacement between Ag layers in Au@Ag core–shell nanoprisms and H2PdCl4 with a coreduction process of silver and palladium ions. Synthesis of AgPd nanodendrite-tipped (4.14–11.47 wt % Pd) and -edged (25.25–31.01 wt % Pd) Au nanoparticles can be controlled simply by tuning the concentration of H2PdCl4. More importantly, these multicomponent AgPd nanodendrite-modified Au nanoparticles show exceptional electrocatalytic performance for CO2 reduction. AgPd nanodendrite-edged Au nanoprisms show more favorable potentials (−0.18 V vs RHE) than previously reported nanocatalysts for the reduction of CO2 to formate, and exhibit higher faradaic efficiencies (49%) than Au, Au@Ag, and AgPd nanodendrite-tipped Au nanoprisms in aqueous electrolytes. Moreover, AgPd nanodendrite-modified Au nanoprisms show much higher selectivity and faradaic efficiency for CO2 reduction to CO (85–87%) than Au and Au@Ag nanoprisms (43–64%) in organic electrolytes. The high performance of these particles for CO2 reduction is attributed to the unique structure of AgPd nanodendrite-modified Au nanoprisms and the synergistic effect of Ag having an affinity for CO2, efficient binding of hydrogen at Pd, and Au as a stable, conductive support. In addition, AgPd nanodendrite-edged Au nanoprisms show highly stable catalytic activity during long-term electrolyses (up to 12 h) and repetitive use. These exciting results indicate that AgPd nanodendrite-modified Au nanoparticles are promising for application in CO2 conversion into useful fuels.

Humic acid (HA) is thought to promote NO conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.

New well‐defined FeII complexes bearing bi‐ and tridentate α‐aminopyridine ligands were synthesized, and their catalytic activity in the oxidation of hydrocarbons and alcohols utilizing peroxide oxidants was investigated. The tridendate bis(picolyl)amine ligand 6 and its benzylated analogue 7 were converted into complexes [FeII(6)2]OTf2 (96 %, X‐ray; OTf=CF3SO3−) and [FeII(7)2]OTf2 (90 %). The bidentate aminopyridine ligand 8 was converted into [FeII(8)2(OTf)2] (93 %, X‐ray). The new complexes are catalytically active in the oxidation of secondary alcohols and benzylic methylene groups to the corresponding ketones, of toluene to benzaldehyde, and of cyclohexene to cyclohexene oxide (3 mol % catalyst, tBuOOH (4 equiv), RT, 2–6 h, 28 to 85 % yield of isolated product). The catalytic oxidation of cyclohexane with ROOH (R=H, tBu) to an alcohol/ketone mixture with low ratio revealed that these oxidations follow largely a radical mechanism, except when [FeII(6)2]OTf2 was employed and H2O2 was added slowly. Together with known bi‐ and tetradendate iron complexes, a comparative study showed slight reactivity differences for the newly prepared complexes, with the highest observed for [FeII(6)2]OTf2 and [FeII(7)2]OTf2. The reaction of the new complexes with peroxides was followed over time by UV/Visible spectroscopy; this revealed a fast reaction between the two reactants within minutes. Ligand‐decomposition pathways were investigated, and revealed that the NCH2 units of the complexes are rapidly oxidized to the corresponding amides NCO. The iron complex [FeII(6)2]OTf2 showed no decrease in catalytic activity and a moderate decrease in selectivity when first subjected to oxidative conditions similar to those employed in catalysis. Thus, oxidative ligand deterioration had a marginal effect on the catalytic activity of the iron complex [FeII(6)2]OTf2.

Catalytic reduction of halogenated organic compounds by electrogenerated nickel(I) complexes first appeared in the literature as a series of publications [1][2][3][4][5] from the laboratory of Derek Pletcher. In this early work, a family of nickel(II) procatalysts (or catalyst precursors) was employed, which included the compound [[2,2′-[1,2-ethanediylbis-(nitrilomethylidyne)]bis [phenolato]]-N,N′,O,O′]nickel(II), hereafter called nickel(II) salen (1). At a variety of cathodes (mercury, glassy carbon, platinum, and gold) and in numerous non-aqueous solventelectrolyte media [for example, dimethylformamide containing tetran-butylammonium tetrafluoroborate (DMF-TBABF 4 ) or acetonitrile containing tetramethylammonium perchlorate (CH 3 CN-TMAP)], chocolate-brown nickel(II) salen (1) undergoes a reversible, metalcentered, one-electron reduction to green nickel(I) salen (2). On the basis of density functional theory, it was established later 6 that reduction of nickel(II) salen can also produce a ligand-reduced form (3) of the parent complex in which a single electron is added to the carbon atom of one imino (C=N) bond of the ligand:Furthermore, the energy of 3 was calculated to be approximately only 2-3 kcal mol -1 higher than that of 2, meaning that both reduced states of 1 are accessible electrochemically-which becomes vitally important in later discussion.Shown in Fig. 1 is a cyclic voltammogram, recorded at 100 mV s -1 on a glassy carbon electrode in dimethylformamide containing tetramethylammonium tetrafluoroborate (TMABF 4 ) as the supporting electrolyte, which reveals the reversible one-electron reduction of nickel(II) salen. For the experimental conditions employed, the cathodic and anodic peak potentials (E pc and E pa ) are -0.95 and -0.84 V, respectively, and the cathodic and anodic peak currrents (I pc and I pa ) are identical. What is not seen in Fig. 1 (and what must be avoided) is that, at more negative potentials, another prominent cathodic peak is observed, due to the fact that the salen ligand itself can undergo further and irreversible degradation-which destroys the desired catalytic ability of 2. Our First Use of Electrogenerated Nickel(I) SalenOur first published effort to employ nickel(I) salen (2), electrogenerated at a reticulated vitreous carbon cathode in dimethylformamide-tetraethylammonium perchlorate (DMF-TEAP), was as a catalyst for the reductive intramolecular cyclizations of two acetylenic halides-namely, 6-bromo-1-phenyl-1-hexyne (4) and 6-iodo-1-phenyl-1-hexyne (5) Our goals were: (a) to overcome problems associated with direct reduction of these two compounds at mercury pool or reticulated vitreous carbon electrodes; and (b) to maximize the yield of the desired product (benzylidenecyclopentane, 6). Earlier, when these two acetylenic halides were reduced directly at a mercury pool cathode, 8 more than seven different products were obtained, and 6 was obtained in a yield below 25%. Then, in a subsequent investigation of the direct reduction of 6-iodo-1-phenyl-1-hexyne at a reticulated...