A series of cobalt bis(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt2–) ring substitutions (S2C6X4 2–) were prepared and adsorbed on inexpensive electrodes ...composed of (a) reduced graphene oxide (RGO) electrodeposited on fluorine-doped tin oxide (FTO) and (b) highly ordered pyrolytic graphite (HOPG). The catalyst-adsorbed electrodes are characterized by X-ray photoelectron spectroscopy. Catalyst loading across the ligand series improved notably with increasing halide substitution from 2.7 × 10–11 mol cm–2 for TBACo(S2C6H4)2 (1) to 6.22 × 10–10 mol cm–2 for TBACo(S2C6Cl4)2 (3) and increasing ring size of the benzenedithiolate ligand up to 3.10 × 10–9 mol cm–2 for TBACo(S2C10H6)2 (6). Electrocatalytic analysis of the complexes immobilized on HOPG elicits a reductive current response indicative of dihydrogen generation in the presence of mildly acidic aqueous solutions (pH 2–4) of trifluoroacetic acid, with overpotentials of around 0.5 V versus SHE (measured vs platinum). Rate constant (k obs) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s–1 with the maximum k obs for TBACo(S2C6H2Cl2)2 (2) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M H2SO4 at −0.5 V versus SHE show impressive initial rate constants of over 500 s–1 under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 106. Electrolysis studies reveal that halide substitution is a central factor in improving the turnover stability, whereas the ring size is less of a factor in optimizing the long-term stability of the heterogeneous catalyst manifolds. Catalyst deactivation is likely caused by catalyst desorption from the electrode surfaces.
Reduced graphene oxide (RGO) films have been prepared by immersion of graphene oxide (GO) films at room temperature in nonaqueous solutions containing simple, outer-sphere metallocene reductants. ...Specifically, solutions of cobaltocene, cobaltocene and trifluoroacetic acid (TFA), and decamethylcobaltocene each showed activity for the rapid reduction of GO films cast on a wide variety of substrates. Each reactant increased the conductivity of the films by several orders of magnitude, with RGO films prepared with either decamethylcobaltocene or cobaltocene and TFA possessing the highest conductivities (∼104 S m–1). X-ray photoelectron spectroscopy suggested that while all three reagents lowered the content of carbon–oxygen functionalities, solutions of cobaltocene and TFA were the most effective at reducing the material to sp2 carbon. Separately, Raman spectra and atomic force micrographs indicated that RGO films prepared with decamethylcobaltocene consisted of the largest graphitic domains and lowest macroscopic roughness. Cumulatively, the data suggest that the outer-sphere reductants can affect the conversion to RGO but the reactivity and mechanism depend on the standard potential of the reductant and the availability of protons. This work both demonstrates a new way to prepare high-quality RGO films on a wide range of substrate materials without annealing and motivates future work to elucidate the chemistry of RGO synthesis through the tunability of outer-sphere reductants such as metallocenes.
p-Type gallium phosphide (GaP) electrodes have been sensitized by triarylmethane dyes physisorbed from aqueous solutions. This work is the first to show light-stimulated hole injection from an ...adsorbed molecular chromophore on native GaP surfaces. Freshly etched p-GaP(100) and p-GaP(111)A electrodes were loaded with physisorbed dye by brief soaking in solutions of Fast Green. X-ray photoelectron spectroscopy, corroborated by Auger electron spectroscopy, indicated that such treatments yield undetectable surface coverages. However, steady-state photoelectrochemical responses consistently showed that sub-bandgap photoresponses were commensurate with light absorption by the adsorbed dye. The photoresponse characteristics were clearly insensitive to the identity and amount of intended redox mediators dissolved in solution at low light intensities. Instead, the data suggest electrochemically active surface states related to the cathodic degradation of GaP can accept electrons from photoexcited physisorbed dye. Measurements at high illumination intensities showed sensitivity toward redox mediators in solution, indicating that the conventional mode of dye regeneration by redox species in solution is possible with p-GaP. Separate measurements with covalently modified p-GaP(111)A photoelectrodes further suggested that deliberate modification to minimize/eliminate surface states is also possible. Collectively, this work indicates that although some types of dye readily adsorb onto native GaP interfaces, the low dye loadings and the susceptibility of the interface to chemical attack during the sensitization process argue against using bare p-GaP photocathodes with physisorbed triarylmethane dyes. Instead, these studies suggest that dye-sensitized photocathodes based on p-GaP require deliberate surface chemical modification methods to overcome the low loading and inhibit unwanted charge transfer between the surface and the photoexcited dye.
Heterogeneous dihydrogen production manifolds comprised of bulk graphite, pencil graphite, graphite powder in Nafion films, graphene, and glassy carbon electrodes with adsorbed proton reduction ...catalyst TBACo(S2C6Cl2H2)2 have been prepared and tested for their efficiency to generate dihydrogen in acidic aqueous media. The catalyst adsorbed on these inexpensive graphitic surfaces consistently displays similar electrocatalytic profiles compared to the same catalyst on highly ordered pyrolytic graphite (HOPG) supports, including high activity in moderately acidic aqueous solutions (pH < 4), moderate overpotentials (0.42 V vs platinum), and some of the highest reported initial turnover frequencies under electrolysis conditions (96 s–1). The exceptions are glassy carbon and single-layer graphene surfaces, which only weakly adsorb the catalyst, with no sustained catalytic current upon acid addition. In particular, the improved stability and good activity observed for the catalyst adsorbed on graphite powder embedded in a Nafion film shows that this is a promising H2 production system that can be assembled at minimal cost and effort.
Synthetic routes to salts containing uranium bis-imido tetrahalide anions U(NR)2X42– (X = Cl–, Br–) and non-coordinating NEt4 + and PPh4 + countercations are reported. In general, these compounds can ...be prepared from U(NR)2I2(THF) x (x = 2 and R = t Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl–, the U(NMe)22+ cation also reacts with Br– to form stable NEt42U(NMe)2Br4 complexes. These materials were used as a platform to compare electronic structure and bonding in U(NR)22+ with UO22+. Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U–Cl bonding interactions in PPh42U(N t Bu)2Cl4 and PPh42UO2Cl4. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7–10% per U–Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.
A series of four lanthanide thenoyltrifluoroacetone (TTA) complexes consisting of two f0 (La3+ and Ce4+) and two f1 (Ce3+) complexes was examined using steady-state and time-resolved spectroscopic ...techniques. The wide range of spectroscopic techniques presented herein have enabled us to discern the nature of the excited states (charge transfer, CT vs ligand localized, LL) as well as construct a Jablonski diagram for detailing the excited state reactivity within the series of molecules. The wavelength and excitation power dependence for these series of complexes are the first direct verification for the presence of simultaneous competing, noninteracting CT and LL excited states. Additionally, a computational framework is described that can be used to support spectroscopic assignments as a guide for future studies. Finally, the relationship between the obtained photophysics and possible photochemical separation mechanisms is described.
Photochemical separation of plutonium from uranium DiMucci, Ida M; Root, Harrison D; Jones, Zachary R ...
Chemical communications (Cambridge, England),
09/2022, Letnik:
58, Številka:
78
Journal Article
Recenzirano
Odprti dostop
Plutonium-based technologies would benefit if chemical hazards for purifying plutonium were reduced. One critical processing step where improvements could be impactful is the adjustment of plutonium ...oxidation-states during separations. This transformation often requires addition of redox agents. Unfortunately, many of the redox agents used previously cannot be used today because their properties are deemed incompatible with modern day processing facilities and waste stream safety requirements. We demonstrated herein that photochemistry can be used as an alternative to those chemical agents. We observed that (1) Pu
4+
Pu
3+
and UO
2
2+
→ U
4+
photoreduction proceeded in HCl
(aq)
and HNO
3(aq)
and (2) photogenerated Pu
3+
(aq)
and U
4+
(aq)
could be separated using anion exchange chromatography (high yield, >90%; good separation factor, 322).
Photochemical reduction and separation of plutonium from uranium in acidic solutions is described as a potential alternative to conventional separations that employ harsh chemical redox agents.
Catalytic MoSx thin films have been directly photoelectrodeposited on GaInP2 photocathodes for stable photoelectrochemical hydrogen generation. Specifically, the MoSx deposition conditions were ...controlled to obtain 8-10 nm films directly on p-GaInP2 substrates without ancillary protective layers. The films were nominally composed of MoS2, with additional MoOxSy and MoO3 species detected and showed no long-range crystalline order. The as-deposited material showed excellent catalytic activity toward the hydrogen evolution reaction relative to bare p-GaInP2. Notably, no appreciable photocurrent reduction was incurred by the addition of the photoelectrodeposited MoSx catalyst to the GaInP2 photocathode under light-limited operating conditions, highlighting the advantageous optical properties of the film. The MoSx catalyst also imparted enhanced durability toward photoelectrochemical hydrogen evolution in acidic conditions, maintaining nearly 85% of the initial photocurrent after 50 h of electrolysis. In total, this work demonstrates a simple method for producing dual-function catalyst/protective layers directly on high-performance, planar III-V photoelectrodes for photoelectrochemical energy conversion.
Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this ...problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl
n
; M = K, Na,
n
= 1; M = Ca,
n
= 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (M
n
+
) on redox reactions accessible to lanthanide ions; Ln
3+
+ e
1−
→ Ln
2+
(Ln = Eu, Yb, Sm; e
1−
= electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu
2+
instantaneously formed when Eu
3+
dissolved in molten chloride salts that had strongly polarizing cations (like Ca
2+
from CaCl
2
)
via
the Eu
3+
+ Cl
1−
→ Eu
2+
+ ½Cl
2
reaction. Conversely, molten salts with less polarizing outer-sphere M
1+
cations (
e.g.
, K
1+
in KCl) stabilized Ln
3+
. For instance, the Eu
3+
/Eu
2+
reduction potential was >0.5 V more positive in CaCl
2
than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard M
n
+
cations (high polarization power) inductively removed electron density from Ln
n
+
across Ln-Cl M
n
+
networks and stabilized electron-rich and low oxidation state Ln
2+
ions. Conversely, less polarizing M
n
+
cations (like K
1+
) left electron density on Ln
n
+
and stabilized electron-deficient and high-oxidation state Ln
3+
ions.
Molten salt matrices were used to evaluate outer-coordination sphere effects on lanthanide redox chemistry. Results were rationalized by correlating the polarization power of the outer-sphere cation with shifts in the Ln
3+
/Ln
2+
reduction potentials.
Catalytic MoS x thin films have been directly photoelectrodeposited on GaInP2 photocathodes for stable photoelectrochemical hydrogen generation. Specifically, the MoS x deposition conditions were ...controlled to obtain 8–10 nm films directly on p-GaInP2 substrates without ancillary protective layers. The films were nominally composed of MoS2, with additional MoO x S y and MoO3 species detected and showed no long-range crystalline order. The as-deposited material showed excellent catalytic activity toward the hydrogen evolution reaction relative to bare p-GaInP2. Notably, no appreciable photocurrent reduction was incurred by the addition of the photoelectrodeposited MoS x catalyst to the GaInP2 photocathode under light-limited operating conditions, highlighting the advantageous optical properties of the film. The MoS x catalyst also imparted enhanced durability toward photoelectrochemical hydrogen evolution in acidic conditions, maintaining nearly 85% of the initial photocurrent after 50 h of electrolysis. In total, this work demonstrates a simple method for producing dual-function catalyst/protective layers directly on high-performance, planar III–V photoelectrodes for photoelectrochemical energy conversion.
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