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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

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 (MCln; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mn+) on redox reactions accessible to lanthanide ions; Ln3+ + e1− → Ln2+ (Ln = Eu, Yb, Sm; e1− = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu2+ instantaneously formed when Eu3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca2+ from CaCl2) via the Eu3+ + Cl1− → Eu2+ + ½Cl2 reaction. Conversely, molten salts with less polarizing outer-sphere M1+ cations (e.g., K1+ in KCl) stabilized Ln3+. For instance, the Eu3+/Eu2+ reduction potential was >0.5 V more positive in CaCl2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mn+ cations (high polarization power) inductively removed electron density from Lnn+ across Ln–Cl⋯Mn+ networks and stabilized electron-rich and low oxidation state Ln2+ ions. Conversely, less polarizing Mn+ cations (like K1+) left electron density on Lnn+ and stabilized electron-deficient and high-oxidation state Ln3+ ions.

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.

Molten salts have found use as solvents in numerous applications including nuclear reactors, batteries, and the extraction and purification of various metals. Unfortunately, understanding of the chemistry of molten salt solutions is limited. In this presentation we explore the use of molten salts as a testbed for understanding both outer and inner coordination sphere effects on dissolved metal ions. The electron transfer reactions available to lanthanides (Eu3+, Sm3+, and Yb3+) and actinides (U3+, U4+, and Th4+) were explored in a series of alkali and alkaline earth halide salts. We present electrochemical data that demonstrate significant shifts in the reduction potentials of these metal ions as a function of the anion and cation identities of the molten salt solvent. We hypothesize that effects on the reduction potential of these metals come from two sources: (1) the primary coordination sphere and (2) the secondary coordination sphere. The influence from the primary coordination sphere is dominated by the degree of covalency in the coordination bonds between the Lnn+ and Ann+ cations and the molten salt anions. The influence of the secondary coordination sphere is dominated by the electron-withdrawing character of the salt cations. EXAFS data and computational results that support these hypotheses are presented. Further, we provide insight into electrodeposition of the An0 metals under these conditions and highlight temperature and molten salt effects that influence these electrodepositions. Specifically, we propose that increased mobility of solid-state atoms at high temperature (> 800°C) influence the properties of electrodeposited metals.

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. ...

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 macros...

A series of cobalt bis(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt2–) ring substitutions (S2C6X42–) 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 TBA[Co(S2C6H4)2] (1) to 6.22 × 10–10 mol cm–2 for TBA[Co(S2C6Cl4)2] (3)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10–9 mol cm–2 for TBA[Co(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 ...

Catalytic MoS

Understanding the coordination chemistry between lanthanide elements and dithiophosphinate ligands is important given prospects for using dithiophosphinate ligands to separate trivalent minor actinides from lanthanides in advanced nuclear fuel cycles. Reported here is the coordination chemistry of a series of lanthanide elements (Ce, Nd, Sm, Eu, Gd, Dy, and Yb) with diphenyldithiophosphinate anions, S2PPh21−. In all cases, LnCl3 was reacted with 4 equivalents of S2PPh21−. The large lanthanides (Ce3+, Nd3+, Sm3+, Eu3+) formed eight coordinate anions of the general formula Ln(S2PPh2)41− and were characterized by 1H and 31P NMR, IR, UV–Vis, elemental analysis, and single crystal X-ray crystallography. For the slightly smaller Gd3+ and Dy3+ ions, a break in reactivity was observed. Here, analyses of reaction solutions by 1H and 31P NMR spectroscopy suggested that a complicated mixture formed from which single crystals of eight coordinate and neutral Gd(S2PPh2)3(NCMe)2 and Dy(S2PPh2)3(NCMe)2 complexes were isolated. Reactions involving even smaller Yb3+ ions provided similar results to Gd3+ and Dy3+, in that reaction solutions also contained a mixture of products. In the Yb3+ case a new heteroleptic species [Ph4P][Yb(S2PPh2)3Cl], was isolated in single crystal form. The structural data have been presented in comparison to that of [Ph4P][Sm(S2PMe2)4] and [Ph4P][Eu(S2PMe2)4], which were also structurally characterized for the first time. Overall, these results demonstrate how subtle differences in lanthanide ionic radii can drastically affect the coordination number, ligand stoichiometry, and charge on the resulting diphenyldithiophosphinate complex.

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U(NR)(2)X(4)](2-) (X = Cl(-), Br(-)) and non-coordinating NEt(4)(+) and PPh(4)(+) countercations are reported. In general, these compounds can be prepared from U(NR)(2)I(2)(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)(2)](2+) cation also reacts with Br(-) to form stable [NEt(4)](2)[U(NMe)(2)Br(4)] complexes. These materials were used as a platform to compare electronic structure and bonding in [U(NR)(2)](2+) with [UO(2)](2+). 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 [PPh(4)](2)[U(N(t)Bu)(2)Cl(4)] and [PPh(4)](2)[UO(2)Cl(4)]. 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.

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 TBA[Co(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 (pH4), 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 alkyl and aryl substituted dithiophosphinate salts that contain non-coordinating PPh(4)(+) counter cations are reported. In general, these compounds can be prepared via a multi-step procedure that starts with reacting secondary phosphines, i.e. HPR(2), with two equivalents of elemental S. The synthetic transformation proceeds by oxidation of the phosphine followed by insertion of S into the H-P bond. This approach was used to synthesize a series of dithiophosphinic acids that were fully characterized, namely HS(2)P(p-CF(3)C(6)H(4))(2), HS(2)P(m-CF(3)C(6)H(4))(2), HS(2)P(o-MeC(6)H(4))(2) and HS(2)P(o-MeOC(6)H(4))(2). Although the insertion step was found to be much slower than the oxidation reaction, the formation of (NH(4))S(2)PR(2) from HPSR(2) occurred rapidly upon addition of NH(4)OH. Subsequent cation exchange reactions proceeded readily with PPh(4)Cl in water, under air and at ambient conditions to provide analytically pure samples of [PPh(4)][S(2)PR(2)] (R = p-CF(3)C(6)H(4), m-CF(3)C(6)H(4), o-CF(3)C(6)H(4), o-MeC(6)H(4), o-MeOC(6)H(4), Ph, and Me, 1b-7b, respectively), which were characterized by elemental analysis, multinuclear NMR, and IR spectroscopy. In addition, S(2)PPh(2)(-) and dithiophosphinates with ortho-substituted aryl groups (3b-6b) were characterized by X-ray crystallography. As opposed to the acids, which have short P=S double bonds and long P-SH single bonds, the metric parameters for the S atoms in S(2)PR(2)(-) are equivalent. In addition, the presence of large non-coordinating PPh(4)(+) cations guard against intermolecular P-S···X interactions and ensure that the P-S bond is isolated. These S(2)PR(2)(-) anions, which can be prepared in large quantities and isolated in crystalline form, are attractive for spectroscopic and theoretical studies because the P-S interaction can be probed independently in the absence of intermolecular interactions.

Herein, we have evaluated relative changes in M-S electronic structure and orbital mixing in Group 6 MS4(2-) dianions using solid- and solution-phase S K-edge X-ray absorption spectroscopy (XAS; M = Mo, W), as well as density functional theory (DFT; M = Cr, Mo, W) and time-dependent density functional theory (TDDFT) calculations. To facilitate comparison with solution measurements (conducted in acetonitrile), theoretical models included gas-phase calculations as well as those that incorporated an acetonitrile dielectric, the latter of which provided better agreement with experiment. Two pre-edge features arising from S 1s → e* and t electron excitations were observed in the S K-edge XAS spectra and were reasonably assigned as (1)A1 → (1)T2 transitions. For MoS4(2-), both solution-phase pre-edge peak intensities were consistent with results from the solid-state spectra. For WS4(2-), solution- and solid-state pre-edge peak intensities for transitions involving e* were equivalent, while transitions involving the t orbitals were less intense in solution. Experimental and computational results have been presented in comparison to recent analyses of MO4(2-) dianions, which allowed M-S and M-O orbital mixing to be evaluated as the principle quantum number (n) for the metal valence d orbitals increased (3d, 4d, 5d). Overall, the M-E (E = O, S) analyses revealed distinct trends in orbital mixing. For example, as the Group 6 triad was descended, e* (π*) orbital mixing remained constant in the M-S bonds, but increased appreciably for M-O interactions. For the t orbitals (σ* + π*), mixing decreased slightly for M-S bonding and increased only slightly for the M-O interactions. These results suggested that the metal and ligand valence orbital energies and radial extensions delicately influenced the orbital compositions for isoelectronic ME4(2-) (E = O, S) dianions.