A.J. Kropf

A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability

A non-precious metal catalyst for oxygen reduction in acid media, enriched in graphene sheets/bubbles during a high-temperature synthesis step, has been developed from an Fe precursor and in situ polymerized polyaniline, supported on multi-walled carbon nanotubes. The catalyst showed no performance loss for 500 hours in a hydrogen/air fuel cell. The improved durability is correlated with the graphene formation, apparently enhanced in the presence of carbon nanotubes.

Performance Durability of Polyaniline-derived Non-precious Cathode Catalysts

This research has focused on performance durability of the newly-developed polyaniline (PANI)-derived non-precious metal cathode catalysts. These catalysts show high oxygen-reduction activity in electrochemical and fuel cell testing, reflected by the onset and half-wave (E1/2) potentials of oxygen reduction in RDE testing of 0.90 V and 0.77 V, respectively. Best-performing catalysts also exhibit insignificant H2O2 yield of less than 1%. Catalyst performance in fuel cell testing strongly depends on the choice of nitrogen precursors, transition metals used, and carbon supports. As expected, catalyst stability is affected by the operating voltage of the fuel cell, with more stable performance observed at low operating voltage and open cell voltage, than at intermediate voltages. Physical and electrochemical characterization of the catalysts, also in the presence of hydrogen peroxide, has been carried out to provide insight into the origin of possible degradation mechanisms.

Direct Observation of Reduction of Cu(II) to Cu(I) by Terminal Alkynes

X-ray absorption spectroscopy and in situ electron paramagnetic resonance evidence were provided for the reduction of Cu(II) to Cu(I) species by alkynes in the presence of tetramethylethylenediamine (TMEDA), in which TMEDA plays dual roles as both ligand and base. The structures of the starting Cu(II) species and the obtained Cu(I) species were determined as (TMEDA)CuCl2 and [(TMEDA)CuCl]2 dimer, respectively.

Labile Cu(I) catalyst/spectator Cu(II) species in copper-catalyzed C-C coupling reaction: operando IR, in situ XANES/EXAFS evidence and kinetic investigations.

Insights toward the Cu-catalyzed C-C coupling reaction were investigated through operando IR and in situ X-ray absorption near-edge structure/extended X-ray absorption fine structure. It was found that the Cu(I) complex formed from the reaction of CuI with β-diketone nucleophile was liable under the cross-coupling conditions, which is usually considered as active catalytic species. This labile Cu(I) complex could rapidly disproportionate to the spectator Cu(II) and Cu(0) species under the reaction conditions, which was an off-cycle process. In this copper-catalyzed C-C coupling reaction, β-diketone might act both as the substrate and the ligand.

Crystal chemistry of uranium (V) and plutonium (IV) in a titanate ceramic for disposition of surplus fissile material

Abstract We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 A and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.

Reduction of plutonium(VI) in brine under subsurface conditions

The redox stability of PuO22+ was investigated in brine under subsurface conditions. In simulated brines, when no reducing agent was present, 0.1 mM concentrations of plutonium(VI) were stable as regards to reduction for over two years, which was the duration of the experiments performed. In these systems, the plutonyl existed as a carbonate or hydroxy-chloride species. The introduction of reducing agents (e.g. steel coupons, and aqueous Fe2+) typically present in a subsurface repository, however, led to the destabilization of the plutonium(VI) complexes and the subsequent reduction to Pu(IV) under most conditions investigated. X-ray Absorption Near-Edge Spectroscopy (XANES) confirmed that the final oxidation state in these systems was Pu(IV). This reduction lowered the overall steady state concentration of plutonium in the brine by 3−4 orders of magnitude. These results show the importance of considering repository constituents in evaluating subsurface actinide solubility/mobility and provide further evidence of the effectiveness of reduced iron species in the reduction and immobilization of higher-valent plutonium species.

Oxidation/reduction of multivalent actinides in the subsurface

The important role of oxidation state and redox chemistry on the mobility of multivalent actinides in the subsurface is beyond dispute. These actinides, primarily uranium, neptunium and plutonium, are likely to exist in more than one oxidation state that in many cases have very different mobility in the subsurface. An understanding of the key factors that influence the oxidation state distribution in the subsurface is essential to predicting actinide migration. Our research here at Argonne has focused on two key factors that help define actinide speciation in the subsurface. These are microbiological interactions and interactions with anthropogenic organic complexants.

Plutonium and uranium disposition in a sodalite/glass composite waste form via XAFS

Sodalite/glass composite waste forms are being developed at Argonne National Laboratory for disposal of radioactive fission elements in salt form from the electrometallurgical treatment of spent EBR-II nuclear reactor fuel. For the purposes of this experiment, a matrix of four sample types was created. Given the previously stated target actinide loading in the process waste form, the decision was made to utilize 3:1 and 1:3 uranium to plutonium ratios in the salt used where the total actinide loading sums to 2 mol%.