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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

15N2 formation and fast oxygen isotope exchange during pulsed 15N18O exposure of MnOx/CeO2

Pulsing 15N18O onto an annealed 1% Mn16Ox/Ce16O2 catalyst resulted in very fast oxygen isotope exchange and 15N2 formation at 295 K. In the 1st 15N18O pulse, due to the presence of large number of surface oxygen defects, extensive 15N218O and 15N2 formations were observed. In subsequent pulses oxygen isotope exchange dominated as a result of highly labile oxygen in the oxide. We gratefully acknowledge the US Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy/Vehicle Technologies Program for the support of this work. The research described in this paper was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Dissecting the steps of CO2 reduction: 1. The interaction of CO and CO2 with γ-Al2O3: an in situ FTIR study

The adsorption of CO2 and CO was investigated on a pure γ-Al2O3 support material that has been used for the preparation of Pd and Ru catalysts for the catalytic reduction of CO2. The adsorption of CO2 resulted in the formation of carbonates, bicarbonates and linearly adsorbed CO2 species. The amount and the nature of the adsorbed species were dependent on the annealing temperature of the alumina support. On γ-Al2O3 annealed at 473 K mostly bicarbonates formed, while no adsorbed CO2 was seen on this highly hydroxylated surface. With increasing calcinations temperature, i.e., increasing extent of dehydroxylation, the amounts of both surface nitrates and linear adsorbed CO2 increased, but still the most abundant surface species were bicarbonates. Surface carbonates and adsorbed CO2 can readily be removed from the alumina surface, while bicarbonates are stable to elevated temperatures. The interaction of CO with γ-Al2O3 is much weaker than that of CO2. At room temperatures CO adsorbs only on Lewis acid sites, and can be readily removed by evacuation. At 100 K CO can probe different defect sites on the alumina surface (both Lewis acid sites and surface hydroxyls). Under no conditions we have observed the formation of any carbonates or bicarbonates upon the interaction of CO with the pure alumina support. In co-adsorption experiments CO competes for adsorption sites with the linearly adsorbed CO2 on the 773 K-annealed γ-Al2O3 surface; but it does not result in the desorption of CO2, rather in the increase of weakly-held carbonate production. After the removal of adsorbed CO, CO2 moves back to its original adsorption sites, i.e., Lewis acidic Al3+ centers. The exposure of a CO2-saturated γ-Al2O3 to H2O did not affect any of the adsorbed surface species. The findings of this study will be used to rationalize the results of our ongoing in situ and in operando studies on the reduction of CO2 on supported Pd and Ru catalysts. Acknowledgements: We gratefully acknowledge the US Department of Energy Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences for the support of this work. The synthesis and catalyst pre-treatment portion of the work described in this manuscript was supported by a Laboratory Directed Research and Development (LDRD) project at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute. J.H.K. also acknowledges the support of this work by the 2013 Research Fund of UNIST (Ulsan National Institute of Science and Technology, Ulsan, Korea).

CO and CO2 adsorption↗
DOE OSTI2014

Dissecting the steps of CO2 reduction: 2. The interaction of CO and CO2 with Pd/γ-Al2O3: an in situ FTIR study

Alumina supported Pd catalysts with metal loadings of 0.5, 2.5 and 10 wt% were investigated by in situ FTIR spectroscopy in order to understand the nature of adsorbed species formed during their exposure to CO2 and CO. Exposing the annealed samples to CO2 at 295 K resulted in the formation of alumina support-bound surface species only: linear adsorbed CO2, bidentate carbonates and bicarbonates. Room temperature exposure of all three samples to CO produced IR features characteristic of both ionic and metallic Pd, as well as bands we observed upon CO2 adsorption (alumina support-bound species). Low temperature (100 K) adsorption of CO on the three samples provided information about the state of Pd after oxidation and reduction. Oxidized samples contained exclusively ionic Pd, while mostly metallic Pd was present in the reduced samples. Subsequent annealing of the CO-saturated samples revealed the facile (low temperature) reduction of PdOx species by adsorbed CO. This process was evidenced by the variations in IR bands characteristic of ionic and metallic Pd-bound CO, as well as by the appearance of IR bands associated with CO2 adsorption as a function of annealing temperature. Samples containing oxidized Pd species (oxidized, annealed or reduced) always produced CO2 upon their exposure to CO, while CO2-related surface entities were observed on samples having only fully reduced (metallic) Pd. Acknowledgements: The catalyst preparation was supported by a Laboratory Directed Research and Development (LDRD) project. This work was supported by the US Department of Energy Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy. JHK also acknowledges the support of this work by the 2013 Research Fund of UNIST (Ulsan National Institute of Science and Technology, Ulsan, Korea).

CO and CO2 adsorption↗
DOE OSTI2014

Geometric and Electronic Properties of Graphene Modified by “External” N-Containing Groups

By means of the first-principles spin polarized density functional theory (DFT) calculations, we investigated structures and electronic properties of “external” nitrogen-containing groups (pyridine derivatives) modified graphene via single or double bonding mode. This study of single-bond pyridine derivatives modified graphene (SBPG) shows that the ortho-carbon is the most favorable adsorption site for pyridine derivatives on graphene, as confirmed by bader charge analysis. The external stability of pyridine derivatives on graphene by [2+2] cycloaddition is caused by the match between frontier orbitals of pyridine derivatives and those of graphene, which leading to the formation of stronger chemical bonds. Interestingly, electronic structure analysis reveals that the spin-up and spin-down parts DOS of SBPG clearly split, while it is not found for double-bonds pyridine derivatives modified graphene (DBPG). This work was supported by the 973 project (2013CB733501) and the National Natural Science Foundation of China ( 21101137, 21136001, 21176221, 21306169 and 91334013). D. Mei is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

electronic structure↗
DOE OSTI2014

Mechanistic Insights into Hydride Transfer for Catalytic Hydrogenation of CO2 with Cobalt Complexes

The catalytic hydrogenation of CO2 to formate by Co(dmpe)2H can proceeds via direct hydride transfer or via CO2 coordination to Co followed by reductive elimination of formate. Both pathways have activation barriers consistent with experiment (~17.5 kcal/mol). Controlling the basicity of Co by ligand design is key to improve catalysis. The research by N.K., D.M.C. and A.M.A. was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The research by S.R. and M.D. was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle.

Kumar, Neeraj↗
DOE OSTI2015

Nickel Complexes of a Binucleating Ligand Derived from an SCS Pincer

A binucleating ligand has been prepared that contains an SCS pincer and three oxygen donor ligands in a partial crown ether loop. To enable metalation with Ni0, a bromoarene precursor was used and resulted in the formation of a nickel-bromide complex in the SCS pincer. Reaction of the nickel complex with a lithium salt yielded a heterobimetallic complex with bromide bridging the two metal centers. The solid-state structures were determined for this heterobimetallic complex and the nickel-bromide precursor, and the two complexes were characterized electrochemically to determine the influence of coordinating the second metal. This research was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. MLH was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

Peterson, Sonja M.↗
DOE OSTI2014

Heterolytic Cleavage of H2 by Bifunctional Manganese(I) Complexes: Impact of Ligand Dynamics, Electrophilicity, and Base Positioning

We report the synthesis, characterization, and reactivity with H2 of a series of MnI complexes of the type [(P-P)Mn(L2)CO]+ (L2 = dppm, bppm, or (CO)2; P-P = PPhNMePPh or PPh2 NBn2 ) that bear pendant amine ligands designed to function as proton relays. The pendant amine was found to function as a hemilabile ligand; its binding strength is strongly affected by the ancillary ligand environment around Mn. Tuning the electrophilicity of the Mn center leads to systems capable of reversible heterolytic cleavage of the H-H bond. The strength of pendant amine binding can be balanced to protect the Mn center while still leading to facile reactivity with H2. Neutral amine-bearing MnIH species were found to react with one-electron oxidants and, after proton and electron transfer reactions, regenerate MnI cationic species. The reactivity presented herein indicate that the Mn complexes we have developed are a promising platform for Mn-based H2 oxidation electrocatalyst development. The research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for DOE.

electrochemistry↗
DOE OSTI2014

A Radar-like Iron based Nanohybrid as an Efficient and Stable Electrocatalyst for Oxygen Reduction

The present study shows a design concept for fabricating Fe-PyNG hybrid via strong coupling between FePc and pyridine-N. The prominent features of the Fe-PyNG hybrid include high electrocatalytic activity, superior durability, and better performance than Pt/C toward ORR in alkaline media. These features potentially make Fe-PyNG an outstanding nonprecious metal cathode catalyst for fuel cells. The incorporation of Fe ion and pyridine-N afforded effective bonding and synergetic coupling effects, which lead to significant electrocatalytic performance. DFT calculations indicate that N-modified Fe is a superior site for OOH adsorption and ORR reaction. Meanwhile, the strong chemical bonding between FePc and pyridyne in PyNG leads to its superior stability. We believe that our present synthetic strategy can be further extended to develop other metal complexes/N-doped carbon materials for broad applications in the field of catalysts, batteries, and supercapacitors. This work was supported by National Basic Research Program of China (973 Program) (2013CB733501), the National Natural Science Foundation of China (NSFC-21306169, 21176221, 21136001 and 21101137), Zhejiang Provincial Natural Science Foundation of China (ZJNSF-R4110345) and the New Century Excellent Talents in University Program (NCET-10-0979). We thank Prof. Youqun Zhu for Instruments support. D. Mei is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. Computing time was granted by the grand challenge of computational catalysis of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). EMSL is a national scientific user facility located at Pacific Northwest National Laboratory (PNNL) and sponsored by DOE’s Office of Biological and Environmental Research.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Nickel Phosphine Catalysts with Pendant Amines for Electrocatalytic Oxidation of Alcohols

Nickel phosphine complexes with pendant amines have been found to be electrocatalysts for the oxidation of primary and secondary alcohols, with turnover frequencies as high as 3.3 s-1. These complexes are the first electrocatalysts for alcohol oxidation based on non-precious metals, which will be critical for use in fuel cells. The research by CJW, ESW, and AMA was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The research by JASR was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Alcohol Oxidation↗
DOE OSTI2015

Effect of Graphene with Nanopores on Metal Clusters

Porous graphene, which is a novel type of defective graphene, shows excellent potential as a support material for metal clusters. In this work, the stability and electronic structures of metal clusters (Pd, Ir, Rh) supported on pristine graphene and graphene with different sizes of nanopore were investigated by first-principle density functional theory (DFT) calculations. Thereafter, CO adsorption and oxidation reaction on the Pd-graphene system were chosen to evaluate its catalytic performance. Graphene with nanopore can strongly stabilize the metal clusters and cause a substantial downshift of the d-band center of the metal clusters, thus decreasing CO adsorption. All binding energies, d-band centers, and adsorption energies show a linear change with the size of the nanopore: a bigger size of nanopore corresponds to a stronger metal clusters bond to the graphene, lower downshift of the d-band center, and weaker CO adsorption. By using a suitable size nanopore, supported Pd clusters on the graphene will have similar CO and O2 adsorption ability, thus leading to superior CO tolerance. The DFT calculated reaction energy barriers show that graphene with nanopore is a superior catalyst for CO oxidation reaction. These properties can play an important role in instructing graphene-supported metal catalyst preparation to prevent the diffusion or agglomeration of metal clusters and enhance catalytic performance. This work was supported by National Basic Research Program of China (973Program) (2013CB733501), the National Natural Science Foundation of China (NSFC-21176221, 21136001, 21101137, 21306169, and 91334013). D. Mei acknowledges the support from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. Computing time was granted by the grand challenge of computational catalysis of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC).

cobalt↗
DOE OSTI2015

Covalent Attachment of Diphosphine Ligands to Glassy Carbon Electrodes via Cu-Catalyzed Alkyne-Azide Cycloaddition. Metallation with Ni(II)

Covalent tethering of PPh2NC6H4C=CH2 ligands (PPh2NC6H4C=CH2 = 1,5-di-(4-ethynylphenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) to planar, azide-terminated glassy carbon electrode surfaces has been accomplished using a CuI-catalyzed alkyne-azide cycloaddition (CuAAC) as the coupling reaction, using a BH3?P protection-deprotection strategy. Deprotected, surface-confined ligands were metallated using [NiII(MeCN)6](BF4)2. X-ray photoelectron spectroscopic measurements demonstrate that metallation introduced 1.3 equivalents NiII per diphosphine onto the electrode surface. Exposure of the surface to a second diphosphine ligand, PPh2NPh2, resulted in the removal of Ni from the surface. Protection, coupling, deprotection, and metallation conditions were optimized using solution-phase model systems, with benzyl azide as a model for the azide-terminated carbon surface; these reactions generate a [NiII(diphosphine)2]2+ complex. Acknowledgment. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The XPS measurements were performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U. S. Department of Energy, Office of Science, Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The x-ray crystal structure analysis was carried out by S.L., who was supported by a grant to Wendy J. Shaw from the Early Career Research Program through the U.S. Department of Energy, Office of Science. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

electrocatalysis, surface attachment, XPS↗
DOE OSTI2015

Water-Assisted Proton Delivery and Removal in bio-inspired Hydrogen Production Catalysts

Electrocatalysts for H2 production are envisioned to play an important role in renewable energy utilization systems. Nickel-based catalysts featuring pendant amines functioning as proton relays in the second coordination sphere of a metal center have led to catalysts achieving turnover frequencies as high as 107 s-1 for H2 production. The fastest rates are observed when water is present in solution, with rates up to 102 times faster than dry conditions. The focus of this paper is to provide mechanistic insight into the unexpected enhancement due to water. Addition of H2 to [Ni(PR2NR´2)2]2+ was previously shown to give three isomers of a Ni(0) product with two protonated amines, where the N-H can be endo or exo to the Ni. By investigating the deprotonation of two N-protonated Ni(0) intermediates resulting from H2 addition to to [Ni(PR2NR´2)2]2+, we observe an enhancement in the rate of deprotonation for protons positioned on the pendant amine next to the metal (endo) vs. protons that are positioned away from the metal (exo). Computational studies suggest that steric accessibility and desolvation energy of the deprotonating base are responsible for limiting endo deprotonation. The significant reduction in this barrier observed in the presence of water has important implications for disfavoring less productive catalytic pathways and increasing catalytic rates. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Ho, Ming-Hsun↗
DOE OSTI2015

Photoswitching a Molecular Catalyst to Regulate CO 2 Hydrogenation

Inspired by nature’s ability to regulate catalysis using physiological stimuli, azobenzene was incorporated into Rh(bis)diphosphine CO 2 hydrogenation catalysts to photoinitiate structural changes to modulate the resulting catalytic activity. The rhodium bound diphosphine ligands (P(Ph 2 )-CH 2 -N(R) CH 2 -P(Ph2)) contain the terminal amine of a non-natural amino acid, either β-alanine (β-Ala) or γ-aminobutyric acid (GABA). For both β-ala and GABA containing complexes, the carboxylic acids of the amino acids were coupled to the amines of diaminoazobenzene, creating a rhodium bound trans-spanning tetraphosphine complex. The photo-induced cis-trans isomerization of the azobenzene-containing complexes imposes structural changes on these complexes, as evidenced by 1H NMR. We found that the CO 2 hydrogenation activity for the β-ala bound Rh complex is 40% faster with azobenzene in the cis configuration (16 s -1 ) than in the trans conformation (11 s -1 ), while the γ-aminobutyric acid containing Rh complex has the same rate (~17 s -1 ) in either the cis or the trans configuration at 27 °C. The corresponding complexes without the attached azobenzene were also prepared, characterized, and catalytically tested for comparison, and have rates of 30 s -1 . Computational studies were undertaken to evaluate the difference in rate between the cis and trans isomers for the β-Ala bound Rh complex, and revealed major structural changes between all cis and trans structures, but only minor structural changes that would be unique to the β-Ala bound Rh complex. We postulate that the slower rate of the azobenzene-containing β-Ala bound Rh complex is due to subtle changes in the bite angle arising from steric strain due to the trans-spanning azobenzene, altering hydricity and consequently rate. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle. A portion of this research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Blending materials composed of boron, nitrogen and carbon to transform approaches to liquid hydrogen stores

Energy storage remains a key challenge for the advancement of fuel cell applications. Because of this, hydrogen has garnered much research attention for its potential as an energy carrier. This can be attributed to its abundance from non-petroleum sources, and its energy conversion efficiency. Our group, among others, has been studying the use of ammonia borane as a chemical hydrogen storage material for the past several years. Ammonia borane (AB, NH3BH3), a solid state complex composed of the light weight main group elements of nitrogen and boron, is isoelectronic with ethane and as such is an attractive hydrogen storage material with a high gravimetric capacity of H2 (19.6 wt%). However, the widespread use of AB as a chemical hydrogen storage material has been stalled by some undesirable properties and reactivity. Most notably, AB is a solid and this presents compatibility issues with the existing liquid fuel infrastructure. The thermal release of H2 from AB also results in the formation of volatile impurities (borazine and ammonia) that are detrimental to operation of the fuel cell. Additionally, the major products in the spent fuel are polyborazylene and amine borane oligomers that present challenges in regenerating AB. This research was funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Characterization of Fe 2+ ions in Fe,H/SSZ-13 zeolites: FTIR spectroscopy of CO and NO probe molecules

The IR spectra of adsorbed CO and NO probe molecules were used to characterize the coordination chemistry of Fe2+ ions in solution ion exchanged Fe,H/SSZ-13 zeolites. The effects of Fe ion exchange levels, as well the sample pre-treatment conditions on the adsorption of these probe molecules were investigated. The ion exchange levels (in the range of the study) did not affect significantly the IR spectra of either probe molecule, the IR features and their intensity ratios were very similar. Experiments with both probe molecules substantiated the presence of two distinct types of Fe2+ ions in cationic positions. We assign these two Fe2+ ions to two distinct cationic positions: Fe2+ in 6R and 8R positions. NO initially adsorbs preferentially onto Fe2+ sites in the 6R position, and then populates sites in the 8R. Fe2+ ions in the 8R positions require the interaction of more than one NO molecule to move them out from their adsorbate-free cationic positions. As soon as they move from their stable positions, they are able to bind to multiple NO molecules, and form mostly tri-nitrosyls. These tri-nitrosyls, however, are only stable in the presence of gas phase NO; under dynamic vacuum they lose one of the NO molecules from their coordination sphere and form stable di-nitrosyls. The adsorption of CO is much weaker on Fe2+ sites than that of NO, and requires cryogenic sample temperatures to initiate CO adsorption. Under the conditions applied in this study, only mono-carbonyl formation was observed. Reduction in H2 at 773 K increased the number of Fe2+ adsorption sites, primarily in the 8R locations. Oxidation by N2O, on the other hand, selectively reduced the adsorption of both CO and NO on the Fe2+ sites in 8R positions. Adsorbed oxygen left behind from the decomposition of N2O at 573 K readily reacted with CO to produce CO2 even at 150 K. The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Program for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.

coordination chemistry of Fe2+ ions↗
DOE OSTI2016

Electronic origin of the dependence of hydrogen bond strengths on nearest-neighbor and next-nearest-neighbor hydrogen bonds in polyhedral water clusters (H 2 O) n , n = 8, 20 and 24

The relative stability and the characteristics of the hydrogen bond networks in the cubic cages of (H2O)8, dodecahedral cages of (H2O)20,and tetrakaidodecahedral cages of (H2O)24 are studied. The charge-transfer and dispersion interaction terms of every pair of the hydrogen bonds are evaluated by using the perturbation theory based on the locally-projected molecular orbital (LPMO PT). Every water molecule and every hydrogen-bonded pair in polyhedral clusters are classified by the types of the adjacent molecules and hydrogen bonds. The relative binding energies among the polyhedral clusters are grouped by these classifications. The necessary condition for the stable conformers and the rules of the ordering of the relative stability among the isomers are derived from the analysis. The O–O distances and the pair-wise charge-transfer terms are dependent not only on the types of the hydrogen donor and acceptor waters but also on the types of the adjacent waters. This dependence is analyzed with Mulliken’s charge-transfer theory. The work is partially supported by the Grant-in-Aid for Science Research of JSPS (SI, DA, MA). SSX was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Battelle operates the Pacific Northwest National Laboratory for the US Department of Energy.

Iwata, Suehiro↗
DOE OSTI2016

Modelling the local atomic structure of molybdenum in nuclear waste glasses with ab initio molecular dynamics simulations

The nature of chemical bonding of molybdenum in high level nuclear waste glasses has been elucidated by ab initio molecular dynamics simulations. Two compositions, (SiO2)57.5 – (B2O3)10 – (Na2O)15 – (CaO)15 – (MoO3)2.5 and (SiO2)57.3 – (B2O3)20 – (Na2O)6.8 – (Li2O)13.4 – (MoO3)2.5 , were considered in order to investigate the effect of ionic and covalent components on the glass structure and the formation of the crystallisation precursors (Na2MoO4 and CaMoO4). The coordination environments of Mo cations and the corresponding bond lengths calculated from our model are in excellent agreement with experimental observations. The analysis of the first coordination shell reveals two different types of molybdenum host matrix bonds in the lithium sodium borosilicate glass. Based on the structural data and the bond valence model, we demonstrate that the Mo cation can be found in a redox state and the molybdate tetrahedron can be connected with the borosilicate network in a way that inhibits the formation of crystalline molybdates. These results significantly extend our understanding of bonding in Mo-containing nuclear waste glasses and demonstrate that tailoring the glass composition to specific heavy metal constituents can facilitate incorporation of heavy metals at high concentrations. K.K. was supported through the Impact Studentship scheme at UCL co-funded by the IHI Corporation and UCL. P.V.S. thanks the Royal Society, which supported preliminary work on this project, and the Laboratory Directed Research and Development program at PNNL, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. Via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗