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Structure, stability and photocatalytic H2 production by Cr-, Mn-, Fe-, Co-, and Ni-substituted decaniobate clusters
Abstract: Here the authors report synthesis and characterization of early transition-metal(TM)-substituted decaniobates as a continuation of the authors' previous report of tetramethylammonium (TMA) salt of FeNb9 and NiNb9: TMA6[H2CrIIINb9O28]�14H2O (1, CrNb9), TMA8[MnIIINb9O28]�29H2O (2, MnNb9) and TMA7[H2CoIINb9O28]�25H2O (3, CoNb9). Among the TM-substituted decaniobates, CoNb9 or NiNb9 exhibit a higher photocatalytic H2 evolution activity in methanol-water mixtures than others.
Son, Jung-Ho; Wang, Jiarui; Casey, William H. Structure, stability and photocatalytic H2 production by Cr-, Mn-, Fe-, Co-, and Ni-substituted decaniobate clusters Dalton Transactions (2014), published online.
The energetics of isomerisation in Keggin-series aluminate cations
Abstract: Electronic-structure calculations show that the epsilon-isomer of the polyoxoaluminate ion in the Keggin structure [AlO4(Al(OH)2(H2O))12]7+ is the thermodynamically favoured one. Direct interconversion between the epsilon- and delta-isomers via cap rotation has a prohibitively high energy barrier in vacuo, suggesting that isomerisation in solution either proceeds via a dissolution-precipitation pathway, or that solvation and/or coordination to counterions reduces the barrier significantly. The implications for the formation of the [Al2O8Al28(OH)56(H2O)26]18+ ion is discussed.
Andre Ohlin, C.; Rustad, James R.; Casey, William H. The energetics of isomerisation in Keggin-series aluminate cations Dalton Transactions (2014), 43, 14533-14536.
A High-Pressure NMR Probe for Aqueous Geochemistry
Abstract: A non-magnetic piston-cylinder pressure cell is presented for solution-state nuclear magnetic resonance (NMR) spectroscopy at geochemical pressures. The probe has been calibrated up to 20 kbar using in situ ruby fluorescence and allows for the measurement of pressure dependencies of a wide variety of NMR-active nuclei with as little as 10 �L of sample in a micro-coil. Initial 11B NMR spectroscopy of the H3BO3- catechol equilibria reveals a large pressure-driven exchange rate and a negative pressure-dependent activation volume, reflecting increased solvation and electrostriction upon boron-catecholate formation. The inexpensive probe design doubles the current pressure range available for solution-NMR spectroscopy and is particularly important to advance the field of aqueous geochemistry.
Pautler, Brent G.; Colla, Christopher A.; Johnson, Rene L.; Klavins, Peter; Harley, Stephen J.; Ohlin, C. Andre; Sverjensky, Dimitri A.; Walton, Jeffrey H.; Casey, William H. A High-Pressure NMR Probe for Aqueous Geochemistry Angewandte Chemie, International Edition (2014), 53(37), 9788-9791.
Hafnium Sulfate Prenucleation Clusters and the Hf18 Polyoxometalate Red Herring
Abstract: In prior studies, aqueous Hf sulfate�peroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH� for SO42�), and finally converted to HfO2 hard masks via annealing. The atomic-level details of the underlying aqueous chemistries of these processes are complex and yet to be understood. Yet a thorough understanding of this specific chemical system will inspire development of design rules for other aqueous-precursor-to-solid-state metal oxide systems. Often-observed crystallization of the Hf18 polyoxometalate from aqueous Hf sulfate�peroxide precursor solutions has led us to believe that Hf18 may represent an important intermediate step in this process. However, via detailed solution studies described here (small-angle X-ray scattering, electrospray ionization mass spectrometry, and Raman spectroscopy), we ascertained that Hf18 is in fact not a prenucleation cluster of Hf sulfate coatings. Rather, the Hf tetramers, pentamers, and hexamers that are the core building blocks of Hf18 are robustly persistent over variable compositions and aging time of precursor solutions, and therefore they are likely the rudimentary building blocks of the deposited thin-film materials. These Hf clusters are capped and linked by sulfate and peroxide anions in solution, which probably prevents crystallization of Hf18 during the rapid dehydration process of spin-coating. In fact, crystallization of Hf18 from the amorphous gel coating would be detrimental to formation of a high-density conformal coating that we obtain from precursor solutions. Therefore, this study revealed that the well-known Hf18 polyoxometalate is not likely to be an important intermediate in the thin-film process. However, its subunits are, confirming the universal importance of deriving information from the solid state, albeit judiciously and critically, to understand the solution state.
Rose E. Ruther, Brenna M. Baker, Jung-Ho Son, William H. Casey, and May Nyman Hafnium Sulfate Prenucleation Clusters and the Hf18 Polyoxometalate Red Herring Inorganic Chemistry, 2014, 53(8), 4234�4242.
Kinetic Studies of the [NpO2(CO3)3]4� Ion at Alkaline Conditions Using 13C NMR
Abstract: Carbonate ligand-exchange rates on the [NpO2(CO3)3]4� ion were determined using a saturation-transfer 13C nuclear magnetic resonance (NMR) pulse sequence in the pH range of 8.1 = pH = 10.5. Over the pH range 9.3 = pH = 10.5, which compares most directly with previous work of Stout et al.,1 we find an average rate, activation energy, enthalpy, and entropy of kex298 = 40.6(�4.3) s�1, Ea =45.1(�3.8) kJ mol�1, ?H� = 42.6(�3.8) kJ mol�1, and ?S� = -72(�13) J mol�1 K�1, respectively. These activation parameters are similar to the Stout et al. results at pH 9.4. However, their room-temperature rate at pH 9.4, kex298 = 143(�1.0) s�1, is 3 times faster than what we experimentally determined at pH 9.3: kex298 = 45.4(�5.3) s�1. Our rates for [NpO2(CO3)3]4� are also faster by a factor of 3 relative to the isoelectronic [UO2(CO3)3]4� as reported by Brucher et al.2 of kex298 = 13(�3) s�1. Consistent with results for the [UO2(CO3)3]4� ion, we find evidence for a proton-enhanced pathway for carbonate exchange for the [NpO2(CO3)3]4� ion at pH < 9.0.
Adele F. Panasci, Stephen J. Harley, Mavrik Zavarin, and William H. Casey Kinetic Studies of the [NpO2(CO3)3]4� Ion at Alkaline Conditions Using 13C NMR Inorganic Chemistry, 2014, 53(8), 4202�4208.
Investigating the behaviour of Mg isotopes during the formation of clay minerals
Abstract We present elemental and isotopic data detailing how the Mg isotope system behaves in natural and experimentally synthesized clay minerals. We show that the bulk Mg isotopic composition (d26Mg) of a set of natural illite, montmorillonite and kaolinite spans a 2&range, and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable Mg. After acid leaching, these natural clays become relatively enriched in isotopically heavy Mg by between 0.2&and 1.6&. Results of exchange experiments indicate that the Mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light Mg compared to the residual clay. The isotopic composition of this exchangeable Mg ( 1.49&to 2.03&) is characteristic of the isotopic composition of Mg found in many natural waters. Further experiments with an isotopically characterized MgCl2 solution shows that the clay minerals adsorb this exchangeable Mg with little or no isotopic fractionation, although we cannot discount the possibility that the uptake of exchangeable Mg does so with a slight preference for 24Mg. To characterize the behaviour of Mg isotopes during clay mineral formation we synthesized brucite (Mg(OH)2), which we consider to be a good analogue for the incorporation of Mg into the octahedral sheet of Mg-rich clay minerals or into the brucitic layer of clays such as chlorite. In our experiment the brucite mineral becomes enriched in the heavy isotopes of Mg while the corresponding solution is always relatively enriched in isotopically light Mg. The system reaches a steady state after 10 days with a final fractionation factor (asolid-solution) of 1.0005 at near-neutral pH. This result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy Mg during their formation. However our results also show that exchangeable Mg is an important component within bulk clay minerals and can have an important influence over the bulk clay d26Mg value. Modeling shows that in certain soils or sediments where the percentage of exchangeable Mg is >30% and the isotopic composition of the exchangeable Mg is around 2&, the generation of bulk d26Mg values of < 0.5&is likely. On a broader scale, Mg-rich minerals such as smectite and illite are likely to impart a stronger control over the Mg budget in clay rich sediments, and their high structural Mg component is likely to result in bulk sediment d26Mg values that are closer in composition to the UCC. Despite this, results of modeling, together with experimental observation suggests that the uptake of exchangeable Mg into these clay rich sediments could cause a decrease in the bulk d26Mg value by up to 0.3�0.4&. This should be accounted for when assessing the d26Mg value of sediments on a crustal scale.
Joshua Wimpenny, Christopher A. Colla, Qing-Zhu Yin, James R. Rustad, William H. Casey Investigating the behaviour of Mg isotopes during the formation of clay minerals, Geochimica et Cosmochimica Acta 128 (2014) 178�194
A tellurium-substituted Lindqvist-type polyoxoniobate showing high H2 evolution catalyzed by tellurium nanowires via photodecomposition
Abstract: A new tellurium-substituted Lindqvist-type polyoxoniobate [H2TeNb5O19]5- was synthesized as a tetramethylammonium salt. When irradiated with a Xe lamp, a water-methanol soln. of this cluster showed exceptionally high H2-evolution activity suggesting cocatalysis by the hexaniobate cluster and metallic tellurium, both of which are formed as photodecompn. products.
Son, Jung-Ho; Wang, Jiarui; Osterloh, Frank E.; Yu, Ping; Casey, William H. A tellurium-substituted Lindqvist-type polyoxoniobate showing high H2 evolution catalyzed by tellurium nanowires via photodecomposition Chemical Communications, 2014,50(7), 836-838.
Computational characterization of the internal bonding and solvation structure for [Nb10O28]6-
Abstract The chem. of polyoxometalate ions is richly explored, but mostly via synthesis and exptl. studies. Less emphasis has been placed on developing a robust computational understanding of their aq. dynamics. In this work, we utilize both a previously created force-field model and ab initio mol. dynamics to explore the solvation structure of [Nb10O28]aq 6-. We examine characteristic behaviors of both cluster-water interactions and intra-cluster bond strengths. We show that cluster-water interactions are dictated by electrostatic interactions, which in turn are dictated by the shape of the cluster; ultimately reflecting a quantitatification of the steric shielding to the cluster's oxygen sites. We also show that bond strengths within the cluster do not correlate with oxygen reactivities, lending credence to previous suggestions that oxygen exchange for decaniobate clusters occurs via intermediates that form by significant structural reorganizations.
Hanken, Benjamin E.; Asta, Mark; Casey, William H. Computational characterization of the internal bonding and solvation structure for [Nb10O28]6- Physical Chemistry Chemical Physics, 2013.15(48), 20929-20936.
Calcium-isotope fractionation between solution and solids with six, seven or eight oxygens bound to Ca(II)
Abstract The aq.-mineral D44/40Casolid-solution fractionation factors were measured for equilibrating solns. and a set of inorg. solids that trap solvated calcium in six, seven and eight coordination as an intact solvated ion in the lattice. The measured values were then compared to D44/40Casolid-solution values predicted from electronic-structure calcns. and vibrational frequencies of mol. models of the solvated ions and their solid hydrates. Exptl., the D44/40Casolid-solution values differed by ~2� for crystals of six- and seven-hydrated calcium, and by about ~2� between crystals having seven- and eighthydrated calcium. In comparison, the calcns. predict D44/40Casolid-solution values between the sixfold- and eightfoldcoordinated aquo ions of 5.4�. Calcns. predict 2.6� fractionation between the sixfold- and sevenfold-coordinated aquo ions. Stirred and unstirred expts. gave similar results in most, but not all cases. In general, measured isotopic fractionations compare well with the predictions from quantum mechanics and vibrational anal. Isotopic fractionation is directly correlated with coordination no. We consider this to be a well-poised test of predictions.
Colla, Christopher A.; Wimpenny, Joshua; Yin, Qing-Zhu; Rustad, James R.; Casey, William H. Calcium-isotope fractionation between solution and solids with six, seven or eight oxygens bound to Ca(II) Geochimica et Cosmochimica Acta, 2013.121, 363-373.
Mechanism, decomposition pathway and new evidence for self-healing of manganese oxides as efficient water oxidizing catalysts: new insights
Abstract The electrochem. water-oxidn. reaction usually requires a catalyst to reduce the overpotential and Earth-abundant catalysts, like MnO2, are attracting much attention. Here we use chemometric anal., EPR and UV-Vis spectroscopies to track Mn(II) and MnO4 - byproducts to the reaction of a MnO2 film in the presence of cerium(IV) ammonium nitrate. Permanganate ion is involved in at least two key reactions: it may oxidize water to O2 or can combine with Mn(II) to remake MnO2 solid. We propose mechanisms for water oxidn. and present a self-healing process for this reaction.
Najafpour, Mohammad Mahdi; Kompany-Zareh, Mohsen; Zahraei, Ali; Jafarian Sedigh, Davood; Jaccard, Hugues; Khoshkam, Maryam; Britt, R. David; Casey, William H.Mechanism, decomposition pathway and new evidence for self-healing of manganese oxides as efficient water oxidizing catalysts: new insights Dalton Transactions, 2013.42(40), 14603-14611.
A decatungstate-type polyoxoniobate with centered manganese: [H2MnIVNb10O32]8- as a soluble tetramethylammonium salt
Abstract A highly sym. MnIV-centered polyoxoniobate [H2MnIVNb10O32]8-(1) was synthesized via hydrothermal methods as a sol. tetramethylammonium salt. The structure is similar to decatungstate structure [W10O32]4-, except for the central heteroatom. The cluster is stable between 4 < pH < 10, and was characterized by ESI-MS and UV-visible spectroscopy.
Son, Jung-Ho; Casey, William H. A decatungstate-type polyoxoniobate with centered manganese: [H2MnIVNb10O32]8- as a soluble tetramethylammonium salt Dalton Transactions, 2013.42(37), 13339-13342.
Structural insights into [Co4O4(C5H5N)4(CH3CO2)4]+, a rare Co(IV)-containing cuboidal complex
Abstract The authors report high-frequency (up to 219 GHz) and correspondingly high-field ESR spectra and x-ray crystal structure of [Co4O4(pyridine)4(acetate)4]+ ([Co4O4(py)4(OAc)4]+) that serves as a structural and spectroscopic model of cobalt-oxide films that are capable of oxidizing water. These results are, in large part, consistent with those from the authors' earlier, lower-frequency study and serve to benchmark future high-field studies on paramagnetic states of the catalyst film.
Stich, Troy A.; Krzystek, J.; Mercado, Brandon Q.; McAlpin, J. Gregory; Ohlin, C. Andre; Olmstead, Marilyn M.; Casey, William H.; David Britt, R. Structural insights into [Co4O4(C5H5N)4(CH3CO2)4]+, a rare Co(IV)-containing cuboidal complex Polyhedron, 2013.64, 304-307.
Dynamics of a Nanometer-Sized Uranyl Cluster in Solution
Johnson, Rene L.; Ohlin, C. Andre; Pellegrini, Kristi; Burns, Peter C.; Casey, William H. Dynamics of a Nanometer-Sized Uranyl Cluster in Solution Angewandte Chemie, International Edition, 2013.52(29), 7464-7467.
Highly Soluble Iron- and Nickel-Substituted Decaniobates with Tetramethylammonium Countercations
Abstract Iron- and nickel-substituted decaniobates, TMA6[H2FeIIINb9O28]�14H2O (1) and TMA6[H3NiIINb9O28]�17H2O (2), were hydrothermally synthesized as tetramethylammonium salts and the structures were detd. by X-ray crystallog. The highly sol. title compds. were characterized by ESI-MS, FT-IR and UV-Vis titrn.
Son, Jung-Ho; Ohlin, C. Andre; Casey, William H. Highly Soluble Iron- and Nickel-Substituted Decaniobates with Tetramethylammonium Countercations Dalton Transactions, 2013.42(21), 7529-7533.
Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation
Abstract Previous measurements show that calcium manganese oxide nanoparticles are better water oxidn. catalysts than binary manganese oxides (Mn3O4, Mn2O3, and MnO2). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodn. stability and a high surface area, their low surface energy suggests relatively loose binding of H2O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidn. enthalpy independent of the Mn3+/Mn4+ ratio and much smaller in magnitude than the Mn2O3-MnO2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidn. states.
Birkner, Nancy; Nayeri, Sara; Pashaei, Babak; Najafpour, Mohammad Mahdi; Casey, William H.; Navrotsky, Alexandra Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation Proceedings of the National Academy of Sciences, 2013.110(22), 8801-8806.
A Soluble Phosphorus-Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups
Abstract A water-sol. tetramethylammonium (TMA) salt of a novel Keggin-type polyoxoniobate was isolated as TMA9[PV2Nb12O42].19H2O (1). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus-contg. polyoxoniobate, [(PO2)3PNb9O34]15-, was known, which is a lacunary Keggin ion decorated with three PO2 units. However, that cluster was isolated as an insol. structure consisting of chains linked by sodium counterions. In contrast, the [PV2Nb12O42]9- cluster in 1 is stable over a wide pH range, as evident by 31P and 51V NMR, UV/visible spectroscopy, and ESI-MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.
Son, Jung-Ho; Ohlin, C. Andre; Johnson, Rene L.; Yu, Ping; Casey, William H. A Soluble Phosphorus-Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups Chemistry - A European Journal, 2013, 19(16), 5191-5197.
Mineral oxides, dissolution of
Casey, William H. Mineral oxides, dissolution of Encyclopedia of Surface and Colloid Science (2nd Edition), 2012, 3982-3991.
Synthesis and characterization
of a soluble vanadium-containing Keggin polyoxoniobate via ESI-MS and 51V NMR: (TMA) 9[V3Nb12O42] ·18H2O
Abstract The vanadium-containing heteropolyoxoniobate (TMA)9[V3Nb12O42]·18H2O was synthesized by hydrothermal reaction of V2O5 and hydrous niobium oxide in tetramethylammonium hydroxide solution. The cluster has an α- Keggin structure with a central VO4 and two trans-bicapped VO5. The water-soluble product was characterised by X-ray crystallography, ESI-MS and both liquid- and solid-state 51V NMR. The solid- and solution-phase 51V NMR spectra indicate two major peaks corresponding to the one VO4 and two VO5 sites.
Son, J.-H., Ohlin, C. A., Larson, E. C., Yu, P., Casey, W. H. Synthesis and characterization of a soluble vanadium-containing Keggin polyoxoniobate via ESI-MS and 51V NMR: (TMA) 9[V3Nb12O42]·18H2O. Eur. J. Inorg. Chem., 2013, 1748-1753.
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