AmericanChemicalSociety.com

Research Progress

This year’s work has been divided between characterization of polymorphs of hybrid compounds containing tetrathiomolybdate anions and hydrothermal and ion-exchange syntheses of niobium and tantalum pyrochlores.  As will be explained in more detail below, the second area represents a change in research direction that allows us to take advantage of new capabilities in our laboratory.

Hybrid inorganic-organic materials based on the tetrathiomolybdate anion, MoS₄^2-, are an interesting target because of their potential to act as precursors for MoS₂ hydrodesulfurization (HDS) catalysts.  First-row transition metals such as Co and/or Ni – frequently used to activate MoS₂ – can be incorporated into the hybrid compounds, providing crystalline, well-defined starting points for studying and optimizing the decomposition.  During the previous year, we had completed synthesis, structure determination, and thermal characterization for the polymorphic Ni(ethylenediamine)₃MoS₄ system as well as Co and Mn analogues of one of the polymorphs.   At the start of this year, we completed the characterization by collecting variable-temperature magnetic data for these compounds.  All four compounds showed classic paramagnetic behavior.  Effective magnetic moments (mu_eff) showed reasonable agreement with predicted spin-only magnetic moments (mu_s), and the compounds obeyed the Curie-Weiss law.  With the characterization complete, we published work on the polymorphic system in Crystal Growth and Design.

Additional work during the grant year focused on preparing new niobium and tantalum pyrochlores through hydrothermal and ion-exchange methods.  Pyrochlore oxides are described by the formula A₂B₂O₆O’, with interpenetrating B₂O₆ and A₂O’ sublattices.  Variability in occupancy in the A₂O’ network gives rise to defect pyrochlores and is one of the reasons for the large compositional range observed within this family.  Pyrochlores with Ta and Nb in the B site are currently being studied as photocatalysts for dye degradation and water-splitting, and the identities of both the A and B site metals can affect catalytic activity.  We are using solution-phase reactions to prepare phases that are not accessible through high-temperature approaches. 

Using hydrothermally synthesized K_1.27Nb₂(O,OH)₆·1.2H₂O and K_1.26Ta₂(O,OH)₆·1.3H₂O as starting points, we have fully or partially exchanged K⁺ with ions including H⁺, Na⁺, Sn²⁺, and Mn²⁺. Following exchange with Na⁺, K⁺ was no longer detectable within the uncertainty of energy-dispersive X-ray spectroscopy (EDS), and Na⁺ was detected at a level comparable to the initial amount of K⁺.  Exchange with Sn²⁺ and Mn²⁺ did not lead to complete replacement of K⁺, even taking into account the difference in charge.  Powder X-ray diffraction (PXRD) showed that in each case, the pyrochlore structure was maintained.  Preliminary Rietveld refinement using a laboratory PXRD pattern of the Na⁺-exchanged material supports occupation of the A-sites by Na⁺ and shows a contraction in the cubic lattice parameter from 10.621 Å to 10.533 Å.  We are currently completing the refinement using high-resolution synchrotron PXRD data obtained through the 11-BM Mail-In Service at Argonne National Laboratory.

Influence on Student and Faculty Professional Development

Support from an ACS-PRF grant continues to be instrumental in helping to set up laboratory infrastructure, recruit summer students, and report on our work at conferences.  In particular, results obtained within this project were included as preliminary data in a successful proposal to the National Science Foundation Major Research Instrumentation program for acquisition of a new powder X-ray diffractometer for Oberlin College.  I am the principal investigator for this instrument grant, along with four co-PIs from the departments of chemistry and biochemistry, physics and astronomy, and geology, and the new instrument presents new possibilities for research and teaching for faculty members in all three departments.

The flexibility to work toward synthetic targets other than those originally discussed in the project proposal has been of great benefit in acting on new ideas and incorporating new capabilities.  While we identified and characterized an interesting polymorphic system within the tetrathiomolybdate project, we have found that reaction systems containing MoS₄^2- anions and additional transition-metal cations frequently produce amorphous binary sulfides rather than the desired network compounds.  Performing syntheses in ethylenediamine, which acts not only as a solvent but also as a strongly chelating ligand, is one of the few means we found of avoiding this pitfall.  Progress toward growing single-crystalline tetrathoimetallate-containing networks with other organic ligands has thus not been as rapid as we had hoped.  Meanwhile, I spent September-December 2009 working at the Materials Research Laboratory at the University of California, Santa Barbara, while on research leave from Oberlin.  While there, I synthesized the new pyrochlore K_1.27Nb₂(O,OH)₆·1.2H₂O hydrothermally and began preliminary experiments with ion-exchange reactions.  In spring 2010, Oberlin acquired its new powder X-ray diffractometer, making it possible to collect data of the quality needed for Rietveld refinement.  Directing our efforts toward synthesis of pyrochlore oxides and analysis of their structures by powder refinement engages students in fulfilling work that takes advantage of our new resources.     

Student involvement continues to be a centerpiece of this research program.  To date, four Oberlin undergraduates have worked on research related to this grant – Hengfeng Tian ‘10 full-time during summer 2008, Hadley Iliff ‘09 for her senior honors project during the 2008-09 year, and Joshua Greenfield ’11 and Gabrielle White-Dzuro ‘12 full-time during summer 2010.  Hadley and Hengfeng gave oral presentations about their work at the Meeting-in-Miniature of the Cleveland Section of the ACS in 2009, and Hadley won an award for an outstanding undergraduate talk.  Hadley, Hengfeng, and Joshua were co-authors on presentations at regional and national conferences, and Hengfeng and Hadley are co-authors on the paper published in Crystal Growth and Design.  Involvement in this work has prepared and perhaps inspired students to continue to graduate school; Hadley and Hengfeng are both starting graduate programs in fall 2010.