Showing papers by "Taner Yildirim published in 2011"

Carbon capture in metal–organic frameworks—a comparative study

Abstract: Metal–organic frameworks (MOFs) have been shown to be excellent materials for storage of carbon dioxide, implying that they could be useful for removal of carbon dioxide from flue gas stacks, however their performance in industrially relevant swing adsorption processes for carbon capture has not been studied. Here we show that the efficacy of MOFs for carbon capture depends dramatically on the process and that some MOFs can provide significant carbon capture under typical pressure and vacuum swing processes. In particular, MOFs that possess coordinatively unsaturated metal centers offer as much as 9 mmol g−1 swing capacity under certain conditions. The results herein clearly show that there is no single ideal compound for carbon capture applications and that different materials can perform better or worse depending on the specific process conditions. In addition to their capture performances, we have also investigated their selectivity to carbon dioxide over that of nitrogen and methane. The analysis provided clearly demonstrates that the performance of a given MOF cannot be determined without also considering the detailed industrial process in which the MOF is to be applied.

Porous graphene oxide frameworks: Synthesis and gas sorption properties

Abstract: We report detailed synthesis of a range of porous graphene oxide frameworks (GOFs) by expansion of graphene oxide (GO) sheets with various linear boronic acid pillaring units in a solvothermal reaction The GOF structures develop through boronate-ester formation as a result of B–O bonding between boronic acids and oxygen functional groups on the GO layers Synthesized GOFs exhibit periodic layered structures with largely expanded interlayer spacing as characterized by X-ray powder diffraction (XRD) The boronate-ester link formation is further evidenced by Fourier transform infrared (FTIR) and Raman spectroscopy Furthermore, the strong boronate-ester bonds between GO layers results in improved thermal stability over the precursor GO The solvent-free, evacuated frameworks provide highly increased accessible surface area for nitrogen adsorption compared to GO alone, which depends on the type and length of the boronic acid, indicating the importance of pillaring unit Both isosteric heat of adsorption (Qst) and the adsorbed hydrogen capacity per surface area are twice as large as typical porous carbon materials and comparable to metal–organic frameworks (MOFs) with open metal centers This enhanced Qst and adsorption capacity is attributed to optimum interlayer spacing between graphene planes such that hydrogen molecules interact with both surfaces Finally, our systematic study reveals the profound effect of both synthesis and activation temperatures to obtain porous framework structures

Nanoconfinement and Catalytic Dehydrogenation of Ammonia Borane by Magnesium-Metal–Organic-Framework-74

Abstract: Ammonia borane (NH3BH3, AB) has recently received much attention as a promising hydrogen-storage medium among a very large number of candidate materials because of its satisfactory air stability, relatively low molecular mass (30.7 gmol ), and remarkably high energy-storage densities (gravimetric and volumetric hydrogen capacities are 19.6wt% and 140 gL , respectively). However, the direct use of pristine AB as a hydrogen energy carrier in onboard/fuel-cell applications is prevented by its very slow dehydrogenation kinetics below 100 8C and the concurrent release of detrimental volatile by-products such as ammonia, borazine, and diborane. Many different methods have been adopted to promote efficient H2 generation from AB, including catalytic hydrolysis in aqueous solution, ionic liquids, organic solvents, and thermodynamic modifications by formation of hybrid structures with transition metals, alkali-, or alkaline-earth metal/hydrides, 12] or nanoconfined phases using porous scaffolds. However, many of these methods rely on the usage of heavy metal catalysts, aqueous or nonaqueous solutions, and ionic liquids, all of which make the hydrogen density of the systems unacceptably low for practical applications. Furthermore, the vigorous reactions, hygroscopic properties, and water solubility of borohydrides have negative impacts on the dehydrogenation performance and make it difficult to control the release of hydrogen. The other approach is made, in particular, nanocomposition of AB within porous scaffoldings. However, systems still suffers one or more of the followings: either the nanocomposite is heavier or cannot prevent the generation of all the volatile by-products. Hence, more work needs to be done to explore the potential role that catalysts can play to further improve the controllable H2-release kinetics under moderate conditions while at the same time preventing the generation of detrimental byproducts. Over the past few years, porous metal–organic frameworks (MOFs) have emerged as promising multifaceted materials, combining such functions as catalytic activity, 24] shape-selectivity, templating, and purification. Crystalline MOF structures are composed of metal sites linked to organic ligands, yielding three-dimensional extended frameworks that often possess considerable porosity. In principle, the combination of nanoporosity and active metal sites in MOFs makes them potentially useful materials for promoting the decomposition of AB. However, until now, such a use of MOFs has been rare and any future success would depend crucially on the particular choices of a suitable metal center, pore structure, and thermal stability. For instance, Li et al. were the first to show that Y-based MOF as a solid state decomposition agent for AB. The main drawback of AB-Y-MOF is largely added weight due to the heavy Y metal. In addition, for the given very narrow pore structure of Y-MOF, as low as approximately 8 wt% of AB loading is achieved for the reported 1:1 mole ratio. Thus, it is highly desirable to have a light weight MOF with stable and suitable nanopore channels that can hold more than one AB molecule. Herein, we show that the porous MgMOF-74 (Mg2ACHTUNGTRENNUNG(DOBDC), DOBDC=2, 5-dioxido-1, 4-benzenedicarboxylate) is a promising candidate for nanoconfinement and catalytic decomposition of AB for clean and efficient H2 generation. Mg-MOF-74 has a rigid framework, composed of one-dimensional (1D) hexagonal channels (Figure 1a) with a nominal diameter of approximately 12 running parallel to the DOBDC ligands. In as-synthesized material, the Mg cations are coordinated with five oxygen atoms from the DOBDC ligands and one oxygen atom from a terminal water molecule. However, upon heating under vacuum, the terminal water molecules can be easily removed, leading to unsaturated (open) Mg metal sites (decorated on the edges of the hexagonal pore channels) with an open pore structure of high surface area (>1000 mg ). The open Mg metal sites play a vital role in enhanced binding of various gas molecules (H2, CH4, C2H2, NO, etc. ) and successfully used to promote molecular separation. Figure 1b represents AB confinement within the MOF pores as obtained [a] Dr. S. Gadipelli, Dr. J. Ford, Dr. W. Zhou, Dr. H. Wu, Dr. T. J. Udovic, Dr. T. Yildirim NIST Center for Neutron Research Gaithersburg MD 20899-6102 (USA) Fax: (+1)301-921-9847 E-mail : taner@seas.upenn.edu gsrini@seas.upenn.edu [b] Dr. S. Gadipelli, Dr. J. Ford, Dr. T. Yildirim Department of Materials Science and Engineering University of Pennsylvania, Philadelphia PA, 19104 (USA) [c] Dr. W. Zhou, Dr. H. Wu Department of Materials Science and Engineering University of Maryland, College Park MD, 20742 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100090.

A highly practical route for large-area, single layer graphene from liquid carbon sources such as benzene and methanol

Abstract: Through a detailed systematic study, we determined the parameters critical for high-quality, single-layer graphene formation and developed a straightforward synthesis that requires no explosive hydrogen or methane gas flow. The synthesis is further simplified by using only a liquid carbon source such as methanol. Of over a dozen liquid carbon sources studied, methanol is found to be unique in that it acts as both a carbon/hydrogen source and an inhibitor to amorphous carbon growth. No deposition of amorphous carbon was observed, regardless of vapor pressure, unlike methane and other hydrocarbons. Finally, we describe a protocol to control graphene growth to a single side or selected location on the copper substrate, which is required for most device applications. Using our novel methods, we have prepared high-quality, single-layer graphene samples at the inch scale that have been thoroughly characterized with Raman spectroscopy, optical transmittance, scanning electron microscopy and sheet resistance measurements. Our method is safe, simple, and economical and will be of value to both fundamental researchers and nanodevice engineers.

Sodium Magnesium Amidoborane: The First Mixed‐Metal Amidoborane.

Abstract: Na2Mg(NH2BH3)4 is synthesized by ball milling (200 rpm, He atmosphere, 80 min) of appropriate mixtures of NaH, MgH2, and NH3·BH3.

Spin-phonon coupling and superconductivity in iron pnictides

Abstract: Submitted for the MAR11 Meeting of The American Physical Society Spin-phonon coupling and superconductivity in iron pnictides TANER YILDIRIM, UPENN & NIST, XUHUI LUO, UIC & NIST, SERDAR OGUT, UIC — Early electron-phonon (el-ph) coupling calculations for iron pnictide system based on standard non-spin-polarized perturbation theory indicate that conventional el-ph coupling cannot explain the observed high Tc in these systems. However, the experimental phonon spectrum indicates features which are not produced in the standard linear response non-magnetic phonon calculations. The magnetic phonon calculations clearly indicate that the observed phonon-DOS at room temperature is much closer to the magnetic phonon-DOS rather than non-magnetic DOS and Fe-magnetism must present in the iron-pnictide systems all the time [12]. Thus we need to calculate the magnetic el-phonon coupling with the Fe-spins included before we can rule out any type of phonon-mediated mechanism. In order to carry out such complex self-consistent magnetic el-ph coupling calculations we are developing a finite-displacement method in which both the phonon energies and the corresponding el-ph coupling constant are easily calculated. Implications of our results on the mechanism of superconductivity in iron pnictides will be discussed. Finally, we will compare our calculations with the available phonon energy and linewidth measurements. [1] T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009). [2] T. Yildirim, Physica C 469, 425-441 (2009). Xuhui Luo UIC & NIST Date submitted: 30 Dec 2010 Electronic form version 1.4