Applications of Nuclear Magnetic Resonance to Metal Hydrogen Materials

Transcription

1 Applications of Nuclear Magnetic Resonance to Metal Hydrogen Materials Part I: Basic Principles & General Applications Robert C. Bowman, Jr. and Son-Jong Hwang California Institute of Technology Pasadena, CA Summer School Lectures MH June

2 Outline Part I: Basic Principles & General Applications -Introduction to Hydrogen Storage Materials -Nuclear Magnetic Resonance (NMR) Concepts & Terminology The basics of NMR Solid State NMR fundamentals (Low Resolution) Dynamics via nuclear relaxation times diffusion phenomenon -References Part II: High Resolution (HR) Solid State NMR -Principles & methods of multinuclear spectroscopy (MAS, CP, MQ) -HR-NMR Studies of MH x materials phase compositions, etc. -Summary & Conclusions -References 2

3 Sorption Methods for Hydrogen Storage (Mostly reversible reactions, compact, mass issues) ABsorption (chemisorption) metal hydride family ADsorption (physisorption) zeolites, carbons, etc. [Images from J. Miller, Chromatography: Concepts and Contrasts (Wiley, NY, 1987)] 3

4 Family tree of hydriding alloys and complexes [1,2]. (TM = transition metal) Elements Alloys Complexes Solid Solutions Intermetallic Compounds Other TM Non-TM Other AB 5 AB 2 AB A 2B Borohydrides Alanates Other Other AB 3, A 2B 7 A 2B 17, etc. Nitrides/Imides Misc. Polyhydrides Stable Metastable A x B y O z, etc. Multiphase Quasicrystalline Amorphous Nanocrystalline [1] G. Sandrock, J. of Alloys and Compounds, , 877 (1999). [2] R.C. Bowman, Jr. and G. Sandrock: Metal Hydrides for Energy Storage and Conversion Applications, ANS/ENS 2003 Winter Meeting, New Orleans, LA, Nov. 19, Extreme range of physical & chemical properties (nearly are all solids!)

5 PART I General Background of NMR Spectroscopy and its Applications to Solids 5

6 What s NMR? Nuclear Magnetic Resonance (NMR) spectroscopy is based upon splitting of nuclear spin (I) energy levels in an external magnetic field (B o ), where transitions between levels are induced via radiofrequency (rf) radiation at the Larmor frequency ν I (2π) ν I = γ I B o I = 1/2 Each non-zero spin I has an unique magnetogyric ratio γ I 6

7 Types & Properties of NMR Active Nuclei No NMR for I = 0 nuclei (i.e., 4 He, 12 C, 28 Si, etc.) NMR active nuclei for M-H Studies I = 1/2: 1 H, 3 H*, 13 C, 15 N, 29 Si, etc. I = 1: 2 H, 6 Li, 14 N I = 3/2: 7 Li, 11 B, 23 Na, etc. I = 5/2: 25 Mg, 27 Al, etc. I = 7/2: 45 Sc, 51 V, etc. Figures from Rob Schurko s Introductory Solid State NMR Notes ( 7

8 Foundation of Pulse FT-NMR Spectroscopy Schematic of Fourier transformation of FID in time domain to NMR spectrum in frequency domain. Schematic view of 3 stages in a simple NMR Experiment: A. Magnetic moment precesses around principal axis of applied Zeeman field B o. B. Sample irradiated at Larmor frequency with plane-polarized rf radiation generating magnetic field B 1 causing net magnetization M to incline. A π/2 pulse orients M perpendicular to B o. C. As the spin system coherently precesses in the transverse plane, a voltage is induced in a rf coil to be detected as the FID (Free Induction Decay) signal. Figures from K. J. D. MacKenzie and M. E. Smith, Multinuclear Solid-State NMR of Inorganic Materials (Pergamon, Amsterdam, 2002). 8

9 Basic NMR Spin Echo Experiment Basic pulse sequence (π/2-τ- π) forming a Spin-Echo with effect on the spin packets. * 9

10 Schematic Description of FT-NMR Spectrometers Bruker DSX 500 solids NMR spectrometer (B o = 11.7 T) in the Caltech Solid State NMR Facility A number of solution and solids NMR probes available. Capability of variable Temperature Exp. (-100 o C ~ +150 o C). Schematic of High Field superconducting magnet [Bruker 4 mm triple resonance CP-MAS probe.] 10

11 Overview of Nuclear Spin Interactions in Solids Notation: I = NMR resonant spin and S = Non-resonant spin(s) B o = Zeeman external magnetic field B 1 = rf magnetic field (NMR pulses) 1: Zeeman interaction of nuclear spins 2: Direct dipolar spin interaction 3: Indirect spin-spin coupling (J-coupling), nuclearelectron spin coupling (paramagnetic), coupling of nuclear spins with molecular electric field gradients (quadrupolar interaction) 4: Direct spin-lattice interactions 3-5: Indirect spin-lattice interaction via electrons 3-6: Chemical shielding and polarization of nuclear spins by electrons 4-7: Coupling of nuclear spins to sound fields 11 Figure & next few charts adapted from:

12 Nuclear Spins Couple to External Magnetic Fields via Interaction Tensors: All NMR interactions are anisotropic - their three dimensional nature can be described by second-rank Cartesian tensors, which are 3 3 matrices. Dipolar Interactions 12

13 Chemical Shielding in NMR of Solids Methods for reducing dipolar broadening and CSA effects to yield high resolution spectra from solids where peaks are narrowed to only σ iso will be given in Part II 13

14 Solid State NMR of Quadrupolar Nuclei I = 5/2 Spectra for C Q (A) > C Q (B) Methods for extracting information from NMR of quadrupolar nuclei will be covered in Part II 14

15 Examples of General NMR Studies for some representative Metal Hydrides 15

16 NMR for Crystal Structure & H-Site Locations NMR spectra can be used to determine the location(s) of protons within the host crystal structure as well as detect different phases (i.e., H 2 gas in the solid or metal particles in the hydride). Second Moments are extracted from the time domain signal G(t) where the rigid-lattice dipolar interactions are used. M 2D = (3/5)C II n (1/r mn ) 6 + (4/15)C IS n' (1/r mn' ) 6 NMR is not as powerful to solve crystal structures as neutron diffraction for deuterides but sometimes an advantage when samples are nanocrystalline or amorphous as dipolar terms rapidly decay with spin separations (~1/r 6 ). Most examples involve protons with one or two locations in rather simple structures. Sometimes there can be complications! 16

17 Variable Temperature Proton Spectra for YH 3 This splitting is anomalous! Not predicted for protons in the known P63cm structure for YH 3 should be just a single peak from dipolar interactions in rigid lattice. Dipolar M 2d for YH 3 in rigid lattice limit (T < 310 K) Reference: G. Majer, et al., Phys. Rev. B 70, (2004) 17

18 Reliable Rigid Lattice 1 H Line Shape Measurements Magic-Echo method will give proton spectra without distortions due to very large dipolar interactions seen in many MH x coming from instrumental recovery limitations. Correct proton Line Shape using Magic Echo peak Line Shape distorted by a 5.0 µs shift of Magic Echo M 2 (E) = 17.5 Oe 2 M 2 (D) = 18.3 Oe 2 Line Shape distorted by a 11.6 µs shift of Magic Echo gives an erroneous doublet spectra as found from a FID signal with similar dead time S. K. Brady, et al., Phys. Rev. B 72 (2005)

19 Many Factors Influence MH x Absorption/Desorption Kinetics Heat transfer (bed design) H 2 gas permeability in powder Surface Reactivity/Activation Native oxide layers (barrier) Selective oxidation (AB 2, AB 5 ) Poisoning (CO, SO 2, etc.) Surface area/decrepitation Permeable layers (Pd, fluorides) Diffusion processes Grain boundaries/microfractures Bulk diffusion (D bulk ) in metal/hydride phases Horizontal Dash Line Indicates: D Bulk = 1.67 x 10-8 cm 2 /s = d 2 /6t = (10 µm) 2 /6(10 s) Condition when D rarely limits Diffusion Coefficient (cm 2 /s) Bulk Diffusion Constants in MH x 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 1.E-11 1.E-12 1.E-13 1.E-14 1.E-15 1.E-16 1.E-17 MgH2 PdH0.7 LaH2.92 ZrH1.58 ZrV2H5.0 TiFeH1.0 Mg2NiH4 LaH2.00 LaNi5H6.0 a-zr3rhh3.5 b-tih0.70 D = 10-8 cm2/s /T (K -1 ) reaction rates in activated MH x [Bowman & Fultz, MRB27 (2002) 688.] Note: While researchers can change/improve many kinetic parameters, but usually do NOT improve the intrinsic diffusion constants except by raising operating temperature! 19

20 Direct Diffusion Constant Measurements via NMR When D(T) > m 2 /s, it can be directly measured without using any models G. Majer, et al., Phys. Rev. B 57 (1998)

21 NMR Relaxation Times & Diffusion (Very General - Focus on Hydrogen) Pulsed NMR Sequences: T 1 T 2 T 1ρ T 1D Spin-Lattice: from 180º-τ-90º and (90º-t) n -τ-90º Spin-Spin: from free induction decays (T 2 *), 2-pulse echoes (T 2 '), and Carr-Purcell-Meiboom-Gill (CPMG) echo trains (T 2m ). Rotating-Frame: from CW (not chopped) spin-locking. Dipolar: (T 1 of dipolar-ordered state, essentially T 1 in zero field) Correlation time, τ c : 1/T 2 = K * M 2 * τ c Mean residence time, τ = 1/ω Η : τ c = τ / 2 T 1, T 2 depend on τ C and M 2 (M 2 = mean-squared dipolar interaction) Diffusion Coefficient: D(T) = f<l 2 >/(6 τ) Activation energy (Arrhenius), E A : 1/ τ = (1/τ o )exp(-e A /kt) τ O -1 = attempt frequency 21

22 Temperature Dependence of NMR Relaxation Rates for an Arrhenius Diffusion Process Idealized BPP Model T 1d = Dipolar (i.e., protons) T 1e = Conduction electrons (metals) T 1p = Paramagnetic species 22

23 Examples: LaNi 5 H 6.0 & LaNi 4.8 Sn 0.2 H 5.8 Diffusion in LaNi 5 H x involves superposition of localized hops within subset of protons in the mid-plane region of the Ni(3) layer as well as long range motion that cause deviation from the Arrhenius relation for the proton relaxation times. G. Majer, et al., Phys. Rev. B 57 (1998)

24 Proton Relaxation Times in LaNi 5 H 6.8 In LaNi 5 H x magnetic precipitates (presumably Ni) distort field uniformity and keep T 2 * of FID very short (~10 µs). The T 2 data is from two-pulse spin echoes. The T 1 data are from MHz (upper) and MHz (lower). [M. P. Mendenhall, et al., Rate of Hydrogen Motion in Ni-Substituted LaNi 5 H x from NMR, J. Alloys Compounds (2007)

25 Guide to Interpreting NMR Results ω H rate of H hopping motion Motional averaging of local dipolar fields is evident as increase of T 2 with temperature. At onset, ω H ~ 10 5 s -1 (approx. the low-t linewidth). At T 1ρ minimum, ω H ~ s -1 (about 1.6 γb 1 ). At T 1 minimum at 21 MHz, ω H ~ s -1 (about 1.6 γb 0 ). T 1D is approx the mean time between hops, T 1D ~ 1/ω H, a strong-collision result. 25

26 Measuring Very Slow Motion T 1D Dipolar order = spins oriented along local dipolar field. Dipolar field varies from site-to-site, almost 100%. If spin jumps to neighbor site, it is no longer oriented along the local dipolar field. Signal decays ~exp(-τ/t 1D ) and T 1D 1/ω H. Measured T 1D detects motions when D(T) is in range ~ cm 2 /s, which is much slower (i.e., 3-6 orders of magnitude) than other relaxation times. J. Jeener and P. Broekaert, Phys. Rev. 157 (1967)

27 Proton NMR Studies of Diffusion in the Mg-Sc-H System Illustrates Effect of Structure Conradi, et al. [J. Alloys Compounds (2007) ] have measured H hopping rate ω H using various proton relaxation times for Mg 65 Sc 35 Pd 2.4 H 220 and compared to MgH 2, ScH 2, and LaNi 5 H x. Hydrogen diffusion in MgH 2 could only be detected using T 1D times 27

28 Proton Hopping Times Comparing ω H give increasing mobility in the sequence: MgH 2, ScH 2, MgScH x, and LaNi 5 H 6.8 E a (ev) LaNi 5 H Mg 65 Sc 35 H ScH MgH Fits are ω H = s -1 exp(-e a /kt) Data points are from T 1 and T 1ρ minima, onset of increase of T 2, and T 1D (ω H = 1/T 1D ). 28

29 Summary & Conclusions for Part I Solid State NMR is a powerful & versatile method to assess numerous properties of hydrogen storage materials Multi-nuclear spectroscopy (hydrogen isotopes & host species) Monitor phase compositions and transformation from spectra and relaxation times. Local structure & site occupancy from dipolar interactions of rigid lattice spectra Diffusion behavior over large dynamic range Direct measurement (D > m 2 /s) via Pulsed Field Gradients Several types of relaxation times are available to cover wide range of rates; however, quantitative analyses are difficult for complex materials with multiple diffusion processes and/or complicated structures. 29

30 Reviews on Mostly Conventional Solid State NMR Studies of Metal Hydrides R. M. Cotts, Nuclear Magnetic Resonance on Metal-Hydrogen Systems in Hydrogen in Metals I Basic Properties, edited by G. Alefeld and J. Völkl, Topics in Appl. Phys. Vol. 28 (Springer, Berlin, 1978) pp R. C. Bowman, Jr., Hydrogen Mobility at High Concentrations in Metal Hydrides, edited by G. Bambakidis (Plenum, New York, 1981) pp R. C. Bowman, Jr., NMR Studies of the Hydrides of Disordered and Amorphous Alloys in Hydrogen in Disordered and Amorphous Solids, edited by G. Bambakidis and R. C. Bowman, Jr. (Plenum, New York, 1986) pp D. Richter, R. Hempelmann, and R. C. Bowman, Jr., Dynamics of Hydrogen in Internetallic Hydrides in Hydrogen in Intermetallic Compounds II Surface and Dynamic Properties, Applications, edited by L. Schlapbach, Topics in Appl. Phys. Vol 67 (Springer-Verlag, Berlin, 1992) pp R. G. Barnes, Nuclear Magnetic Resonance in Metal Hydrogen Systems in Hydrogen in Metals III Properties and Applications, edited by H. Wipf, Topics in Appl. Phys. Vol. 73 (Springer, Berlin, 1997) pp R. Hempelmann and A. Skripov, Hydrogen Motion in Metals in Hydrogen-Transfer Reactions, edited by R. L. Schowen, J. P. Klinmann, J. T. Hynes, and H. H. Limbach (Wiley-VCH, Weinheim, 2007) pp