Experimental Setup XRD Powder Diffraction

Size: px

Start display at page:

Download "Experimental Setup XRD Powder Diffraction"

Ophelia Brooks
7 years ago
Views:

1 Structure Principle of X-Ray Diffraction X-Ray Diffraction (XRD) Crystalline materials (long range order) Neutron scattering H/D containing molecules X-Ray absorption spectroscopy (XAS) Amorphous materials (short range order) Composite materials d θ θ Interference of photons scattered by ordered structures Point lattice with crystalline long range (min. 10 unit cells) Long range order Interference positive for: n λ = d sinθ Bragg s Law X-Ray Powder Diffraction Experimental Setup XRD Powder Diffraction a. b A fraction of the crystallites will be orientated to satisfy the Bragg condition for each set of planes (hkl). These crystallites will be randomly oriented around the incoming beam, so the diffracted beams forms a cone around the incident beam at the angle of θ. KCl NaCl

2 Position of XRD lines XRD: Identification of structure Miller indices Powder XRD of α-quarz Neighboring atoms/ Free valences 1/0 8/4 7/5 9/3 Features of a powder XRD and their origin Application of X-ray diffraction In situ characterization of catalysts MO MnO reference 60 min 40 min Reduced Fe-MnO catalyst 0 min After CO hydrogenation Fe (bcc) converts into Fe-carbides Formation of Pd hydride during Benzene hydrogenation

3 Formation of metal oxide phases in situ XRD Properties of neutrons Mass m=1.675 x 10-4 g Charge = 0 Spin =½ Magnetic moment µ = nuclear magnetons Neutron wavelength range: Å, 1 mev=8.065 cm -1 h π λ = = mv k Reduction of a supported Cu catalysts Wave vector (mag.) Neutron energy Neutron momentum π mv k = = λ k 1 E = mv = m p = mv = k Interaction of neutrons with matter Incoherent and coherent scattering cross-sections Momentum Transfer p = mv = Q = Q k ( k ) 0 k f k 0 Elastic scattering k f = k 0 ћω=0 0 only momentum (Q) is transferred π mv k = = λ Energy Transfer 1 k E = mv = ω = E E 0 m = m ( k k ) k f 0 f Inelastic scattering k f < k 0 (energy loss) H D N Ni Incoherent scattering cross-section Coherent scattering cross-section Absorption cross-section Coherent scattering interference effects between waves scattered from different nuclei Structure and motion of atoms relative to each other Incoherent scattering deviation of individual atom positions from the mean potential Motion of single atoms

4 Elastic neutron scattering Experimental setup Elastic neutron scattering Elastic Neutron scattering is a coherent scattering process analogous to X-ray diffraction. k f = k 0 only momentum is transferred (no energy analysis) Incoherent scattering increases background use deuterated substances Neutrons are scattered by the nuclei, X- rays by the electrons Light nuclei (e.g., H, C) are easier to locate in structures with heavy atoms by neutron diffraction Neutron scattering densities for C 6 D 6 adsorbed on ZSM5, (top) 4 mol/uc, (bottom) 8 mol/uc Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) Secondary Sources FRM-II Ø=43 mm, l=700 mm Thermal neutrons: from D O moderator Hot source: Block of graphite (0 cm diameter, 30 cm high) heated by the gamma radiation to a temperature of ~ 900 K. Spectrum from 100 mev to 1eV Cold source: Liquid D moderator (0 l) temperature about 5 K at a distance of 40 cm from the core axis. The cold neutron spectrum peaked around 5 mev

5 Neutron energies at the FRM-II Spallation source ISIS Energy of neutrons and their application in a research reactor. The size of the colorized area is proportional to the amount of neutrons available for the application. X-Ray absorption spectroscopy X-Ray absorption edge Absorption of X-ray's and promotion of a corelevel electron to continuum

6 X-Ray absorption near edge structure XANES X-Ray absorption near edge structure Density of states (DOS) TiO Fe O 3 Extended X-ray absorption fine structure EXAFS EXAFS Single scattering plane wave approximation Fk ( ) N χ ( ) sin( ( )) r k k = kr + φ k k e σ Constructive (in phase) Destructive (out of phase) FFT outgoing electron wave backscattered electron wave Short range order 0.4 Due to φ(k) the distances are 0.3 shifted to smaller values!!! r[å]

7 . Cluster size and scattering contributions Location of Zn + cations in zeolites FT mag g NiO experimental i t l NiO 1 shell NiO shells NiO 3 shells 0.06 NiO 7 shells r[a] Coordination sites for Zn + in zeolite Beta 6-membered rings 5-membered rings 4-membered rings Preferential location of Zn + in BEA Bimetallic Ni-Rh catalysts (Ni-K edge) Sample N r Zn- BEA NiO mag FFT Zn-BEA ZnO Zn 6MR Zn 5MR FFT Ni/HTC NiRh/HTC Bimetallic catalyst 5% Ni, 0.89 wt% Rh r [Å] Metall Zn 4MR Preferential location of fzn + at t6mr 6-MR positions Ni foil R(Å) Coordination parameters for Ni containing samples sample Ni-O Ni-Ni N R (Å) Δσ (Å ) N R (Å) Δσ (Å ) Ni foil 1.49 NiO Ni/HTC NiRh/HTC

8 Bimetallic Ni-Rh catalysts (Rh-K edge) X-Ray absorption near edge structure Electronic properties of d-metals Rh O 3 Fermi level Rh/HTC Bimetallic catalyst 5% Ni, 0.89 wt% Rh Electron deficient 5d 5/ 5d 3/ FFT NiRh/HTC Ni Pt particles Rh Rh foil R(Å) p 3/ L III p 1/ Coordination parameters for Rh containing samples Sample Rh-O Rh-Rh Rh-Ni N R (Å) Δσ (Å ) N R (Å) Δσ (Å ) N R (Å) Δσ (Å ) Rh foil 1.68 RhO Rh/HTC NiRh/HTC Electron deficient particles show a higher peak above the absorption edge L II Determination of oxidation state by XANES Characterization of S-species (XANES S K-edge) XANES for ZnS, ZnSO 3 and ZnSO 4

9 Comparison EXAFS and XANES EXAFS Information level Single scattering dominates Structural environment Mathematical description using Number and kind of Neighbors phase shifts and amplitudes form Distance experiment or theory Disorder XANES Electronic transitions Multiple scattering Exact description based on quantum-mechanical calculations l Interpretation of characteristic spectral features using references (peak fitting, PCA, correlation spectroscopy) Oxidation state Electronic information DOS in the final state Geometry, distortions Synchrotron Experimental setup XAS Sample Cell Reference Slits Ionization Chambers Monochromator GeV 0 ma Pulslänge: 0.17 ns Pulsbastand 0 ns Radius 15.3m The first accelerators (cyclotrons) were built by particle physicists in the 1930s. The nucleus of the atom was split using the collision of high energy particles. From the results of these collisions the physicists tried to deduce the laws of fundamental physics that govern our world and the whole of the universe. Synchrotron radiation was seen for the first time at the General Electric in the USA in 1947 in a different type of accelerator (synchrotron). It was first considered a nuisance because it caused the particles to lose energy, but in the 1960s exceptional properties as light source were recognized. SRs Daresbry UK MeV 0 ma 600 MeV 0 ma Radius 5 m

10 European Synchrotron Radiation Facilities Development of available X-ray flux ESRF DESY Generation of X-Ray radiation Design of in situ XAS cells Plug flow reactor Bending Magnet Wiggler Undulator CSTR type reactors (Continuous stirred tank reactor) Sample cooling Gas inlet Sample heating Free electron laser Capton windows Gas outlet

X-Ray Diffraction HOW IT WORKS WHAT IT CAN AND WHAT IT CANNOT TELL US. Hanno zur Loye

$X-Ray Diffraction HOW IT WORKS WHAT IT CAN AND WHAT IT CANNOT TELL US. Hanno zur Loye$ X-Ray Diffraction HOW IT WORKS WHAT IT CAN AND WHAT IT CANNOT TELL US Hanno zur Loye X-rays are electromagnetic radiation of wavelength about 1 Å (10-10 m), which is about the same size as an atom. The