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X-Ray Diffraction

X-Ray Diffraction. Principle: interference of photons by reflection by ordered structures. Interference positive for:. Bragg’s law. . n  = 2 d sin . . 2 . . So, identification compound. d. d sin . In real catalysts: line broadening

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X-Ray Diffraction

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  1. X-Ray Diffraction Principle: interference of photons by reflection by ordered structures Interference positive for: Bragg’s law  n = 2d sin   2   So, identification compound d d sin  In real catalysts: line broadening the smaller the crystals the more line broadening So, from XRD crystallite diameter D can be estimated D = K / (b cos) b = peak width K = constant; usually ca. 1 Catalysis and Catalysts - X-Ray Diffraction (XRD)

  2. Schematic of X-Ray Diffractometer Ionisation chamber rotates 2 degrees when the crystal rotates  Crystal  XRD patterns: fingerprints of ordered solids XRD: unambiguous information on crystal structure 2 X-ray source Detector Catalysis and Catalysts - X-Ray Diffraction (XRD)

  3. a. b. 111 200 200 220 220 222 311 222 400 400 Photographic film 420 331 422 Diffraction line 420 Undeviated X-rays 422 440 511 600 333 442 440 620 531 622 600 442 444 Sample 620 640 533 642 622 444 Incident X-rays 711 551 640 Active Phase of Industrial Catalysts is Microcrystalline 2 = 180o 2 = 0o X-Ray Diffraction Data: KCl NaCl Catalysis and Catalysts - X-Ray Diffraction (XRD)

  4. 60 50 40 30 20 10 2 (degrees) X-Ray Diffraction Patterns WO3/SiO2-W wt% WO3 100 37 WO3 (1.5 -100 wt%) WO3/SiO2 20 12 6 3 1.5 WO3/Al2O3-D wt% WO3 29 WO3 (0 -29 wt%) 13 WO3/Al2O3 7.4 3.6 1.8 0.9 0 70 60 50 40 30 20 10 2 (degrees) Catalysis and Catalysts - X-Ray Diffraction (XRD)

  5. Solid-State Diffusion of Co into Al2O3 9.1 wt% CoO/Al2O3 * bulk CoAl2O4 Calcination T: o 1290 K 1025 875 675 380 n m l k j i h g f e d c b a 80 70 60 50 40 30 20 2 (degrees) Catalysis and Catalysts - X-Ray Diffraction (XRD)

  6. 2-line position of “Al2O3 and CoAl2O4” diffraction line at about 80 degrees Above 900 K the intensity of spinel lines increases, Al2O3 lines shift: 79.6 79.4 79.2 79.0 78.8 2 (degrees) 600 800 1000 1200 Calcination temperature (K) Conclusion: above 800 K Co2+ diffuses into the bulk Catalysis and Catalysts - X-Ray Diffraction (XRD)

  7. Thermal Stability of NiO-WO3/Al2O3 • Does Ni2+ diffuse into alumina? • If so, does WO3 influence the rate of diffusion? unit cell size 2d sin  = n T Catalysis and Catalysts - X-Ray Diffraction (XRD)

  8. Change of Unit Cell Size vs Calcination Temperature 791.0 792.7 794.4 796.0 797.7 NiO(4.1)/Al2O3 a NiO(5.1)WO3/Al2O3 calculated 675 875 1075 Calcination temperature (K) Catalysis and Catalysts - X-Ray Diffraction (XRD)

  9. Calculated Concentration Profiles of Ni in Al2O3 Crystallites t = 0 5 4 3 2 1 0 a 875 K 925 K 975 K 1025 K Al2O3 b C/Ce NiAl2O4 c d 0 1 2 3 4 5 6 r (nm) 251 kJ/mol 2.910-9 m2/s Catalysis and Catalysts - X-Ray Diffraction (XRD)

  10. Summary of XRD • A “must” in catalyst characterisation • structure of crystalline phases (nm range) • Crystal dimension • Hydrotreatment: • diffusion of Ni and Co follows Fick’s law • Ni rate of diffusion not influenced by W oxide • Ni and Co diffusion during reaction negligible Catalysis and Catalysts - X-Ray Diffraction (XRD)

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