Why XAS? What can we learn from XAS?
Why XAS? What can we learn from XAS? • Element specific • Metal valence during synthesis and reaction • Sensitive to low concentrations • Metal coordination (0.01-0.1 %) • Applicable under extreme conditions (high-pressure, high temperature, operando) • Applicable to gasses, liquids and solids (+ surfaces, buried interfaces, impurities, etc.) Binuclear centers, (very small) cluster sizes • Metal site symmetry • d-band occupation (3d, 4d or 5d; metal versus oxide, valence) • 20 nm microscopy, 50 ps time-resolved • Local geometric information • Local electronic information 2p edges 3d Normalized Intensity 1 4 3d 2 3d 3 3d 4 3d 2 3d 6 3d 1 3d 8 3d 3 5 CrIII in Al2O3 SrTiO3 0 Calculated pre-edges 3dN→2p53dN+1 • Single configuration • Atomic values for intra-atomic interactions • Cubic crystal field of 1.2 eV 7 3d 0 9 MnO 1 q PRB 42, 5459 (1990) Normalised Intensity 5 Metal L edges 0 -6 -4 -2 0 2 4 6 8 10 12 14 16 635 640 645 650 Energy (eV) Relative Energy (eV) Fe/ZSM5 • proposed for selective oxidation of hydrocarbons ¾ Soft X-ray absorption • proposed for deNOx in Heterogeneous Catalysis • soft X-ray absorption • STEM-EELS Cat. Today 100, 228 (2005) 1 Soft X-rays: bulk and surface probe Fe/ZSM5 Soft X-rays: Penetration depth: Fluorescence yield: Electron yield (escape depth): 100 – 1000 eV 10 µm > 10 µm 4 nm + 4. 5 V InIn-situ Soft XX-ray Absorption InIn-situ Soft XX-ray Absorption manipulator 150 m m ter hea mass spectrometer Fe2O3 butterfly-valve hυ 150 mm sample foreline pump gas inlet from MFC Fe/ZSM5 (O2) Plattenventil UHV gate valve turbo pump φ 100 mm J. Phys. Chem. B. 107, 13069 (2003) Redox behavior of Fe/ZSM-5 J. Phys. Chem. B. 107, 13069 (2003) Redox behavior of Fe/ZSM-5 J. Phys. Chem. B. 107, 13069 (2003) 2 Conclusions Fe/ZSM5 TEMTEM-EELS of calcined Fe/ZSM5 1.0 fe4 1. Fe/ZSM5 in O2: FeIII, octahedral Y Axis Title 0.8 2. Weaker Fe-O bonding than Fe2O3: 10Dq ~ 1.0 eV 0.6 0.4 3. Fast auto-reduction in He at 25 °C 0.2 0.0 700 705 710 715 720 725 730 X Axis Title 4. Reversible oxidation/reduction at 350 °C. B >> Extremely active iron redox system 180000 Y Axis Title 160000 5. Calcined Fe/ZSM5 has Fe2O3 nanoparticles 140000 120000 100000 80000 60000 530 535 540 545 550 555 560 565 X Axis Title 570 Cat. Today 100, 228 (2005) 5. No spectra for 0.25 wt% FeZSM5 Cat. Today 100, 228 (2005) InIn-situ XX-ray Absorption in Catalysis Co/TiO2 FischerFischer-Tropsch catalysts Production of hydrocarbons from CO + H2 (Syngas) 1926: Fischer and Tropsch Charcoal Gas Biomass • used for Fischer-Tropsch synthesis • transformation of CO + H2 to hydrocarbons Partial oxidation Steam reforming Co-based catalyst Syngas (Clean liquid) Synfuels 220°C / ↑ pressure • 5B$ plant being build at Qatar • make clean fuels from natural gas or biomass • Study Mn-doped Co/TiO2 surface • soft X-ray absorption • Follow Co and Mn L2,3 edge • STEM-EELS • Reduction: temperature from RT to 425 º under 5 mbar H2 (Fernando Morales, UU) Co/TiO2 FischerFischer-Tropsch catalysts Co/TiO2 FischerFischer-Tropsch catalysts MnOx HDP-CoMn/TiO2 catalyst Bulk Co3O4 L3 Co0 CoO 425°C 425°C 385°C 350°C 350°C 300°C 300°C 250°C 250°C Co3O4 Bulk Co3O4 reduces to pure Co0 RT RT Co/TiO2 catalysts reduce to CoO + Co0 Cobalt average valency 3 Co3O4 Co3O4 2.5 Co3O4 TiO2 CoO 2 Reduction 1.5 δ+ MnO 1 Co0 Bulk Co3O4 0.5 HDP-CoMn Co0 0 0 100 200 300 400 Reduction temperature (Celsius) 500 Co0 TiO2 Mn promoted catalyst Mn is preventing CoO reduction to Co0 3 Co/TiO2 FischerFischer-Tropsch catalysts Co/TiO2 FischerFischer-Tropsch catalysts Quantify EELS spectra of Ti, Mn and Co 10nm Co/TiO2 FischerFischer-Tropsch catalysts Co/TiO2 FischerFischer-Tropsch catalysts Before reduction (Co,Mn)3O4 particles After reduction Co metal + MnO particles PCCP 7, 568 (2005) J. Catal. Catal. 230, 310 (2005) Transmission X-ray Microscopy icroscopy (TXM) Transmission X-ray Microscopy icroscopy (TXM) 20 nm soon 10 nm 200 nm 1 1 bar 250° 250°C 0.2 eV 0 780 785 790 Energy (eV) 795 4 Co/TiO2 FischerFischer-Tropsch catalysts STEMSTEM-EELS soft XAS UHV Co/TiO2 FischerFischer-Tropsch catalysts STEMSTEM-EELS 10 mbar 0.5 nm / 0.3 eV Bulk / 0.2 eV TXM 1 atm / 450 °C UHV 15x15 nm / 0.2 eV 0.5 nm / 0.3 eV 1 10 nm 10 nm Conclusions Co/TiO2 0 780 785 790 Energy (eV) 795 Charge Transfer in EELS CuIII 1. TiO2 and Mn prevent complete reduction 30% 3d8 2. Mn and Co segregate (depending on preparation method) 3. TXM-XAS allows to track single particle during 1A 1 3d8 ∆<0 catalysis 30% 3d8 3d9L 3A 2 Chem. Phys. Lett. Lett. 297, 321 (1998) EELS and Electronic Structure • Directional variations of 3d-electrons: symmetry: 2S+1LJ, crystal field, charge transfer interplay spin-orbit, exchange, distortions • Hybridization of correlated electrons DFT + Multiplet approaches FeIII(tacn)2: Ground state: 3d5 + 3d6L Energy of 3d6L: Charge transfer energy ∆ 3d6L superconductivity, magnetism, Kondo mixed valence, orbital orderings ∆ 3d5 2p53d7L ∆+U-Q ≈ ∆ 2p53d6 5 DFT + Multiplet approaches DFT + Multiplet approaches ¾ Projection of charge transfer simulation to ‘constituents’ FeIII(tacn)2: Ground state: 3d5 + 3d6L Different mixing for different symmetries ¾ Comparison to DOS from DFT calculation FeIII(tacn)2 • low-spin 3d5 [t2g5] • admixture of 3d6L [t2g5eg1] (eg mixing) • admixture of 3d6L [t2g6] (t2g mixing) JACS 125, 12894 (2003) JACS 125, 12894 (2003) LigandLigand-metal Charge Transfer DFT + Multiplet approaches ¾ Projection of charge transfer simulation to ‘constituents’ FeIII: Ground state: 3d5 + 3d6L ¾ Comparison to DOS from DFT (ADF) calculations M C filled σfilled emptyempty d or p 3d6L 2p53d7L ∆ ∆+U-Q ≈ ∆ 2p53d6 3d5 JACS 125, 12894 (2003) JACS, 128, 10442 (2006) π-bonding + MetalMetal-ligand Charge Transfer FeIII: Ground state: 3d5 + 3d6L + 3d4L 3d4L ∆π-U+Q ≈ ∆π + 2 ∆π 3d6L ∆ 3d5 2p53d7L ∆+U-Q ≈ ∆ - 2 2p53d6 JACS, 128, 10442 (2006) FeIII(tacn)2 10 Normalized Absorption 2p53d5L N C M N 8 Fit X Series2 FeIII(CN)6 6 4 2 0 700 -2 705 710 715 Energy (eV) 720 725 730 JACS, 128, 10442 (2006) 6 Conclusions: Soft XX-ray absorption Conclusions: Metal L edges 8 t2g Fe L edge of FeIII(CN)6 eg ? eg LS+ D3D ? 0 705 710 715 720 Energy (eV) 725 low-spin FeIII 3d5 [2T2] t2g one hole eg empty ¾ ¾ ¾ ¾ ¾ DFT works well for oxygen K edge LFM works well for metal L edge CTM needed for XPS CTM needed for Cu3+ Add MLCT for π-bonding JACS, 128, 10442 (2006) 7
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