X-Ray Electron Spectroscopy (XPS)

1. X-Ray Electron Spectroscopy (XPS) • Applications: • catalyst composition • chemical nature of active phase • dispersion of active phase • Standard technique in catalyst characterisation levels h1 h2 XPS XRF AES Catalysis and Catalysts - XPS

2. Binding Energy • Conservation of energy • EB depends on chemical environment: • element • valence state • coordination (type of ligands, number, tetrahedral, octahedral. …) corrects for potential difference between sample and analyser h = EB + Ekin + Ecorr kinetic energy of emitted electron energy of photons binding energy of emitted electron Catalysis and Catalysts - XPS

3. XPS Equipment Hemispherical electrodes Electron energy analyser Slit Slit X-ray source Electrostatic electron lens Al Electron detector e- Photon Sample Number of emitted electrons measured as function of their kinetic energy Catalysis and Catalysts - XPS

4. XPS Survey of Al2O3 O 1s Al2O3 Mg K 1253 eV O(KVV) 745.3 766.7 N(E) 780.6 Auger transitions E O(KVV) AUGER 805 765 725 C 1s Al 2s Al 2p Satellite Ar 2p3/2 Ar 2s O 2s 1000 900 800 700 600 500 400 300 200 100 0 Catalysis and Catalysts - XPS EB

5. Effect of Valence on Chemical Shift - Al and Al2O3 2p 72.85 Al Al2O3 2p 74.7 86 76 66 ‘Binding energy’ (eV) Peak position determined by element and valence Chemical information on elements Catalysis and Catalysts - XPS

6. 77 eV Influence of F Content on XPS Spectrum of Al2O3 75 eV Al (2p) 1.2 at.% F 6.5 % 10.6 % 20.6 % 75.0 % (AlF3) 82 72 eV Catalysis and Catalysts - XPS

7. Effect of Mo Valence on Chemical Shift and Multiplet Splitting 3d5/2 222.7 Mo MoO3 3d5/2 232.65 3d3/2 3d3/2 3.15 3.2 240 230 220 240 230 220 Binding energy (eV) Binding energy (eV) Catalysis and Catalysts - XPS

8. 100 80 60 Intensity (%) 40 20 8 4.1 0.6 0.5 0.5 0 1253.6 1262.0 1263.8 1271.1 1273.6 1302.1 Photon energy (eV) Satellites in XPS Plots Causes: • non-monochromatic X-Ray source • contamination of X-Ray source • excitation of ion by leaving electron: ‘shake-up’ Mg X-ray emission spectrum: Catalysis and Catalysts - XPS

9. Shake-up Lines in Cu 2p Spectrum 2p3/2 2p1/2 Cu CuO Shake-up lines CuSO4 970 960 950 940 930 Catalysis and Catalysts - XPS Binding energy (eV)

10. Charging of Catalyst Samples Al 2p emission line of Al in alumina: charging results in a shift of 3.3 eV Al2O3 2p 74.7 3.3 86 76 66 Binding energy (eV) Catalysis and Catalysts - XPS

11. Where do Electrons come from? Intensity of a peak depends on: • composition • position where electrons are emitted Electrons interact with the solid • only a fraction of the emitted electrons reach detector with original kinetic energy • the longer the distance, the higher the number of “lost” electrons z = distance travelled by electrons  = escape depth Catalysis and Catalysts - XPS

12. Inelastic Mean Free Path Au 10.0 5.0 1.0 0.5 0.3 Au Au Au Ag Au Mo Ag Inelastic Mean Free Path (nm) Au Ag Au C W C Au Au Ag Be Be Ag Ag Be Be Ag Ag Be C Mo Fe W Ag Ag Mo Be 2 5 10 50 100 500 1000 2000 Electron energy (eV) Catalysis and Catalysts - XPS

13. Is XPS a Bulk or a Surface Technique? 5000 4000 3000 XPS yield 2 nm 2000 1000 0.5 nm 0 0 2 4 6 8 10 12 Thickness of absorbing layer z (nm) XPS is a surface-sensitive technique Catalysis and Catalysts - XPS

14. Monolayer growth: curve “A” Particle growth: curve “B” I A - monolayer (can be modelled theoretically) p XPS I particle size s B - particle growth p /s Bulk Catalyst Structure and XPS Curves Catalysis and Catalysts - XPS

15. Quantitative Model for XPS Signal Intensities • Based on: • model catalyst (parallel sheets of support with promoter crystals) • X-ray intensity uniform throughout catalyst particle • Lambert-Beer type equation for absorption of radiation Model catalyst c = crystal size (promoter) t = thickness ratio of cross sections atomic ratio Catalysis and Catalysts - XPS ratio of detector efficiencies

16. Formulation of Model • one support layer, thickness t • one layer of active phase (“promoter”) • allow for position of layers: surface area atoms/volume cross section (electrons/photons.s.at) fractional coverage of support detector less intensity then Catalysis and Catalysts - XPS

17. Quantitative Model for XPS Signal Intensities Model catalyst c = crystal size (promoter) t = thickness Dispersion coupled with c Highest dispersion corresponds to c 0 (“monolayer” catalyst) c can be calculated from XPS data Catalysis and Catalysts - XPS

18. Calculation of Crystallite Size limit c  0 y = x  a monolayer calculation particle size calculation experimental p/s Catalysis and Catalysts - XPS

19. Example: Re2O7/Al2O3 escape depth  = 1.3 nm I(Re 4f) I(Al 2p)  = 1.8 nm 1.0 0.5 0 0.02 0.04 (Re/Al)bulk Catalysis and Catalysts - XPS

20. Other Catalyst Models Randomly Oriented Support Layers Inhomogeneous promoter distribution: egg-shell catalyst Catalysis and Catalysts - XPS

21. Example: MoO3 and WO3 supported on SiO2 and Al2O3 MoO3/Al2O3 MoO3/SiO2 2 1 1 0.5 WO3/Al2O3 WO3/SiO2 2 1 1 0.5 Catalysis and Catalysts - XPS

22. I(Pt 4f) I(Si 2p) Pt/SiO2 - XPS intensities 0.25 Monolayer prediction 0.2 0.15 Dispersion 34% ? 0.1 Experimental points 0.05 0 0 0.002 0.004 0.006 0.008 0.01 0.012 (Pt/Si)bulk Catalysis and Catalysts - XPS

23. Intensity Decreases with Crystallite Size Fraction monolayer intensity 1 literature Catalysis and Catalysts - XPS

24. Fraction of monolayer intensity 1 Catalysis and Catalysts - XPS

25. 12 10 8 c (nm) 6 4 2 0 0 1 2 3 4 5 6 7 a1 Catalysis and Catalysts - XPS

26. Example: Fluorinated Alumina powdered AlF3 Al2O3 coarse 80 78 76 74 eV Catalysis and Catalysts - XPS

27. Summary of XPS XPS can give valuable information regarding: • catalyst composition, i.e. the elements present • chemical nature of the elements • chemical nature of neighbouring (co-ordinating) atoms • dispersion of active phase and support • location of active phase in the particle Catalysis and Catalysts - XPS