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

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

Catalysis and Catalysts - XPS XPS Survey of Al 2 O 3 N(E)N(E) E EBEB O(KVV) O 1s O 2s Al 2s Al 2p Ar 2p 3/2 Ar 2s C 1s Satellite O(KVV) Auger transitions AUGER 1253 eV Al 2 O 3 Mg K 

Catalysis and Catalysts - XPS 2p p Effect of Valence on Chemical Shift - Al and Al 2 O 3 Peak position determined by element and valence Chemical information on elements Al 2 O 3 Al ‘Binding energy’ (eV)

Catalysis and Catalysts - XPS Influence of F Content on XPS Spectrum of Al 2 O 3 Al (2p) 1.2 at.% F 6.5 % 10.6 % 20.6 % 75.0 % (AlF 3 ) eV 75 eV 77 eV

Catalysis and Catalysts - XPS Effect of Mo Valence on Chemical Shift and Multiplet Splitting MoO 3 3d 3/2 3d 5/ Binding energy (eV) Mo 3d 3/2 3d 5/ Binding energy (eV)

Catalysis and Catalysts - XPS 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: Photon energy (eV) Intensity (%)

Catalysis and Catalysts - XPS Shake-up Lines in Cu 2p Spectrum Shake-up lines 2p 3/2 2p 1/2 Cu CuO CuSO 4 Binding energy (eV)

Catalysis and Catalysts - XPS Charging of Catalyst Samples 2p Al 2 O 3 Binding energy (eV) Al 2p emission line of Al in alumina: charging results in a shift of 3.3 eV

Catalysis and Catalysts - XPS 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 Inelastic Mean Free Path Electron energy (eV) Inelastic Mean Free Path (nm) Au Ag Au Ag Au Ag Mo Be Ag W Be Fe Be C Ag Mo Ag Be Au C C Be Mo W

Catalysis and Catalysts - XPS Is XPS a Bulk or a Surface Technique? XPS is a surface-sensitive technique Thickness of absorbing layer z (nm) XPS yield 2 nm 0.5 nm

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

Catalysis and Catalysts - XPS 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 detector efficiencies ratio of cross sections atomic ratio

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

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

Catalysis and Catalysts - XPS Example: Re 2 O 7 /Al 2 O 3 escape depth = 1.3 nm = 1.8 nm (Re/Al) bulk I(Re 4f) I(Al 2p)

Catalysis and Catalysts - XPS Other Catalyst Models Randomly Oriented Support LayersInhomogeneous promoter distribution: egg-shell catalyst

Catalysis and Catalysts - XPS WO 3 /SiO 2 MoO 3 /Al 2 O 3 MoO 3 /SiO WO 3 /Al 2 O Example: MoO 3 and WO 3 supported on SiO 2 and Al 2 O 3

Catalysis and Catalysts - XPS Pt/SiO 2 - XPS intensities I(Pt 4f) I(Si 2p) (Pt/Si) bulk Dispersion 34% ? Experimental points Monolayer prediction

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

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

Catalysis and Catalysts - XPS  c (nm)

Catalysis and Catalysts - XPS Example: Fluorinated Alumina AlF 3 Al 2 O 3 powdered coarse eV

Catalysis and Catalysts - XPS 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