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2 nd FEZA School On Zeolites 1-2 September 2008, Paris X-ray photoelectron spectroscopy and its use for solid materials Jacques C. Védrine.

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Presentation on theme: "2 nd FEZA School On Zeolites 1-2 September 2008, Paris X-ray photoelectron spectroscopy and its use for solid materials Jacques C. Védrine."— Presentation transcript:

1 2 nd FEZA School On Zeolites 1-2 September 2008, Paris X-ray photoelectron spectroscopy and its use for solid materials Jacques C. Védrine

2 General scheme representing different surface techniques ions ISS SIMS electronsAES photons XPS, UPS XAES ISS: Ion scattering spectroscopy SIMS: Secondary ion mass spectrometry AES: Auger electron spectroscopy

3 Principle of surface techniques e-e- h source of photons analyser ions gun sample h ions

4 Electron distribution for a given element: 1s 2 2s 2 2p 6 3s 2 Transition time10 -7 s; Life time of the hole: s h = E k + E b +  sp

5 Scheme of XPS electron level energy Sample h = E k + E b +  sp

6 XPS spectrum taken at photon energies: 1,486.6 eV (AlK  ) and 3,000 eV Binding energy (eV) Tb 3d Cu 2p O 1s Ce 3d O KLL Ce MNN Ce 4d Tb 4d AlK  3000 eV

7 Illustrative scheme (left) and VG ESCALAB MkII spectrometer (right)

8 Main parameters determined by XPS 1.Binding energy values E b and chemical shifts  E b (spin-orbit coupling, final state and multielectronic effects); Auger peaks 2.Quantitative aspects and surface analysis 3.Case of supported catalysts 4.Applications in heterogeneous catalysis: porous materials, bimetallic catalysts, organometallic compounds, mixed oxides materials, basic catalysts, mixed oxides on CeO 2, lanthanide phosphates, adsorbed species,

9 Chemical shifts corresponding to different oxidation states and environments of Al Al 2 O 3 Al 2p Al 0 Ni 3p J = L + S S= ½ L= 0,1,2,3 for s,p,d,f orbitals spin-orbit coupling varies as Z 5 Two components with 2J+1 relative intensities

10 Chemical shifts observed as a function of oxidation state for several compounds E C (A,B) = K C (q A - q B ) + V A – V B K C the overlapping integral between core and valence electrons q A and q B valence charges of element C in A and B compounds ElementElectronic level CompoundsChemical shift /eV Al2pAl 0 -Al 2 O Si2pSi 0 -SiO Co2p 3/2 Co 0 -CoO Co 0 -Co 3 O Ti2p 3/2 Ti 0 -TiO Ti 0 -Ti 2 O 3 Ti 0 -TiO W4f 7/2 W 0 -WO 2 W 0 -CrWO 4 W 0 -WO

11 XPS Parameters initial state E i with N electrons final state E f with N-1 electrons E b = E f N-1 - E i N and  E b =  E i -  E f charge potential model:  E i = (e 2 r /r) +  V e r charge borne by the element  V change in potential due to neighbouring atoms (e.g. Madelung potential V=  j (q j /R j ) R j distance between C and atom j bearing a charge q j ) K C q = q/r i, with r i average radius of element C Final state effects (Koopmans theorem of sudden approximation relaxation) E i b = -  i + E i R and  E i b = -  i +  E i R Multielectronic effects: plasmons, configuration interaction, shake-up and shake-off processes. np ligand  np transition metal difference in energy beween the background state and the states after photoemission (shake-off if electron ejected in the continuum) Shake-up peaks for paramagnetic ions as Co 2+ (d 7 ), Ni 2+ + (d 8 ) or Cu 2+ (d 9 ) Configuration interaction: e.g. Mn 2+ d 5 ion 6 S initial state (Ar 3s 2 3p 6 3d 5 ) and 7 S final state [with two states 7 S and 5 S depending on the spin orientation] Mn 3+ (Ar 3s 1 3p 6 3d 5 ) and energy splitting  E( 7 S- 5 S) = [(2S+1)/(2L+1)]G(s,d) [G(s,d) exchange intergral] and I( 7 S)/I( 5 S) = 7/5 as spin-orbit coupling between 2p, 3d, 4f peaks and (2L+1) /(2S+1) relative peaks intensities.

12 Principle of the Auger process occurring under photon or electron impact e-e- e - Auger e - / photon Z Y X E c = E X - E Y - E Z E KL2L3 = E K - E L2 – E L3 – F(L2,L3,X) + R(L2,L3)

13 Intensity I = P of the emitted beam as a function of its originating depth x = d from the surface

14 Universal curve of the electron mean free path as a function of the electron kinetic energy value 10 eV100 eV

15 Quantitative aspects dI(  ) = .(NA 0 /sin  )(d .  /d  ).T.exp(-x/ sin  )dx I = I 0 ∫ 0 ∞ exp(-x ).d(x) = I 0 [exp(-x )] 0 ∞ = I 0 I(  ) = ∫ 0 ∞  (NA 0 /sin  )(d .  /d  ).T.exp(-x/ sin  )dx =  NA 0 (d .  /d  ).T. N A /N B = (I A /I B )(  B /  A )(T B /T A )( B / A ) (d  /d  ).( ,h ) = (  /4  ) [1+ (  /2).{(3sin2  )/2)-1}] N A /N B = (I A /I B ).[  B (E k B ) x ] / [  A (E k A ) x ].T B /T A with x = 0.5 to 0.75

16 Scheme of a supported catalyst of high surface area support as proposed by Kerkhof et al 1 2 j Support Promoter Approx.: catalyst particle = infinite number of sheets high surface area (I p /I s ) exp = (N p /N s ) b (  p /  s ){ [1+ exp(-d s /2 s )] / [1-exp(-d s /2 s )]}{[1-exp(-d p / p )] / (d p / p }} d s = 2/S.  I p /I S = (N S /N p ).(     ).(d s /2 )

17 I p /I S = (N S /N p ).(  p /  s ).F(d, p ) F(d, p ) = (3/2).{1-(2 p 2 /d 2 )[1-exp(-d/ p )]+ (2 p /d) exp(-d/ p )} for spherical crystallites of diameter d F(d, p ) = 3{[1-(8 p 2 /d 2 )].[1-exp(-d/2 p )] + (4 p /d) exp(-d/2 p )} for hemispherical crystallites of diameter d F(d, p ) = 1-exp(-d/ p ) for cubic or planar deposits of thickness d

18 Prediction and experimental metal dispersion for Pt/SiO 2 catalysts

19 Schematic models for supported catalysts a) Layer mode (Frank-van der Merwe, 2 D) b) Island mode (Volmer-Weber, 3 D) c) Layer + island mode (Stranski- Krastanov) 3 2 (f = 0.5)

20 Theoretical calculated XPS peak intensity ratio variations for supported catalysts. Case of Cu/MgO

21 Ag 3d XPS spectra of 0.3Pd-0.6Ag/pumice catalyst in a) as synthesised; b) oxidised at 623K; c) reduced at 623K

22 Spectra of MgNd alloys (25wt% Nd) oxidised for 90 min at 773K

23 O1s, Eu 3d 5/2 and Eu 4d XPS spectra of Eu III organometallic compounds

24 Binding energy values in eV of Eu3d 5/2 peaks and of its associated shake-down satellite and ratio of intensities for Eu III compounds Eu III compoundsE b Eu3d 5/2 E b shake down satellite EbEb Eu 2 O 3 Eu 2 (C 2 O 4 ) 3 Eu(acac)3 Eu 2 (CO 3 ) 3 Eu 2 (SO 4 ) 3 Eu(NO 3 ) For europium 4f 6 5d 1 and 4f 7 5d 0 configurations in the final state unoccupied 4f levels are lowered in energy by the potential of the created photohole (Coulomb interaction of the created photohole with the electron system)

25 Correlation between Pauling electronegativity of the heteroatom X and O 1s binding energy values for Eu III compounds

26 Basic oxide catalysts used for propane ODH to propene Rare earth element Mg, Ca, Sr doped with Nd (5 mol% Nd 2 O 3 ) (Nd 3d (left) and Nd 4d (right) core levels from the Nd/CaO sample; insert: Nd 3d 5/2 peak decomposition

27 Nd content as determined from chemical analysis and XPS Nd 3d or Nd 4d, M* = Mg 1s, Ca 2p, Sr 3d peak intensities CatalystsNd/M* Chem Anal. Nd/M* XPS Excess of Nd on the surface (at %) Nd/MgO ± Nd/CaO ± Nd/SrO ± slight enrichment (<5%) of the surface with Nd with respect to the bulk

28 XPS spectra before catalytic testing and their decomposition: Nd/MgO (a), Nd/CaO (b), Nd/SrO (c) c c b a O 1s in oxide (E b ~ eV), in adsorbed water E b ~ eV and from hydroxyls and carbonates (E b ~ eV),

29 C1s XPS peaks before catalytic testing and their decomposition: Nd/MgO (a), Nd/CaO (b), Nd/SrO (c) a b c Carbonate C1s peaks at ~ 285 eV for contamination carbon (adventitious hydrocarbon species) and ~ eV for carbonates

30 Mixed oxides based on CeO 2 Ce 3d experimental spectrum and its decomposition Final state effect:v (u) Ce IV : 3d 9 5d 6 s 0 4f 1 -O2p 5 v’’(u’’): Ce IV : 3d 9 5d 6 s 0 4f 2 -O2p 4 Multiplet splitting: v’’’(u’’’): Ce IV : 3d 9 5d 6 s 0 4f 0 -O2p 6 v o (u o ): Ce III : 3d 9 5d 6 s 0 4f 1 -O2p 6 v’(u’): Ce II : 3d 9 5d 6 s 0 4f 2 -O2p 5 Binding Energy (eV) v’’’ u u’u’u’’ u’’’ v’’ v’v’ v v0v0

31 Concluding remarks XPS is the most currently used « surface » technique Quantitative data are determining but should be used with care, unless the geometry of particles and support are well known Not really useful for porous materials as too sensitive to the surface (1-5nm) of particles usually in  m-mm size range Chemical shifts and multiplet peaks are useful « chemical » indications Challenges for analysis under « pressure » and for spatial resolution and scanning analysis


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