X-ray absorption spectroscopy

Transcription

1 X-ray absorption spectroscopy Jagdeep Singh Jeroen A. van Bokhoven

2 Absorption as function of energy of the x-ray

3 Data-analysis Absorption (a.u.) 2.0 Pre-edge subtraction Energy (ev) a. Absorption (a.u.) Edge energy determination 2.0 b Energy (ev) Absorption (a.u.) χ(k) 0.15 c. d Energy (ev) Background and Normalization k (Å -1 ) EXAFS Function

4 EXAFS formula χ(k) 2 S 0 2R i 2 2 = N i F i (k) exp exp sin 2kR 2 i + ϕ j k kr λ i Scatter power i Damping ( 2σ i k ) () Disorder ( ) 1 st neighbor χ(k) Fourier transformation Fourier transform (k 2 *χ(k)) nd neighbor k(å -1 ) R (Å) radial distribution function

5 S 2R χ( k) = N F( k) exp exp 2σ k sin 2kR + ϕ k i 2 0 i i i 2 i i j kri λ Scatter power Damping ( 2 2) ( ( )) Disorder Backscatterings Amplitude Pt - O Pt - Pt k (Å)

6 Temperature effect on EXAFS / XANES 2 S0 2Ri χ( k) = NiFi( k) exp exp 2 sin 2 2 i kr λ Scatter power i Damping ( 2 2 σ ) ( ( )) ik kri +ϕ j k Disorder Temperature effect 1.0 exp(-2σ 2 k 2 ) Low T σ 2 = High T σ 2 = K (A -1 )

7 Getting structural information from EXAFS S 2R χ( k) = N F( k) exp exp 2σ k sin 2kR + ϕ k i 2 0 i i i 2 i i j kri λ ( 2 2) ( ( )) F j, ϕ j, and S o2 from reference compound or theory Fourier transform (k 3 ) Au/Al 2 O < K < 11.5 A R (Å) Coordination number 6.8 Au-Au distance 2.76 Å ΔDWF C3 9 E-6 C4 3E-6 Added parameter: ΔE 0

8 Abstract EXAFS gives local structure XANES gives geometry and oxidation state (empty density of states)

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10 χ(k) 2 S0 2R i 2 2 = N i F i (k) 2 exp exp kr λ sin 2kR i + ϕ j k i Scatter power i Damping ( 2σ i k ) ( ()) Disorder

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14 Pre edge EXAFS (χ) Extended X-ray Absorption Fine-Structure Single scattering XANES X-ray Absorption Near-Edge Structure Multiple scattering Single Scattering A Multiple Scattering

15 XANES versus EXAFS A EXAFS - Single scattering dominates XANES - Multiple scattering - Electronic transitions - Multiple electron transitions Information - Number & kind of neighbor - Distance -Disorder - Geometry / subtle distortions - Oxidation state - Electronic information -DOS in final state Theoretical description of XANES - Detailed electronic information - Aids interpretation of spectra of unknown compounds - Time-consuming - Needs an expert

16 XAS in Catalysis Goal Local structure of catalysts under well-defined conditions precursor state during / after activation during reaction deactivation time-resolution (few msec) space-resolution (few μm).

17 Palladium hydride formation Normalized Absorption %Pd/SiO 2 10 nm Naked paticle Hydride Difference Normalized Absorption %Pd/Al 2 O 3 <2 nm Naked paticle Hydride Difference Energy (ev) Energy (ev)

18 + ε d Fermi level - Metal d-band reactant level

19 Anti bonding state Normalized Absorption %Pd/SiO 2 Naked paticle Hydride Difference Energy (ev)

20 2.5 DOS vs XANES for PdH 0.64 (93 atoms) s p d s (H) p (H) 1.5 MuX 1.5 XAS DOS vs XANES for PdH 0.64 (93 atoms) ev s p d s (H) p (H) 1.5 MuX ev

21 Catalysis by Gold Partial / complete oxidation of hydrocarbons methane, alkenes, methanol Hydrogenation / dehydrogenation reactions alkenes, alkynes, alkadienes, (un)saturated ketones Methanol synthesis CO + 2H 2 CH 3 OH CO H 2 CH 3 OH + H 2 O WGS H 2 O + CO CO 2 + H 2 Nitric Oxide reduction (with CO, olefins, or H 2 ) CO oxidation 1925: Active in CO oxidation highly active in presence of H 2 : Haruta Catal. Today 36 (1997) 153 Selective CO removal, air purification, high-purity N 2 and O 2

22 Catalysis by Gold Physical properties bulk metallic gold is thermodynamically stable melting point and metallicity of the particle is function of particle size Buffat Phys. Rev. A 13 (1976) 2287

23 Catalysis by Gold Physical properties bulk metallic gold is thermodynamically stable melting point and metallicity of the particle is function of particle size CO oxidation: particle-size effect Goodman Science 281 (1998) 1648 Haruta Cattech 3 (2002) 102

24 Catalysis by Gold Physical properties: bulk metallic gold is thermodynamically stable melting point and metallicity of the particle is function of particle size CO oxidation: particle-size effect Large support effects: SiO 2 : hardly active Al 2 O 3, MgO: moderately active (TiO 2 ) Fe 2 O 3, CeO 2, other reducible supports: very active & less dependent on particle sizes; No clear relation to reducibility of support Active species in gold oxidation catalysis? Carbonate-mechanism excluded Small particles become active as soon as they are non-metallic (Goodman) Oxidic gold (I or III) is active species (Gates) Theory supports both gold-only and support-aided mechanism Support supplies oxygen via molecularly (activated) adsorbed oxygen via Mars van Krevelen

25 Geometry / coordination Density of states Oxidation state Time resolved In situ

26 How is oxygen activated on the catalyst? How can the most inert metal be so active?

27 Nørskov et al. Angew. Chem. 44 (2005) 1824 Small gold particles adsorb oxygen (and react)

28 Structure of gold catalysts Sample Preparation Deposition precipitation HAuCl 4 adjusted ph Washing with a base to remove chlorine Reduction in hydrogen Supports Al 2 O 3, SiO 2, CeO 2, TiO 2, ZrO 2, Nb 2 O 5 Full EXAFS & XANES analyses

29 EXAFS Fitting Results of Reduced Catalysts ±0.02 Au Bond Distance, Å Bulk value 2.88 Å Au-Au CN ± 10-20%

30 EXAFS Fitting Results of Reduced Catalysts Au-Au Distance % 50% 30% Al 2 O 3 TiO 2 SiO 2 Nb 2 O 5 ZrO 2 CeO Particle Size (Å) Strong reduction in Au-Au distance with particle size No visible influence of support

31 Electronic structure from L III XANES 1.4 Normalized Absorption Au(III) Au(0) Energy (ev) Whitelines reflect number of holes in the d-band Gold whiteline: spd-rehybridization results in 5d 10 x 6sp 1+x Mattheiss, L.F. et al. Phys. Rev. B 1980, 22, 1663

32 Whitelines reflect number of holes in the d-band Au/Al 2 O 3 Au/TiO Normalized Absorption Bulk Gold 32 nm 12 nm 5 nm Normalized Absorption Bulk Gold 32 Å 16 Å 11 Å Energy (ev) Energy (kev) 1.1 nm Au Normalized Absorption Au/ZrO 2 Au/Al 2 O 3 Au/TiO 2 Au/Al 2 O 3 Bulk Gold Whiteline is particle-size dependent Energy (kev)

33 Whiteline intensity versus particle size Difference intensity with bulk Difference in WL Intensity Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO 2 Difference in WL Intensity Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO Au-Au Coordination Number Au-Au Distance (Angstron) Six supports, one trend Larger particles fewer d-electrons

34 Exposure to 20% O wt% Au/Al 2 O Normalizd Absorption Reduced (H 2 250C) Reoxidized (20% O 2 RT) Reoxidized (20% O 2 225C) Normalized Absorption Au(III) Au(0) Energy (ev) Energy (ev) CN R(Å) %Au(III) Reduced Reox. RT 3.6 / / Reox. 225C 2.7 / /

35 XAS during CO Oxidation Normalized Absorption Au/Al 2 O 3 Normalized Absorption CO/O 2 2/1 CO/O 2 1/1 CO/O 2 1/2 He Energy (ev) Au whiteline Energy (ev) CN(Au) R(Au) He : : : Small oxygen contribution More intense with more CO: holes in the d-band (anti-bonding states)

36 Gold catalysts and activation of oxygen Under (diluted) O 2 : surface oxidation (Au/Al 2 O 3 & Au/TiO 2 ) Switch to CO/O 2 : CO keeps gold reduced Normalized Absorption Au/Al 2 O 3 Switch Oxygen to CO 2s repetition Energy/eV

37 O O O 2 Slow Au (0) Au (0) Au n+ O C CO + O 2 Fast Au (0) O O O C CO + O 2 Au n+ Au (0) Au (0) Au (0) Van Bokhoven Angew. Chem. (2006) VIP Reduced gold is active phase Gold participates in oxygen activation

38 Rehybridization of spd-orbitals (5d 10 x 6sp 1+x ) Smaller particles have fewer holes in the d-band Particle size dominates support-effect Oxygen is activated on gold particle d-ldos Fermi level Boiling point Gold-Gold distance d electrons Au-Au Distance Al 2 O 3 TiO 2 SiO 2 Nb 2 O 5 ZrO 2 CeO 2 Difference in WL Intensity Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO Particle Size (Å) Au-Au Distance (Angstron)

39 Adsorption sites from XAS Pt/Al 2 O 3 He Traces O 2 1%CO(He)

40 FEFF8 simulation Experimental bare atop He CO

41 fuel cell (PEM) catalytic converter platinum catalyst Structure of the active phase? surface reaction surface patterns surface roughening

42 Single Crystals UHV conditions Langmuir-Hinshelwood

43 Single Crystals UHV conditions high pressure Two reaction regimes Low activity CO poisoning High activity??? Langmuir-Hinshelwood

44 material gap Real Catalysts Real Conditions 1 nm 1.3 nm 1.8 nm pressure gap Single Crystals UHV conditions

45 2wt%Pt/Al 2 O 3 Conversion data High activity region rate of CO conversion (cm 3 /min/gcat) Low activity region O 2 /CO = Temperature (K)

46 Conversion data O 2 /CO ratio ignition or extinction temperature (K) heating cooling hysteresis temperature for onset of conversion (K) yes yes yes 329

47 XAS 3 O 2 /CO = 2 2 heating heating Temperature (K) Energy (ev) 2 O 2 CO 308 K 340 K 386 K 422 K 439 K 455 K 472 K Energy (ev)

48 XAS O 2 /CO = cooling 0.02 cooling Temperature (K) Energy (ev) 2 O 2 CO 308 K 340 K 386 K 422 K 439 K 455 K 472 K Energy (ev)

49 ratio O 2 / CO = 1 ratio O 2 / CO = 2 3 (a) 3 (a) 2 heating 2 heating heating heating Energy (ev) Energy (ev) 3 (a) ratio O 2 / CO = heating heating rate of CO conversion (cm 3 /min/gcat) Temperature (K) Energy (ev) oxidation parallels ignition

50 Kinetics and XAS High activity region partially oxidic surface rate of CO conversion (cm 3 /min/gcat) Low activity region adsorbed CO O 2 /CO = Temperature (K)

51 Kinetics and XAS High activity region partially oxidic surface rate of CO conversion (cm 3 /min/gcat) Low activity region adsorbed CO O 2 /CO = Temperature (K)

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53 Qexafs signals Pt foil L 3, L 2, L 1 edges

54 XAS (QEXAFS) O 2 /CO = 1 CO Conversion % Temperature/ K Energy (ev)

55 Kinetics and XAS heating rate of CO conversion (cm 3 /min/gcat) increasingly oxidized CO poisoning partially oxidic surface 3 2 heating 308 K 340 K 386 K 422 K 439 K 455 K 472 K Temperature (K) 1 heating J. Singh, et al. Angew. Chem. Int. Ed., accepted Energy (ev)

56 Conclusions a highly active state of the catalyst is discovered the catalyst shows different structure in low- and highactivity regime; low-activity region : CO adsorbed on platinum, high-activity region: partially oxidized platinum the catalyst increasingly oxidizes during the ignition high temperature and a high oxygen concentration benefit the formation of the more active partially oxidic catalyst.

57 Other supports (normal XAS) percent CO conversion Pt/SiO 2 Extinction plots temperature, o C normalized absorption Energy, kev 68 o C, 4% conversion 134 o C, 24% conversion 180 o C, 67% conversion 190 o C, 100% conversion normalized absorption Energy, kev 125 o C, 2% conversion 194 o C, 9% conversion 210 o C, 20% conversion 218 o C, 32% conversion 231 o C, 100% conversion normalized absorption Energy, kev 40 o C, 0% conversion 140 o C, 1% conversion 240 o C, 6% conversion 294 o C, 45% conversion 306 o C, 100% conversion TiO 2, 1.3 nm Pt/Al 2 O 3 L, 2 nm SiO 2, nm

58 EXAFS analysis Below and above temperature of ignition 6 4 A Chi Fit 4 B Fourier Transform Fit Chi x k o k (A -1 ) FT (A 3 ) K 3 weighted CHI and Fourier transform 2.5 < k < 12 Å -1 Pt/Al 2 O 3 L reduced, hydrogen removed; He (RT) o o R (A)

59 EXAFS analysis Pt /Al 2 O 3 small particles Conditions atom N DW R(Å) Eo RT, He Pt below ignition Pt rate of CO conversion (cm 3 /min/gcat) above ignition O Pt Temperature (K) RT, O 2 /CO Pt

60 Kinetic oscillations Single Crystals T = 470K p CO = mbar p O2 = mbar Pt (110) M. Eiswirth and G. Ertl, Surface Sci. 177 (1986), 90

61 Kinetic oscillations Single Crystals bulk crystal plane Pt (110)

62 Kinetic oscillations Single Crystals lower adsorption energy of CO higher adsorption energy of CO sticking coefficient of O on 1x1 is about 1.5 times than on 1x2 T. Gritsch et al. Phys. Rev. Lett. 63 (1989) 1086 N. Freyer et al. Surface Sci. 166 (1986) 206

63 Kinetic oscillations - 2wt% Pt/Al 2 O Mass Spec. Signal (a.u.) CO 2 CO O C Pt O C Pt Pt IR Peak Intensity (a.u.) storage of CO that is released in sudden spike of CO 2 CO poisoning; activity inversely proportional to adsorbed CO Temp. (K)

64 Kinetic Oscillations and QEXAFS CO/O 2 catalyst particles O 2 : CO = 19 2 wt% Pt/Al 2 O 3 20 mg catalyst 30 ml/min To exhaust / mass spec activity loss due to reduction of active (oxidized) surface sudden increase in activity parallels oxidation of surface

65 kinetic oscillations originate from the reduction and re-oxidation of the surface

66 Best of luck for your exams

67 EXAFS error margins: N ± % R ± σ 2 ±20%