X-ray absorption spectroscopy - PDF Free Download

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

Absorption as function of energy of the x-ray

Data-analysis Absorption (a.u.) 2.0 Pre-edge subtraction 1.5 1.0 0.5 0.0-0.5 8800 9000 9200 9400 9600 9800 Energy (ev) a. Absorption (a.u.) Edge energy determination 2.0 b. 1.5 1.0 0.5 0.0 8950 8975 9000 9025 Energy (ev) Absorption (a.u.) 1.0 0.8 0.6 0.4 0.2 χ(k) 0.15 c. d. 0.10 0.05 0.00-0.05-0.10 0.0 0 200 400 600 800 Energy (ev) Background and Normalization -0.15 2 4 6 8 10 12 14 k (Å -1 ) EXAFS Function

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) 60 40 20 0-20 Fourier transformation Fourier transform (k 2 *χ(k)) 4 2 0-2 2 nd neighbor -40-60 4 6 8 10 12 14 k(å -1 ) -4 1 2 3 4 5 6 R (Å) radial distribution function

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 0.25 0.20 0.15 0.10 0.05 0.00 Pt - O Pt - Pt 0 5 10 15 20 k (Å)

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 ) 0.8 0.6 Low T σ 2 = 0.001 0.4 High T σ 2 = 0.01 0.2 0.0 0 5 10 15 20 K (A -1 )

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 ) 1.5 1.0 0.5 0.0 0-0.5-1.0-1.5 Au/Al 2 O 3 3.04 < K < 11.5 A-1 0 1 2 3 4 5 6 R (Å) Coordination number 6.8 Au-Au distance 2.76 Å ΔDWF 0.0058 C3 9 E-6 C4 3E-6 Added parameter: ΔE 0

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

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

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

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

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).

Palladium hydride formation Normalized Absorption 1.5 1.0 0.5 0.0 2%Pd/SiO 2 10 nm Naked paticle Hydride Difference Normalized Absorption 1.5 1.0 0.5 0.0 2%Pd/Al 2 O 3 <2 nm Naked paticle Hydride Difference -0.5 3160 3170 3180 3190 3200 Energy (ev) -0.5 3160 3170 3180 3190 3200 Energy (ev)

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

Anti bonding state Normalized Absorption 1.5 1.0 0.5 0.0 2%Pd/SiO 2 Naked paticle Hydride Difference -0.5 3160 3170 3180 3190 3200 Energy (ev)

2.5 DOS vs XANES for PdH 0.64 (93 atoms) 2.0 2.0 s p d s (H) p (H) 1.5 MuX 1.5 XAS 1.0 1.0 0.5 0.5 0.5 DOS vs XANES for PdH 0.64 (93 atoms) 2.0 0.0 0.0-15 -10-5 0 5 10 15 ev s p d s (H) p (H) 1.5 MuX 1.0 0.5 0.0 0.0-15 -10-5 0 5 10 15 ev

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 2 + 3 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

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

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

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

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

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

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

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

EXAFS Fitting Results of Reduced Catalysts ±0.02 Au Bond Distance, Å 2.88 2.86 2.84 2.82 2.80 2.78 2.76 2.74 2.72 Bulk value 2.88 Å 2.70 2 4 6 8 10 12 Au-Au CN ± 10-20%

EXAFS Fitting Results of Reduced Catalysts 2.88 2.84 Au-Au Distance 2.80 2.76 100% 50% 30% Al 2 O 3 TiO 2 SiO 2 Nb 2 O 5 ZrO 2 CeO 2 2.72 0 10 20 30 40 50 60 70 80 Particle Size (Å) Strong reduction in Au-Au distance with particle size No visible influence of support

Electronic structure from L III XANES 1.4 Normalized Absorption 1.2 1.0 0.8 0.6 0.4 Au(III) Au(0) 0.2 0.0 11875 11900 11925 11950 11975 12000 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

Whitelines reflect number of holes in the d-band Au/Al 2 O 3 Au/TiO 2 1.0 1.0 Normalized Absorption 0.8 0.6 0.4 0.2 Bulk Gold 32 nm 12 nm 5 nm Normalized Absorption 0.8 0.6 0.4 0.2 Bulk Gold 32 Å 16 Å 11 Å 0.0 11.91 11.92 11.93 11.94 11.95 Energy (ev) 0.0 11.91 11.92 11.93 11.94 11.95 Energy (kev) 1.1 nm Au Normalized Absorption 0.8 0.6 Au/ZrO 2 Au/Al 2 O 3 Au/TiO 2 Au/Al 2 O 3 Bulk Gold Whiteline is particle-size dependent 0.4 11.92 11.93 Energy (kev)

Whiteline intensity versus particle size Difference intensity with bulk 0.60 0.60 Difference in WL Intensity 0.45 0.30 0.15 0.00 Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO 2 Difference in WL Intensity 0.45 0.30 0.15 0.00 Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO 2 3 4 5 6 7 8 9 10 11 12 Au-Au Coordination Number 2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90 Au-Au Distance (Angstron) Six supports, one trend Larger particles fewer d-electrons

Exposure to 20% O 2 1.0 1.3wt% Au/Al 2 O 3 1.4 Normalizd Absorption 0.8 0.6 0.4 0.2 Reduced (H 2 250C) Reoxidized (20% O 2 RT) Reoxidized (20% O 2 225C) Normalized Absorption 1.2 1.0 0.8 0.6 0.4 0.2 Au(III) Au(0) 0.0 11900 11920 11940 11960 Energy (ev) 0.0 11875 11900 11925 11950 11975 12000 Energy (ev) CN R(Å) %Au(III) Reduced 3.6 2.72 0 Reox. RT 3.6 / 0.3 2.72 / 2.04 10 Reox. 225C 2.7 / 0.5 2.71 / 2.04 15

XAS during CO Oxidation Normalized Absorption 1.2 1.0 0.8 0.6 0.4 0.2 Au/Al 2 O 3 Normalized Absorption 1.0 0.8 0.6 CO/O 2 2/1 CO/O 2 1/1 CO/O 2 1/2 He 0.0 11900 11910 11920 11930 11940 11950 11960 Energy (ev) Au whiteline 0.4 11920 11930 11940 11950 Energy (ev) CN(Au) R(Au) He 6.5 2.77 1:1 5.3 2.73 2:1 5.7 2.73 1:2 5.2 2.77 Small oxygen contribution More intense with more CO: holes in the d-band (anti-bonding states)

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 1.0 0.8 0.6 0.4 0.2 Au/Al 2 O 3 Switch Oxygen to CO 2s repetition 0.0 11910 11920 11930 11940 11950 Energy/eV

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

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 2.88 0.60 Au-Au Distance 2.84 2.80 2.76 2.72 Al 2 O 3 TiO 2 SiO 2 Nb 2 O 5 ZrO 2 CeO 2 Difference in WL Intensity 0.45 0.30 0.15 0.00 Au/Al 2 O 3 Bulk Au/ZrO 2 CeO 2 SiO 2 Nb 2 O 5 TiO 2 0 10 20 30 40 50 60 70 80 Particle Size (Å) 2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90 Au-Au Distance (Angstron)

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

FEFF8 simulation Experimental bare atop He CO

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

Single Crystals UHV conditions Langmuir-Hinshelwood

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

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

2wt%Pt/Al 2 O 3 Conversion data High activity region rate of CO conversion (cm 3 /min/gcat) 0.04 0.02 Low activity region O 2 /CO = 5 2 1 0.00 350 400 450 500 550 Temperature (K)

Conversion data O 2 /CO ratio ignition or extinction temperature (K) heating cooling hysteresis temperature for onset of conversion (K) 1 472 456 yes 340 2 445 440 yes 338 5 433 421 yes 329

XAS 3 O 2 /CO = 2 2 heating heating 1 350 400 450 500 550 600 3 Temperature (K) 0 11560 11570 11580 11590 Energy (ev) 2 O 2 CO 308 K 340 K 386 K 422 K 439 K 455 K 472 K 1 0 11560 11570 11580 11590 Energy (ev)

XAS O 2 /CO = 2 3 0.04 2 cooling 0.02 cooling 1 0.00 3 350 400 450 500 550 600 Temperature (K) 0 11560 11570 11580 11590 Energy (ev) 2 O 2 CO 308 K 340 K 386 K 422 K 439 K 455 K 472 K 1 0 11560 11570 11580 11590 Energy (ev)

ratio O 2 / CO = 1 ratio O 2 / CO = 2 3 (a) 3 (a) 2 heating 2 heating heating heating 1 1 0 11560 11570 11580 11590 Energy (ev) 0 11560 11570 11580 11590 Energy (ev) 3 (a) ratio O 2 / CO = 5 2 1 heating heating rate of CO conversion (cm 3 /min/gcat) 0.04 0.02 5 2 1 0 0.00 350 400 450 500 550 11560 11570 11580 11590 Temperature (K) Energy (ev) oxidation parallels ignition

Kinetics and XAS High activity region partially oxidic surface rate of CO conversion (cm 3 /min/gcat) 0.04 0.02 Low activity region adsorbed CO O 2 /CO = 5 2 1 0.00 350 400 450 500 550 Temperature (K)

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

XAS (QEXAFS) 100 90 O 2 /CO = 1 CO Conversion % 80 70 60 50 40 470 471 472 473 474 475 Temperature/ K 1.5 1.2 0.9 11580 11600 11620 Energy (ev)

Kinetics and XAS heating rate of CO conversion (cm 3 /min/gcat) 0.04 0.02 increasingly oxidized CO poisoning partially oxidic surface 3 2 heating 308 K 340 K 386 K 422 K 439 K 455 K 472 K 0.00 350 400 450 500 550 Temperature (K) 1 heating J. Singh, et al. Angew. Chem. Int. Ed., accepted 0 11560 11570 11580 11590 Energy (ev)

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.

Other supports (normal XAS) percent CO conversion 100 80 60 40 20 2 2 3 Pt/SiO 2 Extinction plots 0 100 150 200 250 300 temperature, o C 1.4 1.8 1.6 1.6 1.4 normalized absorption 1.2 1.0 0.8 0.6 0.4 0.2 0.0 11.55 11.56 11.57 11.58 11.59 11.60 Energy, kev 68 o C, 4% conversion 134 o C, 24% conversion 180 o C, 67% conversion 190 o C, 100% conversion normalized absorption 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 11.55 11.56 11.57 11.58 11.59 11.60 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 1.2 1.0 0.8 0.6 0.4 0.2 0.0 11.55 11.56 11.57 11.58 11.59 11.60 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, 1.5-3 nm

EXAFS analysis Below and above temperature of ignition 6 4 A Chi Fit 4 B Fourier Transform Fit Chi x k 3 2 0-2 -4-6 3 4 5 6 7 8 9 10 11 12 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 2 0-2 -4 1.5 2.0 2.5 3.0 3.5 4.0 o R (A)

EXAFS analysis Pt /Al 2 O 3 small particles Conditions atom N DW R(Å) Eo RT, He Pt 5.7 0.0042 2.72 1.68 below ignition Pt 6.2 0.0054 2.77 0.58 rate of CO conversion (cm 3 /min/gcat) 0.04 0.02 5 2 1 0.5 0.25 above ignition O 2.4 0.0021 1.99 4.36 Pt 3.0 0.0065 2.61 7.19 0.00 350 400 450 500 550 600 Temperature (K) RT, O 2 /CO Pt 5.6 0.0039 2.76 1.87

Kinetic oscillations Single Crystals T = 470K p CO = 3 10-5 mbar p O2 = 2.0 2.7 10-4 mbar Pt (110) M. Eiswirth and G. Ertl, Surface Sci. 177 (1986), 90

Kinetic oscillations Single Crystals bulk crystal plane Pt (110)

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

Kinetic oscillations - 2wt% Pt/Al 2 O 3 0.009 55 Mass Spec. Signal (a.u.) 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 CO 2 CO O C Pt -0.001 385 380 375 370 365 O C Pt Pt 50 45 40 35 30 25 20 15 10 5 0 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)

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

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

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EXAFS error margins: N ± 0.5 20% R ± 0.01 0.03 σ 2 ±20%