Structure of heterogeneous catalysts using X-ray absorption by xww95991

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									Structure of heterogeneous catalysts using X-ray
absorption spectroscopy




             Jeroen A. van Bokhoven

             Swiss Federal Institute of Technology
             (ETH) Zurich
            Abbreviations


XAS      X-ray Absorption Spectroscopy
EXAFS    Extended X-ray Absorption Fine-Structure
XANES    X-ray Absorption Near-Edge Structure
NEXAFS   Near-Edge X-ray Absorption Fine Structure
X-rays
X-ray absorption through matter                                                       25 μm 1 wt% Au/SiO2
                                       I = I0 e-μx                             1.0

                                I0                                             0.8




                                                                Transmission
                         hν
                                                                               0.6

                                                                               0.4

                1.0
                       10-3 mbar
                                                                               0.2
 Transmission




                0.8                                                            0.0
                                                                                     2000 4000 6000 8000 100001200014000
                                                                                                 Energy (eV)
                0.6
                              Air @ 1 bar                                      0.95
                0.4
                                                                               0.94




                                                                Transmission
                0.2       Transmission through 1 cm                                                                  L1
                                                                               0.93
                                                                                                                L2
                0.0                                                            0.92
                   0    2000 4000 6000 8000 10000 12000 14000
                                     Energy (eV)                               0.91
                                                                                                 L3
                                                                               0.90
                                                                                 10000           12000          14000
                       Lambert Beer’s law                                                         Energy (eV)

                       dI = -μ(E)I dx
                       I = I0 exp(-μ(E)x)
X-ray absorption through matter

                  0.95


                  0.94
   Transmission




                  0.93


                  0.92


                  0.91


                  0.90
                     10000   12000          14000
                              Energy (eV)




                                                    Creation of a photo-electron
                                                          Ekin = hν - EBinding


      Element specific
                                  Dipole transition                            Δl=±1
                                                                               ΔS=0 spin neutral
                                  K edge:         1s          p
                                  L edge:         2s          p
                                                  2p          s,d

                                  Quadrupole transition                        Δl=±2
                                  (Much weaker)
                                  K edge          s       d
                                                  p       f


                          Transition Probability
                                                      2

                en
                   R   ul e   P       Ψi T Ψ f            δE       − Ei − hν
             ld                                                f

      ’s   Go
  rmi                         Initial state    Final state
Fe

                                   Transition operator
Transition probability vs Final State
                       2
         P   Ψ i T Ψ f δ E − E −hν       f   i



           absorbing atom                        Ekin = hν - EBinding
                      neighboring atom




           outgoing photo-electron



backscattered wave



      Ψfinal = Ψoutgoing + Ψbackscattered
             Transition probability
                           2
              P  Ψ i T Ψ f δ E − E −hν   f   i




                                   Scatter pattern is function of
       absorbing atom
                                      • # of neighbors, CN
                neighboring atom
                                      • Distance, R
                                      • Kind of neighbor
                                      • Disorder in R and CN, DWF

        outgoing photo-electron


backscattered wave



       Ψfinal = Ψoutgoing + Ψbackscattered
EXAFS (χ)   Extended X-ray Absorption Fine-Structure
            Single scattering
XANES       X-ray Absorption Near-Edge Structure
            Multiple scattering

               μ = μ0 (1 + χ)
Edges over the Periodic Table




                       X-ray data booklet:
                           xdb.lbl.gov



        Element specific
                Information in the spectra?

Local geometry                                         1.4                           3
                                                                                                            H+
   element-specific                                    1.2
                                                                                                               (2p to
                                                                                                      L3 edges(2p to nd) nd)
                                                                                                           L3 edge
                                                                                                   Octahedral Al




                                                             Normalised Absorption
                                                                                     1.66 Å
                                                                                                       Au(0) Reduction of Au(III) in H2
                                                                                         Au(III)                   5d5 6s2




                               Normalized Absorption
                                                                                                   1.89 Å  O
                                                       1.0 1.2 2
Oxidation state                                                                                                    5d6 6s2
                                                                                                                   5d7 6s2
                                                       0.8 1.0 1.66 Å                          Al                       Si
                                                                                                                   5d9 6s1
                                                                                                        Tetrahedral Al
                                                                                                                       5d10 6s1
Electronic state                                       0.6 0.8
                                                               1
                                                                                                  1.66 Å

       s.a. filling of the d-band                      0.4 0.6                                                      70 °C : Au3+
                                                       0.2 0.4
 In-situ data                                                  0
                                                       0.0 0.2 1560                           1570    1580 200 °C : Au0
                                                                                                             1590
                                                        11875                                   Energy 11950 11975 12000
                                                                                         11900 11925 (eV)
                                                          0.0                                       Energy (eV)
                                                                                         11900        11925          11950        11975
                 How are the X-rays made?
                    Synchrotrons provide high flux of X-rays




                     Spring-8 Japan    APS Chicago

                              All light sources
                              http://www.lightsources.org/
                     under construction:
                                                         Diamond
ESRF, Grenoble
                                                         (Oxford)




                      Soleil (Paris)
X-rays
“white beam”
                              Tuning the energy
                       Double crystal monochromator
                                                        sample

                Bragg angle
                                                   I0            I
   X-rays
   white beam                  Crystal [Si(111)]


2d sin θ = nλ
Experimental Hutch




                BM26 (DUBBLE), ESRF Grenoble
The experiment


             μx = -ln (IT/I0)
             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)
        …….
                 Sample environment
            Absorption of X-rays is limiting factor


                                                       Transmission through
Find a good window material                            25 μm and 1 mm
• Size of window                             1.0

• Thickness                                  0.8
                                                                      Be
• Inertness


                              Transmission
• Temperature resistance                     0.6

• Pressure                                   0.4                              Mylar
• Safety
                                             0.2
                                                                                            Al
                                             0.0
                                                   0   2000 4000 6000 8000 10000 12000 14000
      pressure
                                                                      Energy (eV)
      temperature
                                                       Si K   Ag L3   Cr K   Cu K   Pt L3
      environment
      In situ EXAFS cells for gas-solid reactions

    Reaction gas            Reaction gas flows              Small Glass Reactor
    mixture flows           through a catalyst                 with very thin
   around a pellet                pellet                     windows (0.01mm)




   Large dead volume                                       Small dead volume
                            Critical d/l (smaller          Optimal d/l
   Good for stationary
                            effectivity of the catalyst)   Good for structural changes
       conditions
                                                           Structure-activity relations

                 1 x 8 mm                                                     1 x 8 mm


Grunwaldt et al. PCCP 6 (2004) 3037
                         ID26, ESRF
                         Angew. Chem. Int. Ed. 2006, 45, 4651




                          Fluorescence
                          detector                Thermo
                                                  Couple




         Reactor




Exit-tube to mass spec
                            d




                                               Three-phase system
                                               Grunwaldt J. Catal. 213 (2003) 191


Koningsberger Rev.Sci. Instr. 60 (1989) 2635
XANES   X-ray Absorption Near-Edge Structure
        Multiple scattering
EXAFS   Extended X-ray Absorption Fine-Structure
        Single scattering
        XANES


Edge position ≡ oxidation state
      Shape ≡ geometry




 Strategies
 • compare SAMPLE to
      known compounds
 • calculate spectra
                        EXAFS



                                   Scatter pattern is function of
       absorbing atom
                                      • # of neighbors, CN
                neighboring atom
                                      • Distance, R
                                      • Kind of neighbor
                                      • Disorder in R and CN, DWF

        outgoing photo-electron


backscattered wave
                                                       EXAFS data analysis
                                     Pre-edge subtraction                                            Edge energy determination
                            2.0
                                                                                                     2.0
                            1.5
    Absorption (a.u.)




                                                                                Absorption (a.u.)
                                                                                                     1.5
                            1.0
                                                                                                     1.0
                            0.5
                                                                                                     0.5
                            0.0
                                                                                                     0.0
                            -0.5
                              8800   9000   9200       9400     9600   9800                            8950       8975                 9000             9025
                                                  Energy (eV)                                                            Energy (eV)



                                                                                                    0.15
                            1.0
                                                                                                    0.10
        Absorption (a.u.)




                            0.8
                                                                                                    0.05

                            0.6
                                                                              χ(k)

                                                                                                    0.00

                            0.4                                                                     -0.05

                            0.2                                                                     -0.10

                            0.0                                                                     -0.15
                                      0     200        400      600    800                               2    4   6        8      10          12   14
                                                  Energy (eV)
                                                                                                                           K (Å-1)
Background and Normalization                                                                         EXAFS Function
                                                             0.15
Absorption (a.u.)   1.0
                                                             0.10
                    0.8
                                                             0.05
                    0.6




                                                      χ(k)
                                                             0.00
                    0.4
                                                             -0.05
                    0.2                                      -0.10
                    0.0                                      -0.15
                          0   200   400   600   800               2   4   6   8      10   12   14
                                Energy (eV)                                   K (Å-1)

                                                                              1/ 2
                                    ⎡⎛ 8π m ⎞    2
                                                     ⎤
                                k = ⎢⎜ 2 ⎟(hν − E 0 )⎥
                                     ⎜ h ⎟
                                    ⎣⎝      ⎠        ⎦
                                                                                                Radial distribution function
       0.15

                                                                                           4            1st shell




                                                                       Fourier transform
       0.10

                                                                                           2                         3rd 4th
       0.05
                                                                                                               2nd
                                              Fourier transformation
χ(k)




       0.00                                                                                0
       -0.05
                                                                                           -2
       -0.10
                                                                                           -4
       -0.15
            2   4   6   8     10   12    14                                                 1       2      3        4      5   6
                                                                                                               R (Å)
                        K (Å-1)


                                         EXAFS formula
                                        χ(k) = Σ A(k) sin (2kR + φ)


                                      ⎛ −2 Ri ⎞
                                 2
                                                exp ( −2σ i2k 2 ) sin ( 2kRi + ϕ j ( k ) )
                                S0
       χ ( k ) = ∑ N i Fi ( k ) 2 exp ⎜
                 i             kRi    ⎝  λ ⎟  ⎠
                        Scatter               Damping            Disorder
                        power
                               ⎛ −2 Ri ⎞
                          2
                                         exp ( −2σ i2k 2 ) sin ( 2kRi + ϕ j ( k ) )
                         S0
χ ( k ) = ∑ N i Fi ( k ) 2 exp ⎜       ⎟
          i             kRi    ⎝ λ ⎠
             Scatter        Damping                                            Disorder
             power




                                                                    0.25
                                                                                    Pt - O




                                        Backscatterings Amplitude
                                                                    0.20

                                                                    0.15                             Pt - Pt


                                                                    0.10

                                                                    0.05

                                                                    0.00
                                                                           0        5         10        15     20
                                                                                             k (Å)
         Temperature effect on EXAFS / XANES
                               ⎛ −2 Ri ⎞
                          2
                                         exp ( −2σ i2k 2 ) sin ( 2kRi + ϕ j ( k ) )
                         S0
χ ( k ) = ∑ N i Fi ( k ) 2 exp ⎜       ⎟
          i             kRi    ⎝ λ ⎠
             Scatter             Damping                      Disorder
             power                                                         Temperature effect
                       1.0


                       0.8
                                                         Low T
         exp(-2σ2k2)                                     σ2 = 0.001
                       0.6


                       0.4        High T
                                  σ2 = 0.01
                       0.2


                       0.0
                             0    5           10             15       20
                                                   K (A-1)
      Getting structural information from EXAFS

                               ⎛ −2 Ri ⎞
                          2
                                         exp ( −2σ i2k 2 ) sin ( 2kRi + ϕ j ( k ) )
                         S0
χ ( k ) = ∑ N i Fi ( k ) 2 exp ⎜
          i             kRi    ⎝  λ ⎟  ⎠
                                 Fj, ϕj, and So2 from reference compound or theory

                                1.5
                                                           Au/Al2O3
       Fourier transform (k )




                                1.0
       3




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




                                                        Added parameter: ΔE0
                                                                         Fit shell by shell
                                                                4               1st shell
       0.15
                                                                                              3rd     4th
       0.10                                                     2
                                                                                        2nd



                                            Fourier transform
       0.05
χ(k)




       0.00                                                     0
       -0.05

       -0.10                                                    -2

       -0.15
            2   4   6   8    10   12   14
                        K (Å-1)                                 -4

                                                                     1      2       3           4       5   6
                                                                                              R (Å)
                                          Accuracy?
                    4                    1st shell

                                                          3rd     4th
                    2
                                                    2nd
Fourier transform




                    0


                    -2


                    -4

                         1         2            3           4       5          6
                                                          R (Å)


                             Absolute                        Relative
                             R         0.04 Å                R          ≤ 0.015 Å
                             N         20 %                  N          ≤ 10 %
                             DWF       0.002
Summary

  Adsorption of X-rays through matter

  Local structure

  Probing empty DOS

  Measurement conditions (> 1500 K; >200 bar)

  Amorphous, liquid, or crystalline samples

  Time resolution (~msec)

  Less than a monolayer on flat support
    High energy resolution fluorescence detected XAS



Normal XAS process




    hν
               Emission Spectroscopy

                                                      e.g. Pt (L3 edge):
                                                    Lα1 (L3-M5) = 9442




Some transitions more likely to occur then others
High energy resolution fluorescence detected XAS




                                                     Nett transition




                                                        hν
 hν
                                                   • Selective detection of hν
                                                   • Spherically bent crystal




      Corehole Lifetime broadening   Lifetime broadening
  High energy resolution fluorescence detected XAS
                                    L3 edge Gold foil                                                    FEFF8-reproduced
                              1.2
                                                                                                   1.0
      Normalized Absorption




                                                                           Normalised Absorption
                              1.0
                                                                                                   0.8
                              0.8
                                                                                                   0.6
                              0.6                Transmission
                                                 HERFD                                             0.4
                              0.4

                              0.2                                                                  0.2


                              0.0
                                                       ID26 ESRF                                   0.0
                               11910 11920 11930 11940 11950 11960 11970                                 -10   0   10    20      30   40   50
                                                  Energy                                                           Energy (eV)



                                                           Good
                                                           • High energy-resolution
                                                           • Hard X-rays: in-situ

                                                           Not so good
                                                           • Low yield of good photons
                                                           • No time-resolution


Gold whiteline: spd-rehybridization results in 5d10–x6sp1+x
Adsorption of CO on Pt/Al2O3
High energy resolution fluorescence detected XAS
               Pt/Al2O3

                              XAS
                          He
                          Traces O2
                          1%CO(He)

                           HERFD




         ID26, ESRF
                FEFF8 simulation




                                        atop

                                               XAS
                                 bare
Experimental

                          bare          atop
  He
               CO                              HERFD
               Structure of adsorption sites can be determined
               FEFF8 reproduces experiment and provides LDOS



J. Phys. Chem. B accepted
Structure of Aluminum in Zeolites in extreme conditions
       Development of in-situ XAFS for low Z elements
                         Zeolites


                                            8 MR
                                            2.6 x 5.7 Å
                                            (side pockets)

                                              Si

                                            O

                                             Si or Al
  Mordenite

NB kinetic diameter of       12 MR (6.5 x 7.0 Å)
hexane is ~5 Å
                   SEM of Beta



            Beta

 ZSM-5




Y-zeolite
               Acid sites in zeolites



                             H+
Brønsted:
                          Al3+    Si4+

                                   O2-



Lewis:      Octahedral or three coordinate??
            Essentially unknown
Zeolite Activation by Steaming
   • Pore-topology (mesopores)

   • Loss of Brønsted acid sites
   • Lewis acid sites

   • Dealumination of framework
   • Extra-framework aluminum


Structure – performance relations

       Detailed structure?
        Structure - Activity Relationship

Q: What is structure of a zeolite
     under reaction / treatment conditions?


In-situ spectroscopy:
observe changes as function of varying conditions
                  Time resolution


     Aluminum coordination as function of conditions
        Al K-edge X-ray Absorption Spectroscopy
 In-situ Al K-edge XAFS on zeolitic samples

1995
       ♦ No experimental in-situ cell
       ♦ Limited structural information from near-edge spectra
       ♦ No theoretical basis of spectra
                                 In-situ Al K-edge XAFS
                                    Strong absorption of X-rays


                     Transmission through 2 cm air
               1.0


               0.8
Transmission




               0.6                                           Special instrumentation
                                                             Interpretation of spectra
               0.4 Al K-edge


               0.2


               0.0
                  0      1000   2000   3000    4000   5000
                                 Energy (eV)
                     Developed Instrumentation




                                                             5 cm




                    ♠ Fluorescence and electron yield detection
                    ♠ Polymer coated Be windows


van Bokhoven J. Synchr. Rad. 6 (1999) 201
van der Eerden Rev. Sci. Instrum. 71 (2000) 3260
            Developed Instrumentation



                                         X-rays




Installed at Station 3.4 SRS Daresbury
SRS has a cell
                       Al K-edge XAFS
Characteristics:
                                                          3
• Quantitative                                                    Octahedral Al




                                  Normalised Absorption
• Conditions:
        100 < T < 1000K                                             Overlapping peaks
        10-6 < P < 103 mbar                               2
• 5-10 min per spectrum
• Solid -gas / -liquid systems                                          Tetrahedral Al
• Si/Al ratios > 50 (< 1wt% Al)
                                                          1

   Coordinations:
   • Edge position                                        0
                                                          1560   1570    1580       1590
   • Shape
                                                                   Energy (eV)
   • Intensity
                               Al coordinations from XAS spectra
                                                • Edge position
Experimental:                                   • Shape                                   FEFF8 Simulation:
                        3                       • Intensity 5




                                                         Normalised Absorption
                                Octahedral Al
Normalised Absorption




                                   Split                                         4             Split
                                           Peak
                        2
                                                                                 3                 Peak
                                      Tetrahedral Al
                                                                                 2
                        1
                                                                                 1

                        0                                                        0
                        1560   1570    1580       1590                               0   5 10 15       20
                                 Energy (eV)                                             Energy (eV)

van Bokhoven J. Phys. Chem. 103 (1999) 7557
                         Samples
Crystalline zeolites in H-form, in-situ calcination of NH4 zeolite
H-Beta
H-Mordenite
H-Y

Activated zeolites through steam dealumination
H-USY

Amorphous silica-alumina (14% Al2O3)




        Measurements Conditions
Low T: in presence and absence of low Pp water
High T: in vacuum
                                     Al XAFS at low temperature
Measurement conditions:
• Samples in a wet environment: flowing He / H2O
• Room temperature

                                                                                           Difference H-Beta/NH4-Beta




                                                            Normalised Absorption
 Normalised Absorption




                         1.5
                                     H-Beta
                         1.0
                                  NH4-Beta                                                   Reference Octahedral
                         0.5


                         0.0
                          1560     1570       1580   1590                           1560   1570       1580          1590
                                     Energy (eV)                                               Energy (eV)




                               Octahedral Al in H-Beta
                                      Al XAS vs 27Al MAS NMR
    Normalised Absorption


                            1.5                                                27Al
                                                   Octahedral Al
                                                                                      MAS NMR
                                       H-Beta
                            1.0
                                                                                          H Beta
                                    NH4-Beta
                            0.5

                                                                          80    60    40 20   0 -20
                            0.0                                                        ppm
                             1560   1570       1580                1590
                                      Energy (eV)

                            1.6
                                                                                        NH4 beta
Normalised Absorption




                                        NH4 Beta
                                                                                        H-Beta-NH3 (red)
                                        H- Beta-NH3 (NH3 at 100ºC)
                            1.2

                            0.8                                           80    60    40 20   0 -20
                                                                                       ppm

                            0.4

                            0.0
                             1560   1570        1580               1590
                                       Energy (eV)
                                              Al XAFS at low temperature
                                               450°C                                                                     120°C
                    NH4-Zeolite                             H-Zeolite [dry and wet]                                                 H-Zeolite
                                 2.0           H-Y wet                                               2.0




                                                                             Normalised Absorption
         Normalised Absorption




                                               H-Y dry                                                               H-Y dry
                                                                                                                     H-Y wet 120C
                                 1.5                                                                 1.5

                                 1.0                                                                 1.0

                                 0.5                                                                 0.5

                                 0.0                                                                 0.0
                                  1560   1570        1580      1590                                   1560 1565 1570 1575 1580 1585 1590
                                            Energy (eV)                                                            Energy (eV)

                                 2.0                                                                 2.0
Normalised Absorption




                                                H-Beta wet            Normalised Absorption                          H-MOR wet
                                                H-Beta wet, 120C                                                     H-MOR wet, 120C
                                 1.5                                                                 1.5

                                 1.0
                                                                                                     1.0
                                 0.5
                                                                                                     0.5
                                 0.0
                                                                                                     0.0
                                  1560   1570        1580      1590                                   1560      1570       1580        1590
                                            Energy (eV)                                                            Energy (eV)
                                            Resumé
                     H                 H
                             OH                                                     OH
                     O                 O                                 OH
    TO                                           T > 120 C   TO
           Si                Al             Si                     Si               Al
                     Si O              OH         -H2O                       Si O
                                                                                               H
                                                                                               O Si
                         H    O
      TO        OT                Si                          TO        OT               OSi




• Acidic zeolites have octahedral Al only when H2O is present

• Octahedral forms at room temperature
   Wouters J. Am. Chem. Soc. 120 (1998) 11419


• Octahedral aluminium not stable at T > 120 C

• Beta, Mordenite and zeolite Y show same behavior
                    Zeolite Steam-acivation: Octahedral Al

                                                                                H-USY (Steamed)
               40                                                         2.0
                                                                                                      RT (wet)




                                                  Normalised Absorption
               35         Wet condition
                                                                                                      25 C (Vac)
% Octahedral




               30
                                                                                                      100 C
               25                                                                                     200 C
               20                                                         1.5                         400 C
               15                  H-USY
               10
               5             ASA      Vacuum
                    H-Y                                                           Td      Oh
               0                                                          1.0
                    0     100 200 300 400 500                              1565    1570        1575     1580   1585
                          Temperature (Celcius)                                           Energy (eV)



          Coordination changes with condition
   Local Structure of the Zeolitic Catalytically
          Active Site During Reaction
                     In situ EXAFS

What do we know?
• BAS is bridging hydroxyl group
• Theory: distorted tetrahedral
              Haw JACS 124 (2002)10868
• NMR in dehydrated conditions gives extreme broad peaks
              Kentgens JACS 123 (2001) 2925          H+

                                                      O
                                                Al         Si



In situ generation of BAS
Oligomerization of olefins over the BAS
                 NH4                                              H
 TO              O                 OSi            TO              O                 OSi
       Si                   Al           450°C          Si                   Al


  TO        OT       Si O    OSi                   TO        OT       Si O    OSi




                                                                       5% ethylene in He




                                         Al EXAFS at various temperatures




At T>RT C2= oligomerization
Fourier transform, k -weighted
                                 0.3

                                 0.2
                                                           HY plus fit
                                                                                                   1.89 Å
1




                                 0.1

                                 0.0

                                 -0.1                                                      C2=, 323 K oligomerization
                                 -0.2

                                 -0.3             Fit range
                                        0   1       2           3        4
                                                   R (Å)
                                                                                                       1.81 Å
                                            CN    R (Å)
                                 NH4-Y      4.4   1.68
                                 (RT)
                                                                                                 High T coking
                                 H-Y        3.1   1.66
                                 (RT)       1.1   1.89

                                 H-Y C2=    3.4   1.64
                                 (323 K)    1.1   1.81                                          1.9Å


                                 H-Y C2=    2.9   1.65
                                 (625 K)    1.1   1.91        van Bokhoven   JACS 2004 126(14) 4506
                                                                             Science 304 (2004) 18
                     Catalysis by Gold
Partial / complete oxidation of hydrocarbons
methane, alkenes, methanol

Hydrogenation / dehydrogenation reactions
alkenes, alkynes, alkadienes, (un)saturated ketones

Methanol synthesis
CO + 2H2       CH3OH
CO2 + 3 H2      CH3OH + H2O

WGS
H2O + CO       CO2 + H2

Nitric Oxide reduction (with CO, olefins, or H2)

CO oxidation
1925: Active in CO oxidation
highly active in presence of H2: Haruta Catal. Today 36 (1997) 153
Selective CO removal, air purification, high-purity N2 and O2
                 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:
SiO2: hardly active
Al2O3, MgO: moderately active
(TiO2) Fe2O3, CeO2, 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
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)
    EXAFS Cell




• LN < T > 600 ºC
• 10-6 < P > 3 bars
• Catalysts measured as
         sieve fraction / pellet
• Combined with mass-spectrometer
• (Near) plug-flow characteristics
   Structure of gold catalysts



Sample Preparation
• Deposition precipitation HAuCl4 adjusted pH
• Washing with a base to remove chlorine

Supports
Al2O3, SiO2, CeO2, TiO2, ZrO2, Nb2O5


 Full EXAFS & XANES analyses
                                        Temperature-Programmed Reduction


                               HAuCl4 on Al2O3
                        1.4

                        1.2                                                                     1.4
Normalized Absorption




                                                                                                1.2




                                                                        Normalized Absorption
                        1.0                                                                                                  Au(0)
                                                                                                1.0
                                                                                                           Au(III)
                        0.8
                                                                                                0.8

                        0.6                                                                     0.6

                        0.4                                                                     0.4

                                                                                                0.2
                        0.2
                                                                                                0.0
                                                                                                 11875   11900       11925     11950   11975   12000
                        0.0
                         11875      11900   11925    11950   11975   12000                                            Energy (eV)
                                              Energy (eV)



                              Direct reduction from Au(III) to Au(0) (on all supports)
                              Reduction temperature is strong function of support
    EXAFS Fitting Results of Reduced Catalysts


                          2.88
                                                                        Bulk value
                                                                         2.88 Å
                          2.86
± 0.01
                          2.84
    Au Bond Distance, Å




                          2.82

                          2.80

                          2.78

                          2.76

                          2.74

                          2.72

                          2.70
                                 2   4   6           8        10   12
                                             Au-Au CN ± 10%
                 EXAFS Fitting Results of Reduced Catalysts
                 2.88



                 2.84
Au-Au Distance


                                                           Al2O3
                 2.80                                      TiO2
                                                           SiO2
                                                           Nb2O5
                                                           ZrO2
                 2.76
                            100%     30%                   CeO2
                               50%

                 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
                                                     Exposure to 20% O2
                             1.3wt% Au/Al2O3
                       1.0                                                                         1.4
Normalizd Absorption




                                                                                                   1.2




                                                                           Normalized Absorption
                       0.8                                                                                                      Au(0)
                                                                                                   1.0
                                                                                                              Au(III)

                       0.6                                                                         0.8

                                             Reduced (H2 250C)                                     0.6
                       0.4
                                             Reoxidized (20% O2 RT)
                                             Reoxidized (20% O2 225C)                              0.4
                       0.2
                                                                                                   0.2

                       0.0                                                                         0.0
                        11900        11920           11940         11960                            11875   11900       11925     11950   11975   12000
                                         Energy (eV)                                                                     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
                            Gold Oxidation under Oxygen
                 2.88



                 2.84                                 0%
                                            0%
                                       0%
Au-Au Distance




                                                            Al2O3
                 2.80                  5%                   TiO2
                                                            SiO2
                                                            Nb2O5
                                                            ZrO2
                 2.76
                                                            CeO2

                                 10%
                 2.72            10% RT
                                 15% 225ºC
                        0   10    20    30       40    50   60      70   80
                                            Particle Size (Å)

                             At most 10-15% reoxidation
                                                    XAS during CO Oxidation
                              Au/Al2O3                                                                   1.0
                        1.2




                                                                                 Normalized Absorption
Normalized Absorption




                        1.0


                        0.8
                                                                                                         0.8
                        0.6
                                                                                                                                 CO/O 2/1
                                                                                                                                     2

                        0.4
                                                                                                                                 CO/O 1/1
                                                                                                                                     2
                                                                                                                                 CO/O 1/2
                                                                                                                                     2
                        0.2
                                                                                                         0.6                     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                                                         More intense with more CO:
                          1:1      5.3      2.73                                                         holes in the d-band (anti-bonding states)
                          2:1      5.7      2.73
                          1:2      5.2      2.77
                         Small oxygen contribution
                                               Au L3: spectra simulation


                        1.0
                                Au10-CO
Normalized Absorption




                              Au10-Ox
                                              Au10
                        0.5                                       Charge transfer: d-band holes
                                                                  CO different from oxygen

                        0.0
                              -20   -10   0      10     20   30
                                          Energy (eV)
Full multiple scattering calculations (FEFF8)
                          Au / TiO2
Difference spectra with reduced Au / TiO2
1-2 nm particles
                                                                    1.0    Au10-CO




                                            Normalized Absorption
                                                                          Au10-Ox
                                                                    0.5




                                                                    0.0
                                                                           -20   -10   0     10      20   30
                                                                                       Energy (eV)
Gold catalysts and activation of oxygen

• Under (diluted) O2: surface oxidation (Au/Al2O3 & Au/TiO2)

• Switch to CO/O2: CO keeps gold reduced




                               Au/Al2O3
                               Switch Oxygen to CO
                         1.0   2s repetition
 Normalized Absorption




                         0.8

                         0.6


                         0.4

                         0.2


                         0.0
                          11910   11920   11930    11940   11950
                                            Energy/eV
                 O O
                                                                    O
                                                                        C
                                 O2                          CO + O2
                                                     Au n+

                 Au (0)        Slow        Au (0)            Fast           Au (0)




         O O



         O
             C
                               CO + O 2              Au n+

                 Au (0)                     Au (0)                      Au (0)




                          Reduced gold is active phase
                          Gold participates in oxygen activation
Van Bokhoven Angew. Chem. (2006) VIP
         Bulk gold is most inert metal: Single Au crystals are unreactive

                          What is different in small particles?

                                                   1.4

                                                   1.2




                           Normalized Absorption
                                                                                Au(0)
                                                   1.0
                                                              Au(III)

                                                   0.8

                                                   0.6

                                                   0.4

                                                   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 5d10–x6sp1+x


Mattheiss, L.F. et al. Phys. Rev. B 1980, 22, 1663
                                  Whitelines reflect number of holes in the d-band
                              Au/Al2O3                                                                       Au/TiO2
                        1.0                                                                            1.0
Normalized Absorption




                                                                               Normalized Absorption
                        0.8                                                                            0.8

                                                           Bulk Gold                                                                      Bulk Gold
                        0.6                                32 nm                                       0.6                                32 Å
                                                           12 nm                                                                          16 Å
                                                            5 nm                                                                          11 Å
                        0.4                                                                            0.4


                        0.2                                                                            0.2


                        0.0                                                                            0.0
                         11.91     11.92     11.93        11.94        11.95                            11.91     11.92     11.93        11.94        11.95
                                           Energy (eV)                                                                    Energy (keV)

                              1.1 nm Au

                        0.8
                                                                               Whiteline is particle-size dependent
Normalized Absorption




                                                     Au/ZrO2
                                                     Au/Al2O3
                        0.6
                                                     Au/TiO2
                                                     Au/Al2O3
                                                     Bulk Gold


                        0.4
                                   11.92                  11.93
                                           Energy (keV)
                                                     Whiteline intensity versus particle size
                                                                Difference intensity with bulk


                                                                                                                    0.60
                              0.60
Difference in W L Intensity




                                                                                       Difference in WL Intensity
                                                                                                                    0.45
                              0.45


                                                                                                                    0.30
                              0.30           Au/Al2O3                                                                         Au/Al2O3
                                             Bulk                                                                             Bulk
                                             Au/ZrO2                                                                          Au/ZrO2
                              0.15           CeO2
                                                                                                                    0.15
                                                                                                                              CeO2
                                             SiO2                                                                             SiO2
                                             Nb2O5                                                                            Nb2O5
                              0.00           TiO2                                                                   0.00      TiO2

                                     3   4       5      6   7   8   9   10   11   12                                  2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90
                                                 Au-Au Coordination Number                                                               Au-Au Distance (Angstron)



                                                   Six supports, one trend
                                               Larger particles fewer d-electrons
                                                           FEFF8 simulations
                        1.0                                                                                1.0
Normalized Absorption




                                                                                   Normalized Absorption
                        0.8                                                                                0.8

                                                                                                                                           Bulk Gold
                        0.6                                                                                0.6                             32 nm
                                                           Bulk Gold                                                                       12 nm
                                                           Ten-atom Cluster                                                                 5 nm
                        0.4                                                                                0.4


                        0.2                                                                                0.2


                        0.0                                                                                0.0
                          -10   -5   0   5      10    15       20     25      30                            11.91   11.92    11.93        11.94        11.95
                                             Energy (eV)                                                                    Energy (eV)




                                Smaller particles have different electronic structure
                                         affected by coordination number and s,p,d-bands

                                No support effect on number of d-electrons
                                 Projected DOS

        8                                         Bulk Gold
                                                  Ten-atom Cluster
                                                  Ten-atom Cluster
        6   smaller                               Dimer
                                                                              Fermi level
d-DOS




                                                                     d-LDOS
        4



        2



        0
        -10    -8     -6   -4       -2        0       2      4
                                Energy (eV)
                                                                                                                                                                            Fermi level
• Rehybridization of spd-orbitals                       (5d10–x6sp1+x)                       d-LDOS

• Smaller particles have fewer holes in the d-band

• Particle size dominates support-effect

• Oxygen is activated on gold particle


       Boiling point                                      Gold-Gold distance                                                                       d electrons
                                             2.88
                                                                                                                                   0.60




                                                                                                      Difference in WL Intensity
                                             2.84
                                                                                                                                   0.45
                            Au-Au Distance




                                                                                    Al2O3
                                             2.80                                   TiO2                                           0.30
                                                                                    SiO2                                                     Au/Al2O3
                                                                                    Nb2O5                                                    Bulk
                                                                                    ZrO2                                                     Au/ZrO2
                                             2.76                                                                                  0.15
                                                                                    CeO2                                                     CeO2
                                                                                                                                             SiO2
                                                                                                                                             Nb2O5
                                             2.72                                                                                  0.00      TiO2

                                                    0    10   20   30   40    50    60      70   80                                  2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90
                                                                    Particle Size (Å)                                                                   Au-Au Distance (Angstron)
                           XAS
• Structure: bond lengths, coordination numbers, DWF
• Geometry
• Oxidation states
• In-situ conditions
• Element specific

              Sample damage
              Model free data fitting
              Asymmetric distribution functions
              Mixtures (Bulk technique)
              ... ... ...
                                                                   Summary
                                      4             1st shell

                                      2                            3rd    4th

                  Fourier transform
                                                            2nd                                     1.89 Å

                                      0


                                      -2
                                                                                            C2=, 323 K oligomerization
                                      -4

                                          1    2       3            4      5    6
                                                                  R (Å)

     OO
                                                                                                        1.81 Å


     O
      C
                                  CO + O
                                       2                      Aun+                                High T coking
            (0)                                      Au(0)                            (0)
          Au                                                                        Au


                                                                                                 1.9Å
                                              Fermi level
d-LDOS
Acknowledgement
Eveline Bus and Professor Prins (ETH)

Dr. Weiher (ETH)

Dr. Miller (BP)

Drs. Glatzel and Safonova (ESRF)

Dr. Tromp (Southampton University)

Professor Ramaker (GWU)

Professor Koningsberger (UU)


Swiss National Fonds ($$)

								
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