XAFS

XAFS Basics and catalyst Characterization.

X線吸収微細構造

Kiyotaka Asakura

Catalysis Research Center

Hokkaido University

Characterization techniques of

Catalysts

Kiyotaka Asakura

Institute for Catalysis,

Hokkaido University

2013 AGS

How to characterize the Catalysts



Quality, Quantity and State Analysis



Imaging, Spectroscopy and Diffraction

2013 AGS

Three categories

Diffraction

Absorption

Transmission

Scattering

Probe

M

ate

ria

l

Diffraction

Spectroscopy

Imaging



Materials are illuminated by probe(light, electron,

neutron, muon, He, ion and sharp tip)

metal wire tip A STM tunneling current conductor metal wire tip A STM tunneling current conductor i f

hv

E

E





Emission

2013 AGS

Characterization techniques

NMR Nuclear Magnetic Resonance Nuclear spin Structure

ESR Electron Magnetic Resonance Electron spin Structure

IR Infrared Vibration Adsobate

RAMAN Raman scattering Vibration Structure

UV-VIS Ultraviolet and visible absorption Electron absorption Electronic state

XPS X-ray photoelectron spectroscopy Photoelectron mission Electronic State

XRD X-ray diffraction Diffraction Structure

XSAS X-ray small angle scattering Scattering Long range order

XAFS X-ray absorption fine structure X-ray absorption Local structure

Mössbauer Gamma-ray absorption Electronic state

TEM Transmission electron microscopy Electron Morphology

SPM Scanning probe microscopy Probe tip Morphology

EPMA Electron probe microanalysis Fluorescent x-ray Local composition

XRF X-ray fluorescence Fluorescent X-ray Composition

PEEM Photoemission electron microscopy Photoemission WF

In –situ Characterization

2013 AGS



What is in situ ?



In situ is a Latin phrase, “in position”



In catalyst, it means “as it is”.



Why is in situ analysis necessary?



Catalyst active site is not stable.



Catalyst active site structure changes

with conditions.



Catalyst active site structure should be

different from before and after the

Rh dimer catalyst



Active and selective for ethylene hydroformylation reaction at

low pressure.

Catalyst

TOF/10

-4

_min

-11

Selectivity/%

Total Ethane

Propanal

Propanal/total

Impreg

Rh dimer

10.9

36.9

9.9

4.1

0.96

32.8

8.8

88.9

C

₂

H

₄

:CO:H

₂

=1:1:1 total pressure , 40.0 kPa

CO insertion reaction is usually done under high CO pressure

C

₂

H

₄

+H

₂

+CO C

₂

H

₆

+C

₂

H

₅

CHO

In –situ Characterization

2013 AGS 4 6 8 10 12 14 16 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 k  (k ) K / 10nm-1 Rh Rh C C O O C₂H₅ Cｐ* O O SiO2 Rh Rh C O C2H5 Cｐ* O O SiO2 4 6 8 10 12 14 16 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 k  (k ) k /10 nm-1 +CO Heat, -CO +H2 RCHO +CO, +C2H4 0.270 nm [RhCp*(CH3)]2(m-CH2)2 IR 1710 cm-1 IR 2032,1969 cm-1

EXAFS with adsorbed CO EXAFS after CO desorption No bond

X-ray Absorption Fine Structure

Fine structure appearing around X-ray absorption edge

extending upto about 1000eV

m / a rb . u n it s 24500 24000 23500 23000 Photon energy/ eV EXAFS (40-1000 eV) XANES m = ln ( I₀ / I ) I₀ I X -r ay a bs o rp tio n e dg e

XANES（X-ray Absorption Near Edge Structure) EXAFS（Extended X-ray Absorption Fine Structure）

x-ray

 outgoing electron and incoming electron interfere with each other

 Outgoing and incoming electrons have a wavelength (or wave number

X-ray Absorbing atom

X-ray scattering atom      0 1 0 0 2 ) ( 262 . 0 ) ( ) (/ 150 2 2 2 ) ( E eV h A k E eV h E h m k A         _      

amplifiers & computer

Beamline

BL9C

, KEK-PF

SR

I

₀

I

_t

optics slits detectors

sample double crystal

monochromator Si(311) n= 2dsinq

The experimental hutch

0 2 2

2

m

E

E

k







K: wave vector、

: Plank Const

E：Photon energy E0:threshold



Enhancement

suppression

XAFS gives you bond distance and coordination number

When the coordination number increases, the amplitude also increases

mt/ a rb . u n it s Photon energy/ eV

When the bond length

increases, the frequency of the EXAFS oscillation will higher mt/ a rb . u n it s Photon energy/ eV

The EXAFS equation

R₀ Outgoing Photoelectron Scattered Photoelectron Scattering atom Absorbing atom )) ( 2 sin( ) ( ) ( ) ( ) ( ) ( 2 ₂ 2 2 2 0 0 i i i i k i _{i i} i i i s _{e k r k} r k k F N S E E E k i i  m m m  _  _



  _ shift Phase : * amblitude ring Backscatte : ) ( / 2 i k r i i i e F    

Theorptically or empirically derived Parameters

XAFS oscillation Absorbance Smooth backgroun

Edge-jump Curve-Fitting Parameters N_i Coordination number _i2 _DWfactor E₀ energy shift r distance  / ) ( 2m E E₀ k  _e 

Sketch of ＸＡＦＳ

analysis

8500 9000 9500 10000 0.0 0.5 1.0 1.5 2.0 a) m_pre m₀ m_post A bs or ba nc e E / eV 4 6 8 10 12 14 -30 -20 -10 0 10 20 30 b) k 3 ( k) k / Å-1 0 1 2 3 4 5 6 -30 -20 -10 0 10 20 30 Fourier filter c) F ou ri er tr an sf or m r / Å 4 6 8 10 12 14 -20 -15 -10 -5 0 5 10 15 20 d) k 3  (k ) k / Å-1 Data Photon energy to k Background removal kn weight Fourier transform k curve fitting Fourier filtering R curve fitting CookSayers -evaluation Model structure

40 30 20 10 0 Amplitude/a.u. 5 4 3 2 1 0

Supported nano Gold catalsyts

High activity for CO oxidation at room temperature when it is in nanosize

Au foil

Highly active catalysts

CO + O2 _CO2

Low activity catalysts

EXAFS does not require long range order!!

Amorphous, liquid, Enzyme, powder

RT

M.Haruta,GoldBull.37,27(2004).

Time resolved measurements

1. QXAFS Quick XAFS

2. DXAFS Dispersive XAFS

P R q Sample X-ray Ring Focus size = 0.3 mm 0.8

Quick XAFS

QXAFS

– Less than a few second

– CRC constructed BL for QXAFS

dedicated to catalysis.

Supported by Grant-in-aid for scientific research S

Ni2P Ni(OH)2+ H3PO4 Redispersion process Reduction and formation of Ni-P Highly dispersed Ni species

θ= 9.4 degree (Rh K-edge) p = 30 m

q = 0.35 m R = 4.2 m

1/p + 1/q = 2/(R sinq DE/E= L sinq cotq /p

P

R

q

Sample

X-ray

Ring

Focus size = 0.3 mm

Diagram of energy dispersive XAFS(DXAFS)

technique

22800 22000 mt 0 0.2 0.4 0.6 0.8 0 20 40 Time / s 22400 E / eV 60

Re catalysts during activation process

（Courtesy of Prof. M.Tada and Prof. Y.Iwasawa)

Tada,M. Iwasawa, Y. (2006). Angew. Chem.-45, 448. (2007); J.Phys.Chem.C 111, 10095触媒反応温度での触媒反応温度でのNHNH₃₃による活性による活性Reクラスターの形成過程Reクラスターの形成過程

4 3 2 1 0 P h en o l fo rm at io n r a te /m m o l g R e -1 s -1 120 100 80 60 40 20 0

NH₃-treatment time /min

100 80 60 40 20 0 P h e n o l s el e ct iv ity % -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 0 min Re-Re CN= 0 -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 0 min -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 0 min Re-Re CN= 0 -20 -10 0 10 20 F T [k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 30 min CN= 1.3 -20 -10 0 10 20 F T [k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 30 min -20 -10 0 10 20 F T [k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 30 min CN= 1.3 -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 60 min CN= 2.8 -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 60 min -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 60 min CN= 2.8 -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 120 min CN= 5.2 -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 120 min -20 -10 0 10 20 F T [ k 3 ( k) ] 5 4 3 2 1 0 R /10-1 nm 120 min CN= 5.2 フェノール選択性 90-94% Re L_III-edge EXAFS 553 K, pNH3=75 kPa (A) (B) Re Re Re Re Re Re Re Re Re Re N N O O O O O O Al HZSM-5 O O O Et O O O O HZSM-5 Phenol selectivity =90-94%

Hydrodesulfurization(HDS) catalyst

It removes sulfur compounds from fossil fuel.

Reactions are conducted under high pressure

condition in the presence of oil.

s

s

H

₂

Development of hydrodesulfurization catalyst

Sulfur is contained in cruid oil(1%)

→Poisoning of Automobile catalyst, corrosion, SOx Fuel cell electrode

石油連盟HPより

Legal regulation of S content In fuel

The legal regulation to reduce the sulfur content in fossil

fuel in spite of the increasing sulfur content in

available crude oil motivate the development of high

performance hydrodesulfurization catalysts.

– Improvement of conventional catalysts(NiMoS, CoMoS).

– To find a new type of catalyst.

Ni

₂

P is a new class of catalyst to show the high HDS

activity.

– S. T. Oyama, J.Catal. 216, 343 (2003).

Nickel phosphosulfide (Ni

₂

PS) is an active phase.

It has a strong support effect.

0 25 50 75 100 DDS Sel. HYD Sel. Conv. Ni₂P/SiO₂-H 0 25 50 75 100 HYD Sel. Conv. DDS Sel. Ni2P/SiO2-L 0 10 20 30 40 50 60 0 25 50 75 100 DDS Sel. Conv.

HYD Sel. Ni-Mo-S/Al₂O₃

Time / h 0 25 50 75 100 DDS Sel. HYD Sel. Conv. _Ni 2P/MCM-41 C on ve rs io n a nd S el ec tiv ity / % Conv. = 68% Conv. = 29% Conv. = 57% Conv. = 95% HYD sel. = 34% HYD sel. = 50% HYD sel. = 65% HYD sel. = 72%

Activity in HDS of 4-6 DMDBT (573 K, 30 atm)

12.2 wt % Ni₂P on SiO₂ S. Ted Oyama and Yong-Kul Lee J.Cat 2008 in press

Stable structure might be the long life time

In-situ XAFS

We conducted the active site structure in the presence

of reactants(oil, hydrogen) under high pressure and

high temperature conditions.

Is it easy?

– NO!!

– Why?

In situ EXAFS

princilple put window away from furnace.

.

High pressure cell J.Phys. Chem.93,4213(1989) Z.Phys.Chem.,144,10 5(1985). Thermocouple Heater Water Gas Gas Gas Window (Acrylic resin) O-ring Quartz Tube Sample Water Water Water X-ray Io I J.Synchro.Rad.8, 581(2001). J.Chem.Phys. 70 (1979) 4849. X-ray absorption, Principles, applications, techniques of EXAFS, SEXAFS, and XANES, New York, John Wiley & Sons, 1988.

in situ high pressure & high temperature gas phase XAFS



Y-zeolite supported Rh catalysts during H

₂

reduction

Flow rate :20 % H₂/Ar 100ml/min

Total pressure 3 MPa Temperature rate: 7K/min

XANES : Oxidized Rh species  Rh metal EXAFS : Rh-O  Rh-Rh

In situ EXAFS application to HDS

In situ EXAFS is a powerful tool.

The hydrodesulfurization reactions are often carried out in the presence of oil under high-pressure and high temperature conditions.

Large X-ray absorbance of liquid phase.

We have to put the windows close to the sample.

Windows must be tolerable against high pressure, high temperature and keeping the high X-ray transparency.

S Hydrodesulfurization

Answers found in literature

Demands are “window materials must stand for

high pressure and high temperature at the same

time”.

– Capillary cells

• But curved window causes thickness effects

– Diamond, Be and Al Flat windows.

• Expensive (diamond) or toxic (Be)

• Diamond, Be and Al are chemically and thermally not so stable.

Al window

N. Weiher et.al., J. Synchrotron Rad.

12 (2005) 675. J. D. Grunwaldt, et al. _{Rev. Sci. Instrum. 76, (5),}

054104 (2005).

G. Sankar, and J. M. Thomas, Topics.in Catal. 8, 1 (1999).

Cubic BN

cBN

hBN

Cubic BN is Second hardest material

Tensile strength = 1078.7 MPa

Diamond=2000 Mpa;Be=260 MPa

Previously cBN was made using binders and stregths was reduced.

Recently direct formation of cBN without binders by directly conversion at high pressure (7.7 GPa ) and high temperature(2400 K).

(Sumitomo Electro Engineering Co. Dr. Sumiya)

It is thermally stable upto 1273 K and chemically also stable.

Attenuation length 600 mm at 8 keV tolerable against 30 MPa.

A cell drawing

Cross section

SUS315 3.5 kg Bolt

Fixed screw hole Cartridge heater

ｃ- BN

Cell installed in the BL7C

EXAFS experiments were carried out at BL7C at Photon Factory

Synthesis of Ni

₂

P on MCM41(12.4 wt%)

Impregnation Drying/Calcination Reduction to phosphides Passivation in 0.5%O₂ TPR in H₂ Phosphorus and metal compounds 300K 923K Supported phosphate precursors 773K TPR in H₂ Reduction to phosphides Reaction TPR In H₂

Temperature sequences

0 200 400 900 1100 1300 0 100 200 300 400 500 b c d e a

T

e

m

p

e

r

a

t

u

r

e

/

℃

Time /min.

Reduction

Ramping rate=1.4 K/min

H

₂

flow rate=50 cc/min

Structure of passivated Ni

₂

P on MCM-41

Ni - O CN : 5.9 ±0.8 R : 0.204 ± 0.001 nm dE : -2.8 ± 2.0 eV DW : 0.0084 ± 0.0018 nm Ni - Ni CN : 4.6 _{R : 0.310} ±0.9_{± 0.001 nm} dE : 2.6 ± 1.5 eV DW : 0.0104 ± 0.0015 nm The structure

coincides with bulk

Ni(OH)₂

Crsytal structure of Ni(OH)₂

Impregnation Drying/Calcination Reduction to phosphides Passivation in 0.5%O2 TPR in H2 Phosphorus and metal compounds 300K 923K Supported phosphate precursors 773K TPR in H2 Reduction to phosphides Reaction TPR In H2 Impregnation Drying/Calcination Reduction to phosphides Passivation in 0.5%O2 TPR in H2 Phosphorus and metal compounds 300K 923K Supported phosphate precursors 773K TPR in H2 Reduction to phosphides Reaction TPR In H2 TPR In H2

Temperature sequences

0 200 400 900 1100 1300 0 100 200 300 400 500 b c d e a

T

e

m

p

e

r

a

t

u

r

e

/

℃

Time /min.

Reduction

Ramping rate=1.4 K/min

H

₂

flow rate=50 cc/min

Water desorption during reduction.

Temperature sequences 0 200 400 900 1100 1300 0 100 200 300 400 500 b c d e a Te m pe ra t ur e / ℃ 0 200 400 900 1100 1300 0 100 200 300 400 500 0 200 400 900 1100 1300 0 100 200 300 400 500 b c d e a b c d e a Te m pe ra t ur e / ℃ Time /min. Reduction XAFS measurement XAFS measurement

Structure after the reduction

0 1 2 3 4 5 6 0 5 10 15

After the reduction

Ni₂P F T r / 0.1 nm Ni₂P/MCM-41 N r/ A D2_/nm2 Ni-P 1.9 2.23 0.0 Ni-Ni 4.9 2.63 0.001 Ni₂P bulk N r/ A Ni-P 2.0 2.23 Ni-Ni 7.0 2.63 0 5 10 15 20 0.4 0.6 0.8 1.0

Calculated CN for Ni-P Calculated CN for Ni-Ni Ni-P given by XAFS Ni-Ni given by XAFS

Ni-P Ni-Ni SiO₂ MCM41 N (p a ri tc le )/ N (b u lk ) diameter / nm 1-2 nm Ni₂P nanoparticle Ni₂P is formed in MCM-41

c

a

b

Ni2P is formed after the activation

Ni

P

S. T. Oyama, J.Catal. 216, 343 (2003). Structure of Ni₂P space group P -6 2 m lattice constant a = 5.859 b = 5.859 c = 3.382 a= 90 b = 90 g = 120

Synthesis of Phosphides

Thiophene hydrodesulfurization reaction at various temperature H₂ 50 sccm with thiophene 0.004 ml/min, 12.2 wt % Ni₂P /MCM-41

Impregnation Drying/Calcination Reduction to phosphides Passivation in 0.5%O₂ TPR in H₂ Phosphorus and metal compounds 300K 923K Supported phosphate precursors 773K TPR in H₂ Reduction to phosphides Reaction TPR In H₂

QXAFS measurement during the reaction

Thiophnene input

_{Thiophnene input}

No big change during the reaction was found.

0 25 50 75 100 DDS Sel. HYD Sel. Conv. Ni₂P/SiO₂-H 0 25 50 75 100 HYD Sel. Conv. DDS Sel. Ni2P/SiO2-L 0 10 20 30 40 50 60 0 25 50 75 100 DDS Sel. Conv.

HYD Sel. Ni-Mo-S/Al₂O₃

Time / h 0 25 50 75 100 DDS Sel. HYD Sel. Conv. _Ni 2P/MCM-41 C on ve rs io n a nd S el ec tiv ity / % Conv. = 68% Conv. = 29% Conv. = 57% Conv. = 95% HYD sel. = 34% HYD sel. = 50% HYD sel. = 65% HYD sel. = 72%

Activity in HDS of 4-6 DMDBT (573 K, 30 atm)

12.2 wt % Ni₂P on SiO₂ S. Ted Oyama and Yong-Kul Lee J.Cat 2008 in press

Stable structure might be the long life time

EXAFS of Ni₂P in the presence of Oil

J. Cat. 2006, 241, 20-24.

Rev. Sci. Instrum. 2008, 79,014101

2 4 6 8 10 12 -0.1 0.0 0.1 0.2

b)

a)

 (k ) k / A-1

Before reaction

Working conditions

4 6 8 10 -0.004 -0.002 0.000 0.002 0.004  (k ) k / 10 nm-1 Ni-S Ni-S=0.224 nm

z z z z buffer vent X-ray sample HDS reactor NaOHaq absorber silicagel ZnO absorber Burner Exhaust gas duct Pressure gage z IR Exhaust gas duct

Exhaust gas duct Gas sensor Gas sensor MFC H₂ He Thiophene bubbler Cell Gas switch Hutch QMS

Experimental Setup

MCT

Interferometer

Optical faiber

IR

X-ray

QMS

Gas controller

Activitiy

QMS

Gas flow

Structure

QXAFS

Adsorption

FT-IR

Reaction Cell

KEK, PF BL-9C

45

Experimental Setup

46

X-ray

Infrared

Thermocouple

Gas injection bulb

Heater

Sample : Ni

₂

P/MCM-41

35 mg, 15 mm Φ disk

45 deg tilted against Xray and

IR.

加熱機構

反応セル全体をヒーターで包み、

最大803 Kで加熱可能。

ガス導入機構

四方からガスを導入し、

中央でガスを取り出している。

Sample

C

₄

H

₄

S + 2H

₂

→ C

₄

H

₈

S

C

₄

H

₈

S + H

₂

→ H

₂

S + C

₄

H

₈

Difference spectra in HDS – before reaction

Ni P Ni P P P Ni S _P Ni P Ni P P P Ni P

• CF results of Ni-S

R=0.227 nm； CN=0.1.

• Judging from the ratio of

surface Ni to the bulk ~0.5

S/Ni=0.2±0.2

• Little reaction temperature

dependence

k (k) / 10 n m -1 0.00 0.02 0.04 0.06 0.08 0.10 0.12 k / 10 nm-1 3 4 5 ₆ 7 8 9 10 Ni – S (FEFF) CN = 0.1, R = 0.227 nm, 2_{= 0.0064 Å}2 HDS at 513 K HDS at 553 K HDS at 593 K

How does Ni-S bond change?

250 300 350 400 450 500 550 600 0.0 0.1 N N i-S T / K 0 10 20 30 40 50 60 0.0 0.1 0.2 t / min 0.0 0.1 0.2  N i-S 0.0 0.1 0.2 513 K 553 K 593 K T / K 513 553 593 v/min-1 _{0.47(3) 0.8(3) 2.5(3)}

•Ni-S is rapidly formed and saturated.

•Saturation number is 0.1 •Ea=53 kJ/mol for formation reaction of Ni-S

0.1

Time and temperature dependence of Ni-S formation

Formation rate

XANES, IR and MS changes during 513 K

49

XANES, IR and MS changes during 513 K

50 HDS反応開始 3000 2950 2900 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 a b so rb an ce wave number /cm-1 513 K HDS 0.8 min 11 min 21 min 31 min 57 min 77 min 98 min 109 min 123 min

XANES, IR and MS changes during 513 K

51 HDS反応開始 Ⅲ _{: STEADY} STATE Ⅰ _{: Ni-SFORMATION} Ⅱ _{: REACTION}

Reaction mechanisms

52

NiPS Formation processes

Ⅰ _{: Ni-S Formation}

Reaction mechanisms

Hydorgenation processes _{C-S bond cleavage}

C-S bond cleavage H₂S

2H₂

NiPS Formation processes

Ⅱ _{: REACTION}

H₂S

2010年9月16日 2B06 53

Conclusions

Phys. Conference Series 2009, 190, 012158; Journal of Catalysis 2009, 268, 209-222; Physical Chemistry C 2011, 115, 7466-7471 Journal of Catalysis 2012, 286, 165-171 J.Synchrotro.Rad. 2012, 19, 205-209 S S H H H Ni P Ni Ni S P S S Ni P Ni Ni S P S Ni P Ni Ni S P S H2 S H H H Ni P Ni Ni S P S Ni P Ni Ni S P S S H Ni P Ni Ni S P S S H + -H -H -H H2 + -Ni P Ni Ni S P S H H H H + -S Ni P Ni Ni S P S H + S H + 2--H2S E2 E2 Ni P Ni P P P Ni P Sulfur compounds

53 kJ/mol

S removes

_{more than 620 K}

Ni P Ni P P P Ni S _P

1. Very small Ni₂P cluster (~1 nm) is present in MCM41. 2. Ni₂PS phase is formed during the reaction conditions. 3. The formation energy of Ni-S is 53 kJ/mol.

4. Ni-S is stable in the reaction conditions. 5. Ni-S is an active site to extract H.

XAFS

Powerful for dynamic analysis of catalyst surface.

Strong X-ray sources are preferable like Synchrotron

radiation.