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Understanding Dynamic Chemistry
at the Catalytic Interfaces
Debabrata Bagchi
17/01/2019
NCU Student Topic Seminar
 Conclusions & future aspects
 Challenge in probing the dynamics at Catalytic Interface
 Dynamic behavior at Catalytic Interface
 What is Catalytic Interface
2
Outline
 In Situ Techniques to probe the Dynamics at Interface
 Difficulty of using Conventional Spectroscopy & Microscopy
 Environmental Transmission Electron Microscopy (ETEM)
 High Pressure Scanning Tunneling Microscopy (HP STM)
What is Interface?
Solid
Gas
Liquid
Solid/Gas
Solid/Liquid
Gas/Liquid
 When different phases exist together, the boundary
between two of them is called Interface.
Wenpei Gao et al. Acc. Chem. Res. 2017, 50, 787-795. 3
Semiconductor Device Fuel Cell Surface Coating Catalysis Interface
Herbert Kroemer
Nobel Prize in Physics, 2000
The interface is the device
Catalysis
4
What is Catalysis?
Homogeneous Catalysis
Enzyme catalyst Organometallic catalyst
• Single phase
• Typically Liquid
Heterogeneous Catalysis
• Multi phase
• Solid-Liquid or Solid-Gas
Zeolite catalyst Catalyst powders
5
Where is Catalytic Interface?
Thermochemical Catalysis
Solid-Gas Interface
Electrochemical Catalysis
Solid-Liquid Interface
Photochemical Catalysis
Solid-Liquid Interface
Yong Yang et al. Chem. 2018, 4, 2054−2083.
6
Dynamic chemistry happening in Catalytic Interface
Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027.
----
----
Catalyst Surface
Reactant
adsorption
Product
desorption
Surface
Reaction
Sabatier principle
 The catalyst-reactant interface can contain
Intact molecules having Van der Waals interaction
with the surface
Reactant molecules having chemically bonded with
catalyst surface
Intermediates of a catalytic reaction adsorbed on the surface of the catalyst
Physisorption
Chemisorption
 Elementary Steps of Heterogeneous Catalysis
7Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027.
Catalyst-Gas Interface
Changes occurred in the catalyst surface…
Surface restructuring
(forming kinks & steps)
Stepped atoms are having lower coordination
& form bonds with adsorbate.
Oxidation or reduction
of catalyst
Metal based catalyst can be oxidised or
reduced at harsh condition which usually
decrease catalytic activity.
Metal atoms can
migrate to the interface
For alloy catalyst the metal having high
affinity towards adsorbed gas can migrate
to the surface.
Reactant gas can react
with catalyst at interface
Certain metals are reactive towards
gases which poison the catalyst.
Sintering of Metal
Nanoparticle catalyst
The agglomeration and gradual growth of
nanoparticles which decrease the activity.
8
Changes occurred in the catalyst surface…
Particle Detachment
Decrease catalytic activity
Metal Dissolution
Usually decease the activity
Dealloying
Sometimes increase the activity by
creating strain in the system
Oswald Ripening
Reduce the activity
Reshaping
May increase the activity by
exposing active facet
Agglomeration
Decease the activity
Nejc Hodnik et al. Acc. Chem. Res. 2016, 49, 2015−2022
Catalyst-Liquid Interface
9
Time & length scales for dynamic processes
in Catalytic Interface
 Blue: Molecular processes at the active site.
 Green: Processes involving solid state catalysts.
 Yellow: Transport processes of reactants and products.
Kai F. Kalz et al. ChemCatChem 2017, 9, 17−29.
10
How to probe dynamic processes in Catalysis?
?
 How can we make the structural changes visible &
deduce their implications for the catalytic activity of
the system?
 How fast are the dynamic changes?
 How can more active phases be kinetically stabilized
& regenerated?
 How to reduce the structural changes that deactivate
the catalytic activity?
In situ or Operando Studies
Spectroscopic Study Conducted in Reaction Condition
(High pressure, Temperature or at some specific condition)
Kai F. Kalz et al. ChemCatChem 2017, 9, 17−29.
11
In Situ Techniques to probe the dynamics
Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027.
Photon based
Spectroscopy
Electron based
Microscopy
 In Situ X-ray Absorption Spectroscopy
(XAS)
 Ambient Pressure X-Ray Photoelectron
Spectroscopy
 Surface Enhanced Raman
Spectroscopy (SERS)
 In Situ Infrared (IR) Spectroscopy
 High Pressure Scanning Tunneling
Microscopy (HP-STM)
 Environmental Transmission
Electron Microscopy(ETEM)
Used to know the chemical environment
(oxidation state or coordination no. of metal)
or chemical property of adsorbed species
Can image dynamic behaviour
happened at the catalytic interface
 For visualisation of dynamic nature of
interface, we need microscopy
12
In Situ X-Ray Absorption Spectroscopy(XAS)
In Situ XAS: Evolution of the structure, coordination & oxidation state of metal
centre during catalysis.
Pd@ZrO2
Kristof Paredis et al. J. Am. Chem. Soc. 2011, 133, 13455–13464.
 At ≤120°C a Pdδ+ species were detected from initial Pd
metallic species. This process parallels the high production
of N2O observed.
 At ≥150°C the selectivity shifts mainly toward N2 (∼80%) the
Pd atoms aggregate again into metallic Pd NPs.
Reduction of NO with H2
13Franklin Tao et al. Chem. Commun., 2012, 48, 3812–3814.
In Situ X-Ray Photoelectron Spectroscopy(XPS)
Evolution of Ce 3d XPS spectra of
CeO2 at different reaction conditions
In Situ XPS: measures the elemental composition, empirical formula, chemical state
& electronic state of the elements that exist within a material.
 Oxygen vacancies on a CeO2 surface are active
sites for dissociative or molecular adsorption during
catalysis.
Ceria (CeO2)
 At 400°C in O2 Most of the cerium atoms exist in the
form of Ce4+
 Upon purging hydrogen and then filling oxygen, the
Ce4+ is partially reduced to Ce3+
 From XPS Spectra
 Operando XPS of CeO2 shows the dynamic
change of oxidation state of Ce.
14
In Situ Electron Microscopy
 Conventional Electron Microscopy needs UHV
 Electron must transit from the sample to a detector
without scattering from any background gas over a
flight path on the order of 1 m
 Microchannel plates in detector do not tolerate moisture
Electron energy analyser
Vacuum chamber
(10-9 Torr/mBar )
 Mean free path
At p = 1atm =1.013 bar = 760 Torr, l = 70 nm;
Whereas at 10-10 Torr l = 500 km
T. W. Hansen et al. Catal. Lett. 2002, 84, 7-9.
15
Environmental Transmission Electron Microscopy
T. W. Hansen et al. Mat. Sci. Technol. 2010, 26, 1338–1344.
Ernst Ruska
Nobel Prize in Physics
in 1986 for inventing TEM
Main purpose:
To confine the reactant to the vicinity of the sample thus making the
gas path length along the direction of the electrons as short as possible.
16
Environmental TEM study
Emmanuel Auger et al. Science 2001, 294, 1508-1511.
Ba-Ru/BN
Barium promoted ruthenium catalyst on a
support of boron nitride
Thin film of BN covers most Ru crystals which
can’t be active
The distance between the BN layers is 0.34
nm corresponding to the (002) planes..
 The increased activity is suggested to
be related to a barium-oxygen overlayer
on the Ru crystals.
Operando Condition: NH3 Synthesis
3:1 H2/N2 mixture at 550°C & 5.2 mbar
ETEM image of Ba-Ru/BN
The lattice spacing is 0.23 nm (100) planes of Ru.
On the edge of the Ru crystal, a small monolayer
patch of a barium oxide phase is observed.
Conventional TEM
3H2 + N2 2NH3
17G. Krinner at al. Nature 2004, 427, 426−429.
Growth of CNF at Interface during catalysis
 Carbon Nano Fibres from methane decomposition was developed through
reshaping of Ni nanocrystal.
Ni/MgAl2O4 catalyst used for steam reforming
Carbon formation can destroy the catalyst pellets
causing blockage of the reactor with detrimental result
CH4:H2= 1:1 at P = 2.0 mbar T = 500-540°C
5 nm5 nm5 nm5 nm
3.5 mbar H2, 430°C
 The graphene overlayer helps the formation of Ni steps, and hence, the release of Ni
adatoms, which can diffuse along the interface towards the free surface &CNF forms.
18
ETEM Study of (100)Au@CeO2 & Interface CO
Hideto Yoshida et al. Science 2012, 335, 317−319.
 CO adsorption induces a reconstruction of a (100) surface facet to a
(100)-hex facet on Au NPs.
At 45 Pa, RT
At 45 Pa, RT
Au/CeO2 catalyst showed high catalytic activity for the oxidation of CO at RT
At Vacuum
19
 By combining ab initio calculations with image simulations, it is confirmed that
CO molecules only bind with reconstructed hexagonal Au top layers on the
(100) surface.
 Such selective absorption implies dissimilar reaction rates on different surface
facets can be applied to elucidate reaction mechanisms.
1 volume % CO in air at 100Pa
Hideto Yoshida et al. Science 2012, 335, 317−319.
ETEM Study of (100)Au@CeO2 & Interface CO
20
Sintering at the Catalytic Interface
S. B. Simonsen et al. J. Am. Chem. Soc. 2010, 132,7968–7975.
 Pt/Al2O3/Si3N4 Catalyst
 Understand the fundamental of Sintering
 10 mBar air at 650 °C
 Oxygen-induced Pt Nanoparticle Sintering
Visualisation of Sintering by ETEM
 Here sintering is happening by Oswald Ripening process which means diffusion of
mass from smaller nanoparticle to immobile larger nanoparticles.
The diameters of the selected particles presented
as a function of time
21
Dynamics of Pt/CNT on O2 & H2O Using ETEM
Langli Luo et al. ACS Catal. 2017, 7, 7658−7664.
 Particle migration and coalescence is the dominant coarsening mechanism.
 In comparison with the case of H2O, O2 promotes Pt nanoparticle migration on
the carbon surface.
 The strong oxygen chemisorption on Pt nanoparticles weakens the interaction
between Pt and the CNT surface, leading to a fast migration in O2.
0.01 mbar
Schematic Of ETEM Set Up
22
Dynamics of Pt/CNT on O2 & H2O Using ETEM
Langli Luo et al. ACS Catal. 2017, 7, 7658−7664.
 Particle migration and coalescence is the dominant coarsening mechanism.
 In comparison with the case of H2O, O2 promotes Pt nanoparticle migration on
the carbon surface.
 The strong oxygen chemisorption on Pt nanoparticles weakens the interaction
between Pt and the CNT surface, leading to a fast migration in O2.
0.01 mbar
Schematic Of ETEM Set Up
23Langli Luo et al. ACS Catal. 2017, 7, 7658−7664.
Dynamics of Nafion/Pt/CNT on O2 & H2O Using ETEM
Pt/CNTs Nafion/Pt/CNTs
 Nafion is used as a membrane for PEMFC
by permitting hydrogen ion transport.
Nafion/Pt/CNTs
 Nafion electrolyte layer creates a mechanical confinement to Pt/CNTs, which reduces
the Pt migration rate in O2.
 This mechanical confinement is largely relieved by introducing H2O, and a lubricated
interface is created leading to a faster migration rate of Pt in H2O than that in O2.
24
Probing Nano particle interface: Challenge
J. A. Rodriguez et al. Science 2007, 318, 1757-1760.
Single crystals of metal,
bimetallic, oxide, carbide, or
sulfide with specific
crystallographic faces
What is Model Catalyst
Elimination of complexity of variations in particle size, shape,
and irregular defect structures of high-surface-area systems.
Role of Model Catalyst…
To identify atomic lateral
packing of the topmost surface
of catalyst nanoparticles with a
size of 1-10 nm
Challenge
25
Scanning Tunneling Microscope (STM)
STM is an electron microscope that transmit 3D
images of the electron cloud around the nucleus
STM allows the inspection of the properties of
a conductive solid surface at an atomic size.
Fe atoms on Cu(111)
Nobel Prize in Physics in 1986
for inventing STM
Gerd Binning Heinrich Rohrer
STM image of Graphite
26
Basic Set-Up of STM
 The basis of STM is the Quantum Tunneling theory
 There is a finite probability that an electron will “jump”
from one surface to the other of lower potential.
STM includes,
1. Scanning tip
2. Piezoelectric controlled scanner
3. Distance control & scanning unit
4. Vibration isolation system
5. Computer
27
Tunneling Current in STM
2
(0, ) kz
t s FI V E e 

 If the distance(z) between tip and sample increases, It will decrease exponentially.
 Atoms of different elements of a catalyst surface or of adsorbates can be
readily distinguished as It depends on local density of states s (0, EF) of the sample
surface.
Constant Height Mode Constant Current Mode
28
Creating In Situ Condition in HP-STM
Luan et al. Rev. Sci. Instrum. 2013, 84, 034101.
The main feature is isolation of the gas
Environment from the UHV environment
General Features
 Sample is heated by the
irradiation from Halogen
Lamp through the window
• Differential pumping system can
keep the STM chamber at UHV,
while keeping pressure in the
reaction cell 100 Torr
• Volume of reactor is only 10mL
• A dome made of Mo having small
aperture (tor movement of tip) is
installed between STM room & reactor
Let’s talk about application…
29
Restructuring due to surface lattice strain in
Hex-Pt(100) & CO Interface
Feng Tao et al. Nano Lett. 2009, 9, 2167−2171.
CO at pressures 5x10-9 Torr.
CO at pressures 10-5 Torr
CO molecules are bound to Pt nanoclusters through a tilted on-
top configuration with a separation of ∼3.7-4.1 Å
The phenomenon of restructuring of metal catalyst surfaces
induced by adsorption
DFT Models of Pt(557) covered by CO
30
Behaviour of stepped catalyst & gas interface
Pt(557) & CO
Feng Tao et al. Science, 2010, 327, 850–853.
P=10-10Torr P=10-8Torr,CO
P=1Torr of CO
STM images of Pt(557)
Low-coordination Pt edge sites in nanoclusters
relieves the strong CO-CO repulsion in the
highly compressed adsorbate film
As CO coverage approaches 100%, the flat terraces of Pt(557) break up
into nm-sized clusters & the process is reversible
CO + H2O CO2 + H2
Water-gas shift reaction
31E. K. Vestergaard et al. Phys. Rev. Lett. 2005, 95, 126101.
Dynamics in a Metal Alloy-gas Interface
Au/Ni(111) surface alloy & CO
At high pressure surface is covered with small irregular clusters,
persisting even after the high-pressure CO is pumped away
At 1000 mbar of COAt UHV
0 min 25 min 50 min
75 min 100 min 125 min
At 13 mbar of CO
Ni-carbonyl formation is responsible for the removal of the Ni atoms in the
surface layer.
Scale: (800x800 Å2)
Inset: (50x50 Å2)
STM Image Analysis
(1000x1000 Å2)
Inset: (60x60 Å2) reveals a
clean Ni(111) surface)
STM images (1000x1000 Å2)
taken from an STM movie.
 Movie reveals that the Au cluster
formation starts at the Ni steps. Ni
atoms are removed and Au clusters
are nucleated and left behind.
32Baran Eren et al. Science 2016, 351, 475–478.
Restructuring due to low cohesive energy by HP-STM
Cu(111) & CO interface
Cu (111), most compact & lowest energy surface of Cu, became
unstable and formed cluster at the terraces when exposed to CO gas
Restructuring mainly results from the relatively weak metal-metal bond, as indicated
by the low cohesive energy of 3.5 eV and energy gain from CO binding to low
coordinated Cu atoms
In UHV micro terraces
are observed
Clusters form
at step edges
Clusters form
on the terraces
High density of clusters with
adsorbed CO molecule
33
Effect of clustering on surface reactivity for the
Water Gas Shift reaction
CO + H2O CO2 + H2
Water-gas shift reaction
Baran Eren et al. Science 2016, 351, 475–478.
 Water does not adsorb on the Cu(111)
surface at room temperature
 In the presence of 2 × 10−9 Torr of H2O,
the cluster-covered surface was very
active in dissociating water, as shown
by the increasing oxygen peak.
 A key step in the water-gas i.e.
dIssociation of H2O shift reaction,
becomes highly activated as a result of
the CO-induced clustering.
APXPS experiments of H2O
adsorption on Cu(111)
34A. E. Baber et al. J. Am. Chem. Soc. 2013, 135, 16781–16784.
Gas enhanced mass transport at the surface
Cu2O-Cu(111) & CO Interface
 Cu2O(111)-like thin films grown
on Cu(111) appear as rows.
Cu(111) Cu2O/Cu(111)
UHV-STM images
Inset Scale=1nm Inset Scale=3nm
The metallic Cu is observed growing from the upper step
edge after exposing to CO for 281s.
Cu atoms are released from the reduction of Cu2O by CO, &
then diffuse to the step edges to form metallic Cu terraces.
P=10 mTorr of CO scale bar = 5nm
nm
46 s 281 s 374 s 490 s
603 s 715 s 828 s
 The reduction from the Cu2O oxide rows to the glass
like hex/5-7 ring oxide, and then to metallic Cu is
observed.
35
Formation of self assembled hydrocarbon at Interface
Co(0001)-Gas interface during reaction
Violeta Navarro et al. Nat. Chem. 2016, 8, 929–934.
220°C, 4bar, (PH2:PAr = 1:4)
220°C, 40 min after exposing gas at 4bar, (PCO:PH2:PAr = 1:2:4)
The growth of a hydrocarbon takes place by the repeated
addition of individual CH2 monomers at the steps and the
terrace sites store the alkyl units.
Particles below a certain size may be too small to comfortably
accommodate the long hydrocarbon molecules on their nano terraces.
n CO + (2n+1) H2
CnH2n+2 + n H2
Fischer-Tropsch Reaction
36G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728.
CO induced coalescence of isolated Pd adatoms
at the Fe3O4(001)
Fe3O4(001) surface
CO induces the mobility in
the Pd/Fe3O4 system
STM images(6.5×8.5 nm2) of Pd on the
Fe3O4(001) at 6×10−11 mbar of CO pressure
37G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728.
CO induced coalescence of isolated Pd adatoms
at the Fe3O4(001)
Fe3O4(001) surface
CO induces the mobility in
the Pd/Fe3O4 system
STM images(6.5×8.5 nm2) of Pd on the
Fe3O4(001) at 6×10−11 mbar of CO pressure
Mobile species also interact with surface hydroxyl groups and form stable Pd-OH
STM images (10×10 nm2)
38
Effect of higher pressure of CO on the Pd/Fe3O4
G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728.
1.33×10-4 mbar.sec of COBefore After
(30×30nm2)
(a) Before: Isolated Pd adatoms and hydroxyl groups are observed, as
well as five OH-Pd species (marked by red x) a few small clusters.
(b) After: All Pd adatoms have disappeared, many several large clusters
have formed, as well as 22 OH-Pd species.
CO is linked to Pd adatom sintering and OH-Pd species are significantly
more resistant to CO induced sintering than bare Pd atoms.
39
HP-STM & ETEM for In Situ Studies of Interface
HP-STM ETEM
Single crystal model catalyst Catalyst Nano particle catalyst
ResolutionLateral: 0.1 Å, vertical 0.1 Å Lateral: 0.1-1 Å, vertical 0.1-1Å
Information
depth
1−2 atomic layers Subsurface and bulk
At least milliseconds
Time per
image of
10 nm ×10 nm
Nanosecond to seconds
Potential
damage
No chance Possible
Lateral & vertical packings of
atoms of the topmost surface
Function
Atomic packing of subsurface
& chemical information
Franklin. Tao et al. Chem. Rev. 2016, 116, 3487-3539.
40
Challenge & Future Aspects
 Probing the transient phenomena happening in the interface is an issue.
Study of Solid Liquid interface is still challenging at reaction condition.
Thermal drift is always a unavoidable problem for high Temperature
reaction dynamic study.
Deconvolution of beam effects (for ETEM).
Effects might be directly to sample or by ionization of gases.
Ultrafast Laser Spectroscopy can be utilized to
probe the transient intermediate species & exact
mechanism.
A wide range of operando imaging and
spectroscopy techniques along with theoretical
understanding will dramatically improve our
ability to establish structure−reactivity relations
.
Thank you…
God made the bulk; the surface was
invented by the evil.
Wolfgang Pauli
Nobel Prize in Physics, 1945

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Understanding Dynamic chemistry at the Catalytic Interface

  • 1. Understanding Dynamic Chemistry at the Catalytic Interfaces Debabrata Bagchi 17/01/2019 NCU Student Topic Seminar
  • 2.  Conclusions & future aspects  Challenge in probing the dynamics at Catalytic Interface  Dynamic behavior at Catalytic Interface  What is Catalytic Interface 2 Outline  In Situ Techniques to probe the Dynamics at Interface  Difficulty of using Conventional Spectroscopy & Microscopy  Environmental Transmission Electron Microscopy (ETEM)  High Pressure Scanning Tunneling Microscopy (HP STM)
  • 3. What is Interface? Solid Gas Liquid Solid/Gas Solid/Liquid Gas/Liquid  When different phases exist together, the boundary between two of them is called Interface. Wenpei Gao et al. Acc. Chem. Res. 2017, 50, 787-795. 3 Semiconductor Device Fuel Cell Surface Coating Catalysis Interface Herbert Kroemer Nobel Prize in Physics, 2000 The interface is the device
  • 4. Catalysis 4 What is Catalysis? Homogeneous Catalysis Enzyme catalyst Organometallic catalyst • Single phase • Typically Liquid Heterogeneous Catalysis • Multi phase • Solid-Liquid or Solid-Gas Zeolite catalyst Catalyst powders
  • 5. 5 Where is Catalytic Interface? Thermochemical Catalysis Solid-Gas Interface Electrochemical Catalysis Solid-Liquid Interface Photochemical Catalysis Solid-Liquid Interface Yong Yang et al. Chem. 2018, 4, 2054−2083.
  • 6. 6 Dynamic chemistry happening in Catalytic Interface Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027. ---- ---- Catalyst Surface Reactant adsorption Product desorption Surface Reaction Sabatier principle  The catalyst-reactant interface can contain Intact molecules having Van der Waals interaction with the surface Reactant molecules having chemically bonded with catalyst surface Intermediates of a catalytic reaction adsorbed on the surface of the catalyst Physisorption Chemisorption  Elementary Steps of Heterogeneous Catalysis
  • 7. 7Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027. Catalyst-Gas Interface Changes occurred in the catalyst surface… Surface restructuring (forming kinks & steps) Stepped atoms are having lower coordination & form bonds with adsorbate. Oxidation or reduction of catalyst Metal based catalyst can be oxidised or reduced at harsh condition which usually decrease catalytic activity. Metal atoms can migrate to the interface For alloy catalyst the metal having high affinity towards adsorbed gas can migrate to the surface. Reactant gas can react with catalyst at interface Certain metals are reactive towards gases which poison the catalyst. Sintering of Metal Nanoparticle catalyst The agglomeration and gradual growth of nanoparticles which decrease the activity.
  • 8. 8 Changes occurred in the catalyst surface… Particle Detachment Decrease catalytic activity Metal Dissolution Usually decease the activity Dealloying Sometimes increase the activity by creating strain in the system Oswald Ripening Reduce the activity Reshaping May increase the activity by exposing active facet Agglomeration Decease the activity Nejc Hodnik et al. Acc. Chem. Res. 2016, 49, 2015−2022 Catalyst-Liquid Interface
  • 9. 9 Time & length scales for dynamic processes in Catalytic Interface  Blue: Molecular processes at the active site.  Green: Processes involving solid state catalysts.  Yellow: Transport processes of reactants and products. Kai F. Kalz et al. ChemCatChem 2017, 9, 17−29.
  • 10. 10 How to probe dynamic processes in Catalysis? ?  How can we make the structural changes visible & deduce their implications for the catalytic activity of the system?  How fast are the dynamic changes?  How can more active phases be kinetically stabilized & regenerated?  How to reduce the structural changes that deactivate the catalytic activity? In situ or Operando Studies Spectroscopic Study Conducted in Reaction Condition (High pressure, Temperature or at some specific condition) Kai F. Kalz et al. ChemCatChem 2017, 9, 17−29.
  • 11. 11 In Situ Techniques to probe the dynamics Jian Dou et al. Chem. Soc. Rev. 2017, 46, 2001-2027. Photon based Spectroscopy Electron based Microscopy  In Situ X-ray Absorption Spectroscopy (XAS)  Ambient Pressure X-Ray Photoelectron Spectroscopy  Surface Enhanced Raman Spectroscopy (SERS)  In Situ Infrared (IR) Spectroscopy  High Pressure Scanning Tunneling Microscopy (HP-STM)  Environmental Transmission Electron Microscopy(ETEM) Used to know the chemical environment (oxidation state or coordination no. of metal) or chemical property of adsorbed species Can image dynamic behaviour happened at the catalytic interface  For visualisation of dynamic nature of interface, we need microscopy
  • 12. 12 In Situ X-Ray Absorption Spectroscopy(XAS) In Situ XAS: Evolution of the structure, coordination & oxidation state of metal centre during catalysis. Pd@ZrO2 Kristof Paredis et al. J. Am. Chem. Soc. 2011, 133, 13455–13464.  At ≤120°C a Pdδ+ species were detected from initial Pd metallic species. This process parallels the high production of N2O observed.  At ≥150°C the selectivity shifts mainly toward N2 (∼80%) the Pd atoms aggregate again into metallic Pd NPs. Reduction of NO with H2
  • 13. 13Franklin Tao et al. Chem. Commun., 2012, 48, 3812–3814. In Situ X-Ray Photoelectron Spectroscopy(XPS) Evolution of Ce 3d XPS spectra of CeO2 at different reaction conditions In Situ XPS: measures the elemental composition, empirical formula, chemical state & electronic state of the elements that exist within a material.  Oxygen vacancies on a CeO2 surface are active sites for dissociative or molecular adsorption during catalysis. Ceria (CeO2)  At 400°C in O2 Most of the cerium atoms exist in the form of Ce4+  Upon purging hydrogen and then filling oxygen, the Ce4+ is partially reduced to Ce3+  From XPS Spectra  Operando XPS of CeO2 shows the dynamic change of oxidation state of Ce.
  • 14. 14 In Situ Electron Microscopy  Conventional Electron Microscopy needs UHV  Electron must transit from the sample to a detector without scattering from any background gas over a flight path on the order of 1 m  Microchannel plates in detector do not tolerate moisture Electron energy analyser Vacuum chamber (10-9 Torr/mBar )  Mean free path At p = 1atm =1.013 bar = 760 Torr, l = 70 nm; Whereas at 10-10 Torr l = 500 km T. W. Hansen et al. Catal. Lett. 2002, 84, 7-9.
  • 15. 15 Environmental Transmission Electron Microscopy T. W. Hansen et al. Mat. Sci. Technol. 2010, 26, 1338–1344. Ernst Ruska Nobel Prize in Physics in 1986 for inventing TEM Main purpose: To confine the reactant to the vicinity of the sample thus making the gas path length along the direction of the electrons as short as possible.
  • 16. 16 Environmental TEM study Emmanuel Auger et al. Science 2001, 294, 1508-1511. Ba-Ru/BN Barium promoted ruthenium catalyst on a support of boron nitride Thin film of BN covers most Ru crystals which can’t be active The distance between the BN layers is 0.34 nm corresponding to the (002) planes..  The increased activity is suggested to be related to a barium-oxygen overlayer on the Ru crystals. Operando Condition: NH3 Synthesis 3:1 H2/N2 mixture at 550°C & 5.2 mbar ETEM image of Ba-Ru/BN The lattice spacing is 0.23 nm (100) planes of Ru. On the edge of the Ru crystal, a small monolayer patch of a barium oxide phase is observed. Conventional TEM 3H2 + N2 2NH3
  • 17. 17G. Krinner at al. Nature 2004, 427, 426−429. Growth of CNF at Interface during catalysis  Carbon Nano Fibres from methane decomposition was developed through reshaping of Ni nanocrystal. Ni/MgAl2O4 catalyst used for steam reforming Carbon formation can destroy the catalyst pellets causing blockage of the reactor with detrimental result CH4:H2= 1:1 at P = 2.0 mbar T = 500-540°C 5 nm5 nm5 nm5 nm 3.5 mbar H2, 430°C  The graphene overlayer helps the formation of Ni steps, and hence, the release of Ni adatoms, which can diffuse along the interface towards the free surface &CNF forms.
  • 18. 18 ETEM Study of (100)Au@CeO2 & Interface CO Hideto Yoshida et al. Science 2012, 335, 317−319.  CO adsorption induces a reconstruction of a (100) surface facet to a (100)-hex facet on Au NPs. At 45 Pa, RT At 45 Pa, RT Au/CeO2 catalyst showed high catalytic activity for the oxidation of CO at RT At Vacuum
  • 19. 19  By combining ab initio calculations with image simulations, it is confirmed that CO molecules only bind with reconstructed hexagonal Au top layers on the (100) surface.  Such selective absorption implies dissimilar reaction rates on different surface facets can be applied to elucidate reaction mechanisms. 1 volume % CO in air at 100Pa Hideto Yoshida et al. Science 2012, 335, 317−319. ETEM Study of (100)Au@CeO2 & Interface CO
  • 20. 20 Sintering at the Catalytic Interface S. B. Simonsen et al. J. Am. Chem. Soc. 2010, 132,7968–7975.  Pt/Al2O3/Si3N4 Catalyst  Understand the fundamental of Sintering  10 mBar air at 650 °C  Oxygen-induced Pt Nanoparticle Sintering Visualisation of Sintering by ETEM  Here sintering is happening by Oswald Ripening process which means diffusion of mass from smaller nanoparticle to immobile larger nanoparticles. The diameters of the selected particles presented as a function of time
  • 21. 21 Dynamics of Pt/CNT on O2 & H2O Using ETEM Langli Luo et al. ACS Catal. 2017, 7, 7658−7664.  Particle migration and coalescence is the dominant coarsening mechanism.  In comparison with the case of H2O, O2 promotes Pt nanoparticle migration on the carbon surface.  The strong oxygen chemisorption on Pt nanoparticles weakens the interaction between Pt and the CNT surface, leading to a fast migration in O2. 0.01 mbar Schematic Of ETEM Set Up
  • 22. 22 Dynamics of Pt/CNT on O2 & H2O Using ETEM Langli Luo et al. ACS Catal. 2017, 7, 7658−7664.  Particle migration and coalescence is the dominant coarsening mechanism.  In comparison with the case of H2O, O2 promotes Pt nanoparticle migration on the carbon surface.  The strong oxygen chemisorption on Pt nanoparticles weakens the interaction between Pt and the CNT surface, leading to a fast migration in O2. 0.01 mbar Schematic Of ETEM Set Up
  • 23. 23Langli Luo et al. ACS Catal. 2017, 7, 7658−7664. Dynamics of Nafion/Pt/CNT on O2 & H2O Using ETEM Pt/CNTs Nafion/Pt/CNTs  Nafion is used as a membrane for PEMFC by permitting hydrogen ion transport. Nafion/Pt/CNTs  Nafion electrolyte layer creates a mechanical confinement to Pt/CNTs, which reduces the Pt migration rate in O2.  This mechanical confinement is largely relieved by introducing H2O, and a lubricated interface is created leading to a faster migration rate of Pt in H2O than that in O2.
  • 24. 24 Probing Nano particle interface: Challenge J. A. Rodriguez et al. Science 2007, 318, 1757-1760. Single crystals of metal, bimetallic, oxide, carbide, or sulfide with specific crystallographic faces What is Model Catalyst Elimination of complexity of variations in particle size, shape, and irregular defect structures of high-surface-area systems. Role of Model Catalyst… To identify atomic lateral packing of the topmost surface of catalyst nanoparticles with a size of 1-10 nm Challenge
  • 25. 25 Scanning Tunneling Microscope (STM) STM is an electron microscope that transmit 3D images of the electron cloud around the nucleus STM allows the inspection of the properties of a conductive solid surface at an atomic size. Fe atoms on Cu(111) Nobel Prize in Physics in 1986 for inventing STM Gerd Binning Heinrich Rohrer STM image of Graphite
  • 26. 26 Basic Set-Up of STM  The basis of STM is the Quantum Tunneling theory  There is a finite probability that an electron will “jump” from one surface to the other of lower potential. STM includes, 1. Scanning tip 2. Piezoelectric controlled scanner 3. Distance control & scanning unit 4. Vibration isolation system 5. Computer
  • 27. 27 Tunneling Current in STM 2 (0, ) kz t s FI V E e    If the distance(z) between tip and sample increases, It will decrease exponentially.  Atoms of different elements of a catalyst surface or of adsorbates can be readily distinguished as It depends on local density of states s (0, EF) of the sample surface. Constant Height Mode Constant Current Mode
  • 28. 28 Creating In Situ Condition in HP-STM Luan et al. Rev. Sci. Instrum. 2013, 84, 034101. The main feature is isolation of the gas Environment from the UHV environment General Features  Sample is heated by the irradiation from Halogen Lamp through the window • Differential pumping system can keep the STM chamber at UHV, while keeping pressure in the reaction cell 100 Torr • Volume of reactor is only 10mL • A dome made of Mo having small aperture (tor movement of tip) is installed between STM room & reactor Let’s talk about application…
  • 29. 29 Restructuring due to surface lattice strain in Hex-Pt(100) & CO Interface Feng Tao et al. Nano Lett. 2009, 9, 2167−2171. CO at pressures 5x10-9 Torr. CO at pressures 10-5 Torr CO molecules are bound to Pt nanoclusters through a tilted on- top configuration with a separation of ∼3.7-4.1 Å The phenomenon of restructuring of metal catalyst surfaces induced by adsorption
  • 30. DFT Models of Pt(557) covered by CO 30 Behaviour of stepped catalyst & gas interface Pt(557) & CO Feng Tao et al. Science, 2010, 327, 850–853. P=10-10Torr P=10-8Torr,CO P=1Torr of CO STM images of Pt(557) Low-coordination Pt edge sites in nanoclusters relieves the strong CO-CO repulsion in the highly compressed adsorbate film As CO coverage approaches 100%, the flat terraces of Pt(557) break up into nm-sized clusters & the process is reversible CO + H2O CO2 + H2 Water-gas shift reaction
  • 31. 31E. K. Vestergaard et al. Phys. Rev. Lett. 2005, 95, 126101. Dynamics in a Metal Alloy-gas Interface Au/Ni(111) surface alloy & CO At high pressure surface is covered with small irregular clusters, persisting even after the high-pressure CO is pumped away At 1000 mbar of COAt UHV 0 min 25 min 50 min 75 min 100 min 125 min At 13 mbar of CO Ni-carbonyl formation is responsible for the removal of the Ni atoms in the surface layer. Scale: (800x800 Å2) Inset: (50x50 Å2) STM Image Analysis (1000x1000 Å2) Inset: (60x60 Å2) reveals a clean Ni(111) surface) STM images (1000x1000 Å2) taken from an STM movie.  Movie reveals that the Au cluster formation starts at the Ni steps. Ni atoms are removed and Au clusters are nucleated and left behind.
  • 32. 32Baran Eren et al. Science 2016, 351, 475–478. Restructuring due to low cohesive energy by HP-STM Cu(111) & CO interface Cu (111), most compact & lowest energy surface of Cu, became unstable and formed cluster at the terraces when exposed to CO gas Restructuring mainly results from the relatively weak metal-metal bond, as indicated by the low cohesive energy of 3.5 eV and energy gain from CO binding to low coordinated Cu atoms In UHV micro terraces are observed Clusters form at step edges Clusters form on the terraces High density of clusters with adsorbed CO molecule
  • 33. 33 Effect of clustering on surface reactivity for the Water Gas Shift reaction CO + H2O CO2 + H2 Water-gas shift reaction Baran Eren et al. Science 2016, 351, 475–478.  Water does not adsorb on the Cu(111) surface at room temperature  In the presence of 2 × 10−9 Torr of H2O, the cluster-covered surface was very active in dissociating water, as shown by the increasing oxygen peak.  A key step in the water-gas i.e. dIssociation of H2O shift reaction, becomes highly activated as a result of the CO-induced clustering. APXPS experiments of H2O adsorption on Cu(111)
  • 34. 34A. E. Baber et al. J. Am. Chem. Soc. 2013, 135, 16781–16784. Gas enhanced mass transport at the surface Cu2O-Cu(111) & CO Interface  Cu2O(111)-like thin films grown on Cu(111) appear as rows. Cu(111) Cu2O/Cu(111) UHV-STM images Inset Scale=1nm Inset Scale=3nm The metallic Cu is observed growing from the upper step edge after exposing to CO for 281s. Cu atoms are released from the reduction of Cu2O by CO, & then diffuse to the step edges to form metallic Cu terraces. P=10 mTorr of CO scale bar = 5nm nm 46 s 281 s 374 s 490 s 603 s 715 s 828 s  The reduction from the Cu2O oxide rows to the glass like hex/5-7 ring oxide, and then to metallic Cu is observed.
  • 35. 35 Formation of self assembled hydrocarbon at Interface Co(0001)-Gas interface during reaction Violeta Navarro et al. Nat. Chem. 2016, 8, 929–934. 220°C, 4bar, (PH2:PAr = 1:4) 220°C, 40 min after exposing gas at 4bar, (PCO:PH2:PAr = 1:2:4) The growth of a hydrocarbon takes place by the repeated addition of individual CH2 monomers at the steps and the terrace sites store the alkyl units. Particles below a certain size may be too small to comfortably accommodate the long hydrocarbon molecules on their nano terraces. n CO + (2n+1) H2 CnH2n+2 + n H2 Fischer-Tropsch Reaction
  • 36. 36G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728. CO induced coalescence of isolated Pd adatoms at the Fe3O4(001) Fe3O4(001) surface CO induces the mobility in the Pd/Fe3O4 system STM images(6.5×8.5 nm2) of Pd on the Fe3O4(001) at 6×10−11 mbar of CO pressure
  • 37. 37G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728. CO induced coalescence of isolated Pd adatoms at the Fe3O4(001) Fe3O4(001) surface CO induces the mobility in the Pd/Fe3O4 system STM images(6.5×8.5 nm2) of Pd on the Fe3O4(001) at 6×10−11 mbar of CO pressure Mobile species also interact with surface hydroxyl groups and form stable Pd-OH STM images (10×10 nm2)
  • 38. 38 Effect of higher pressure of CO on the Pd/Fe3O4 G. S. Parkinson et al. Nat. Mater. 2013, 12, 724–728. 1.33×10-4 mbar.sec of COBefore After (30×30nm2) (a) Before: Isolated Pd adatoms and hydroxyl groups are observed, as well as five OH-Pd species (marked by red x) a few small clusters. (b) After: All Pd adatoms have disappeared, many several large clusters have formed, as well as 22 OH-Pd species. CO is linked to Pd adatom sintering and OH-Pd species are significantly more resistant to CO induced sintering than bare Pd atoms.
  • 39. 39 HP-STM & ETEM for In Situ Studies of Interface HP-STM ETEM Single crystal model catalyst Catalyst Nano particle catalyst ResolutionLateral: 0.1 Å, vertical 0.1 Å Lateral: 0.1-1 Å, vertical 0.1-1Å Information depth 1−2 atomic layers Subsurface and bulk At least milliseconds Time per image of 10 nm ×10 nm Nanosecond to seconds Potential damage No chance Possible Lateral & vertical packings of atoms of the topmost surface Function Atomic packing of subsurface & chemical information Franklin. Tao et al. Chem. Rev. 2016, 116, 3487-3539.
  • 40. 40 Challenge & Future Aspects  Probing the transient phenomena happening in the interface is an issue. Study of Solid Liquid interface is still challenging at reaction condition. Thermal drift is always a unavoidable problem for high Temperature reaction dynamic study. Deconvolution of beam effects (for ETEM). Effects might be directly to sample or by ionization of gases. Ultrafast Laser Spectroscopy can be utilized to probe the transient intermediate species & exact mechanism. A wide range of operando imaging and spectroscopy techniques along with theoretical understanding will dramatically improve our ability to establish structure−reactivity relations .
  • 41. Thank you… God made the bulk; the surface was invented by the evil. Wolfgang Pauli Nobel Prize in Physics, 1945

Editor's Notes

  1. TEM:.2A, Light micro 200nm
  2. Nobel prize for developing semiconductor heterostructure used in high speed and optoelectronics
  3. Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst. And what it does , it directs the reaction in different path and reduce the activation energy of the reaction.
  4. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts
  5. approximating approach, also called an indirect approach of surface science, was put forward for simulating the surface of a catalyst to achieve a fundamental understanding of catalysis. T, P, Nm and mm are the same as described above, Na is the total number of adsorbed reactant molecules on the surface and ma is the chemical potential of the adsorbed reactant molecule on the surface
  6. bimetallic surface could restructure at high temperature in UHV. The driving force typically is the decrease of the surface energy through segregating the atoms of a constituting metal whose surface energy is lower than the other constituting metal.
  7. eactant-related species during catalysis can be tracked with vibrational spectroscopy or an NMR technique in a gas or even liquid. They are mainly used to track the reactant-relevant surface species derived from reactants, including adsorbed reactants, or their dissociated species or intermediates towards the formation of products
  8. Usually, a promoter is defined as a substance that causes a more than proportional increase in activity or selectivity when added to the catalyst promoter alone is completely inactive in the catalytic process, Lattice fringes are resolved in both the Ru particles and BN support material. Under these conditions, the BN is not covering the Ru crystals. Still, the crystal lattice of Ru is well-resolved just as the layered structure of BN could be easily observed. the morphology of the Ru crystals was not altered by the presence of Ba, which suggests that the number of active sites was not substantially altered and that Ba is an electronic pro In situ electron energy-loss spectroscopy (EELS) at the O K edge and the Ba M 4,5 edge demonstrates that the Ba is present in an oxide structure
  9. develop through a reactioninduced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites ). Between a pair of such step edges, an additional graphene layer grows as the Ni steps move towards the ends of the Ni cluster and vanish. Clearly, this process involves transport of C atoms towards and Ni atoms away from the graphene–Ni interface.
  10. (CO) molecules caused the {100} facets of a gold nanoparticle to reconstruct during CO oxidation at room temperature In vacuum, the distance between the topmost and the second topmost {100} surface layers of 0.20 nm was the same as the interplanar distance of the {200} planes in crystalline bulk gold.
  11. A and B, is parallel to the {100} facet and it is off the <110> zone axis, which decreased the image contrast of the crystalline GNP and enhanced that of the light-element atoms such as carbon and oxygen An image simulation was performed in combination with ab initio electronic calculations with regard to CO adsorption on the Au{100}- hex reconstructed surface
  12. the dynamic migration of three couples of Pt nanoparticles smaller particle migrates toward the larger one, indicating a higher apparent migration barrier for large particles. coalescence is the rotation of the particle at the left side, as seen from the 0 s image, followed by the merging of the gap between these two nanoparticles, as seen in images of 56 s, and finally the formation of one larger single Pt NP, as shown in the image of 118 s
  13. the dynamic migration of three couples of Pt nanoparticles smaller particle migrates toward the larger one, indicating a higher apparent migration barrier for large particles. coalescence is the rotation of the particle at the left side, as seen from the 0 s image, followed by the merging of the gap between these two nanoparticles, as seen in images of 56 s, and finally the formation of one larger single Pt NP, as shown in the image of 118 s
  14. TEM images showing the nonreversible expansion of the Nafion layer upon water introduction
  15. Figure 1. Schematics of structure of a catalyst nanoparticle. (a) Catalyst nanoparticle (gray cube) loaded on a support (light blue plate). (b) Surface (gray), subsurface (yellow), and bulk (green) of a catalyst nanoparticle supported on a support; the characterization techniques of structures of different sections are marked on the schematic. Scanning tunneling microscopy (STM) is the technique to visualize the topmost surface of a catalyst. Environmental transmission electron microscopy (ETEM) provides projected structural and chemical information on surface, subsurface, and bulk. (c) Schematic of a topmost surface of a catalyst nanoparticle consisting of different types of catalytic sites that can be identified with scanning tunneling microscopy because STM can visualize both the lateral and vertical arrangements of atoms on the topmost surface of a catalyst. (d) Schematic of arrangements of catalyst atoms of the topmost surface of a nanoparticle. port. During catalysis, the structure of a catalyst may vary in a complex way along with changes in pressure of reactants and temperature of a catalyst. To establish an intrinsic correlation between structure of a catalyst and its corresponding catalytic performance, it is crucial to characterize the surface (Figure 1c and d) and bulk structures (Figure 1a and b) of the catalyst at the atomic level under reaction conditions or during catalysis
  16. the catalytic surface below the adsorbate layer could significantly restructure in order to release the increased intermolecular repulsion of the adsorbate layer at a high coverage of molecules. At these pressures, the surface is only partially reconstructed, with the lifted atoms forming clusters along the [110] direction in the same terrace showing numerous islands of 0.5-3.5 nm in size and ∼0.23 nm high Each at 10-6 island consists of an approximately square arrangement of maxima along both the [01-1] and [011] directions with a corrugation of ∼0.1 nm. Th
  17. at low pressure, the initially straight steps (Fig. 1A) became wavy (Fig. 1B) The nanoclusters have a roughly triangular shape with the vertex pointing to the lower terrace clusters are rectangular or roughly parallelogram shaped We thus propose that the observed break-up of the surface is driven by the relaxation of the repulsive CO-CO interaction, restructuring provides a substantial increase in the number of low-coordinated edge atoms where CO molecules can tilt away from the center and thus decrease their mutual repulsion. Pt(557) is restructured into nanoclusters with a thickness of 2–3 atomic layers under reaction condition (1 Torr CO or above)
  18. The preferential formation of nickel carbonyl at the edges of Ni–Au surface leaves Au atoms with a quite low coordination and thus they aggregate to form Au nanoclusters. As no nucleation of Ni islands is observed, it is thus proposed that the surface Ni atoms are
  19. Under UHV conditions, the Cu(111) surface exhibited micrometre-scale terraces, atomic steps and screw dislocations. After exposing the surface to 0.1 Torr CO, the step edges became rough, while the rest of the surface remained atomically flat. With increasing the pressure to 0.2 Torr, the terraces were covered with nanoclusters that increased in density along with the CO pressure
  20. More than half of the heterogeneous catalysts used in the chemical industries are oxides. Cu is observed growing from the upper step edge after exposing to CO for 281 s.
  21. After exposure to the reactive gases for 30 min, islands with a height of 0.11 0.03 nm were observed on the Co surface. aligned with the three h1010i equivalent directions