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Understanding Dynamic chemistry at the Catalytic Interface

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The presentation talks about the In Situ techniques to probe the dynamic behaviour happening at the Catalytic Interfaces.

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Understanding Dynamic chemistry at the Catalytic Interface

  1. 1. Understanding Dynamic Chemistry at the Catalytic Interfaces Debabrata Bagchi 17/01/2019 NCU Student Topic Seminar
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 41. Thank you… God made the bulk; the surface was invented by the evil. Wolfgang Pauli Nobel Prize in Physics, 1945

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