IC T Characterzation of Catalysts Investigate Structuremorphology Surface

IC T Characterzation of Catalysts Investigate: • Structure/morphology • Surface area • Number of active sites • Pore distributions • Nature of active site • Overall reactivity • Turn over frequency (TOF) • Selectivity • Stability • Heat and mass transport • ? IC-1/35 Lecture-5 07 -10 -2004

IC T The Surface Science Approach • Simpler system - Detailed studies • Fundamental insight • Input to catalyst design • The structure gap • The pressure gap • The materials gap Single crystal surfaces as model catalysts. IC-2/35 Lecture-5 07 -10 -2004

IC T Catalysts are complex Nano-materials IC-3/35 Lecture-5 07 -10 -2004

IC T Most often used methods IC-4/35 Lecture-5 07 -10 -2004

IC T X-Ray Diffraction (XRD) Bragg ´s Law X-rays in X-rays out catalyst Can be used in situ Gives information on phases and sizes of particles if big enough. IC-5/35 Lecture-5 07 -10 -2004

IC T X-Ray Diffraction (XRD) Position reveals crystalline structure Width of peaks reveals particle size 4. 5 Å 2. 5 Å IC-6/35 Lecture-5 07 -10 -2004

IC T X-rays X-ray Photoelectron Spectroscopy (XPS) Photo-electrons catalyst Can be not used in situ as it require vacuum Ekin=hn-EB-F Gives information on elementary coposition and chemical state IC-7/35 Lecture-5 07 -10 -2004

IC T Decays following creation of core hole L X-ray emission (Electron dispersive x-ray emission - EDX): (hn’)=Eb(K)-Eb(L) K After ~10 -14 s Auger electron emission: L Photoelectron: Ekin=hn – Eb - F Ekin=Eb(K)-Eb(L 1)-Eb(L 2) K IC-8/35 Lecture-5 07 -10 -2004

IC T Surface Sensitive Electrons traveling though a solid has a short mean free path I(z)=I 0 exp(-z/l) IC-9/35 Lecture-5 07 -10 -2004

IC T X-ray Photoelectron Spectroscopy (XPS) Notice how the EB is influenced by chemical environnement IC-10/35 Lecture-5 07 -10 -2004

IC T X-ray Photoelectron Spectroscopy (XPS) On combustion catalyst the loading of Pt may be so small that it cannot be detected by XPS (0. 1% and 2 -5 nm particles). IC-11/35 Lecture-5 07 -10 -2004

IC T Extended X-Ray Absorption Fine Structure (EXAFS) catalyst Measure adsorbtion An in situ method Gives information on local atomic distances and coordination number IC-12/35 Lecture-5 07 -10 -2004

IC T Extended X-Ray Absorption Fine Structure (EXAFS) IC-13/35 Lecture-5 07 -10 -2004

IC T Extended X-Ray Absorption Fine Structure (EXAFS) IC-14/35 Lecture-5 07 -10 -2004

IC T Electron Microscopy Does not work in situ Gives information on partiles size, shape, composition. IC-15/35 Lecture-5 07 -10 -2004

IC T TEM images of Cu on Zn. O (a model methanol catalyst). IC-16/35 Lecture-5 07 -10 -2004

IC T Environnemental EM from Haldor Topsøe A/S Ni Graphite ~1000 Å IC-17/35 Lecture-5 07 -10 -2004

IC T An in situ method Mössbauer spectroscopy hn=14. 4 ke. V DE=10 -9 e. V Rather specialized method since it works only on Fe, Sm, . . IC-18/35 Lecture-5 07 -10 -2004

IC T Mössbauer spectroscopy example Reveals oxidation state and chemical sourroundings of particular Fe and Mo catalysts IC-19/35 Lecture-5 07 -10 -2004

IC T Ions in Ion spectroscopy: SIMS Ions out catalyst SIMS is not an in situ method Is very sensitive for some elements on the ppm level IC-20/35 Lecture-5 07 -10 -2004

IC T Ions in Ion spectroscopy: LEIS Ions out catalyst Why a shoulder? Very surface sensitive IC-21/35 Lecture-5 07 -10 -2004

IC T Temperature programmed reduction, oxidation and sulfidation IC-22/35 Lecture-5 07 -10 -2004

IC T Temperature programmed reduction, oxidation and sulfidation Sulfidation in a H 2 S/H 2 mixture IC-23/35 Lecture-5 07 -10 -2004

IC T Infrared spectroscopy IR out IR in catalyst IR through An in situ method IC-24/35 Lecture-5 07 -10 -2004

IC T Infrared spectroscopy IR is used for identifying intermediates Active sites by adsorption of probe molecules like NO, CO, . . Line position may reveal bonding geometry and bonding strength Surface Science metod is HREELS High Reflection Electron Energy Loss Spectroscopy - photons replaced by electrons IC-25/35 Lecture-5 07 -10 -2004

IC T A Surface Science Approach to Heterogeneous Catalysis Natural Gas CH 4+H 2 O Ni CO + 3 H 2+(CO 2) Cu CH 3 OH N 2+Fe, Ru H 2 NH 3 IC-26/35 Lecture-5 07 -10 -2004

IC T Low Electron Energy Diffraction (LEED) Requires well-defined surfaces under UHV IC-27/35 Lecture-5 07 -10 -2004

IC T LEED picture Reconstruction of a Fe(111) surface into a 5 x 5 N-Fe(111) surface IC-28/35 Lecture-5 07 -10 -2004

IC T LEED Crystal Screen Diffracted beams (-1, 1) (0, 1) (1, 1) (-1, 0) (0, 0) a 2 (1, 0) a 2* a 1 Primary beam a 1* (-1, -1) (0, -1) (1, -1) IC-29/35 Lecture-5 07 -10 -2004

IC T The Scanning Probe Methods Requires well-defined surfaces. For STM they must also be conducting y surface z x current Piezo electric elements tip IC-30/35 Lecture-5 07 -10 -2004

IC T Scanning tunneling microscopy (STM) Length scales ”Seeing” atoms Electron tunneling Hardware realisation Applications Basic research L. P. Nielsen University of Århus (80Åx 80Å) IC-31/35 Lecture-5 07 -10 -2004

IC T Crystallites and STM-tip SEM image A sharp tip 35 micrometer A. Emundts und H. P. Bonzel IC-32/35 Lecture-5 07 -10 -2004

IC T STM Au/Ni(111) Let you sometime, but not always, see the atoms 0. 02 ML Au 0. 8 Ml Au IC-33/35 Lecture-5 07 -10 -2004

IC T Pt diffusion on Pt(110) IC-34/35 Lecture-5 07 -10 -2004

IC T STM Model Catalyst Mo. S 2 on Au(111) Sulfur removed by atomic hydrogen IC-35/35 Lecture-5 07 -10 -2004

IC T Manipulating atoms Fe/Cu(111) Kanji ”atom” CO/Pt(111) ”CO-man” 5 nm Scanned at very low temperatures http: //www. almaden. ibm. com/vis/stm/gallery. html IC-36/35 Lecture-5 07 -10 -2004

IC T Quantum Corrals Fe/Cu(111) http: //www. almaden. ibm. com/vis/stm/gallery. html IC-37/35 Lecture-5 07 -10 -2004