Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment Photons: In situ spectroscopy in the soft X-ray energy range Axel Knop-Gericke

Outline Photons for the investigation of heterogeneous catalytic processes Synchrotron radiation diffraction : single slit, double slit, grating in situ XAS in the soft energy range examples: methanol oxidation over copper n-butane oxidation over VPO catalysts

Spectrum of electromagnetic radiation

UV-vis : A. Brückner Vibrational spectroscopy (IR spectroscopy, Raman: G. Rupprechter ) Electron spectroscopy: R. Schlögl X-ray diffraktion: I. Erran EXFAS/NEXAFS in the hard X-ray range: T.Ressler Other lectures in this field :

Synchrotron radiation

Undulator

Synchrotron radiation

Single Slit Diffraction Princip of Huygens Minimum: g= k k=1,2,3,...<a/ sin  k = g/a ; tan  k =d k /l

Diffraction Pattern

Double Slit Diffraction Expected diffraction pattern: Actually observed diffraction pattern

Double Slit Diffraction l>>a : waves normals related to P k on the screen are parallel, sin  k = g/a, tan  k = d k /l  k < 8  : sin  k = tan  k = d k /l = g/a  d k = gl/a Maximum (g= 0,, 2, 3,...) d k = k l/a k= 0,1,2,3.. Minimum (g= /2, 3/2, 5/2,...) d k =(2k+1) l/2a k=0,1,2,3,..

From single slit to grating: Diffraction pattern

XAS in the soft energy range Soft energy range: eV which elements: C(1s), N(1s), O(1s), transition metal (2p)....  L edge / 2p XPS peaks of transition metal are sensitive to details of chemical XAS is a local process not restricted to material with long range order surface sensitive when applied in electron yield mode  pellets can be investigated under reaction conditions orientation of molecules on single crystal surfaces can be estimated

Anregung von Rumpfniveauelektronen mittels Röntgenstrahlung Relaxation mittels Fluoreszenzstrahlung oder durch Aussenden eines Auger- Elektrons (TEY, PEY, AEY) Untersuchung der unbesetzten Zustände Anregung von Rumpfniveauelektronen mittels Röntgenstrahlung Relaxation mittels Fluoreszenzstrahlung oder durch Aussenden eines Auger- Elektrons (TEY, PEY, AEY) Untersuchung der unbesetzten Zustände Spektroskopische Methode Prinzip der Röntgenabsorptionsspektroskopie (XAS) Auger-Elektron Fluoreszenz

Mean free path of electron in solids

Experimental Technique In situ methods are required to investigate heterogeneous catalytic reactions since the structure of a catalyst estimated ex situ might differ from the structure revealed by in situ studies Material gap single crystal vs real catalyst Pressure gap UHV vs p > 1bar XAS in the soft energy range represent surface sensitive spectroscopic methods, which can be applied in the mbar pressure range

Transmission of 20 cm air

heating up to 900 K pressure up to 20 mbar batch- and flow-through-mode angular dependent measurements heating up to 900 K pressure up to 20 mbar batch- and flow-through-mode angular dependent measurements Experimental Set-Up properties of the set-up heating

In situ XAS Detector system Simultaneous detection of gas phase- and sample signal

Gas phase subtraction NEXAFS of the O K-edge Analysis of the Near Edge X-ray Absorption Fine Structure (NEXAFS) Total electron yield of the gas phase dominates all signals, therefore only small differences in the detector signals Substraction allows to separate the absorption signal of the surface of the catalyst Total electron yield of the gas phase dominates all signals, therefore only small differences in the detector signals Substraction allows to separate the absorption signal of the surface of the catalyst

2 CH 3 OH + O 2 2 CH 2 O + 2 H 2 O CH 3 OHCH 2 O + H 2 2 CH 3 OH + 3 O 2 2 CO H 2 O oxidative dehydrogenation dehydrogenation total oxidation Methanol oxidation

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany Cu L 3 - NEXAFS Increased activity for gas flow ratios: O 2 / CH 3 OH  0.5 Increased activity for gas flow ratios: O 2 / CH 3 OH  0.5 Catalytic Activity NEXAFS at the Cu L 3 -edge Transition from an oxidic copper-phase to the metallic state

NEXAFS of the active state is completly different from the NEXAFS of the known copper-oxides 2 oxidic- and 1 suboxidic species can be distinguished NEXAFS of the active state is completly different from the NEXAFS of the known copper-oxides 2 oxidic- and 1 suboxidic species can be distinguished NEXAFS at the O K-edge less active very active

Intensity of the suboxide species increases with increasing temperature Intensity of the suboxide species is positively correlated to the yield of CH 2 O and CO Intensity of the suboxide species increases with increasing temperature Intensity of the suboxide species is positively correlated to the yield of CH 2 O and CO Variation of temperature at O 2 / CH 3 OH = 0.2 Correlation between the SuboxideSpecies and CH 2 O

Intensity of the oxidic species Ox surf decreases with increasing CO 2 -yield 2 areas of activity can be distinguished Intensity of the oxidic species Ox surf decreases with increasing CO 2 -yield 2 areas of activity can be distinguished Correlations between oxidic species and CO 2

Model Proposed model of the copper surface under reaction conditions for methanol oxidation

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany n-Butane Oxidation to MA by Vanadium Phosphorus Catalysts +3,5 O 2 +4 H 2 O 1,5 Vol%air C 4 H 10 +6,5 O 2 4 CO 2 +5 H 2 O C 4 H 10 +4,5 O 2 4 CO+5 H 2 O VPO 400 °C, 1 bar O O O Maleic Anhydride (MA) Active phase: highly ordered vanadyl pyrophosphate (VO) 2 P 2 O 7 ) ?

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany The VPO V L 3 -NEXAFS V1 V2 V3 V5 V4 V6 V7 Analysis of spectral shape by unconstrained least squares fit V valence Details of the local chemical bonding Local geometric structure

Interpretation of V L 3 NEXAFS Å V1*V1* Å V2O5V2O5 V6V6 V1V1 V2V2 V3V3 V4V4 VPO V4*V4* V2*V2* V3*V3* V5*V5* V6*V6*  NEXAFS resonances appear in a sequence of V-O bond lengths Experimental finding:

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany Changes of NEXAFS while heating Relative spectral Intensity of V 5 at V L 3 -edge Proportion of int. intensity of V5 Spectra number V5V5 RT 400°C Decrease while active MA Yield (a. u.)

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany Interpretation of V L 3 NEXAFS O(1a) O(2) O(1b) O(3) V V 2 O 5 as model substance for VPO V 2 O 5 : Close relationship between geometric and electronic structure at V L 3 -absorption edge DFT calculation of DOS (V 2 O 5 !)*:  V6: O(1a)  V5: ? (estimated value of bond length between O(2) and O(1a): 1.72 Å)  V6: O(1a)  V5: ? (estimated value of bond length between O(2) and O(1a): 1.72 Å) Identification of resonances (V5, V6):  main contributions to NEXAFS resonances appear in a sequence of V-O bond length  main contributions to NEXAFS resonances appear in a sequence of V-O bond length