Bluhm, H.

Hävecker, M.

Knop-Gericke, A.

Teschner, D.

English

MPG.PuRe

High pressure XPS: A tool for the investigation of heterogeneous catalytic processes

1IDECAT WP3 SeminarPragNovember 28,2006High pressure XPS: A tool for the investigation of heterogeneous catalytic processesA. Knop-Gericke , D.Teschner, H. Bluhm, M. Hävecker(knop@fhi-berlin.mpg.de)Fritz-Haber-Institut, Berlin, Department of Inorganic Chemistry2• Very few methods can investigatethe solid-gas interfaceat high pressures- non-linerar optics (SFG, SHG)- scanning probe microscopies- X-ray diffractionWhy in situ XPS ?15105-40 -20 0 20temperature (ºC)vaporpressure(torr)water Tm• Photoelectron spectroscopy is very powerful⇒ Goal:  XPS at pressures of at least 5 torr• Many processes cannot beinvestigated in UHV:“Pressure Gap”- environmental chemistry- catalysis- corrosion- electrochemistry- biological samples3Fundamental limit:elastic and inelasticscattering of electronsin the gas phaseTechnical issues: - Differential pumping to keep analyzer in high vacuum- Sample preparation and control in a flow reactorIn situ XPS:  obstacles30x10-21 2520151050Ionization cross section (m2)101102103104105106Electron kinetic energy (eV)in situSEM in situTEMin situEXAFSin situXPSO2 exp. data from Schram et al. (1965) extrapolation of Schram's data4• Photons enterthrough a window• Electrons and a gasjet escape throughan aperture tovacuumIn situ XPS:  basic concept5H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 319 (1973)2• H. Siegbahn et al. (1973- )• M.W. Roberts et al. (1979)• M. Faubel et al. (1987) • M. Grunze et al. (1988)• P. Oelhafen (1995)H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 205 (1981)24In situ XPS instruments:  previous designsIn situ XPS using differentially pumped electrostatic lensesD.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci. Instrum. 73 (2002) 3872.to pump to pumpX-rays from synchrotronGas phase composition canbe measured by XPS.gas phase signal:  1 torr·mm ~ a few monolayersClose-up of sample-first aperture regiongaszdhνe-p01.00.50.0-2 -1 0 1 2z [d]p/p0z=2 mm=1 mm8In situ XPS systemExperimental cellsupplied by gas lines (p0)X-rays enter the cell at 55° incidence through an SiNx window(thickness ~ 1000 Å)Analyzerinput lensFocal point of analyzerinput lensFirst differentialpumping stage (10-4p0) Second differentialpumping stage (10-6p0) Third differentialpumping stage (10-8p0) Hemisphercalelectronanalyzer(10-9p0)mass spectrometerand additional pumping9IntroductionLiterature carbon laydown selective hydrogenation“similar” catalysts different activity & selectivity(structure sensitivity?)Selectivity issue: what defines selectivity?[     ]+ H210Summary1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation2. Surface H : could be selective (spillover)3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes4. C uptake is significantly more in the selective regime 11Reaction in the mbar p region (in-situ XPS)-~2~20~5SelectivityPentane [%]100~98~80~95SelectivityPentene [%]<1~2.5~5~ 10Conversion[%]Pd(111)Pd foil3% Pd/Al2O35% Pd/CNT+ H2Recation conditions: C5/H2 = 1:9, 1 mbar, 358 K12In-situ XPS: Pd 3d (hν: 720 eV)13In-situ XPS: Pd 3d depth profilingNot only adsorbate-induced surface core level shift !But on-top location!14In-situ XPS: C1s (Switching off experiments)1: reaction mix.2: C5 off3: H2 off4: C5 gas-phaseTeschner et al.J. Catal. 242 (2006) 26-3715In-situ XPS: Pd 3d (Switching off experiments)1: reaction mix.2: C5 off3: H2 off4: reaction; high T523 K16In-situ XPS: Pd vs. C depth profiling100 200 300 400 500 600 700 8002025303540455055606570A  Palladium Inelastic Mean Free Path [Å]Carbon Content [%]Electron Kinetic Energy [eV] Experiment: 358 K Model: 358 K Model: 3.5 ML Experiment: C5 off Model: C5 off5.25 7.15 9.05 15.15202530354045505560657075100 200 300 400 500 600 700 800 22.3513.2510.25 Electron Kinetic Energy [eV]Carbon content [%]Carbon Inelastic Mean Free Path [Å] Epxeriment: 523K Model: 523K Model: 4.2 ML7.25B0.3 nm1.4 nm14 nm17Summary1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation2. Surface H : could be selective (spillover)3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes4. C uptake is considerably more in the selective regime 5. Pd-C surface phase forms in the early stage of selective pentyne hydrogenation & there is significant amount of subsurface Cbelow of it18Model (during the reaction)19Summary1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation2. Surface H : could be selective (spillover)3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes4. C uptake is considerably more in the selective regime 5. Pd-C surface phase forms during selective hydrogenation of pentyne & there is significant amount of subsurface C below of it6. Dynamic behaviour of Pd-C and subsurface C 20M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSYInstallation of a beamline exclusively used for in situ spectroscopy in the soft X-ray rangeInstallation of infrastructure optimized for these kind of experiments on site (e.g. chemical lab, gas supply, gas analytics)ISISS:Innovative Station for In Situ Spectroscopy A project of BESSY and the Dep. Inorganic Chemistry, Fritz-Haber-Institut Later, further implementation of other in situ spectroscopy techniques: multi wavelength Raman, UV-Vis, fluorescence yield ?!Start of user operation of the beamline: 200721M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSYISISS:Sketch of laboratory: dr. heinekamp, Berlin, Germany22M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY1,0E+091,0E+101,0E+110 200 400 600 800 1000 1200 1400Photon Energy / eVflux (phot./s/0.1A/0.1%BW)1200 l/mm, 80 μm1200 l/mm, 20 μm600 l/mm, 80 μm600 l/mm, 20 μm100030005000700090000 200 400 600 800 1000 1200 1400Photon Energy / eVResolution (E/dE)1200 l/mm, 80 μm1200 l/mm, 20 μm600 l/mm, 20 μm600 l/mm, 80 μm23M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY24M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY25M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY26M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY27M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, GermanyOutlook: In situ XPS / XASThe future at BESSY28Thanks to:• Detre Teschner, Elaine Vass, Michael Hävecker, EvgueniKleimenov,,Spiros Zafeiratos, Péter Schnörch, Hermann Sauer, Robert Schlögl  (FHI, Dept. AC)• Mounir Chamam, Attila Wootsch (Institute of Isotops, Budapest) • Arran S. Canning, Jonathan J. Gamman, S. David Jackson (Glasgow University)• James McGregor, Lynn F. Gladden (Cambrigde University)• Fred Senf, Rolf Follath, Walter Braun, Jens Blume, Jan Schmidt (BESSY) 

High pressure XPS: A tool for the investigation of heterogeneous catalytic processes

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