Bluhm, H.

Fadley, C.

Hussain, Z.

Hävecker, M.

Knop-Gericke, A.

Ogletree, D.

Requejo, F.

Salmeron, M.

Schlögl, R.

English

MPG.PuRe

High-pressure XPS: a new tool for environmental science and catalysisD.F. Ogletree, F.G. Requejo, C.S. Fadley1, Z. Hussain2, M. Salmeron H. Bluhm, A. Knop-Gericke, M. Hävecker, R. SchlöglLawrence Berkeley National Laboratory, Materials Sciences Division, Berkeley, CA 94720.1also at Department of Physics, University of California, Davis, CA 95616.2 Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA 94720.Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Anorganische Chemie, Faradayweg 4-6,D-14195 Berlin, Germany.The Instrument Premelting of IceObserving Catalysts at WorkIntensity (a.u.)560555550545540535530Incident photon energy (eV)292 288 284Fluorescence Yield,Ice, -25 ºCcontamination peakfree hydrogen peak-4 ºC-25 ºCXPSC1s-25 ºC0.400.350.300.250.200.150.10Free hydrogen peak area0.40.30.20.10.0Contamination peak areaWater-----------------------------------Ice-2 C -10 C-4 C -15 C-5 C -25 C-7 C -39 CWhy high-pressure XPS?Many processes and samples cannot be investigated in UHV (“Pressure Gap”)• catalysis, corrosion, some oxidation• environmental chemistry• biological samples under wet conditions (high vapor pressure of water!)LEED-type calculation of elastic scattering cross sections for Oxygen atoms1 Torr-mm of water vapor is 3.3 monolayersFundamental limit in high-pressure XPS:Elastic and inelastic scattering of electrons by gas molecules0 200 400 600 800 10000246Elastic Mean Free Path in Oxygen GasElectron Kinetic Energy [eV]Mean Free Path [Torr-mm]The solution: Differentially pumpedelectrostatic lens systemExperimental cellsupplied by gas linesX-rays enter the cell at15° incidence through asilicon nitride window(100 nm thick, 4x4 mm wide, Δp ~50 torr)„Nozzle“„Control Electrode“„Skimmer“„Condenser“„Quadrupole“Analyzer input lens;analyzer is pumpedindependentlyFocal point of analyzerinput lensFirst differentialpumping stageSecond differentialpumping stageIceliquidTmeltTonsetAbove a certain temperature the ice surface is covered bya thin liquid filmOur goal is the investigation of the influence of hydrocarbon contamination (using XPS) on the thickness (using NEXAFS)of the liquid-like layer.Normalized Auger yield (a.u.)555550545540535530Incident photon energy (eV)-25 ºC-15 ºC-10 ºC-5 ºC-4 ºCice, -2 ºCwater, -2 ºC-39 ºC“Clean” Ice SurfaceAuger yield NEXAFS spectra showing the premelting transition302520151050LLL thickness (Å)-40 -30 -20 -10 0Temperature (ºC)Thickness of the liquid layer on “clean” ice samples as determined from the NEXAFS spectraenergy windowfor Auger Yield NEXAFSphotoelectronsfor hv=530...560 eVE      of valencekin2a11b3a111b2Oxygen KLL Auger spectra550500450400supercooled water-10 ºC, 1.95 torrice, -10 ºC, 1.95 torrice, -85 ºC, p < 0.01 torr(hv = 560 eV)vapor, 0.5 torr2.0 torrNormalized Auger yield (a.u.)Kinetic Energy (eV) (hv = 569 eV)Comparison of NEXFAS and XPS: At both temperatures  the more contaminatedice surface shows stronger signs of  premelting  than the less contaminatedice surface.Area of the free hydrogen peak (535 eV) vs. contamination peak area (532 eV). Premelting of the ice surface thus depends strongly on both temperature and hydrocarbon contamination. Acknowledgements: Gennadi Lebedev (GETOM, Petaluma, CA), initial lens design; John Bozek, Ed Wong, ALS; Eli Rotenberg, XPS software; Anders Nilsson, Satych Myneni for helpful discussions.Example: oxidation of methanol over a copper catalyst.The two main reactions are:(1) CH3OH + ½O2 CH2O+ H2O(2) 2CH3OH + 3O2 2CO2+ 4H2OReaction (1) (partial oxidation) is an important process in the chemical industry. Our goal is to identify the composition and chemical nature of the surface of the copper catalyst for formaldehyde (CH2O) production under working conditions.Contaminated Ice Surface545 540 535 530Binding Energy (eV)O2 (g)CH3OH (g)CH3OH (surface)Cu2O CH3O-XPS O1s spectrum of a Cu foil in a gas mixture of 0.07 torr O2 + 0.34 torr CH3OH @ 275 °C.Due to the work function of the sample (4.5 eV) the surface peaks are shifted towards the gas phase peaks.0,10,20,3150 200 250 300 350 400 450Temperature (°C)00,10,20,30,40,50,6CH2 O yield (a.u.)O1s Cu2O intensity (a.u.)Comparison of mass spec and XPS data. A more metallic (i.e., less oxidic) Cu surface and higher temperatures favor the production of formaldehyde (CH2O).

High-pressure XPS: a new tool for environmental science and catalysis

https://pure.mpg.de/pubman/item/item_741236_2/component/file_3241251/trieste.pdf

High-pressure XPS: a new tool for environmental science and catalysis

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe