Polycrystalline Vanadium

download Polycrystalline Vanadium

of 7

  • date post

    16-Aug-2015
  • Category

    Documents

  • view

    13
  • download

    2

Embed Size (px)

description

ciencia

Transcript of Polycrystalline Vanadium

Water adsorption on polycrystalline vanadium from ultra-high vacuumto ambient relative humidityC. Rameshana,, M.L. Ngb, A. Shavorskiyc, J.T. Newbergd, H. BluhmcaInstitute of Materials Chemistry, Technische Universitt Wien, Getreidemarkt 9, 1060 Vienna AustriabSUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park CA 94025 USAcChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USAdDepartment of Chemistry & Biochemistry, University of Delaware, Newark, DE 19716 USAabstract arti cle i nfoArticle history:Received 2 April 2015Accepted 3 June 2015Available online 10 June 2015Keywords:HydroxylationAmbient pressure photoelectron spectroscopyWater adsorptionVanadiumIn-situ spectroscopyWe have studied the reaction of water vapor with a polycrystalline vanadium surface using ambient pressureX-ray photoelectron spectroscopy (AP-XPS) which allows the investigation of the chemical composition of thevanadium/water vapor interface at p(H2O) in the Torr range. Water dissociation on the vanadium surface wasstudied under isobaric conditions at p(H2O) ranging from 0.01 to 0.50 Torr and temperatures from 625 K to260 K, i.e. up to a relative humidity (RH) of ~15%. Water vapor exposure leads to oxidation and hydroxylationof the vanadium foil already at a pressure of 1 106Torr at 300 K (RH ~ 4 106%). The vanadium oxidelayer on the surface has a stoichiometry of V2O3. Initial adsorption of molecular water on the surface is observedat RHN 0.001%. Above a RHof 0.5% the amount of adsorbed water increases markedly. Experiments at increasingtemperatures show that the water adsorption process is reversible. Depth prole measurements show a thick-ness for the vanadium oxide layer of 35 mono layers (ML) and for vanadium hydroxide of 11.5 ML over thewhole RH range in the isobar experiments. The thickness of the adsorbed water layer was found to be in thesub-ML range for the investigated RH's. 2015 Elsevier B.V. All rights reserved.1. IntroductionTheinteractionof watervaporwithsolidsurfacesat ambientconditions of temperature and relative humidity plays a major role intechnological applications and in the environment and is thus a highlyinterdisciplinary eld. Hitherto researchhas focusedonthe role of inter-facial water in heterogeneous catalysis [13], atmospheric chemistry[4,5], environmental science [6], corrosion chemistry [7] and electro-chemistry[8,9]. Themechanismandkineticsof surfacechemicalprocesses are strongly inuenced by the presence of adsorbed water[10,11]. Water can be a participant or product in surface chemical reac-tions, as in the water gas shift reaction (CO+H2OCO2+H2) or it canbe a spectator and still inuence the reaction through blocking of activesites or hindering the adsorption of reactants. On the other hand, traceamounts of H2O can promote CO oxidation on Pt(111) [12] and Aunanoparticles supported on TiO2 [13,14]. Most surfaces, in particularthe polar ones, are covered by a water layer with thicknesses from afew (aerosol particles in troposphere) to innite thickness (particlesin solution) under ambient relative humidities [1517]. Despite its im-portance the growth mechanismof water and water layers on differentmaterials (metallic, mineral, oxide) is still not fully understood for allsurfaces.The interaction of water with solid surfaces has been intensivelystudiedbyusingsurfacesciencetechniquesinultrahighvacuum(UHV) and at low temperatures. These studies provide detailed infor-mation on the water/solid interface at a molecular level [1820]. Mostprocesses of interest in real systems take place at elevated temperaturesand at ambient or even higher pressures, as in heterogeneous catalysis.The fundamental questionis if the informationthat is gained under UHVand low temperatures can be extrapolated to realistic conditions. Thestructure and chemical composition of the surface in equilibrium withgases at ambient pressure can be different from those in UHV. Further-more, chemical reactions can be kinetically hindered at low tempera-tures. Thisisoftenreferredtoasthepressuregap. Inordertoclose this gap, surface chemical reactions including those involvingwater have to be investigated in situ at as close to realistic operatingconditions as possible.Synchrotronbasedinsituambient pressureXPS (AP-XPS) isanexcellent experimental tool for water adsorption studies on surfaces atambient relative humidities since it allows the investigation of surfacesat water vapor pressures in the Torr range (equilibrium water vaporpressure at 273 K is 4.6 Torr) and up to a RH of 100% [21]. Furthermoreit provides information on the elemental composition at the samplesurface as well as on the local chemical environment (e.g., oxidationSurface Science 641 (2015) 141147 Corresponding author.E-mail address: christoph.rameshan@tuwien.ac.at (C. Rameshan).http://dx.doi.org/10.1016/j.susc.2015.06.0040039-6028/ 2015 Elsevier B.V. All rights reserved.Contents lists available at ScienceDirectSurface Sciencej our nal homepage:www. el sevi er . com/ l ocat e/ suscstates and functional groups) [22]. Recently, AP-XPS has been used toinvestigatetheinteractionofwaterwithCumetal [23]andmetaloxide surfaces, including -Fe2O3(0001) [24], Fe3O4(001) [25],MgO(100)/Ag(100) [26], Cu2O [27], Al2O3 [27], TiO2 [15] and SiO2 [28].Here we discuss the interaction of water vapor with a polycrystallinevanadium surface.Vanadium is used in a wide range of applications. Aside from steelproduction, vanadiummetal is used as a coating material, an alloy com-ponent in functional materials [29] and it is also a promising alternativeto more costly metals (such as Pd) in H2purication processes [30]. Va-nadium oxides are part of electrical and optical switching devices, lightdetectors, sensors and in heterogeneous catalysis. The high variety ofoxidation states of vanadium (V0V5+) make it suitable for numerouscatalytic reactions [31,32].Extended research has focused on the properties of vanadiumoxide,as described in the review of Surnev et al. [33]. There is, however, notmuchinformationyetontheinteractionof vanadiummetal withwater vapor, although this is highly relevant for hydrogen puricationprocesses, including vanadium membranes, and for catalytic reactions.Jaegeretal. [34]studiedthechemisorptionofwateronvanadiumclusters by infrared photodissociation (IR-PD) spectroscopy. On thebasis of their measurements they postulated that on the V+-clusters(3 to 18 atoms) water is mainly adsorbed as intact molecule; it couldnot be excluded, however, that some dissociative chemisorption ofwater is present on the clusters because hydroxyl groups would notexhibit any bending mode resonance in IR-PD [34].Here we report on the interaction of water vapor with a polycrystal-line vanadium foil, which we have studied using AP-XPS by measuringuptake and desorption isobars at water pressures of 0.05, 0.10, 0.25and0.50Torr. Thequantitativeanalysisof thepeakareasduetoadsorbed water molecules, hydroxide groups and vanadium oxide pro-vides information on the degree of oxidation and hydroxylation of thevanadium surface as well as the thickness of the adsorbed water layeras a function of RH. We show that hydroxylation occurs atRH b 106%, while molecular water is already present at RH as low as103%. These results imply that the vanadium surface is covered by asignicant amount of hydroxyl groups and molecular water moleculesunder most realistic operating conditions, which need to be taken intoaccount in models of the heterogeneous surface chemistry of vanadiumin catalytic reactions.2. Materials and methodsThe experiments were performed at the Molecular EnvironmentalScience beamline (11.0.2) at the Advanced Light Source (ALS) at Law-rence Berkeley National Laboratory [21], using the ambient pressureX-ray photoelectron spectrometer endstation [35]. AP-XPS is based ona differentially-pumped electrostatic lens system, which minimizesthepathlengthofelectronsthroughthehigh-pressureregionandthusscattering ofelectrons bygas molecules, as wellas maintainshigh vacuum conditions in the electron energy analyzer [21,36].Polycrystalline vanadium foil (Alfa Aesar, 99.5% purity, 0.15 mmthickness) was cleaned prior to the experiments by several sputter-anneal cycles (105Torr of Ar, 1.5 keV, 4 mA) followed by annealingto 1200 K for 2 min. The cleaning progress was monitored by XPS.Allimpuritiescouldberemovedexceptforsmalltracesofoxygen(less than ~0.15 ML equivalent, chamber base pressure was~8 1010Torr). The high reactivity of vanadium towards oxygenand water vapor in the residual gas makes the preparation of oxygen-free surfaces extremely difcult. The required equipment for the prepa-ration of oxygen-free vanadium, a titanium sublimation pump and acooling trap held at liquid nitrogen temperature as it is described inthe literature [37], is not compatible with the experimental setup usedin the present investigations.After cleaning, the vanadium foil was transferred from the prepara-tion chamber to the spectroscopy chamber for XPS analysis of the initialstate of the foil prior to water vapor exposure. XPS data were collectedfor V 2p, O 1s and C 1s core levels at a kinetic energy (KE) of ~210 eVwith incident photon energies of 720 eV, 735 eV and 490 eV, respective-ly. For the depth proling the O 1s and V 2p spectra were taken insequence with kinetic energies between 115 eV and 715 eV in 100 eVincrements. All binding energies were referenced to the V Fermi edge,which was measured after every change of the incident photon energy.Water vaporfromHPLCgradewater wasintroducedintothemeasurement chamber through a precision leak valve. Prior to the ex-periments the water was puried in multiple freezepumpthawcyclesfollowed by direct pumping on the water source at room temperature.The relative humidity is dened by RH = 100(p/p0), where p is thewater vapor pressure in the spectroscopy chamber and p0the equilibri-um water vapor pressure (calculated from Eq. 2.5 of Wagne